Gas Sweetening Process - Problems And Remedial Measures

Abstract

Summary Gas sweetening plant of an LNG train is a vital operating unit, where the sour contents (H2S and CO2) of the feed gas are removed to make it sweet for processing downwards to produce LNG. Any improper removal of sour gases from the feed natural gas would directly affect the quality of LNG and pose operational problem in Cryogenic column, where the LNG is produced. Besides the above, any problem in the Gas sweetening plant would reduce the gas handling capacity and in turn reduce the LNG production rate, which means loss of revenue. A severe operational problem was encountered during early 2001, in ADGAS' LNG Train-3 Gas sweetening plant. Through this article ADGAS shares their operating experience and describes the evaluation of the problem, successful remedial measures adopted and benefits derived. Abu Dhabi Gas Liquefaction Company (ADGAS), as a part of ADNOC (Abu Dhabi National Oil Company) group, was formed in the year 1973 and started production in 1977 from their industrial facilities located on Das Island. Presently ADGAS operates at a capacity of around 8 million tons per year of LNG, LPG, C5+ and Liquid Sulphur as co-products. ADGAS was a pioneer in setting up the first combined LNG and LPG production facility in the Middle East. ADGAS operates three LNG Trains. The first two Trains (Train 1 &2) have been in operation since 1977, each operating at a capacity of 180 tons per hour of LNG. The third one (Train-3) was commissioned in 1994 with a capacity of 380 tons per hour of LNG. Role of ‘Gas sweetening process’ in LNG Production For producing LNG, major portion of feed natural gas, in associated and non-associated forms, is received from Abu Dhabi Marine Operating Company (ADMA OPCO), which is also a part of ADNOC group and engaged in the production of Oil and Gas and a small portion from other offshore operating Companies. The feed natural gas undergoes a series of process steps at different plants in an LNG train to produce the end product. The Gas sweetening plant (named as Plant-2) is a key process unit in the production of LNG, especially when the feed contains higher percentage of sour gases (H2S and CO2). The sour content of the feed gas varies depending upon the source of oil and gas wells. The sour gases present in the feed natural gas are removed in Gas sweetening plant and make it sweet and suitable for processing downwards to produce LNG. If the sour content exceeds the permissible limit in the sweetened gas, which would directly affect the quality of LNG product and pose operational problem in Cryogenic column, where the LNG is produced. Thus, Gas sweetening plant is a vital unit in the LNG production route. The Gas Sweetening Process in brief In the Gas Sweetening process, the sour contents present in the feed natural gas are removed by chemical absorption technique. The feed gas for train-3 is HP gas (780 psig); the sour content H2S and CO2 would be at 2.2% and 4.0% respectively. And the sweetened gas (per design) would have 5 ppm H2S and 50 ppm CO2. The H2S and CO2 gases present in feed natural gas are normally soluble in water to form acidic solutions, hence they are referred as acid gases and the Gas sweetening process is also known as Acid Gas removal plant. ADGAS' Train-3, Gas Sweetening plant is a "Benfield Hi Pure" design supplied by UOP, and is a hybrid arrangement of the basic Benfield carbonate unit and Amine technologies. The Hi pure design takes advantage of these two solvent (carbonate and amine) systems.