Method for ethane recovery, using a refrigeration cycle with a mixture of at least two coolants, gases obtained by said method, and installation therefor

Abstract

The invention concerns a method and an installation for refrigerating gas mixtures, for cryogenic separation of a pressurised gas ( 1 ) constituents. The method comprises a refrigerating cycle wherein a fluid ( 13 ) is separated in a separator tank (B 2 ) into a less volatile fraction ( 4 ), for producing refrigeration at a relatively high first temperature in an exchanger (E 1 ), and into a more volatile second fraction ( 5 ) for producing refrigeration at a relatively low temperature in an exchanger (E 2 ). The heated and expanded fractions ( 4, 5 ) are brought together then compressed in a compressor (K 1 ). A fraction ( 26 ) derived from said compressor (K 1 ) is cooled to supply the fraction ( 13 ).

1 . A process for recovering the ethane contained in a pressurized gas ( 1 ) containing methane and C 2  and higher hydrocarbons, operating a refrigerating cycle in which a relatively less-volatile first refrigerant ( 2 ) is compressed, cooled and expanded in order thereafter to be used for cooling said pressurized gas ( 1 ) to be separated or first separation products to a relatively high first temperature, and in which a relatively more-volatile second refrigerant ( 3 ) is compressed, cooled and expanded in order thereafter to be used for cooling at least second separation products obtained from said pressurized gas ( 1 ) to a relatively low second temperature * 1 , * 2  characterized in that the first and second refrigerants ( 2 ,  3 ) are used as a mixture when they are compressed and cooled, in that this mixture then undergoes a separation into a first fraction ( 4 ) essentially containing the relatively less-volatile first refrigerant ( 2 ) and into a second fraction ( 5 ) essentially containing the relatively more-volatile second refrigerant ( 3 ), in that the first refrigerant is used in the form of the first fraction, for cooling to the relatively high first temperature, and in that the second refrigerant is used in the form of the second fraction, for cooling to the relatively low second temperature.  
     
     
         2 . The process as claimed in  claim 1 , characterized in that the first fraction ( 4 ) is cooled in a first exchanger (E 1 ), expanded to give a first expanded fraction ( 6 ), then warmed in said first exchanger, before being introduced into a low-pressure stage ( 7 ) of a compressor (K 1 ).  
     
     
         3 . The process as claimed in  claim 2 , characterized in that the second fraction ( 5 ) is cooled in the first exchanger (E 1 ) and then in a second exchanger (E 2 ), expanded, then warmed in said second exchanger and mixed with the expanded first fraction ( 6 ).  
     
     
         4 . The process as claimed in  claim 2  or  claim 3 , characterized in that a third fraction ( 8 ) is tapped off the first fraction ( 4 ) after it has been cooled in the first heat exchanger (E 1 ) and in that said third fraction ( 8 ) is expanded and warmed in said first exchanger (E 1 ) in order to deliver an expanded and warmed fourth fraction ( 9 ), which is introduced into a medium-pressure stage ( 10 ) of the compressor (K 1 ).  
     
     
         5 . The process as claimed in  claim 4 , characterized in that a gaseous fifth fraction ( 11 ) is tapped off from the refrigerants undergoing compression in the compressor (K 1 ) at a medium pressure, slightly above that of the expanded and warmed fourth fraction ( 9 ), and is cooled and expanded to the same pressure as said fourth fraction ( 9 ) and then mixed with the latter.  
     
     
         6 . The process as claimed in any one of the preceding claims, characterized in that said first and second refrigerants ( 2 ,  3 ) are used as a mixture with a third refrigerant ( 12 ).  
     
     
         7 . The process as claimed in  claim 6 , characterized in that the refrigerants are methane, ethylene and propane.  
     
     
         8 . A methane-enriched gas obtained by the process as claimed in one of the preceding claims.  
     
     
         9 . An ethane-enriched product obtained by the process as claimed in one of  claims 1  to  7 .  
     
     
         10 . A product enriched with C 2  and higher hydrocarbons, obtained by the process as claimed in one of  claims 1  to  7 .  
     
     
         11 . A plant for recovering the ethane contained in a pressurized gas ( 1 ) containing methane and C 2  and higher hydrocarbons, this plant comprising means for compressing, cooling and expanding a relatively less-volatile first refrigerant ( 2 ), means for cooling, by means of the first refrigerant, said pressurized gas ( 1 ) to be separated or first separation products to a relatively high first temperature, and means for compressing, cooling and expanding a relatively more-volatile second refrigerant ( 3 ), means for cooling, by means of the second refrigerant ( 3 ), at least second separation products obtained from said pressurized gas ( 1 ) to a relatively low second temperature, characterized in that the first and second refrigerants ( 2 ,  3 ) are used as a mixture when they are compressed and cooled and in that this plant comprises means for this mixture to undergo a separation into a first fraction ( 4 ) essentially containing the relatively less-volatile first refrigerant ( 2 ) and into a second fraction ( 5 ) essentially containing the relatively more-volatile second refrigerant ( 3 ), the first refrigerant being used in the form of the first fraction for cooling to the relatively high first temperature and the second refrigerant being used in the form of the second fraction for cooling to the relatively low second temperature.