FUNDAMENTALS OF GAS CHROMATOGRAPHY, GAS QUALITY & TROUBLESHOOTING Larry Ewing Chandler Engineering Company LLC

Transcription

1 FUNDAMENTALS OF GAS CHROMATOGRAPHY, GAS QUALITY & TROUBLESHOOTING Larry Ewing Chandler Engineering Company LLC 2001 N. Indianwood Avenue, Broken Arrow, OK INTRODUCTION Measurement of the quality of natural gas requires a variety of instrumentation, only one of which is the gas chromatograph. Contractual requirements frequently define the energy content, relative density, and moisture content of the gas being sold. The sale of natural gas is performed on the basis of the heating value per unit volume of the gas. For these reasons, the industry uses instruments to monitor the quality of the gas at the point of sale or at strategic locations along a pipeline. The following instruments are commonly found in the field and in the laboratory: Gas Chromatographs Moisture Analyzers Gravitometers Hydrogen Sulfide Monitors Others Many of the instruments listed above are portable and many are installed as online instruments at custody transfer points. In many cases, sample cylinders are used to take a sample of the gas to a lab chromatograph for analysis. If the method used to capture a gaseous sample in the cylinder is not optimal, an inaccurate sample could be obtained. For this reason, the need for proper sampling systems and portable or online instrumentation has increased. With modern instrumentation, the analysis of the gas is taking place in the field without transferring the sample to a laboratory. In this article we will restrict our discussions to gas chromatography (Refs. 2,6,8) used to measure the composition and heating value of the natural gas. WHAT IS GAS CHROMATOGRAPHY? Gas chromatography, color writing, is a technique used to separate the components of a gas sample for analysis. After separation, the quantity, or mole %, of each component is accurately measured. Once the gas composition has been determined, the heating value, relative density, and other characteristics of the sample are calculated. Typical natural gas components are nitrogen, carbon dioxide, methane, ethane, propane, iso and n-butane, neo, iso, and n-pentane, and C 6 +. The industry requirement for chromatograph repeatability is ±0.05%, commonly expressed as ±0.50 BTU per 1000 Scf of gas. The accuracy of any instrument does not exceed the accuracy of the associated calibration standard. For this reason, the accuracy of the chromatograph relies on the accuracy of the calibration gas blend. All chromatographs include the following major components: Sampling System Column Oven Packed or Capillary Columns Sample Loop Column Switching Valve(s) Detector and associated electronics Carrier Gas Computer used for data acquisition, calibration, communication The specific details and instrument performance vary with manufacturer but the overall operational concepts are nearly identical. Sampling System The chromatograph sampling system is the most important part of the instrument. The sampling system must extract a representative gaseous sample from the pipeline, reduce the pressure, filter the sample, prevent sample condensation, and efficiently deliver the sample to the chromatograph. More measurement inaccuracies occur due to improper sampling systems than any other reason. To prevent sampling errors one must study the phase behavior of the gas to determine the temperatures and pressures at which condensation may occur and design accordingly. Oven The chromatograph oven usually contains the columns, the column switching valve(s), the sample loop, and the detector. The temperature of the oven is precisely controlled since the performance of the columns and detector is affected by changes in temperature. Columns The packed columns are usually 1-10 meter lengths of 4 or 8 mm stainless steel tubing that is filled with coated solid material. The choice of the column packing material depends on the sample being analyzed and the desired component separations. After sample injection, the time PAGE 102

2 required for each component to emerge, or elute, from the columns is proportional to the molecular weight and boiling point of the component. Capillary columns are also used in lab chromatographs. The advantage of capillary columns is speed of component separation; the disadvantages are column fragility, maintenance, and cost. The difference in the individual component transfer rates through the columns causes the component separation and allows individual component detection. BASIC GAS CHROMATOGRAPH Valves The sample valve introduces a fixed volume of the gas sample into the chromatograph columns. For reliable operation, the valve must inject a reproducible sample as rapidly as possible. The sample volume varies with instrument but