Abstract:
Hydrogen and carbon monoxide are separated from a condensate-containing gaseous mixture thereof by using (a) a “first” stripping column ( 8 ) to remove the hydrogen content of the CO-loaded methane stream ( 6 ) obtained by washing CO from the gaseous mixture, or the vapor portion from a phase separation thereof, ascending a methane wash column ( 2 ) and (b) a “second” stripping column or a flash separator ( 30 ) to remove the hydrogen content of the feed gas condensate ( 9 ) obtained from the methane wash column ( 2 ), or the phase separation. The vapor stream ( 32 ) from the second stripping column or flash separator ( 30 ) is fed to the first stripping column ( 8 ). The liquid stream from the first stripping column and the liquid stream ( 33 ) from the second stripping column or flash separator ( 30 ) are fed ( 16, 17, 19  &amp;  20; 34  to  39 ) to different locations of a separation column ( 18 ) providing a gaseous carbon monoxide product stream ( 21 ) and a liquid methane wash recycle stream ( 3 ). The process improves the efficiency of the separation by avoiding dilution of the CO concentration of the feed gas condensate ( 9 ) with the CO-loaded methane stream ( 6 ) which occurs in prior art CO/hydrogen separations using a methane wash.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a process and apparatus for separation of hydrogen and carbon monoxide from gaseous mixtures thereof. It has particular, but not exclusive, application to the recovery of hydrogen and carbon monoxide from synthesis gas. 
     BACKGROUND OF THE INVENTION 
     Carbon monoxide is usually obtained by separation from synthesis gas produced by catalytic conversion or partial oxidation of natural gas, oils or other hydrocarbon feedstock. Synthesis gas consists primarily of hydrogen and carbon monoxide and, depending on the level of purity, typically also contains small amounts of inter alia methane and nitrogen. It is well known to separate carbon monoxide from synthesis gas by a cryogenic separation process in which carbon monoxide is removed by a low temperature scrubbing step using liquid methane in a wash column to provide a CO-loaded methane liquid containing some, typically 2 to 4%, hydrogen. Residual hydrogen is removed from the CO-loaded methane liquid in, for example, a stripping column or flash separator to meet the required carbon monoxide product specification and the resultant hydrogen-stripped CO-loaded methane liquid is separated into a gaseous carbon monoxide product and liquid methane in a separation column. The bulk of the liquid methane is recycled to provide the methane wash liquid and a portion of the carbon monoxide product can be recycled to provide a heat pump stream. 
     The CO-loaded methane liquid can be withdrawn entirely from the sump of the methane wash column, in which case it is admixed with condensate from the feed gas. 
     However, the condensate usually has a much higher CO concentration (typically 60 to 70% CO) than the CO-loaded methane (typically 20 to 30% CO), which higher concentration is diluted by admixture with the CO-loaded methane thereby decreasing the efficiency of the subsequent separation of CO from methane. In order to mitigate this reduction in potential efficiency, the CO-loaded methane can be separately withdrawn from a location above the synthesis gas feed and fed to a stripping column at a higher location than the feed gas condensate. 
     European Patent Publication No. 0895961A discloses the separation of synthesis gas, or other gaseous mixtures of hydrogen and carbon monoxide, by a process in which the CO-loaded methane and feed gas condensate are separately fed from the methane wash column to vertically spaced locations of a stripping column. The stripping column is refluxed with a methane-rich scrubbing liquid withdrawn from an intermediate location of the methane wash column. Preferably, the hydrogen-stripped CO-loaded methane liquid is split into two substreams. One substream is subcooled and the subcooled liquid introduced into the separation column. The other substream is at least partially vaporized and introduced into the separation column at a location below that of the subcooled substream. 
     U.S. Pat. No. 5133793 discloses the separation of synthesis gas, or other gaseous mixtures of hydrogen and carbon monoxide, by a process in which feed gas condensate is separated from the feed prior to the methane wash column. Only the vapor portion from that separation is fed to the wash column. The condensate is vaporized and fed to the stripping column at a location below that of the sump liquid from the wash column. The hydrogen-stripped CO-loaded methane liquid is subcooled and split into three substreams. One substream is introduced, at about its bulb temperature, at upper location of the separation column. Another substream is vaporized and introduced, at about its dew point, at a lower location of the separation column. The third substream is vaporized and introduced, at a temperature intermediate that of the other two substreams, at an intermediate location of the separation column. 
     Although the feeding of feed gas condensate to a stripping column separately from the CO-loaded methane does increase the efficiency with which hydrogen is removed in that column, it does not obviate the loss in potential efficiency in reduction of the CO concentration of the condensate on admixture with the CO-loaded methane. However, the necessity of this loss has been accepted in the art. 
