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 January 30, 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, Darin L. George, Ph.D. Senior Research Engineer Flow Measurement Section Approved: 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) D. M. Deffenbaugh, SwRI David Bromley, BP (for PRCI) D. L. George, SwRI Ron Brunner, GPA T. A. Grimley, SwRI Paul Burnett, PMC (for Michell) R. A. Hart, SwRI Ben Ho, BP (for PRCI) Report Copy B Dean Osmar, Ametek D. Todd, SwRI Dan Potter, Ametek T. Walvoord, SwRI 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 1 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 January 2008

3 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.

4 TESTS OF INSTRUMENTS FOR MEASURING HYDROCARBON DEW POINTS IN NATURAL GAS STREAMS, PHASE 1 D. L. George, Ph.D. R. C. Burkey 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. An experimental HCDP research apparatus, first designed to gather reference hydrocarbon dew point 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 tested on gravimetrically prepared gas blends chosen to simulate a transmission-quality gas and a production gas. Multiple HCDP measurements were made with each instrument at gas pressures below, at, and above the cricondentherm of the gas blend. Analyzer results were 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 based on gas chromatographic (GC) analyses of the test gases. The repeatability of the Ametek 241 CE II was found to be somewhat better than the manual chilled mirror, while the Michell Condumax II exhibited the best repeatability of the three instruments tested. Measurement uncertainties were assessed where possible, using calibration data and manufacturer specifications. For the leaner transmission-quality test gas, the Ametek 241 CE II consistently 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 richer production-quality gas, the relationship shifted with gas pressure, such that the Michell unit reported slightly higher dew point temperatures than the Ametek at the test pressure above the cricondentherm, while the Ametek measurements were 2 F to 3 F (1 C to 2 C) above Michell Instruments values at the test pressure below the cricondentherm. Trends in the analyzer and manual chilled mirror measurements suggest that the differences in performance between the automated units are related to their measurement techniques and default 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 hydrocarbon dew point, which occurs a few degrees below the iridescent ring dew point. iii

5 Elevated temperatures involved in measuring dew points on the richer gas blend had no effect on the overall cycle time of the Michell Condumax II, but the overall cycle time for the Ametek decreased when the unit measured dew points for the richer gas blend. The total cycle time of the Condumax II is preset by the user, but operation of the Ametek unit is governed by usercontrolled temperature set points, so that as the dew point of the gas stream increases, the span between initial and final mirror temperatures (and the cycle time) decreases. It was also concluded that the filters used on both the Michell and Ametek units did not adversely impact dew point accuracy. iv

6 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. 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 commercial instruments sold worldwide use optical methods to objectively detect condensed hydrocarbons and measure the hydrocarbon dew point. The first phase of a Joint Industry Project has independently evaluated the accuracy of these commercially-available devices, and provided information that will guide the use of these devices in custody transfer situations. Two commercially-available hydrocarbon dew point 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 hydrocarbon dew point 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 hydrocarbon dew points 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 liquefied natural gas (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 exhibiting the best repeatability of the three. Measurement uncertainties were also assessed, v

7 where possible, using analyzer specifications and calibration data provided by the manufacturers, along with uncertainties of instrumentation in the test apparatus. The table below summarizes the estimated measurement uncertainties of the devices examined in this work, after adjustment for small changes in test gas compositions. Table ES-1. Estimated measurement uncertainties of individual dew point pressures and temperatures determined by each device at each test condition, after adjustment for changes in test gas composition. Manual chilled mirror Ametek 241 CE II Michell Condumax II Quantity Temperature Pressure Temperature Pressure Temperature Pressure Measurement uncertainty estimated for test article (95% confidence level) ±2.4 F (±1.3 C) ±3 psi (±21 kpa) ±2 F (±1 C) ±3 psi (±21 kpa) ±0.9 F (±0.5 C) ±4.8 psi (±33.1 kpa) For the leaner 1,050-Btu/scf (39.12-MJ/Nm 3 ) 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 richer 1,145- Btu/scf (42.66-MJ/Nm 3 ) 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 Ametek 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 hydrocarbon dew point, which occurs a few degrees below the iridescent ring dew point. The table below summarizes the test conditions and measurements by each device during the research (with 95% confidence intervals indicating measurement repeatability). 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. Bureau of Mines chilled mirror Line pressure Iridescent ring dew points Droplet dew points Ametek 241 CE II Michell Condumax II (psia) T ( F) P (psia) T ( F) P (psia) T ( F) P (psia) T ( F) P (psia) 1,050 Btu (94.75% methane, 0.1% C 6+) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.4 1,145 Btu (85.25% methane, 0.25% C 6+) 1, , ± 0.7 1,000.7 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.5 vi

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. Bureau of Mines chilled mirror Line pressure Iridescent ring dew points Droplet dew points Ametek 241 CE II Michell Condumax II (bara) T ( C) P (bara) T ( C) P (bara) T ( C) P (bara) T ( C) P (bara) MJ/Nm 3 (94.75% methane, 0.1% C 6+) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± MJ/Nm 3 (85.25% methane, 0.25% C 6+) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.03 The measurement cycle times for the Michell Condumax II were consistent at ten minutes throughout the tests. Since the total cycle time of the Condumax II is preset by the user, 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 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. Further tests are now planned to assess the impact of moisture and methanol in the gas stream on analyzer performance. The follow-on work will modify the test apparatus to introduce known or controlled amounts of each contaminant separately to the known gas stream. The Ametek and Michell analyzers will then be tested to determine their ability to accurately measure hydrocarbon dew points when H 2 O or CH 3 OH is present in the gas stream. These tests will provide valuable information on analyzer performance under adverse conditions, data that will be useful to the natural gas industry in the creation of guidelines for their use and possible U.S. industry standards. Completion of this additional work will determine the ability of these automated units to easily and accurately replace the Bureau of Mines chilled mirror device for routine dew point determination at field sites. vii

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10 TABLE OF CONTENTS 1. INTRODUCTION THE NEED FOR OBJECTIVE DEW POINT MEASUREMENT PROJECT OBJECTIVES SCOPE OF WORK AND TECHNICAL APPROACH MODIFICATION OF APPARATUS FOR HCDP ANALYZER TESTS DESIGN CRITERIA TEST ARTICLES Bureau of Mines Manual Chilled Mirror Ametek Model 241 CE II Michell Condumax II FINAL APPARATUS DESIGN Layout Warm Box Pressure and Temperature Instrumentation Gas Chromatograph Modifications to the Test Articles TEST GAS COMPOSITIONS TEST PROCEDURES PREHEATING OF GAS BLENDS AND EQUIPMENT GAS CHROMATOGRAPH CALIBRATION INTRODUCTION OF A NEW TEST GAS 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 MOISTURE CONTENT COMMENTS ON HCDP CALCULATIONS ,050-BTU/SCF (39.12-MJ/NM 3 ) TEST RESULTS ,145- BTU/SCF (42.66-MJ/NM 3 ) TEST RESULTS 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 Automated Analyzer Response Times Measurement Uncertainty and Analyzer Repeatability CONCLUSIONS AND PLANS FOR FURTHER WORK...72 APPENDIX: APPROVED TEST PROCEDURE FOR HYDROCARBON DEW POINT EXPERIMENTS...79 Page ix

11 FIGURE 2-1. FIGURE 2-2. LIST OF FIGURES Page A CHANDLER CHANSCOPE II CHILLED MIRROR DEW POINT TESTER SIMILAR TO THE UNIT USED IN THIS RESEARCH... 6 EXAMPLES OF HYDROCARBON DEW POINT FORMATIONS ON A MANUAL CHILLED MIRROR. LEFT, AN IRIDESCENT RING DEW POINT; RIGHT, A DROPLET DEW POINT... 7 FIGURE 2-3. AN AMETEK MODEL 241 CE II AUTOMATED DEW POINT TESTER SIMILAR TO THE UNIT USED IN THIS RESEARCH... 8 FIGURE 2-4. CUTAWAY VIEW OF THE MEASUREMENT CELL WITHIN THE AMETEK 241 CE II (AMETEK, 2005)... 8 FIGURE 2-5. DETAIL OF THE AMETEK 241 CE II SAMPLE FILTRATION SYSTEM... 9 FIGURE 2-6. FIGURE 2-7. FIGURE 2-8. FIGURE 2-9. A MICHELL CONDUMAX II AUTOMATED DEW POINT TESTER SIMILAR TO THE UNIT USED IN THIS RESEARCH...10 REPRESENTATION OF THE REFLECTED LIGHT PATTERN FROM THE CONDUMAX II CHILLED MIRROR WHEN HYDROCARBON CONDENSATE FORMS...10 LAYOUT OF THE TEST SYSTEM...12 SCHEMATIC OF THE TEST SYSTEM...13 FIGURE TEST SYSTEM RECIRCULATING LOOP IN THE INTERIOR OF THE WARM BOX...13 FIGURE DESIGN OF THE MAGNETICALLY-COUPLED RODLESS CYLINDER USED TO DRIVE THE FLOW OF THE TEST GAS...14 FIGURE INTERNAL SCHEMATIC OF THE PISTON ACCUMULATOR USED TO CONTROL LOOP PRESSURE AND MIX THE TEST GAS...15 FIGURE SCHEMATIC OF THE GAS HANDLING SIDE OF THE TEST SYSTEM FIGURE SCHEMATIC OF THE TEST ARTICLE SIDE OF THE TEST SYSTEM FIGURE VARIAN CP-4900 PORTABLE GAS CHROMATOGRAPH USED TO ANALYZE TEST GASES...19 FIGURE AMETEK 241 CE II STANDARD SAMPLING SYSTEM SCHEMATIC...22 FIGURE AMETEK 241 CE II MODIFIED SAMPLING SYSTEM SCHEMATIC...22 FIGURE PHOTOGRAPH OF THE AMETEK 241 CE II MODIFIED SAMPLING SYSTEM...23 FIGURE MICHELL CONDUMAX II STANDARD SAMPLING SYSTEM SCHEMATIC...24 FIGURE MICHELL CONDUMAX II MODIFIED SAMPLING SYSTEM SCHEMATIC...24 FIGURE PHOTOGRAPH OF THE MICHELL CONDUMAX II MODIFIED SAMPLING SYSTEM...25 FIGURE 4-1. FIGURE 4-2. FIGURE 4-3. FIGURE 4-4. TRENDS IN ANALYZED NITROGEN CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND DURING TEST PREPARATIONS...39 TRENDS IN ANALYZED DECANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND DURING TEST PREPARATIONS...40 TRENDS IN ANALYZED NITROGEN CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND DURING ANALYZER TESTS...41 TRENDS IN ANALYZED NONANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND DURING ANALYZER TESTS...41 x

12 FIGURE 4-5. TRENDS IN ANALYZED DECANE CONTENT OF THE 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS BLEND DURING ANALYZER TESTS...42 FIGURE 4-6. COMPARISON OF 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS DEW POINTS MEASURED WITH THE BUREAU OF MINES CHILLED MIRROR WITH DEW POINT CURVES PREDICTED FROM GC ANALYSES...44 FIGURE 4-7. COMPARISON OF 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS DEW POINTS MEASURED BY THE AMETEK ANALYZER WITH DEW POINT CURVES PREDICTED FROM GC ANALYSES...45 FIGURE 4-8. COMPARISON OF 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS DEW POINTS MEASURED BY THE MICHELL ANALYZER WITH DEW POINT CURVES PREDICTED FROM GC ANALYSES...47 FIGURE 4-9. COMPARISON OF 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS DEW POINTS MEASURED BY ALL INSTRUMENTS AND PREDICTED FROM ALL GC ANALYSES...48 FIGURE COMPARISON OF 1,050-BTU/SCF (39.12-MJ/NM 3 ) GAS DEW POINTS MEASURED BY ALL INSTRUMENTS, AFTER ADJUSTMENTS FOR CHANGES IN GAS COMPOSITION DURING TESTS...50 FIGURE TRENDS IN ANALYZED NITROGEN CONTENT OF THE 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS BLEND DURING ANALYZER TESTS...52 FIGURE TRENDS IN ANALYZED NONANE CONTENT OF THE 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS BLEND DURING ANALYZER TESTS...53 FIGURE TRENDS IN ANALYZED DECANE CONTENT OF THE 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS BLEND DURING ANALYZER TESTS...53 FIGURE COMPARISON OF 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS DEW POINTS MEASURED WITH THE BUREAU OF MINES CHILLED MIRROR WITH DEW POINT CURVES PREDICTED FROM GC ANALYSES...55 FIGURE COMPARISON OF 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS DEW POINTS MEASURED BY THE AMETEK ANALYZER WITH DEW POINT CURVES PREDICTED FROM GC ANALYSES...56 FIGURE COMPARISON OF 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS DEW POINTS MEASURED BY THE MICHELL ANALYZER WITH DEW POINT CURVES PREDICTED FROM GC ANALYSES...58 FIGURE COMPARISON OF 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS DEW POINTS MEASURED BY ALL INSTRUMENTS AND PREDICTED FROM ALL GC ANALYSES...59 FIGURE COMPARISON OF 1,145- BTU/SCF (42.66-MJ/NM 3 ) GAS DEW POINTS MEASURED BY ALL INSTRUMENTS, AFTER ADJUSTMENTS FOR CHANGES IN GAS COMPOSITION DURING THE TESTS FIGURE FILTERS FROM THE AUTOMATED ANALYZERS AS FOUND AFTER THE TESTS, SHOWING NORMAL COLORATION: (A) THE FILTER MEMBRANE FROM THE MICHELL CONDUMAX II; (B) FIRST-STAGE MEMBRANE FROM THE AMETEK 241 CE II, SHOWING SMALL PARTICLES NEAR THE TOP AND RIGHT EDGES BUT NO DISCOLORATION; (C) SECOND- STAGE MEMBRANE FROM THE AMETEK UNIT, SHOWING DIMPLING BUT NO DISCOLORATION; (3) THIRD-STAGE FILTER FROM THE AMETEK UNIT, SHOWING NORMAL COLORATION...66 xi

