S O U T H W E S T R E S E A R C H I N S T I T U T E

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1 S O U T H W E S T R E S E A R C H I N S T I T U T E 6220 CULEBRA ROAD POST OFFICE DRAWER SAN ANTONIO, TEXAS, USA (210) TELEX Department of Fluids Engineering September 12, 2008 To: From: Reference: Participants of the Hydrocarbon Dew Point Analyzer Evaluation JIP Darin L. George Testing of Methods for Measuring Hydrocarbon Dew Points in Natural Gas Streams FINAL REPORT SwRI Proposal No A, Project No Attached is a copy of the Final Report for the Hydrocarbon Dew Point Analyzer Joint Industry Project. If you have any questions or comments about the report, please feel free to contact me at (210) or via at darin.george@swri.org. Respectfully submitted, ORIGINAL IS SIGNED Darin L. George, Ph.D. Senior Research Engineer Flow Measurement Section Approved: ORIGINAL IS SIGNED Terrence A. Grimley, Program Coordinator Flow Measurement Group cc: Ray Adcock, Ametek E. B. Bowles, SwRI Andrew J. Benton, Michell J. C. Buckingham, SwRI David F. Bergman, Ph.D., BP (for GPA) R. C. Burkey, SwRI Ilia Bluvshtein (Union Gas) T. A. Grimley, SwRI David Bromley, BP (for PRCI) Report Copy B Ron Brunner, GPA D. Todd, SwRI Paul Burnett, PMC (for Michell) T. Walvoord, SwRI Ben Ho, BP (for PRCI) Dean Osmar, Ametek Dan Potter, Ametek William H. Ryan, El Paso (for API) Andrew Stokes, Michell Mike Whelan, PRCI

2 Tests of Instruments for Measuring Hydrocarbon Dew Points in Natural Gas Streams, Phase 2 FINAL REPORT Prepared by: D. L. George, Ph.D. R. A. Hart SOUTHWEST RESEARCH INSTITUTE Mechanical and Fluids Engineering Division 6220 Culebra Road San Antonio, Texas, USA Prepared for: Michell Instruments Inc. 11 Old Sugar Hollow Road Danbury, CT September 12, 2008

3 Tests of Instruments for Measuring Hydrocarbon Dew Points in Natural Gas Streams, Phase 2 FINAL REPORT Prepared by: D. L. George, Ph.D. R. C. Burkey SOUTHWEST RESEARCH INSTITUTE Mechanical and Fluids Engineering Division 6220 Culebra Road San Antonio, Texas, USA Prepared for: Michell Instruments Inc. 11 Old Sugar Hollow Road Danbury, CT September 12, 2008

4 DISCLAIMER This report was prepared by Southwest Research Institute (SwRI ) as an account of contracted work sponsored by Michell Instruments Inc. (Michell). Neither SwRI, Michell, members of these organizations, nor any person acting on their behalf: a. Makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, methods, or process disclosed in this report may not infringe upon privately owned rights; or b. Assumes any liability with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process disclosed in this report. References to trade names or specific commercial products, commodities, or services in this report does not represent or constitute an endorsement, recommendation, or favoring by SwRI or Michell of the specific commercial product, commodity, or service.

5 TESTS OF INSTRUMENTS FOR MEASURING HYDROCARBON DEW POINTS IN NATURAL GAS STREAMS, PHASE 2 D. L. George, Ph.D. R. A. Hart SOUTHWEST RESEARCH INSTITUTE Mechanical and Fluids Engineering Division 6220 Culebra Road San Antonio, Texas, USA ABSTRACT Research has assessed the accuracy of two commercially-available hydrocarbon dew point (HCDP) analyzers, an Ametek Model 241 CE II and a Michell Condumax II. During a previous phase of this project, both automated analyzers, along with a Bureau of Mines chilled mirror device serving as a reference, were tested on gravimetrically-prepared gas blends chosen to simulate a transmission-quality gas and a production gas. The measurement repeatability of both units was found to be better than the manual chilled mirror. Trends in the analyzer and manual chilled mirror measurements suggested that differences in performance between the automated units were related to their measurement techniques and default set points. During the second phase of the project, the Ametek and Michell automated analyzers were tested again on the transmission-quality test gas used in Phase 1, but with specific levels of contamination added to gain knowledge of their performance under adverse conditions. In one round of tests, water vapor was added to simulate a transmission gas with water vapor levels above common tariff specifications. In the second round of tests, the test gas contained both methanol and water vapor, simulating a stream to which methanol has been added to prevent hydrates. Contaminants were added to the test gas stream in amounts such that, depending upon the pressure of the test stream, the contaminant dew point would be reached first, the HCDP would be reached first, or the two phases would condense simultaneously. Multiple HCDP measurements were made with the analyzers to determine their ability to accurately measure HCDPs under these adverse conditions. Analyzer results were again compared to HCDP measurements taken with the Bureau of Mines chilled mirror device and a digital video camera. Results were adjusted for small changes in the heavy hydrocarbon content of the test gases over time, using predictions from an equation of state and gas chromatographic analyses of the test gases. The repeatabilities of the Ametek 241 CE II and the Michell Condumax II in the presence of contaminants were comparable to one another. Both automated units generally showed tighter repeatabilities than that of the Bureau of Mines chilled mirror, as was the case in the uncontaminated streams. Measurements by the Bureau of Mines chilled mirror demonstrated the potential problems involved when the same mirror surface is subject to both hydrocarbon and contaminant condensation. The first condensate to form on the mirror can provide nucleation iii

6 sites for later phases, or may reduce the number of nucleation sites, causing the second condensate to appear at different temperatures than would occur in the absence of the first condensate. In one instance, formations on the mirror, surmised to be natural gas hydrates, appeared to provide nucleation sites for a hydrocarbon iridescent ring to form at a higher temperature than in the absence of water contamination. Comparisons of measurements by the Ametek 241 CE II to analogous measurements by the Bureau of Mines chilled mirror were limited, as very few iridescent ring dew points were observed on the chilled mirror during these tests. However, results suggest that the various contaminants have an effect on HCDP measurements by the Ametek unit, ranging from -6.2 F to +2.9 F (-3.4 C to +1.7 C). The Michell Condumax II HCDP measurements show similar changes as the Ametek 241 CE II HCDP measurements when water vapor is introduced to the stream. However, the presence of methanol-water contamination leads to higher HCDP measurements from the Michell unit, with shifts up to +8.2 F (+4.6 C) observed during tests. The cause of this upward shift has not been confirmed, but may be due to a methanol-water mixture condensing on the Michell unit chilled mirror and producing a false HCDP measurement. Most significant to the natural gas industry is the performance of the Bureau of Mines manual chilled mirror in the contaminated streams. When both methanol and water were present in the stream, upward shifts in droplet HCDP temperatures as high as 19.8 F (11.1 C) were observed. This change in HCDP may be related to the presence of a methanol-water condensate on the mirror, and as such, may indicate potential operational problems for natural gas pipelines. The increase in HCDP values from the Michell unit may be due to a similar phenomenon. With the conclusion of Phase 2 of the project, more information on analyzer performance is now available. In transmission-quality and production-quality natural gas streams free of water and methanol contaminants, the automated analyzers are in generally good agreement with the Bureau of Mines manual chilled mirror, particularly with the types of HCDPs the units were designed to detect. Phase 2 data from the Bureau of Mines unit demonstrate the difficulties in measuring HCDPs in the presence of water and methanol, which may condense on the chilled mirror and bias the measured HCDPs. Depending upon the design of the automated units, they may also be susceptible to bias from contaminants. The results of this research are offered to the natural gas industry, to assist in creating U.S. industry standards. Such standards can guide the industry in using the automated analyzers in processed and unprocessed natural gas streams, and in determining where the automated units may replace the Bureau of Mines chilled mirror device for routine dew point determination at field sites. Further research is suggested to identify the causes of measurement changes in the presence of contaminants. iv

7 EXECUTIVE SUMMARY The Bureau of Mines chilled mirror dew point tester is the most commonly-accepted instrument for measuring dew points of natural gas streams at field sites. In this device, a natural gas sample flows continuously from a pipeline across the surface of a small mirror. An operator slowly decreases the mirror temperature while watching through an eyepiece for hydrocarbon condensation on the mirror surface. When condensation is seen, the operator reads the mirror temperature and records it as the hydrocarbon dew point (HCDP). This device is often used to resolve custody transfer disputes over natural gas quality. However, its use requires skill and training to operate, and measurement techniques can vary from operator to operator. Some advances in operator training are being made through the use of video technology to record the condensation process. Because the dew point is detected visually, however, the chilled mirror is still considered a subjective instrument. An ideal dew point instrument would rely on objectively measured properties of the gas flow to identify the dew point, and could have the potential for improved repeatability, tighter measurement uncertainties, and less bias than the chilled mirror device. Two commercially-available HCDP analyzers, the Ametek Model 241 CE II and the Michell Condumax II, use optical methods to objectively detect condensed hydrocarbons and measure the HCDP. Phase 1 of a Joint Industry Project recently evaluated the repeatability and measurement uncertainties of these commercially-available devices on production-quality and transmission-quality hydrocarbon gas blends. Results from the units were compared to measurements of the same gas streams taken with a Bureau of Mines chilled mirror. As documented previously in the Phase 1 report, the repeatability of both the Ametek 241 CE II and the Michell Condumax II were both found to be tighter than that of the Bureau of Mines chilled mirror. The Ametek 241 CE II records the temperature at which a thin hydrocarbon film is detected, analogous to the iridescent ring dew point of the Bureau of Mines device. The default trip point for the Condumax II was chosen by Michell Instruments to agree with manual visual chilled mirror measurements of the droplet HCDP, which occur a few degrees below the iridescent ring dew point. Measurements by both instruments were consistent with analogous measurements from the Bureau of Mines chilled mirror. In Phase 2 of the project, documented here, tests assessed the impact of water and methanol contaminants in the gas stream on analyzer performance. The test apparatus used in Phase 1 was modified to introduce known amounts of each contaminant separately to the test gas stream. Tests were performed using the same hydrocarbon gas blend prepared for Phase 1 tests to resemble a transmission-quality gas (heating value of 1,050 Btu/scf, MJ/Nm 3 ). Contaminants were added to the test gas stream in amounts such that, depending upon the pressure of the stream, the contaminant dew point would be reached first, the HCDP would be reached first, or the two phases would condense simultaneously. Multiple HCDP measurements were made with the Ametek and Michell analyzers at the various pressures, to determine their ability to accurately measure HCDPs when water or a combination of water and methanol could potentially interfere with the measurement process. Analyzer results were again compared to HCDP measurements taken with the Bureau of Mines chilled mirror dew point tester and recorded with a digital video camera. Effects of small changes in the gas composition during the tests were approximated using dew point temperatures predicted by common cubic equations of state and gas chromatographic analyses of the test v

8 gases. As in Phase 1, the repeatabilities of the various instruments were determined using sequential repeat measurements at each dew point measurement condition. As Table ES-1 shows, the repeatabilities of the Ametek 241 CE II and the Michell Condumax II in the presence of contaminants were comparable to one another. Both automated units generally showed tighter repeatabilities than that of the Bureau of Mines chilled mirror. Measurement uncertainties were assessed, where possible, using analyzer specifications and calibration data provided by the manufacturers, along with uncertainties of instrumentation in the test apparatus. These values were found to be unchanged from Phase 1 tests. Table ES-1. Measurement repeatabilities of hydrocarbon dew point temperatures determined by each device at each test condition, after adjustment for changes in test gas composition. Contaminants Line Pressure Bureau of Mines Chilled Mirror Droplet Dew Points Ametek 241 CE II Michell Condumax II Water 750 psia (51.7 bara) 4.7 F (2.6 C) 0.4 F (0.2 C) 0.3 F (0.2 C) 500 psia (34.5 bara) 1.0 F (0.6 C) 1.5 F (0.8 C) 0.4 F (0.2 C) 300 psia (20.7 bara) 0.6 F (0.3 C) 0.1 F (0.1 C) 0.5 F (0.3 C) Methanol and water 750 psia (51.7 bara) 2.6 F (1.4 C) 0.4 F (0.2 C) 1.6 F (0.9 C) 500 psia (34.5 bara) 1.4 F (0.8 C) 1.3 F (0.7 C) 0.4 F (0.2 C) 300 psia (20.7 bara) 3.1 F (1.7 C) 0.5 F (0.3 C) 0.5 F (0.3 C) Unlike the automated devices, the manual chilled mirror has a single surface dedicated to the measurement of both HCDPs and water vapor dew points (WVDPs). The first condensate to form on the mirror can provide nucleation sites for later phases to condense at different temperatures than would occur in the absence of the first phase, causing bias in the dew points of the phases appearing at lower temperatures. With water alone as a contaminant, the average change in droplet HCDP temperatures over corresponding Phase 1 HCDPs without contaminants ranged from +0.2 F (+0.1 C) to +6.0 F (+3.4 C).. When both methanol and moisture were present in the stream, more significant upward shifts in droplet HCDP temperatures as large as 19.8 F (11.1 C) were observed. The presence of water and methanol-water in the gas stream significantly increased the droplet HCDP observed with the manual chilled mirror; this is potentially significant to the natural gas industry, since this may represent an increased risk of liquids in the pipeline. Equation-of-state estimations of HCDP normally do not show such increases, since compositional analyses are limited to hydrocarbon compounds, and often do not include water or methanol. Also, the manual chilled mirror results at 750 psia (51.7 bara) with only moisture in the stream were unique, in that the first formation on the mirror was surmised to be natural gas hydrates. At this pressure, a frost circle was observed first in the mirror center, with a hydrocarbon iridescent ring forming on top of the frost layer at lower temperatures. Comparisons of measurements by the Ametek 241 CE II to analogous measurements by the Bureau of Mines chilled mirror were limited, as only two iridescent ring dew points were observed on the chilled mirror during these tests. The average net effect of water contamination on the Ametek unit HCDP measurements ranges from -6.2 F to +2.9 F (-3.4 C to +1.7 C), vi

9 compared to the Ametek unit measurement uncertainty of ±2 F (±1 C). When both methanol and moisture were present in the stream, the net effect ranged from -6.2 F to -3.1 F (-3.4 C to C). These results suggest that the various contaminants have some effect on HCDP measurements by the Ametek unit, though the effect on the results may be positive or negative. In the tests on streams with water and methanol-water contaminants, the Michell unit reported HCDP temperatures lower than the manual chilled mirror droplet HCDPs in the majority of test conditions. After corrections for compositional changes during tests, the Michell unit HCDPs and chilled mirror droplet HCDPs showed statistically-significant differences from each other at every test condition. In the case of water contamination, the average net effect on HCDP measurements ranged from -7.2 F to +3.0 F (-4.0 C to +1.7 C), compared to the reported accuracy of ±0.9 F (±0.5 C). By comparison, average shifts in manual chilled mirror measurements, caused by water condensation on the mirror, ranged from +0.2 F (+0.1 C) to +6.0 F (+3.4 C). When both methanol and water vapor are present, the net effect on the Michell unit is a change in the average observed HCDP (relative to the case without contaminants) ranging from -1.5 F to +8.2 F (-0.9 C to +4.6 C). By comparison, the effect of methanol and water vapor on the average manual chilled mirror HCDP values ranges from +4.5 F (+19.8 C) to F (+11.1 C). As with the chilled mirror, the presence of methanol and water together in the stream is linked to significant, observable increases in the HCDP temperatures from the Michell unit, and the shifts for both units are larger at higher pressures, where the formation of a combined methanol-water dew point is expected to occur at higher temperatures. It may be that the formation of a low-surface-tension film of methanol-water condensate is influencing the response of both the Michell and the manual chilled mirror, with the manual chilled mirror responding more strongly to the contaminant. Inspection of the filters in the automated units and GC analyses of the test streams ruled out filtration of gas stream components as the cause of the observed shifts. Table ES-2 lists the HCDPs measured by each device over the course of tests, adjusted for small changes in test gas composition during repeat tests at each pressure. Tables ES-3 through ES-5 list the observed shifts in HCDP measurements relative to tests on the same gases without contaminants in Phase 1. Corrections are made for the slight differences between hydrocarbon gas compositions during Phase 1 tests without contaminants and Phase 2 tests with contaminants, using predictions of the HCDP values for each gas composition with the SRK equation of state and the Multiflash software package. Both the Ametek unit and the Michell unit show very similar changes in measurement results between the uncontaminated gas in Phase 1 and the water-contaminated gas in Phase 2. Any interference of moisture appears to have a similar effect on both analyzers, changing measured values by approximately -7 F to +3 F (-4 C to +2 C). Under the same conditions, changes in manual chilled mirror HCDP values range from +0.2 F (+0.1 C) to +6.0 F (+3.4 C). By comparison, the effects of methanol-water contamination in the gas are distinctly different for the Ametek and Michell units. The Ametek unit tends to read lower HCDP temperatures than the comparable values from uncontaminated streams in Phase 1, while the Michell unit reads significantly higher HCDPs in the presence of water and methanol. As noted above, this may be due to the formation of a combined methanol-water condensate on the mirror, with similar characteristics to hydrocarbon condensates. The shifts in HCDPs from the manual chilled mirror are even larger, suggesting a higher response to the methanol-water condensate. vii

10 Table ES-2a. Comparison of averages and 95% confidence intervals of dew point pressures and temperatures measured by each device at each test condition, after adjustment for changes in test gas composition. Contaminants Bureau of Mines Chilled Mirror Droplet Dew Points Ametek 241 CE II Michell Condumax II Line Pressure (psia) T ( F) P (psia) T ( F) P (psia) T ( F) P (psia) Water ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.5 Methanol and water ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.8 Table ES-2b. Comparison of averages and 95% confidence intervals of dew point pressures and temperatures measured by each device at each test condition, after adjustment for changes in test gas composition. Contaminants Bureau of Mines Chilled Mirror Droplet Dew Points Ametek 241 CE II Michell Condumax II Line Pressure (bara) T ( C) P (bara) T ( C) P (bara) T ( C) P (bara) Water ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.03 Methanol and water ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.06 viii

11 Table ES-3. Comparison of droplet hydrocarbon dew points measured with the Bureau of Mines chilled mirror on uncontaminated and contaminated transmission-quality gas streams, compensating for shifts due to changes in hydrocarbon gas composition predicted from GC analyses and the SRK equation of state. Contaminant(s) Test Pressure 193 ppmv H 2 O 750 psia (51.7 bara) 229 ppmv H 2 O 500 psia (34.5 bara) 300 ppmv H 2 O 300 psia (20.7 bara) 805 ppmv CH 3 OH, 215 ppmv H 2 O 834 ppmv CH 3 OH, 249 ppmv H 2 O 866 ppmv CH 3 OH, 311 ppmv H 2 O 750 psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) Phase 1 Result Without Contaminants, Average and 95% Confidence Interval 25.4 F ± 0.5 F (-3.7 C ± 0.3 C) 31.7 F ± 2.1 F (-0.2 C ± 1.2 C) 31.4 F ± 1.4 F (-0.3 C ± 0.8 C) 25.4 F ± 0.5 F (-3.7 C ± 0.3 C) 31.7 F ± 2.1 F (-0.2 C ± 1.2 C) 31.4 F ± 1.4 F (-0.3 C ± 0.8 C) Phase 2 Result With Contaminants, Average and 95% Confidence Interval 33.4 F ± 4.7 F (0.8 C ±2.6 C) 38.3 F ± 1.0 F (3.5 C ±0.6 C) 32.6 F ± 0.6 F (0.3 C ±0.3 C) 43.7 F ± 2.6 F (6.5 C ±1.4 C) 38.1 F ± 1.4 F (3.4 C ±0.8 C) 36.5 F ± 3.1 F (2.5 C ±1.7 C) Average Measurement Shift with Contaminant Added Approximate Shift in Predicted HCDP Curve Net Average Measurement Shift +8.0 F ( +4.4 C) +2 F (+1 C) +6 F (+3.4 C) +6.6 F (+3.7 C) +1 F (+0.5 C) +5.6 F (+3.1 C) +1.2 F (+0.7 C) +1 F (+0.5 C) +0.2 F (+0.1 C) F (+10.2 C) -1.5 F (-1 C) F (+11.1 C) +6.4 F (+3.6 C) -3 F (-1.5 C) +9.4 F (+5.2 C) +5.1 F (+2.8 C) +0.5 F (+0.3 C) +4.5 F (+2.5 C) Table ES-4. Comparison of hydrocarbon dew points measured by the Ametek 241 CE II on uncontaminated and contaminated transmission-quality gas streams, compensating for shifts predicted from GC analyses and the SRK equation of state. Contaminant(s) Test Pressure 193 ppmv H 2 O 750 psia (51.7 bara) 229 ppmv H 2 O 500 psia (34.5 bara) 300 ppmv H 2 O 300 psia (20.7 bara) 805 ppmv CH 3 OH, 215 ppmv H 2 O 834 ppmv CH 3 OH, 249 ppmv H 2 O 866 ppmv CH 3 OH, 311 ppmv H 2 O 750 psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) Phase 1 Result Without Contaminants, Average and 95% Confidence Interval 31.5 F ± 0.5 F (-0.3 C ± 0.3 C) 37.4 F ± 2.3 F (3.0 C ± 1.3 C) 35.0 F ± 1.0 F (1.7 C ± 0.6 C) 31.5 F ± 0.5 F (-0.3 C ± 0.3 C) 37.4 F ± 2.3 F (3.0 C ± 1.3 C) 35.0 F ± 1.0 F (1.7 C ± 0.6 C) Phase 2 Result With Contaminants, Average and 95% Confidence Interval 33.2 F ± 0.4 F (0.7 C ± 0.2 C) 37.0 F ± 1.5 F (2.8 C ± 0.8 C) 32.8 F ± 0.1 F (0.4 C ± 0.1 C) 26.7 F ± 0.4 F (-2.9 C ± 0.2 C) 33.2 F ± 1.3 F (0.7 C ± 0.7 C) 33.9 F ± 0.5 F (1.1 C ± 0.3 C) Average Measurement Shift with Contaminant Added Approximate Shift in Predicted HCDP Curve Net Average Measurement Shift +1.7 F (+1.0 C) -1.2 F (-0.7 C) +2.9 F (+1.7 C) -0.4 F (-0.2 C) +3.5 F (+2 C) -3.9 F (-2.2 C) -2.2 F (-1.3 C) +4 F (+2 C) -6.2 F (-3.4 C) -4.8 F (-2.7 C) +0.5 F (+0.3 C) -5.3 F (-2.9 C) -4.2 F (-2.3 C) +2 F (+1 C) -6.2 F (-3.4 C) -1.1 F (-0.6 C) +2 F (+1 C) -3.1 F (-1.6 C) ix

