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Fundamentals of Gas Chromatography Class # 5170-1 John Renfrow ABB Totalflow 7051 Industrial Blvd Bartlesville, OK. 74006 John.s.renfrow@us.abb.com INTRODUCTION The objective of the analyzation of natural gas by gas chromatography is to obtain a sample from the system in question and analyze the product without changing the composition or environment. There are 3 basic systems in obtaining anatural gas analysis by gas chromatography: 1. Sampling Systems 2. Gas Chromatography 3. Calculation of data. In this paper we will discuss each system. SAMPLING SYSTEMS All sampling systems have certain requirements Sample probes are always recommended to be used and inserted into the middle 1/3 of the pipeline when taking a sample of any type. Sample probes should be 5 times the pipe diameterdownstream of any obstruction. The sample system temperature must be 20-50 degrees above the sample hydrocarbon dew point. The 3 types of sampling natural gas are: 1. Spot Sampling 2. Continuous Sampling 3. On-line Sampling with Chromatography Spot Sampling Spot sampling is the process of obtaining an instantaneous sample from the system. There are 2 types of spot sampling: 1. Constant volume cylinder2. Portable chromatography Gas Processor’s Association Method 2166 explains in detail the 8 different procedures for taking spot samples using a constant volume. It is important to consult with laboratory personnel and agree on the method of spot sampling to be used. The cleaning of spot sample cylinders is an important part of obtaining a good sample. G.P.A 2166(Appendix A) explains the proceduresfor cleaning cylinders. When transporting sample containers D.O.T. regulations should be reviewed by a company safety department to insure technicians comply with the shipping regulations. Portable chromatographs are the second type of spot sampling. The chromatograph is either taken to the point of sample or sample systems are taken to a vehicle where the instrument resides. Instead of samplingwith cylinders, insertion probes are used with tubing to remove the product from the sample point. Sample lines must be 20-50 degrees above the hydrocarbon dew point of the sample gas.

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Continuous Sampling A mechanical sampling device is used in continuous sampling. There are 2 types of continuous sampling: 1. Time proportional sampling uses a timer to actuate the mechanical sampler. 2.Proportional to flow sampling requires a sampler actuated by a flow computer or other flow calculation device.

Both proportional to flow and time proportional sampling requires an approximate estimation of the gas flow to determine the sampling intervals. The continuous sampler uses 2 types of cylinders: 1. Constant volume cylinders 2. Constant pressure cylinders Constant volume cylindersrequire a single port sample probe. Constant pressure cylinders require a dual port sample probe. The continuous sampler usually produces an average sample over a predetermined flow or time period. On-line Chromatography On-line chromatography is the use of a permanently installed gas chromatograph at the sample point of the system. The chromatograph may be installed on the system piping, in anenvironmental enclosure or an analyzer building. Probe regulators are recommended to be installed to transfer sample to the chromatograph. The pressure of the probe regulator is usually 15 PSIG. The probe regulator and calibration standard regulator should be the same outlet pressure. It is recommended to install the chromatograph as close to the sample point as possible. Regulatory codes may restrictwhere the chromatograph is installed in regard to sample point. Carrier and calibration gas cylinders and accessories are onsite at these installations. Real Time measurement is the use of gas chromatography communicating analysis data to a flow computer. The communication of data between chromatograph and flow computer enables measurement calculations to be performed in the field. Remote...