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Carbon Dioxide Measurement in Natural Gas

Carbon Dioxide Measurement in Natural Gas
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Carbon Dioxide Measurement in Natural Gas - 1

Application Note Carbon Dioxide (CO2) Measurement in Natural Gas Application Concentrations of carbon dioxide (CO2) across gas pipelines and distribution networks is strictly enforced and kept at low levels. If CO2 concentrations are higher than desired, removal techniques such as amine washers, polymeric membranes, or pressure swing absorptions (PSAs) are installed. To monitor CO2 levels, the Rosemount Analytical X-STREAM flameproof analyzer can be equipped to provide continuous analysis of CO2 for natural gas applications. If CO2 has to be measured in the percent range, the error from cross interferences by the background gases is negligible. Calibration can be performed with the mean background concentration of the natural gas components. Even calibration in nitrogen might be possible. For ppm CO2 ranges, calibration with mixed background gases or in a nitrogen background is not possible. Varying gas composition in natural gas would lead to varying errors in CO2 measurement, which are too high for ppm measurement ranges. Background Natural gas mainly consist of methane (CH4 70–90 %). Other components are ethane (C2H6), propane (C3H8), butane (C4H10) and higher hydrocarbons (up to 20 %). Carbon dioxide (CO2) can go from several ppm up to 8 %. Further components are nitrogen (N2), hydrogen sulfide (H2S) and noble gases as helium (He), argon (Ar), and neon (Ne). Solution for ppm CO2 Measurement in Natural Gas To overcome the problem of varying background in natural gas, a special gas analyzer setup with a CO2 absorbing agent can be used. Pipeline transmission legislation sets limits for the CO2 con ent of t natural gas. Therefore, CO2 has to be measured and, if necessary, reduced by technical processes. CO2 measurement in natural gas can be done with complex gas chromatographs (GC), as well as with optical non-disper ive infrared s (NDIR) measurement, which allows a continuous measurement. Gas Analyzer Application Methane, ethane, propane and other hydrocarbons in natural gas are influencing the CO2 measurement. This cross interfer nce is low for e methane (approx. 1:700), but larger for ethane (approx. 1:100) and higher hydrocarbons. Natural Gas Composition In an external sample handling system the sample gas stream is split and the CO2 in one stream is removed with a scrubber. This stream is flowing through the reference side. The other stream which still contains the CO2 is applied to the measure ent side of the analyzer m cell. With this configuration, variations in the background of natu ral gas will affect both sides of the analysis cell and will therefore be cancelled out. The external sample handling system is shown in Figure 1. The left vessel contains the CO2 absorbing material, whereas the right vessel is a blank vessel filled with glass beads to equalize the flow. Figure 1 - External Sample Handling System

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Carbon Dioxide Measurement in Natural Gas - 2

System Configuration CO2 measurement for natural gas applications often takes place in hazardous areas and therefore an X-STREAM flameproof analyzer by Rosemount Analytical (Figure 2) is recommended. Depending of the application, an X-STREAM general purpose housing may be used instead as long as the analyzer is configured properly and meets the safety requirements specified. Figure 3 below shows a flow diagram of the recommended analyzer system. When measuring sample gas, V3 is open and the gas stream is divided into unscrubbed and scrubbed (CO2 removed) gas streams. The unscrubbed gas...

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Carbon Dioxide Measurement in Natural Gas - 3

Emerson Process Management Rosemount Analytical Analytical Center of Excellence Emerson Process Management Rosemount Analytical Analytical Center of Excellence 10241 West Little York, Suite 200 MIDDLE EAST AND AFRICA Emerson Process Management Jebel AN Free Zone Dubai, United Arab Emirates, P.O. Box 17033 Emerson Process Management Asia Pacific Private Limited ©2013 Emerson Process Management. All rights reserved. The Emerson logo is a trademark and service mark of Emerson Electric Co. Rosemount Analytical is a mark of one of the Emerson Process Management family of companies. All other...

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