12 Table ES-5. Comparison of hydrocarbon dew points measured by the Michell Condumax II on uncontaminated and contaminated transmission-quality gas streams, compensating for shifts due to changes in hydrocarbon gas composition predicted from GC analyses and the SRK equation of state. Contaminant(s) Test Pressure 193 ppmv H 2 O 750 psia (51.7 bara) 229 ppmv H 2 O 500 psia (34.5 bara) 300 ppmv H 2 O 300 psia (20.7 bara) 805 ppmv CH 3 OH, 215 ppmv H 2 O 834 ppmv CH 3 OH, 249 ppmv H 2 O 866 ppmv CH 3 OH, 311 ppmv H 2 O 750 psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) Phase 1 Result Without Contaminants, Average and 95% Confidence Interval 27.7 F ± 0.6 F (-2.4 C ± 0.3 C) 33.2 F ± 0.9 F (0.7 C ± 0.5 C) 30.3 F ± 0.8 F (-0.9 C ± 0.4 C) 27.7 F ± 0.6 F (-2.4 C ± 0.3 C) 33.2 F ± 0.9 F (0.7 C ± 0.5 C) 30.3 F ± 0.8 F (-0.9 C ± 0.4 C) Phase 2 Result With Contaminants, Average and 95% Confidence Interval 30.2 F ± 0.3 F (-1.0 C ± 0.2 C) 30.4 F ± 0.4 F (-0.9 C ± 0.2 C) 26.6 F ± 0.5 F (-3.0 C ± 0.3 C) 35.9 F ± 1.6 F (2.2 C ± 0.9 C) 35.2 F ± 0.4 F (1.8 C ± 0.2 C) 31.3 F ± 0.5 F (-0.4 C ± 0.3 C) Average Measurement Shift with Contaminant Added Approximate Shift in Predicted HCDP Curve Net Average Measurement Shift +2.5 F (+1.4 C) -0.5 F (-0.3 C) +3.0 F (+1.7 C) -2.8 F (-1.6 C) +4 F (+2.2 C) -6.8 F (-3.8 C) -3.7 F (-2.1 C) +3.5 F (+1.9 C) -7.2 F (-4.0 C) +8.2 F (+4.6 C) ~ F (+4.6 C) +2.0 F (+1.1 C) +2.5 F (+1.4 C) -0.5 F (-0.3 C) +1.0 F (+0.5 C) +2.5 F (+1.4 C) -1.5 F (-0.9 C) x

13 With the conclusion of Phase 2 of the project, more information on analyzer performance is now available. In transmission-quality and production-quality natural gas streams free of water and methanol contaminants, the automated analyzers are in generally good agreement with the Bureau of Mines manual chilled mirror, particularly with the types of HCDPs the units were designed to detect. Phase 2 data from the Bureau of Mines unit demonstrates the difficulties in measuring HCDPs in the presence of water and methanol, which may condense on the chilled mirror and bias the measured HCDPs. Depending upon the design of the automated units, they may also be susceptible to bias from the contaminants tested here. The results of this research are offered to the natural gas industry, to assist in creating U.S. industry standards. Such standards can guide the industry in using the automated analyzers in processed and unprocessed natural gas streams, and in determining where the automated units may replace the Bureau of Mines chilled mirror device for routine dew point determination at field sites. Further research is suggested in two areas. The first project would involve analysis of the total condensed phase at various pressures, temperatures, and gas compositions tested in Phase 2. The purpose would be to identify the exact composition of condensates being observed by the various analyzers, in order to resolve questions about interference between the various dew points. The second proposed project would test the analyzers using different flow rates across their mirror surfaces, to help identify causes of disagreement between the various units. xi

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15 TABLE OF CONTENTS 1. INTRODUCTION THE NEED FOR OBJECTIVE DEW POINT MEASUREMENT PREVIOUS FINDINGS FROM PHASE 1 OF THE PROJECT PROJECT OBJECTIVES SCOPE OF WORK AND TECHNICAL APPROACH TEST APPARATUS TEST SYSTEM OVERVIEW System Hardware Instrumentation TEST ARTICLES Bureau of Mines Manual Chilled Mirror Ametek Model 241 CE II Michell Condumax II APPARATUS MODIFICATIONS FOR PHASE Injection Bypass Loop Gas Chromatograph Analyzer Modifications TEST GAS COMPOSITIONS TEST PROCEDURES PREHEATING OF GAS BLENDS AND EQUIPMENT GAS CHROMATOGRAPH CALIBRATION Procedure for Calibration on Hydrocarbons Procedure for Calibration on Methanol Procedure for Calibration on Water Vapor INTRODUCTION OF TEST GAS AND CONTAMINANTS Loop Preparation and Test Gas Introduction Addition of Water Contamination Addition of Methanol-Water Contamination DEW POINT MEASUREMENT PROCEDURES Procedure for the Bureau of Mines Chilled Mirror Procedure for the Ametek 241 CE II Procedure for the Michell Condumax II RESULTS AND DISCUSSION MEASUREMENTS OF CONTAMINANT CONTENT DURING TESTS Measurements of Water Vapor as Sole Contaminant Simultaneous Measurements of Methanol and Water Vapor COMMENTS ON DEW POINT CALCULATION METHODS Hydrocarbon Dew Point Calculations Water Vapor Dew Point Calculations Methanol-Water Dew Point Calculations TEST RESULTS FOR THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS WITH WATER CONTAMINATION...35 Page xiii

16 TABLE OF CONTENTS (CONT'D) Page 4.4 TEST RESULTS FOR THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS WITH WATER AND METHANOL CONTAMINATION ANALYSIS OF RESULTS Results from the Bureau of Mines Chilled Mirror Results from the Ametek 241 CE II Results from the Michell Condumax II Effect of Filtration on Analyzer Accuracy Measurement Uncertainty and Analyzer Repeatability CONCLUSIONS...63 APPENDIX A: APPROVED TEST PROCEDURE FOR HYDROCARBON DEW POINT EXPERIMENTS, PHASE PREHEATING OF GAS BLENDS:...69 CALIBRATION OF THE GC:...69 CHARGING THE SYSTEM WITH A TEST GAS:...70 STARTING TESTS AT A NEW PRESSURE...76 ADDITIONAL DATA TO RECORD IN THE LOGBOOK:...80 AT THE END OF TESTS AT A GIVEN PRESSURE:...80 AT THE END OF THE DAY:...80 APPENDIX B: DETAILED TEST RESULTS FOR HYDROCARBON DEW POINT ANALYZERS ,050-BTU/SCF (39.12-MJ/NM 3 ) GAS WITH WATER CONTAMINANT TEST RESULTS FOR INDIVIDUAL ANALYZERS ,050-BTU/SCF (39.12-MJ/NM 3 ) GAS WITH WATER AND METHANOL CONTAMINANT TEST RESULTS FOR INDIVIDUAL ANALYZERS...87 APPENDIX C: GAS STREAM ANALYSES...92 xiv

17 LIST OF FIGURES Page FIGURE 2-1. LAYOUT OF THE HCDP ANALYZER TEST SYSTEM....7 FIGURE 2-2. FIGURE 2-3. FIGURE 2-4. FIGURE 2-5. FIGURE 2-6. FIGURE 4-1. FIGURE 4-2. FIGURE 4-3. FIGURE 4-4. FIGURE 4-5. FIGURE 4-6. FIGURE 4-7. FIGURE 4-8. A BUREAU OF MINES CHILLED MIRROR DEW POINT TESTER SIMILAR TO THE UNIT USED IN THIS RESEARCH...10 EXAMPLES OF HYDROCARBON DEW POINT FORMATIONS ON A MANUAL CHILLED MIRROR. LEFT, AN IRIDESCENT RING DEW POINT; RIGHT, A DROPLET DEW POINT...10 AN AMETEK MODEL 241 CE II AUTOMATED DEW POINT TESTER SIMILAR TO THE UNIT USED IN THIS RESEARCH...11 A MICHELL CONDUMAX II AUTOMATED DEW POINT TESTER SIMILAR TO THE UNIT USED IN THIS RESEARCH...12 BYPASS LOOP USED FOR INJECTION OF CONTAMINANTS, SHOWN PRIOR TO THE INSTALLATION OF INSULATION...14 COMPARISON OF WATER PEAK AREAS FROM VARIAN GC ANALYSES AFTER THE MICHELL UNIT HCDP MEASUREMENTS TO MOISTURE MEASUREMENTS BY THE MICHELL ANALYZER COMPARISON OF AVERAGE WATER PEAK AREAS FROM VARIAN GC ANALYSES AFTER ALL HCDP MEASUREMENTS TO MOISTURE MEASUREMENTS BY THE MICHELL ANALYZER HYDROCARBON AND METHANOL-WATER DEW POINT CURVES PREDICTED USING THE SRK EOS AND MULTIFLASH, SHOWING PROBLEMS WITH CONVERGENCE AND DEVIATIONS FROM EXPECTED HCDP CURVE BEHAVIOR HYDROCARBON AND METHANOL-WATER DEW POINT CURVES PREDICTED USING THE PSRK EOS AND MULTIFLASH, SHOWING UNEXPECTED CRICONDENTHERMS IN THE METHANOL-WATER CURVES TRENDS IN ANALYZED NITROGEN CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) TEST GAS DURING ANALYZER TESTS WITH MOISTURE CONTAMINATION...36 TRENDS IN ANALYZED NONANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) TEST GAS DURING ANALYZER TESTS WITH MOISTURE CONTAMINATION...36 TRENDS IN ANALYZED DECANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) TEST GAS DURING ANALYZER TESTS WITH MOISTURE CONTAMINATION...37 DEW POINTS DETERMINED USING ALL THREE MEASUREMENT DEVICES COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING MOISTURE...38 xv

18 FIGURE 4-9. LIST OF FIGURES (CONT'D) DEW POINTS DETERMINED USING ALL THREE MEASUREMENT DEVICES COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING MOISTURE. RESULTS HAVE BEEN ADJUSTED FOR CHANGES IN GAS COMPOSITION DURING TESTS...39 FIGURE TRENDS IN ANALYZED NITROGEN CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) TEST GAS DURING ANALYZER TESTS WITH METHANOL AND WATER CONTAMINATION FIGURE TRENDS IN ANALYZED NONANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) TEST GAS DURING ANALYZER TESTS WITH METHANOL AND WATER CONTAMINATION FIGURE TRENDS IN ANALYZED DECANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) TEST GAS DURING ANALYZER TESTS WITH METHANOL AND WATER CONTAMINATION FIGURE DEW POINTS DETERMINED USING ALL THREE MEASUREMENT DEVICES COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING METHANOL AND WATER VAPOR...45 FIGURE DEW POINTS DETERMINED USING ALL THREE MEASUREMENT DEVICES COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING METHANOL AND WATER VAPOR. RESULTS HAVE BEEN ADJUSTED FOR CHANGES IN GAS COMPOSITION DURING TESTS...47 FIGURE EXAMPLES OF PRESUMED HYDRATE FORMATIONS AND IRIDESCENT RING FORMATIONS ON THE BUREAU OF MINES CHILLED MIRROR. LEFT, AN ESTABLISHED HYDRATE FORMATION; CENTER, INITIAL FORMATION OF THE IRIDESCENT HYDROCARBON RING ON THE HYDRATE LAYER, INWARD FROM THE EDGE; RIGHT, EXPANSION OF THE IRIDESCENT RING TOWARD THE CENTER OF THE MIRROR...51 FIGURE FILTERS FROM THE AUTOMATED ANALYZERS AS FOUND AFTER THE TESTS, SHOWING NORMAL COLORATION: (A) FILTER MEMBRANE FROM THE MICHELL CONDUMAX II; (B) FIRST-STAGE MEMBRANE FROM THE AMETEK 241 CE II; (C) SECOND-STAGE MEMBRANE FROM FIGURE B-1. FIGURE B-2. THE AMETEK UNIT; (D) THIRD-STAGE FILTER FROM THE AMETEK Page UNIT...59 DEW POINTS DETERMINED USING THE BUREAU OF MINES CHILLED MIRROR COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF A 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING MOISTURE...82 DEW POINTS DETERMINED USING THE AMETEK AUTOMATED ANALYZER COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF A 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING MOISTURE...84 xvi

19 FIGURE B-3. FIGURE B-4. FIGURE B-5. FIGURE B-6. LIST OF FIGURES (CONT'D) Page DEW POINTS DETERMINED USING THE MICHELL AUTOMATED ANALYZER COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF A 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING MOISTURE...85 DEW POINTS DETERMINED USING THE BUREAU OF MINES CHILLED MIRROR COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF A 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING METHANOL AND MOISTURE DEW POINTS DETERMINED USING THE AMETEK AUTOMATED ANALYZER COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF A 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING METHANOL AND MOISTURE DEW POINTS DETERMINED USING THE MICHELL AUTOMATED ANALYZER COMPARED TO DEW POINT CURVES PREDICTED FROM GC ANALYSES OF A 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS STREAM CONTAINING METHANOL AND MOISTURE xvii

20 LIST OF TABLES Page TABLE 2-1. TABLE 2-2. TABLE 3-1. TABLE 4-1. TABLE 4-2. TABLE 4-3. TABLE 4-4. TABLE 4-5. TABLE 4-6. TABLE 4-7. TABLE 4-8. TABLE 4-9. SPECIFICATIONS OF THE MANUAL CHILLED MIRROR AND ANALYZERS TESTED IN THIS RESEARCH....9 NOMINAL COMPOSITION AND CALCULATED PROPERTIES OF THE HYDROCARBON GAS BLEND USED AS THE BASIS FOR THE TEST GASES IN PHASE 2. COMPONENT VALUES ARE IN UNITS OF MOLE PERCENT EQUIPMENT TEMPERATURES AND TEST CONDITIONS FOR TESTS WITH WATER AND METHANOL-WATER CONTAMINATION...24 WATER VAPOR CONTENT OF THE TEST LOOP, MEASURED BY THE MICHELL UNIT MOISTURE SENSOR DURING TESTS OF THE MICHELL ANALYZER ON THE GAS STREAM WITH ONLY WATER CONTAMINATION...29 CONTAMINANT CONTENT OF THE TEST LOOP DURING TESTS ON THE GAS STREAM WITH BOTH METHANOL AND WATER VAPOR PRESENT...31 COMPARISON OF DROPLET HYDROCARBON DEW POINTS FROM TESTS ON UNCONTAMINATED AND CONTAMINATED TRANSMISSION- QUALITY GAS STREAMS, AS MEASURED WITH THE BUREAU OF MINES CHILLED MIRROR, AND COMPENSATING FOR SHIFTS DUE TO CHANGES IN HYDROCARBON GAS COMPOSITION PREDICTED FROM GC ANALYSES AND THE SRK EQUATION OF STATE COMPARISON OF HYDROCARBON DEW POINTS MEASURED BY THE AMETEK 241 CE II ON UNCONTAMINATED AND CONTAMINATED TRANSMISSION-QUALITY GAS STREAMS, COMPENSATING FOR SHIFTS DUE TO CHANGES IN HYDROCARBON GAS COMPOSITION PREDICTED FROM GC ANALYSES AND THE SRK EQUATION OF STATE COMPARISON OF HYDROCARBON DEW POINTS MEASURED BY THE MICHELL CONDUMAX II ON UNCONTAMINATED AND CONTAMINATED TRANSMISSION-QUALITY GAS STREAMS, COMPENSATING FOR SHIFTS DUE TO CHANGES IN HYDROCARBON GAS COMPOSITION PREDICTED FROM GC ANALYSES AND THE SRK EQUATION OF STATE...56 PUBLISHED UNCERTAINTIES IN DEW POINT MEASUREMENTS BY EACH DEVICE...60 CALIBRATION UNCERTAINTIES IN TEMPERATURE AND PRESSURE INSTRUMENTATION USED WITH DEW POINT MEASUREMENT DEVICES...60 AVERAGES AND 95% CONFIDENCE INTERVALS OF HCDP PRESSURES AND TEMPERATURES, FOR EACH INSTRUMENT AND TEST CONDITION, USED TO QUANTIFY HCDP MEASUREMENT REPEATABILITY (BRITISH UNITS)...61 AVERAGES AND 95% CONFIDENCE INTERVALS OF HCDP PRESSURES AND TEMPERATURES, FOR EACH INSTRUMENT AND TEST CONDITION, USED TO QUANTIFY HCDP MEASUREMENT REPEATABILITY (SI UNITS)...62 xviii

21 TABLE C-1. TABLE C-2. TABLE C-3. TABLE C-4. TABLE C-5. TABLE C-6. LIST OF TABLES ANALYSES OF TEST GAS STREAMS DURING TESTS OF AUTOMATED Page ANALYZERS ON THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND CONTAINING WATER VAPOR CONTAMINANT AT 750 PSIA (51.7 BARA). ALL VALUES ARE IN MOLE PERCENT...92 ANALYSES OF TEST GAS STREAMS DURING TESTS OF AUTOMATED ANALYZERS ON THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND CONTAINING WATER VAPOR CONTAMINANT AT 500 PSIA (34.5 BARA). ALL VALUES ARE IN MOLE PERCENT...93 ANALYSES OF TEST GAS STREAMS DURING TESTS OF AUTOMATED ANALYZERS ON THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND CONTAINING WATER VAPOR CONTAMINANT AT 300 PSIA (20.7 BARA). ALL VALUES ARE IN MOLE PERCENT...94 ANALYSES OF TEST GAS STREAMS DURING TESTS OF AUTOMATED ANALYZERS ON THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND CONTAINING METHANOL AND WATER VAPOR CONTAMINANTS AT 750 PSIA (51.7 BARA). ALL VALUES ARE IN MOLE PERCENT ANALYSES OF TEST GAS STREAMS DURING TESTS OF AUTOMATED ANALYZERS ON THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND CONTAINING METHANOL AND WATER VAPOR CONTAMINANTS AT 500 PSIA (34.5 BARA). ALL VALUES ARE IN MOLE PERCENT ANALYSES OF TEST GAS STREAMS DURING TESTS OF AUTOMATED ANALYZERS ON THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND CONTAINING METHANOL AND WATER VAPOR CONTAMINANTS AT 300 PSIA (20.7 BARA). ALL VALUES ARE IN MOLE PERCENT xix

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23 ACKNOWLEDGMENTS The success of Phase 2 of this project was possible only through the aid of many individuals, companies, and participant representatives, and the authors wish to thank them for their guidance and help. Foremost are participant representatives and members of the Project Advisory Committee, which guided the project scope of work and defined its goals: Ray Adcock (Ametek Process and Analytical Instruments) Andy Benton (Michell Instruments Ltd.) Dave Bergman (BP, representing Gas Processors Association) Ilia Bluvshtein (Union Gas Limited) Dave Bromley (BP, representing Pipeline Research Council International, Inc.) Ron Brunner (Gas Processors Association) Paul Burnett (representing Michell Instruments Ltd.) Ben Ho (BP, representing Gas Processors Association) Dean Osmar (Ametek Process and Analytical Instruments) Dan Potter (Ametek Process and Analytical Instruments) William Ryan (El Paso Energy, representing Pipeline Research Council International, Inc. and liaison to the American Petroleum Institute) Andrew Stokes (Michell Instruments Ltd.) Kenny Wheat (Gas Processors Association) Mike Whelan (Pipeline Research Council International, Inc.) Several of these members took time from their schedules for planning meetings, test observation, and assistance in test preparation, and their efforts are greatly appreciated. Others associated with the natural gas industry are recognized for providing supplies and technical assistance to the project. The authors would like to thank Andy Tomich of Questar Applied Technology Services for briefing SwRI on industry procedures for dew point measurement in contaminated gas streams. DCG Partnership and Scott Specialty Gases donated test gases and gas chromatograph calibration gases for the project, and their support of the project is gratefully acknowledged. Last but not least, the authors would like to thank Dave Spears of the SwRI technical staff. Without Dave s advice and efforts in gathering data, and particularly in operating the Bureau of Mines chilled mirror, the accomplishments of this project would not have been possible. xxi

