Method For the Liquefaction of a Hydrocarbon-Rich Stream

Abstract

The present invention relates to a method for the liquefaction of a hydrocarbon-rich stream, preferably a natural gas containing stream, by heat exchanging against a refrigerant ( 1 a - d ). The liquid refrigerant ( 19 ) is evaporated using heat from the hydrocarbon-rich stream, thereby obtaining an evaporated refrigerant ( 3 a - d ). The evaporated refrigerant ( 3 a - d ) is subsequently compressed ( 5 ), cooled ( 10 ) against ambient thereby fully condensing the compressed refrigerant. Next, the fully condensed compressed refrigerant ( 12 ) is further sub-cooled ( 14 ) by indirect heat exchange against an auxiliary refrigerant being cycled in an auxiliary refrigerant. Then the subcooled refrigerant ( 16 ) is expanded ( 18 ) thereby forming the liquid refrigerant ( 19 ).

1 . A method for the liquefaction of a hydrocarbon-rich stream, wherein the hydrocarbon-rich stream to be liquefied is heat exchanged against a refrigerant, the method at least comprising the steps of:
 (a) evaporating a liquid refrigerant using heat from the hydrocarbon-rich stream, thereby obtaining an evaporated refrigerant;   (b) compressing the evaporated refrigerant, thereby obtaining a compressed refrigerant;   (c) cooling the compressed refrigerant against ambient thereby fully condensing the compressed refrigerant;   (d) expanding the fully condensed compressed refrigerant thereby forming said liquid refrigerant;   wherein, before expanding in step (d), the fully condensed compressed refrigerant is further sub-cooled by indirect heat exchange against an auxiliary refrigerant being cycled in an auxiliary refrigerant cycle comprising an auxiliary compressing step followed by drawing heat from the fully condensed compressed refrigerant for its further sub-cooling,   wherein the auxiliary refrigerant is selected to have a higher bubble point temperature than the liquid refrigerant when determined under equal-pressure condition.   
     
     
         2 . The method of  claim 1 , wherein the further sub-cooling is performed to a temperature that is lower than ambient temperature. 
     
     
         3 . The method of  claim 1 , wherein the auxiliary refrigerant is selected to have a higher heat of evaporation than the liquid refrigerant. 
     
     
         4 . The method of  claim 1 , wherein the pressure drop of the refrigerant between the cooling in step (d) and the sub-cooling against the auxiliary refrigerant is less than 10 bar. 
     
     
         5 . The method of  claim 1 , wherein the refrigerant comprises >90 mol % propane and the auxiliary refrigerant comprises >90 mol % butane. 
     
     
         6 . The method of  claim 1 , wherein expanding the fully condensed compressed refrigerant in step (d) is performed in at least consecutive first and second sub-stages, wherein the further sub-cooling of the fully condensed compressed refrigerant is performed to a temperature on or above a bubble point temperature of the refrigerant after the subsequent expanding and before expanding in the second sub-stage. 
     
     
         7 . The method of  claim 6 , wherein part of the liquid refrigerant is evaporated in the first sub-stage, using heat from the hydrocarbon-rich stream, after expanding in the first sub-stage and before expanding in the second sub-stage, wherein a liquid fraction of the liquid refrigerant is retained and separated from the evaporated part and further expanded in the second sub-stage, wherein the liquid fraction is further sub-cooled by indirect heat exchange against a second auxiliary refrigerant before the further expanding in the second sub-stage. 
     
     
         8 . The method of  claim 1 , wherein the hydrocarbon-rich stream is a natural gas containing stream. 
     
     
         9 . The method of  claim 2 , wherein the auxiliary refrigerant is selected to have a higher heat of evaporation than the liquid refrigerant. 
     
     
         10 . The method of  claim 2 , wherein the pressure drop of the refrigerant between the cooling in step (d) and the sub-cooling against the auxiliary refrigerant is less than 10 bar. 
     
     
         11 . The method of  claim 3 , wherein the pressure drop of the refrigerant between the cooling in step (d) and the sub-cooling against the auxiliary refrigerant is less than 10 bar. 
     
     
         12 . The method of  claim 2 , wherein the refrigerant comprises >90 mol % propane and the auxiliary refrigerant comprises >90 mol % butane. 
     
     
         13 . The method of  claim 3 , wherein the refrigerant comprises >90 mol % propane and the auxiliary refrigerant comprises >90 mol % butane. 
     
     
         14 . The method of  claim 4 , wherein the refrigerant comprises >90 mol % propane and the auxiliary refrigerant comprises >90 mol % butane. 
     
     
         15 . The method of  claim 2 , wherein expanding the fully condensed compressed refrigerant in step (d) is performed in at least consecutive first and second sub-stages, wherein the further sub-cooling of the fully condensed compressed refrigerant is performed to a temperature on or above a bubble point temperature of the refrigerant after the subsequent expanding and before expanding in the second sub-stage. 
     
     
         16 . The method of  claim 3 , wherein expanding the fully condensed compressed refrigerant in step (d) is performed in at least consecutive first and second sub-stages, wherein the further sub-cooling of the fully condensed compressed refrigerant is performed to a temperature on or above a bubble point temperature of the refrigerant after the subsequent expanding and before expanding in the second sub-stage. 
     
     
         17 . The method of  claim 4 , wherein expanding the fully condensed compressed refrigerant in step (d) is performed in at least consecutive first and second sub-stages, wherein the further sub-cooling of the fully condensed compressed refrigerant is performed to a temperature on or above a bubble point temperature of the refrigerant after the subsequent expanding and before expanding in the second sub-stage. 
     
     
         18 . The method of  claim 5 , wherein expanding the fully condensed compressed refrigerant in step (d) is performed in at least consecutive first and second sub-stages, wherein the further sub-cooling of the fully condensed compressed refrigerant is performed to a temperature on or above a bubble point temperature of the refrigerant after the subsequent expanding and before expanding in the second sub-stage. 
     
     
         19 . The method of  claim 15 , wherein part of the liquid refrigerant is evaporated in the first sub-stage, using heat from the hydrocarbon-rich stream, after expanding in the first sub-stage and before expanding in the second sub-stage, wherein a liquid fraction of the liquid refrigerant is retained and separated from the evaporated part and further expanded in the second sub-stage, wherein the liquid fraction is further sub-cooled by indirect heat exchange against a second auxiliary refrigerant before the further expanding in the second sub-stage. 
     
     
         20 . The method of  claim 16 , wherein part of the liquid refrigerant is evaporated in the first sub-stage, using heat from the hydrocarbon-rich stream, after expanding in the first sub-stage and before expanding in the second sub-stage, wherein a liquid fraction of the liquid refrigerant is retained and separated from the evaporated part and further expanded in the second sub-stage, wherein the liquid fraction is further sub-cooled by indirect heat exchange against a second auxiliary refrigerant before the further expanding in the second sub-stage.