Dual nitrogen expansion process

Abstract

A method of natural gas liquefaction comprising first and second nitrogen refrigerant streams, each stream under-going a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first, intermediate pressure and the second nitrogen stream is expanded to a second, lower pressure, and the heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams is in heat exchanging relationship with natural gas, wherein, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams. The liquefaction can occur in three stages: in an initial stage the heated, expanded first nitrogen stream and the heated, expanded second nitrogen stream are used to cool the natural gas; in an intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used to cool the natural gas; and in a final stage the compressed, second nitrogen stream is expanded to a low pressure and used to cool the natural gas.

1 . A method of natural gas liquefaction comprising first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, during which the first nitrogen stream is expanded to a first, intermediate pressure and the second nitrogen stream is expanded to a second, lower pressure, and the heating occurs in one or more heat exchangers in which at least one of the expanded nitrogen streams—is in heat exchanging relationship with natural gas, wherein, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in a heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams. 
     
     
         2 . A method as claimed in  claim 1  wherein the liquefaction method comprises three cooling stages; a final cooling stage, in which the second nitrogen stream is expanded to a low pressure and placed in heat exchanging relationship with the natural gas, an intermediate stage, in which the first nitrogen stream is expanded to an intermediate pressure and placed in heat exchanging relationship with the natural gas to cool the natural gas prior to the final stage, and an initial cooling stage in which the first and second nitrogen streams, after undergoing heating in the final and/or intermediate stages, are placed in heat exchanging relationship with the natural gas to cool this prior to the intermediate stage. 
     
     
         3 . A method as claimed in  claim 2  wherein the second nitrogen stream, after being heated in the final stage, is used within the intermediate stage to provide further cooling to the natural gas. 
     
     
         4 . A method as claimed in  claim 2  wherein the intermediate stage provides cooling for the compressed second nitrogen stream, prior to its expansion and use in the final stage. 
     
     
         5 . A natural gas liquefaction method in which natural gas is cooled by first and second nitrogen refrigerant streams, each stream undergoing a cycle of compression, cooling, expansion and heating, the method comprising three stages in which: in an initial stage the heated, expanded first nitrogen stream and the heated, expanded second nitrogen stream are used to cool the natural gas; in an intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used to cool the natural gas; and in a final stage the compressed, second nitrogen stream is expanded to a low pressure and used to cool the natural gas. 
     
     
         6 . A natural gas liquefaction method as claimed in  claim 5  wherein, in the intermediate stage the compressed first nitrogen stream is expanded to an intermediate pressure and used, together with the heated expanded second nitrogen stream, to cool the natural gas and the compressed second nitrogen stream. 
     
     
         7 . A natural gas liquefaction method as claimed in  claim 5  wherein the first and second compressed streams are combined during the initial stage and during compression. 
     
     
         8 . A natural gas liquefaction method as claimed in  claim 5 , wherein in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas. 
     
     
         9 . A natural gas liquefaction method as claimed in  claim 5 , wherein the expanded first nitrogen stream is compressed from said intermediate pressure after cooling the natural gas in the initial and intermediate stages. 
     
     
         10 . A method as claimed in  claim 1  wherein the first nitrogen stream comprises a larger volume of nitrogen than the second nitrogen stream. 
     
     
         11 . A method as claimed in  claim 1  wherein the first and second nitrogen streams are compressed in a three stage compression process. 
     
     
         12 . A method as claimed in  claim 1  further comprising the step of removing C 3+  hydrocarbons from the natural gas after pre-cooling. 
     
     
         13 . A method as claimed in  claim 1  wherein the method provides complete cooling of the natural gas. 
     
     
         14 . A natural gas liquefaction apparatus comprising one or more heat exchangers for placing the natural gas in a heat exchanging relationship with first and second nitrogen refrigerant streams; one or more compressors for compressing the first and second nitrogen refrigerant streams; a first expander for expanding the first nitrogen stream to a first pressure and a second expander for expanding the second nitrogen stream to a second, lower pressure; wherein the apparatus is arranged such that, in at least one of said one or more heat exchangers, the first and second expanded nitrogen streams are in heat exchanging relationship with the natural gas and both the first and second compressed nitrogen streams. 
     
     
         15 . A natural gas liquefaction apparatus arranged to carry out the method of  claim 1 . 
     
     
         16 . A natural gas liquefaction apparatus as claimed in  claim 14  wherein the apparatus is arranged to provide complete cooling of the natural gas. 
     
     
         17 . A method as claimed in  claim 3  wherein the intermediate stage provides cooling for the compressed second nitrogen stream, prior to its expansion and use in the final stage. 
     
     
         18 . A natural gas liquefaction method as claimed in  claim 6  wherein the first and second compressed streams are combined during the initial stage and during compression. 
     
     
         19 . A natural gas liquefaction method as claimed in  claim 6  wherein in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas. 
     
     
         20 . A natural gas liquefaction method as claimed in  claim 7  wherein in the initial stage the expanded first nitrogen stream and the expanded second nitrogen stream are used to cool the compressed first and second nitrogen streams as well as the natural gas.