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25 1. INTRODUCTION 1.1 THE NEED FOR OBJECTIVE DEW POINT MEASUREMENT The Bureau of Mines chilled mirror dew point tester is the most commonly-accepted instrument for measuring dew points of natural gas streams at field sites. With this device, a natural gas sample flows continuously across the surface of a small mirror, while a flow of low-temperature gas (usually carbon dioxide or nitrogen) cools the mirror from the other side. A resistive temperature device or thermocouple detects the mirror temperature as it is cooled, and this temperature is displayed within the field of view of the eyepiece. To measure the dew point of the natural gas sample, an operator slowly decreases the mirror temperature by adjusting the flow rate of the low-temperature gas. The operator then watches through an eyepiece for hydrocarbon condensation on the mirror surface. When condensation is detected, the operator reads the mirror temperature and records it as the hydrocarbon dew point (HCDP). This device is sometimes used to resolve custody transfer disputes over natural gas quality. Recent investigations into the accuracy of HCDPs predicted using equations of state have also led to the recommendation that the HCDP be measured using a chilled mirror device where there is uncertainty in the calculated values (American Petroleum Institute, 2006). However, use of the chilled mirror device requires skill and training to operate, and measurement techniques can vary from operator to operator. Errors can be made by inexperienced users in controlling the flow of the low-temperature gas, in identifying condensation when it first appears, or in identifying the source of condensation (hydrocarbons versus water vapor). Some advances in operator training are being made through the use of video footage to record the condensation process. However, because the dew point is detected visually, the chilled mirror is still considered a subjective instrument. An ideal dew point instrument would rely on objectively-measured properties of the gas flow, such as density, optical absorption, or temperature changes with pressure drop, to identify the dew point. Such an instrument could have the potential for improved repeatability and tighter measurement uncertainties than the chilled mirror device, which is subject to operator bias. An ideal HCDP instrument would also be useful as a method for settling custody transfer disputes quickly and impartially, or for improving the efficiency of liquefied natural gas (LNG) blending. Easily-installed remote instruments could be used to monitor conditions at custody transfer points, alerting parties to gas quality problems in real time. The ability to identify problems before they lead to disputes and shut-ins would make such instruments a cost-effective alternative to hand-operated chilled mirror devices that must be transported to the site. Optical methods of detecting water vapor dew points (WVDPs) have been incorporated into sensors for various applications, but the use of these methods to detect HCDPs is in its early stages. Two commercial instruments sold worldwide use different optical methods to detect condensed hydrocarbons and measure the HCDP directly. Research sponsored by this Joint Industry Project (JIP) has independently evaluated the accuracy of these commercially-available devices, in order to provide information for a U.S. industry standard that will guide the use of these devices in custody transfer situations. 1

26 1.2 PREVIOUS FINDINGS FROM PHASE 1 OF THE PROJECT Results from the first phase of this JIP have been documented in an earlier report (George and Burkey, 2008). Two commercially-available HCDP analyzers, an Ametek Model 241 CE II and a Michell Condumax II, were provided by JIP participants for testing. An experimental HCDP research apparatus, first designed at Southwest Research Institute to gather reference HCDP data, was modified to test the automated analyzers. Both automated analyzers, along with a Bureau of Mines chilled mirror device serving as a reference, were installed in the apparatus. Gravimetrically-prepared gas blends containing hydrocarbons through decane were used as test gases, and a small warm box was built to keep the test gases above their HCDPs at various simulated line pressures. Tests were performed using two well-characterized hydrocarbon gas blends, one prepared to resemble a transmission-quality gas (heating value of 1,050 Btu/scf, MJ/Nm 3 ), the other a production gas (1,145 Btu/scf, MJ/Nm 3 ). The production gas composition was chosen to provide information on analyzer performance in gas streams where marginally-rich gas supplies or LNG have entered the transmission system instead of conventional lean gas supplies. Multiple HCDP measurements were made with each instrument at gas pressures below, at, and above the cricondentherm of each gas blend. Analyzer results were compared to HCDP measurements taken with the Bureau of Mines chilled mirror dew point tester and recorded with a digital video camera. Effects of small changes in the gas composition during the tests were approximated using dew point temperatures predicted by common cubic equations of state and gas chromatographic analyses of the test gases. While the scope of work did not allow for assessment of the instruments reproducibility (closeness of dew point measurements under the same conditions at different times), their repeatability was determined using sequential repeat measurements at each dew point measurement condition. The repeatability of both the Ametek 241 CE II and the Michell Condumax II were tighter than that of the Bureau of Mines chilled mirror, with the Condumax II exhibiting the best repeatability of the three. Measurement uncertainties were also assessed, where possible, using analyzer specifications and calibration data provided by the manufacturers, along with uncertainties of instrumentation in the test apparatus. The reader is referred to the Phase 1 report (George and Burkey, 2008) for detailed results. For the transmission-quality test gas, the Ametek 241 CE II reported dew point temperatures 1 F to 6 F (1 C to 3 C) higher than those measured by the Michell Condumax II. On the production gas, this relationship shifted with gas pressure, such that the Michell unit reported slightly higher dew point temperatures ( 1 F or 0.6 C higher) at the highest test pressure of 1,000 psia (68.9 bara), but the Ametek unit dew points were 2 F to 3 F (1 C to 2 C) above the Michell unit values at the lowest pressure of 300 psia (20.7 bara). These differences are thought to be related to the measurement techniques of the two automated devices and their set points. The Ametek 241 CE II records the temperature at which a thin hydrocarbon film is detected, analogous to the iridescent ring dew point of the Bureau of Mines chilled mirror. The default trip point for the Condumax II was chosen by Michell Instruments to agree with manual visual chilled mirror measurements of the droplet HCDP, which occur a few degrees below the iridescent ring dew point. Data were also gathered on the effect of dew point temperatures on the length of each test unit s measurement cycle. Since the total cycle time of the Michell Condumax II is preset by the user, 2

27 the elevated temperatures needed to measure dew points on the richer gas blend had no effect on either the overall cycle time or the cooling time of the Condumax II. For the Ametek 241 CE II, the maximum mirror temperature after warming, the mirror cooling rates, and the unit purge time are preset parameters. The overall cycle time for the Ametek unit shortened from 20 minutes to 17 minutes when the unit measured dew points for the richer gas blend, because of the smaller span between the initial mirror temperature and the final dew point temperature. It was also concluded that the filters used on both the Michell and Ametek units did not impact dew point accuracy. 1.3 PROJECT OBJECTIVES The primary objective of the Hydrocarbon Dew Point Analyzer JIP was to evaluate existing, cost-effective instruments capable of repeatable, objective dew point measurements that may easily replace the Bureau of Mines chilled mirror device for routine HCDP determination at remote sites. Specific objectives of Phase 1 of the project were to: Determine the instruments repeatability at various dew point measurement conditions. Determine the measurement uncertainties of the instruments. Obtain information on the general capabilities of automated HCDP instruments that may also be confirmed with data on natural gases in the field, and eventually used in preparing U.S. industry standards on their use. Assess the impact of sample filtration on instrument performance and accuracy in natural gas streams free of particulates, water, and alcohols. Determine the intervals needed for repeat measurements by the instruments, and verify the recovery time of the instruments after exposure to elevated HCDP temperatures. During Phase 1, plans began for further tests to assess the impact of water and methanol in the gas stream on analyzer performance. While the Phase 1 tests evaluated the automated analyzers in gas streams free of contaminants, units in the field may encounter unprocessed or poorly processed production streams with high levels of water vapor. In some production streams, methanol is added to prevent the formation of hydrates, which can plug lines and damage pipeline equipment. If present in the gas stream in large quantities, both water vapor and methanol can condense on chilled mirror surfaces, interfering with chilled mirror measurements of HCDPs. For Phase 2 of the project, the following objective was added: Assess the impact of water vapor and methanol in the gas stream on instrument performance. To this end, the Ametek and Michell automated analyzers were again tested on the transmissionquality test gas used in Phase 1, but with specific levels of contamination added to gain knowledge of their performance under adverse conditions. In one round of tests, water vapor was added to simulate a production gas with water vapor levels above common tariff specifications. In the second round of tests, the test gas contained both methanol and water vapor, simulating a stream to which methanol has been added to prevent hydrates. As before, the automated measurements were compared to (1) measurements made with a Bureau of Mines chilled mirror device, and (2) equation-of-state dew point predictions based on gas 3

28 chromatographic analyses of the test gases. The results of the Phase 2 tests are documented in this report for use by the natural gas industry in creating guidelines for dew point determination in poor-quality natural gas streams. 1.4 SCOPE OF WORK AND TECHNICAL APPROACH Water vapor is a common contaminant in natural gas streams, and knowledge of any effect of water vapor condensation on automated HCDP analyzers will help users to decide where they can be installed with confidence. Under certain pipeline conditions, water and hydrocarbons can combine in the gas stream to form hydrates, an ice-like crystalline substance in which small, non-polar molecules (such as methane) are trapped inside a lattice of hydrogen-bonded water molecules. Since hydrates can block pipelines and severely damage equipment, gas processors often add methanol to gas streams to prevent hydrate formation, but at sufficiently high levels, the methanol and water can also form a mixed condensate, one whose effects on dew point analyzer accuracy may also be of concern. The original scope of work for Phase 2 of the project called for tests on one stream with only water as the contaminant, and a second stream with only methanol impurities. After review of reference data made available by Gas Processors Association (GPA), the JIP Project Committee chose to change the second test condition from methanol alone to methanol and water in a 3:1 ratio, the ratio typically seen in the gas phase when combined methanol-water (CH 3 OH-H 2 O) dew points (OHDPs) are reached in pipelines. Work on Phase 2 of the project began with revision of the Phase 1 test protocol to evaluate the analyzers on the two contaminated gas streams. The JIP participants agreed to use the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend from Phase 1 as the base gas for the contaminated streams, and to perform measurements at the same line pressures as in Phase 1. This consistent test plan allowed results from Phase 2 to be compared with HCDP measurements of the uncontaminated stream in Phase 1. For the first round of contaminated stream tests, it was decided to add water to the stream to levels just above common tariff limits. For the second contaminated stream, the approach was to add methanol or a 3:1 methanol-water blend to the stream to achieve the target contaminant ratio. For both streams, the JIP participants chose target contaminant levels such that at the highest test pressure, the contaminants would condense out at a higher dew point temperature than the HCDP. At the lowest test pressure, the natural gas HCDP would occur at a higher temperature than the contaminants. At the intermediate pressure, the hydrocarbons and contaminants would condense at approximately the same temperature. After the test procedure was finalized, the experimental apparatus used for Phase 1 was modified to test the automated analyzers in the contaminated streams. A bypass loop was added to the test apparatus to introduce controlled amounts of water and methanol to the test gas stream. The Varian portable gas chromatograph, used to monitor the natural gas composition during Phase 1, was modified to also monitor levels of methanol and water in the stream. Section 2 describes the apparatus modifications for the Phase 2 tests, while Section 3 reviews the revised test procedures. Once modifications were complete and the apparatus had gone through a shakedown period, the HCDP analyzers were tested to determine their ability to accurately measure HCDPs in the presence of contaminants. Multiple HCDP measurements were made with each analyzer at gas 4

29 pressures below, at, and above the cricondentherm of each gas blend. Where possible, measurements of the WVDP and OHDP were also recorded. Analyzer results were compared to corresponding measurements taken with the Bureau of Mines chilled mirror dew point tester and recorded with a digital video camera, and to predictions from equations of state based on gas chromatographic analyses of the test gases. Section 4 reviews these results and assesses the analyzers uncertainty and repeatability. Finally, Section 5 summarizes the test findings and their implications for use of the various devices (including the Bureau of Mines chilled mirror) at field sites containing low-quality gas streams. 5

30 2. TEST APPARATUS Since this project was performed to extend the results obtained from the Phase 1 work, it was possible to utilize the existing test apparatus with only a few minor modifications. Most of these modifications were made to facilitate the addition of water and methanol contaminants to the system and to quantify the resulting concentrations. Additional steps taken to prepare the equipment for this round of testing included calibration of the key instruments and maintenance of the automated analyzers. To maintain consistency with the Phase 1 work, system modifications were done only where necessary, so that the test system remained as nearly identical as possible to that used in the earlier investigation. 2.1 TEST SYSTEM OVERVIEW Due to the similarities of the test apparatus to that used in the Phase 1 work, a complete description of the test system design is not presented in this report. Full documentation of the test apparatus can be found in the report on the Phase 1 project (George and Burkey, 2008). A brief overview of the key components is given here, followed by a detailed discussion (in subsection 2.3) of the parts of the test apparatus that were modified from the Phase 1 configuration System Hardware The test apparatus is a closed-loop system in which a continuous, recirculating gas stream at a controlled flow rate is routed to each test article individually. Figure 2-1 is a photograph of the test system, showing the layout and the major components. Flow is induced in the loop at approximately 5 standard cubic feet per hour (scfh) (2.5 normal liters per minute (Nl/min)) by a magnetically-coupled, rodless piston/cylinder device. The piston is actuated by a stepper motor and control system, and a series of pneumatically-actuated solenoid valves are activated by the motor control system to direct flow in one direction around the loop as the piston is stroked back and forth. The speed of the motor can be adjusted to vary the flow rate of gas through the loop. All wetted parts of the test apparatus are made of stainless steel or Viton elastomers to minimize adsorption of heavy hydrocarbons from the gas into the equipment. Most of the equipment in the test system is housed inside a heated, insulated enclosure. This warm box is equipped with fans and heaters that can be used to increase the equipment temperature when testing with certain gases, to ensure that the test gas within the test loop remains in the gas phase at all test pressures. Although the box was not actively heated in the Phase 2 experiments, its temperature was typically between 80 F to 90 F (27 C to 32 C) due to the heat output of the equipment housed inside. Since the warm box contains electrical equipment that is not rated for use in a hazardous area, a continuous nitrogen purge is maintained inside the box to reduce the oxygen concentration once the loop is pressurized with flammable gas. In addition, safety shutdowns de-energize all electrical equipment in the box in the event of a loss of positive purge gas pressure, or if flammable gas is detected inside the box. The static pressure in the loop is produced and maintained by a piston accumulator, located inside the warm box. The test gas is delivered from the low-pressure storage cylinder to the test loop, which includes the top side of the piston accumulator. Pressurant (compressed nitrogen gas) is applied to the bottom side of the piston accumulator in order to reduce the test loop volume and to pressurize the test gas to the desired test pressure. By varying the pressure of the compressed nitrogen pressurant, a range of loop pressures can be achieved. For the Phase 2 testing, all three test pressures were achieved with a single charge of test gas to the system (i.e., 6

31 no gas was added to or removed from the test system, with the exception of gas chromatograph (GC) analysis samples, during the course of testing with each contaminant). Ametek 241 CE II Michell Condumax II Gas Chromatograph Gas Storage Cylinders Dew Scope Warm Box Liquid Nitrogen Supply Auxiliary Chiller Primary Chiller Figure 2-1. Layout of the HCDP analyzer test system. The gases needed for preparation and operation of the apparatus are stored in cylinders located outside of the warm box. These include the test gas, the calibration gases, helium carrier gas for the GC, and various other gases used for loop preparation and utility purposes. Several valve manifolds and tubing lines are used to route the various gases to and from the test system. By the use of the valves in the system, the gases can also be discharged into the vent header. All components (tubing, valves, and test and calibration gas cylinder pressure regulators) carrying test gas or calibration gas outside the warm box are heat traced and insulated. Temperature set points for the heat-traced lines were maintained at a value near the temperature of the warm box, except for the line carrying the methanol calibration gas to the gas chromatograph, which was maintained at 86.5 F ± 0.5 F (30.3 C ± 0.3 C) to keep the methanol concentration stable as it traveled to the GC. The test gas and calibration gas cylinders were wrapped in regulated heating blankets to control the gas temperature, and insulated covers were placed over the top of each cylinder and its regulator to minimize cooling. These gas cylinders were also placed on insulated pads to thermally isolate them from the laboratory floor. The heating blankets were regulated using a sensor attached to the wall of the cylinder about halfway up its height. To ensure that the gas in the cylinder was heated thoroughly, cylinder heating continued for a minimum of 24 hours after the set point temperature was reached before the cylinder was first opened. Each cylinder was heated continuously until the conclusion of testing. Gas from the test system inside the warm box was connected to the test articles through supply and return lines that terminated at a valve manifold. During tests, the valves in the manifold were set so that flow was routed though only one device at a time. In particular, the unit under test was valved into the test loop and experienced flow, while the other two units not under test were valved out of the test loop and did not experience flow. The units not under test remained 7

32 filled with test gas while waiting their turn for testing. The valves in the manifold could also be positioned so that flow passed through the automated analyzers and the Bureau of Mines chilled mirror dew point tester either in series or in parallel. These configurations were used for some loop preparation and transition steps. As with the lines connecting the gas cylinders to the test apparatus, all components (tubing, manifold valves, and the manual dew point tester) carrying test gas outside of the warm box were heat traced and insulated. Temperature set points for the heat traced lines were maintained at a value near the temperature of the warm box. Heaters were installed inside the automated analyzer enclosures, but because of the low dew point temperatures of the gases used in Phase 2 tests, they were not used Instrumentation Data from all loop instrumentation were recorded at two-second intervals with an HP 34970A data acquisition unit and HP 34902A multifunction multiplexer cards. Test system pressures upstream and downstream of the test articles were measured with Rosemount Model 3051TC pressure transducers with a full-scale range of 4,000 psi (276 bara) and a manufacturer s stated accuracy of 0.075% of full scale. The dew point pressure at the manual dew point tester and the Ametek 241 CE II were recorded using the upstream test system pressure transducer. The Michell Condumax II includes a pressure transducer on board to measure dew point pressures. The dew point temperature at the manual dew point tester was measured with a 100-Ohm, DIN Class A, platinum resistance temperature detector. Other pressure and temperature sensors were installed at various locations in the system for operational and diagnostic purposes; values from these sensors were also logged by the HP 34970A data acquisition unit. The gas in the loop was analyzed by a Varian CP-4900 portable GC. The GC is permanently connected to the loop by a sample line that operates at a pressure of approximately 50 psig (3.4 barg). The pressure-reducing regulator installed on this line is heated to avoid potential distortion of the sample composition by Joule-Thomson cooling. As with the Phase 1 tests, the Varian included a 0.4-meter Haysep A column with backflush capability for detecting diluents and light hydrocarbons through propane, and a CP-Sil-5CB 8-meter packed column for detection of heavier hydrocarbons. In the Phase 2 experiments, an additional column (discussed in detail below) was added to the GC for the detection of methanol and water contaminants. Injection pressures, column pressures and temperatures, and elution periods are controlled by the GC to allow analysis of hydrocarbons through C 10 at a measurement resolution of 5 ppmv (parts per million by volume). 2.2 TEST ARTICLES The two commercially-available, automated dew point analyzers evaluated in Phase 2 were as follows: Ametek Model 241 CE II Michell Condumax II The test articles were the same units tested in Phase 1 of the project, and were unchanged for Phase 2. The manufacturers of each of these analyzers were members of the Joint Industry Project, and provided the analyzers for testing. Comparisons were made to measurements with a manual Bureau of Mines chilled mirror device. Results were also compared to dew points estimated using the Multiflash software package (Infochem, 2007a) with the Soave-Redlich- 8

33 Kwong equation of state (SRK EOS) (Soave, 1972) and the Multiflash default binary interaction parameters. Table 2-1 compares the specifications of the three units tested in this research. Table 2-1. Specifications of the manual chilled mirror and analyzers tested in this research. Device Bureau of Mines Chilled Mirror Ametek 241 CE II Michell Condumax II Reported accuracy ±2.4 F (±1.3 C)* ±2 F (±1 C) ±0.9 F (±0.5 C) Sample flow rate 5 scfh (2.5 Nl/min) 2 scfh to 10 scfh (1 Nl/min to 5 Nl/min) Maximum sample pressure 3,000 psig (207 barg)** Operating environment 40 F to 120 F (4 C to 49 C) 1 scfh (0.5 Nl/min) (HCDP) 2 scfh to 10 scfh (1 Nl/min to 5 Nl/min) (WVDP) 2,000 psig (138 barg) 1,500 psig (103 barg) 50 F to 104 F (10 C to 40 C) -4 F to 140 F (-20 C to 60 C) Maximum gas temperature at inlet no specification 104 F (40 C) 122 F (50 C) * See Warner et al. (2001). ** Specification of SwRI unit Bureau of Mines Manual Chilled Mirror The Bureau of Mines chilled mirror dew point tester shown in Figure 2-2 is a commonly-used instrument for measuring dew points of natural gas streams. With this device, a natural gas sample flows continuously across the surface of a small mirror, while a flow of low-temperature gas (usually carbon dioxide or nitrogen) cools the mirror from the other side. A resistance temperature detector (RTD) in physical contact with the mirror displays the mirror temperature as it is cooled, and a digital readout displays the temperature within the field of view of the eyepiece. To measure the dew point of the natural gas sample, an operator slowly decreases the mirror temperature by adjusting the flow rate of the low-temperature gas. The operator then watches through an eyepiece for hydrocarbon condensation on the mirror surface. When condensation is detected, the operator reads the mirror temperature and records it as the HCDP. Two different types of condensation on the mirror surface may be identified by different chilled mirror users as the HCDP, as shown in Figure 2-3. Depending upon the standards and requirements of the user, either formation may be cited as the HCDP. One type, referred to here as the iridescent ring dew point, is observed when rainbow-like colors form in the center of the mirror and quickly expand to cover the mirror. This formation is generally believed to be a thin layer of hydrocarbon condensate, and indeed bears a visual resemblance to an oil slick moving across the surface of water. The other formation, referred to in this report as the droplet dew point, occurs when small droplets of hydrocarbon liquids nucleate and form on the mirror within the iridescent ring or near the edges of the mirror. In typical applications, the iridescent ring dew point is observed first as the mirror temperature is lowered, followed by the droplet dew point as more condensation occurs and nucleation begins. Both types of dew points were recorded, where possible, in the Phase 2 tests for comparison with dew points measured by the automated devices. 9