     It is an object of the present invention to improve the efficiency of separation of carbon monoxide from a mixture with hydrogen. This is achieved by reducing the extent to which the CO concentration in the feed gas condensate is diluted prior to the separation of CO and methane. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improvement in a process for separating hydrogen and carbon monoxide from a condensate-containing gaseous mixture thereof in which the gaseous mixture, or a vapor portion from phase separation thereof, is scrubbed with a liquid methane wash stream in a methane wash column to provide a gaseous hydrogen product stream and a liquid CO-loaded methane stream. This CO-loaded methane stream is separated into a gaseous hydrogen-rich stream and a liquid CO/methane stream in a first stripping column and the resulting CO/methane stream is separated into a gaseous carbon monoxide stream and a liquid methane stream in a separation column. The improvement consists in that feed gas condensate from the methane wash column, or from the phase separation, is separated into a second gaseous hydrogen-enriched stream and another (“second”) liquid CO/methane stream in a second stripping column or a flash separator; the second gaseous hydrogen-enriched stream is fed to the first stripping column; and the second liquid CO/methane stream is fed to the separation column. 
     The invention also provides an improved apparatus for separating hydrogen and carbon monoxide from a condensate-containing gaseous mixture. The apparatus comprises a methane wash column; means for feeding the gaseous mixture to the methane wash column and optionally including phase separation means for removing a feed gas condensate from the gaseous mixture; a first stripping column; a carbon monoxide/methane separation column; means for feeding CO-loaded methane from the methane wash column to the stripping column; means for feeding liquid CO/methane from the stripping column to the separation column; and means for recycling liquid methane from the separation column to the methane wash column. The improvement consists in that the apparatus further comprises second stripping column or a flash separator; means for feeding feed gas condensate from the methane wash column, or from the phase separation means, to the second stripping column or flash separator; 
     means for feeding a second gaseous hydrogen-enriched stream from the second stripping column or flash separator to the first stripping column; and means for feeding a second liquid CO/methane stream from the second stripping column or flash separator to the separation column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a process and apparatus in accordance with the teaching of European Patent Publication No. 0895961A; 
     FIG. 2 is a schematic representation of a preferred embodiment of the process and apparatus of the present invention; and 
     FIG. 3 is a schematic representation of another embodiment of the process and apparatus of the invention. 
     The same reference numerals are used to identify the same or equivalent items in all three drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Accordingly, in a first aspect of the invention, there is provided a process for separating hydrogen and carbon monoxide from a condensate-containing gaseous mixture thereof, said process comprising: 
     (a) scrubbing the gaseous mixture, or a vapor portion from phase separation thereof, with a liquid methane wash stream in a methane wash column to provide a gaseous hydrogen product stream and a “first” liquid CO-loaded methane stream; 
     (b) separating said CO-loaded methane stream into a gaseous hydrogen-rich stream and a liquid CO/methane stream in a “first” stripping column; and 
     (c) separating said first liquid CO/methane stream into a gaseous carbon monoxide stream and a liquid methane stream in a separation column, 
     wherein feed gas condensate from the methane wash column, or from the phase separation, is separated into a “second” gaseous hydrogen-enriched stream and a “second” liquid CO/methane stream in a “second” stripping column or a flash separator; the second gaseous hydrogen-enriched stream is fed to the first stripping column; and the second liquid CO/methane stream is fed to the separation column. 
     In a second aspect of the invention there is provided an apparatus for separating hydrogen and carbon monoxide from a condensate-containing gaseous mixture thereof by a process of the first aspect, said apparatus comprising: 
     (a) a methane wash column; 
     (b) means for feeding the gaseous mixture to the methane wash column and optionally including phase separation means for removing a feed gas condensate from the gaseous mixture; 
     (c) a “first” stripping column; 
     (d) a carbon monoxide/methane separation column; 
     (e) means for feeding CO-loaded methane from the methane wash column to the first stripping column; 
     (f) means for feeding liquid CO/methane from the first stripping column to the separation column; and 
     (g) means for recycling liquid methane from the separation column to the methane wash column, 
     wherein the apparatus further comprises 
     a “second” stripping column or a flash separator; 
     means for feeding feed gas condensate from the methane wash column, or from the phase separation means, to the second stripping column or flash separator; 
     means for feeding a “second” gaseous hydrogen-enriched stream from the second stripping column or flash separator to the first stripping column; and 
     means for feeding a “second” liquid CO/methane stream from the second stripping column or flash separator to the separation column. 
     The invention has particular application to the separation of carbon monoxide from synthesis gas produced by catalytic conversion or partial oxidation of natural gas, oils or other hydrocarbon feedstock. However, it is of general application to the cryogenic separation of other gaseous mixtures containing hydrogen and carbon monoxide, especially those consisting primarily of hydrogen and carbon monoxide. 
