©  Copyright 2013 In fact, associated hydrocarbons, known as 'natural gas liquids' (NGLs) can be very valuable by-products of natural gas  processing. NGLs include ethane, propane, butane, iso-butane, and natural gasoline. These NGLs are sold separately and have a variety of different uses; including enhancing oil recovery in oil wells, providing raw materials for oil refineries or  petrochemical plants, and as sources of energy. A Natural Gas Processing Plant While some of the needed processing can be accomplished at or near the wellhead (field processing), the complete  processing of natural gas takes place at a processing plant, usually located in a natural gas producing region. The  extracted natural gas is transported to these processing plants through a network of gathering pipelines, which are small-  diameter, low pressure pipes. A complex gathering system can consist of thousands of miles of pipes, interconnecting the  processing plant to upwards of 100 wells in the area. According to the American Gas Association's Gas Facts 2000, there  was an estimated 36,100 miles of gathering system pipelines in the U.S. in 1999.  In addition to processing done at the wellhead and at centralized processing plants, some final processing is also  sometimes accomplished at 'straddle extraction plants'. These plants are located on major pipeline systems. Although the  natural gas that arrives at these straddle extraction plants is already of pipeline quality, in certain instances there still exist  small quantities of NGLs, which are extracted at the straddle plants.  The actual practice of processing natural gas to pipeline dry gas quality levels can be quite complex, but usually involves  four main processes to remove the various impurities:     Oil and Condensate Removal     Water Removal     Separation of Natural Gas Liquids     Sulfur and Carbon Dioxide Removal In addition to the four processes above, heaters and scrubbers are installed, usually at or near the wellhead. The  scrubbers serve primarily to remove sand and other large-particle impurities. The heaters ensure that the temperature of  the gas does not drop too low. With natural gas that contains even low quantities of water, natural gas hydrates have a  tendency to form when temperatures drop. These hydrates are solid or semi-solid compounds, resembling ice like crystals.  Should these hydrates accumulate, they can impede the passage of natural gas through valves and gathering systems. To reduce the occurrence of hydrates, small natural gas-fired heating units are typically installed along the gathering pipe  wherever it is likely that hydrates may form.    Chemical Engineering & Gas Processing