34 Pressure gauge Temperature readout Eyepiece Heat exchanger RTD Pressure chamber Figure 2-2. A Bureau of Mines chilled mirror dew point tester similar to the unit used in this research. Figure 2-3. Examples of hydrocarbon dew point formations on a manual chilled mirror. Left, an iridescent ring dew point; right, a droplet dew point Ametek Model 241 CE II The Ametek Model 241CE II analyzer, shown in Figure 2-4, uses an automated optical detection method to identify dew points. The Ametek unit uses a Peltier thermoelectric cooler to control the temperature of a two-sided mirror in a sample chamber. Each side of the mirror is used to identify different dew points: hydrocarbon condensation is identified on one side of the mirror with a roughened surface, while water vapor condensation can be identified on the other side, which has a polished, reflective surface. The lower surface tension of hydrocarbon liquids causes them to form a thin film on the roughened side, while water condenses into smaller droplets on the polished side. When a thin hydrocarbon film is detected, the unit records the temperature of the mirror as the HCDP. The detection parameters are set so that the measured dew point temperature is analogous to the iridescent ring dew point recorded by Bureau of Mines chilled mirror users. Under normal operation, the WVDP measurement is used only as a diagnostic. Samples entering the analyzer pass through a multiple-stage filtration system designed to protect the analyzer from aerosols, particulates, and liquid slugs. 10

35 Figure 2-4. An Ametek Model 241 CE II automated dew point tester similar to the unit used in this research. The Ametek unit measurement cycle consists of three stages: a cooling stage, during which the mirror temperature is lowered until condensation is detected; a warming stage, during which the condensate is revaporized; and a purge stage, during which sample gas flows through the sample system and measuring cell to completely purge the measuring cell. When the unit enters the cooling stage, a valve closes, trapping a sample of gas in the measurement chamber. During the cooling stage, a microcontroller controls the cooling rate of the mirror until a photodetector detects changes in light intensity caused by liquid condensation on the mirror surfaces. The cooling rate of the mirror is controlled so that it cools rapidly at first, but then switches to a slower cooling rate as the expected HCDP temperature is approached. The user can select among three operational modes in which the analyzer halts the cooling cycle after finding an HCDP, a WVDP, or both dew points. The mirror temperature is measured using a platinum RTD Michell Condumax II The Michell Condumax II analyzer shown in Figure 2-5 also uses an automated optical detection method to identify HCDPs by detecting the amount of condensate forming on a cooled optical surface. The stainless steel "mirror" surface contains a central conical-shaped depression, onto which collimated red light is focused. In the dry condition, the incident light beam is dispersed by the acid-etched mirror surface, providing a base signal to the optical detector. During a measurement cycle, a Peltier heat pump lowers the mirror temperature at a controlled rate. When the dew point is reached, hydrocarbon condensate forms on the optical surface and it becomes reflective, due to the low surface tension of the condensate. A ring of light forms around the detector, the scattered light intensity within the central region is reduced, and the 11

36 output from the photodetector falls accordingly. The Condumax II determines the HCDP as the temperature of the optical surface when a set signal trip point value (photodetector voltage output) is reached during the measurement cooling cycle. Figure 2-5. A Michell Condumax II automated dew point tester similar to the unit used in this research. The Michell Condumax II HCDP measurement cycle consists of a measurement stage during which the mirror temperature is lowered until condensation is detected, and a recovery stage in which the optical surface is actively heated to evaporate condensates back into the flowing gas sample. During the measurement stage, flow to the HCDP cell is interrupted by a solenoid valve closed downstream of the HCDP measuring cell, and the unit analyzes a fixed sample of the natural gas stream within the cell. This technique is advocated by Michell Instruments as a way to prevent preferential drop-out of heavy components from a flowing sample. After an initial range-finding cycle, the Condumax II determines an optimized cooling rate that causes the sensor surface to cool quickly at first, then to follow a cooling rate of 0.1 F/sec (0.05 C/sec) as the sensor approaches the HCDP. The Michell Condumax II also includes a ceramic moisture sensor that operates on a gas sample in parallel with the HCDP measurement cell to provide a WVDP measurement. The gas supplied to the Michell Condumax II is split between the hydrocarbon and WVDP measurement paths, and is recombined after the measurement prior to leaving the Condumax II. The WVDP sample gas flow is continuous and is not affected by the interruption of the HCDP measurement flow. The WVDP measurement is updated at one-second intervals. Several parameters in the Condumax II measurement cycle are adjustable. The length of the entire HCDP measurement cycle was left at the default setting of ten minutes during these tests, resulting in a measurement stage of 2 to 3 minutes and a recovery stage of 7 to 8 minutes. The factory default trip point setting of 275mV at 400 psig (27.6 barg) has been found through field experience to give good agreement with manual chilled mirror measurements of the droplet HCDP at the same analysis pressure. For analysis pressures higher or lower than 400 psig 12

37 (27.6 barg), the actual sample volume within the sensor cell changes proportionately with pressure. Michell Instruments recommends adjusting the default 275mV trip point setting in proportion to the pressure change, in order to maintain consistent measurement sensitivity in terms of mass of condensate per unit volume of sample gas. The signal trip points corresponding to the three test pressures used in this project are listed in the test procedures in Appendix A. 2.3 APPARATUS MODIFICATIONS FOR PHASE 2 As part of the planning for the Phase 2 project, several options for modifying the Phase 1 test system were investigated by SwRI. During teleconferences with the JIP members, these options were discussed, and a plan for both the required design modifications and a new test procedure were finalized. The majority of these modifications were related to developing a method to add contaminants to the system, and to quantifying the resulting levels of moisture and methanol in the gas stream. Although included in the planned modifications, the addition of another phosphorous pentoxide (P 2 O 5 ) desiccant cylinder upstream of the Varian GC was not implemented. The intended purpose of this system was to protect the GC columns by removing moisture from the gas stream before analysis. However, after discussions with Varian and the JIP members, it was decided that the expected moisture levels would not cause any harm to the GC. In addition, there was concern that the desiccant material could preferentially remove some heavy hydrocarbon components from the gas blend. Two similar desiccant cylinders already exist on lines used to fill the loop from the supply cylinders, and no evidence of heavy hydrocarbon loss was found during the Phase 1 tests. In the final design, however, the lines that deliver contaminants to the loop and deliver samples from the loop to the GC do not incorporate desiccant cylinders, so contaminant and heavy hydrocarbon concentrations in the test gas could not be altered before GC analysis. No specific hardware or instrumentation was added to the system for the measurement of water concentrations. Instead, the JIP participants agreed that the moisture sensor in the Condumax II analyzer would be used to quantify the water content of the stream for the tests with water as the only contaminant. The ceramic moisture sensor uses a variation on a capacitive measurement method to measure water in natural gas; measurements are traceable to both the National Physical Laboratory (NPL) of the UK and the National Institute of Standards and Technology (NIST) in the US. Because there was some concern that the Michell unit moisture sensor could be adversely affected by methanol, the Varian GC was used to measure both the water and methanol concentrations for the tests with both contaminants. The use of the data from the Michell analyzer and the GC to determine contaminant concentrations is discussed in further detail in subsection 4.1. Test preparations also included the recalibration of all instruments and equipment used for the measurement of critical test parameters. These instruments included the RTD used to sense the mirror temperature in the manual chilled mirror dew point tester, the pressure sensors upstream and downstream of the test articles, and the HP data logger used to measure and record data from these instruments Injection Bypass Loop The primary loop modification needed for the Phase 2 work was the addition of a system to add the water and methanol contaminants to the gas stream. It was desirable for the system to allow these changes to be made while the loop remained pressurized, so that the contaminant levels 13

38 could be adjusted to achieve the correct concentrations with a minimum of effort and wasted gas. A photograph of the injection bypass loop that was installed is shown in Figure 2-6. The bypass loop was installed so that the gas returning to the warm box from the test article manifold flowed through the bypass loop. In this position, the contaminants were injected downstream of the analyzers and upstream of the equipment in the warm box. Vent Pressure Gauge and Transducer Injection Septum Injection Flow Path Normal Flow Path Return to Loop Injection Loop Isolation Valves From Manifold Figure 2-6. Bypass loop used for injection of contaminants, shown prior to the installation of insulation. The bypass loop consisted of two flow paths, either of which could be selected by opening or closing isolation valves. The first flow path was the normal path used during loop operation and testing. This path simply connected the supply and return lines. The second flow path was the injection path that was used for introducing contaminants into the system. Contaminants were measured using an appropriately-sized syringe and needle and injected through a chromatography septum installed in the line. Since the septum could withstand only a limited amount of pressure, the procedure for adding a contaminant began with the closing of two isolation valves and reduction of the pressure in the line between the two valves by use of a vent valve. Once the pressure reached approximately 30 psig (2 barg), the vent was closed, the valve below the injection septum was opened, and the needle was passed through the septum to inject the water and/or methanol. The septum valve was then closed, the isolation valves were opened, and gas was allowed to flow through the 14

39 injection path for a sufficient time to uniformly distribute the contaminant throughout the system. During this distribution step, the normal flow path was closed to direct all of the flow through the injection path. As with the lines elsewhere in the system, all of the bypass loop lines and valves were insulated and heat traced Gas Chromatograph To quantify the methanol concentration in the loop, the Varian GC was upgraded to include a third channel with a 4-meter CP-Wax column for the detection of alcohols. The Varian GC used during the Phase 1 tests included a chassis with room for only two channels; these were both occupied by channels for the detection of hydrocarbons. The addition of a third channel required the exchange of the existing chassis for one with room for four channels. Two methods for calibrating the GC on methanol were considered. The first method was the use of a calibration gas generator with a methanol permeation tube that could be calibrated traceable to NIST. The second alternative was the use of a certified methanol-in-helium gas blend. A calibration gas generator was loaned to SwRI by Fesco Ltd., a member of the API Alternate Sampling Methods Working Group, for use in this project. However, it was determined that the unit was out of calibration, and the length of time needed to recalibrate the gas generator would have severely delayed the project schedule. It was decided instead by the JIP Project Committee to calibrate the GC on methanol using the certified gas blend. The disadvantage of this method is the potential for the delivered methanol concentration to drift, due to methanol adsorption and desorption from the equipment walls as the cylinder temperature and pressure change. To minimize these effects the following steps were taken: A gas cylinder heating blanket and heat tracing on the transfer lines were used to closely control the temperature of the methanol blend in the cylinder and lines. Temperatures of the equipment were maintained near the value at which the blend was certified. This precaution minimized fluctuations in methanol levels due to temperature-related adsorption and desorption on the cylinder walls and lines. Care was taken to limit use of the methanol standard, to minimize potential changes in methanol levels caused by falling pressure within the cylinder. A dedicated delivery line for the methanol calibration gas was connected directly to the back of the GC, avoiding the possibility of non-equilibrium methanol levels that could occur if the flow was directed though a line that was used for other gases. The methanol gas calibration line was allowed to remain at the delivery pressure (40 psig, 2.8 barg) for at least 30 minutes prior to the start of a calibration. This provided time for adsorption and desorption of methanol at the tubing walls to reach equilibrium, and avoided fluctuations in the methanol level of the stream during flow. Prior to calibrating the GC on methanol or analyzing the test gas from the loop, the gas stream was allowed to flow through the GC for two minutes to purge the lines and establish equilibrium conditions for the methanol Analyzer Modifications Because the automated analyzers were not designed to operate in a fixed-pressure, recirculating closed-loop configuration, modifications were made to each of the automated analyzers sampling systems in order to integrate the analyzers into the test system before the beginning of 15

40 the Phase 1 tests (George and Burkey, 2008). In general, the modifications included removing pressure regulators, because the test pressure and flow were controlled by the test system (specifically, the rodless piston and piston accumulator). Any other unnecessary components that could cause pressure drops in the sampling system were also removed. Filters in the automated devices were not modified, and remained in the flow path as in a standard installation. No changes were made to the HCDP measurement hardware installed in the analyzers. Complete documentation of the analyzer modifications can be found in the Phase 1 project report (George and Burkey, 2008). Since all of the changes needed to integrate the analyzers into the test system were made as a part of the Phase 1 testing, no additional modifications were made to the manual chilled mirror or automated analyzer hardware for the Phase 2 testing. The units used for the Phase 1 testing remained in the test apparatus after the conclusion of that project, and were used again in the Phase 2 work. All of the adjustable setup and configuration parameters (timing, temperature ramp rates, etc.) on each instrument were also left at the same values as in the Phase 1 work. The only exception to this was the setting on the Ametek analyzer that determines when the analyzer halts the measurement cooling cycle. The Ametek unit was originally configured to end the cooling cycle after finding an HCDP; for Phase 1, the unit was reprogrammed to stop the cooling cycle after both an HCDP and a WVDP were found. Prior to the Phase 2 testing, routine maintenance was done on each of the automated analyzers at the manufacturers requests. In the case of the Ametek analyzer, the filter was replaced. On the Michell unit, the WVDP sensor module was replaced. 2.4 TEST GAS COMPOSITIONS For Phase 2, it was decided that tests should be performed using only a single hydrocarbon gas composition. The chosen test gas was the leaner of the two gases used in Phase 1. This gas has a heating value of 1,050 Btu/scf (39.12 MJ/Nm 3 ) and is blended to represent a transmissionquality gas stream. Gravimetrically-prepared gas blends of this nominal composition were provided by DCG Partnership Ltd. and Scott Specialty Gases for the Phase 1 testing. Sufficient quantities of these gases remained so that the Phase 2 tests could be performed using test gas and GC calibration gas taken from the same cylinders as in Phase 1. No new gases were used, nor were gases from more than one supply cylinder mixed in the loop. Table 2-2 lists the composition and volume-based heating value of the base hydrocarbon test gas. The heating value was computed using data from the latest editions of GPA Standard 2145 (Gas Processors Association, 2007) and GPA Technical Report TP-17 (Gammon, 1988), assuming ideal gas behavior and standard conditions of psia and 60 F ( kpa and 15.6 C). The table also includes the cricondentherm (maximum dew point temperature over all pressures) and cricondenbar (maximum dew point pressure over all temperatures) of the gas, as predicted by Multiflash (Infochem, 2007a). The certified composition of the blend in each cylinder (test gas and calibration gas) was used for reference purposes during tests and data analysis. 16

41 Table 2-2. Nominal composition and calculated properties of the hydrocarbon gas blend used as the basis for the test gases in Phase 2. Component values are in units of mole percent. Component Value methane ethane 2.00 propane 0.75 isobutane 0.30 n-butane 0.30 isopentane 0.15 n-pentane ,2-dimethylbutane (neohexane) methylpentane (isohexane) ,3-dimethylbutane methylpentane cyclohexane methylcyclopentane n-hexane ,4-dimethylpentane (cyclohexane) ethylpentane methylcyclohexane n-heptane ,2,4-trimethylpentane (isooctane) methylheptane n-octane n-nonane n-decane CO nitrogen 1.00 Total Total C 6+ content Total diluent content 1.50 Heating value 1,050.4 Btu/scf (39.12 MJ/Nm 3 ) Predicted cricondentherm 43.4ºF (6.3ºC) Predicted cricondenbar 1,298.3 psia (89.5 bara) 17

42 3. TEST PROCEDURES Test procedures for Phase 1 of the project were developed with the assistance of the JIP members. For Phase 1, specific procedures were created to preheat test gas blends before use, calibrate the Varian GC, remove unwanted impurities from the test loop before tests, add and remove test gases from the loop, supply the test stream to each analyzer at the correct flow rate, and collect data from each analyzer after the loop had reached stable conditions. The test plan for Phase 1 called for dew point measurements of each gas to be performed with each instrument at gas pressures below, at, and above the cricondentherm. Phase 2 called for tests with the same transmission-quality gas as in Phase 1, at the same pressures above, at, and below the cricondentherm, but with specific levels of contaminants added to the stream. New procedures were created to calibrate the GC on these contaminants, to add them to the test gas in specific amounts and confirm those amounts, and to measure both HCDPs and contaminant dew points of the mixed stream. The complete test procedures, with annotations by SwRI personnel, are listed in Appendix A. This chapter briefly reviews the procedures used during Phase 2 tests, with emphasis on new procedures related to methanol and water vapor. For more detail on the existing procedures from Phase 1, the reader is referred to the report by George and Burkey (2008). 3.1 PREHEATING OF GAS BLENDS AND EQUIPMENT A key step in test preparations was the preheating of the hydrocarbon gas blends in their delivery cylinders, and heating of the transfer lines to the test apparatus. This avoided condensation of the heavy components and distortion of the gas compositions from their certified values. For Phase 2, a reference gas blend containing 1,500 ppmv methanol in helium was added to the system. This gas blend was used to calibrate the GC before analyzer tests with methanol and water vapor in the gas stream. The methanol standard cylinder was kept at a closely-controlled temperature, since fluctuations in the cylinder temperature could potentially cause methanol to adsorb and desorb from the cylinder walls, biasing the GC calibration. The hydrocarbon and methanol gas blend cylinders were placed on insulated pads to thermally isolate them from the laboratory floor. The cylinders were wrapped in regulated heating blankets to control the gas temperature, and insulated covers were placed over the top of each cylinder and its regulator to minimize convection cooling. The heating blankets were regulated using an RTD attached to the wall of the cylinder, placed halfway up its height. Each hydrocarbon gas storage cylinder and its outlet regulator were heated to a temperature approximately 30 F (17 C) above the predicted HCDP of the gas at its pressure in the cylinder. Since the pressures of the hydrocarbon gases in the delivery cylinders were below their cricondentherm pressures, heating above the cricondentherm temperature was not necessary. The methanol gas blend cylinder was heated to a temperature of 86.5 F ± 0.5 F (30.3 C ± 0.3 C), using a digital controller chosen for its narrow deadband about the set point. After the wall of each cylinder reached the target temperature, cylinder heating continued for a minimum of 24 hours before any gas was withdrawn from the cylinder to calibrate the GC or to charge the test loop. This requirement ensured that the core temperature of the gas in the cylinder had also reached the target temperature. The gas cylinders were heated continuously until the conclusion of tests. 18

43 3.2 GAS CHROMATOGRAPH CALIBRATION The Varian GC was calibrated at the beginning of each test day. With contaminants present in the test streams for Phase 2, a column set capable of analyzing methanol and water vapor content was added to the GC before tests, and new procedures were created to calibrate the unit on these components. Other GC modifications allowed for faster calibration and analysis on hydrocarbon streams than in Phase Procedure for Calibration on Hydrocarbons The procedure for calibrating the GC on hydrocarbons was essentially unchanged from Phase 1. A reference hydrocarbon gas blend of the same nominal composition as the hydrocarbon test gas, provided by a different blender than the test gas itself, was used in each GC calibration. After the hydrocarbon calibration gas cylinder and its outlet regulator had been preheated according to the procedure described in subsection 3.1, the hydrocarbon calibration gas cylinder was opened to supply gas to the GC at approximately 80 psia (552 kpa). Because only the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blends were used during Phase 2, and their HCDP temperatures were well below room temperature at all pressures, heating of the gas manifold and connecting lines was not needed. However, a preheater on the GC constantly heated each sample coming from the manifold to 212 F (100 C), as a final step to ensure that the sample entering the GC was in the gas phase. Six consecutive eight-minute calibration runs were performed on the hydrocarbon calibration gas, and data from the last five runs were used to generate calibration factors for each day s analyses. These response factors were checked for linear detector response and for repeatability, and only calibrations with acceptable response factors were used to analyze the test gas. After the last hydrocarbon calibration gas sample was drawn from the cylinder, the valve on the cylinder was closed, and the hydrocarbon gas was allowed to vent from the manifold and through the GC sample bypass stream. This final step was added for Phase 2 to prevent crosscontamination of the hydrocarbon calibration gas with the new methanol calibration gas, discussed in the following subsection Procedure for Calibration on Methanol During the planning stages for Phase 2 of the project, the decision was made to add a CP-Wax channel to the Varian CP-4900 MicroGC to measure methanol levels in the test stream. Two methods of calibrating the GC on methanol were pursued, as discussed in subsection The preferred method was to use a standard calibration gas generator, which uses a methanol permeation tube and a carrier gas at closely-controlled flow rates and temperatures to produce a reference stream with methanol content traceable to NIST. The secondary method was to employ a certified methanol-in-helium gas blend. This approach was less preferable than the gas generator, due to the potential for the delivered methanol concentration to drift with changes in the temperature and pressure in the cylinder. However, equipment for both calibration methods was obtained during preparations. A gravimetrically-blended reference gas blend of 1,500 ppmv methanol-in-helium was purchased, and a standard gas generator was obtained on loan from a member of the API Alternate Sampling Method Working Group. The calibration of the gas generator was found to have expired, however, and having the unit recalibrated would have significantly delayed the start of tests, so the JIP Project Committee agreed that the certified gas blend should be used to calibrate the GC on methanol. 19

