Purification of Carbon Dioxide

Abstract

Impurities that are less volatile than carbon dioxide, e.g. hydrogen sulphide, are removed from crude carbon dioxide by processes involving sub-ambient distillation of said crude carbon dioxide in a distillation column system operating at super-atmospheric pressure(s) to produce carbon dioxide-enriched overhead vapour and bottoms liquid enriched with said impurities. Where such processes involve at least one heat pump cycle, significant savings in power consumption are realised when the process uses more than one recycle pressure in the heat pump cycle(s).

1 . A process for purifying crude carbon dioxide comprising at least one impurity that is less volatile than carbon dioxide, said process comprising:
 feeding crude carbon dioxide feed at sub-ambient temperature to a distillation column system operating at super-atmospheric pressure(s) for separation to produce carbon dioxide-enriched overhead vapour and bottoms liquid enriched with said at least one impurity;   providing carbon dioxide-enriched liquid as reflux for said distillation column system;   at least partially re-boiling a portion of said bottoms liquid by indirect heat exchange to provide vapour for said distillation column system;   removing carbon dioxide-enriched overhead vapour from said distillation column system; and   removing a further portion of said bottoms liquid, or a liquid derived from bottoms liquid, from said distillation column system,   
       wherein re-boiling duty for said distillation column system is provided at least in part by indirect heat exchange against recycle fluids from at least one heat pump cycle using a carbon dioxide-containing fluid from said distillation column system as working fluid, at least one of said recycle fluids having a different pressure from the other recycle fluid(s). 
     
     
         2 . The process of  claim 1  comprising a single heat pump cycle comprising a least a first recycle fluid and a second recycle fluid, said second recycle fluid having a pressure that is greater than that of said first recycle fluid. 
     
     
         3 . The process of  claim 2 , wherein the pressure of said second recycle fluid is at least 10% greater than the pressure of said first recycle fluid. 
     
     
         4 . The process of  claim 2 , wherein the pressure of said first recycle fluid is from about 15 bar to about 30 bar. 
     
     
         5 . The process of  claim 2 , wherein the pressure of said second recycle fluid is from about 20 bar to about 70 bar. 
     
     
         6 . The process of  claim 2 , wherein said working fluid comprises carbon dioxide-enriched gas generated by warming said carbon dioxide-enriched overhead vapour by indirect heat exchange. 
     
     
         7 . The process of  claim 6 , wherein at least a portion of the duty required to warm said carbon dioxide-enriched overhead vapour is provided by indirect heat exchange against at least one of said recycle streams. 
     
     
         8 . The process of  claim 2 , wherein said recycle fluids are recycled to an appropriate location in said distillation column system after suitable pressure reduction. 
     
     
         9 . The process of  claim 2 , wherein ratio of molar flow of said first recycle fluid to said second recycle fluid is from about 0.1 (i.e. 1:10) to about 10 (i.e. 10:1). 
     
     
         10 . The process of  claim 9 , wherein said ratio is from about 3 (i.e. 3:1) to about 10 (i.e. 10:1). 
     
     
         11 . The process of  claim 2 , wherein said working fluid comprises crude carbon dioxide gas generated by evaporating “intermediate” liquid taken from an intermediate location in said distillation column system by indirect heat exchange after suitable pressure reduction. 
     
     
         12 . The process of  claim 11 , comprising at least partially condensing carbon dioxide-enriched overhead vapour by indirect heat exchange to produce at least partially condensed carbon dioxide-enriched overhead vapour as said reflux for said distillation column system. 
     
     
         13 . The process of  claim 12 , wherein at least a portion of the duty required to at least partially condense said carbon dioxide-enriched overhead vapour is provided by indirect heat exchange against at least one “cold” process stream. 
     
     
         14 . The process of  claim 11 , wherein at least a portion of the duty required to evaporate said “intermediate” liquid is provided by indirect heat exchange against at least one of said recycle fluids. 
     
     
         15 . The process of  claim 11 , wherein said “intermediate” liquid has a composition that is at least substantially identical to said crude carbon dioxide feed. 
     
     
         16 . The process of  claim 11 , wherein said first recycle fluid is recycled as part of said feed to said distillation column system. 
     
     
         17 . The process of  claim 11 , wherein said second recycle fluid is recycled as part of said working fluid for said heat pump cycle after suitable pressure reduction. 
     