13 FIGURE A1. FIGURE A2. TEST SETUP SCHEMATIC...84 TEST SETUP SCHEMATIC DEW POINT DEVICE DETAIL...85 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. SPECIFICATIONS OF THE MANUAL CHILLED MIRROR AND ANALYZERS TESTED IN THIS RESEARCH... 6 NOMINAL COMPOSITIONS AND CALCULATED PROPERTIES OF NATURAL GAS BLENDS CHOSEN FOR THE SWRI HYDROCARBON DEW POINT TESTS. COMPONENT VALUES ARE IN UNITS OF MOLE PERCENT...26 EQUIPMENT TEMPERATURES AND TEST CONDITIONS FOR EACH TEST GAS COMPOSITION...31 WATER VAPOR CONTENT OF THE TEST LOOP BEFORE AND DURING HYDROCARBON DEW POINT TESTS...36 AVERAGES AND 95% CONFIDENCE INTERVALS OF CYCLE TIMES (IN MINUTES AND SECONDS) FOR EACH AUTOMATED INSTRUMENT AT EACH TEST CONDITION...67 PUBLISHED UNCERTAINTIES IN DEW POINT MEASUREMENTS BY EACH DEVICE...68 CALIBRATION UNCERTAINTIES IN TEMPERATURE AND PRESSURE INSTRUMENTATION USED WITH DEW POINT MEASUREMENT DEVICES...68 AVERAGES AND 95% CONFIDENCE INTERVALS OF DEW POINT PRESSURES AND TEMPERATURES, FOR EACH INSTRUMENT AND TEST CONDITION, USED TO QUANTIFY MEASUREMENT REPEATABILITY...70 xii

14 ACKNOWLEDGMENTS The success 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) Dave Bromley (BP, representing Pipeline Research Council International, Inc.) Ron Brunner (Gas Processors Association) Paul Burnett (representing Michell Instruments Ltd.) 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.) 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 with the project. The authors would like to thank Andy Tomich of Questar Applied Technology Services and Fred D Angelo of DGC Partnership for their guidance in the initial calibration of the Varian gas chromatograph. Appreciation is also extended to Mr. Tomich and Questar Applied Technology Services for the loan of a spare gas chromatograph module to SwRI, which allowed project preparations to continue on schedule. DCG Partnership and Scott Specialty Gases donated test gases and GC 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. xiii

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16 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. This device is sometimes used to resolve custody transfer disputes over natural gas quality. Recent investigations into the accuracy of hydrocarbon dew points 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 moisture (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 hydrocarbon dew point instrument would also be useful as a method for settling custody transfer disputes quickly and impartially, or for improving the efficiency of 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 have been incorporated into sensors for various applications, but the use of these methods to detect hydrocarbon dew points is in its early stages. Two commercial instruments sold worldwide use different optical methods to detect condensed hydrocarbons and measure the hydrocarbon dew point directly. Research has been proposed to independently evaluate the accuracy of these commercially-available devices, and to provide information for a U.S. industry standard that will guide the use of these devices in custody transfer situations. This report describes the first phase of a Joint Industry Project to perform such an independent evaluation. 1

17 1.2 PROJECT OBJECTIVES The primary objective of the Hydrocarbon Dew Point Analyzer JIP is to evaluate existing costeffective instruments capable of repeatable, objective dew point measurements that may easily replace the Bureau of Mines chilled mirror device for routine hydrocarbon dew point determination at remote sites. The instruments are evaluated based on their accuracy and performance in different gas compositions representing production and transmission gas streams. Specific objectives of the project are as follows: 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 hydrocarbon dew point 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 hydrocarbon dew point temperatures. This report describes Phase I of the project, in which devices have been tested against (1) measurements made with a Bureau of Mines chilled mirror device, and (2) equation-of-state dew point predictions based on gas chromatograph analyses of the test gases. Phase II of the project, under way as this report is being published, will provide additional data to meet these objectives, as well as assess the impact of moisture and methanol in the gas stream on instrument performance. 1.3 SCOPE OF WORK AND TECHNICAL APPROACH Work on the project began with creation of a protocol for testing commercially-available automated hydrocarbon dew point analyzers. The protocol was closely based on a similar procedure used with the same apparatus to obtain reference hydrocarbon dew point data (George et al., 2005b; George and Burkey, 2005c). In this case, test gas composition(s), test pressures, and evaluation criteria for the analyzers and the Bureau of Mines chilled mirror were chosen with input from representatives of the JIP participants. The details of the protocol are described in Section 3 of this report. After discussions with members of the PRCI Measurement Technical Committee, discussions with contacts in the natural gas industry, and literature searches, two commercially-available hydrocarbon dew point analyzers were selected for testing. Two of the JIP participants each provided one test unit: Ametek Model 241 CE II Michell Condumax II An experimental HCDP research apparatus at Southwest Research Institute was modified to test these automated analyzers. This apparatus was originally designed to closely control flowing gas 2

18 conditions in a Bureau of Mines chilled mirror device, as part of tests to provide reference hydrocarbon dew point data. For this research, the apparatus was modified to supply a controlled gas stream to the automated analyzers as well as the manual chilled mirror device. With help from the analyzer manufacturers and other JIP participants, the apparatus was modified to provide each device with the same gas stream at its own specified flow conditions. Minor modifications were made to the Ametek and Michell units, primarily downstream of the dew point sensor locations, to allow the analyzers to operate on the recirculating gas stream. A small warm box was also built to maintain the test apparatus and lines to the analyzers above the dew point of production-quality gas compositions. Section 2 describes the apparatus design and the changes to the Michell and Ametek units for these tests. 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 hydrocarbon dew points. Tests were performed using two well-characterized, gravimetrically-prepared gas blends containing hydrocarbons through C 10. One blend was prepared to resemble a transmissionquality gas (heating value of 1,050 Btu/scf, MJ/Nm 3 ), the other a production gas (1,145 Btu/scf, MJ/Nm 3 ). The higher heating value 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, 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. Data on response and recovery time of the analyzers are also presented. Finally, Section 5 summarizes the test findings and discusses plans for Phase II of this project, which will evaluate the analyzers performance in gas streams carrying condensable contaminants. 3

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20 2. MODIFICATION OF APPARATUS FOR HCDP ANALYZER TESTS The test apparatus used in the 2005 Gas Technology Institute (GTI)/Pipeline Research Council International (PRCI) hydrocarbon dew point tests (George et al., 2005b; George and Burkey, 2005c) was modified to provide a controlled test system for the automated analyzers and the manual chilled mirror. The test apparatus provided a continuous, recirculating gas stream at a controlled flow rate to each test article individually. 2.1 DESIGN CRITERIA The test apparatus used in the 2005 GTI/PRCI hydrocarbon dew point tests provided most of the functionality required for the current research and only relatively minor modifications and/or additions were required. The key final design parameters for the current embodiment of the test apparatus included: Capability to test the 1,050-Btu/scf (39.12-MJ/Nm 3 ) and 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas specified for the and 2005 experiments. Capability to route flow to one test article at a time during testing. Test pressures from near atmospheric pressure to 1,500 psia (103 bara) [limited by pressure rating of test article; 1,000 psia (68.9 bara) maximum planned test pressure]. Minimized system volume (currently approximately 4,250 cm 3, 0.15 ft 3 ). Flow rate across test articles: 5 standard cubic feet per hour (scfh) (2.5 Nl/min). Pressure measurements with a pressure transducer accurate to ±3 psi (±21 kpa) at the one-sigma level. Chilled mirror dew point tester temperature measurements with a resistance temperature device (RTD) accurate to ±0.2 F (±0.1 C) at the one-sigma level. 2.2 TEST ARTICLES The primary objective of this Joint Industry Project is to evaluate existing cost-effective automated dew point analyzers capable of repeatable, objective dew point measurements that may easily replace the Bureau of Mines chilled mirror device for routine HCDP determination at field sites. Two such analyzers were identified through discussions with members of the PRCI Measurement Technical Committee, discussions with contacts in the natural gas industry, and literature searches. These included: Ametek Model 241 CE II Michell Condumax II The manufacturers of each of these analyzers became members of the JIP and provided analyzers for testing. In addition, comparison measurements were made with a manual Bureau of Mines chilled mirror device. Results were also compared to dew points estimated using the Multiflash software package (Infochem, 2007) with the Soave-Redlich-Kwong equation of state and the Multiflash default binary interaction parameters. Table 2-1 compares the specifications of the three units tested in this research. 5

21 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., ** Specification of SwRI unit Bureau of Mines Manual Chilled Mirror 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. On some older units, a thermometer in physical contact with the mirror displays the mirror temperature as it is cooled; on newer units, a resistive temperature device or thermocouple detects the mirror temperature and a digital readout displays the temperature within the field of view of the eyepiece (Figure 2-1). 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. Pressure gauge Temperature readout Eyepiece Heat exchanger RTD Pressure chamber Figure 2-1. A Chandler Chanscope II chilled mirror dew point tester similar to the unit used in this research. 6

22 Two different types of condensation on the mirror surface may be identified by different chilled mirror users as the hydrocarbon dew point, as shown in Figure 2-2. Depending upon the standards and requirements of the user, either formation may be cited as the hydrocarbon dew point. One type, referred to here as the iridescent ring dew point, is observed when rainbowlike 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 a body 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. Usually, but not always, 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. While the droplet dew point is generally cited as the dew point for custody transfer purposes in the United States (George, 2006), many manual chilled mirror users in Europe use the iridescent ring dew point as the reference dew point (e.g., Cowper, 2002). Both types of dew points were recorded where possible in these tests, for comparison with dew points measured by the automated devices. Figure 2-2. 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 (Figure 2-3) uses an automated optical detection method to identify dew points, avoiding the subjectivity involved with human operation. The 241 CE II uses a Peltier thermoelectric cooler to control the temperature of a two-sided mirror in a sample chamber (Figure 2-4). 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 hydrocarbon dew point; this is analogous to the iridescent ring dew point recorded by Bureau of Mines chilled mirror users. Under normal operation, the 241 CE II uses the water dew point measurement only as a diagnostic. The placement of the mirrors at the top of the sample chamber prevents the build-up of liquids on the mirror surfaces in cases where the dew point of the stream approaches ambient temperatures. Samples entering the analyzer pass through a multiple-stage filtration system designed to protect the analyzer from aerosols, particulates and liquid slugs (Figure 2-5). 7

23 Figure 2-3. An Ametek Model 241 CE II automated dew point tester similar to the unit used in this research. Figure 2-4. Cutaway view of the measurement cell within the Ametek 241 CE II (Ametek, 2005). 8

24 Figure 2-5. Detail of the Ametek 241 CE II sample filtration system. The Ametek 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. 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 mirror temperature is measured using a platinum RTD. The cooling rate of the mirror is controlled so as to provide repeatable measurements. In particular, the temperature of the mirror is decreased linearly at the fast cooling rate (default 5 C/min, 9 F/min) from the mirror upper set point temperature until the mirror temperature reaches a preset temperature (default 10 C, 18 F) above the last determined HCDP temperature. After this temperature is reached, the mirror is then cooled at the slow cooling rate (default 2 C/min, 3.6 F/min) until either the HCDP is detected or the mirror lower set point temperature is reached. In the initial measurement cycle after power-up or a system reset, the mirror is cooled at the slow cooling rate for the entire cooling stage. The temperature of all components in the sampling system must be at least 18 F (10 C) above the highest expected hydrocarbon dew point temperature of the gas stream for efficient operation of the Peltier cooler. The default purge time is ten minutes and the complete measurement cycle takes approximately 20 minutes. The purge time, cooling rates, and the switchover set point can be adjusted from the default values for specific applications, thereby modifying the overall length of the measurement cycle as well. These adjustments may be made from the front panel of the analyzer, using a PC interface, or remotely via Modbus communications Michell Condumax II The Michell Condumax II analyzer (Figure 2-6) also uses an automated optical detection method to identify dew points. The detection configuration of the Condumax II is essentially quantitative, responding to the amount of condensate forming on a cooled optical surface, and is designed for sensitivity on the order of 5 mg/m 3 (0.31 lb m /MMcf) of condensate. The stainless 9