is usually less than 0.50 ml at low pressure. Refer to Figure 1. For the determination of the gas heating value, there are two chromatographic techniques used to separate the components, the backflush (BF) and extended analysis (EA) methods. The BF method is commonly used to reduce the total analysis time by backflushing the heavier components (C 6, C 7, C 8, etc..) to the detector. Since these components would pass through the long measuring column slowly, the backflushing technique reduces the analysis time without sacrificing accuracy. In this way, the C 6, C 7, and C 8 components are combined and treated as a composite, the C 6 + component. Refer to Figure 1. The EA method provides the same information as the BF method except that the C 6 + composite is separated into the individual components. The EA method, when using packed columns, usually causes long sample analysis times. When capillary columns are used, the EA method can be used effectively with great speed. There are advantages to both chromatographic methods. In most cases, the relative composition of the C 6 + components does not vary sufficiently at a specific location to require the longer analysis times or the expense of a capillary column chromatograph. Although one could assume that an extended analysis of the gas yields more reproducible or accurate results, this is frequently not the case. The technique of measuring a composite C 6 + component instead of separating C 6 + into the myriad of sub-components has merit. Not only does backflushing the sample reduce the analysis time and simplify the design of the instrument, but it provides excellent results. The foundation for this observation is that an extended analysis of C 6 + usually produces over 50 components, all of which have associated measurement inaccuracies. The cumulative error in terms of BTU/Scf usually rivals the results from a chromatograph that uses the backflush technique. One of the uses of the extended analysis result at a given location is to allow refinement of the value for the C 6 + mole% heating value. Carrier Gas A carrier gas, typically helium, is used to transport the gas sample through the columns. The carrier gas must be pure (99.995%) or a degradation of the sensitivity of the detector will occur. Computer An internal computer provides all of the instrument control and the integration of the chromatogram peak areas. Once the compositional analysis of the sample is completed, the computer calculates the heating value, relative density, and other parameters of the gas using ASTM, GPA, AGA, and ISO methods and component heating value data (Refs. 1,3,5,9). The computer also handles the data communication and storage needs that are frequently used as a part of SCADA systems. FIGURE 1. Typical chromatography valve PAGE 103

3 A PORTABLE INSTRUMENT FIGURE 2. A portable chromatograph, also referred to as a BTU Analyzer, is a light-weight, rugged, chromatograph (Figure 2) that is used to analyze a natural gas sample onsite and determine the gas composition and heating value. Portable chromatographs have internal sampling system components that allow the chromatograph to be connected directly to the gas pipeline sample probe. In all other respects, a portable chromatograph provides all of the measurement features available in a single stream online instrument. The analysis results are displayed using a digital display and stored in memory until they can be printed at a later time (see report at the end of this article). WHAT IS AN ONLINE CHROMATOGRAPH? An online chromatograph is basically the same as a portable chromatograph with the main differences listed as follows (Refer to Figure 3.): FIGURE 3. Typical Online Chromatograph System Automatic analysis cycling Automatic calibration Automatic sample selection Remote control and data communication features Current loop (4-20 ma) outputs Parameter alarm features These instruments and related sample switching systems are packaged in environmental resistant enclosures (NEMA 4X or equivalent) to allow placement of the instrument at pipeline locations. Many times the instrument is located outdoors. The analysis results may be printed locally and the data is automatically transferred to a central computer system using a data acquisition system known as a SCADA system. Advanced systems provide features that allow full remote control and diagnostics for the chromatograph. In most cases, the portable or online chromatograph and related equipment are located in Class I, Division 2, Group C & D hazardous environments. For this reason, all equipment should be approved for use in these environments by a standards organization such as Factory Mutual Research (FMRC), Canadian Standards Association (CSA), Underwriters Laboratory (UL), or equivalent