Natural gas in its liquid state, is called LNG, or liquid natural gas. Liquid Natural Gas (LNG) comprises of liquid hydrocarbons that are recovered from natural gases in gas processing plants, and in some cases, from field processing facilities. These hydrocarbons involves propane, pentanes, ethane, butane and some other heavy elements.
LNG accounts for about 4% of natural gas consumption worldwide, and is produced in dozens of large-scale liquefaction plants. It is produced by cooling natural gas to a temperature of minus 260 degrees F (minus 160 Celsius). At this temperature, natural gas becomes liquid and its volume reduces 615 times. LNG occupies 1/600th the volume of natural gas at atmospheric temperature and pressure. The gas have high energy density, which makes it useful for energy storage in double-walled, vacuum-insulated tanks.
The production process of LNG starts with, Natural Gas, being transported to the LNG Plant site as feedstock. After filtration and metering in the feedstock reception facility, the feedstock gas enters the LNG plant and is distributed among the identical liquefaction systems. Each LNG process plant consists of reception, acid gas removal, dehydration/mercaptan removal, mercury removal, gas chilling and liquefaction, refrigeration, fractionation, nitrogen rejection and sulfur recovery units
The process through which Liquefied Natural Gas is produced consists of tree main steps, namely:-
Transportation of Gas (feedstock)
The best place to install the plant is near the gas source. The gas is basically transported through pipelines or by truck and barge.
Pretreatment of Gas.
The liquefaction process requires that all components that solidify at liquefaction temperatures must be removed prior to liquefaction. This step refers to the treatment the gas requires to make it liquefiable and includes compression, filtering of solids, removal of liquids and gases that would solidify under liquefaction, and purification which is removal of non-methane gases.
And finally, Liquefaction of Gas.
The LNG Plant
There exists a vast number of natural gas liquefaction plants designs, but, all are based on the combination of heat exchange and refrigeration. The gas being liquefied, however, takes the same liquefaction path. The dry, clean gas enters a heat exchanger and exits as LNG. The capital invested in a plant and the operating cost of any liquefaction plant is based on the refrigeration techniques.
Though, Liquefied Natural Gas can also be extracted from cryogenic hydrocarbon extraction and petro-chemical processes, but it requires careful consideration at these facilities to assure the process gas is liquefiable.