25 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, and the scattered light intensity within the central region is reduced (Figure 2-7). The resulting signal change is quantified by the measurement electronics in units of millivolts. The Condumax II determines the HCDP as the temperature of the optical surface when a set signal trip point value is reached during the measurement cooling cycle. The measurement cell is rated to 100 barg (1,450 psig) and is specified to have a lowest measurable hydrocarbon dew point of approximately -35 C (-31 F). Figure 2-6. A Michell Condumax II automated dew point tester similar to the unit used in this research. Figure 2-7. Representation of the reflected light pattern from the Condumax II chilled mirror when hydrocarbon condensate forms. 10

26 The Michell Condumax II 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. During start-up, the Condumax II performs a measurement cycle at a standard chill rate in order to "range-find" the hydrocarbon dew point. On subsequent cycles, the previously measured HCDP value is used to determine an optimized chill rate that will cause the sensor surface to cool quickly in the initial phase. The chill rate is then reduced to 0.1 F/sec (0.05 C/sec) as the sensor approaches the hydrocarbon dew point. Several parameters in the Condumax II measurement cycle are adjustable. The length of the entire HCDP measurement cycle can be set by the user, and the minimum (and default) cycle time is ten minutes. This typically results 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 visual chilled mirror measurements of the droplet hydrocarbon dew point at the same analysis pressure. Adjustment of the trip point setting can allow users to fine tune the measurement sensitivity of the Condumax II to harmonize with the measurement practices applied by their individual companies, such as the definition of an iridescent ring dew point, a droplet dew point, or a predefined volume of liquid condensate as the official dew point. For analysis pressures higher or lower than 400 psig (27.6 barg), the actual sample volume within the sensor cell will change proportionately with pressure. Michell Instruments recommends adjusting the default 275mV trip point setting in proportion to the pressure change to maintain consistent measurement sensitivity in terms of mass of condensate per unit volume of sample gas. Details on the relationship between the measurement trip point and the mass of condensate per unit volume of gas are described by Panneman, The Michell Condumax II also includes a Michell ceramic moisture sensor that operates on a gas sample in parallel with the hydrocarbon dew point measurement cell to provide a water dew point measurement (see Figure 2-19 and Figure 2-20 later in this section). The gas supplied to the Michell Condumax II is split between the hydrocarbon and water dew point measurement paths and is recombined after the measurement prior to leaving the Condumax II. The water dew point sample gas flow is continuous and is not affected by the hydrocarbon dew point measurement cycle. The water dew point measurement is updated at one-second intervals. 2.3 FINAL APPARATUS DESIGN The design of the test system was planned based on input from the JIP members at the test protocol development meeting held at SwRI in January 2007 and reviewed in detail with the JIP members at the design review teleconference held in March In addition, in late January 2007, Ametek Process and Analytical Instruments and Michell Instruments demonstrated the operation of the analyzers and provided training to SwRI staff concerning the operation of the analyzers. 11

27 2.3.1 Layout The final design of the test apparatus, including the warm box that houses a recirculating loop, the test articles, and all ancillary equipment, is shown in its entirety in Figure 2-8 and Figure 2-9. A recirculating, closed loop (Figure 2-10), placed inside the warm box, flows the test gas through the test articles. Flow is induced through the closed loop and through the test at approximately 5 scfh (2.5 Nl/min) by a magnetically coupled, rodless cylinder chosen to eliminate rod seals (Figure 2-11). The rodless cylinder is actuated by a worm-gear-driven linear slide, powered by a stepper motor and control system located outside the chamber. A bank of four pneumatically-actuated solenoid valves are activated by the motor control system to direct flow in one direction around the closed loop as the rodless cylinder is stroked back and forth. All wetted parts of the test apparatus are made of stainless steel or Viton elastomers to avoid adsorption of heavy hydrocarbons from the gas into the equipment. Samples of the gas in the loop are sent to a gas chromatograph and moisture analyzer at a pressure of approximately 50 psig (3.4 barg); the regulator through which the samples pass is heated to avoid the potential distortion of the sample composition by Joule-Thomson cooling. Ametek 241 CE II Michell Condumax II Gas Chromatograph LN2 Supply Auxiliary Chiller Primary Chiller Dew Scope Warm Box Figure 2-8. Layout of the test system. 12

28 LT1 SW2 PS2 AUXILLARY CHILLER PRIMARY CHILLER T1 DEW SCOPE VIDEO CAMERA 5V D C MV1 MV2 INSULATED WARM BOX SW1 PS1 1 2 V DC PR2 N2 TV VCR MIXER HP DATALOGGER VIDEO DATA CAMERA COMPUTER AUTOMATED ANALYZER #1 (AMETEK) MV7 MV3 MV4 P1 P2 T2 SV1(a) SV1(b) T4 SSR1 SV4 PR7 LN2 LN2 REMOTE INTERFACE UNIT AUTOMATED ANALYZER #2 (MICHELL) MV8 MV5 MV6 N.O. SV3 N.C. N.C. SV3 N.O. SV2 ACCUMULATOR D2 VENT N2 P3 PRESSURANT BV1 BV2 CV4 VENT MOTOR CONTROL COMPUTER MOTOR CONTROLLER STEPPER MOTOR CGD1 PRIME MOVER T5 PR1 T6 D1 PR6 DV1 P4 DV2 DV3 DV4 CV3 GAS CHROMATOGRAPH GC COMPUTER CV1 DV6 VACUUM PUMP CV2 BV4 BV3 PR3 PR4 PR5 LT2 T7 CAL NG PC2 T3 HT DV5 TEST NG CH4 He DPS1 PS3 12VDC MR1 LARM SSD SSD PC1 SSD SSR3 SSD BT2 SSD SSR2 BT1 SSD Figure 2-9. Schematic of the test system. Warm Box (Interior) Pressure Transducers Combustible Gas Detector Magnetically Coupled, Rodless Piston Stepper Motor Desiccant Cylinders Heater/Fan Heated Pressure Regulator Accumulator Figure Test system recirculating loop in the interior of the warm box. 13

29 Figure Design of the magnetically-coupled rodless cylinder used to drive the flow of the test gas. The flow is ported through an accumulator (Figure 2-12) such that the gas is well mixed in the accumulator before it reaches the test articles. The static pressure in the closed loop is produced and maintained by this piston accumulator system, located inside the warm box. The test gas is delivered from the low-pressure delivery 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. As installed in the current test system, the piston accumulator has sufficient displacement relative to the internal volume of the remainder of the test system to increase the test system pressure by a factor of approximately 6.5 above the pressure delivered to the system with the accumulator fully expanded. Thus, for the current testing for each gas blend, all test pressures were achieved with a single charge of test gas to the test system (i.e., no gas was added or removed from the test system, with the exception of GC analysis samples, during the course of testing each gas blend). The test system has provisions to pressurize the test gas to more than 6.5 times the maximum delivery pressure of the delivery cylinder by means of multiple fills and strokes of the piston accumulator. However, this functionality was not needed or used for the current testing. 14

30 Pressurant 35.3 Exit Piston Sliding Bushing Inlet Figure Internal schematic of the piston accumulator used to control loop pressure and mix the test gas. The schematic in Figure 2-13 shows the layout of equipment on the gas handling side of the test system. The gas handling equipment includes the test gas cylinder; the gas chromatograph, along with calibration gases and associated helium carrier gases; the methane cylinder used to purge the test loop; the nitrogen pressurant cylinder used to pressurize the accumulator; a second nitrogen cylinder used to control the pneumatic solenoid valves; the vacuum pump; the liquid nitrogen (LN2) dewars used to purge the interior of the warm box; and the manifold and vent lines used to route the various gases to and from the test system. The test gas and calibration gas cylinders were placed on insulated pads to thermally isolate them from the laboratory floor. 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 convection cooling. The heating blankets were regulated using an RTD attached to the wall of the cylinder, placed halfway up its height. After the wall of the cylinder reached the specified temperature, cylinder heating continued for a minimum of 24 hours before the cylinder was first opened. This requirement was imposed to ensure that the core temperature of the gas in the cylinder had reached a temperature at least 30 F (16.7 C) above its calculated hydrocarbon dew point. The gas was heated continuously until the conclusion of testing. In addition, all components (tubing, valves, and test and calibration gas cylinder pressure regulators) carrying test gas or calibration gas outside the warm box were heat traced and insulated to the same time and temperature requirements. 15

31 SW2 SW1 PR2 PS1 12VDC N2 SSR1 SV4 LN2 PR7 LN2 VENT N2 P3 PRESSURANT BV1 BV2 CV4 VENT INSULATED WARM BOX CAL NG PR6 PC2 DV1 DV3 HT T3 P4 DV2 DV4 DV5 CV3 GAS CHROMATOGRAPH GC COMPUTER CV1 DV6 PR3 TEST NG CV2 PR4 BV4 CH4 VACUUM PUMP BV3 PR5 He BT2 SSR2 BT1 SSD Figure Schematic of the gas handling side of the test system. The schematic in Figure 2-14 shows the layout of the equipment on the test article side of the test system. The test system equipment includes the three test articles; the data acquisition (DAQ) system collecting temperature and pressure data; the glycol chillers controlling the chilled mirror temperature; the video system recording the view on the dew scope; and the manifold used to route flow to each of the test articles. In addition, all components (tubing, manifold valves, and manual dew point tester) carrying test gas or calibration gas outside the warm box or the Ametek and Michell heated enclosures were heat traced and insulated. 16

32 PS2 5VDC AUXILLARY CHILLER PRIMARY CHILLER T1 DEW SCOPE LT1 VIDEO CAMERA MV1 MV2 TV VCR MIXER VIDEO CAMERA HP DATALOGGER DATA COMPUTER AUTOMATED ANALYZER #1 (AMETEK) REMOTE INTERFACE UNIT AUTOMATED ANALYZER #2 (MICHELL) MV7 MV3 MV4 MV8 MV5 MV6 INSULATED WARM BOX MOTOR CONTROL COMPUTER MOTOR CONTROLLER Figure Schematic of the test article side of the test system. Two laboratory glycol chillers were required in order to achieve the low coolant temperatures required for the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas tests. An auxiliary chiller was used to cool air blown over the condenser coils of the primary chiller to reduce the primary chiller s coolant bath temperature. The schematic in Figure 2-14 shows that the three test articles may be connected in parallel to the test system. When changing test system pressure, the test articles were connected in parallel by opening all manifold valves. The manifold used to route flow to each of the test articles also allows flow to be routed though one device at a time during testing. 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 did remain filled with test gas while waiting their turn for testing. The manifold used to route flow to each of the test articles also included two bypass valves (MV7 and MV8) that allowed the three test articles to be connected in series; this feature was used during loop preparation steps (including 17

33 the methane and helium fill/flush cycles) only. The bypass valves (MV7 and MV8) were always closed during testing Warm Box An insulated warm box was constructed in order to maintain the test loop at a temperature of approximately 110 F (43 C) for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas tests. This was done to ensure that the 1,145- Btu/scf (42.66-MJ/Nm 3 ) test gas within the test loop would remain in the gas phase at all test pressures. The warm box was constructed using 3/4 (19-mm) plywood and includes 3/4 (19-mm) fiberboard insulation on all six interior surfaces. Four 400W heater and fan assemblies warm and circulate the nitrogen within the warm box. The temperature in the warm box is maintained using a temperature controller that supplies power to the heaters, as necessary, based on the measured nitrogen temperature in the warm box. Power to the fans is supplied continuously to maintain circulation within the warm box. Surface temperatures of several key components (e.g., the rodless piston and accumulator) are monitored to ensure appropriate warming. Electrical and plumbing access to and from the warm box is completed through 1/2 (13-mm) thick PVC plates on each end of the warm box. A clear acrylic window in the warm box front allows visualization of the motor and rodless piston motion during testing. Due to the use of non-intrinsically safe hardware (e.g., the stepper motor, heaters, and fans) within the warm box, two safety shutdowns were implemented (in addition to the thorough helium leak checking conducted during the test system setup). The two safety shutdowns were wired in series such than a fault in either safety shutdown would shut down all power sources inside the warm box. First, the warm box was purged with nitrogen (boil-off from a LN2 dewar) to reduce the oxygen concentration to less than 5% (as measured with a handheld multigas detector) before any test gas was introduced to the test setup. The positive pressure nitrogen purge was maintained within the warm box for the duration of testing and measured oxygen concentrations were reduced to less than 1.5%. A low-range differential pressure switch was installed to shut down all power sources within the warm box if the positive pressure nitrogen purge was not maintained. Second, a combustible gas detector using infrared sensor technology monitored the interior of the warm box for the presence of natural gas (from a leak in the test system into the warm box). The combustible gas detector would shut down all power sources within the warm box if it detected methane at levels greater than 10% of the lower explosive limit Pressure and Temperature Instrumentation All equipment used to collect test data was calibrated at the SwRI Calibration Laboratory prior to installation in the test system. Test system pressures (P1, upstream of test articles; and P2, downstream of test articles) were measured with Rosemount Model 3051TC pressure transducers with a full-scale range of 4,000 psi (276 bar) and a manufacturer s stated accuracy of 0.075% of full scale. The dew point pressure for the manual dew point tester and the Ametek 241 CE II were recorded using the upstream test system pressure (P1) transducer. The Michell Condumax II contained a pressure transducer on board to measure dew point pressure. The dew point temperature for the manual dew point tester was measured with a 100-Ohm, DIN Class A, platinum RTD. Data were recorded with an HP 34970A data acquisition unit and HP 34902A 18

