Process For Recovering Hydrogen And Carbon Dioxide

Abstract

The present invention provides a process for recovering hydrogen and carbon dioxide from a process stream utilizing a carbon dioxide separation unit and two membrane separation units. The present invention further provides a process within a hydrogen generation plant to increase recovery of hydrogen and capture equal to or greater than 80% of the carbon dioxide in the syngas stream. By using the process of the present invention, especially in terms of a hydrogen generation plant, it is possible to increase recovery of hydrogen and capture of the carbon dioxide in the syngas stream by balancing the recycle of the hydrogen rich permeate from the hydrogen membrane separation unit to the process unit and/or the water gas shift as capacity allows when a carbon dioxide separation unit, a carbon dioxide membrane separation unit and a hydrogen membrane separation unit are utilized.

1 . A process for recovering hydrogen and carbon dioxide from a process stream ( 1 ) of a process unit ( 0 ), the process stream containing at least carbon dioxide, hydrogen and methane and the process comprising the steps of:
 a) optionally compressing at least a portion of the process stream ( 1 ) in a first compressor ( 2 );   b) cooling the optionally compressed portion of the process stream ( 1 ) in a heat exchanger ( 3 ) to a temperature equal to or less than −10° C.;   c) separating and purifying the cooled process stream ( 1 ) in a carbon dioxide separation unit ( 4 ) to produce a carbon dioxide rich liquid stream ( 6 ) and a carbon dioxide lean non-condensable stream ( 5 );   d) withdrawing the carbon dioxide rich liquid stream ( 6 ) as a carbon dioxide product for further use;   e) withdrawing the carbon dioxide lean non-condensable stream ( 5 ) from the carbon dioxide separation unit ( 4 ) and passing the carbon dioxide lean non-condensable stream ( 5 ) through a hydrogen selective membrane separation unit ( 7 ) to form a hydrogen rich permeate stream ( 8 ) with the remaining components in the carbon dioxide lean non-condensable stream ( 5 ) forming a hydrogen lean residue stream ( 9 );   f) passing the hydrogen lean residue stream ( 9 ) through a carbon dioxide selective membrane separation unit ( 10 ) to form a carbon dioxide enriched permeate stream ( 11 ) with the remaining components in the hydrogen lean residue stream ( 9 ) forming a carbon dioxide depleted residue stream ( 12 );   g) optionally compressing the hydrogen rich permeate stream ( 8 ) in a second compressor ( 13 ) and recycling the hydrogen rich permeate stream ( 8 ) for use as a supplemental feed stream in the process unit ( 0 ) or for use in other processes upstream from the process unit ( 0 );   h) recycling the carbon dioxide enriched permeate stream ( 11 ) to the process stream ( 1 ) prior to the compressor ( 2 ) or within the compressor ( 2 ) between stages of compression or optionally compressing the carbon dioxide enriched permeate stream ( 11 ) and recycling the carbon dioxide enriched permeate stream ( 11 ) to be used in the carbon dioxide separation unit ( 4 ) and recycling the carbon dioxide depleted residue stream ( 12 ) to be used as a supplemental feed stream or as both a fuel and a feed stream in the present process or in other processes.   
     
     
         2 . The process of  claim 1 , wherein the process unit ( 0 ) is a pressure swing adsorption unit and the process stream ( 1 ) is a tail gas from the pressure swing adsorption unit. 
     
     
         3 . The process of  claim 1 , wherein the hydrogen rich permeate stream ( 8 ) is recycled to be used as a supplemental feed stream upstream from the process unit ( 0 ). 
     
     
         4 . The process of  claim 1 , wherein the hydrogen rich permeate stream ( 8 ) is recycled to be used as a supplemental feed stream in the process unit ( 0 ). 
     
     
         5 . The process of  claim 2 , wherein the hydrogen rich permeate stream ( 8 ) is split into a first hydrogen rich permeate fraction ( 8 . 1 ) to be used as a supplemental feed stream upstream from the process unit ( 0 ) and a second hydrogen rich permeate fraction ( 8 . 2 ) to be used as a supplemental feed stream to be added to the feed gas ( 15 ) prior to being introduced into the pressure swing adsorption unit ( 0 ) with the proportion of each fraction depending upon the percentage of production in the process upstream of the pressure swing adsorption unit ( 0 ). 
     
     
         6 . The process of  claim 1 , wherein the process stream ( 1 ) is compressed to a pressure above 35 bar, the compression occurring either 1) completely in the compressor ( 2 ) prior to step f), 2) completely in a second compressor that is part of the carbon dioxide separation unit ( 4 ) with the compression occurring just prior to the separation and purification of step g), or 3) partially in the compressor ( 2 ) prior to step f) and then the remaining compression in the second compressor that is a part of the carbon dioxide separation unit ( 4 ) in order to reach a pressure level that is equal to or greater than 35 bar. 
     
     
         7 . The process of  claim 1 , wherein the hydrogen rich permeate stream ( 8 ) is subjected to a shift reaction in an optional second water gas shift reactor ( 32 ) prior to being recycled to the water gas shift reactor ( 21 ), the process unit ( 0 ) or both the water gas shift unit ( 21 ) and the process unit ( 0 ). 
     
     
         8 . The process of  claim 1 , wherein the water gas shift reactor ( 32 ) is a low temperature water gas shift reactor. 
     
     
         9 . The process of  claim 1 , wherein the carbon dioxide lean non-condensable stream ( 5 ) is subjected to a shift reaction in an optional third water gas shift reactor ( 33 ) prior to being passed thorough the hydrogen and carbon dioxide membrane separation units ( 7 ,  10 ). 
     
     
         10 . The process of  claim 9 , wherein the water gas shift reactor ( 33 ) is a low temperature water gas shift reactor.