     In a presently preferred embodiment, the condensate-containing gaseous mixture is fed to the methane wash column and the CO-loaded methane and feed gas condensate are separately removed from vertically spaced locations of said column. According to the alternative embodiment, the condensate-containing gaseous mixture is phase separated to provide the feed gas condensate; the uncondensed (vapor) portion is fed to the methane wash column; and the condensate is fed to the second stripping column or flash separator. 
     It is preferred that the feed gas condensate is partially vaporized before being fed to the second stripping column or flash separator. 
     The second liquid CO/methane stream can be split into at least two substreams; one substream being subcooled and the subcooled liquid introduced into the separation column and another substream being at least partially vaporized and introduced into the separation column at a location below that of the subcooled substream. 
     The first liquid CO/methane stream also can be split into at least two substreams; one substream being subcooled and the subcooled liquid introduced into the separation column and another substream being at least partially vaporized and introduced into the separation column at a location below that of said subcooled substream. 
     The first stripping column can be refluxed with a methane-rich liquid stream withdrawn from an intermediate location of the methane wash column above the level of removal of the CO-loaded methane therefrom as taught in European Patent Publication No. 0895961 A, the entirety of the disclosure of which is incorporated by this reference. 
     Referring first to FIG. 1, partially condensed crude synthesis gas is fed via conduit  1  to the bottom of methane wash column  2 . The vapor rising up through the wash column trays or packing is scrubbed with liquid methane introduced at the top of the column via conduit  3 . This dissolves carbon monoxide into the liquid methane and produces an overhead hydrogen product in conduit  4 . The heat of solution of carbon monoxide in the wash methane is typically removed by indirect heat exchange with at least part of a liquid carbon monoxide heat pump stream in heat exchanger(s)  5 . This can typically be accomplished by at least one contactor heat exchanger as described in U.S. Pat. No. 3,813,889 and is shown only schematically here. The number of contactor heat exchangers, their position and configuration within the methane wash column stages, is such as to most economically provide near isothermal operation of the column. 
     The CO-loaded methane from the bottom stage of the methane wash column, (which typically contains 2% to 4% H 2 ), is removed via conduit  6 , reduced in pressure by control valve  7 , and introduced into stripping column  8 , containing trays or packing, where hydrogen is stripped from the liquid in order to achieve the required carbon monoxide product purity specification. Condensate in the crude synthesis gas feed is removed from the sump of the methane wash column via conduit  9 , reduced in pressure by control valve  10 , and partly vaporized in heat exchanger  11 , preferably by indirect heat exchange with at least part of the crude synthesis gas upstream of conduit  1 . Alternatively other heat exchange means could be provided. The partly vaporized liquid is then fed to stripping column  8  several stages below the introduction of the liquid in conduit  6  to provide part of the stripping vapor for hydrogen removal from the latter stream. A reboiler  12  in the bottom of the stripping column  8  provides stripping vapor for the liquid in both feed streams. The liquid introduced via conduit  6  also serves to scrub some of the carbon monoxide from the vapor passing through the hydrogen stripping column. A methane rich scrubbing liquid is withdrawn from an appropriate stage of the methane wash column via conduit  13 , reduced in pressure by control valve  14 , and used to provide wash liquid to the top of the stripping column  8  to further reduce carbon monoxide losses in the reject hydrogen stream from conduit  15 . 
     Liquid from the bottom of the stripping column  8  is subcooled in heat exchanger  16  and then divided into two substreams. The first substream is reduced in pressure by control valve  17 , and introduced to carbon monoxide/methane separation column  18 . The second substream is reduced in pressure by control valve  19 , partially vaporized in heat exchanger  20 , and introduced to separation column  18  several stages below the subcooled liquid from control valve  17 . The two feeds are separated in separation column  18  into carbon monoxide and methane streams in conduits  21  and  22  respectively. The column  18  is reboiled by reboiler  23 , and reflux is provided by direct introduction of liquid carbon monoxide via control valve  24  and conduit  25 . Heat transfer in heat exchangers  16  and  20  is accomplished by indirect heat exchange with other process streams and is not detailed here. Purified methane liquid in conduit  22  is subcooled in subcooler  26  by indirect heat exchange with other process streams, not detailed here, and then divided. The major part of stream  22  is pumped by pump  27  to methane wash column pressure, further subcooled in heat exchanger  28 , and introduced to the top of the methane wash column  2  via conduit  3 . The minor portion of stream  22  is removed from the distillation system via control valve  29 . 
     Table 1 summarizes a mass balance for a typical application of the process of FIG.  1 . 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Stream 
               