34 multifunction multiplexer cards. Many Type K thermocouples were used throughout the test setup to confirm proper heating of the test equipment Gas Chromatograph A Varian CP-4900 portable gas chromatograph (Figure 2-15) capable of 5-ppmv (parts per million by volume) measurement resolution was purchased with SwRI capital equipment funds for use in the tests. The dual channel unit includes 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. Injection pressures, column pressures and temperatures, and elution periods are controlled to allow analysis of hydrocarbons through C 10 in parts per million by volume (ppmv) quantities. This is the same model and column configuration used in the 2005 hydrocarbon dew point tests sponsored by Gas Technology Institute and PRCI (George et al., 2005b; George and Burkey, 2005c). Figure Varian CP-4900 portable gas chromatograph used to analyze test gases. The choice of the Varian micro-gc was guided by several factors. It is well known that the thermodynamic dew point (the condition at which hydrocarbons just begin to condense from the stream) depends upon the heaviest hydrocarbon present. Typical field gas chromatographs are C 6+ units that report a single total amount of hexanes and heavier hydrocarbons without reporting the individual amounts of heptanes, octanes, etc. While this analysis is sufficient for prediction of heating values and other 19

35 gas properties needed for custody transfer, details of the heavy component distribution are needed for accurate dew point predictions by equations of state. During the and 2005 hydrocarbon dew point experiments (George et al., 2005b; George and Burkey, 2005c), the test gases were designed to resemble transmission and production gases of interest to the industry. These gas compositions, used again in these tests and shown in Table 2-2 (presented later in this section), contain normal nonane and decane in amounts on the order of 10 ppmv to 40 ppmv. The Varian CP-4900 is capable of hydrocarbon analyses through C 10 to 5 ppmv resolution, making it suitable for use with these test gases. (While C 11 and C 12 would also be present in natural gas streams, their small amounts would make certifying similar quantities in the test gas blends difficult.) Gas chromatographs using flame ionization detector (FID) technology are generally accepted as having better precision and repeatability in the ppmv range than GCs with thermal conductivity detector (TCD) technology, such as the Varian CP Flame ionization detector GCs are often used in laboratories where sub-ppmv analyses are required, and such resolution is often desired when dew point predictions are to be made for gases containing hydrocarbons through C 12. However, the expense of FID-based GCs and the safety considerations associated with the hydrogen carrier gas, made the use of an FID GC impractical for this study Modifications to the Test Articles In typical field installations, the automated analyzers operate in an open-loop, single-passthrough mode in which the high-pressure gas to be analyzed is taken from its source, routed though the sampling and measurement systems of the analyzer, and then dumped to a low pressure vent. In contrast, the test system developed for the current work evaluated the performance of automated dew point analyzers in a laboratory-controlled setting. This was accomplished by providing a continuous, closed-loop, recirculating flow at a controlled static pressure to each test article individually (i.e., the analyzers experienced high static pressure at both their inlets and exits). This allowed all test articles to be tested with the same batch of gas in a controlled fashion. However, the recirculating, closed-loop configuration of the test system required some modification to each of the automated analyzers sampling systems in order to integrate the automated analyzers into the test system. 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), and involved removing any other unnecessary components that could cause pressure drops in the sampling system. Any filters in the devices were not modified and remained in the flow path as in a standard installation. Whenever possible, 1/8 (3-mm) tubing was used for any additional tubing added to the analyzers as part of the modifications, in order to minimize system volume. No changes were made to the hydrocarbon dew point measurement hardware installed in the analyzers. The modifications to the sampling systems of each test article are described in detail in the following subsections. The modifications to the sampling systems of each test article were carefully planned and implemented with the advice, consent, and oversight of all of the members 20

36 of the JIP, including the analyzer manufacturers. In addition, several modifications to the Bureau of Mines chilled mirror device were made in order to record visual observations and mirror temperature data. These modifications were originally implemented during the 2005 GTI/PRCI hydrocarbon dew point tests. The modified unit was reused in the current work, and the modifications are described in detail below. Bureau of Mines Chilled Mirror. The dew point tester s RTD was replaced by an RTD whose signal was carried back to a data acquisition system. A digital camcorder focused on the mirror surface was used to remotely observe condensation on the chilled mirror. The chilled mirror image from the camcorder was recorded on digital videotape for later review and eventual storage in DVD format. Simultaneously, the chilled mirror image and the temperature, pressure, and time readouts from the data acquisition computer were recorded on VHS videotape using an analog video mixer. The chilled mirror temperature was controlled by glycol coolant flow from a laboratory chiller. In addition, the light illuminating the dew point tester s optical train was powered by an external, adjustable DC power supply so that the light level could be adjusted to optimize visualization of the chiller mirror. Ametek 241 CE II. Figure 2-16 and Figure 2-17 show schematics of the Ametek 241 CE II (with sampling system details) in the standard, as-delivered configuration and the modified, as-tested configuration, respectively. Figure 2-18 also shows a picture of the Ametek 241 CE II in the modified, as-tested configuration. The modifications from the as-delivered configuration to the as-tested configuration included the following: The flow restrictors on the inlet and exit of the filter block assembly were removed. The pressure regulator and check valve between the measuring cell and solenoid valve were removed. The trickle purge bypass line and valve were removed, and the input to the filter block assembly for this bypass was plugged. The above modifications were completed by Ametek Process and Analytical Instruments when the 241 CE II was returned to Ametek for installation in a heated enclosure prior to integration of the analyzer into the test system. After the analyzer was returned to SwRI, the following modification was completed: A flow meter (rotameter RT1) was added between the measuring cell outlet and the solenoid valve (SV1). While Ametek Process and Analytical Instruments made these changes to the analyzers to accommodate the recirculating test apparatus, they note that the changes eliminated the normally large differential pressure between the inlet and vent, and that this may affect the readings and repeatability of the 241CE II. Having a low vent pressure relative to the inlet increases the flow velocity in the test cell for a given volumetric flow rate, greatly assisting in the evaporation of liquids from the mirror and the purge of the cell. 21

37 Figure Ametek 241 CE II standard sampling system schematic. HEATED ENCLOSURE MEASURING CELL FLAME ARRESTOR FLAME ARRESTOR RT1 FILTER BLOCK ASSEMBLY COALESCING FILTER LOW FLOW MEMBRANE FILTER INLET HIGH FLOW MEMBRANE FILTER SV1 FLOW FLOW FLOW RESTRICTOR RESTRICTOR RESTRICTOR FR3 FR2 FR1 S OUTLET TEST SYSTEM Figure Ametek 241 CE II modified sampling system schematic. 22

38 Ametek 241 CE II Heated Enclosure Heat Pipe Electronics Enclosure Measuring Cell Rotameter Filter Block Assembly Heater Figure Photograph of the Ametek 241 CE II modified sampling system. Michell Condumax II. Figure 2-19 and Figure 2-20 show schematics of the Michell Condumax II (with sampling system details) in the standard, as-delivered configuration and the modified, as-tested configuration, respectively. Figure 2-21 also shows a picture of the Michell Condumax II in the modified, as-tested configuration. The modifications from the as-delivered configuration to the as-tested configuration include: The upstream heated pressure regulator (PR1) and pressure gauge (PG1) were removed from the flow path, because pressure was controlled by the test system accumulator. The sample normally filtered through F1 was routed to the hydrocarbon dew point measurement path only. The bypass from the filter (F1) was routed to the water dew point measurement path only. All other components in the standard bypass/gas letdown flow path were not used. The pressure regulator (PR3), non-return valves (check valves NRV1 and NRV3), and flow meters (rotameters FM1 and FM3) were removed from the hydrocarbon and water dew point downstream flow paths. The ball valves (BV1 and BV2) were removed from the analyzer inlet and exit flow paths. Two needle valves (NV1 and NV2) and a flow meter (rotameter RT2) were added to the hydrocarbon and water dew point downstream flow paths. The needle valves were added in order to adjust the relative restrictions in the hydrocarbon and water dew point flow paths and ensure that the hydrocarbon measurement cell received adequate flow during 23

39 the purge stage of the analyzer s measurement cycle. The flow meter was added to the water dew point path to indicate flow in the hydrocarbon dew point path; this was done by observing the change in water dew point path flow as the analyzer opened and closed the hydrocarbon dew point flow path during its measurement cycle. During a visit by the Michell JIP representative just prior to testing, the needle valves were adjusted to ensure adequate hydrocarbon dew point path flow at a low test pressure. The needle valves remained at these settings for the duration of testing. Figure Michell Condumax II standard sampling system schematic. MICHELL CONDUMAX II - SAMPLING SYSTEM RECONFIGURE DETAIL HEATED ENCLOSURE CONDUMAX II "MEASURING CELL" FILTER (F1) HCDP NV1 INLET FILTERED S SV1 OUTLET BYPASS WDP NV2 RT2 TEST SYSTEM Figure Michell Condumax II modified sampling system schematic. 24

40 Filter Electronics Enclosure (HCDP, WDP Measurement) Needle Valves Heater Rotameter Figure Photograph of the Michell Condumax II modified sampling system. During preparations for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) tests, it was discovered that the deadband in the thermostat provided for the Michell heated enclosure allowed the temperature within the enclosure (near the gas inlet line) to fall unacceptably low (approximately 82 F or 28 C). An acceptable alternative thermostat could not be obtained without significant delay in the test schedule. Therefore, the temperature within the heated enclosure was controlled with a temperature controller supplied by SwRI powering the enclosure heater, using the feedback of a Type T thermocouple mounted near the thermostat location. 2.4 TEST GAS COMPOSITIONS The JIP members agreed that the initial scope of work should include tests of the two leanest nominal gas compositions specified for the and 2005 hydrocarbon dew point experiments (George et al., 2005b; George and Burkey, 2005c). These gas blends represent both transmission-grade and production-grade gases, with heating values of 1,050 Btu/scf (39.12 MJ/Nm 3 ) and 1,145 Btu/scf (42.66 MJ/Nm 3 ), respectively. Gravimetrically-prepared gas blends were chosen for testing over samples of natural gas streams from the field based on two advantages. First, certified gravimetric compositions have smaller relative uncertainties than analyzed compositions of natural gas streams, and thus have smaller uncertainties in the predicted HCDPs against which measurements are compared. Second, although preparation of gas blends with representative amounts of heavy hydrocarbons beyond decane may be difficult, the ability to specify the gas compositions allowed test conditions and hydrocarbon dew points to be tuned to meet the research goals. Table 2-2 lists the compositions and volume-based heating values of the two test gases. The heating values were computed using data from the latest editions of GPA Standard 2145 (Gas 25

41 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 each gas, as predicted by Multiflash (Infochem, 2007). Table 2-2. Nominal compositions and calculated properties of natural gas blends chosen for the SwRI hydrocarbon dew point tests. Component values are in units of mole percent. Component 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas methane ethane propane isobutane n-butane isopentane 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 Total Total C 6+ content Total diluent content Heating value Predicted cricondentherm Predicted cricondenbar 1,050.4 Btu/scf (39.12-MJ/Nm 3 ) 43.4ºF (6.3ºC) 1,298.3 psia (89.5 bara) 1,146.5 Btu/scf (42.66-MJ/Nm 3 ) 83.6ºF (28.7ºC) 1,614.4 psia (111.3 barg) 26

