Heating and cooling is used in many of the processes that make up the production and refining of oil and gas, including fractionation and adsorption. Extracting natural gas liquids (NGLs) like ethane and propane from natural gas streams is no exception. In particular, the use of the principles of cryogenics — the study and use of low-temperature physics — proves vital to NGL processing facilities.

Prior to cryogenic processing, cascaded refrigeration and Joule-Thomson pressure manipulation were used to separate out ethane and other NGLs. A need for greater efficiency in processing brought about the turbo-expander process, which uses high-speed centrifugation to expand gas to temperatures nearing cryogenic levels of -150°F, increasing NGL yields further and generating additional compression power for moving gas ready for sale. This application originates from Ortloff Corporation’s gas-subcooled process (GSP), which has evolved since the ’70s to become even more efficient today. Combining the turbo-expanded GSP process with auxiliary refrigeration and a molecular sieve adsorption unit, producers can now feasibly recover 90+ percent of ethane and 99 percent of methane and propane under optimal conditions.

After running through a gas-liquid separator, GSP essentially expands a gas without changing thermodynamic entropy by pushing the gas stream through a high-speed turbine, lowering the gas stream temperature anywhere from -130°F to -150°F before entering into a low-temperature distillation column called a demethanizer. The NGLs drop out of the column eventually, helping to cool the incoming gas stream before being stored; the distribution-quality gas then exits the top of the column to be expanded and compressed with a little help from the turbo-expander. Auxiliary refrigeration may also be applied to gas streams rich in NGL components. Other custom variants of this process exist, but in general, many producers continue to use this sort of cryogenic cooling for more efficient NGL separations and recoveries.