             
          
           
               
                   
                 1 
                 3 
                 4 
                 6 
                 9 
                 13 
                 15 
                 21 
                 22 
                 25 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 bar (a) 
                 21.7 
                 22.6 
                 21.4 
                 21.6 
                 21.6 
                 21.5 
                 5.5 
                 2.8 
                 3.0 
                 2.8 
               
               
                 Temperature 
                 deg C. 
                 −180 
                 −181 
                 −181 
                 −173 
                 −180 
                 −178 
                 −166 
                 −182 
                 −146 
                 −182 
               
               
                 Flowrate 
                 kgm/h 
                 100.0 
                 39.0 
                 69.4 
                 49.0 
                 17.5 
                 3.0 
                 2.8 
                 31.7 
                 42.9 
                 7.9 
               
               
                 Hydrogen 
                 mol % 
                 70.3 
                   
                 98.7 
                 2.2 
                 3.6 
                 1.8 
                 62.5 
                   
                   
                 0 
               
               
                 Nitrogen 
                 mol % 
                 1.4 
                   
                 0.2 
                 1.4 
                 3.1 
                 2.2 
                 4.2 
                 5.0 
                   
                 5.0 
               
               
                 Carbon monoxide 
                 mol % 
                 23.2 
                   
                   
                 23.0 
                 66.8 
                 7.1 
                 21.4 
                 95.0 
                   
