Abstract:
An apparatus separating carbon dioxide from combustion gas using separation membranes, which includes: a first separation membrane in which combustion gas is injected into an inlet side of the first separation membrane; a second separation membrane in which residue gas of the first separation membrane is injected into an inlet side of the second separation membrane; and a third separation membrane in which permeate gas of the first separation membrane is injected into an inlet side of the third separation membrane, wherein at least a part of permeate gas of the third separation membrane is captured, and residue gas of the third separation membrane is injected into the inlet side of the first separation membrane or the second separation membrane. The present invention can be easily applied to an actual process by efficiently separating carbon dioxide using separation membranes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2015-0076621, filed on May 29, 2015 and Korean Patent Application No. 10-2015-0076622, filed on May 29, 2015 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to an apparatus for separating carbon dioxide from combustion gas and, more particularly, to an apparatus for efficiently separating carbon dioxide from combustion gas using multiple separation membranes and for allowing easy application to an actual process. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, technologies for capturing carbon dioxide that contributes to greenhouse effects have been developed recent years. The technologies for capturing carbon dioxide can be classified into post-combustion capture, pre-combustion capture, and oxyfuel combustion capture. Post-combustion capture can be classified into chemical absorption using an aqueous amine or ammonia absorbent, dry-type absorption using a solid absorbent instead of an existing aqueous absorbent, and a membrane separation using a separation membrane, etc. The membrane separation method has an advantage of being an environmentally friendly process. However, only membrane materials and modules have been mainly developed, and studies on post-combustion capture have been conducted by only a minority of companies. 
         [0006]    U.S. Pat. No. 7,964,020 discloses gas separation processes using membranes with permeate sweep to remove carbon dioxide from combustion gases. 
         [0007]    U.S. Pat. No. 7,964,020 includes processes of injecting a flue gas stream to be treated into a feed side of a membrane, injecting sweep gas, usually air, to a permeate side of the membrane, and passing a permeate stream and the sweep gas to a combustor. In this way, the permeate stream of the membrane is withdrawn to a boiler with the sweep gas, thereby building up carbon dioxide concentration on the feed side of the membrane. However, the above processes disclosed in U.S. Pat. No. 7,964,020 have a limited application to an actual process for a power plant since the processes cause a change in efficiency of the boiler. 
         [0008]    U.S. Pat. No. 4,264,338 discloses a method for separating gases by a process using multiple stages of membrane separation, whereby an unpermeated residue of a second stage membrane or a third stage membrane is recovered, and a permeant mixture is passed to an inlet of a first stage membrane and therefore a separation factor of the first stage membrane is increased. However, the gas separation method recovering and using an unpermeated residue has a limitation in increasing carbon dioxide separation performance of a gas separation system. 
       DOCUMENTS OF RELATED ART 
       [0009]    (Patent Document 1) U.S. Pat. No. 7,964,020 (Gas separation process using membranes with permeate sweep to remove CO2 from combustion gases) 
         [0010]    (Patent Document 2) U.S. Pat. No. 4,264,338 (Method for separating gases) 
         [0011]    (Patent Document 3) U.S. Pat. No. 5,102,432 (Three-stage membrane gas separation process and system) 
         [0012]    (Thesis Document 1) A parametric study of CO2/N2 gas separation membrane processes for post-combustion capture (L Zhao, E Riensche, R Menzer, L Blum; Journal of Membrane, 2008, Elsevier) 
       SUMMARY OF THE INVENTION 
       [0013]    Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and, in one aspect, the present invention is intended to propose an apparatus for efficiently separating carbon dioxide from combustion gas using multiple separation membranes. 
         [0014]    In another aspect, the present invention is intended to propose an apparatus for separating carbon dioxide from combustion gas, which can be easily applied to an actual process. 
