Method for liquefying natural gas.

Abstract

The invention pertains to technologies for liquefaction of natural gas to be subsequently transported by river or sea. The technical result of the proposed method consists in more stable operation of a plant by virtue of the fact that the end boiling point of a refrigerant is lower than the post-expansion temperature of the mixed refrigerant of the preceding circuit, which makes it impossible to cool down the natural gas in the preceding liquefaction circuit to the temperatures which correspond to a mixed refrigerant existence in the two-phase area at any flow rate or other fluctuations of the technological process parameters. Prepared natural gas is compressed, the heat of compression is removed, the natural gas is cooled by three circuits containing mixed refrigerants, the pressure of the cooled gas is reduced to produce a vapour-liquid mixture, and the liquefied gas is withdrawn. In each circuit, a mixed refrigerant is compressed, the heat of compression is removed, the refrigerant is subcooled, the pressure thereof is lowered to produce in each circuit a low-pressure mixed refrigerant, and said low-pressure mixed refrigerant is used to cool the natural gas. ln the first circuit, the first mixed refrigerant is sub-cooled through evaporation of the first low-pressure mixed refrigerant. ln the second circuit, the second mixed refrigerant is also cooled through evaporation of the first low-pressure mixed refrigerant and subcooled through evaporation of the second low-pressure mixed refrigerant. In the third circuit, the third mixed refrigerant is cooled through evaporation of the second low-pressure mixed refrigerant and sub-cooled through evaporation of the third low-pressure mixed refrigerant. It is possible to precool the third mixed refrigerant through evaporation of the first mixed refrigerant. The end boiling point of the second mixed refrigerant at the pressure at which the compression thereof commences is lower than the temperature of the first mixed refrigerant after the pressure of such first mixed refrigerant has been reduced, and the end boiling point of the third mixed refrigerant at the pressure at which the compression thereof commences is below the temperature of the second mixed refrigerant after the pressure of such second mixed refrigerant has been reduced.