42 DCG Partnership Limited and Scott Specialty Gases were both approached to provide the gas blends to be used in the research. One of these facilities had been visited by SwRI staff to review their blending practices as part of a 2003 study of reference gas blend accuracy (George, 2003). In preparation for the 2005 dew point experiments, the other facility was also visited to review their reference gas blending procedures. While several suggestions were made to improve the company s compliance with the upcoming revision to API MPMS Chapter 14.1, no flaws in their blending procedures were found that would affect the accuracy of the gravimetrically-prepared test blends. Both companies were able to donate one or more cylinders of each gas blend to SwRI for the tests. All cylinders of a given nominal composition were blended to the same specifications by both companies, but the certified composition of the blend in each cylinder was used for reference purposes during tests and data analysis. Calculations were performed to confirm that the amount of gas in a single cylinder from either company would be adequate to charge the test loop to the pressure required for tests of that gas. This ensured that mixing of gases from multiple cylinders in the test loop would not be necessary, and that the certified composition of the gas could be used in evaluating its dew point behavior. Of the two companies, one provided delivery cylinders containing a higher total mass of each gas blend than the other company. It was decided to use the gas blends delivered in smaller amounts from one provider to calibrate the Varian GC and to use the larger-quantity gas blends from the other provider as the test gases in the loop. Since it was unlikely that both companies would make identical errors in their blending processes, analyzing the test gases from one company using a GC calibration based on a gas from the other company, made any errors in the certified composition of the test gas more likely to be identified before the tests. 27

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44 3. TEST PROCEDURES The test procedures, including preparations of the apparatus for each test and the chosen test conditions, were developed with the assistance of the JIP members and were based on the test procedures developed for the 2005 tests. Based on the scope of work, the test plan called for dew point measurements of each gas to be performed with each instrument at gas pressures below, at, and above the cricondentherm. The test procedures, as approved by the JIP members, are included in the Appendix. Annotations by SwRI personnel to aide in the conduct of testing are marked in the procedure. 3.1 PREHEATING OF GAS BLENDS AND EQUIPMENT The tests began with preheating of the gas blends in their delivery cylinders, heating of the transfer lines to the test apparatus, and heating of the apparatus itself as necessary. This was done to avoid condensation of the heavy components and distortion of the gas composition from the certified values. Input from Scott Specialty Gases and DCG Partnership was used to create this procedure. Each gas storage cylinder and its outlet regulator were heated to a temperature approximately 30 F (17 C) above the predicted hydrocarbon dew point of the gas. The gas cylinder was heated to the required temperature and allowed to stabilize for 24 hours before any gas was withdrawn from the cylinder to calibrate the GC or to charge the test loop. The expected delivery temperature and pressure of each gas were used to compute the temperature and pressure to which the cylinder and its contents would be heated. The maximum temperature for all gas storage cylinders allowed by procedure was 125 F (51.7 C), based on Compressed Gas Association (CGA) and American Petroleum Institute (API) specifications (American Petroleum Institute, 2006). The storage 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 reduce convection cooling. The heating blankets were regulated using an RTD attached to the wall of the cylinder, placed halfway up its height. After the wall of the cylinder reached the specified temperature, cylinder heating continued for a minimum of 24 hours before the cylinder was first opened. The requirement was intended to ensure that the core temperature of the gas in the cylinder had reached a temperature at least 30 F (17 C) above its calculated hydrocarbon dew point. The gas cylinder was heated continuously until the conclusion of testing. 3.2 GAS CHROMATOGRAPH CALIBRATION The Varian GC was calibrated at the beginning of each test day. A gas blend of the same nominal composition as the gas to be tested, provided by a different blender than the test gas itself, was used in each GC calibration. After the calibration gas storage cylinder, its outlet regulator, and the gas manifold and connecting lines had been preheated according to the procedure described in the previous section, the calibration gas storage cylinder was valved to supply gas to the GC at approximately 80 psia (552 kpa). A preheater on the GC constantly heated the sample 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 nine-minute calibration runs were 29

45 performed on the calibration gas, and data from the last five runs were used to generate calibration factors for that 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. Per the test procedure, the test gas was analyzed directly from the delivery cylinder before its introduction to the test loop, from the test loop prior to any testing, and from the test loop after testing was completed for each analyzer at each pressure. 3.3 INTRODUCTION OF A NEW TEST GAS Before the initial tests, several precautions were taken to avoid contamination of the test gas and ensure the integrity of the apparatus. For the 2005 HCDP experiments, all equipment was cleaned using wet steam or acetone to remove potential contaminants before first use. In 2007, all new and modified equipment (including the accumulator, fittings, and tubing) was again cleaned using wet steam or acetone to remove potential contaminants. The system was leaktested at the maximum test pressure (1,000 psia, or 68.9 bara) using helium gas. A thermal conductivity leak detector was used to find any helium leaking from the test system during these leak tests. Also, 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 water dew point measurement cell was used to check the moisture content in the loop. 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 gas chromatograph 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 interior volume of the warm box was purged with nitrogen, and remained purged with nitrogen throughout the tests. (Safety systems were incorporated into the apparatus to shut down all power inside the warm box in the event that a leak from the loop was detected or the nitrogen purge was not maintained.) 3. The system was evacuated to 27 -Hg (13.3-psi) vacuum for a minimum of one-half hour. The temperature of the environmental chamber was brought to a temperature specific to the gas about to be tested. The specified temperature was 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. 4. 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 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. 30

46 5. 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. 6. The system was filled with helium at approximately 80 psig (552 kpa gauge), and circulated in the loop for 30 minutes. 7. The GC and Michell Condumax II water dew point measurement cell were used to check the helium in the loop for contaminants (remaining hydrocarbons, nitrogen, carbon dioxide, water vapor, etc.). A GC analysis was performed of the loop contents to confirm that no contaminants were present. If contaminants were found, steps 5 and 6 were repeated. 8. 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. 9. The system was filled with test gas by performing five fill/flush cycles with test gas at 25 psig to 50 psig (170 kpa 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. 10. Finally, the contents of the loop were analyzed by 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. 3.4 DEW POINT MEASUREMENT PROCEDURES Once the equipment was at the specified temperature and the test gas composition in the loop had been confirmed, dew point measurements of the gas 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. Table 3-1. Test gas 1,050 Btu/scf (39.12 MJ/Nm 3 ) 1,145 Btu/scf (42.66 MJ/Nm 3 ) Equipment temperatures and test conditions for each test gas composition. Gas cylinder, regulator and manifold temperature Loop temperature Test pressures 110 F (43 C) Room temperature 750, 500, 300 psia (51.7, 34.5, 20.7 bara) 110 F (43 C) 110 F (43 C) 1,000, 650, 300 psia (68.9, 44.8, 20.7 bara) 31

47 After the desired pressure had been achieved in the loop, the stroke speed of the rodless cylinder 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) Procedure for the Bureau of Mines Chilled Mirror The manifold used to route flow to each of the test articles was configured to allow flow to the dew point tester only (MV1 and MV2 open; MV3, MV4, MV5, MV6, MV7, and MV8 closed (see Figure 2-14)). 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 no greater than 1ºF (0.6ºC) per minute, per the recommendations of ASTM Standard D 1142 (ASTM, 1995). The temperature was lowered until hydrocarbon condensation was observed, at which point the dew point temperature was recorded. 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 may be cited as the hydrocarbon dew point. Using the interpretations described in Subsection 2.2.1, the operator noted the time, line pressure, and mirror temperature at which the droplet HCDP was first observed. If the iridescent ring HCDP was observed before the droplet HCDP, its time, pressure, and temperature were also recorded. Six droplet dew point measurements were performed at each test pressure, with iridescent ring dew points measured as they were observed. 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 at each pressure, the flow was allowed to continue circulating for 30 minutes to ensure that all condensate was re-vaporized prior to GC analysis. After the tests at each pressure, four consecutive nine-minute GC analysis runs were performed on the test gas from the loop, and the results from the last three analyses were checked to determine if the gas composition had significantly changed over the course of the tests Procedure for the Ametek 241 CE II The manifold used to route flow to each of the test articles was configured to allow flow to the Ametek 241 CE II only (MV3 and MV4 open; MV1, MV2, MV5, MV6, MV7, and MV8 closed (see Figure 2-14)). 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 effectively no flow path existed through the analyzer. The analyzer was allowed to perform nine measurements of hydrocarbon dew point. Per the manufacturer s instructions, the first three measurements after initiating testing with the analyzer were disregarded, and only the final six measurements were recorded. For the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas tests, the default upper and lower mirror temperature set points were used [68 F and 18 F (20 C and -7.8 C), respectively]. For the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas 32

48 tests, the upper and lower mirror temperature set points were adjusted to 100 F and 50 F (37.8 C and 10 C), respectively. The operator recorded the time, line pressure, and dew point measurement 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 flow was allowed to continue circulating for 30 minutes to ensure that all condensate was re-vaporized prior to GC analysis. After the tests at each pressure, four consecutive nine-minute GC analysis runs were performed on the test gas from the loop, and the results from the last three analyses were checked to determine if the gas composition had significantly changed over the course of the tests Procedure for the Michell Condumax II The manifold used to route flow to each of the test articles was configured to allow flow to the Michell Condumax II only (MV5 and MV6 open; MV1, MV2, MV3, MV4, MV7, and MV8 closed (see Figure 2-14)). 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 (see the Appendix) 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 measurements of hydrocarbon dew point. Per the manufacturer s instructions, the first three measurements after initiating testing with the analyzer were disregarded, and only the final six measurements were recorded. The operator recorded the time, hydrocarbon dew point measurement, water dew point measurement, and line pressure 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 in order to ensure that all condensate was re-vaporized prior to GC analysis. After the tests at each pressure, four consecutive nine-minute GC analysis runs were performed on the test gas from the loop, and the analysis results from the last three analysis runs were checked to determine if the gas composition had significantly changed over the course of the tests. 33

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50 4. RESULTS AND DISCUSSION Preparation of the test loop was completed during July 2007, and dew point measurements were taken with the automated analyzers and the manual chilled mirror during July and early August. Dew point measurements of each test gas blend were performed at three line pressures, one each above, at, and below the predicted cricondentherm of the gas blend. Representatives of Michell Instruments, Ametek Process and Analytical Instruments, and PRCI came to SwRI during this period to witness the tests. This section discusses the results of these tests, comparing measurements from the Bureau of Mines chilled mirror and the automated analyzers to one another, and to hydrocarbon dew point curves predicted using the analyzed gas compositions and the Soave-Redlich-Kwong (SRK) cubic equation of state within Multiflash. [The SRK equation was specifically chosen over the commonly used Peng-Robinson (PR) equation of state for reasons discussed in Subsection 4.2.] Confidence intervals are shown with each measured dew point, determined from the uncertainties of each device listed in Table 2-1 and Table 4-3 (presented later in this section). Confidence intervals have also been placed on each predicted dew point curve, as described in Subsection 4.2. Temperatures and pressures have been recorded at which both iridescent ring dew points and droplet dew points were observed on the Bureau of Mines chilled mirror, to address questions on the interpretation of chilled mirror observations. The preliminary data were sent to participants for review and comment, and their feedback has been incorporated into this section. The SwRI project manager and technical staff also reviewed the videotapes of the chilled mirror measurements to confirm the observed dew points. Less than one half of the observed dew points were changed as a result of the review, the changes being made when additional droplet nucleation sites were found in the video. Of those that were revised, two-thirds were increased by only 0.1 F or 0.2 F (0.05 C or 0.1 C); the largest change in a droplet dew point was +0.6 F (+0.3 C). No changes to the iridescent ring dew points were required after review. 4.1 MEASUREMENTS OF MOISTURE CONTENT Before, during, and after each series of HCDP tests, data were also collected on the moisture content of the gas in the test loop. The moisture content was determined using the water vapor dew point sensor incorporated into the Michell Condumax II and software provided by Michell Instruments. The intent was to keep the moisture dew point well below the hydrocarbon dew point, to avoid interference in HCDP measurements. A general tariff specification for natural gas pipelines is that moisture content be no more than seven pounds of water per million standard cubic foot of gas (7 lb m /MMscf) (112 mg/nm 3 ), or approximately 150 ppmv. At a line pressure of 1,000 psia (68.9 bara), this moisture level corresponds to a water dew point of 32 F (0 C). Table 4-1 shows the water vapor dew points measured by the Condumax II at various points during the tests and the moisture content determined from the dew point values. The general trend of increasing moisture content with decreasing pressure is the result of water vapor desorbing from the walls of the test loop and entering the test gas. Note, however, that the highest moisture level of 43.8 ppmv (about 2 lb m /MMscf or 33 mg/nm 3 ) is well below the common tariff limit, and that all measured water vapor dew points are well below the hydrocarbon dew points reported in this section. 35

51 Table 4-1. Water vapor content of the test loop before and during hydrocarbon dew point tests. Event Temperature ( F) Measured water vapor dew point Pressure (psig) Temperature ( C) Pressure (kpa gauge) Water vapor content (ppmv) Helium flush before 1,050-Btu/scf (39.12-MJ/Nm 3 ) tests ,050-Btu/scf (39.12-MJ/Nm 3 ) gas test at 750 psia (51.7 bara) ,050-Btu/scf (39.12-MJ/Nm 3 ) gas test at 500 psia (34.5 bara) ,050-Btu/scf (39.12-MJ/Nm 3 ) gas test at 300 psia (20.7 bara) Helium flush before 1,145-Btu/scf (42.66-MJ/Nm 3 ) tests ,145- Btu/scf (42.66-MJ/Nm 3 ) gas test at 1,000 psia (68.9 bara) ,145- Btu/scf (42.66-MJ/Nm 3 ) gas test at 650 psia (44.8 bara) ,145- Btu/scf (42.66-MJ/Nm 3 ) gas test at 300 psia (20.7 bara) After final tests