         [0015]    In order to achieve the above objects, according to the present invention, an apparatus for separating carbon dioxide from combustion gas using multiple separation membranes includes: a first separation membrane in which combustion gas is injected into an inlet side of the first separation membrane; a second separation membrane in which residue gas of the first separation membrane is injected into an inlet side of the second separation membrane; and a third separation membrane in which permeate gas of the first separation membrane is injected into an inlet side of the third separation membrane, wherein at least a part of permeate gas of the third separation membrane is captured, and residue gas of the third separation membrane is injected into the inlet side of the first separation membrane or the second separation membrane. 
         [0016]    In the separation apparatus, at least another part of the permeate gas of the third separation membrane may be injected into the inlet side of the third separation membrane, and the residue gas of the third separation membrane may be injected into the inlet side of the first separation membrane. At this time, permeate gas of the second separation membrane may be injected into the inlet side of the first separation membrane. Further, selectively at least another part of the permeate gas of the third separation membrane may be injected into the inlet side of the third separation membrane, and the residue gas of the third separation membrane may be injected into the inlet side of the second separation membrane. 
         [0017]    Further, in the separation apparatus, at least a part of permeate gas of the second separation membrane may be injected into the inlet side of the second separation membrane, and at least another part of the permeate gas of the second separation membrane may be injected into the inlet side of the first separation membrane. 
         [0018]    Furthermore, a sensor that senses carbon dioxide concentration of the permeate gas of the second separation membrane or the residue gas of the third separation membrane, which is injected into the inlet side of the first separation membrane, may be provided. A pressure unit may be provided on at least one inlet side of the first to the third separation membranes, and a decompression unit may be provided on at least one permeate side of the first to the third separation membranes. 
         [0019]    Meanwhile, the separation apparatus further includes a fourth separation membrane, in which permeate gas of the second separation membrane may be injected into an inlet side of the fourth separation membrane, residue gas of the fourth separation membrane may be injected into the inlet side of the second separation membrane, and permeate gas of the fourth separation membrane may be injected into the inlet side of the first separation membrane. 
         [0020]    In addition, the apparatus for separating carbon dioxide from combustion gas using multiple separation membranes of the present invention is provided with: a separation membrane; a first line in which the combustion gas is injected into an inlet side of the separation membrane; a second line in which a residue gas of the separation membrane is discharged; and a third line in which a permeate gas of the separation membrane is injected into the inlet side of the separation membrane. 
         [0021]    According to the present invention as described above, the present invention can efficiently separate carbon dioxide from combustion gas using multiple separation membranes. Further, the present invention can provide an apparatus for separating carbon dioxide from combustion gas, which can be easily applied to an actual process. 
       DESCRIPTION OF REFERENCE NUMERALS 
       [0000]    
       
         
           
               104 ,  106 ,  107 ,  113 ,  206 ,  402 ,  405 ,  414 ,  424 ,  510 : separation membrane 
               102 ,  111 ,  122 ,  204 ,  410 ,  420 : compressor 
               109 ,  116 ,  120 ,  202 ,  208 ,  512 : vacuum pump 
           
         
       
     
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
           [0026]      FIG. 1  is a block diagram showing a carbon dioxide separation system according to a first embodiment of the present invention; 
           [0027]      FIG. 2  is a block diagram showing a carbon dioxide separation system according to a second embodiment of the present invention; 
           [0028]      FIG. 3  is a block diagram showing a carbon dioxide separation system according to a third embodiment of the present invention; 
           [0029]      FIG. 4  is a block diagram showing a carbon dioxide separation system according to a fourth embodiment of the present invention; and 
           [0030]      FIG. 5  is a block diagram showing a carbon dioxide separation system according to a fifth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0031]    Hereinbelow, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
         [0032]    In the present invention, self-recycle means that permeate gas of a specific separation membrane is injected into an inlet side of itself. Further, inlet gas means gas that is injected into the inlet side of the specific separation membrane, permeate gas means gas that is discharged by passing through the specific separation membrane, and residue gas means gas that is discharged without passing through a permeate side of a separation membrane. 
