Pollution Discharge Elimination System program enforced by EPA and individual 

 states. The sources and concentrations of contaminants in cooling water ef- 

 fluent for a typical petrochemical plant are tiven in Table 20. 



Additional major cooling water problems related to petrochemical plants 

 may occur where electrical power is generated to meet the needs of the plant. 



3.9 GAS PROCESSING PLANTS 



3.9.1 Profile 



Gas processing plants are designed to remove impurities such as carbon 

 dioxide and hydrogen sulfide, and to recover valuable liquid hydrocarbons such 

 as methane, butane, and propane, from raw gas streams before they enter com- 

 mercial gas distrib ution systems. 



Natural gas passes through several stages of processing from the time it 

 is produced at the wellhead until it enters a commercial distribution system. 

 If the raw gas stream contains much oil or water, these are usually separated 

 from the gas on the production platform. The oil m ay be pumped ashore; the 

 water is treated to remove hydrocarbons and then discharged into the sea. The 

 separated gas is pumped to shore through a separate pipeline for processing 

 at a gas plant. 



At the gas plant, three alternative processes may be used to recover natu- 

 ral gas liquids from a gas stream: (1) lean oil absorption; (2) mechanical 

 refrigeration, and (3) cryogenic refrigeration. The oldest and most common is 

 the lean oil absorption process, in which the gas stream bubbles through a 

 gas-oil medium, similar to kerosene, which absorbs the natural gas liquids. 

 The natural gas liquids are then stripped from the "medium" by heating in 

 another vessel. In the mechanical refrigeration process, the gas stream 

 is cooled (typically to -40°F), which causes the liquefiable hydrocarbons to 

 condense. The cryogenic process cools the gas stream (typically to -150°F) 

 through the use of compressors and turbo-expanders, resulting in greater re- 

 covery rates for the lighter liquefied hydrocarbons such as ethane and pro- 

 pane [7]. Gas plants may utilize a single process or, more commonly, a com- 

 bination of them. 



Modern gas processing plants are highly automated. Their capacity may 

 range from 2 MMCFD to 2 BCFD. There are no standard sizes or designs for 

 gas processing plants; a plant is specifically designed for the volume and 

 composition of the gas stream it processes. 



Gas processing plants require relatively small tracts of level land, but 

 often much larger tracts are acquired to provide space for buffer zones and 

 for possible future expansion. A gas plant's principal components include 

 storage tanks, separators, processing units, fractionators, and gas trans- 

 mission lines. 



If a gas stream is "sour" (high sulfur content), a "sweetening" 

 treatment process is needed to remove the sulfur compounds (particularly 

 hydrogen sulfide gas) before the gas can be sold. The treatment may take 

 place at the gas processing plant or at a separate sulfur treatment and 

 recovery facility. If only a small volume of hydrogen sulfide is removed 

 from the natural gas, commercial sulfur recovery is unprofitable and the 

 hydrogen sulfide is flared or incinerated as a waste product. 



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