52 4.2 COMMENTS ON HCDP CALCULATIONS In situations where the hydrocarbon dew point of a natural gas stream is impractical to measure, the industry often uses equations of state and GC analyses of the stream to predict the dew point. Many different calculational methods are available for predicting hydrocarbon dew points, including various software packages, equations of state, and methods for characterizing heavy hydrocarbon distributions that field GCs cannot resolve. These various methods often produce results that are inconsistent from one method to another, and for some gas streams, calculational methods have been found to significantly underpredict hydrocarbon dew point temperatures. This has led, in part, to the recent emphasis on direct dew point measurements and measurement accuracy. However, for this work, predicted dew points for the test gases were still required for choosing test conditions and for comparison to the various measurements. 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) evaluated existing and new approaches for predicting hydrocarbon dew points of natural gas streams using GC compositional data as input. The research evaluated the accuracy of several C 6+ characterization methods, software packages, generic equations of state, and gas property datasets in predicting dew points for a wide range of production, transmission, and distribution gases. The findings of these research reports were as follows: By far, the calculational variable with the largest influence on predicted dew point accuracy is the method of characterizing heavy hydrocarbons. Characterization refers to the process of assigning amounts to individual heavy hydrocarbon components (heptane, benzene, isooctane, etc.) when only the total amount of hydrocarbons above a certain carbon number is known. Some software users, unfamiliar with dew point prediction methods and given only a total C 6+ fraction from a process GC, will often characterize the total hexanes-plus fraction as normal hexane. Using this C 6+ characterization instead of the complete analytical gas composition beyond C 9 was found to bias the computed dew point by as much as 70ºF (40ºC). An adaptation of a Gaussian characterization method used by the petroleum industry was found to best simulate actual distributions of hexane and heavier components in many natural gases, but limited experimental dew point data from the literature prevented the creation of a versatile and generally acceptable characterization method. In the most recent study, characterizations were tested using the GERG-2004 equation of state, and the Peng-Robinson and Soave-Redlich-Kwong (SRK) cubic equations of state (EOS) as implemented by the Multiflash software package. The SRK equation of state was found to have advantages over the other equations in predicting dew points. In particular, the SRK equation consistently predicted dew point temperatures a few degrees higher than the Peng-Robinson equation. Comparisons of several EOS results to experimental dew point data (George et al., 2005a; George and Burkey, 2005c) have shown that, particularly for production-quality gases with high amounts of hexane and heavier hydrocarbons, commonly-used equations of state can underpredict actual hydrocarbon dew points by as much as 30 F (17 C). The use of the SRK equation of state can eliminate at least part of this underprediction error, by virtue of its higher predicted dew points. 37

53 The 2005 research sponsored by Gas Technology Institute found that different software packages and different sets of binary interaction parameters (BIPs) produce hydrocarbon dew points that differ by relatively small amounts compared to the effect of heavy hydrocarbon characterizations and EOS selections. Four software packages computed dew point temperatures in agreement to within ±5ºF (±3 C) of one another for the same gas compositions and pressures, and using the same equations of state. Similarly, calculations with the Multiflash package using the default BIPs from the Dechema Data Series (Knapp et al., 1982) and other BIP sets assuming BIP values of zero between dissimilar molecules, found the impact of changing BIPs to be only ±4ºF (±2ºC). Based on these findings, it was decided to use the SRK equation of state in this project to predict hydrocarbon dew points for experimental design and for comparison to measured values. It was also expected from previous experience with the apparatus that small changes could occur in the heavy hydrocarbon content of the gases over the course of the tests, and that these changes could affect their dew point temperatures, as discussed in the next section. Predicted dew point curves have been used in this section to estimate biases in measured dew points caused by these small shifts in composition. All predicted HCDP curves were computed using the Multiflash software package with BIPs from the Dechema Data Series (the default set used by Multiflash). All components, 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). Even so, the reader is reminded that the dew points produced by equations of state are predicted values, and should not be taken as an absolute reference value that measurements must unconditionally agree with. To emphasize this, confidence intervals at the 95% level have been estimated for each predicted HCDP curve and shown in this section 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%. Comparisons of measured and predicted dew points will involve comparisons of their confidence intervals as well, to identify instances where the probability of agreement is high even though the observed and calculated values do not agree exactly ,050-BTU/SCF (39.12-MJ/NM 3 ) TEST RESULTS Dew point measurements with the various devices were made in succession on the same gas stream, but as noted above, differences among the various measurements can be expected. In previous experience with the test apparatus (George et al., 2005b; George and Burkey, 2005c), small changes in the heavy hydrocarbon content were observed over time. The most significant changes occurred in the cyclic hydrocarbons benzene and toluene, which were not included in the gases tested here. Smaller decreases were also observed in normal heavy hydrocarbons such as nonane and decane, which were included in the test gases for this work. 38

54 Gas chromatograph analyses were performed before and after dew point measurements with each device, and the results were used with the Multiflash software package and the SRK equation of state to predict hydrocarbon dew point 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. Uncertainty budgets were also applied to the GC analyses, incorporating uncertainties in certified gas compositions and GC repeatability. 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. Figure 4-1 and Figure 4-2 show two examples of component histories from preparations for the 1,050-Btu/scf (39.12-MJ/Nm 3 ) tests. While many components were checked for signs of sudden change, data for two components of interest, nitrogen and decane, are shown here. Nitrogen was monitored for indication of leaks of the pressurant from below the accumulator piston into the test gas, while the decane plot is typical of data for the heavy hydrocarbons in the gas blend. A statistically-significant change in component content is indicated when the 95% confidence intervals for successive amounts do not overlap. For nitrogen, the analyzed amount is consistent throughout all of the pretest analyses. For decane, the small amount present leads to noticeable changes over the pretest analyses, but the 95% confidence intervals overlap and show no statistically-significant difference. nitrogen mole percent nitrogen cylinder analysis Pretest check of gas in loop at 750 psia (51.7 bara) Pretest check with Michell analysis loop in line Second check with Michell analysis loop in line Check after blowdown and second charge to loop Day 2 pretest check of gas in loop at 750 psia (51.7 bara) Day 2 repeat pretest check of gas in loop at 750 psia (51.7 bara) analysis Figure 4-1. Trends in analyzed nitrogen content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend during test preparations. 39

55 n-decane mole percent n-decane cylinder analysis Pretest check of gas in loop at 750 psia (51.7 bara) Pretest check with Michell analysis loop in line Second check with Michell analysis loop in line Check after blowdown and second charge to loop Day 2 pretest check of gas in loop at 750 psia (51.7 bara) Day 2 repeat pretest check of gas in loop at 750 psia (51.7 bara) analysis Figure 4-2. Trends in analyzed decane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend during test preparations. The next three figures present similar results during the tests of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas itself. The GC was used to analyze the gas immediately after dew point measurements with each device; the horizontal axis notes the conditions of the test immediately preceding the GC analysis, in the format test gas heating value date pressure in psia instrument used. Figure 4-3 shows that the nitrogen content was again relatively constant during all tests, confirming that no N 2 pressurant had leaked into the system. Figure 4-4 suggests a small decrease in the nonane content occurred over the course of the tests, but the changes are not considered statistically significant. In Figure 4-5, the average amount of decane found by the GC analysis dropped noticeably during tests at 500 psia (34.5 bara) and 300 psia (20.7 bara), but the large 95% confidence intervals (which extend below zero mole percent) again mean that the changes are not considered statistically significant. These changes could be explained by small amounts of the heavy hydrocarbons from the gas stream adsorbing onto the walls of the apparatus during the tests. It was unclear, because of the large confidence intervals on the nonane and decane amounts, whether the levels were stabilizing asymptotically to an equilibrium level, or whether they would have eventually depleted through adsorption if the tests had continued. 40

56 nitrogen mole percent nitrogen Btu DewScope 1050Btu Ametek 1050Btu Michell 1050Btu DewScope 1050Btu Ametek2 1050Btu Michell 1050Btu Pretest 1050Btu Dewscope 1050Btu Ametek 1050Btu Michell analysis Figure 4-3. Trends in analyzed nitrogen content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend during analyzer tests. n-nonane mole percent n-nonane Btu DewScope 1050Btu Ametek 1050Btu Michell 1050Btu DewScope 1050Btu Ametek2 1050Btu Michell 1050Btu Pretest 1050Btu Dewscope 1050Btu Ametek 1050Btu Michell analysis Figure 4-4. Trends in analyzed nonane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend during analyzer tests. 41

57 n-decane Btu DewScope 1050Btu Ametek 1050Btu Michell 1050Btu DewScope 1050Btu Ametek2 1050Btu Michell 1050Btu Pretest 1050Btu Dewscope 1050Btu Ametek 1050Btu Michell mole percent n-decane analysis Figure 4-5. Trends in analyzed decane content of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend during analyzer tests. The next several figures in this section compare dew points measured with the Bureau of Mines chilled mirror, the Ametek automated analyzer, and the Michell automated analyzer. The measurements are compared to one another, and to dew point curves predicted using the GC analyses before and after tests with each instrument. Dashed lines shown in Figure 4-6 through Figure 4-10 represent the confidence intervals on the predicted dew point curves, as discussed in Section 4.2. Comparison of the curves themselves suggest the fraction of the difference in measured dew points that can be attributed to the changes in composition indicated by Figure 4-1 through Figure 4-5. Additional information on the uncertainties in the dew point measurements, indicated by the error bars on the data points, can be found in Section Some trends are evident in the data: As the test pressure falls, both the iridescent ring and droplet dew points recorded using the Bureau of Mines chilled mirror device shift position relative to the corresponding analytical curve (Figure 4-6) and to the measurements from the automated devices (Figure 4-9). For example, at 750 psia (51.7 bara), the droplet dew points are about 5 F (3 C) lower than the analytical dew point predicted from the corresponding GC analysis taken after the chilled mirror measurement. The droplet dew points are also lower than the measurements from both the automated devices, though the disagreement with the values from the Condumax II is not statistically significant. At 300 psia (20.7 bara), the droplet dew points fall between the Ametek and Michell values, and are typically 2 F (1 C) higher than the corresponding analytical dew point. Similarly, the difference in iridescent ring dew point temperatures relative to the analytical curves range from -2 F at 750 psia (-1 C at 51.7 bara) to +9 F at 300 psia (+5 C at 20.7 bara). 42

58 Shifts with pressure in the bias between the droplet dew point and the analytical dew point predicted by cubic equations of state have been observed before with this gas composition (George et al., 2005b; George and Burkey, 2005c). It was found here that the iridescent ring dew point (which occurs a few degrees above the droplet dew point) shows a similar shift in bias relative to the predicted dew point curves with changing pressure. The Ametek unit consistently reported dew point temperatures higher than the corresponding analytical dew point temperature (Figure 4-7), while the Michell unit reported dew point temperatures at or below the corresponding analytical dew point temperature (Figure 4-8). 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 trip point is calibrated to give good agreement with manual chilled mirror measurements of the droplet hydrocarbon dew point, which occurs at a lower temperature than the iridescent ring dew point. However, note the error bars showing uncertainties in dew point measurements from each of the instruments, and the dashed lines indicating uncertainties in the predicted dew point curves due to uncertainties in GC analyses of the test gases. The confidence intervals on corresponding measured and predicted dew points overlap in each figure, indicating that the observed differences are not statistically significant. While the Ametek unit reported hydrocarbon dew points 4 F to 7 F (2 C to 4 C) higher than the Michell unit at each test condition, differences in the dew point curves predicted from the corresponding gas analyses are 1 F (0.5 C) or less. This suggests that most of the difference in measurements between the two instruments is due to differences in their measurement techniques, not due to the changing gas composition. In Figure 4-10, the measurements from the three instruments have been corrected for changes in the gas composition over the course of the tests. The adjustment was made using the following assumptions: o The effect of the lost heavy hydrocarbons on the true dew point was accurately estimated by the shift in the dew point curves predicted by Multiflash and the SRK equation of state. o 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. o The effects of lost heavy hydrocarbons accumulated over all tests at a given pressure, so that all measurements were corrected back to the compositional analysis before tests with all instruments at that pressure. After the adjustment, the difference between the Ametek and Michell dew points was reduced to a range of 1 F to 6 F (0.6 C to 3 C), confirming that the majority of the differences between measurements is due to differences in the automated instruments themselves. 43