     
         18 . The process of  claim 1  comprising at least a first heat pump cycle and a second heat pump cycle, each heat pump cycle comprising at least one recycle fluid, said recycle fluid of said first heat pump cycle or, where the first heat pump cycle has more than one recycle fluid, at least one of said recycle fluids, having a pressure that is greater than that of a recycle fluid of said second heat pump cycle. 
     
     
         19 . The process of  claim 18 , wherein the pressure of said recycle fluid of said first heat pump cycle is at least 10% greater than the pressure of said recycle fluid of said second heat pump cycle. 
     
     
         20 . The process of  claim 18 , wherein the pressure of said recycle fluid of said first heat pump cycle is from about 15 bar to about 60 bar. 
     
     
         21 . The process of  claim 18 , wherein said working fluid of said first heat pump cycle comprises carbon dioxide-enriched gas generated by warming said carbon dioxide-enriched overhead vapour by indirect heat exchange. 
     
     
         22 . The process of  claim 21 , wherein at least a portion of the duty required to warm said carbon dioxide-enriched overhead vapour is provided by indirect heat exchange against at least one of said recycle streams. 
     
     
         23 . The process of  claim 18 , wherein the pressure of said recycle fluid of said second heat pump cycle is from about 10 bar to about 25 bar. 
     
     
         24 . The process of  claim 18 , wherein said working fluid of said second heat pump cycle comprises crude carbon dioxide gas generated by warming “intermediate” vapour taken from an intermediate location of said distillation column system by indirect heat exchange. 
     
     
         25 . The process of  claim 24 , wherein at least a portion of the duty required to warm said “intermediate” vapour is provided by indirect heat exchange against at least one of said recycle fluids. 
     
     
         26 . The process of  claim 24 , wherein said crude carbon dioxide gas has a composition that is at least substantially identical to said crude carbon dioxide feed. 
     
     
         27 . The process of  claim 18  wherein said recycle streams are recycled to appropriate locations in said distillation column system after suitable pressure reduction if required. 
     
     
         28 . The process of  claim 1 , wherein liquid from an intermediate location in said distillation column system is at least partially re-boiled by indirect heat exchange to provide additional vapour for said distillation column system. 
     
     
         29 . The process of  claim 28 , wherein at least a portion of the re-boiling duty is provided by indirect heat exchange against at least one of said recycle fluids. 
     
     
         30 . The process of  claim 1 , wherein a portion of said working fluid is purged from said process. 
     
     
         31 . The process of  claim 1 , wherein said reflux for said distillation column system is provided by at least one recycle fluid condensate after suitable pressure reduction. 
     
     
         32 . The process of  claim 31 , wherein the refrigeration duty required to cool and at least partially condense at least one recycle fluid is provided by indirect heat exchange against at least one “cold” process stream. 
     
     
         33 . The process of  claim 1 , wherein said reflux for said distillation column system is provided by condensed overhead vapour. 
     
     
         34 . The process of  claim 33 , wherein the refrigeration duty required to cool and at least partially condense overhead vapour is provided by indirect heat exchange against at least one “cold” process stream. 
     
     
         35 . The process of  claim 1 , wherein said crude carbon dioxide feed is crude carbon dioxide fluid derived from a natural source of carbon dioxide and expanded prior to feeding to said distillation column system. 
     
     
         36 . The process of  claim 35 , wherein, prior to said expansion, said crude carbon dioxide fluid is at a super-critical pressure and a sub-critical temperature. 
     
     
         37 . The process of  claim 35 , wherein said crude carbon dioxide fluid is cooled by indirect heat exchange prior to expansion. 
     
     
         38 . The process of  claim 37 , wherein at least a portion of the duty required to cool said crude carbon dioxide fluid is provided by indirect heat exchange against at least one “cold” process stream. 
     
     
         39 . The process of  claim 35 , wherein said expanded crude carbon dioxide is used as a “cold” process stream to provide refrigeration duty for said process. 
     
     
         40 . The process of  claim 35 , wherein said expanded crude carbon dioxide fluid is fed directly to said distillation column system. 
     
     
         41 . The process of  claim 1 , wherein said feed is derived from supercritical crude carbon dioxide fluid and carbon dioxide-enriched liquid is produced as a product. 
     
