After the water and acid gas has been removed from the feed, the Natural Gas Liquids (NGL) are recovered due to their high value. The most common recovery method is cryogenic distillation. The distillation columns are specified as a demethanizer, deethanizer, depropanizer, and debutanizer that extract methane, ethane, propane, and butane respectively and result in a residual stream of pentane and other heavier hydrocarbons out the bottom and dry gas consisting of methane and leftover inert gas out the top of the fractionation train.
Cryogenic Distillation
The components of the process include external refrigeration, expansion, heat exchange, and fractionation. The goal of external refrigeration is to cool the gas stream using propane as the refrigerant, the saturated propane is compressed, condensed, expanded, and evaporated, as illustrated in the refrigeration module.
In expansion, the gas is cooled to lower temperatures using a Joule-Thomson (J-T) valve, or a turboexpander. A turboexpander is a centrifugal compressor that runs in reverse and performs work but J-T expansion is an isenthalpic process where no work is done by or on the gas. A turboexpander provides maximum heat removal from a system while generating work that is used to drive compressors or generators.
After the gas is expanded, it enters a heat exchanger as a third step in lowering its temperature to cryogenic levels. The types of heat exchangers used include the generic shell and tube, plate-fin exchanger, and circuit heat exchangers, described in the heat exchangers module.
Once the gas has been cooled to the required temperature, it enters the fractionation process. The first fractionation step involves a demethanizer, a stripping column used to remove the methane and nitrogen from the NGL bottoms product. A demethanizer has distinct features compared to a normal distillation column, including an increased diameter to accommodate the predominantly vapor feed to the top, so that it functions primarily as a stripping column with no condenser reflux. It may also have several side reboilers and liquid feeds.
Acknowledgments
- Sundyne Corporation
References
- Mokhatab, S.; Poe, W. A.; & Mak, J. Y. Handbook of Natural Gas Transmission and Processing. Boston: Gulf Professional Publishing, 2015, 191-217
- Kidnay, A.; Parrish, W. Fundamentals of Natural Gas Processing. Boca Raton: CRC Press, 2006
- Carroll, J. Natural Gas Hydrates – A Guide for Engineers; 3rd ed.; Elsevier, 2018.
- Engineering data book: volumes I & II: SI version; Gas Processors Suppliers Association: Tusla, OK, 2012.
Developers
- Nuramani Saiyidah Binti Ramli