Process for liquefying natural gas and carbon dioxide

Abstract

A process for producing liquefied natural gas ( 14 ) and liquid carbon dioxide (CO 2 ) ( 15 ) comprising at least the following steps: Step a): separating a natural gas feed gas ( 1 ), containing hydrocarbons and carbon dioxide in a treatment unit ( 2 ), into a CO 2 -enriched gas stream ( 4 ) and a CO 2 -depleted natural gas stream ( 3 ); step b): liquefying the CO 2 -depleted natural gas stream ( 3 ) resulting from step a) in a natural gas liquefaction unit ( 5 ) comprising at least a main heat exchanger ( 8 ) and a system ( 9 ) for producing frigories; step c): simultaneous liquefying of the CO 2 -enriched gas stream ( 4 ) resulting from step a) in a CO 2 liquefaction unit ( 6 ); characterized in that all of the refrigeration necessary for the liquefaction of the CO 2 -enriched gas stream ( 4 ) and for the liquefaction of the natural gas is supplied by said frigorie-producing system ( 9 ) of the natural gas liquefaction unit ( 5 ).

What is claimed is: 
     
         1 . A process for producing liquefied natural gas) and liquid carbon dioxide (CO 2 ) comprising the following steps:
 Step a): separating a natural gas feed gas, containing hydrocarbons and carbon dioxide in a treatment unit, into a CO 2 -enriched gas stream and a CO 2 -depleted natural gas stream;   Step b): liquefying the CO 2 -depleted natural gas stream resulting from step a) in a natural gas liquefaction unit comprising at least a main heat exchanger and a system for producing frigories;   Step c): simultaneous liquefying of the CO 2 -enriched gas stream resulting from step a) in a CO 2  liquefaction unit;   wherein all or some of the refrigeration necessary for the liquefaction of the CO 2 -enriched gas stream and for the liquefaction of the natural gas is supplied by said frigorie-producing system of the natural gas liquefaction unit); and wherein the CO 2 -enriched gas stream resulting from step a) is purified prior to step c), the refrigeration necessary for this purification being provided by said frigorie-producing system of the natural gas liquefaction unit.   
     
     
         2 . The process according to  claim 1 , wherein the feed gas comprises from 0.1 mol % to 5 mol % of CO 2 . 
     
     
         3 . The process according to  claim 1 , wherein the CO 2 -enriched gas stream resulting from step a) comprises at least 95 mol % of CO 2 . 
     
     
         4 . The process according to  claim 1 , wherein prior to step b), the natural gas stream resulting from step a) is pretreated in a pretreatment unit. 
     
     
         5 . The process according to  claim 1 , wherein said treatment unit used in step a) is an amine scrubbing unit. 
     
     
         6 . The process according to  claim 5 , wherein the CO 2 -enriched stream thus purified comprises at least 99.5 mol % of CO 2 . 
     
     
         7 . The process according to  claim 1 , wherein said system for producing frigories comprises at least one compressor. 
     
     
         8 . The process according to  claim 1 , wherein said natural gas liquefaction unit comprises at least one refrigeration cycle fed by a refrigerant stream containing at least one of the constituents selected from the group consisting of nitrogen, methane, ethylene, ethane, propane, ammonia, butane and pentane. 
     
     
         9 . The process according to  claim 1 , wherein the refrigeration necessary for the liquefaction of the CO 2 -enriched gas stream originates from a refrigerant fluid precooled in the heat exchanger of the natural gas liquefaction unit. 
     
     
         10 . The process according to  claim 9 , wherein the refrigerant fluid is an ammonia and/or propane fluid. 
     
     
         11 . A device for producing liquefied natural gas and liquefied CO 2  comprising a feed gas treatment unit, producing at least a CO 2 -enriched gas stream and a CO 2 -depleted natural gas stream, and a natural gas liquefaction unit, said natural gas liquefaction unit comprising at least a main heat exchanger and a system for producing frigories, wherein the system for producing frigories is capable of and designed for liquefying both the CO 2 -enriched stream resulting from the treatment unit and the natural gas stream circulating in the natural gas liquefaction unit; and
 further comprising a CO 2 -enriched gas purification and liquefaction unit comprising at least one compression means, one purification means and at least one distillation column, wherein said device is designed so that the refrigeration necessary for the use of the CO 2 -enriched gas purification and liquefaction unit originates partly or completely from a refrigerant fluid cooled in said main heat exchanger of the natural gas liquefaction unit.   
     
     
         12 . The device according to the  claim 11 , wherein said system for producing frigories comprises at least one compressor. 
     
     
         13 . The device according to  claim 12 , wherein said system for producing frigories comprises a refrigeration cycle, comprising a compressor driven by an electric or heat engine, a circuit for circulating a refrigerant fluid.