Method and apparatus for cooling and/or liquefying a hydrocarbon stream

Abstract

A refrigerant stream is provided at a refrigerant pressure, and passed through at least three heat exchange steps operating at different pressures. A hydrocarbon stream is passed through at least two of these heat exchange steps to provide a cooled stream. A fraction of the refrigerant stream is expanded and evaporated at each heat exchange step to a different pressure, to provide a first evaporated refrigerant stream at a first evaporation pressure, and at least two other evaporated refrigerant streams at evaporation pressures lower than the first. The first evaporated refrigerant stream is compressed through a highest-pressure compressor to provide at least a fraction of the refrigerant stream at the refrigerant pressure, and the other evaporated refrigerant streams are compressed through at least two parallel lower pressure compressors to provide two or more part-compressed refrigerant streams, all of which part-compressed refrigerant streams are passed through the highest pressure compressor.

1 . A method of cooling a hydrocarbon stream, comprising at least the steps of:
 (a) providing a refrigerant stream at a refrigerant pressure;   (b) passing the refrigerant stream through at least three heat exchange steps operating at different pressure levels;   (c) passing a hydrocarbon stream through at least two of the heat exchange steps of step (b) thereby progressively lowering the temperature of the hydrocarbon stream to provide a cooled hydrocarbon stream;   (d) expanding and evaporating a fraction of the refrigerant stream at each heat exchange step to a different pressure, to provide a first evaporated refrigerant stream at a first evaporation pressure, and at least two other evaporated refrigerant streams at evaporation pressures lower than the first evaporation pressure;   (e) compressing the first evaporated refrigerant stream through a highest-pressure compressor stage of a single compressor casing to the refrigerant pressure to provide at least a fraction of the refrigerant stream at the refrigerant pressure of step (a);   (f) compressing the other evaporated refrigerant streams through at least two parallel lower pressure compressor stages to provide two or more part-compressed refrigerant streams; and   (g) passing all of the part-compressed refrigerant streams through said highest pressure compressor stage of step (e).   
     
     
         2 . The method as claimed in  claim 1 , wherein said passing of the hydrocarbon stream through at least two of the heat exchange steps of step (b) comprises passing the hydrocarbon stream through at least three of the heat exchange steps of step (b). 
     
     
         3 . The method as claimed in  claim 1 , comprising four or five heat exchange steps, and wherein expanding and evaporating the refrigerant stream provides four or five evaporated refrigerant streams at four or five different pressures respectively. 
     
     
         4 . The method as claimed in  claim 3 , wherein the refrigerant stream and the hydrocarbon stream pass through the same heat exchange steps. 
     
     
         5 . The method as claimed  claim 1  wherein evaporating a fraction of the refrigerant stream in at least two of the heat exchange steps of step (d) comprises exchanging heat with the hydrocarbon stream passing through said heat exchange steps. 
     
     
         6 . The method as claimed in  claim 1  wherein the refrigerant stream is propane. 
     
     
         7 . The method as claimed in  claim 1 , further comprising:
 expanding and evaporating a first fraction of the refrigerant stream at a first heat exchange step to provide a high-high-pressure evaporated refrigerant stream being the first evaporated refrigerant stream, and having a high-high pressure being the first evaporation pressure,   expanding and evaporating a second fraction of the refrigerant stream at a second heat exchange step to provide a high-pressure evaporated refrigerant stream having a high pressure lower than the high-high pressure,   expanding and evaporating a third fraction of the refrigerant stream at a third heat exchange step to provide an intermediate-pressure evaporated refrigerant stream having an intermediate pressure lower than the high pressure,   expanding and evaporating a fourth fraction of the refrigerant stream at a fourth heat exchange step to provide a low-pressure evaporated refrigerant stream having a low pressure lower than the intermediate pressure,   compressing the high-high-pressure evaporated refrigerant stream through a high-high-pressure compressor stage being said highest pressure compressor stage of step (e), to provide a refrigerant stream at the refrigerant pressure of step (a),   compressing the low pressure-evaporated refrigerant stream through a low-pressure compression stage,   compressing the intermediate-pressure evaporated refrigerant stream through an intermediate-pressure compressor stage, and   compressing the high-pressure evaporated refrigerant stream through a high-pressure compression stage,   wherein the intermediate-pressure compressor stage and the high-high-pressure compressor stage are comprised in a first compressor train, and   the low-pressure compression stage and the high-pressure compression stage are comprised in a second compressor train at least partly separate from the first compressor train.   
     
     
         8 . The Method as claimed in  claim 7 , wherein the second compressor train has an outlet to provide a part-compressed refrigerant stream, and wherein said part-compressed refrigerant stream is passed to the inlet of the high-high-pressure compressor stage in the first compressor train. 
     
     
         9 . The method as claimed in one or more of the preceding claims, wherein at least two of the lower pressure compressor stages are in at least two compressor trains. 
     
     
         10 . The method as claimed in  claim 1  wherein the hydrocarbon stream comprises, preferably essentially consists of, natural gas. 
     
     
         11 . A method of liquefying a hydrocarbon stream to provide a liquefied hydrocarbon stream, comprising cooling of the hydrocarbon stream in accordance with a method as claimed in one or more of  claims 1  to  10 . 
     
     
         12 . An apparatus for cooling a hydrocarbon stream, comprising:
 a refrigerant stream at a refrigerant pressure;   at least three heat exchangers comprising pressure reduction means for operating the heat exchange steps at different pressure levels   refrigerant passage means for passing the refrigerant stream passes through the at least three heat exchangers;   hydrocarbon passage means for passing a hydrocarbon stream through at least two of the heat exchangers steps for progressively lowering the temperature of the hydrocarbon stream to provide a cooled hydrocarbon stream;   a first evaporated refrigerant stream at a first evaporation pressure,   at least two other evaporated streams at evaporation pressures lower than the first evaporation pressure;   a highest-pressure compressor stage in a single compressor casing for compressing the first evaporated refrigerant stream to provide at least a fraction of the refrigerant stream at the refrigerant pressure;   at least two parallel lower pressure compressor stages for compressing the other evaporated refrigerant streams to provide one or more part-compressed refrigerant streams; and   a pathway to pass all of the part-compressed refrigerant streams through the highest pressure compressor stage in said single compressor casing.   
     
     
         13 . The apparatus as claimed in  claim 12 , wherein at least two of the lower pressure compressor stages are in at least two at least partly mutually separate compressor trains.