         [0033]      FIG. 1  is a block diagram showing a carbon dioxide separation system  100  according to a first embodiment of the present invention. As shown in  FIG. 1 , the carbon dioxide separation system  100  includes three separation membranes  104 ,  106 , and  113 . Compressors  102 ,  112 , and  122  may be provided on inlet sides of the separation membranes  104 ,  106 , and  113 , and vacuum pumps  109 ,  116 , and  120  may be selectively provided on permeate sides of the separation membranes  104 ,  106 , and  113 . The compressor may be substituted with a pressure unit, such as a blower, and the vacuum pump may be substituted with a decompression unit, such as a fan. The pressure unit or the decompression unit may be omitted in accordance with pressure conditions. 
         [0034]      FIG. 1  illustrates the configuration of the separation membrane  104  in which combustion gas is injected into an inlet side of the separation membrane  104 , the separation membrane  106  in which residue gas of the separation membrane  104  is injected into an inlet side of the separation membrane  106 , and the separation membrane  113  in which permeate gas of the separation membrane  104  is injected into an inlet side of the separation membrane  113 , wherein at least a part of permeate gas of the separation membrane  113  is captured, and residue gas of the separation membrane  113  is injected into the inlet side of the separation membrane  104 . 
         [0035]    Specifically, combustion gas discharged from a boiler, etc., is injected into the compressor  102  via line  101 , the compressor  102  compresses the combustion gas and injects the combustion gas into the inlet side of the separation membrane  104  via line  103 . Carbon dioxide of the combustion gas that is injected into the inlet side of the separation membrane  104  flows into a permeate side of the separation membrane  104 . The vacuum pump  109  applies a vacuum to the permeate side of the separation membrane  104  via line  108  to smoothly flow the combustion gas that is injected into the inlet side of the separation membrane  104  into the permeate side of the separation membrane  104 . Residue gas that does not pass through the separation membrane  104  is injected into the inlet side of the separation membrane  106  via line  105  from a residue of the separation membrane  104 . 
         [0036]    Residue gas of the separation membrane  106  is discharged via line  107 . The residue gas has carbon dioxide removal rate of approximately equal to or greater than about 80% and has carbon dioxide concentration of approximately equal to or less than about 3%. Permeate gas of the separation membrane  106  is suctioned by the vacuum pump  120  and supplied to the compressor  102  via line  119  and line  101 . Further, a part of permeate gas of the separation membrane  106  is compressed by the compressor  122  and may be injected into the inlet side of the separation membrane  106  via line  121  and line  105 . The permeate gas of the separation membrane  106  is injected into the inlet side of the separation membrane  106  via line  121  and line  105  and therefore the separation membrane  106  has a self-recycle loop. The self-recycle loop increases carbon dioxide concentration of the inlet side of the separation membrane  106  and therefore separation performance of the separation membrane  106  is improved. 
         [0037]    The permeate gas of the separation membrane  104  is suctioned by the vacuum pump  109  and supplied to the compressor  111  via line  110 , and compressed by the compressor  111  and injected into the inlet side of the separation membrane  113  via line  112 . The residue gas of the separation membrane  113  is injected into the inlet side of the separation membrane  104 . The permeate gas of the separation membrane  113  is suctioned by the vacuum pump  116  via line  115  and supplied to a carbon dioxide capture apparatus (not shown in the drawing). Further, a part of the permeate gas of the separation membrane  113  is supplied to the compressor  111  via line  118  and may be injected into the inlet side of the separation membrane  113 . The permeate gas of the separation membrane  113  is injected into the inlet side of the separation membrane  113  and therefore carbon dioxide selectivity of the separation membrane  113  is also improved by the self-recycle loop. 
