Method for optimising liquefaction of natural gas

Abstract

A method for liquefying a hydrocarbon stream such as natural gas starting from a feed stream.

1 .- 8 . (canceled) 
     
     
         9 . A method for liquefying a hydrocarbon stream such as natural gas starting from a feed stream comprising at least the following steps:
 Step a): passing the feed gas against a mixed refrigerant stream through a heat exchanger to supply an at least partially liquefied hydrocarbon stream having a temperature below −140° C.;   Step b): withdrawing a mixed refrigerant stream from the heat exchanger from an outlet where the temperature in the heat exchanger is highest;   Step c): introducing the mixed refrigerant resulting from step b) into a phase separating means in order to produce a gaseous refrigerant stream and a first liquid refrigerant stream;   Step d): passing the first liquid refrigerant stream resulting from step c) in the heat exchanger starting from a first inlet and up to a so-called intermediate outlet, beyond which the refrigerant stream thus obtained is expanded, the temperature T1 at said outlet being such that said expansion produces a gas fraction below 20%;   Step e): in parallel with step d), compressing the gaseous refrigerant stream resulting from step c) and then cooling before introducing the refrigerant stream thus obtained into a phase separating means in order to produce a gaseous refrigerant stream and a second liquid refrigerant stream;   Step f): passing the second liquid refrigerant stream resulting from step e) in the heat exchanger starting from a second inlet and up to an outlet, beyond which the refrigerant stream thus obtained is expanded, the temperature T2 at said outlet being above T1 and such that said expansion produces a gas fraction below 20%;   Step g): passing the gaseous refrigerant stream resulting from step e) in the heat exchanger starting from a third inlet and up to an outlet at a temperature T3, the level of which is the lowest of the temperature levels of said heat exchanger in order to produce a liquefied stream, and then expanding the stream thus obtained;   Step h): passing the stream resulting from step g) in the heat exchanger from an inlet at a temperature T3 up to an outlet at a temperature approximately equal to the temperature T2;   Step i): mixing the refrigerant stream resulting from step h) with the refrigerant stream resulting from step f), then passing the mixture thus obtained in the heat exchanger from an inlet having a temperature approximately equal to T2 up to an outlet having a temperature approximately equal to T1;   Step j): mixing the refrigerant stream resulting from step i) with the refrigerant stream resulting from step d) and then passing the mixture thus obtained in the heat exchanger up to the outlet.   
     
     
         10 . The method as claimed in  claim 9 , wherein the mixed refrigerant stream circulates in the closed-cycle refrigeration circuit. 
     
     
         11 . The method as claimed in  claim 9 , further comprising a step before step c) of compressing the mixed refrigerant resulting from step b) followed by cooling. 
     
     
         12 . The method as claimed in  claim 9 , wherein T1 is between −30° C. and −50° C. 
     
     
         13 . The method as claimed in  claim 9 , wherein T2 is between −80° C. and −110° C. 
     
     
         14 . The method as claimed in  claim 9 , wherein T3 is between −140° C. and −170° C. 
     
     
         15 . The method as claimed in  claim 9 , wherein the mixed refrigerant stream contains constituents selected from the group consisting of nitrogen, methane, ethylene, ethane, butane and pentane. 
     
     
         16 . The method as claimed in  claim 9 , wherein a pump is not used.