European Community recognized laboratories (CENELEC). SAMPLING SYSTEMS As stated previously, the importance of an efficient and maintained sampling system can not be overemphasized. The sampling system for the chromatograph must perform the following functions: Extract the sample from the pipeline Resist the corrosive nature of the gas Reduce the pressure of the sample without liquid condensation Filter the sample Transport the sample to the chromatograph Control the sample flow rate and provide appropriate sample venting The sample probe must not be located near flow disruptions (orifice plate, elbow, etc.) that will induce flow turbulence and cause inaccurate samples to be collected. The end of the sample probe must be located in the center 1/3 of the pipeline. All components of the sampling system must be made from 300 series stainless steel to resist possible corrosive attack from H 2 S, CO 2, and water. The sample pressure must be reduced using a heated sample regulator or an insertion probe/regulator if the phase behavior of the gas indicates condensation of liquids due to the Joule-Thomson effect or low ambient temperatures. Normally, a coalescing or membrane filter is used to remove particulates and entrained liquids from the sample. For this reason, the sampling system and related lines may require heating to prevent liquid condensation problems. If the ambient temperature approaches the gas hydrocarbon dew-point temperature, the sampling system must be heated or buried. The sampling system design should provide a shut-in valve, a sample flow control valve, bypass loops, and venting. The specific requirements will vary with the choice of components and the chromatograph requirements. Refer to Figure 4. PAGE 104

4 PROBE/REGULATOR MEMBRANE FILTER GLYSORB FILTER peak areas to the known component concentration in the calibration gas. A constant, or response factor (RF), is determined for each component in the calibration gas. The following equation describes the response factor calculation: TO SAFE AREA VENT TO ANALYZER RF = Comp. Concentration Peak area FIGURE 4. Typical Sampling System SAMPLE SWITCHING An online chromatograph is used to analyze samples from different pipelines using a sample switching system. Each pipeline must have a sample probe, regulator, filter, and related hardware. The design of the sample switching system varies with manufacturer but the principle of operation does not change. The sample switching system must provide a sample from each pipeline sample probe to the chromatograph without any mixing or carryover from the previous sample. The sample switching system must also provide a connection for the chromatograph calibration gas and a connection for a sample cylinder if required. The computer in the chromatograph can be programmed to sequence the sample solenoid valves as required. Typical components used in a sample switching system are intrinsically safe 3-way solenoid valves. These solenoid valves may be used in the double block and bleed (DBB) configuration. The DBB method prevents sample mixing due to the possibility of an internally leaking solenoid valve. CALIBRATION The accuracy of the results from a chromatograph is totally dependent on the accuracy of a certified blend of gases known as calibration gas. A calibration gas blend may be obtained from equipment suppliers or sources with NIST traceable gravimetric gas mixing equipment. A certified analysis report is included with each bottle of calibration gas. Most analysis reports will also include a statement of heating value and relative density at standard temperature and pressure. Ideally, these sources should provide proof of traceability to a primary standard. The concentration of each component in the calibration gas should be similar to the pipeline gas being measured. If the temperature of the calibration gas blend drops below the hydrocarbon dew-point, condensation of the heavier components in the calibration gas will occur. Although the actual minimum temperature will depend on the composition and pressure of the blend, as a general rule, the temperature of the gas should never be allowed to drop below 50 F. In some areas, with certain blends, the bottle may require a heater. A chromatograph is calibrated by analyzing the calibration gas and relating the individual component Once the chromatograph has been calibrated, the individual response factors are used to determine unknown component concentrations from measured peak areas. The frequency of calibration of a chromatograph will depend on manufacturer recommendations. Advanced online chromatographs are programmed to automatically recalibrate at predetermined time intervals. In many cases, custody transfer contracts require chromatograph recalibration every 24 