The process stream (natural gas) goes through a main cryogenic heat exchanger (coldbox) for liquefaction by rejecting heat to the mixed refrigerant streams. The mixed refrigerant is partially condensed by ambient cooling utilities (air or cooling water) upon the completion of multi-stage compression.
The vapour and liquid phases of the mixed refrigerant are then separated in a flash unit. By mixing a fraction of the vapour product and a fraction of the liquid product, the light refrigerant (LR) and the heavy refrigerant (HR) streams are produced. The light refrigerant preferably has a higher mixing ratio of the vapour product, and the heavy refrigerant preferably has a higher mixing ratio of the liquid product. Both refrigerant streams (LR and HR) go through the coldbox and get sub-cooled.
The light refrigerant is preferably sub-cooled to a lower temperature level than the heavy refrigerant. After sub-cooling in the coldbox, both refrigerant streams are expanded in a typical expansion device (throttle valve or expander) and go back to the coldbox.
The heavy refrigerant is expanded to an intermediate pressure level, preferably higher than the pressure that the light refrigerant is expanded to.
The two refrigerant streams evaporate in the coldbox over different temperature ranges and liquefy the process stream (natural gas). After absorbing heat in the coldbox, the light refrigerant is compressed to the intermediate pressure level and mixes with the heavy refrigerant from the coldbox.
The mixed refrigerant then is further compressed in another compression stage and circulated in the loop.