To convert the pretreated feed gas into LNG, the gas is first cooled with propane refrigerant, after which it is stripped of natural gas liquids (NGLs) via cryogenic distillation in the scrub column. The NGLs are, in turn, stripped of light gases (mostly methane) that are recycled through the main cryogenic heat exchanger (MCHE). Following NGL removal, the primary feed gas from the scrub column is also sent to the MCHE, where it is further cooled inside tube bundles, each made up of several tube circuits, by a lower-temperature mixed refrigerant that flows outside the tubes.
As the feed gas flows up the tubes, it starts condensing by transferring heat to the liquid/vapor mixed refrigerant, which warms up and vaporizes as it flows down the outside of the tubes. The heated mixed refrigerant is then cooled by ambient air, compressed and subsequently chilled by propane refrigerant in heat exchangers, where a portion of the refrigerant condenses. After separating the vapor and liquid streams of mixed refrigerant, both streams are depressurized and admitted to the MCHE to provide cooling for the conversion of methane-rich gas into LNG. The liquid mixed refrigerant stream is depressurized through a liquid expander to increase the overall process efficiency. The high-pressure LNG exiting the MCHE is depressurized through a liquid expander and delivered to the LNG storage tank at near-ambient pressure. Once in the storage tank, the LNG can be pumped through the plant piping to the dock to be loaded onto ships for export.