59 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 750-psia (51.7-bara) Bureau of Mines chilled mirror data pressure (psia) Pretest analyses Analysis after chilled mirror measurements 620 chilled mirror 'iridescent ring' chilled mirror 'droplets' temperature (deg. F) 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 500-psia (34.5-bara) Bureau of Mines chilled mirror data pressure (psia) Pretest analyses Analysis after chilled mirror measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' temperature (deg. F) Figure 4-6. Comparison of 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas dew points measured with the Bureau of Mines chilled mirror with dew point curves predicted from GC analyses. 44

60 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 300-psia (20.7-bara) Bureau of Mines chilled mirror data pressure (psia) Pretest analyses Analysis after chilled mirror measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' temperature (deg. F) Figure 4-6 (continued). 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 750-psia (51.7-bara) Ametek data pressure (psia) Pretest analyses Analysis after Ametek measurements Ametek temperature (deg. F) Figure 4-7. Comparison of 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas dew points measured by the Ametek analyzer with dew point curves predicted from GC analyses. 45

61 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 500-psia (34.5-bara) Ametek data pressure (psia) Pretest analyses Analysis after Ametek measurements Ametek temperature (deg. F) 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 300-psia (20.7-bara) Ametek data Pretest analyses Analysis after Ametek measurements Ametek 340 pressure (psia) temperature (deg. F) Figure 4-7 (continued). 46

62 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 750-psia (51.7-bara) Michell data pressure (psia) Pretest analyses Analysis after Michell measurements Michell temperature (deg. F) 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 500-psia (34.5-bara) Michell data pressure (psia) Pretest analyses Analysis after Michell measurements Michell temperature (deg. F) Figure 4-8. Comparison of 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas dew points measured by the Michell analyzer with dew point curves predicted from GC analyses. 47

63 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 300-psia (20.7-bara) Michell data pressure (psia) Pretest analyses Analysis after Michell measurements Michell temperature (deg. F) Figure 4-8 (continued). 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 750-psia (51.7-bara) data pressure (psia) Pretest analyses Analysis after chilled mirror measurements Analysis after Ametek measurements Analysis after Michell measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' Ametek Michell temperature (deg. F) Figure 4-9. Comparison of 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas dew points measured by all instruments and predicted from all GC analyses. 48

64 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 500-psia (34.5-bara) data pressure (psia) Pretest analyses Analysis after chilled mirror measurements Analysis after Ametek measurements Analysis after Michell measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' Ametek Michell temperature (deg. F) 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 300-psia (20.7-bara) data pressure (psia) Pretest analyses Analysis after chilled mirror measurements Analysis after Ametek measurements Analysis after Michell measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' Ametek Michell temperature (deg. F) Figure 4-9 (continued). 49

65 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 750-psia (51.7-bara) data, adjusted for changes in gas composition pressure (psia) Pretest analyses chilled mirror 'iridescent ring' chilled mirror 'droplets' Ametek Michell temperature (deg. F) 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 500-psia (34.5-bara) data, adjusted for changes in gas composition pressure (psia) Pretest analyses chilled mirror 'iridescent ring' chilled mirror 'droplets' Ametek Michell temperature (deg. F) Figure Comparison of 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas dew points measured by all instruments, after adjustments for changes in gas composition during tests. 50

66 1,050-Btu/scf (39.12 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 300-psia (20.7-bara) data, adjusted for changes in gas composition pressure (psia) Pretest analyses chilled mirror 'iridescent ring' chilled mirror 'droplets' Ametek Michell temperature (deg. F) Figure 4-10 (continued) ,145- BTU/SCF (42.66-MJ/NM 3 ) TEST RESULTS For tests of the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas, the same general approach was used, with the key difference that heat was applied to all equipment to maintain it at a temperature at least 30 F (17 C) above the expected cricondentherm temperature of the gas. A change was also made to the testing order at the request of the JIP participants. During the 1,050-Btu/scf (39.12-MJ/Nm 3 ) tests, measurements were collected in the order: chilled mirror, Ametek, Michell. It was requested that for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) tests, the order be changed to: chilled mirror, Michell, Ametek. The same types of data were collected, including GC analyses of the test gas before, between, and after dew point measurements with each device, and measured dew points from all tests were compared to one another and to hydrocarbon dew point curves predicted from the GC analyses. Figure 4-11 through Figure 4-13 present component history data gathered before and during the 1,145- Btu/scf (42.66-MJ/Nm 3 ) tests. In Figure 4-11, the nitrogen content of the gas is shown as analyzed directly from the delivery cylinder, and from the test loop before and between tests. Because of the number of analyses involved in GC calibration and verification of the test gas, the confidence intervals on the check of the test gas straight from the cylinder are larger than the confidence intervals once the gas has been introduced to the loop. A difference in the average nitrogen content between the cylinder and the loop appears to be present, but as with the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas, the 95% confidence intervals overlap among the various analyses, and the difference cannot be considered statistically significant. Note also that the 51

67 nitrogen level remains constant throughout the tests, indicating no leak of N 2 from the pressurant side of the accumulator. nitrogen mole percent Cylinder analysis Pretest verification of test gas in loop , 1000 psia (68.9 bara) pretest analysis , 1000 psia (68.9 bara) after dewscope tests , 1000 psia (68.9 bara) after Michell tests , 1000 psia (68.9 bara) after Ametek tests , 650 psia (44.8 bara) pretest analysis , 650 psia (44.8 bara) after dewscope tests , 650 psia (44.8 bara) repeat after dewscope tests , 650 psia (44.8 bara) after Michell tests , 650 psia (44.8 bara) after Ametek tests , 300 psia (20.7 bara) pretest analysis , 300 psia (20.7 bara) after dewscope analysis , 300 psia (20.7 bara) after Michell analysis , 300 psia (20.7 bara) after Ametek analysis nitrogen analysis Figure Trends in analyzed nitrogen content of the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas blend during analyzer tests. Figure 4-12 and Figure 4-13 present analysis histories for the heavy hydrocarbons nonane and decane. As in the case of the 1,050-Btu/scf (39.12-MJ/Nm 3 ) tests, the amounts analyzed from the delivery cylinder and from the test loop before the start of the tests do not differ statistically. The amounts of these components decline slowly (and possibly asymptotically) over the course of the tests, and while the results from one test to the next have overlapping confidence intervals, the confidence intervals on the amounts observed at the beginning and the end of the 1,145-Btu/scf (42.66-MJ/Nm 3 ) tests do not overlap. The change over the entire course of the tests is therefore considered significant, and the resulting change in predicted dew point must be considered in evaluating the performance of the various analyzers. If tests with this gas composition or richer gases are repeated in the future, ways of stabilizing or preventing the depletion of heavy hydrocarbons will be investigated to minimize the potential for bias in HCDP determination. 52

68 n-nonane mole percent n-nonane Cylinder analysis Pretest verification of test gas in loop , 1000 psia (68.9 bara) pretest analysis , 1000 psia (68.9 bara) after dewscope tests , 1000 psia (68.9 bara) after Michell tests , 1000 psia (68.9 bara) after Ametek tests , 650 psia (44.8 bara) pretest analysis , 650 psia (44.8 bara) after dewscope tests , 650 psia (44.8 bara) repeat after dewscope tests , 650 psia (44.8 bara) after Michell tests , 650 psia (44.8 bara) after Ametek tests , 300 psia (20.7 bara) pretest analysis , 300 psia (20.7 bara) after dewscope analysis , 300 psia (20.7 bara) after Michell analysis , 300 psia (20.7 bara) after Ametek analysis analysis Figure Trends in analyzed nonane content of the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas blend during analyzer tests. n-decane mole percent n-decane Cylinder analysis Pretest verification of test gas in loop , 1000 psia (68.9 bara) pretest analysis , 1000 psia (68.9 bara) after dewscope tests , 1000 psia (68.9 bara) after Michell tests , 1000 psia (68.9 bara) after Ametek tests , 650 psia (44.8 bara) pretest analysis , 650 psia (44.8 bara) after dewscope tests , 650 psia (44.8 bara) repeat after dewscope tests , 650 psia (44.8 bara) after Michell tests , 650 psia (44.8 bara) after Ametek tests , 300 psia (20.7 bara) pretest analysis , 300 psia (20.7 bara) after dewscope analysis , 300 psia (20.7 bara) after Michell analysis , 300 psia (20.7 bara) after Ametek analysis analysis Figure Trends in analyzed decane content of the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas blend during analyzer tests. 53

69 Figure 4-14 through Figure 4-17 compare dew points measured with the Bureau of Mines chilled mirror, the Ametek automated analyzer, and the Michell automated analyzer for the 1,145-Btu/scf (42.66-MJ/Nm 3 ) gas. As before, measurements from each instrument are compared to one another and to dew point curves predicted using the GC analyses before and after tests with each instrument. Several differences in analyzer performance are noted for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) tests. For this gas composition with higher C 6+ content, both the Ametek and Michell units consistently identified dew point temperatures higher than their corresponding analytical dew point curves. For the Ametek analyzer, the measured dew point temperatures are about 2 F to 4 F (1 C to 2 C) above the predicted HCDP temperatures at all pressures. For the Michell unit, the measured values are above the analytically-predicted temperatures by amounts ranging from less than 1 F at 300 psia (0.6 C at 20.7 bara) to over 4 F at 1,000 psia (2 C at 68.9 bara). This trend in the Michell measurements with respect to the analytical curves is similar to that in the data from the Bureau of Mines chilled mirror (Figure 4-14). Because of the larger amounts of heavy hydrocarbons in the 1,145-Btu/scf (42.66-MJ/Nm 3 ) gas, the uncertainties in the gravimetric blend compositions, GC calibrations, and test gas analyses are smaller. Notably, at the highest pressure of 1,000 psia (68.9 bara), confidence intervals on the Ametek measurements barely overlap the confidence band on the corresponding analytical curve (Figure 4-15). The confidence intervals on the 1,000-psia (68.9-bara) Michell data (Figure 4-16) do not overlap those of its analytical curve at all, meaning that the differences are statistically significant. Again, this is similar to the performance of the Bureau of Mines chilled mirror device in richer hydrocarbon gases (George and Burkey, 2005c). Measurement biases between the various devices are much smaller for this richer gas blend. Before corrections were made for changes in the gas composition during the tests, differences among all of the dew point temperatures at each pressure from the three devices (Figure 4-17) were no more than 3.5 F (1.9 C). By comparison, the largest change in predicted dew points from the test gas analyses is slightly over 1 F (0.6 C). As with the 1,050-Btu/scf (39.12-MJ/Nm 3 ) data, the measured values were adjusted for the observed gas composition changes, producing the results in Figure At the highest test pressure, the Michell unit reports the highest HCDP temperatures of the three instruments, but the span between the Michell dew points and the Bureau of Mines droplet dew points is 3.5 F (1.9 C) after adjustment, compared to 2.5 F (1.4 C) before adjustment. At 650 psia, the dew points reported by all three analyzers span a range of less than 2.5 F (1.4 C), both before and after adjustment. At 300 psia (20.7 bara), biases between the various devices are also essentially unaffected by the adjustment; the Michell measurements lie on top of the manual chilled mirror droplet dew points, while the iridescent ring dew point falls within 1 F (0.6 C) of the Ametek analyzer results. In summary, measured dew points from all of the devices at a given pressure, both before and after adjustment for the changing gas composition, are within a 3.5 F (1.9 C) band on the 1,145- Btu/scf (42.66-MJ/Nm 3 ) test gas, compared to a 7 F (3.9 C) band for the 1,050-Btu/scf (39.12-MJ/Nm 3 ) test gas. 54

70 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 1,000-psia (68.9-bara) Bureau of Mines chilled mirror data pressure (psia) Pretest analysis Analysis after chilled mirror measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' temperature (deg. F) 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 650-psia (44.8-bara) Bureau of Mines chilled mirror data pressure (psia) Pretest analysis Analysis after chilled mirror measurements Repeat analysis after chilled mirror measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' temperature (deg. F) Figure Comparison of 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas dew points measured with the Bureau of Mines chilled mirror with dew point curves predicted from GC analyses. 55

71 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 300-psia (20.7-bara) Bureau of Mines chilled mirror data pressure (psia) Pretest analysis Analysis after chilled mirror measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' temperature (deg. F) Figure 4-14 (continued). 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 1,000-psia (68.9-bara) Ametek data pressure (psia) Pretest analysis Analysis after Ametek measurements Ametek temperature (deg. F) Figure Comparison of 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas dew points measured by the Ametek analyzer with dew point curves predicted from GC analyses. 56

72 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 650-psia (44.8-bara) Ametek data pressure (psia) Pretest analysis Analysis after Ametek measurements Ametek temperature (deg. F) 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 300-psia (20.7-bara) Ametek data pressure (psia) Pretest analysis 240 Analysis after Ametek measurements 220 Ametek temperature (deg. F) Figure 4-15 (continued). 57