     
         42 . The process of  claim 41 , wherein said carbon dioxide-enriched liquid is removed from said distillation column system, pumped and warmed by indirect heat exchange to produce warmed carbon dioxide-enriched liquid as said product. 
     
     
         43 . The process of  claim 42 , wherein at least a portion of the duty required to warm said pumped carbon dioxide-enriched liquid is provided by indirect heat exchange against at least one of said recycle fluids. 
     
     
         44 . The process of  claim 42 , wherein said pumped carbon dioxide-enriched liquid is used as a “cold” process stream to provide refrigeration duty for the process. 
     
     
         45 . The process of  claim 1 , wherein said feed is derived from crude carbon dioxide vapour and carbon dioxide-enriched gas is produced as a product. 
     
     
         46 . The process of  claim 45 , wherein a portion of said carbon dioxide-enriched vapour is warmed by indirect heat exchange to produce said carbon dioxide-enriched gas. 
     
     
         47 . The process of  claim 46 , wherein at least a portion of the duty required to warm said carbon dioxide-enriched overhead vapour is provided by indirect heat exchange against at least one of said recycle fluids. 
     
     
         48 . The process of  claim 46 , wherein said carbon dioxide-enriched overhead vapour is used as a “cold” process stream to provide refrigeration duty for the process. 
     
     
         49 . The process of  claim 1 , wherein said further portion of bottoms liquid, or said liquid derived from bottoms liquid, is pumped and warmed by indirect heat exchange to provide impurity-rich waste liquid. 
     
     
         50 . The process of  claim 49 , wherein at least a portion of the duty required to warm said pumped bottoms liquid is provided by indirect heat exchange against at least one of said recycle fluids. 
     
     
         51 . The process of  claim 49 , wherein said further portion of said bottoms liquid, or said liquid derived from bottoms liquid, is used as a “cold” process stream to provide refrigeration duty for the process. 
     
     
         52 . The process of  claim 1 , wherein the operating pressure(s) said distillation column system is from about 10 bar to about 25 bar. 
     
     
         53 . The process of  claim 1 , wherein said at least one impurity is hydrogen sulphide (H 2 S). 
     
     
         54 . The process of  claim 1 , wherein said process is auto-refrigerated. 
     
     
         55 . A process for purifying crude carbon dioxide comprising at least one impurity that is less volatile than carbon dioxide, said process comprising:
 feeding crude carbon dioxide feed at sub-ambient temperature to a distillation column system operating at super-atmospheric pressure(s) for separation to produce carbon dioxide-enriched overhead vapour and bottoms liquid enriched with said at least one impurity;   removing said carbon dioxide-enriched overhead vapour from said distillation column system and warming at least a portion of said carbon dioxide-enriched overhead vapour by indirect heat exchange to produce warmed carbon dioxide-enriched gas;   compressing a first working fluid comprising said warmed carbon dioxide-enriched gas to produce at least one compressed carbon dioxide-enriched gas;   cooling and at least partially condensing at least a portion of said compressed carbon dioxide-enriched gas as a first recycle fluid by indirect heat exchange to produce carbon dioxide-enriched fluid;   expanding said carbon dioxide-enriched fluid to produce expanded carbon dioxide-enriched fluid and feeding said expanded carbon dioxide-enriched fluid to said distillation column system, at least a portion of which being used as reflux;   compressing a second working fluid comprising carbon dioxide-rich gas from said distillation column system to produce at least one second recycle fluid;   cooling and optionally condensing at least a portion of said second recycle fluid by indirect heat exchange to produce cooled carbon dioxide-rich fluid;   after expansion as required, feeding at least a portion of said cooled carbon dioxide-rich fluid to said distillation column system;   at least partially re-boiling a portion of said bottoms liquid by indirect heat exchange to produce vapour for said distillation column system; and   removing a further portion of said bottoms liquid, or a liquid derived from bottoms liquid, from said distillation column system,   
       wherein re-boiling duty for said distillation column system is provided at least in part by indirect heat exchange against said first and second recycle fluids, said first recycle fluid having a different pressure from said second recycle fluid. 
     
     
         56 . The process of  claim 55 , wherein said compressed carbon dioxide-enriched gas is divided into at least a first portion and a second portion, wherein said first portion is said first recycle fluid(s), and wherein said second portion is said carbon dioxide-rich gas for compression to produce said second recycle fluid(s). 
     
