Method and apparatus for liquefying a hydrocarbon stream and floating vessel or offshore platform comprising the same

Abstract

A method of liquefying a hydrocarbon stream, such as natural gas, comprising at least the steps of : (a) partially liquefying a hydrocarbon feed stream ( 10 ) to provide a partially liquefied hydrocarbon stream ( 20 ); (b) passing the partially liquefied hydrocarbon stream ( 20 ) through a first gas/liquid separator (B) to provide a methane-enriched gaseous overhead stream ( 30 a ) and a mixed C 2 + enriched liquid bottom stream ( 30 b ); (c) adding at least a part ( 30 c ) of the mixed C 2 + enriched bottom stream ( 30 b ) to a mixed refrigerant circuit ( 4 ) to change the C 2 component inventory of the mixed refrigerant ( 100 ) in the mixed refrigerant circuit ( 4 ); and (d) liquefying the methane-enriched gaseous overhead stream ( 30 a, 30 ) against at least a fraction ( 110, 125, 135 ) of the mixed refrigerant ( 100 ) in the mixed refrigerant circuit ( 4 ) to provide a liquefied hydrocarbon stream ( 50 ).

1 . A method of liquefying a hydrocarbon stream, comprising at least the steps of:
 (c) circulating a mixed refrigerant through a mixed refrigerant circuit:   (a) partially liquefying a hydrocarbon feed stream in a first cooling stage to provide a partially liquefied hydrocarbon stream by passing the hydrocarbon feed stream against at least a first fraction of the mixed refrigerant of the mixed refrigerant circuit in at least one first heat exchanger and reducing the temperature of the hydrocarbon feed stream from 0° C. or higher to in the range −20° C. to −70° C.;   (b) passing the partially liquefied hydrocarbon stream through a first gas/liquid separator to provide a methane-enriched gaseous overhead stream and a mixed C 2 + enriched liquid bottom stream;   (d) adding, without fractionation, at least a part of the mixed C 2 + enriched bottom stream to the mixed refrigerant circuit to change the C 2 + component inventory of the mixed refrigerant in the mixed refrigerant circuit; and   (e) liquefying the methane-enriched gaseous overhead stream in a second cooling stage by passage through at least one second heat exchanger and heat exchanging against at least a second fraction of the mixed refrigerant circulating in the mixed refrigerant circuit, to provide a liquefied hydrocarbon stream, wherein in the temperature of the methane-enriched gaseous overhead stream in the second cooling stage is reduced to below −100° C.;   
       which method does not use fractionation to change the mixed refrigerant inventory. 
     
     
         2 . (canceled) 
     
     
         3 . The method as claimed in  claim 1 , wherein the methane-enriched gaseous overhead stream is partially liquefied during passage of through the at least one second heat exchanger to form a further partially_liquefied hydrocarbon stream, the method further comprising passing the further partially-liquefied hydrocarbon stream through a second gas/liquid separator to provide a second methane-enriched gaseous overhead stream, and a second liquid bottom stream; and
 using at least a part of the second liquid bottom stream to change the component inventory of the mixed refrigerant circuit.   
     
     
         4 . The method as claimed in  claim 1 , further comprising the steps of:
 (f) passing at least one fraction of the mixed refrigerant through at least one heat exchanger to provide one or more cooled mixed refrigerant streams;   (g) passing at least one of the cooled mixed refrigerant streams through at least one or more refrigerant gas/liquid separator to provide one or more gaseous overhead refrigerant streams and one or more liquid bottom refrigerant streams; and   (h) removing at least a part of at least one of the gaseous overhead refrigerant streams and the liquid bottom refrigerant streams to change the component inventory of the mixed refrigerant circuit.   
     
     
         5 . The method as claimed in  claim 1 , wherein at least one fraction of the mixed refrigerant is removed from the mixed refrigerant circuit to change the component inventory of the mixed refrigerant. 
     
     
         6 . The method as claimed in  claim 5 , wherein the component inventory of at least one stream added to the mixed refrigerant circuit is different from the component inventory of at least one fraction of the mixed refrigerant being removed from the same mixed refrigerant circuit. 
     
     
         7 . (canceled) 
     
     
         8 . The method as claimed in  claim 1 , wherein the first fraction of the mixed refrigerant in the at least one first heat exchanger also provides an at least partly evaporated mixed refrigerant stream; the method further comprising:
 passing the at least partly evaporated mixed refrigerant stream through a second refrigerant gas/liquid separator to provide a gaseous overhead refrigerant stream and a liquid bottom refrigerant stream; and   removing at least a part of the liquid bottom refrigerant stream from the mixed refrigerant circuit to change the component inventory of the mixed refrigerant.   
     
     
         9 . The method as claimed in  claim 1 , further comprising:
 at least partially evaporating at least one of the at least one fraction of the mixed refrigerant in the first and second heat exchangers to provide at least one of a liquid refrigerant bottom stream and at least one gaseous refrigerant overhead stream from at least one of the first and second heat exchangers; and   removing at least a part of at least one of the gaseous refrigerant overhead streams and the liquid refrigerant bottom streams to change the component inventory of the mixed refrigerant circuit.   
     
     
         10 . The method as claimed in  claim 1 , wherein the mixed refrigerant comprises at least two of the group comprising: nitrogen, methane, ethane, ethylene, propane, propylene, butanes, pentanes. 
     
     
         11 . The method as claimed in  claim 1 , wherein the hydrocarbon feed stream comprises natural gas, and wherein the liquefied hydrocarbon stream is LNG. 
     
     
         12 . The method as claimed  claim 1 , further comprising:
 passing the liquefied hydrocarbon stream through an end flash valve and an end gas/liquid separator, to provide a gaseous overhead stream and a liquid bottom stream; and   combining the gaseous overhead stream from the end gas/liquid separator and at least a part of the gaseous overhead refrigerant stream from one of the refrigerant gas/liquid separators, to provide a combined stream for compression and use as a fuel gas.   
     
     
         13 . The method as claimed in  claim 1 , wherein the mixed refrigerant is initially formed of natural gas from which the C 5 + components have been removed. 
     
     
         14 . The method as claimed in  claim 1 , performed on a floating vessel or off-shore platform. 
     
     
         15 . (canceled) 
     
     
         16 . The method as claimed in  claim 1 , wherein the mixed C 2 + enriched liquid bottom stream is provided in step (b) without fractionation of the partially liquefied hydrocarbon stream. 
     
     
         17 . The method as claimed in  claim 1 , wherein the method comprises a single mixed refrigerant circuit to provide cooling to the at least one first heat exchanger and the at least one second heat exchanger. 
     
     
         18 . The method as claimed in  claim 1 , wherein the hydrocarbon feed stream, essentially consists of natural gas, and wherein the liquefied hydrocarbon stream is LNG.