44 The reference methanol gas blend was installed in the test apparatus. A heating blanket, heat trace, and a digital controller were used to control the temperatures of the gas blend cylinder, regulator, and transfer lines to within ±0.5 F. The purpose of the temperature control in this case was to keep the level of methanol adsorbed on the internal walls of the cylinder and tubing at a constant level. This, in turn, would keep the methanol in the reference stream at an equilibrium level, so that a stable methanol concentration could be delivered to the GC during calibration. Since the methanol reference blend was used solely to calibrate the GC, and not introduced into the test loop, the methanol cylinder was not connected to the sampling manifold. Instead, the delivery line from the methanol calibration gas cylinder was connected to a union just upstream of the GC preheater inlet. This minimized the length of tubing over which the methanol level in the stream had to be kept in equilibrium with the methanol adsorbed to the tubing walls. For the first calibration of the GC, the methanol cylinder and its outlet regulator were preheated according to the procedure described in subsection 3.1. The methanol calibration gas cylinder was opened, and gas flowed continuously to the GC at approximately 80 psia (552 kpa). The stream was analyzed by the GC at regular intervals until the methanol content had stabilized. A valve just upstream of the union was then closed, holding the methanol reference gas at a stable temperature and pressure in the delivery line, and maintaining the equilibrium methanol level of the calibration gas stream. The delivery line was kept at pressure and temperature throughout all of the Phase 2 tests, with the valve just ahead of the union used to control flow of the calibration gas to the GC. During preliminary tests with the reference methanol blend, it was confirmed that the CP-Wax channel could quantify methanol as intended. However, a methanol elution peak was also discovered on the CP-Sil channel used to measure heavy hydrocarbons, partly overlapping the isobutane peak. Because this created the potential for errors in isobutane measurements, the following approach was devised to subtract the methanol peak from the isobutane peak in the analyses. During calibration, the GC was first calibrated on the hydrocarbon reference gas to obtain a response factor for isobutane on the CP-Sil channel. Next, the GC was calibrated using the methanol-in-helium reference gas, and response factors were obtained for methanol on both the CP-Wax channel (where methanol could be accurately quantified) and the CP-Sil channel (where methanol and isobutane coelute). During analyses of gas streams with methanol contaminant, the CP-Wax channel was used to quantify the methanol in the stream. This amount was used with the methanol response factor on the CP-Sil channel to compute the area of the combined methanolisobutane peak generated by methanol alone. This methanol-only area was subtracted from the methanol-isobutane combined peak area on the CP-Sil channel. The remaining area was assigned to isobutane, and its amount was computed with the isobutane-only response factor to quantify isobutate. One-minute analyses with the Varian GC were sufficient to analyze a gas stream for methanol content. Six consecutive one-minute calibration runs were performed on the methanol calibration gas, and data from the last five runs were used to generate response factors for each day s analyses. Sequential analyses were compared to confirm the stability of the methanol content before the calibration was accepted. 20

45 3.2.3 Procedure for Calibration on Water Vapor The original scope of work for Phase 2 called for tests on one stream containing only water vapor, and another stream containing only methanol. Because of the traceability of the moisture sensor in the Michell Condumax II (discussed later in this subsection), it was originally decided to use the Michell unit to measure moisture content during tests on the first stream, and the Varian GC to measure methanol content during tests on the second stream. Later, the work scope was changed so that the second round of tests would employ a gas stream contaminated with both methanol and water vapor. However, Michell Instruments expressed reservations about the accuracy of the moisture sensor in streams containing methanol. While the methanol reference blend was successfully used with close temperature control to calibrate the GC on methanol, use of a water vapor reference gas blend was not considered, as such blends are considered very susceptible to large variations in content from small changes in pressure and temperature in the cylinder. Also, as noted earlier, a traceable gas standard generator was not available. It was eventually decided to use the Michell unit ceramic moisture sensor as a reference to calibrate the GC on water vapor, before the combined methanol-water contaminant tests took place. The Michell Condumax II includes a ceramic moisture sensor that operates on a gas sample stream in parallel with the HCDP measurement stream. The stream through the ceramic sensor flows continuously to provide a WVDP measurement unaffected by the HCDP measurement cycle. The sensor operates on a capacitive measurement principle, and its calibration is traceable both to NPL and to NIST. This sensor was used before and during tests with only water contamination present to verify the water vapor content of the gas stream. Since the Michell unit ceramic sensor provided traceable water vapor measurements, additional measurements of water vapor in the stream by the GC were not necessary. However, it was determined during the water-only tests that the Varian GC could be calibrated on water using the Michell unit measurements as the reference values. Moisture peaks in the chromatograms from the CP-Wax channel, collected during tests on the water vapor-only stream, were integrated and used with moisture measurements by the Michell unit during the same tests to arrive at a water vapor response factor for the GC. During tests of the gas stream containing both methanol and water vapor, this response factor was applied to GC analyses of the test stream, so that the GC could quantify both contaminants in the stream as well as the hydrocarbon content. This approach eliminated concerns about possible errors in moisture measurements by the Michell unit ceramic sensor with methanol present. 3.3 INTRODUCTION OF TEST GAS AND CONTAMINANTS A finalized work scope for the testing portion of the project was approved by the JIP members during a teleconference on March 13, This work scope called for testing with a single gas and two contaminants: water and a methanol-water mixture. The major steps followed for producing the two contaminated test gas mixtures were as follows: 1. Clean and prepare the test loop. 2. Introduce the hydrocarbon test gas. 3. Add water to the system to achieve a coincident HCDP and WVDP at 500 psia (34.5 bara). 21

46 4. Add methanol and water to the existing mixture from Step 3 to achieve a coincident HCDP and OHDP at 500 psia (34.5 bara) and a 3:1 molar ratio of methanol to water Loop Preparation and Test Gas Introduction Preparation of the loop and charging it with the test gas followed the basic procedure that was developed and utilized during the Phase 1 testing. The key steps are briefly summarized here. All of the steps are detailed in the complete test procedure contained in Appendix A. The purpose of the loop preparation measures was to avoid contamination of the test gas and ensure the integrity of the apparatus. Any tubing and equipment that was to be added to the system was cleaned using acetone to remove potential contaminants before installation. After assembly was complete, the system was leak-tested with helium gas at a pressure above the maximum test pressure. A thermal conductivity leak detector was used to inspect all fittings and joints to find any helium leaks from the test system. The apparatus was pre-conditioned before use by heating it under vacuum and introducing a trickle flow of boil-off nitrogen from a liquid nitrogen Dewar to remove any residual water vapor. The Michell Condumax II WVDP measurement sensor was used to check the moisture content in the loop. Since the first series of experiments involved tests with moisture, there was no need to remove moisture from the loop to the extent that was done in the Phase 1 project. However, these steps were still performed to clean the loop and to ensure that the moisture level was sufficiently low so that water could be added later to bring the system to the exact moisture level required. Also, all gases were introduced into the system through a desiccant chamber filled with phosphorus pentoxide, so as to remove any water vapor from the gases. After calibration of the GC and preheating of the equipment as described in the previous sections, several steps were performed to ensure the integrity of the new test gas as it was charged to the apparatus. 1. The calibrated GC was used to verify the test gas composition directly from the test gas delivery cylinder. If the composition in the cylinder did not agree with the certified composition, the cause was determined before work proceeded. 2. The system was evacuated to approximately 27 -Hg (13.3-psi) vacuum for a minimum of one-half hour. The temperature of the warm box was brought to a temperature at least 10 F (5.6 C) above the boiling point of water at the pressure of the system, in order to encourage vaporization of any traces of moisture left in the loop. The test loop temperature was then allowed to stabilize. 3. The system was purged with methane to help extract any heavy hydrocarbon residues from previous tests. The purge involved five fill/flush cycles with methane at 25 psig to 50 psig (170 kpa gauge to 345 kpa gauge). The methane was circulated through the loop for five minutes during each fill cycle. After each of the methane flushes, a vacuum of 27 Hg (13.3 psi) was imposed on the system for 15 minutes. 4. The system was purged with helium to help extract any remaining contaminants. The purge involved five fill/flush cycles with helium at approximately 80 psig (552 kpa gauge). The helium was circulated through the loop for five minutes during each fill cycle. After each of the helium flushes, a vacuum of 27 Hg (13.3 psi) was imposed on the system for five minutes. 22

47 5. The system was filled with helium at approximately 80 psig (552 kpa gauge), and circulated in the loop for 30 minutes. 6. The GC and Michell Condumax II WVDP measurement cell were used to check the helium in the loop for contaminants (remaining hydrocarbons, nitrogen, carbon dioxide, water vapor, etc.). A GC analysis of the loop contents was performed to confirm that contaminant levels were within acceptable limits. 7. The helium was vented from the test system and a vacuum of 27 Hg (13.3 psi) was imposed on the system for 30 minutes. 8. The system was filled with test gas by performing five fill/flush cycles with test gas at 25 psig to 50 psig (170 kpa gauge to 345 kpa gauge). The test gas was circulated through the loop for five minutes during each fill cycle. After each of the flushes, a vacuum of 27 Hg (13.3 psi) was imposed on the system for five minutes. After the final flush, the system was filled with test gas to a pressure that would allow the system to reach all planned test pressures with a single charge of gas. 9. Finally, the contents of the loop were analyzed by the GC to confirm the gas composition in the loop before the tests began. As before, if the composition in the test system did not agree with the certified composition, the cause was determined before work proceeded Addition of Water Contamination After determining that the composition of the test gas in the loop was acceptable, the next objective was to add water to the gas until the HCDP and WVDP coincided at a system pressure of 500 psia (34.5 bara). This objective took into account the fact that the amount of water in the gas stream varies with pressure, due to adsorption and desorption of water molecules from the walls of the apparatus. Consequently, a specified value for the water concentration can only be maintained at one pressure. The objective in this project was to vary the pressure so as to have test conditions where the WVDP occurred at a temperature above, below, and equal to the HCDP. Maintaining a specific moisture content was not the intent of this test plan, and doing so would have required the addition or removal of moisture each time the pressure was changed. The method used for achieving the correct water concentration involved an iterative process of adding a small amount of water (relative to the quantity of water that was expected to be required), circulating the contents of the loop to distribute the moisture, and making a series of observations with the manual chilled mirror dew point tester at various pressures to establish where the WVDP curve would intersect the HCDP curve. Distilled water was added using the bypass loop as described in subsection 2.2.1, then a series of chilled mirror dew point measurements were performed. These measurements were typically made at three pressures between 600 psia and 900 psia (41.4 bara and 62.1 bara), all of which were high enough to ensure that the water would condense out of the gas stream first. When plotted alongside the HCDP curve, the trend of these three test points could be extrapolated to determine an approximate point of intersection of the two dew point curves. If the point of intersection was higher than 500 psia (34.5 bara), more water was added and the process was repeated. The Michell analyzer was also used to confirm the observed dew points at the 500-psia (34.5-bara) condition. After several cycles of this process, the contamination level was such that the two dew points at 500 psia (34.5 bara) were judged to be reasonably close, and testing commenced. 23

48 3.3.3 Addition of Methanol-Water Contamination In the second series of experiments, it was again desired to have the two dew point curves coincide at 500 psia (34.5 bara). For these tests, it was expected that an HCDP and a combined OHDP would be observed (i.e., the water and methanol would not condense separately). Also, to represent field conditions as closely as possible, there was an additional requirement that the concentrations of methanol and water in the contaminant condensation be in a 3:1 molar ratio at the 500-psia (34.5-bara) condition. The same procedure used for the water addition was followed to add laboratory-grade methanol (CH 3 OH) to the system, except that the Varian GC was used to determine the concentrations of moisture and methanol from each iteration. Because the addition of methanol changed the dew point temperature of the methanol-water mixture, additional water also had to be added to the system (beyond the amount present from the first series of experiments) to cause the two dew points to coincide at 500 psia (34.5 bara). Experience showed that better measurements of methanol and water levels were obtained when the system was allowed to stabilize overnight after methanol addition. Because of the need to meet these two simultaneous constraints, this process was time-consuming, but it eventually yielded a composition that was judged to be as close as practically achievable to meeting both objectives. 3.4 DEW POINT MEASUREMENT PROCEDURES Once the test loop temperature, pressure, and gas contaminant levels were judged to be acceptable, each series of dew point measurement experiments began. The pressure in the test loop was brought to the level specified in the test plan (Table 3-1) by changing the pressure of the nitrogen pressurant on the back side of the accumulator piston. After the desired pressure had been achieved in the loop, the stroke speed of the rodless piston was adjusted to produce the required standard volumetric flow rate of 5 scfh (2.5 Nl/min). The stroke speed was computed using the density of the gas at its pressure and temperature as computed by the AGA-8 equation of state (American Gas Association, 1994). Each test day began with calibration of the GC (using the hydrocarbon and methanol standards) and an analysis of the gas in the test loop. All tests at a given pressure were completed in a single day, without interruption. Therefore, each series of tests (water only and water/methanol) required three days to complete. The key steps of the dew point measurement procedure used with each device are briefly summarized below. All of the steps are detailed in the complete test procedure, contained in Appendix A. Table 3-1. Equipment temperatures and test conditions for tests with water and methanol-water contamination. Gas cylinder, regulator and manifold temperature Loop temperature Test pressures 110 F (43 C) Room temperature or above 750, 500, 300 psia (51.7, 34.5, 20.7 bara) Procedure for the Bureau of Mines Chilled Mirror Testing with the Bureau of Mines chilled mirror began by configuring the valves in the manifold used to route flow to the test articles such that flow was sent to the dew point tester only. The stepper motor was started to maintain a standard volumetric flow rate of 5 scfh (2.5 Nl/min). The chiller system was then adjusted to gradually lower the chilled mirror temperature at a rate 24

49 no greater than 1ºF (0.6ºC) per minute, per the recommendations of ASTM Standard D 1142 (ASTM, 1995). As the mirror temperature decreased, the operator noted the time, line pressure, and mirror temperature at which various events were first observed. These events corresponded to the occurrence of a WVDP, an OHDP, an HCDP, or possibly other phenomena, such as hydrate formation. Also, as discussed in Section 2, both iridescent ring and droplet dew points may be observed with the chilled mirror, and depending upon the standards and requirements of the user, either kind may be cited as the HCDP. Using the interpretations described in subsection 2.2.1, the operator noted the conditions corresponding to both types of dew points, if they were present. In each case, the mirror temperature was reduced until evidence was seen of both water and an HCDP, or until the limits of the chiller were reached, typically around 20 F (-7 C). The dew point measurements were repeated a total of six times at each test pressure. A data logger also recorded temperature and pressure data during the tests, and video recordings of the view on the dew scope were made as well. The data files were reviewed after the tests to confirm the dew point conditions noted by the operator, and the operator and project manager reviewed the video recordings after the tests to confirm the observations. After the sixth dew point measurement, the gas stream was allowed to continue circulating for 30 minutes, to ensure that all condensate was re-vaporized prior to performing a GC analysis of the gas in the loop. After this 30-minute period, four consecutive GC analysis runs were performed on the test gas in the loop, and the results from the last three analyses were used in analyzing the test results. For the test cases with methanol, the initial four analyses were followed by five additional methanol-only GC runs, to ensure that a stable condition had been obtained for the methanol measurement Procedure for the Ametek 241 CE II Tests with the Ametek 241 CE II began by configuration of the valves in the manifold used to route flow to the test articles, such that flow was sent to this analyzer only. The stepper motor was started to maintain a standard volumetric flow rate of 5 scfh (2.5 Nl/min). Flow was maintained during each of the analyzer s ten-minute purge times. However, flow was stopped during the analyzer s measuring and warming phases, when the analyzer s solenoid valve was closed and no flow path existed through the analyzer. The analyzer was allowed to perform nine consecutive dew point measurements, with the analyzer set to stop the measurement cycle only after both an HCDP and a WVDP were found. For these tests, the default upper and lower mirror temperature set points were used [68 F and 18 F (20 C and -7.8 C), respectively]. Per the manufacturer s instructions, the first three measurements after the start of tests with the analyzer were disregarded, and only the final six measurements were recorded. The operator recorded the time, line pressure, and dew point measurements indicated by the analyzer. The dew point measurements were also downloaded from the analyzer s log file after the tests. After the last dew point measurement, the gas was allowed to continue circulating for 30 minutes, to ensure that all condensate was re-vaporized prior to performing a GC analysis of the gas in the loop. After this 30-minute period, four consecutive GC analysis runs were performed on the test gas in the loop, and the results from the last three analyses were used in analyzing the test results. For the test cases with methanol, the initial four analyses were followed by five additional methanol-only analyses, to ensure that a stable condition had been obtained for the methanol measurement. 25

50 3.4.3 Procedure for the Michell Condumax II Tests with the Michell Condumax II began by configuration of the valves in the manifold used to route flow to the test articles, such that flow was sent to this analyzer only. The stepper motor was started to maintain a standard volumetric flow rate of 5 scfh (2.5 Nl/min). As noted in subsection 2.2.3, the analyzer s signal level trip point was adjusted for each new test pressure. [The trip points used in these tests (as given in the test procedures in Appendix A) were developed by the manufacturer based on the expected gas quality, and may not be generally applicable to other measurement conditions.] The analyzer was allowed to perform nine dew point measurements. Per the manufacturer s instructions, the first three measurements after the start of tests with the analyzer were disregarded, and only the final six measurements were recorded. The operator recorded the time, HCDP measurement, WVDP measurement, and line pressures indicated by the analyzer. These data were also downloaded from the analyzer s log file after the tests. After the last dew point measurement, the flow was allowed to continue circulating for 30 minutes, to ensure that all condensate was re-vaporized prior to performing a GC analysis of the gas in the loop. After this 30-minute period, four consecutive GC analysis runs were performed on the test gas in the loop, and the results from the last three analyses were used in analyzing the test results. For the test cases with methanol, the initial four analyses were followed by five additional methanol-only analyses, to ensure that a stable condition had been obtained for the methanol measurement. 26

51 4. RESULTS AND DISCUSSION Preparation of the test loop for tests in contaminated gas streams was completed during May 2008, and dew point measurements were taken with the automated analyzers and the manual chilled mirror during June and July. Dew point measurements of each test gas stream were performed at three line pressures, one each above, at, and below the predicted hydrocarbon cricondentherm of the gas blend. Target contaminant levels were selected so that above the hydrocarbon cricondentherm temperature, the dew point of the water vapor or methanol-water mixture (the OHDP) would occur at a higher temperature than the HCDP and cause the contaminant to condense first, while at the cricondentherm, the contaminant and the hydrocarbon gas would condense out simultaneously. These situations have the potential to interfere with HCDP measurements, and were selected to test the effect of the contaminants on analyzer performance. This section discusses the tests results, comparing HCDP measurements from the Bureau of Mines chilled mirror and the automated analyzers to one another. Measurements of WVDPs or methanol-water mixture dew points from the various devices are also reported here, but the discussion of these values focuses on their impact on the primary objective of accurate HCDP measurement. As in the Phase 1 report, HCDP measurements are also compared to HCDP curves predicted using the analyzed gas compositions and the SRK EOS (Soave, 1972). Relevant WVDP curves and methanol-moisture dew point curves, computed using appropriate correlations and equations of state, are also compared to the measured contaminant dew point values. Confidence intervals are shown with each measured dew point, determined from the uncertainties of each device listed in Table 2-1 and Table 4-6 (presented later in this section). Confidence intervals have also been placed on dew point curves predicted with equations of state, as described in subsection 4.2. As in Phase 1, HCDPs observed on the Bureau of Mines chilled mirror have been identified as iridescent ring dew points or droplet dew points, to address questions on the interpretation of chilled mirror observations. The SwRI project manager and technical staff reviewed the videotapes of the chilled mirror measurements after tests to confirm the observed dew points. Several phenomena unique to the contaminated test gases were observed on the chilled mirror, making confirmation of the HCDP values particularly important. Less than one-half of the observed HCDPs were changed as a result of the review. The observed phenomena are discussed in this section, with images of the condensation on the chilled mirror included in support of the discussion. 4.1 MEASUREMENTS OF CONTAMINANT CONTENT DURING TESTS Before, during, and after each series of tests, data were gathered on the contaminant content of the gas in the test loop. For all Phase 2 tests, the 1,050-Btu/scf (39.12-MJ/Nm 3 ) transmissionquality gas blend involved in Phase 1 was used as the baseline hydrocarbon gas. The test loop was first filled with hydrocarbon gas from the same supply cylinder used in Phase 1. Controlled quantities of contaminants were then injected into the loop until the target contamination level had been reached Measurements of Water Vapor as Sole Contaminant In the first set of tests, water was added to produce a WVDP curve that intersected the HCDP curve at approximately 500 psia (34.5 bara). The water vapor content of the stream was 27

