OPERATIONS OF ON-LINE CHROMATOGRAPHS Paul E. Kizer Applied Automation, Inc. Pawhuska Road, Bartlesville, Oklahoma, 74005
INTRODUCTION Gas Marketing or Gas Brokering organizations are putting additional demands on the gas measurement systems. On-line Chromatography has at least two definitions When a company can warrant a gas quality, that today. The most common is that the Chromatograph, company can have a leg up on the competition to sell commonly known as a “GC,” is “on-line” when it extracts gas to a particular market. The company that has gas a sample from a continuously flowing line, injects this composition and BTU information via remote telemetry sample and analyzes it for composition, and then will have a competitive advantage over the one who is perhaps calculates BTU, SG, Relative Density, and/or “just guessing.” Wobbe index or other parameters. This data is used on site or at a remote location to calculate volumes for custody transfer of the natural gas. This is the definition that will be used in this paper. The other definition of “on-line” is that the Chromatograph data is fed or downloaded to a flow computer(s) or RTU(s) on the flowing stream that was analyzed at the measurement site. This definition is illustrated in picture, Fig. 1 and the diagram in Fig. 2, seen below. Most gas composition analysis data, however, can be applied to more flow measurement sites than just the one where the gas is extracted from the flowing stream. This is because the gas stream will not change in composition unless there is another source of gas or there is drastic change in temperature or pressure. Some companies use what they call “Zoning” to assign a particular composition or BTU to a set of flow computers. With either definition, the gas analysis information must be applied to the gas FIGURE 1. Picture of a multi-tube meter site with EFM and analysis volume somewhere to produce the energy amount that the meter station is flowing. After all, what the natural gas industry is selling is energy, not volume. When natural To summarize, on-line gas chromatography is today gas is burned, the following chemical reactions are the being chosen more often than any other analytical primary ones that take place: method in the natural gas industry for monitoring of gas quality for the following reasons: ➔ CH4 + 2O2 CO2 + 2H2O + Heat (1010 BTU/CF) 2C H + 7O ➔ 4 CO + 6 H 0 + Heat (1769 BTU/CF) • The calculations of the gas volumes in modern 2 6 2 2 2 5,10,13 ➔ electronic flow meters requires not only BTU C3H8 + 5O2 3CO2 + 4H2O + Heat (2516 BTU/CF) information, but specific gravity, Mol. % CO2 and Mol. % N as well. Even today, most natural gas custody transfer contracts 2 2 • The current AGA-8 compressibility calculation use MMBTU rather than MCF as the accounting units equations also require a complete analysis for of gas transfer. A BTU is the acronym for British Thermal 16 the “detailed method” of calculation of FPV . Unit. One BTU is the quantity of heat required to raise • With remote telemetry, Gas Marketers have the temperature of one pound of water from 58.5°F to 3 nearly instantaneous gas quality information at 59.5°F (about 1055.056 joules (SI.)) The higher the BTU their finger tips. value, the more energy can be obtained from burning • While not nearly as instantaneous as the gas. It doesn’t take as many cubic feet of gas to calorimeters, GCs with modern micro-packed heat the hot water tank if the gas is 1090 BTU instead of columns are providing faster analysis times. 940 BTU per SCF. The BTU, then, is the measure of the • It is well known that the installation requirements actual amount of heat energy contained in a cubic foot 2 for Chromatographs are less stringent than (CF) of this natural gas. An MMBTU is calculated by: calorimetric methods. BTU/CF * MMCF = MMBTU
2001 PROCEEDINGS PAGE 69 AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY LABORATORY VERSUS ON-LINE GCs is that the gas pressure is reduced to 15 PSIG at the probe for transport to the analyzer. This is to drop the There are two general classes of chromatography; effective dew point of the sampled gas in order to prevent
laboratory chromatography and “on-line” or process the loss of any part of the heavy component (C6+) prior chromatography. The scope of this paper is limited to to injection into the analyzer, and to minimize the number on-line chromatography. The basics of each are very of molecules in the sample transport tubing (thus similar. The differences are mostly “location,” not only shortening the sample transport lag). The gas sample is the location of the GC but the location of the sample then analyzed on the site at regular intervals 24 hours a point or sampling method. Laboratory Chromatographs day, 365 days a year. The analysis data can be stored on are located in the “lab.” Because of this, they must have site for manual retrieval or it can be streamed to a remote their samples brought to them. This process of obtaining location via phone modems, radios, cellular phones and a sample in the field is called “spot” or “grab” sampling even satellite links. The chromatographic process is the or perhaps proportional (to flow or time) or composite same with both, but due to the location, the construction sampling. With spot or composite samples care is taken of the on-line GC must be very rugged. to maintain pipeline conditions. These samples