ABSTRACT:

A plant to liquefy a natural gas (LNG plant) is a succession of gas treating processes to bring the natural gas from its raw composition to the required quality for sales. Some impurities are undesirable because they are technically incompatible with the liquefaction process and some others as they are unacceptable with regards to LNG specification. The gas treatment upstream of the liquefaction process mainly involves acid gas removal, dehydration and mercury removal. Then, in the pre-cooling and cryogenic sections the LNG specification is achieved by extraction of natural gas liquids (NGLs) and nitrogen. Existing LNG plants were developed with feed gas containing relatively small quantities of contaminants and inerts. With the gas price increase as well as the gas demand increase, the development of fields containing a large content of contaminants and inerts (and the liquefaction of the resulting gas through an adapted LNG process) might be economically viable. Total made a field development study with simultaneously two different input feeds. One contains a low percentage (about 1%) of carbon dioxide (CO2) and nitrogen, while the second feed contains more than 40% of CO2 and more than 10% of nitrogen. This high percentage of CO2 complicates the pre-treatment process and the nitrogen content impacts the liquefaction process itself. This paper, limited to the LNG train area, will present the approach made on the process schemes for CO2 and nitrogen removal, as well as comparison with present "typical" liquefaction plants. CO2 and nitrogen rejection aspects have been studied. The nitrogen removal process was developed with Costain Oil, Gas & Process. Upstream facilities, storage and shipping matters have also been studied but are not presented in this paper

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