73 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 1,000-psia (68.9-bara) Michell data pressure (psia) Pretest analysis Analysis after Michell measurements Michell temperature (deg. F) 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 650-psia (44.8-bara) Michell data pressure (psia) Pretest analysis Analysis after Michell measurements Michell temperature (deg. F) Figure Comparison of 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas dew points measured by the Michell analyzer with dew point curves predicted from GC analyses. 58

74 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) 300-psia (20.7-bara) Michell data pressure (psia) Pretest analysis Analysis after Michell measurements Michell temperature (deg. F) Figure 4-16 (continued). 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 1,000-psia (68.9-bara) data pressure (psia) Pretest analysis Analysis after chilled mirror measurements Analysis after Michell measurements Analysis after Ametek measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' Michell Ametek temperature (deg. F) Figure Comparison of 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas dew points measured by all instruments and predicted from all GC analyses. 59

75 pressure (psia) ,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 650-psia (44.8-bara) data Pretest analysis Analysis after chilled mirror measurements Repeat analysis after chilled mirror measurements Analysis after Michell measurements Analysis after Ametek measurements chilled mirror 'iridescent ring' chilled mirror 'droplets' Michell Ametek temperature (deg. F) 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 300-psia (20.7-bara) data pressure (psia) Pretest analysis 280 Analysis after chilled mirror measurements Analysis after Michell measurements 260 Analysis after Ametek measurements 240 chilled mirror 'iridescent ring' chilled mirror 'droplets' 220 Michell Ametek temperature (deg. F) Figure 4-17 (continued). 60

76 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 1,000-psia (68.9-bara) data, adjusted for composition change 1100 pressure (psia) Pretest analysis chilled mirror 'iridescent ring' chilled mirror 'droplets' Michell Ametek temperature (deg. F) 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 650-psia (44.8-bara) data, adjusted for composition changes 750 Pretest analysis 730 chilled mirror 'iridescent ring' 710 chilled mirror 'droplets' pressure (psia) Michell Ametek temperature (deg. F) Figure Comparison of 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas dew points measured by all instruments, after adjustments for changes in gas composition during the tests. 61

77 1,145-Btu/scf (42.66 MJ/Nm 3 ) HCDP data and curves computed from analyses (SRK EOS) All 300-psia (20.7-bara) data, adjusted for composition changes pressure (psia) Pretest analysis chilled mirror 'iridescent ring' 260 chilled mirror 'droplets' 240 Michell 220 Ametek temperature (deg. F) Figure 4-18 (continued). 4.5 ANALYSIS OF RESULTS Results from the Bureau of Mines Chilled Mirror The measurements made in this work with the Bureau of Mines chilled mirror are consistent with previous studies (George et al., 2005a and 2005b; George and Burkey, 2005c). For the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend, Figure 4-6 shows that confidence intervals of droplet hydrocarbon dew points overlap the confidence intervals of analytical dew point curves predicted using the gas compositions and the SRK equation of state, though the values themselves do not always fall within the curve confidence intervals. (Since the analyses were made at approximately the same time as the measurements, adjustments of the measurements for changes in gas composition over time are not necessary for valid comparisons.) Measured droplet dew points range from about five degrees below the analytical curve at the highest pressure to one or two degrees above the curve at the lowest pressure. For the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas blend (Figure 4-14), the measured droplet dew points at 1,000 psia (68.9 bara) shift to higher temperatures relative to the predicted curve, but confidence intervals still overlap one another. This is consistent with the key observation from the 2005 work that as line pressure and heavy hydrocarbon content increase, the SRK cubic equation of state with common binary interaction parameters begins to underpredict droplet HCDPs measured with the chilled mirror. (Notably, the Peng-Robinson cubic equation of state, more often used by the industry than the SRK EOS, underpredicts measured dew points by a larger margin than SRK at these conditions.) 62

78 Unlike previous tests with the Bureau of Mines chilled mirror, separate measurements were made, where possible, of the iridescent ring dew point temperature. Since the iridescent ring dew point appears at a higher temperature than the droplet dew point, it should be expected that these measurements would compare differently to the EOS predictions. For the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend, the iridescent rings all appeared at temperatures within the confidence interval of the SRK curve. Differences between measured and predicted dew point temperatures ranged from one or two degrees below the curve at the highest pressure to as much as ten degrees above the curve at the lowest pressure. For the 1,145- Btu/scf (42.66-MJ/Nm 3 ) blend, iridescent ring dew points appeared above the predicted curves at all pressures, but by a margin of no more than four degrees. Some in the natural gas industry advocate that the iridescent ring formation should be specified as the hydrocarbon dew point, while others advocate the droplet dew point. Since the iridescent ring dew points appear first (i.e., at a higher temperature than the droplet dew points), some may consider the use of the iridescent ring dew point a conservative approach to avoiding hydrocarbon condensation in pipelines. In cases of higher heavy hydrocarbon content and higher line pressure, where the cubic equations of state tend to underpredict observed dew points, use of the iridescent ring dew point would also make the differences between predicted and measured dew points even larger. On the other hand, the data show that the temperature differences between the iridescent ring formation and the droplet formation are generally larger for the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas than for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas. This can be explained by noting that to obtain a certain amount of liquid condensate, a leaner natural gas blend must be cooled further below its theoretical dew point (where condensation first begins) than a richer gas blend with more heavy hydrocarbons present. Hypothesizing that the iridescent ring dew point, like the droplet dew point, becomes visible to the eye once a particular threshold level of hydrocarbons form on the mirror, the difference between the two kinds of observed dew points should be smaller for the richer gas composition, as observed here. This trend, however, would reduce the conservative advantage of the iridescent ring dew point criterion in avoiding condensation at higher pressures and for richer gases Results from the Ametek 241 CE II As discussed in Section 2, the Ametek unit is designed to detect the first appearance of condensation, analogous to the iridescent ring dew point on the Bureau of Mines chilled mirror. The Ametek dew point temperature measurements were always higher than the corresponding dew point temperatures predicted by Multiflash using the SRK equation of state and GC analyses of the test gas. Recall that in most cases, the iridescent ring dew points measured by the Bureau of Mines chilled mirror device were also above the corresponding dew point temperatures predicted by SRK. For the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas, the Ametek dew point measurements in Figure 4-7 were anywhere from 2 F to 5 F (1 C to 3 C) above the corresponding SRK curve at each pressure, while for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas, the Ametek unit consistently produced measurements 2 F to 4 F (1 C to 2 C) above the predicted dew point curve at all pressures (Figure 4-15). By comparison, the iridescent ring dew points from the manual chilled mirror in Figure 4-14 showed biases relative to the corresponding SRK predictions that increased with C 6+ content and line pressure. 63

79 For the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas, Figure 4-17 and Figure 4-18 show that both before and after adjustment for composition changes, the Ametek measurements were in statistical agreement with results from both the Michell analyzer and the Bureau of Mines chilled mirror device (both iridescent ring and droplet dew points). For the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas, the agreement between the Ametek and the Bureau of Mines chilled mirror device depended on test pressure. After adjustment for composition changes (Figure 4-10), Ametek measurements at 300 psia (20.7 bara) were in statistical agreement with all of the manual chilled mirror droplet dew points, and in all but one case, were also in statistical agreement with the iridescent ring dew points, the type of dew point for which agreement would be expected. At 500 psia (34.5 bara), the Ametek results were within statistical agreement of the majority of Bureau of Mines iridescent ring dew points, but significantly higher (2 F to 8 F, or 1 C to 4 C) than the majority of Bureau of Mines droplet dew points after adjustment for composition changes over time. Similarly, at 750 psia (51.7 bara), the Ametek measurements were significantly (5 F to 6 F, or 3 C) higher than the manual chilled mirror measurements, but still in statistical agreement with the iridescent ring dew points. Ametek Process and Analytical Instruments concurs with these comparisons to the Bureau of Mines results. They attribute the larger differences between results for the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas to the sensitivity of the optical detector and the larger temperature drop needed to obtain a given amount of condensate from a leaner gas blend, as discussed in the previous subsection Results from the Michell Condumax II The performance of the Michell Condumax II was similar to the Bureau of Mines chilled mirror in several respects. Dew point temperatures measured by the Condumax II and droplet dew point temperatures measured with the manual chilled mirror on the same nominal gas blend at the same pressure were nearly always in statistical agreement with one another (that is, their uncertainty bands overlapped one another), both with and without adjustments for the changing gas composition. Notably, the measurements from the Michell analyzer also followed two trends observed in the 2005 tests with the manual chilled mirror: (1) Michell dew point measurements for the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend were in statistical agreement with the corresponding dew point temperatures predicted by the SRK equation of state (Figure 4-8); and (2) for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas blend, the SRK equation of state used by Multiflash with common interaction parameters underpredicts the observed dew point by an increasing margin with increasing pressure (Figure 4-16). This latter trend in the Condumax II measurements was also noted in an earlier comparison to the Bureau of Mines chilled mirror device (Brown et al., 2007), particularly on natural gas streams with heavier C 6+ content and methane content below 90%. These similarities may be explained by the trip point setting used with the dark spot measurement technique of the Condumax II. As described in Section 2, the optical detector is quantitative, in that the output signal changes with the amount of condensate that forms on the optical surface during cooling. The factory default trip point of 275mV at 400 psig (27.6 barg) 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 (as described in Subsection 2.2.3). Michell Instruments expected their readings using the default trip point to fall somewhere between the iridescent ring dew points and droplet dew points measured using the Bureau of Mines chilled mirror at each test condition; this held true 64

80 for some of the test cases in this investigation, but not all. Still, the agreement between the Bureau of Mines droplet dew points and Michell dew points is notable, as is the agreement between the SRK equation of state and the Michell measurements for the leaner test gas Effect of Filtration on Analyzer Accuracy One goal of this research is to identify any impact of filtration on analyzer accuracy. Both 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. It was concluded that the filters used on both the Michell and Ametek units have negligible impact on dew point accuracy. This conclusion is based upon the following observations: 1. The dew points measured by the Michell analyzer with its filtration are in good agreement with the droplet dew points from the unfiltered manual chilled mirror. Of a total of 36 dew points measured by the Michell unit, 30 agree to within experimental uncertainty with all chilled mirror droplet dew points at the same pressure and gas composition, after adjustment for differences in heavy hydrocarbon content. The other six agree statistically with all but the lowest droplet dew point at the same pressure and gas composition, with the Michell values being higher. If the filtration on the Michell unit were removing heavy hydrocarbons from the stream, it should produce dew point temperatures lower than the unfiltered chilled mirror, not higher. 2. The Michell data and manual chilled mirror droplet dew points show similar trends relative to their corresponding predicted dew point curves. Notably, for the 1,145- Btu/scf (42.66-MJ/Nm 3 ) gas at the highest test pressure, the Michell results actually deviate slightly more from the corresponding analytical curve than the Bureau of Mines droplet dew point data. If the filtration on the Michell unit were removing heavy hydrocarbons from the stream, it should produce dew point temperatures closer to EOSpredicted values than the unfiltered chilled mirror, not dew point temperatures further from the predicted curve. 3. At four of six test conditions, the Ametek analyzer with its filtration consistently reported dew point temperatures with confidence intervals that overlapped the iridescent ring dew points and its confidence intervals measured with the unfiltered manual chilled mirror. At each of the other two test conditions the 1,050-Btu/scf (39.12-MJ/Nm 3 ) gas blend at 300 psia (20.7 bara) and 500 psia (34.5 bara) the Ametek dew points were still in statistical agreement with all but one of the five iridescent ring dew points (Figure 4-10). 4. GC analyses of the test gases, before and after tests with each HCDP analyzer at each test condition, 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 gas composition. In all but two cases, the shift in predicted hydrocarbon dew points for analyzed compositions before and after each test was less than 1 F (0.6 C). Over the course of all tests on a particular gas composition, significant changes in heavy hydrocarbon content were observed, but this occurred over several days as the test gas was recirculated through the entire loop. The slow decreases in heavy hydrocarbon content were observed both during tests on the unfiltered Bureau of Mines chilled mirror device, and during tests on the filtered, automated analyzers, suggesting 65

81 that the filtration in the Ametek and Michell analyzers was not responsible for these changes. 5. The filters on both automated analyzers were disassembled and visually inspected after all tests were completed. The Michell filter showed no coloration and was in like new condition. Some small, black particulates were found on the first membrane of the threestage Ametek filter assembly, and a dimple was found in the second stage membrane, but the coloration of the three filters was identical to a new set of filters provided by Ametek as spares. The normal coloration indicated that no heavy hydrocarbons had been trapped in the filters during testing. (a) (b) (c) Figure Filters from the automated analyzers as found after the tests, showing normal coloration: (a) the filter membrane from the Michell Condumax II; (b) first-stage membrane from the Ametek 241 CE II, showing small particles near the top and right edges but no discoloration; (c) second-stage membrane from the Ametek unit, showing dimpling but no discoloration; (3) third-stage filter from the Ametek unit, showing normal coloration. (d) 66