52 determined using the NPL- and NIST-traceable WVDP sensor incorporated into the Michell Condumax II, and software provided by Michell Instruments. The Michell unit software uses the IGT-8 correlation (Bukacek, 1955) to produce a WVDP temperature from the measured moisture content. The IGT-8 correlation was used here to reverse the Michell unit calculation and determine the moisture content from the reported WVDP (see subsection 4.2.2). Calculations before tests, using two versions of the SRK EOS and the IGT-8 correlation, predicted that a water vapor level anywhere from 10.5 pounds of water per million standard cubic foot of gas to 34 pounds of water per million standard cubic foot of gas (10.5 lb/mmscf to 34 lb/mmscf) (168 to 544 mg/nm 3 ), equivalent to parts per million by volume to 714 parts per million by volume (220.5 ppmv to 714 ppmv), would produce the target condition of HCDP and WVDP curves intersecting at 500 psia (34.5 bara). These levels are well above the general tariff limit of 7 lb m /MMscf (112 mg/nm 3 ), or 147 ppmv. The wide range of predicted water vapor levels were of concern; further, changes in line pressure were expected to alter the water vapor content of the gas at the different test pressures, as water vapor desorbed from the walls of the test rig with decreasing pressure, and adsorbed to the walls with increasing pressure. Rather than rely on the predicted values, it was decided to add water to the test loop in small amounts until the WVDP and HCDP at 500 psia (34.5 bara) were seen to coincide on the Bureau of Mines chilled mirror. The actual water vapor content was then measured using the Michell unit moisture sensor. Table 4-1 shows the water vapor content determined from the WVDPs measured by the Condumax II at various points during the water-only tests. The average water vapor content at 500 psia (34.5 bara), where the HCDP and WVDP coincided, was 10.9 lb/mmscf (174 mg/nm 3 ), at the lower end of the range of predicted values. At 750 psia (51.7 bara), adsorption reduced the average water vapor content of the stream to 9.2 lb/mmscf (147 mg/nm 3 ), and at 300 psia (20.7 bara), desorption increased the average water vapor content of the stream to 14.3 lb/mmscf (229 mg/nm 3 ). Although the water vapor content in the test gas changed with pressure as expected, it is worth noting that the experiments still achieved the goal of testing the analyzers at conditions where the water vapor condensed first, the hydrocarbons condensed first, and the two dew points coincided. Chromatograms from the Varian GC were reviewed to see if an elution peak on the CP-Wax channel, used for methanol and other alcohols, would correlate to water vapor content. One peak was found in each chromatogram whose area correlated well with water vapor content. Varian provided SwRI with chromatograms from other users of this type of channel, which confirmed that this was a water elution peak. The water vapor peaks in each chromatogram were integrated, and uncertainty analyses were performed on the peak areas. Figure 4-1 compares the relative peak areas from GC analyses, after tests of the Michell unit, to water vapor content measurements by the Michell unit. Figure 4-2 compares the average of peak areas from GC analyses of the loop after tests with all analyzers to the Michell unit measurements. The trends in the GC peak areas and the Michell unit moisture measurements were consistent, and when the average peak areas were scaled properly, the two measurements of water vapor content at each pressure had overlapping 95% confidence intervals. This confirmed the relative trends in the Michell unit moisture dew points, and the moisture content of the loop as computed from the Michell unit measurements and IGT-8. 28

53 Table 4-1. Water vapor content of the test loop, measured by the Michell unit moisture sensor during tests of the Michell analyzer on the gas stream with only water contamination. Measured Water Vapor Dew Point Water Vapor Content Event Temperature ( F) Pressure (psig) Temperature ( C) Pressure (barg) (lb/mmscf) (mg/nm 3 ) Tests at 750 psia (51.7 bara) Average over all tests at pressure Tests at 500 psia (34.5 bara) Average over all tests at pressure Tests at 300 psia (20.7 bara) Average over all tests at pressure Moisture measurements relative peak areas Average GC peak areas after Michell tests Michell measurements Michell measurement (lb/mmscf) loop pressure (psia) Figure 4-1. Comparison of water peak areas from Varian GC analyses after the Michell unit HCDP measurements to moisture measurements by the Michell analyzer. 29

54 Moisture measurements relative peak areas Michell measurement (lb/mmscf) average GC peak areas after all tests at pressure Michell measurements loop pressure (psia) Figure 4-2. Comparison of average water peak areas from Varian GC analyses after all HCDP measurements to moisture measurements by the Michell analyzer Simultaneous Measurements of Methanol and Water Vapor To reflect conditions found in natural gas production streams, the JIP Project Committee asked that tests be performed on a stream containing both methanol and water. The target test condition was a 3:1 ratio of methanol to water, with a combined OHDP curve that intersected the HCDP curve at 500 psia (34.5 bara). A review of data made available by GPA indicated that the 3:1 methanol:water molar ratio is typical in the gas phase of a stream where methanol has been injected to avoid hydrate formation. As discussed in subsection 4.2.3, attempts were made to predict the required amounts of methanol and water vapor beforehand, using various equations of state within the Multiflash software package (Infochem, 2007a). However, the equations failed to converge in many instances, or predicted OHDP curves that contradicted expectations. It was decided to follow the same approach used for adding moisture alone, and add small amounts of contaminants until the target conditions were reached. The contaminant addition process involved four steps: 1. Methanol or a methanol-moisture blend was injected into the loop in a predetermined amount. 2. The loop contents were circulated for at least one-half hour, and then allowed to stabilize overnight. 3. The OHDP was measured at pressures approaching 500 psia (34.5 bara) using the Bureau of Mines chilled mirror, to determine the trend in the OHDP with pressure and the expected intersection of the OHDP and HCDP curves. 4. The methanol and water content in the loop at 500 psia (34.5 bara) were measured using the Varian GC. 30

55 Initially, methanol was added to reach the 3:1 target ratio, but it was soon found that the total contaminant level was too high, with an OHDP temperature well above the HCDP temperature at 500 psia (34.5 bara). A limited volume of gas was vented from the system, and then an amount of the uncontaminated, reference hydrocarbon gas blend was added to the system to replace the vented volume. At this point, a 3:1 methanol-water blend was added in small amounts to work toward the target conditions. At each step, OHDP temperatures and HCDP temperatures were monitored using the Bureau of Mines chilled mirror, and contaminant levels were measured using the Varian GC. After several days, the dew points were judged to be as close as practically achievable, and a methanol-water ratio of 2.9 was observed, so testing at 750 psia (51.7 bara) began the following day. Table 4-2 shows the methanol and water vapor content determined by the Varian GC at various stages of the tests. As with the water-only tests, the water content during these tests increased as loop pressure decreased and water vapor desorbed from the walls. The methanol content also increased with decreasing pressure, but desorption of methanol from the walls into the gas stream was less significant than for the water vapor, so that the methanol-water ratio decreased with decreasing pressure. Contaminant and dew point measurements at 500 psia (34.5 bara) before tests indicated that the target dew point intersection had been attained with a methanolwater ratio of 2.9. After tests began at 750 psia (51.7 bara), a shift in the ratio and the OHDP was discovered, such that the HCDP and OHDP curves intersected at a pressure between 300 psia (20.7 bara) and 500 psia (34.5 bara). Still, tests of the analyzers included conditions where the contaminant phase condensed first, and where the two dew points approximately coincided, the most severe test conditions for the analyzers. Table 4-2. Contaminant content of the test loop during tests on the gas stream with both methanol and water vapor present. Test Pressure Stage of Test Methanol Content (ppmv) Water Content (ppmv) Methanol-water Ratio 750 psia (51.7 bara) Before start of tests After Bureau of Mines tests After Ametek tests After Michell tests Average over all tests at pressure psia (34.5 bara) Before start of tests After Bureau of Mines tests After Ametek tests After Michell tests Average over all tests at pressure psia (20.7 bara) Before start of tests After Bureau of Mines tests After Ametek tests After Michell tests Average over all tests at pressure

56 4.2 COMMENTS ON DEW POINT CALCULATION METHODS Hydrocarbon Dew Point Calculations The natural gas industry often uses equations of state and GC analyses of a natural gas stream to predict its HCDP in situations where the dew point is impractical to measure. In this work, predicted dew points for the test gases were required for selecting test conditions and for comparison to the various measurements. The report on Phase 1 of this project (George and Burkey, 2008), as well as two recent studies sponsored by Gas Technology Institute, Pipeline Research Council International, Inc., and the U.S. Minerals Management Service (George et al., 2005a; George, 2007) discuss various approaches to predicting HCDPs of natural gas streams using GC compositional data as input. Those studies found that, given inaccuracies in commonly-used equations of state, the SRK EOS (Soave, 1972) has advantages over others in predicting HCDPs. The SRK EOS was used in this project for selecting test conditions, for predicting dew points to be compared to measured values, and for estimating biases in measured dew points caused by small shifts in gas composition over the course of tests. All predicted HCDP curves were computed using the Multiflash software package (Infochem, 2007a) with binary interaction parameters drawn from the DECHEMA Data Series (Knapp et al., 1982), the default set used by Multiflash. All components in the stream, including all normal and non-normal hydrocarbons in the certified and analyzed gas compositions, were input to the equation of state. Earlier comparisons of HCDP data measured on these nominal gas compositions and predicted using this equation of state, software, and inputs indicated that the experimental and analytical dew points would be in reasonable agreement (George et al., 2005a and 2005b). Because dew points produced by equations of state are predicted values, confidence intervals at the 95% level have been estimated for each predicted HCDP curve and shown as dashed lines. The 95% confidence intervals were estimated by moving the certified or analyzed amounts of hexane and heavier hydrocarbons to the lower end (or upper end) of their own 95% confidence intervals, then proportionately shifting the amounts of lighter hydrocarbons and diluents in the other direction to maintain a total composition of 100% Water Vapor Dew Point Calculations Issues related to the presence of water vapor in natural gas streams predate the development of cubic equations of state for predicting phase behavior. As early as the 1950s, it was known that the moisture content of natural gases influenced the efficient operation of gas production and transmission equipment. A study sponsored by the Institute of Gas Technology (Bukacek, 1955) investigated the relationship between the equilibrium water vapor content of a natural gas stream and its hydrocarbon composition. Experimental data on a range of synthesized and natural gas blends produced a correlation for water vapor content as a function of only the WVDP pressure and temperature. This formula, known as the IGT-8 correlation, became generally accepted as a method for determining WVDPs of natural gas streams. The correlation takes the form: A( T ) W = + B( T ) P where W is the water vapor content of the stream in pounds mass per million standard cubic feet of gas (lb m /MMscf), P is the observed WVDP pressure in psia, T is the observed WVDP temperature in degrees F, and A and B are temperature-dependent parameters. Effects of gas 32

57 composition on equilibrium water vapor content were of the same order as the experimental uncertainty in the data, and so were not separately characterized. The IGT-8 correlation has been generally accepted for determining the water vapor content of gas streams from measured dew points, or conversely, for determining the WVDP of streams with known water vapor content. The IGT-8 correlation was incorporated into the original ASTM standard for the use of the Bureau of Mines chilled mirror and its subsequent revisions (ASTM, 1995). The software interface for the Michell Condumax II also employs an iterative form of the IGT-8 equation to report a WVDP temperature, using the moisture content measured by the ceramic sensor and the sample pressure as input. While cubic equations of state, such as the SRK EOS, are capable of predicting WVDP curves, they are not always considered to be as accurate as IGT-8 on water vapor. During the first round of Phase 2 tests, water vapor was the sole contaminant in the gas stream. WVDP pressures and temperatures reported by the Michell unit moisture sensor were used with IGT-8 to determine the water vapor content of the stream, effectively reversing the calculations by the Michell unit software to produce the moisture values measured directly by its ceramic sensor. The IGT-8 correlation was also used to produce the WVDP curves appearing in the graphs of test results later in this section. Because of this, each IGT-8 curve passes directly through the measured values Methanol-Water Dew Point Calculations Methanol is added during the processing of natural gas streams containing water vapor to prevent the formation of hydrates or water ice within pipelines. In such streams, it is generally held that the water and methanol will condense together in a liquid phase separate from any hydrocarbon condensates. For Phase 2, it was necessary to calculate the OHDP curve that intersects the HCDP curve at 500 psia (34.5 bara), so as to predict the target concentrations of methanol and water in the gas blend. However, the natural gas industry has little experience modeling methanol behavior in natural gas streams, and finding a useful equation of state proved difficult. Predictions of the required amounts of methanol and water vapor were attempted using the SRK EOS and the Predictive Soave-Redlich-Kwong equation of state (PSRK EOS) within Multiflash (Infochem, 2007a). Cubic equations of state such as the SRK EOS are in common use by the natural gas industry, as they have been proven accurate for ideal and slightly non-ideal gases such as hydrocarbons, carbon dioxide, and nitrogen. However, the standard cubic equations do not model non-ideal chemical systems well, among them alcohol-water systems. The PSRK EOS is an extension of the SRK EOS with specific mixing rules to model the phase behavior of polar compounds (Infochem, 2007b). Both equations of state were used to predict the amounts of methanol and water, in a 3:1 ratio, for which the combined OHDP curve would intersect the HCDP curve at 500 psia (34.5 bara). However, the software failed to converge in many instances, or predicted OHDP curves contrary to expectations. In calculating an OHDP curve, for example, the SRK EOS failed to converge at temperatures and pressures beyond the HCDP phase envelope. At the point where the two curves intersected, the predicted HCDP curves often deviated from the expected behavior (Figure 4-3). Iterative flash calculations (predictions of gas and liquid compositions in equilibrium at a given temperature and pressure) were used to estimate contaminant levels where the curve calculations failed to converge. Flash calculations with the SRK EOS predicted that a contaminant mixture of 450-ppmv methanol and 150-ppmv water would achieve the target 33

58 conditions. The PSRK EOS, surprisingly, produced OHDP curves with cricondentherms, and failed to converge for total contaminant amounts above 1,300 ppmv. Using iterative flash calculations, a target contaminant mixture of about 1,550 ppmv methanol and 516 ppmv water was predicted (Figure 4-4) Btu/scf (39.12 MJ/Nm 3 ) gas with methanol and moisture Dew point curves predicted using SRK EOS and Multiflash 22927AW as certified ppm MeOH, 100 ppm H2O HCDP 300 ppm MeOH, 100 ppm H2O -OHDP 450 ppm MeOH, 150 ppm H2O HCDP Pressure (psia) ppm MeOH, 150 ppm H2O -OHDP 600 ppm MeOH, 200 ppm H2O HCDP 600 ppm MeOH, 200 ppm H2O -OHDP Figure Temperature (F) Hydrocarbon and methanol-water dew point curves predicted using the SRK EOS and Multiflash, showing problems with convergence and deviations from expected HCDP curve behavior Btu/scf gas, 22927AW, with methanol and moisture PSRK EOS 22927AW as certified ppm MeOH, 175 ppm H2O HCDP 525 ppm MeOH, 175 ppm H2O -OHDP Pressure (psia) ppm MeOH, 225 ppm H2O HCDP 675 ppm MeOH, 225 ppm H2O -OHDP 825 ppm MeOH, 275 ppm H2O -OHDP 975 ppm MeOH, 325 ppm H2O HCDP 975 ppm MeOH, 325 ppm H2O -OHDP 1549 ppm MeOH, 516 ppm H2O -OHDP (extrapolated) 250 Figure Temperature (F) Hydrocarbon and methanol-water dew point curves predicted using the PSRK EOS and Multiflash, showing unexpected cricondentherms in the methanol-water curves. 34

59 Given the large disagreement in predicted contaminant amounts between the two equations of state, it was decided to proceed carefully and rely solely on experimental observations of the OHDP to reach the target condition, as was done to prepare for the tests with water as the only contaminant. 4.3 TEST RESULTS FOR THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS WITH WATER CONTAMINATION Dew point measurements with the manual chilled mirror and the two automated analyzers were made in succession on the same gas stream at fixed pressures of 750 psia (51.7 bara), 500 psia (34.5 bara), and 300 psia (20.7 bara). For the first round of testing, the test gas was a 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas to which water vapor had been added. Each series of dew point measurements made with a given device was bracketed by GC analyses of the gas stream, which yielded the following test sequence: 1. Analysis of test loop gas. 2. Tests with manual chilled mirror. 3. Analysis of test loop gas. 4. Tests with Ametek automated analyzer. 5. Analysis of test loop gas. 6. Tests with Michell automated analyzer. 7. Analysis of test loop gas. In previous experience with the test apparatus (George et al., 2005b; George and Burkey, 2005c; George and Burkey, 2008), small changes in the heavy hydrocarbon content were observed over time. In particular, decreases were noted in normal heavy hydrocarbons such as nonane and decane. Average analyzed amounts in components from successive GC analyses, including 95% confidence intervals on the averages, were compared to find any sudden or statistically-significant changes in the test gas composition over the course of the tests. Data for three components of interest, nitrogen, nonane, and decane, are presented here as illustrative examples. Nitrogen was monitored for indication of leaks of the pressurant from below the accumulator piston into the test gas, while the nonane and decane were representative of the heavy hydrocarbons in the gas blend. Figure 4-5 through Figure 4-7 present component histories for nitrogen, nonane, and decane over the course of the moisture contamination tests. The horizontal axis notes the date, pressure, and the point in the test procedure at which the test gas was analyzed. A statistically-significant change in component content is indicated when the 95% confidence intervals for successive amounts do not overlap. Figure 4-5 shows that the nitrogen content was relatively constant during all of the tests, confirming that no nitrogen pressurant had leaked into the test system. The plots for nonane and decane show that these two components varied somewhat over the course of the testing, but the 95% confidence intervals (which extend below zero mole percent in some cases) indicate that the changes are not statistically significant. These changes are most likely the result of small amounts of the heavy hydrocarbons from the gas stream adsorbing onto the walls of the apparatus during the tests. No definite trend in the changes can be discerned. 35

60 mole percent psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) /24/08, Pretest, 750 psia 6/24/08, After dew scope tests, 750 psia 6/24/08, After Ametek tests, 750 psia 6/24/08, After Michell tests, 750 psia 6/25/08, Pretest, 500 psia 6/25/08, After dew scope tests, 500 psia 6/25/08, After Ametek tests, 500 psia 6/25/08, After Michell tests, 500 psia 6/26/08, Pretest, 300 psia 6/26/08, After dew scope tests, 300 psia 6/26/08, After Ametek tests, 300 psia 6/26/08, After Michell tests, 300 psia analysis Figure 4-5. Trends in analyzed nitrogen content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas during analyzer tests with moisture contamination mole percent psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) /24/08, Pretest, 750 psia 6/24/08, After dew scope tests, 750 psia 6/24/08, After Ametek tests, 750 psia 6/24/08, After Michell tests, 750 psia 6/25/08, Pretest, 500 psia 6/25/08, After dew scope tests, 500 psia 6/25/08, After Ametek tests, 500 psia 6/25/08, After Michell tests, 500 psia 6/26/08, Pretest, 300 psia 6/26/08, After dew scope tests, 300 psia 6/26/08, After Ametek tests, 300 psia 6/26/08, After Michell tests, 300 psia analysis Figure 4-6. Trends in analyzed nonane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas during analyzer tests with moisture contamination. 36

61 mole percent psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) /24/08, Pretest, 750 psia 6/24/08, After dew scope tests, 750 psia 6/24/08, After Ametek tests, 750 psia 6/24/08, After Michell tests, 750 psia 6/25/08, Pretest, 500 psia 6/25/08, After dew scope tests, 500 psia 6/25/08, After Ametek tests, 500 psia 6/25/08, After Michell tests, 500 psia 6/26/08, Pretest, 300 psia 6/26/08, After dew scope tests, 300 psia 6/26/08, After Ametek tests, 300 psia 6/26/08, After Michell tests, 300 psia analysis Figure 4-7. Trends in analyzed decane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas during analyzer tests with moisture contamination. Figure 4-8 and Figure 4-9 present the results obtained from the dew point measurements made on the gas stream with water vapor using the Bureau of Mines chilled mirror, the Ametek automated analyzer, and the Michell automated analyzer. In these plots, the measurements are compared to one another, and also to dew point curves predicted using the composition of the gas obtained from GC analyses performed before and after tests with each instrument. Confidence intervals are shown with each measured dew point, determined from the measurement uncertainties of each device. Additional information on the uncertainties in the dew point measurements, indicated by the error bars on the data points, can be found in subsection To better visualize the performance of each individual device, the information presented in Figure 4-8 and Figure 4-9 has been replotted to show the results for each device separately. This series of plots can be found in Appendix B. Data from the GC analyses of the gas stream were used with the Multiflash software package and the SRK EOS to predict HCDP curves for each test condition. Comparisons of these curves allow the effect of any changes in gas composition to be estimated and compared to the differences in measured dew points between analyzers. The dashed lines in the figures represent the confidence intervals on the predicted dew point curves, as discussed in subsection Comparisons of the curves themselves suggest the fraction of the difference in measured dew points that can be attributed to the small changes in gas composition during tests. 37

62 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture Data and curves computed from stream analyses All 750-psia (51.7-bara) data pressure (psia) temperature (deg. F) SRK HCDP curve, pretest analyses SRK HCDP curve, after chilled mirror measurements SRK HCDP curve, after Ametek measurements SRK HCDP curve, after Michell measurements chilled mirror 'iridescent ring' HCDP chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP predicted hydrate curve chilled mirror 'hydrates' IGT-8 WVDP curve, 9.2 lb/mmscf (193 ppmv) Michell WVDP 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture Data and curves computed from stream analyses All 500-psia (34.5-bara) data pressure (psia) ` SRK HCDP curve, pretest analyses SRK HCDP curve, after chilled mirror measurements 490 SRK HCDP curve, after Ametek measurements SRK HCDP curve, after Michell measurements 480 chilled mirror 'droplet' HCDP Ametek HCDP 470 Michell HCDP predicted hydrate curve IGT-8 WVDP curve, 10.9 lb/mmscf (229 ppmv) 460 chilled mirror 'WVDP' Michell WVDP temperature (deg. F) Figure 4-8. Dew points determined using all three measurement devices compared to dew point curves predicted from GC analyses of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas stream containing moisture. 38

