Liquefaction of natural gas for plant capacities ranging from small peak shaving applications, up to mid-size plants (4,000 tpd, 1.4 million tpa) using a mixed refrigerant cycle.
Products: Liquefied natural gas (LNG) at atmospheric pressure. Natural gas liquids (NGL) on larger facilities.
Pretreated natural gas is cooled and condensed by a multi-stage mixed-refrigerant cycle. The refrigerant consists of a blend of nitrogen and hydrocarbons from methane through pentane.
The refrigerant is compressed by a two-stage machine (1) (normally a gas turbine-driven centrifugal type depending upon plant capacity). The high pressure mixed refrigerant is cooled (2) in the main exchanger (3), which normally consists of multiple, brazed aluminum plate-fin heat exchangers, against returning low-pressure mixed refrigerant (4). The subcooled refrigerant is then let down in pressure and evaporated to provide cooling. Liquids from refrigerant compression are cooled separately (5) in the main exchanger, let down in pressure and evaporated to provide increased process efficiency.
The natural gas is cooled (6) in the main exchanger prior to entering a hydrocarbon knockout pot (7) to remove components which would otherwise freeze in the downstream process. On large plants, the knockout pot may be replaced by a demethanizer column. NGLs recovered at this stage may be processed and used to provide refrigerant makeup. The natural gas leaving the knockout pot re-enters (8) the main exchanger and is condensed and subcooled against low-pressure refrigerant.
The subcooled LNG then enters a two-stage flash system (9) where it is let down in pressure before being pumped to storage at near atmospheric pressure. The LNG flash gas is fed to a flash gas compressor system to be used as fuel.
Economics: The mixed refrigerant cycle is often the most cost-effective process for LNG production, combining reasonable initial cost with low power requirements. The use of plate-fin heat exchangers allows the plant to be designed with high efficiency. For a 1.4 million tpy-facility, a total installed cost of $300 per tpy is feasible.
For small (peak-shaving) installations, expander cycles using nitrogen or methane may be cost-effective. Expander cycles can also be the technology of choice for offshore applications. For high efficiency on base-load installations, the cascade cycle, which uses multiple levels of pure refrigerants, can be the optimum choice.
Licensor: Costain Oil, Gas & Process Ltd.