         [0038]    The permeate gas of the separation membrane  106  is injected into the inlet side of the separation membrane  106  by dividing the permeate gas of the separation membrane  106 , and therefore carbon dioxide selectivity of the separation membrane  106  is improved by increasing carbon dioxide concentration of inlet gas of the separation membrane  106 . Further, the permeate gas of the separation membrane  106  having high carbon dioxide concentration is injected into the inlet side of the separation membrane  104 . Moreover, the residue gas of the separation membrane  113  having the self-recycle loop is injected into the inlet side of the separation membrane  104 . As a result, carbon dioxide selectivity of the separation membrane  104  is improved by increasing carbon dioxide concentration of inlet gas of the separation membrane  104 . By this action, carbon dioxide purity of permeate gas of the separation membrane  113  is improved equal to or greater than 80%. 
         [0039]    Composition of the combustion gas supplied to the compressor  102  via line  101  are determined by performance of a combustion apparatus, and typically carbon dioxide concentration of the combustion gas is approximately 10% to 30%. In the permeate gas of the separation membrane  106  supplied to the compressor  102  via line  119  and the residue gas of the separation membrane  113  injected to the inlet side of the separation membrane  104  via line  114 , when carbon dioxide concentration is equal to or greater than initial injection concentration via line  101 , carbon dioxide separation performance of the overall system  100  can be improved. Thus, a carbon dioxide concentration sensor may be provided on line  119  and/or line  114  to sense carbon dioxide concentration of permeate gas of the separation membrane  106  and residue gas of the separation membrane  113 . When carbon dioxide concentration is equal to or less than initial injection concentration, operation of the compressors  102 ,  112 , and  122 , or the vacuum pumps  109 ,  116 , and  120  can be controlled. The compressor may be substituted with the pressure unit, such as the blower, and the vacuum pump may be substituted with the decompression unit, such as the fan. The pressure unit or the decompression unit may be omitted in accordance with pressure conditions. 
         [0040]      FIG. 2  is a block diagram showing a carbon dioxide separation system  200  according to a second embodiment of the present invention. As shown in  FIG. 2 , the carbon dioxide separation system  200  includes four separation membranes  104 ,  106 ,  113 , and  206 . Compressors  102 ,  111 , and  204  may be provided on inlet sides of the separation membranes  104 ,  106 ,  113 , and  206 , and vacuum pumps  109 ,  116 ,  202 , and  208  may be selectively provided on permeate sides of the separation membranes  104 ,  106 ,  113 , and  206 . 
         [0041]    Combustion gas is injected into the compressor  102  via line  101 , the compressor  102  compresses the combustion gas and injects the combustion gas into an inlet side of the separation membrane  104  via line  103 . Carbon dioxide of the combustion gas injected into the inlet side of the separation membrane  104  flows into a permeate side of the separation membrane  104 . The vacuum pump  109  applies vacuum to the permeate side of the separation membrane  104  via line  108  to smoothly flow the combustion gas injected into the inlet side of the separation membrane  104  into the permeate side of the separation membrane  104 . Residue gas that does not pass through the separation membrane  104  is injected from a residue side of the separation membrane  104  into an inlet side of the separation membrane  106  via line  105 . 
         [0042]    Residue gas of the separation membrane  106  is discharged via line  107 . The gas discharged via line  107  has a carbon dioxide removal rate of approximately equal to or greater than about 80% and has carbon dioxide concentration of approximately equal to or less than about 3%. Permeate gas of the separation membrane  106  is suctioned by the vacuum pump  202 , and compressed by the compressor  204  and injected into an inlet side of a separation membrane  206 . Residue gas of the separation membrane  206  is injected into the inlet side of the separation membrane  106  via line  212  and line  105 . 
         [0043]    When comparing the system  100  with the system  200 , in the separation system  100 , the permeate gas of the separation membrane  106  is injected into the inlet side of the separation membrane  106  via line  121  and line  105 . However, in the separation system  200 , the permeate gas of the separation membrane  106  passes through the additional separation membrane  206 , and the residue gas of the separation membrane  206  is injected into the inlet side of the separation membrane  106 . 