63 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture Data and curves computed from stream analyses All 300-psia (20.7-bara) data pressure (psia) ` SRK HCDP curve, pretest analyses SRK HCDP curve, after chilled mirror measurements SRK HCDP curve, after Ametek measurements SRK HCDP curve, after Michell measurements chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP predicted hydrate curve IGT-8 WVDP curve, 14.3 lb/mmscf (300 ppmv) Michell WVDP Figure 4-8 (continued). 800 temperature (deg. F) 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture HDP data and curves computed from stream analyses 750-psia (51.7-bara) data, adjusted for changes in gas composition pressure (psia) SRK HCDP curve, pretest analyses chilled mirror 'iridescent ring' HCDP chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP temperature (deg. F) Figure 4-9. Dew points determined using all three measurement devices compared to dew point curves predicted from GC analyses of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas stream containing moisture. Results have been adjusted for changes in gas composition during tests. 39

64 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture HDP data and curves computed from stream analyses 500-psia (34.5-bara) data, adjusted for changes in gas composition pressure (psia) SRK HCDP curve, pretest analyses chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP temperature (deg. F) 350 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture HDP data and curves computed from stream analyses 300-psia (20.7-bara) data, adjusted for changes in gas composition pressure (psia) SRK HCDP curve, pretest analyses chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP temperature (deg. F) Figure 4-9 (continued). 40

65 In all of the tests, small changes were observed in the gas stream composition before and after the testing of each device. To account for the resulting effect on HCDPs in evaluating the performance of the various analyzers, the measurements from the three instruments have been corrected for changes in the gas composition over the course of the tests. The adjusted HCDPs at each test pressure are shown in Figure 4-9. The adjustments were made using the following assumptions: The effect of the changes in heavy hydrocarbons on the true HCDP was accurately estimated by the shift in the resulting dew point curves predicted by Multiflash and the SRK EOS. The overall shift in dew point between the start and end of the tests with each device occurred at a constant rate. Under this assumption, for example, the second of nine measurements with an analyzer would be corrected by applying two-ninths of the total dew point shift predicted from GC analyses at the beginning and end of tests with the analyzer. The effects of changes in heavy hydrocarbon content accumulated over the course of all tests at a given pressure, so that all measurements were corrected back to the original compositional analysis before tests with all instruments at that pressure. Figure 4-8 also contains curves based on the IGT-8 standard (Bukacek, 1955) indicating the average WVDP of the gas stream at each of the three test pressures. The moisture content values determined in each test, using data from the Michell unit moisture sensor and the IGT-8 correlation, were listed earlier in Table 4-1. Average values at each pressure were used to generate the WVDP curves in Figure 4-8. The Michell unit software uses the IGT-8 correlation to calculate a WVDP temperature from the measured moisture content. The IGT-8 correlation was used here to reverse the Michell unit calculation and determine the water vapor content from the reported WVDP. No confidence intervals have been plotted for these curves, as the IGT-8 correlation does not have a clearly documented uncertainty analysis. As anticipated, changes in pressure alter the water vapor content of the stream through adsorption and desorption of water on the apparatus walls. The trend seen in Table 4-1 is consistent with decreasing water vapor content in the gas as pressure increases and moisture adheres to the walls. Although the water vapor content in the test gas changed with pressure, it is worth noting that the experiments still achieved the goal of testing the analyzers at three conditions: where the water vapor condensed first, the hydrocarbons condensed first, and the two dew points coincided. Some key observations, based on the experimental results obtained using the natural gas blend containing significant amounts of water vapor contamination, are as follows: Nearly all HCDP measurements from all instruments fall within the confidence intervals of the corresponding equation of state curves. The exception is the chilled mirror at 750 psia (51.7 bara), where most HCDPs are above the upper confidence interval of the analytical curve, and the difference between predicted and measured values is statistically significant. Although the Ametek analyzer was configured to report both HCDPs and WVDPs, only HCDP values were recorded by the unit during these tests. Even though the analyzer 41

66 cooled to its lower temperature limit of 18 F (-8 C) in each cycle, it did not report a WVDP. The first formations on the chilled mirror at 750 psia (51.7 bara) (the filled triangles in Figure 4-8) are believed to be natural gas hydrates. Because this is not conclusive, the corresponding data are labeled in quotation marks. Predicted curves for hydrate formation (created using Multiflash with a variant on the SRK EOS) are also graphed for comparison to these data. Hydrates typically form only when free water is present in a system, although this is not always the case. The hydrate formations observed on the chilled mirror appear well below the predicted hydrate curve, but just a few degrees above the predicted WVDP curve. They also appear at about the same conditions where the Michell analyzer reports the WVDP, so their identity is uncertain. The formations believed to be hydrates are not observed at the other pressures. After the first phase has condensed on the mirror, the second phase can be difficult to visualize, if it appears at all. The following data and observations suggest that the first phase to condense on the mirror may alter the conditions under which the second phase appears: o Formations believed to be water vapor frost were observed at 500 psia (34.5 bara). At this pressure, the WVDP values observed with the chilled mirror are several degrees below the IGT-8 curve. It is possible that hydrocarbon condensates on the mirror are partly interfering with condensation of the water vapor. No evidence of a WVDP was seen at 300 psia (20.7 bara), another test condition where the hydrocarbons had condensed first. o Only in the case of the 750 psia (51.7 bara) test was an iridescent ring HCDP observed on the Bureau of Mines chilled mirror. At the other pressures, only droplet HCDPs were seen. Furthermore, the observed iridescent ring HCDPs differed from their usual appearance on the mirror. Because the hydrocarbon was the second phase to condense at 750 psia (51.7 bara), the ring of rainbow-like colors was observed to be forming on top of the layer of frost or hydrate that was already present on the mirror. The Ametek unit consistently reported HCDP temperatures higher than the Michell unit for the same conditions. The difference between the results for the two analyzers is more pronounced at the middle and low pressures than at the maximum test pressure. This trend is expected, given that the Ametek 241 CE II defines the dew point as the first appearance of a film of condensate on its mirror similar to the iridescent ring dew point identified by manual chilled mirror users and the Michell Condumax II trip point is calibrated to give good agreement with manual chilled mirror measurements of the droplet HCDP, which occurs at a lower temperature than the iridescent ring dew point. After adjustment for gas composition changes, the difference between the Ametek unit and Michell unit dew points remain, confirming that the majority of the differences between these HCDP measurements are due to differences in the automated instruments themselves. The manual chilled mirror observations of HCDPs agree most closely with the Ametek unit measurements at all but the highest pressures. At 750 psia (51.7 bara), the manual chilled mirror measurements span the readings obtained with both automated analyzers. 42

67 4.4 TEST RESULTS FOR THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS WITH WATER AND METHANOL CONTAMINATION For the second round of testing, the dew point measurement devices were put through an identical series of tests to those performed in the first round, except that the test gas was a 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas to which methanol and water vapor had been added. The sequence of testing of the three devices was unchanged (i.e., chilled mirror first, followed by the Ametek and Michell automated analyzers). As with the water-only tests, trends in the composition of the gas blend were monitored over the course of tests with methanol and water vapor. Figure 4-10 through Figure 4-12 present the results for nitrogen, nonane, and decane. No significant differences are seen in the average nitrogen content of the gas stream, indicating no leak of nitrogen from the pressurant side of the accumulator into the test system. As was seen previously, the heavy hydrocarbons nonane and decane vary somewhat over the course of testing, but the overlapping confidence intervals indicate that the changes are not significant (especially considering the small quantities involved) and no significant trends are present mole percent psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) /25/08, Pretest, 750 psia 7/25/08, After dew scope tests, 750 psia 7/25/08, After Ametek tests, 750 psia 7/25/08, After Michell tests, 750 psia 7/28/08, Pretest, 500 psia 7/28/08, After dew scope tests, 500 psia analysis Figure Trends in analyzed nitrogen content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas during analyzer tests with methanol and water contamination. 7/28/08, After Ametek tests, 500 psia 7/28/08, After Michell tests, 500 psia 7/29/08, Pretest, 300 psia 7/29/08, After dew scope tests, 300 psia 7/29/08, After Ametek tests, 300 psia 7/29/08, After Michell tests, 300 psia 43

68 mole percent psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) /25/08, Pretest, 750 psia 7/25/08, After dew scope tests, 750 psia 7/25/08, After Ametek tests, 750 psia 7/25/08, After Michell tests, 750 psia 7/28/08, Pretest, 500 psia 7/28/08, After dew scope tests, 500 psia 7/28/08, After Ametek tests, 500 psia 7/28/08, After Michell tests, 500 psia 7/29/08, Pretest, 300 psia 7/29/08, After dew scope tests, 300 psia 7/29/08, After Ametek tests, 300 psia 7/29/08, After Michell tests, 300 psia analysis Figure Trends in analyzed nonane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas during analyzer tests with methanol and water contamination mole percent psia (51.7 bara) 500 psia (34.5 bara) /25/08, Pretest, 750 psia 7/25/08, After dew scope tests, 750 psia 7/25/08, After Ametek tests, 750 psia 7/25/08, After Michell tests, 750 psia 7/28/08, Pretest, 500 psia 7/28/08, After dew scope tests, 500 psia 7/28/08, After Ametek tests, 500 psia 7/28/08, After Michell tests, 500 psia 7/29/08, Pretest, 300 psia 7/29/08, After dew scope tests, 300 psia 7/29/08, After Ametek tests, 300 psia 7/29/08, After Michell tests, 300 psia 300 psia (20.7 bara) analysis Figure Trends in analyzed decane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas during analyzer tests with methanol and water contamination. 44

69 Figure 4-13 and Figure 4-14 present the HCDP measurements made on the gas stream with methanol and water by the Bureau of Mines chilled mirror, the Ametek automated analyzer, and the Michell automated analyzer. In these plots, the measurements are compared to one another, and also to HCDP curves predicted using the gas compositions obtained from GC analyses performed before and after tests with each instrument. Since the methanol and water are believed to condense out of the gas stream together, the term OHDP has been adopted to denote the combined contaminant dew point, rather than referring to this event as the WVDP. To better visualize the performance of each individual device, the information presented in both Figure 4-13 and Figure 4-14 has been plotted to show the results for each device separately. This series of plots can be found in Appendix B. The data analysis and the presentation of the results follow the same procedure as used for the results of the moisture-only tests. The above discussion regarding confidence intervals, predicted HCDP curves, and adjustments to the data to account for differences in gas composition over the course of the testing, applies to the methanol and water test results shown here. The only exception is that the IGT-8 curves have been omitted, as they are not applicable to a methanol and water mixture ,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture and methanol Data and curves computed from stream analyses All 750-psia (51.7-bara) data pressure (psia) temperature (deg. F) SRK HCDP curve, pretest analyses SRK HCDP curve, after chilled mirror measurements SRK HCDP curve, after Ametek measurements SRK HCDP curve, after Michell measurements chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP chilled mirror OHDP Michell OHDP Ametek OHDP PSRK OHDP curve Figure Dew points determined using all three measurement devices compared to dew point curves predicted from GC analyses of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas stream containing methanol and water vapor. 45

70 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture and methanol Data and curves computed from stream analyses All 500-psia (51.7-bara) data pressure (psia) temperature (deg. F) SRK HCDP curve, pretest analyses SRK HCDP curve, after chilled mirror measurements SRK HCDP curve, after Ametek measurements SRK HCDP curve, after Michell measurements chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP chilled mirror OHDP Michell OHDP PSRK OHDP curve 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture and methanol Data and curves computed from stream analyses All 300-psia (20.7-bara) data pressure (psia) temperature (deg. F) SRK HCDP curve, pretest analyses SRK HCDP curve, after chilled mirror measurements SRK HCDP curve, after Ametek measurements SRK HCDP curve, after Michell measurements chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP chilled mirror OHDP Michell OHDP PSRK OHDP curve Figure 4-13 (continued). 46

71 800 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture and methanol HDP data and curves computed from stream analyses 750-psia (51.7-bara) data, adjusted for changes in gas composition pressure (psia) SRK HCDP curve, pretest analyses chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP temperature (deg. F) 550 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture and methanol HDP data and curves computed from stream analyses 500-psia (51.7-bara) data, adjusted for changes in gas composition pressure (psia) SRK HCDP curve, pretest analyses chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP temperature (deg. F) Figure Dew points determined using all three measurement devices compared to dew point curves predicted from GC analyses of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas stream containing methanol and water vapor. Results have been adjusted for changes in gas composition during tests. 47

72 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with moisture and methanol HDP data and curves computed from stream analyses 300-psia (20.7-bara) data, adjusted for changes in gas composition pressure (psia) SRK HCDP curve, pretest analyses chilled mirror 'droplet' HCDP Ametek HCDP Michell HCDP Figure 4-14 (continued). temperature (deg. F) The Varian GC was used to measure the levels of methanol and water in the gas stream during this round of tests, as discussed in subsections 3.2 and The contaminant levels measured in each test were listed earlier in Table 4-2. As can be seen from the table, the methanol-water ratio at 500 psia (34.5 bara) appears to have shifted from a value of 2.9 during preparations to an average of 3.3 during the chilled mirror tests. While the HCDP and OHDP curves were judged to coincide at 500 psia (34.5 bara) before tests, the OHDP was measured to be 4 F to 5 F (2 C to 2.8 C) higher than the HCDP during experiments with the chilled mirror at this pressure. At 300 psia (20.7 bara), the chilled mirror indicated that the OHDP was 1 F to 2 F (0.6 C to 1 C) lower than the HCDP. This would place the intersection of the two curves during the tests somewhere between 300 psia (20.7 bara) and 500 psia (34.5 bara). As these data indicate, maintaining a fixed methanol-water ratio is very difficult, due to the tendency of these two substances to adsorb and desorb from the apparatus walls. As noted in subsection 4.2.3, during calculations to estimate the amount of methanol and water vapor needed for the tests, it was found that the SRK EOS calculation of the OHDP curve would only converge at temperatures and pressures within the HCDP curve. As an alternative, the PSRK EOS, considered accurate for water vapor phase behavior, was used. However, it yielded unreasonably low predictions of the OHDP temperatures. PSRK EOS curves have been plotted in Figure 4-13 for comparison to the experimental data. These curves correspond to the average water vapor and methanol content measured by the GC during tests at each pressure, listed in Table 4-2. Key observations, based on the experimental results obtained using a natural gas blend containing significant amounts of methanol and water vapor contamination, are as follows. Comparisons are based on the measurements after adjustment for changes in gas stream 48

73 composition during tests. Possible explanations for the behavior of the measured dew points are given in subsection 4.5. HCDP measurements from the Ametek analyzer fall within the confidence intervals of the corresponding SRK EOS curves at all pressures. For the Michell unit, HCDP measurements fall within the corresponding equation of state confidence intervals at 300 psia (20.7 bara) and 500 psia (34.5 bara), but are higher than the equation of state curve limits at 750 psia (51.7 bara). For the manual chilled mirror, measurements are higher than the upper limit of the equation of state confidence intervals at 750 psia (51.7 bara) and 300 psia (20.7 bara), and in agreement with the curve at 500 psia (34.5 bara) (for the non-adjusted values). As in the moisture-only tests, the Ametek unit was configured to report both hydrocarbon and WVDPs. During the tests with methanol and water vapor, the Ametek unit successfully recorded WVDPs during the tests at 750 psia (51.7 bara), but not at the two lower pressures. The Michell analyzer consistently reported HCDP temperatures higher than the Ametek unit for the tests at 750 psia (51.7 bara) and 500 psia (34.5 bara). However, this difference was only statistically significant at 750 psia (51.7 bara). It is interesting to note that this trend was the opposite of what was observed in all of the moisture-only tests. At the lowest pressure, the Ametek unit gave the higher HCDP temperature, but the differences were not statistically significant. The manual chilled mirror HCDP observations were generally higher than the values reported by both of the automated analyzers. This was especially true at 750 psia (51.7 bara). At this pressure, the chilled mirror HCDPs were 6 F to 10 F (3.5 C to 5.6 C) above those of the Michell unit, which, in turn, were 8 F to 10 F (4.5 C to 5.6 C) above those of the Ametek unit. At the two lower pressures, the confidence intervals of a few of the chilled mirror observations overlap the confidence intervals of the Ametek unit measurements. At the two lower pressures, the spread in the readings from three devices is not as pronounced as it is at 750 psia (51.7 bara). For the 500 psia (34.5 bara) and 300 psia (20.7 bara) tests, all three units produced HCDPs within 5 F (2.8 C) of one another and some overlap of the confidence intervals is seen. In contrast to the observations made with the manual chilled mirror during the moistureonly tests, the formation of an iridescent-ring on top of a previously condensed phase was not seen in this series of experiments. At the higher pressures, the methanol and water vapor formed a frost-like condensate with somewhat indistinct edges at the center of the mirror. As the temperature was lowered, a dark ring (not iridescent) was observed at the edge of the frost spot, but remained stationary as the frost expanded toward the edges of the mirror. As the temperature was lowered further, hydrocarbon droplets were observed near the outside edge of the mirror. At 300 psia (20.7 bara), hydrocarbon droplets appeared first in the majority of tests, before the methanol-water frost appeared. As with the tests in the presence of water vapor contamination, the first phase to condense on the mirror may alter the conditions under which subsequent phases appear. 49

74 4.5 ANALYSIS OF RESULTS The primary goal of the Phase 2 testing was to assess the impact of water vapor and methanol in the gas stream on instrument performance. This was done by comparing HCDP measurements by each device in these tests to baseline measurements by the same device, on the same transmission-quality gas, without contaminants present. The baseline measurements on contaminant-free gas streams are presented in the Phase 1 report (George and Burkey, 2008), and the reader is referred to that report for details. This section describes the consistency of measurements by each device with and without contaminants present, as well as differences in measurements and performance by the various devices. Although HCDP measurement accuracy is the concern of this research, OHDP measurements are also discussed here, as are comparisons (where possible) between measured and predicted dew points Results from the Bureau of Mines Chilled Mirror The Bureau of Mines chilled mirror device was designed to measure WVDPs, and has since been adapted to measure HCDPs. Unlike the automated devices, the manual chilled mirror has a single surface dedicated to the measurement of both HCDPs and WVDPs. When the mirror surface is altered by one condensing phase, a second phase may condense at a different temperature than would occur in the absence of the first phase. Thus, in instances when multiple phases condense on the Bureau of Mines chilled mirror, the dew points of the phases at lower temperatures may be biased due to interference by the first condensate to form on the mirror. Alternatively, it may be that the mirror surface becomes completely coated with the first condensing phase, which prevents the second phase from condensing or becoming visible. This was believed to be the case with the moisture-only tests at 300 psia (20.7 bara), where the mirror surface was presumably completely covered with hydrocarbon condensate and no WVDP was seen. The manual chilled mirror results at 750 psia (51.7 bara) with only moisture in the stream were unique in that the first formation on the mirror was surmised to be natural gas hydrates. At this pressure, a frost circle was observed first in the mirror center, at temperatures not far above the WVDP reported by the Michell unit. In most of the tests, the first sign of hydrocarbon condensation was an iridescent ring forming on top of the frost circle and moving inward (Figure 4-15). Iridescent rings were not observed in any other Phase 2 tests with water or methanol present in the stream. Comparisons of the frost dew points to predicted hydrate curves, and the unique behavior of the iridescent ring, led to the theory that the first condensation had a surface with nucleation sites for a hydrocarbon film, and may have been hydrates. This is not conclusive, given that the observed frost points are within a few degrees of the WVDPs reported by the Michell unit under the same conditions, and that the dew point temperatures of the piggyback iridescent rings were only 1 F to 3.5 F (0.6 C to 1.9 C) higher than the iridescent ring HCDP temperatures of the uncontaminated streams in Phase 1. When water alone was added to the test gas, a majority of droplet HCDPs at 750 psia (51.7 bara) were above the confidence interval on the predicted HCDP curve. When methanol was added, all droplet HCDPs at 750 psia (51.7 bara) were well above the confidence interval on the predicted HCDP curve, as were several measurements at 300 psia (20.7 bara). Differences at the other two pressures were not statistically significant in either case. It should be remembered, however, that the HCDP curves computed from GC analyses are not absolute references; their primary purpose is to confirm that changes in hydrocarbon gas composition have a minimal 50