of gas are contained in cylinders that must be carried to the DESCRIPTION OF A TOTAL ENERGY METERING lab. When it is time to analyze the sample, it must be STATION heated to 120°F for several hours before the analysis can be done. The diagram in Fig. 2. shows how an energy metering station might be configured. The gas flow for the metering On-line Chromatographs, Fig. 1, however, are on some station is accumulated in the primary tube flow computer flowing line. In this industry, it would be a natural gas for one, two or all three of the flow computers shown pipe line. The flowing gas is sampled continuously by on the drawing. Each flow computer independently use of a probe installed in the line. A major difference measures and calculates the gas flow on its particular when sampling for an on-line Chromatograph, however, meter tube and maintains its own data base.
Comm Link RS-485 TOTALFLOW
AIU HCIA
TOTALFLOW Proportional Valve Control Signal(s)
6713 FCU Tube No. 1
On/Off Valve Control Signal(s) TOTALFLOW
TFLocal.BUS 6410 FCU Tube No. 2
On/Off Valve Control Signal(s) TOTALFLOW 6410 FCU
Tube No. 3
FIGURE 2. Typical three tube gas orifice meter station with energy measurement.
PAGE 70 2001 PROCEEDINGS AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY The On-line Chromatograph takes a sample of the Transmitter Stream 1 Sample flowing gas from a common header and performs an Probe analysis as fast as once every three minutes. This data F5 F4 F3 F2 F1 RELIEF VALVE
PRESSURE can be accumulated as an hourly (or at least daily) Detector vents GAUGE average data base. This type of data base is required to Sample Vent Stream 1 Mtg. Brkt. provide for the analysis audit trail, to match the hourly G1
50 cc/min flow computer data base. In Fig. 2, it is stored in a stand- Purge Coil alone box called an Analyzer Interface Unit (AIU). F6 F7 HV1 Stream 1 In
At the top of each hour, the AIU down loads this 400cc/min. Purge Coil average of the Methane, Carbon Dioxide, Nitrogen Liquid Drain concentrations, the Relative Density and BTU data to each of the flow computers at the station. This allows Cal. Gas the gas calculation to be done more precisely in the flow Carrier Gas He computers and allows the digital display on the FCU to Mounting plate show actual energy flow. FIGURE 3.Diagram of Sample Conditioning System The differential pressure in the primary tube is used to calculate which of the other tubes needs to be turned This procedure is as essential for good sampling for on- on or off. The primary tube flow computer sends a line GCs as it is for spot or composite samples. command to tell the secondary units to open the control valve and start calculating flow or to close it and there- Sometimes it is useful to have a sample bypass loop to by stop calculating. reduce sample transport lag times, as is shown in Fig. 3. Note that filtering and liquid coalescing is done if needed Both the flow and analysis information can be sent out on to protect the integrity of the analyzer columns and to communication links using serial digital communication ensure a single phase sample. Sometimes double block protocols to PC based host software programs. This and bleed sample switching is used to prevent cross allows the gas marketers to have not only instant quantity contamination by the standard or other sample streams.8. but also instant quality information as well. Gas Chromatograph Oven: The primary tube flow computer software can either The GC oven provides an isothermal environment for calculate the needed flow rate using the nominations the elements indicated in Fig. 4 below. Once the natural volume from the gas sales contract for this station, or it gas enters the oven, a precise sample is measured by a can be computed by a host software program like WinCCU sample inject valve. This valve must work very repeatably (In Fig. 2, there is remote telemetry, so this is very to obtain the needed ±1 BTU or better precision5. The reasonable.) or the flow rate can be calculated by hand. Chromatograph controller then blocks the sample flow Once this is done, the new flow or energy rate set point and allows it to equilibrate to atmospheric pressure. This can be downloaded remotely to the flow computer which ensures even greater sample size precision. This will calculate a signal for the station total rate control valve. atmospheric relief function is called “sample shut off” or The instrumentation shown in the drawing allows for either SSO. The sample is then swept on to a set of carefully pressure control (with differential pressure overrides), selected chromatographic columns by a stream of helium differential pressure control (with static pressure overrides), gas for sequential separation of each component in the or rate control on a volume or BTU rate basis (with both natural gas mixture. differential and static pressure overrides). Since temperature control is critical for most A plug-in or stand alone RTU can be added to the flow Chromatograph applications there is usually a mandrel computer if 4-20 mA analog signals are needed for type heater (airless oven) to maintain this temperature communicating to other devices on the station. RTUs of control. The oven components, such as valves, columns, this type can usually perform custom Ladder Logic and detectors, are usually attached to this mandrel. The control and math for later analysis.