     
         57 . The process of  claim 55 , wherein carbon dioxide-rich vapour is removed from an intermediate location in the distillation column system and warmed by indirect heat exchange to produce said carbon dioxide-rich gas for compression to produce said second recycle fluid(s). 
     
     
         58 . The process of  claim 55 , wherein liquid from an intermediate location in said distillation column system is at least partially re-boiled by indirect heat exchange to provide additional vapour for said distillation column system. 
     
     
         59 . A process for purifying crude carbon dioxide comprising at least one impurity that is less volatile than carbon dioxide, said process comprising:
 feeding crude carbon dioxide feed at sub-ambient temperature to a distillation column system operating at super-atmospheric pressure(s) for separation to produce carbon dioxide-enriched overhead vapour and bottoms liquid enriched with said at least one impurity;   condensing a portion of said carbon dioxide-enriched overhead vapour by indirect heat exchange to provide reflux for said distillation column system;   removing a further portion of said carbon dioxide-enriched overhead vapour from said distillation system;   removing carbon dioxide-rich liquid from an intermediate location in said distillation column system and expanding said liquid to produce expanded carbon dioxide-rich liquid;   warming and evaporating said expanded carbon dioxide-rich liquid by indirect heat exchange to provide warmed carbon dioxide-rich gas;   compressing a working fluid comprising said warmed carbon dioxide-rich gas to produce at least one compressed carbon dioxide-rich gas as a first recycle fluid and at least one further compressed carbon dioxide-rich gas as a second recycle fluid;   cooling and optionally at least partially condensing said first recycle fluid by indirect heat exchange to produce cooled first carbon dioxide-rich fluid;   combining said cooled first carbon dioxide-rich fluid with crude carbon dioxide fluid to produce said crude carbon dioxide feed for the distillation column system;   cooling and at least partially condensing said second recycle fluid by indirect heat exchange to produce cooled second carbon dioxide-rich fluid;   expanding said cooled second carbon dioxide-rich fluid to produce expanded carbon dioxide-rich fluid;   combining said expanded carbon dioxide-rich fluid with a fluid selected from the group consisting of said carbon dioxide-rich liquid, said expanded carbon dioxide-rich liquid, and said warmed carbon dioxide-rich gas;   at least partially re-boiling a portion of said bottoms liquid by indirect heat exchange against at least one “warm” process stream to produce vapour for said distillation column system; and   removing a further portion of said bottoms liquid, or a liquid derived from bottoms liquid, from said distillation column system,   
       wherein said re-boiling duty is provided at least in part by indirect heat exchange against said first and second recycle fluids and wherein, in embodiments in which said expanded carbon dioxide-rich fluid is combined with said warmed carbon dioxide-rich gas, said expanded carbon dioxide-rich fluid is first warmed and evaporated by indirect heat exchange to produce further warmed carbon dioxide-rich gas for said combination with said warmed carbon dioxide-rich gas. 
     
     
         60 . Apparatus for carrying out the process of  claim 55 , said apparatus comprising:
 a distillation column system for operation at super-atmospheric pressure(s) for separating crude carbon dioxide feed at sub-ambient temperature to produce carbon-dioxide-enriched vapour and bottoms liquid enriched with said at least one impurity;   a first heat exchanger arrangement for warming at least a portion of said carbon dioxide-enriched overhead vapour by indirect heat exchange to produce warmed carbon dioxide-enriched gas;   a conduit arrangement for removing carbon dioxide-enriched overhead vapour from said distillation column system and feeding said vapour to said first heat exchanger arrangement;   a first compressor system for compressing said warmed carbon dioxide-enriched gas to produce at least one compressed carbon dioxide-enriched gas;   a second heat exchanger arrangement for cooling and at least partially condensing at least a portion of said compressed carbon dioxide-enriched gas as a first recycle fluid by indirect heat exchange to produce carbon dioxide-enriched fluid;   a first expansion device for expanding said carbon dioxide-enriched fluid to produce expanded carbon dioxide-enriched fluid for feeding to said distillation column system as reflux;   a second compressor system for compressing a carbon dioxide-rich gas from said distillation column system to produce at least one second recycle fluid;   a third heat exchanger arrangement for cooling and optionally condensing at least a portion of said second recycle fluid by indirect heat exchange to produce cooled carbon dioxide-rich fluid for feeding to said distillation column system;   an optional expansion device for expanding said cooled carbon dioxide-rich fluid to produce expanded carbon dioxide-rich fluid prior to being fed to said distillation column system;   a fourth heat exchanger arrangement for at least partially re-boiling said bottoms liquid by indirect heat exchange against at least one of said recycle streams to produce vapour for said distillation column system; and   a conduit arrangement for removing a further portion of said bottoms liquid, or a liquid derived from bottoms liquid, from said distillation column system,   
       wherein said first and second compression systems are capable of compressing said warmed carbon dioxide-enriched gas and said carbon dioxide-rich gas respectively to different pressures. 
     