hours although most chromatographs do not require this frequency of calibration. In many cases, calibration every 30 days is adequate. HEATING VALUE CALCULATION Once the chromatograph has determined the pipeline gas composition, the heating value, relative density, and compressibility values are calculated using the internal computer. Each pure component in the natural gas has a known heating value and density. These heating values are obtained from tables published in current GPA, AGA, or ISO standards. The heating value may be determined from the gas compositional analysis using the following equations: where, Heating Value = H ideal = x ih i i+l H ideal x i H i Z n H ideal Z = Ideal heating value = Component concentration = Component heating value/unit volume = Compressibility of sample Similarly, the relative density may be determined from the gas compositional analysis using the following equations: Heating Value = G ideal = x ig i i=l n H ideal Z PAGE 105

5 where, G ideal = Ideal relative density x i = Component concentration G i = Component relative density Z a = Compressibility of air Z = Compressibility of sample The compressibility of the gas may be determined using an older method defined as NX-19 or using the more recent AGA 8 method (Ref. 5). The AGA 8 method uses the composition of the gas in the determination of gas compressibility. Due to the complexity of the calculation, a relatively powerful computer is required to use the current AGA 8 method. DATA COMMUNICATION Online chromatographs must be able to transmit the analysis data to a host computer, flow computer, or other process using serial (Modbus) and current loop (4-20 ma) interfaces. Advanced chromatographs may be remotely controlled and data acquired using serial (RS232 or RS485) communication and appropriate software (Figure 5). The current loop (4-20 ma) outputs from the online chromatograph may also be used to transmit defined data values to a host computer, flow computer, control room, process, or SCADA system. The data gathering system design will vary with installation and instrument capabilities. Since the heating value is expressed as kj/m 3 or BTU/ Scf, the volume of the gas must be considered. Unfortunately, there is not wide agreement for the values of standard pressure and temperature. In the U.S.A., the standard or base pressure standard may vary from to Psia depending on location. In Europe, the standard temperature standard varies. Most chromatograph computers are configurable for different standard temperatures and pressures. For this reason, before comparing heating value results from different chromatograph systems, verify that the standard conditions at which the gas volume is calculated are the same. The heating value of natural gas is reported as gross, or net values: Gross heating values The gross gas heating values are reported as dry or saturated values. The dry result assumes that the gas contained no water prior to combustion. The saturated result assumes the gas is saturated with moisture at standard temperature and pressure prior to combustion. The saturated result accounts for the difference in energy released during a complete and ideal combustion of the gas that includes the heating value (enthalpy of condensation) of water. All water formed by the reaction condenses to a liquid (Ref. 7). Refer to the sample analysis report at the end of this article for a comparison of the gross dry and saturated results. Net heating value The net heating value represents the energy released from the total, ideal combustion of the gas at standard temperature and pressure where all the water formed by the reaction remains in the vapor state. The condensation of excess water vapor does not contribute to the heating value (Ref. 7). In both cases, gross or net values, the difference between the ideal and corrected values referenced on typical chromatograph reports is due to the effect of compressibility of the gas. TROUBLESHOOTING The following items must be considered as a part of operating and maintaining a chromatograph. Carrier Gas: Poor quality carrier gas or leaks in the carrier gas system are responsible for more chromatograph problems than any other cause. Most chromatographs used for natural gas analysis require zero-grade (99.995%) helium as a carrier gas. Problems with a chromatograph may occur when a depleted carrier gas bottle is replaced. The problems may be the result of a carrier gas leak that entrains air, low purity helium, contaminants, or air trapped in the carrier gas tubing and regulator. Air in the carrier gas will change the chromatogram baseline and may oxidize the filaments in the detector, causing premature detector failure or loss of sensitivity and stability. PAGE 106