HOW CHROMATOGRAPHS WORK
The theory of operation of a modern microprocessor controlled on-line Chromatograph has been thoroughly discussed in previous papers.4,7,8,9,13,14,15,16 However, it would be helpful to review briefly how a GC works.
Sample Conditioning System: To obtain a representative sample of gas from a pipe line, a probe should be inserted into the middle third of FIGURE 4. Diagram of GC Oven the flowing gas stream in the pipeline.6
2001 PROCEEDINGS PAGE 71 AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY IC4 NC4 45000 Start Cycle 400000
Valve 350000 35000 C2 & Reverse Lighter 300000 25000 C3 250000
200000 Neo Start 15000 Cycle IC5 150000 C1 NC5 N2 Valve CO2 Reverse 5000 100000 C6+ C4+ C3+ C2 50000 -5000 0 123456789 10 11 12 13 14 15 -50000 FIGURE 5b. Example Chromatogram from Det. 2 12345678910 11 12 13 14 15 FIGURE 5a. Example Chromatogram from Det. 1 +1 BTU or better at 1000 BTU/CF. Sometimes a cross temperature of these components is maintained by correlation may be used, if needed, to correct areas from conduction through metal to metal contact. As each one detector set to the other on dual train systems. This column does its specified separation job, it is switched may not be needed if the sample valves are reliable, out of the helium stream. The separated components injection of the sample occurs on the same sample at make their way through the thermal conductivity cell the same time, and adequate calibration is done. The detector where they can be measured. In some GC microprocessor may also calculate the specific Chromatograph systems, two separate column trains are gravity, compressibility, condensable liquids, wobbie used to speed up the analysis cycle time. There would index, as well as the BTU. be two sample valves, two sets of Chromatographic columns and two sets of detectors. A example of a HOW THE BTU IS CALCULATED detector output from just such a dual column train is shown in Figures 5a and 5b. The methods for calculation of the natural gas BTU from analysis data are well documented in GPA 2172-96 These peaks are “gated” and the area under each peak and example calculations have been published integrated in order to be quantified14,15. All the timing previously.2,3,4,10,11,13,14,15 The results of these calculations functions, such as sample inject, column reverse, auto may then be made available to the “outside” world. An calibration and results logging are done by the RS-232, RS 485, and/or an RS 422 serial digital output microprocessor controller board. In addition, this device can log results to a variety of devices such as FCUs, will detect each peak, open and then close integration RTUs, AIUs, or DCS systems. This then allows gates, identify the peak, and assign the correct response composition data for not only MMBTU computation but factor to it. During the calibration run, the Response also AGA-8 compressibility calculations. Factor (RF) for each component is calculated. The formula is as follows: The C6 and heavier component values can be used in the calculation of the detailed method of the now current RF = Concentration/Peak Area AGA-8 compressibility calculation procedure16. They are also of concern when calculating the heating value, BTU, In some systems, RF is inverted, but both are valid of the natural gas ( discussed previously ). This is because calculations. In the case above, the unknown the molar heat content rises dramatically with rising
concentration is calculated by: carbon number ( C6, C7, C8, etc. ). Certain Industry standards pertaining to the calculation of gas flow and Sample concentration = Peak Area X RF heating value from the AGA and API allow the use of individual concentration values for the heavier
