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With increasing demand for more energy, stranded natural gas reserves that would have been marginal or unprofitable years ago are now being developed. The main challenge in today's natural gas treating plants is to remove high concentrations of carbon dioxide (and sometimes sulfur components) from sour gases produced from these remote or offshore stranded gas fields to meet stringent export specifications.
A number of methods are available for the removal of acid gases. However, membrane-based systems offer some clear advantages and opportunities for the efficient removal ofhigh fractions ofacid gases from these fields. The authors focus on the use of membrane technologies to make a large bulk cut of the acid gases, as well as to remove some mercaptans from raw natural gas. They also address hybrid membrane-amine systems, in which the amine process is used to perform the final purification.
Natural gas, which consists mainly of methane (CH4) and light hydrocarbons, may also contain acid gases, such as hydrogen sulfide (H2S) and carbon dioxide (CO2). In addition to acid gases, natural gas may comprise other sulfur contaminants, such as mercaptans, carbonyl sulfide (COS) and carbon disulfide (CS2). In addition to lowering heating value, acid gases can cause corrosion to carbon steel natural gas processing equipment and pipelines if free water is present. Combustion of these sulfur compounds produces sulfur oxide (SOx) air pollutants that must be limited to protect the environment and to prevent health problems.
CO2 is an inert gas that does not have any heating value, but it is the main source of greenhouse gas.1 Pipeline specifications may require the removal of acid gases to low levels. The added cost to remove such contaminants decreases the financial outcome of natural gas exploration and production. CO2 and H2S in the sour feed gas streams must be removed to specific levels to meet different application requirements (typically < 4 ppmv H2S and < 2% CO2 for pipeline-quality gas, and < 50 ppmv CO2 for LNG). Additionally, COS, mercaptans and other organic sulfur species that contribute to sulfur emissions must typically be removed to 30 ppm total sulfur.
A number of processes are available to remove acid gases from natural gas streams. Some are suitable for bulk acid gas removal. Others are effective for deep...