6 Temperature: All chromatographs have an internal oven that is maintained at a very constant temperature. For this reason, optimum accuracy will be achieved if wide variations in ambient temperatures are avoided. As a general rule, a temperature controlled environment (±20 F) will improve the reliability and longevity of the instrument. Sampling System: If the sampling system is deficient, the analysis results will be unpredictable and the chromatograph may be contaminated by samples that contain liquids (heavy hydrocarbons, water, glycol, etc.). In most instances, to repair the chromatograph involves disassembly and a thorough cleaning. The chromatograph columns may also require replacement if they are contaminated. Vents: All chromatographs have vent ports for the carrier gas and sample. Some chromatographs require a constant flow of the sample through the sample loop. Other chromatographs require brief sample flow times. In any case, proper venting of the sample to a safe area is important. Major variations in the backpressure of the venting system may cause operational problems with the chromatograph since the sample loop volume used by the chromatograph is dependent on the sample pressure. The sample loop pressure will be affected by changes in vent backpressure, atmospheric pressure, or a change in the flow rate of the sample through the sample loop. Minor changes in the sample volume do not cause analysis inaccuracy since the effect of sample volume is removed by normalizing the data. Flow Adjustments: The carrier gas flow rates may require periodic adjustments. Typically, these flow rates measure less than 20 ml/min and adjustments require the use of a low flow measurement instrument. Usually, there are two adjustments of the carrier gas flow rates, the measuring and reference adjustments. Although the specific requirements will vary with equipment, the measuring and reference flow rates are adjusted to be equal. Changes in these flow rates may cause misidentification of components, changes in sample analysis time, valve timing shifts, and calibration changes. Once a chromatograph has been installed and calibrated, changes in the carrier gas flow rates should be avoided. Valve Timing and Integrator Gates: Some chromatographs require valve timing and integrator gate adjustments. These instruments can be more difficult to maintain due to the critical nature of these adjustments and the multiple valves. The valve timing adjustments define the time intervals for the column valve(s) switching. The integrator gates enable or inhibit the integrator at the anticipated start or end of a peak. These adjustments must be made after the carrier gas flow rate and oven temperature is set. More recent chromatograph designs provide a single valve time adjustment and the integrator gates are assigned automatically during calibration. During the troubleshooting of a chromatograph, a strip chart recorder or a diagnostic report containing peak data is commonly used. The shape of the peaks and relative component elution times and valve switches are used to diagnose problems and make adjustments. Comparing the reports with past records usually provides insights into problems. Analyzing a known gas blend such as calibration gas is useful when evaluating the performance and accuracy of the instrument. CONCLUSION Gas chromatography is in common use for the direct measurement of the heating value of natural gas. The advantages of gas chromatography include improved repeatability and accuracy, and the ability to measure the gas composition directly. Once the gas composition is known, the gas heating values, relative density, compressibility, and liquid volumes may be determined. Online or portable chromatographs may be used in the Class I, Division 2, Group C and D environments that are found near gas pipeline installations. Chromatograph designs that have been approved by FMRC, CSA, UL, or CENELEC are usually the best choice. Gas chromatographs are available as portable or online instruments. The portable version of the instrument is used to make onsite gas composition measurements, eliminating the need for sample cylinders and possible sampling problems. The analysis results are stored internally and communicate with SCADA systems using the industry standard Modbus protocol or printed using common printers. Portable chromatographs must be easy to use, reliable, and designed to withstand rough treatment. Potential problems with gas chromatography may be avoided through adequate attention to the sampling system design. Advanced systems have state-of-the-art software and data communication features. It is not unusual to have complete control of a remote online chromatograph by using a computer, modems, and communication software. The methods and heating values associated with gas chromatography governed by AGA, GPA, ASTM, and ISO are very similar with the principal differences being the values of standard temperature and pressure (Refs. 1,3,5,9). Periodically, updated heating values are published causing a need to adjustments to the constants used in the chromatographs. The choice of a specific online or portable chromatograph must be made on the basis of performance, reliability, manufacturer support, ease of use, and cost. The performance issue usually involves a decision on repeatability, C 6 + backflush analysis, extended analysis, PAGE 107