This function is performed on each peak and hence the components of natural gas such as C6, C7, C8, and C9. controller calculates the mole fraction of each These are sometimes called “Pseudo Components” component. Since these peaks are quite arbitrary in size because each represents a family of components rather due to variability of equipment setup and environment, than one discrete component (or many “peaks” rather high quality standards should be used to calibrate these than one “peak”). Please note the use of the word “allow” instruments. Calibration assures the most accurate as opposed to the word “require”. Using C6+ is also determination of mole fraction possible. Indeed, all BTU allowed and is perhaps more precise. While it may seem determination methods require high quality standard intuitive that “more is better” when it comes to values samples. On a BTU GC, the peaks should also be entered into the AGA-8 formula, what you see is not normalized to compensate for barometric pressure necessarily what you get. changes (which cause sample inject size variations). This allows the repeatability of the calculation to be as low as One very well accepted way to adjust for the variable
component content of the C6+ fraction would be to have
PAGE 72 2001 PROCEEDINGS AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY a laboratory do what is called an “extended analysis” from time to time to adjust the BTU contribution characteristics of this C6+ fraction. For a complete discussion of this method in developing a custom BTU factor see references.14,15 However, the calculation below briefly outlines how this composite or pseudo C6+ BTU value was obtained from an actual extended analysis as explained in a previous paper15:
BTU/CF = X1 * C1 + X2 * C2 + X3 * C3 ...... X11 * C11 Where:
X1 = Mole fraction of C1 or methane
X2 = Mole fraction of C2 or ethane FIGURE 6. Set Up Screen for Accessing Remote GC X3 = Mole fraction of C3 or propane “ “ heat the probe and heat trace the sample transport “ “ tubing. Also, the analyzer would then require some sort of heated enclosure to prevent condensation of the liquid X11 = Mole fraction of C6 + hexanes and heavier and: hydrocarbon prior to injection. C = Molar heating value of C , or methane (1010 BTU/CF)1 1 1 Remote Monitoring: 1 C2 = Molar heating value of C2, or ethane (1769 BTU/CF) The GC must have the ability to be monitored and trouble “ “ shot from some central office. This allows the technician “ “ to carry the right parts to the field when problems occur. This means any diagnostics that can be done while on C = Molar heating value of C + or hexanes and heavier 11 6 site must be done remotely as well. 1,15 (made up of CW,CX,CY, and CZ) (~4943 BTU/CF) Fig. 6 shows a modem link to the remote GC. If there is no CW = Molar heating value of nC6, or normal hexane [4755.9 BTU/CF]1,15 phone service available, one should be able to use radios, cellular, CDPD, satellite, or any other communication means C = Molar heating value of nC , or normal heptane X 7 currently used with the flow computer data retrieval system. [5502.5 BTU/CF]1,15
CY = Molar heating value of nC8, or normal octane [6248.9 Remote Mounting: BTU/CF]1,15 In order that gas quality measurement equipment can be justified economically on smaller and more remote CZ = Molar heating value of nC9, or normal nonane [6996.5 BTU/CF]1,15 meter stations, the equipment must be weather proof and able to mount directly to the meter tube. This reduces 14,15 the cost of installation because an analyzer shelter can The “virtual” concentrations of C6, C7, C8, and C9 can be easily calculated using an extended analysis of be as much as the measurement equipment itself. An enclosure can be a “Gas Trap.” This usually upgrades the real C6+ peak for your gas. See the section titled “C6 Plus Considerations” in the above referenced papers 14,15 the hazardous area classification. Going from a Div. 2 to for a complete discussion of this. The Specific Gravity a Div. 1 hazardous area classification can add thousands and compressibility are calculated in similar ways to the of dollars to the installation costs and severely limit the method above. versatility of the equipment.
