Natural gas producers have several methods at their disposal for extracting natural gas liquids (NGLs) from feeds. The process and equipment used largely depends on the makeup of the gas as well as economic concerns. For example, natural gas streams can at times be lean in lighter hydrocarbons like propane and butane but still contain significant quantities of heavier hydrocarbons like pentanes-plus (iso-pentane, natural gasoline, plant condensate) that can be captured without special containment. When water content needs to be controlled and roughly 60 percent or more of those heavier hydrocarbons need to be captured, producers can use the relatively simple three-tower short-cycle dry desiccant process. A separator removes liquids like water and condensate to prevent damage to the desiccate beds of the towers, with one adsorbing while the others run through heating and cooling cycles. The heaver hydrocarbon liquids fall out upon condensing while the lighter hydrocarbons escape in the regeneration stream.
When a more robust extraction process is required, traditional refrigeration or cryogenic turboexpander methods are employed. In 2011, Oil and Gas Journal reported nearly 80 percent of the world’s operating natural gas processing plants were using these extraction methods. Straight refrigeration of the natural gas stream using propane, ethane, or ammonia doesn’t require much capital, though the downside to it is unless modified, the capturing of NGL constituents isn’t all that efficient; the base system can’t get much colder than -35°F. For much more efficient NGL collection, turboexpanders can be used, though at a higher installation and maintenance cost. The turboexpander essentially expands high-pressure gas, which in turn drives a compressor. Low-pressure exhaust gas exits, being cooled significantly in the process. Designs can vary significantly, but the end result is much lower temperatures and in some cases more than 95 percent NGL capture.
Not that important today, but worth mentioning anyway, is an older NGL extraction process called refrigerated lean oil absorption. Feed gas is refrigerated using propane or something similar. The gas enters an absorber that uses a lean oil to absorb many of the hydrocarbons from the gas. A demethanizer strips the oil of methane and lighter constituents, and then a still separates the heavier NGLs from the oil, capturing most propane and nearly half the available ethane. This process is energy intensive, however, and simple external refrigeration (without the oil) can provide similar extraction results.