7 and analysis cycle time. The reliability issue is paramount since the instrument may be used continuously 24 hours per day for years and downtime and repairs are expensive. The ease of use of the instrument is important since it reduces the amount of operator training required to use the instrument efficiently and reduces the chance of operator error. Ideally, the gas chromatograph (BTU Analyzer) operator will not require extensive training in chromatography. The cost of the chromatograph systems varies with product complexity. GLOSSARY Gas composition: The chemical content of the natural gas. A natural gas will contain varying amounts of methane, ethane, propane, nitrogen, carbon dioxide and other components. Heating value: The energy content of the natural gas. The units are BTU/Scf or kj/m 3 and are reported as Net, Dry, or Saturated values depending on the natural gas moisture content. Relative density (Specific gravity): The ratio of the density of the gas to the density of dry air, with normal CO 2 content, at the same temperature and pressure. BTU (British Thermal Unit): A measure of energy. A BTU is the defined International Tables British thermal unit. The term is defined as the quantity of heat required to raise the temperature of one pound of water at, or near its point of maximum density. 1 Btu/lbm = J/g 1 lbm = g Standard temperature and pressure (STP): The standard (reference) temperature and pressure at which gas volumes are calculated. In the U.S.A., the base temperature is 60?F and the base pressure will vary (14.65, , Psia) depending on local standards. When comparing gas volumes, verify that the base conditions are the same. Joule-Thomson Effect: The cooling that occurs when a natural gas at high pressure is throttled through an orifice to a lower pressure. The cooling, in many cases, will cause condensation of liquids (water and liquid hydrocarbons). Dew-point: The dew-point represents the temperature and pressure at which liquid water or hydrocarbons condense from a gas. The dewpoints for water and hydrocarbons in natural gas exist at different temperatures and pressures. The relationship between dew-point (water) and moisture content may be obtained from ASTM Method D1142. REFERENCES 1. GPA Table of Physical Constants of Paraffin Hydrocarbons and Other Components of Natural Gas. 2. GPA Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography. 3. GPA Calculation of Gross Heating Value, Relative Density and Compressibility Factor for Natural Gas Mixtures from Compositional Analysis (except AGA 8 is used for supercompressibility). 4. GPA Method for Converting Mass of Natural Gas Liquids and Vapors to Equivalent Liquid Volumes 5. AGA Transmission Measurement Committee Report No.8 Compressibility and Supercompressibility for Natural Gas and Other Hydrocarbon Gases. 6. ASTM D-1945 (Volume 5.05, Gaseous Fuels; Coal and Coke, 1990) - Standard Method for Analysis of Natural Gas by Gas Chromatography. 7. ASTM D (Volume 5.05, Gaseous Fuels; Coal and Coke, 1992) - Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density (Specific Gravity) of Gaseous Fuels 8. ISO 6568, Natural gas - Simple analysis by gas chromatography, ISO 6976, Natural gas - Calculation of calorific value, density and relative density, 1983 TYPICAL ANALYSIS REPORT The following report is representative of the information that is typically provided by an online chromatograph used to determine the composition, heating value, and relative density of a natural gas sample. BTU Analyzer Test time: Mar :11 Calibration #: 3 Test #:165 Stream#:1 Location No Standard/Dry Analysis Saturated /Wet Analysis Mole% BTU* R.Den.* GPM** Mole% BTU* R.Den.* Methane Ethane Propane i-butane n-butane i-pentane n-pentane (C6+) Moisture Nitrogen (CO2) Total *Uncorrected for compressibility at 60 F & PSIA **Liquid volume reported at 60 F Standard/Dry Analysis Saturated/Wet Analysis Molar Mass = Relative Density = Compressibility Factor = Heating Value = Btu/lb Btu/lb Heating Value = Btu/CF Btu/CF Absolute Gas Density = lbm/1000cf lbm/1000cf Unnormalized Total: Last Calibrated with Calgas of Btu/CF Jan :31 C6+ Last Update: Mar :11 C6+ BTU/CF , C6+ lbm/gal , and C6+ Mol.Wt PAGE 108