                 95.0 
               
               
                 Methane 
                 mol % 
                 5.0 
                 100 
                 1.1 
                 73.3 
                 26.4 
                 89.0 
                 11.9 
                   
                 100.0 
                 0 
               
               
                 Vapor fraction 
                   
                 0.8232 
                 0 
                 1 
                 0 
                 0 
                 0 
                 1 
                 1 
                 0 
                 0.0462 
               
               
                   
               
             
          
         
       
     
     Referring now to FIG. 2, the illustrated preferred embodiment of the invention differs from the process and apparatus of FIG. 1 in that the partially vaporized condensate from heat exchanger  11  is fed to the top of second stripping column  30 . This column  30  contains trays or packing and reboiler  31  in the bottom of the column provides stripping vapor to strip hydrogen from the liquid in order to achieve the required CO product purity specification. Vapor from second stripping column  30  is fed via conduit  32  to stripping column  8  several stages below the introduction of the CO-loaded methane from conduit  6  to provide part of the stripping vapor for hydrogen removal from the CO-loaded methane. 
     Liquid in conduit  33  from the bottom of the second stripping column  30  is subcooled in heat exchanger  34  and divided into two substreams  35  and  36 . Substream  35  is reduced in pressure by control valve  37  and introduced to column  18  several stages above the subcooled liquid from control valve  17 . Substream  36  is reduced in pressure by control valve  38 , partially vaporized in heat exchanger  39 , and introduced to column  18  at about the same location as the subcooled liquid from control valve  17 . The four feeds are separated in column  18  into the purified CO and methane streams in conduits  21  and  22  respectively. Heat transfer in heat exchangers  34  and  39  is accomplished by indirect heat exchange with other process streams and is not detailed here. 
     Table 2 summarizes a mass balance for a typical application of the process of FIG.  2 . 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Stream 
               
             
          
           
               
                   
                 1 
                 3 
                 4 
                 6 
                 9 
                 32 
                 13 
                 15 
                 21 
                 22 
                 25 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 bar (a) 
                 21.7 
                 22.6 
                 21.4 
                 21.6 
                 21.6 
                 5.6 
                 21.5 
                 5.5 
                 2.8 
                 3.0 
                 2.8 
               
               
                 Temperature 
                 deg C. 
                 −180 
                 −181 
                 −181 
                 −173 
                 −180 
                 −168 
                 −178 
                 −167 
                 −182 
                 −146 
                 −182 
               
               
                 Flowrate 
                 kgm/h 
                 100.0 
                 39.1 
                 69.4 
                 48.2 
                 17.5 
                 7.4 
                 3.9 
                 2.8 
                 30.3 
                 43.0 
                 6.5 
               
               
                 Hydrogen 
                 mol % 
                 70.3 
                   
                 98.7 
                 2.2 
                 3.6 
                 8.6 
                 1.8 
                 62.4 
                   
                   
                   
               
               
                 Nitrogen 
                 mol % 
                 1.4 
                   
                 0.1 
                 1.4 
                 3.1 
                 4.6 
                 2.1 
                 4.7 
                 5.0 
                   
                 5.0 
               
               
                 Carbon monoxide 
                 mol % 
                 23.2 
                   
                   
                 23.3 
                 66.8 
                 80.8 
                 7.2 
                 21.3 
                 95.0 
                   
                 95.0 
               
               
                 Methane 
                 mol % 
                 5.0 
                 100.0 
                 1.1 
                 73.0 
                 26.4 
                 6.0 
                 88.9 
                 11.5 
                   
                 100.0 
                   
               
               
                 Vapor fraction 
                   
                 0.8232 
                 0 
                 1 
                 0 
                 0 
                 1 
                 0 
                 1 
                 1 
                 0 
                 0.0462 
               
               
                   
               
             
          
         
       
     
     Referring now to FIG. 3, another embodiment of the present invention differs from that of FIG. 2 in that condensate is separated from the feed  1  by a phase separator  40  and only the vapor portion fed to the wash column  2  via conduit  41 . The CO-loaded methane and any condensate from the vapor portion feed is withdrawn from the sump of the column  2  and fed, via conduit  6  and control valve  7 , to stripping column  8 . The condensate separated in phase separator  40  is fed, via conduit  9 , control valve  10  and heat exchanger  11 , to the second stripping column  30 . 
     Table 3 summarizes a mass balance for a typical application of the process of FIG.  3 . 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Stream 
               
             
          
           
               
                   
                 1 
                 3 
                 4 
                 6 
                 9 
                 32 
                 13 
                 15 
                 21 
                 22 
                 25 
               
               
                   
                   
               
             
          
           
               
                 Pressure 
                 bar (a) 
                 21.7 
                 22.6 
                 21.4 
                 21.6 
                 21.6 
                 5.6 
                 21.5 
                 5.5 
                 2.8 
                 3.0 
                 2.8 
               
               
                 Temperature 
                 deg C. 
                 −180 
                 −181 
                 −181 
                 −173 
                 −180 
                 −168 
                 −178 
                 −167 
                 −182 
                 −146 
                 −182 
               
               
                 Flowrate 
                 kgm/h 
                 100.0 
                 39.1 
                 69.4 
                 48.2 
                 17.5 
                 7.4 
                 3.9 
                 2.8 
                 30.3 
                 43.0 
                 6.5 
               
               
                 Hydrogen 
                 mol % 
                 70.3 
                   
                 98.7 
                 2.2 
                 3.6 
                 8.6 
                 1.8 
                 62.4 
                   
                   
                   
               
               
                 Nitrogen 
                 mol % 
                 1.4 
                   
                 0.1 
                 1.4 
                 3.1 
                 4.6 
                 2.1 
                 4.7 
                 5.0 
                   
                 5.0 
               
               
                 Carbon monoxide 
                 mol % 
                 23.2 
                   
                   
                 23.3 
                 66.8 
                 80.8 
                 7.2 
                 21.3 
                 95.0 
                   
                 95.0 
               
               
                 Methane 
                 mol % 
                 5.0 
                 100.0 
                 1.1 
                 73.0 
                 26.4 
                 6.0 
                 88.9 
                 11.5 
                   
                 100.0 
                   
               
               
                 Vapor fraction 
                   
                 0.8232 
                 0 
                 1 
                 0 
                 0 
                 1 
                 0 
                 1 
                 1 
                 0 
                 0.0462 
               
               
                   
               
             
          
         
       
     
     It will be appreciated that the invention is not restricted to the specific details described above with reference to FIGS. 2 and 3 but that numerous modifications can be made without departing from the spirit or scope of the invention as defined in the following claims. For example, one or more of the following modifications can be made to the process and apparatus of either FIG. 2 or FIG.  3 : 
     the second stripper  30  could be replaced by a flash separator; 
     the methane wash could be omitted from the stripping column  8 ; 
     the CO-loaded methane in conduit  6  could be preheated by indirect heat exchange after pressure reduction via valve  7 ; 
     one or more of heat exchangers  11 ,  16 ,  20 ,  34 , and  39  could be omitted; 
     one of heat exchangers  26  and  28  could be omitted; and 
     heat of solution could be removed by indirect heat exchange with at least part of a heat pump stream in a contactor heat exchanger located at the top of the stripping column  8  to achieve higher CO recovery or reduce the quantity of methane rich liquid used for washing. 
     It will be appreciated from the preceding description of the present invention that it differs from the prior art by utilizing a second stripper column or a flash separator to reject hydrogen, thus keeping the CO-richer condensed feed liquid separate from the CO-leaner methane wash column liquid. The invention therefore benefits by reducing the energy required for CO/methane separation. Consequently the recycle heat pump flowrate that is required for the regeneration column separation is reduced, resulting in a compressor power reduction of about 3% to 5%.