REMOTE INSTALLATION CONSIDERATIONS Remote Power: An on-line GC that is designed to be both 12Vdc and For a Chromatograph to be useful for on-line energy very low power can reduce utility costs. It also lends measurement, it must have the capability to be used in itself more readily to installations off-shore and in areas remote locations. It must also be cost effective to install, where AC power is not available. A battery powered maintain, and use in these remote locations. And it must Chromatograph that has 10 days autonomy, that will run be very reliable, since unattended operations is assumed. on 100 Watts of solar panel, and 200 amp hours of Discussions of these remote analysis considerations are: batteries in most locations in the USA is truly unique.
Gas Quality Requirements: Remote Enclosure: The natural gas itself must also meet some requirements In many cases, the standard blend bottle and the sample if the remote installation is to be successful. If the gas is transport tubing require more protection from the too wet, then an all solar installation may not work in environment than the common on-line GC does. In northern latitudes. For instance, if the gas has a 70 degree latitudes south of 33 Deg. (approximately Dallas), one dew point at 15 psi, then electricity may be required at does not need to pay too much attention to heating the the site. This electric power would be used to electrically standard or the sample transport tubing. However, for
2001 PROCEEDINGS PAGE 73 AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY REFERENCES
1. GPA Standard 2145-00 Rev.2, P. 2
2. GPA Reference Bulletin, Heating Value As A Basis For Custody Transfer of Natural Gas (1984 Draft)
3. AGA Gas Measurement Manual, Section 11A.2 Determination of Heating Value of Gas, Page 11A2.1, ff.
4. “Analysis of Natural Gas by Gas Chromatography,” ASTM D1945 P. 133 ff.
5. “Measuring Gas; Chromatography Bests Calorimetry,” Leisey, Potter, and McKoy, Oil and Gas Journal, July 1977
FIGURE 7. Pipe Mounted Heated GC Enclosure 6. “Sampling and Analysis of Natural Gas,” Schepens, Kilmer and Bernos, GPA proceedings #61 everyone else care must be taken. The enclosure shown in Fig. 7. is unique in that it can, by mounting on the 7. “Energy Measurement Utilizing On-Line meter tube, cover the sample probe, the sample transport Chromatographs,” L. N. Cox, Prod. of Inter. School tubing, and the standard blend, not to mention the GC. of Hydrocarbon Meas. #64, 1989, P469 ff. This system mounts to the pipe similar to how the old chart covers used to in the days of round charts. 8. “BTU Measurement in Natural Gas Using Process Gas Chromatography,” Kizer and Sund, Proc. of CONCLUSION Gulf Coast Meas. Soc. #24, 1989, p. 72 ff.
Gas chromatography has become the device of choice 9. “How to Measure BTU Effectively,” Staff, Gas to determine the BTU of natural gas. The additional Industries, August 1989 composition information is now demanded for the new AGA gas volume calculations. Sampling system design 10. “Calculations of Gross Heating Value, Relative is critical to analytical precision as analyzer design. C6+ Density and Compressibility Factor For Natural analysis is preferred for BTU precision over summing Gas Mixtures From Compositional Analysis,” C6, C7, C8, and C9. In order to have a true on-line GC, GPA Std. 2172-96 the column design must include a back-flush to the detector. This is important because first, the entire sample 11. “Gas Heating Value: What It Is and How To must be analyzed, and second, the GC must be cleaned Measure, Calculate,” D. N. McClanahan, Oil and of sample before the next analysis is started. Digital Gas Journal, Feb. 20, 1967, P. 84 ff. communication links are essential in today’s gas marketing environment. Unique features are required to 12. AGA report No. 3 (ANSI 2530)_ install on-line GCs with ±1 BTU @ 1000 BTU in remote locations. “Instantaneous” data, rather than waiting for 13. Kizer, “Natural Gas Energy Determination days for a lab analysis result, can mean the difference Review,” ISA Proceedings, 1991, p. 777 between profit and loss in a gas marketing situation. 14. “Energy Measurement Utilizing On-Line Chromatography,” Kizer, Proc. of the 69th International School of Hydrocarbon Measure- ment, 1994, p. 478 ff.
15. “Energy Measurement Using On-Line Chromatography”, Kizer, Proc. of the71st International school of Hydrocarbon Measure- ment, 1996, p. 544 ff.
16. AGA Report No. 8
PAGE 74 2001 PROCEEDINGS AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY