Liquefied natural gas (LNG) refers to natural gas that has been cooled to a temperature of approximately -161 oC and atmospheric pressure, resulting in its conversion to a liquid state. This liquefaction process reduces the volume of the gas by about 600 times, making it more economically viable for transportation across continents using specifically designed ocean vessels instead of traditional pipelines. One notable advantage of LNG is its superior environmental sustainability compared to other fossil fuels. When natural gas is burned, it emits significantly less CO2, NOx, and SOx, and produces minimal amounts of ash, dust, smoke, or particulate matter. Furthermore, in comparison to diesel engine-powered cars, the use of LNG can potentially reduce engine noise by up to 50% while providing vehicles with a range of about 1,500 kilometers.
In general, there are three main categories for LNG liquefaction technologies: mixed refrigerant technology (MR), cascade technology (Cascade), and expander-based technology (EXP). The MR process involves the use of a combination of coolants with varying boiling points. One major advantage of this method is that the cryogenic heat exchanger experiences minimal exergy loss due to the low-temperature difference between the hot natural gas stream and the refrigerant mixture. Moreover, the composition of the refrigerant can be altered to minimize energy consumption, even when there are changes in process conditions. Commonly used MR processes include single mixed refrigerant (SMR), dual-mixed refrigerant (DMR), and propane pre-cooled mixed refrigerant (C3MR). For limited-scale LNG production, SMR technology is preferred due to lower equipment and fixed costs, as well as its relatively simple process with few components. The refrigerant used in SMR processes typically consists of nitrogen and hydrocarbon, compressed through a multistage compressor. These characteristics render it an ideal choice for smaller-scale operations.