75 effect on observed HCDPs. Furthermore, the HCDP curves were calculated from the hydrocarbon components only, and should not be expected to reflect any effects of contaminant content. Figure Examples of presumed hydrate formations and iridescent ring formations on the Bureau of Mines chilled mirror. Left, an established hydrate formation; center, initial formation of the iridescent hydrocarbon ring on the hydrate layer, inward from the edge; right, expansion of the iridescent ring toward the center of the mirror. Of more concern is the fact that at nearly all pressures, droplet HCDPs were measured at significantly higher temperatures when water or methanol-water contamination was present than during the Phase 1 tests on the same gas without contaminants present. The method of determining net shifts in measured droplet HCDPs from changes in hydrocarbon gas composition between tests has been applied in Table 4-3. With water alone as a contaminant, the average change in droplet HCDP temperatures ranged from +0.2 F (+0.1 C) to +6.0 F (+3.4 C). When both methanol and moisture were present in the stream, even larger upward shifts in droplet HCDP temperatures as high as 19.8 F (11.1 C) were observed at the two higher test pressures. Interestingly, positive biases occurred across all pressures, even at 300 psia (20.7 bara) where the hydrocarbons condensed first. In summary, the presence of contaminants produced observable increases in the HCDP temperatures measured with the Bureau of Mines chilled mirror. This is of concern to the natural gas industry and natural gas users, due to the operational risks posed by such liquids. As an aside, presumed hydrate points, WVDPs and OHDPs were also observed on the manual chilled mirror. Although the hydrate formations at 750 psia (51.7 bara) in the water-only stream were observed at temperatures about 20 F (11 C) below the predicted hydrate curve, hydrates have been known to appear suddenly at temperatures well below the predicted formation point, with a sudden release of the heat of condensation. Small upward jumps in the mirror temperature were observed at about the time these formations appeared, further supporting their identification as hydrates. At 500 psia (34.5 bara), with water as the only contaminant, a majority of WVDPs were observed on the chilled mirror a few degrees below the WVDP reported by the Michell unit ceramic moisture sensor. The discrepancy between the two units may be due to interference by the hydrocarbon condensate on the chilled mirror, possibly preventing the moisture from condensing until a lower temperature than usual. OHDPs were observed on the manual chilled mirror within 5 F (2.8 C) of HCDPs at all pressures during tests with methanol and water contamination. No accurate predictions of OHDPs are available for comparison, and due to differences in the measurement principles of the automated analyzers, their results cannot be compared directly. 51

76 Table 4-3. Contaminant(s) Comparison of droplet hydrocarbon dew points from tests on uncontaminated and contaminated transmissionquality gas streams, as measured with the Bureau of Mines chilled mirror, and compensating for shifts due to changes in hydrocarbon gas composition predicted from GC analyses and the SRK equation of state. Test Pressure 193 ppmv H 2 O 750 psia (51.7 bara) 229 ppmv H 2 O 500 psia (34.5 bara) 300 ppmv H 2 O 300 psia (20.7 bara) 805 ppmv CH 3 OH, 215 ppmv H 2 O 834 ppmv CH 3 OH, 249 ppmv H 2 O 866 ppmv CH 3 OH, 311 ppmv H 2 O 750 psia (51.7 bara) 500 psia (34.5 bara) 300 psia (20.7 bara) Phase 1 Result Without Contaminants, Average and 95% Confidence Interval 25.4 F ± 0.5 F (-3.7 C ± 0.3 C) 31.7 F ± 2.1 F (-0.2 C ± 1.2 C) 31.4 F ± 1.4 F (-0.3 C ± 0.8 C) 25.4 F ± 0.5 F (-3.7 C ± 0.3 C) 31.7 F ± 2.1 F (-0.2 C ± 1.2 C) 31.4 F ± 1.4 F (-0.3 C ± 0.8 C) Phase 2 Result With Contaminants, Average and 95% Confidence Interval 33.4 F ± 4.7 F (0.8 C ±2.6 C) 38.3 F ± 1.0 F (3.5 C ±0.6 C) 32.6 F ± 0.6 F (0.3 C ±0.3 C) 43.7 F ± 2.6 F (6.5 C ±1.4 C) 38.1 F ± 1.4 F (3.4 C ±0.8 C) 36.5 F ± 3.1 F (2.5 C ±1.7 C) Average Measurement Shift with Contaminant Added Approximate Shift in Predicted HCDP Curve Net Average Measurement Shift +8.0 F ( +4.4 C) +2 F (+1 C) +6 F (+3.4 C) +6.6 F (+3.7 C) +1 F (+0.5 C) +5.6 F (+3.1 C) +1.2 F (+0.7 C) +1 F (+0.5 C) +0.2 F (+0.1 C) F (+10.2 C) -1.5 F (-1 C) F (+11.1 C) +6.4 F (+3.6 C) -3 F (-1.5 C) +9.4 F (+5.2 C) +5.1 F (+2.8 C) +0.5 F (+0.3 C) +4.5 F (+2.5 C) 52

77 4.5.2 Results from the Ametek 241 CE II As discussed in Section 2, the Ametek unit is designed to detect the first appearance of hydrocarbon condensation, analogous to the iridescent ring HCDP on the Bureau of Mines chilled mirror. During tests on the uncontaminated 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas in Phase 1, all Ametek unit HCDP temperature measurements were higher than the corresponding temperatures predicted by Multiflash using the SRK EOS and GC analyses of the test gas, though nearly all were within the confidence intervals of the SRK EOS curves. During the Phase 2 tests on the contaminated streams, this trend was repeated; all HCDP measurements fell within the confidence intervals of the SRK EOS curves, and except for tests with methanol and water contamination at 750 psia (51.7 bara), all Ametek unit HCDP measurements were slightly above the predicted values. Again, the HCDP curves computed from GC analyses are not absolute references, but merely estimate the effect of changes in hydrocarbon gas composition during tests on observed HCDPs. For the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas with water contamination, the Ametek unit HCDP measurements were from 2 F to 3.5 F (1 C to 2 C) above the corresponding SRK EOS curve at each pressure, while for the same gas blend with methanol and water added, the Ametek unit consistently produced HCDP measurements within -1.5 F to +4 F (-0.8 C to 2.2 C) of the predicted HCDP curve at all pressures. Unfortunately, the analogous iridescent ring HCDPs were only observed with the manual chilled mirror in tests with water vapor present at 750 psia (51.7 bara), and only during two of six tests at this condition. In these two cases, however, the Ametek unit and manual chilled mirror data have overlapping confidence intervals, and are not statistically different. After adjustment for slight composition changes between tests of the various analyzers on the water-contaminated stream, the Ametek unit HCDP results were found to be consistent with both iridescent ring and droplet HCDP values from the manual chilled mirror. As in the Phase 1 tests on uncontaminated streams, the Ametek unit HCDP temperatures in the presence of water contamination were consistently above the values from the Michell unit, with approximate differences ranging from 3 F to 8 F (1.5 C to 4.5 C). Indeed, the Ametek and Michell units showed very similar changes in results between the uncontaminated gas stream in Phase 1 and the water contaminated gas in Phase 2, suggesting that the presence of moisture has a similar effect on both analyzers. For the test stream with both moisture and methanol, however, the Ametek unit reported lower HCDPs than the Michell unit at 750 psia (51.7 bara) and 500 psia (34.5 bara). The difference shifts with pressure, moving from Ametek unit values being 10 F (5.6 C) lower at 750 psia (51.7 bara) to being 3 F (1.7 C) higher at 300 psia (20.7 bara). This same swap between the order of the Ametek unit and Michell unit HCDPs was observed, though to a smaller degree, during Phase 1 tests on the production-quality gas stream. Net shifts in HCDPs measured by the Ametek unit relative to the uncontaminated stream are listed in Table 4-4. With water alone as a contaminant, the average net effect on HCDP measurements ranges from -6.2 F to +2.9 F (-3.4 C to +1.7 C), compared to the Ametek unit measurement uncertainty of ±2 F (±1 C). When both methanol and moisture were present in the stream, the net effect ranged from -6.2 F to -3.1 F (-3.4 C to -1.6 C). These results suggest that the various contaminants have some effect on HCDP measurements by the Ametek unit, though the effect on the results may be positive or negative. As will be discussed in the next subsection, the effect of methanol and water contaminants together appears to be quite different for the two automated analyzers, as the Michell unit reports significantly higher HCDPs in the presence of both methanol and water. 53

78 Table 4-4. Comparison of hydrocarbon dew points measured by the Ametek 241 CE II on uncontaminated and contaminated transmission-quality gas streams, compensating for shifts due to changes in hydrocarbon gas composition predicted from GC analyses and the SRK equation of state. Contaminant(s) Test Pressure Phase 1 Result Without Contaminants, Average and 95% Confidence Interval Phase 2 Result With Contaminants, Average and 95% Confidence Interval Average Measurement Shift with Contaminant Added Approximate Shift in Predicted HCDP Curve Net Average Measurement Shift 193 ppmv H 2 O 750 psia (51.7 bara) 31.5 F ± 0.5 F (-0.3 C ± 0.3 C) 33.2 F ± 0.4 F (0.7 C ± 0.2 C) +1.7 F (+1.0 C) -1.2 F (-0.7 C) +2.9 F (+1.7 C) 229 ppmv H 2 O 500 psia (34.5 bara) 37.4 F ± 2.3 F (3.0 C ± 1.3 C) 37.0 F ± 1.5 F (2.8 C ± 0.8 C) -0.4 F (-0.2 C) +3.5 F (+2 C) -3.9 F (-2.2 C) 300 ppmv H 2 O 300 psia (20.7 bara) 35.0 F ± 1.0 F (1.7 C ± 0.6 C) 32.8 F ± 0.1 F (0.4 C ± 0.1 C) -2.2 F (-1.3 C) +4 F (+2 C) -6.2 F (-3.4 C) 805 ppmv CH 3 OH, 215 ppmv H 2 O 750 psia (51.7 bara) 31.5 F ± 0.5 F (-0.3 C ± 0.3 C) 26.7 F ± 0.4 F (-2.9 C ± 0.2 C) -4.8 F (-2.7 C) +0.5 F (+0.3 C) -5.3 F (-2.9 C) 834 ppmv CH 3 OH, 249 ppmv H 2 O 500 psia (34.5 bara) 37.4 F ± 2.3 F (3.0 C ± 1.3 C) 33.2 F ± 1.3 F (0.7 C ± 0.7 C) -4.2 F (-2.3 C) +2 F (+1 C) -6.2 F (-3.4 C) 866 ppmv CH 3 OH, 311 ppmv H 2 O 300 psia (20.7 bara) 35.0 F ± 1.0 F (1.7 C ± 0.6 C) 33.9 F ± 0.5 F (1.1 C ± 0.3 C) -1.1 F (-0.6 C) +2 F (+1 C) -3.1 F (-1.6 C) 54

79 The Ametek automated device was configured to collect WVDP measurements during the Phase 2 tests. However, the unit only reported successful measurements of a WVDP during tests with methanol and water at 750 psia (51.7 bara). At this condition, water is expected to form a combined condensate with methanol that has different properties than water alone, and is not expected to condense on the mirror polished specifically for water condensate. However, the Ametek unit WVDP measurements were in good agreement with WVDP measurements by the Michell Condumax II, which uses a ceramic capacitance sensor to detect water vapor content. At all other test conditions, the unit reached the lower temperature limit of its water vapor mirror before condensation was detected. To date, discussions with Ametek have not yet determined a cause for the lack of WVDP data at most test conditions Results from the Michell Condumax II As described in Section 2, the trip point for the optical detector of the Condumax II has been selected by Michell Instruments, based on field experience, to give good agreement with visual chilled mirror observations of the first formation of hydrocarbon droplets at the same pressure. During Phase 1, this agreement was observed for many of the tests on uncontaminated gas streams. However, in the Phase 2 tests on streams with water and methanol-water contaminants, the Michell unit reported HCDP temperatures lower than the manual chilled mirror droplet HCDPs in the majority of test conditions. After correcting for compositional changes during tests, the Michell unit HCDPs and chilled mirror droplet HCDPs showed statistically-significant differences at every test condition, with the Michell unit values differing from the chilled mirror values by as much as -12 F (-6.7 C). Two chilled mirror HCDP measurements of the watercontaminated stream at 750 psia (51.7 bara) and three in the methanol-water stream at 500 psia (34.5 bara) were in statistical agreement with the Michell unit values, but these were the exceptions. Comparisons of the Michell values with measurements by the Ametek unit appear in the previous subsection. During tests on the uncontaminated 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas in Phase 1, all Michell unit HCDP temperature measurements were in statistical agreement with the corresponding HCDP temperatures predicted by Multiflash using the SRK EOS and GC analyses of the test gas. During the Phase 2 tests on the contaminated streams, this was again the case, except for tests with methanol and water contamination at 750 psia (51.7 bara), where all Michell unit HCDP measurements were approximately 7 F to 9 F (4 C to 5 C) above the predicted values. As noted above, the HCDP curves computed from GC analyses are not absolute references, but merely estimate the effect of changes in hydrocarbon gas composition during tests on observed HCDPs. Net shifts in HCDPs measured by the Michell unit relative to the uncontaminated stream are listed in Table 4-5. With water alone as a contaminant, the average net effect on HCDP measurements ranged from -7.2 F to +3.0 F (-4.0 C to +1.7 C), compared to the reported accuracy of ±0.9 F (±0.5 C). When both methanol and moisture are present, the net effect ranges from -1.5 F to +8.2 F (-0.9 C to +4.6 C). As with the manual chilled mirror, the presence of methanol and water together in the stream is linked to significant, observable changes in the HCDP temperatures from the Michell unit. 55

80 Table 4-5. Comparison of hydrocarbon dew points measured by the Michell Condumax II on uncontaminated and contaminated transmission-quality gas streams, compensating for shifts due to changes in hydrocarbon gas composition predicted from GC analyses and the SRK equation of state. Contaminant(s) Test Pressure Phase 1 Result Without Contaminants, Average and 95% Confidence Interval Phase 2 Result With Contaminants, Average and 95% Confidence Interval Average Measurement Shift with Contaminant Added Approximate Shift in Predicted HCDP Curve Net Average Measurement Shift 193 ppmv H 2 O 750 psia (51.7 bara) 27.7 F ± 0.6 F (-2.4 C ± 0.3 C) 30.2 F ± 0.3 F (-1.0 C ± 0.2 C) +2.5 F (+1.4 C) -0.5 F (-0.3 C) +3.0 F (+1.7 C) 229 ppmv H 2 O 500 psia (34.5 bara) 33.2 F ± 0.9 F (0.7 C ± 0.5 C) 30.4 F ± 0.4 F (-0.9 C ± 0.2 C) -2.8 F (-1.6 C) +4 F (+2.2 C) -6.8 F (-3.8 C) 300 ppmv H 2 O 300 psia (20.7 bara) 30.3 F ± 0.8 F (-0.9 C ± 0.4 C) 26.6 F ± 0.5 F (-3.0 C ± 0.3 C) -3.7 F (-2.1 C) +3.5 F (+1.9 C) -7.2 F (-4.0 C) 805 ppmv CH 3 OH, 215 ppmv H 2 O 750 psia (51.7 bara) 27.7 F ± 0.6 F (-2.4 C ± 0.3 C) 35.9 F ± 1.6 F (2.2 C ± 0.9 C) +8.2 F (+4.6 C) ~ F (+4.6 C) 834 ppmv CH 3 OH, 249 ppmv H 2 O 500 psia (34.5 bara) 33.2 F ± 0.9 F (0.7 C ± 0.5 C) 35.2 F ± 0.4 F (1.8 C ± 0.2 C) +2.0 F (+1.1 C) +2.5 F (+1.4 C) -0.5 F (-0.3 C) 866 ppmv CH 3 OH, 311 ppmv H 2 O 300 psia (20.7 bara) 30.3 F ± 0.8 F (-0.9 C ± 0.4 C) 31.3 F ± 0.5 F (-0.4 C ± 0.3 C) +1.0 F (+0.5 C) +2.5 F (+1.4 C) -1.5 F (-0.9 C) 56

81 Comparing the shifts in dew points by the Ametek and Michell analyzers in the presence of contaminants identifies similarities and differences in their responses. For both analyzers, changes in measurement results between the uncontaminated gas streams from Phase 1 and the water-contaminated gas streams in Phase 2 have similar magnitudes, directions, and trends with line pressure; thus, any interference from moisture appears to have a similar effect on both units. However, the effects of combined methanol-water contamination in the gas are distinctly different for the two units. The Ametek unit tends to report significantly lower HCDP temperatures than in the comparable case without contaminants, while the Michell unit reads values that are either significantly higher than in the baseline case, or change only within the measurement uncertainty of the unit. The Bureau of Mines chilled mirror unit exhibits a large, positive shift in the presence of methanol and moisture, particularly at the highest line pressure, where the formation of a combined OHDP would occur at the highest temperature; the Michell unit may be responding to the same phenomenon as the manual chilled mirror, though to a lesser degree. The three units tested here the Bureau of Mines chilled mirror, the Michell Condumax II, and the Ametek 241 CE II all have mirrored surfaces with different finishes intended to collect liquids with specific surface tensions (hydrocarbons or water). The estimated surface tension of a 3:1 CH 3 OH-H 2 O liquid mixture (25 27 dynes/cm 2 ) is nearly the same as that of liquid octane (21 dynes/cm 2 ), while liquid water has a much higher surface tension (72 dynes/cm 2 ). It is hypothesized that a water-methanol condensate may be forming on the Michell mirror, which is acid-etched to preferentially attract hydrocarbon condensate. The reflection of light from this condensate may be producing an OHDP response that the Michell unit interprets as an HCDP. If this is so, differences in this Michell unit OHDP from OHDP values observed with the manual chilled mirror may be related to the tuning method used to match the Michell unit response to hydrocarbons with that of the manual chilled mirror. During tests with both water and methanol-water contamination, the Michell unit also provided a WVDP based on measurements by its traceable ceramic moisture sensor. For the tests with water alone, these readings were used as reference measurements of the water content of the gas stream. For the methanol-water tests, the gas chromatograph was used to ascertain the methanol and water content of the stream, as listed in Table 4-2. IGT-8 WVDP curves for the water levels observed by the GC were then compared to the WVDP values from the Michell unit sensor. Although the water is expected to combine with methanol when condensing, these WVDP curves provide a check of the accuracy of the Michell unit WVDP measurements. The Michell unit WVDP values correspond to IGT-8 curves for moisture levels 21% to 28% higher than are present in the gas stream, as shown in Figure B-6 of Appendix B. Michell has stated that methanol may cause an increase in the moisture level reported by the sensor, with the increased moisture reading equal to approximately 10% of the methanol level in the stream. The results above correspond to a false positive reading equal to 7% to 8% of the methanol level, consistent with the Michell statement. Interestingly, the Michell unit WVDP readings are statistically the same as the Ametek unit WVDP readings in the methanol-water stream at 750 psia (51.7 bara), and the manual chilled mirror measurements of the OHDP at 500 psia (34.5 bara) and 300 psia (20.7 bara). Both the Ametek unit and chilled mirror measurements of WVDP are optically based, while the Michell unit measurement is derived from a capacitance sensor. 57

82 Most notable, however, is the agreement between the IGT-8 WVDP curves for the moisture levels reported by the GC and the Michell unit HCDP measurements, as shown in Figure B-6 of Appendix B. In the presence of water vapor contamination alone, the Michell unit HCDPs did not coincide with the IGT-8 WVDPs. As noted earlier, the methanol and water are expected to condense together into a liquid phase separate from the hydrocarbons. A reliable prediction of OHDP with an equation of state could not be obtained during this research, but it is unlikely that the OHDP for the contaminant levels in these tests coincide with the theoretical WVDP of the water alone. An explanation for this coincidence has not been found as of the date of this report. A topic for possible future research would be to repeat the test conditions and analyze the condensates producing these readings, so as to identify them as hydrocarbons, water, or a methanol-water mixture Effect of Filtration on Analyzer Accuracy One goal of this research is to identify any impact of filtration on analyzer accuracy. Both of the automated analyzers incorporate filtration to remove any particulates or liquids from the gas stream before it reaches the dew point sensors. While manual chilled mirrors may be used with filtration, none was placed upstream of the Bureau of Mines chilled mirror device in the apparatus for these tests, which provided a reference case to address this question. During the Phase 1 tests on natural gas streams free of water and methanol contamination, it was concluded that the filters used on both the Michell and Ametek units have negligible impact on HCDP accuracy. Similar observations were made during this phase to identify any interactions between the filters and the natural gas stream that could bias HCDP measurements or contaminant measurements. GC analyses of the test gases, before and after tests with each HCDP analyzer at each test condition, again did not show statistically significant composition changes. All analytical dew point curves fell within the confidence intervals of all other analytical curves at a given pressure and contaminant loading. The shift in predicted HCDPs for analyzed compositions before and after each test was within ±2 F (±1.1 C). Over the course of all tests on a particular gas composition, no long-term decreases in heavy hydrocarbon components were observed, confirming the conclusion from Phase 1 that the filtration in the Ametek and Michell analyzers does not remove these components from the gas stream. Similarly, Table 4-2 shows that the contaminant levels after tests with each instrument decreased over the course of some tests and increased over the course of others, but did not consistently decrease, as would have happened if some of the contaminants were trapped by the filters. As in Phase 1, the filters on both automated analyzers were disassembled and visually inspected after all tests were completed. Both the Michell unit filter and the three membranes of the Ametek unit filter assembly showed no discoloration and were in like new condition, indicating that no heavy hydrocarbons had been trapped in the filters during testing. 58

83 (a) (b) (c) (d) Figure Filters from the automated analyzers as found after the tests, showing normal coloration: (a) filter membrane from the Michell Condumax II; (b) first-stage membrane from the Ametek 241 CE II; (c) second-stage membrane from the Ametek unit; (d) third-stage filter from the Ametek unit Measurement Uncertainty and Analyzer Repeatability An uncertainty analysis was performed to obtain the confidence intervals on all individual dew point measurements shown in Section 4.3, Section 4.4, and Appendix B. This analysis considered the following sources of measurement uncertainty: Published uncertainties in pressure and temperature found in the documentation for the automated units, and in published studies on Bureau of Mines chilled mirror performance (Warner et al., 2001; George et al., 2005a). 95% confidence intervals on calibration data provided by Ametek Process and Analytical Instruments and Michell Instruments with the test units. 95% confidence intervals on calibrations of pressure transducers and RTDs installed in the test loop by SwRI. 59