     
         61 . Apparatus of  claim 60  comprising a conduit arrangement for feeding compressed carbon dioxide enriched-gas from said first compressor system as feed to said second compressor system. 
     
     
         62 . Apparatus of  claim 60  comprising:
 a fifth heat exchanger arrangement for warming a carbon dioxide-rich vapour by indirect heat exchange to produce warmed carbon dioxide-rich gas; 
 a conduit arrangement for feeding carbon dioxide-rich vapour from an intermediate location in said distillation column system to said fifth heat exchanger arrangement; and 
 a conduit arrangement for feeding warmed carbon dioxide-rich gas from said fifth heat exchanger arrangement to said second compressor system. 
 
     
     
         63 . Apparatus of  claim 60 , comprising a sixth heat exchanger arrangement for at least partially re-boiling liquid from an intermediate location in said distillation column system to provide additional vapour for said distillation column system. 
     
     
         64 . Apparatus for carrying out the process of  claim 59 , said apparatus comprising:
 a distillation column system for operation at super-atmospheric pressure(s) for separating crude carbon dioxide feed at sub-ambient temperature to produce carbon-dioxide-enriched vapour and bottoms liquid enriched with said at least one impurity;   a first heat exchanger arrangement for cooling and partially condensing said carbon dioxide-enriched overhead vapour by indirect heat exchange to produce at least partially condensed carbon dioxide-enriched overhead vapour as reflux for said distillation column system;   a conduit arrangement for removing carbon dioxide-enriched overhead vapour from said distillation column system;   a first expansion device for expanding carbon dioxide-rich liquid to produce expanded carbon dioxide-rich liquid;   a conduit arrangement for feeding carbon dioxide-rich liquid from an intermediate location in said distillation column system to said first expansion device;   a second heat exchange arrangement for warming and evaporating said expanded carbon dioxide-rich liquid by indirect heat exchange to provide warmed carbon dioxide-rich gas;   a compressor system for compressing a working fluid comprising said combined carbon dioxide-rich gas to produce compressed carbon dioxide-rich gas as a first recycle fluid and at least one further compressed carbon dioxide-rich gas as a second recycle fluid;   a third heat exchange system for cooling and optionally at least partially condensing said first recycle fluid by indirect heat exchange to produce cooled first carbon dioxide-rich fluid;   a conduit arrangement for combining said cooled first carbon dioxide-rich fluid with crude carbon dioxide fluid to produce said crude carbon dioxide feed for the distillation column system;   a fourth heat exchange arrangement for cooling said second recycle fluid by indirect heat exchange to produce cooled second carbon dioxide-rich fluid;   a second expansion device for expanding said cooled second carbon dioxide-rich fluid to produce expanded carbon dioxide-rich fluid;   a conduit arrangement for combining said expanded carbon dioxide-rich fluid with a fluid selected from the group consisting of said carbon dioxide-rich liquid, said expanded carbon dioxide-rich liquid, and said warmed carbon dioxide-rich gas;   a fifth heat exchanger arrangement for at least partially re-boiling a portion of said bottoms liquid by indirect heat exchange against at least one of said recycle streams to produce vapour for said distillation column system; and   a conduit arrangement for removing a further portion of said bottoms liquid, or a liquid derived from bottoms liquid, from said distillation column system,   
       wherein, in embodiments in which said expanded carbon dioxide-rich fluid is combined with said warmed carbon dioxide-rich gas, said apparatus comprises sixth heat exchanger arrangement for warming expanded carbon dioxide-rich fluid by indirect heat exchange to produce further warmed carbon dioxide-rich gas for said combination with said warmed carbon dioxide-rich gas. 
     
     
         65 . Apparatus of any of  claims 60  to  64 , wherein said heat exchanger arrangements are passages within a single main heat exchanger.