         [0044]    The permeate gas of the separation membrane  106  is injected into the inlet side of the separation membrane  106  via the separation membrane  206  and therefore the separation membrane  106  has a self-recycle loop. The self-recycle loop increases carbon dioxide concentration of the inlet side of the separation membrane  106  and improves separation performance of the separation membrane  106 . Permeate gas of the separation membrane  206  having high carbon dioxide concentration is suctioned by the vacuum pump  208  and injected into the inlet side of the separation membrane  104  via line  210 . 
         [0045]    Permeate gas of the separation membrane  104  is suctioned by the vacuum pump  109  and supplied to the compressor  111  via line  110 , and compressed by the compressor  111  and injected into an inlet side of a separation membrane  113  via line  112 . Permeate gas of the separation membrane  113  is suctioned by the vacuum pump  116  via line  115  and supplied to a carbon dioxide capture apparatus (not shown in the drawing). A part of permeate gas of the separation membrane  113  is supplied to the compressor  111  via line  118  and injected into the inlet side of the separation membrane  113 . The permeate gas of the separation membrane  113  is injected into the inlet side of the separation membrane  113  via line  118  and therefore carbon dioxide selectivity of the separation membrane  113  is also improved by the self-recycle loop. Residue gas of the separation membrane  113  is injected into the inlet side of the separation membrane  104 . 
         [0046]    The permeate gas of the separation membrane  106  is injected into the inlet side of the separation membrane  106  via the separation membrane  206  and therefore carbon dioxide selectivity of the separation membrane  106  is improved by increasing carbon dioxide concentration of inlet gas of the separation membrane  106 . Further, permeate gas of the separation membrane  206  having high carbon dioxide concentration is injected into the inlet side of the separation membrane  104 . Moreover, the residue gas of the separation membrane  113  having the self-recycle loop is injected into the inlet side of the separation membrane  104 . As a result, carbon dioxide selectivity of the separation membrane  104  is improved by increasing carbon dioxide concentration of inlet gas of the separation membrane  104 . By this action, carbon dioxide purity of the permeate gas of the separation membrane  113  is improved equal to or greater than 80%. 
         [0047]      FIG. 3  is a block diagram showing a carbon dioxide separation system  300  according to a third embodiment of the present invention.  FIG. 3  illustrates the configuration of a separation membrane  104  in which combustion gas is injected into an inlet side of the separation membrane  104 , a separation membrane  106  in which residue gas of the separation membrane  104  is injected into an inlet side of the separation membrane  106 , and a separation membrane  113  in which permeate gas of the separation membrane  104  is injected into an inlet side of the separation membrane  113 , wherein at least a part of permeate gas of the separation membrane  113  is captured, and residue gas of the separation membrane  113  is injected into the inlet side of the separation membrane  106 . In other words, the system  300  compared to the system  100  and  200  is different in that the residue gas of the separation membrane  113  is injected into the inlet side of the separation membrane  106  rather than the separation membrane  104 . 
         [0048]    In addition, in the carbon dioxide separation system  300 , carbon dioxide selectivity of the separation membrane  104  is improved since permeate gas of the separation membrane  206  is injected into the inlet side of the separation membrane  104 . Carbon dioxide selectivity of the separation membrane  106  is improved since permeate gas of the separation membrane  106  is injected into the inlet side of itself via the additional separation membrane  206 , and the residue gas of the separation membrane  113  is injected into the inlet side of the separation membrane  106 . Carbon dioxide selectivity of the separation membrane  113  is improved since the part of the permeate gas of the separation membrane  113  is injected into the inlet side of the separation membrane  113 . 
         [0049]      FIG. 4  is a block diagram showing a carbon dioxide separation system  400  according to a fourth embodiment of the present invention. As shown in  FIG. 4 , the carbon dioxide separation system  400  includes five separation membranes  104 ,  402 ,  405 ,  414 , and  424 . Compressors  102 ,  111 ,  410 , and  420  may be provided on inlet sides of the separation membranes  104 ,  402 ,  405 ,  414 , and  424 , and vacuum pumps  109  and  116  may be selectively provided on permeate sides of the separation membranes  104 ,  402 ,  405 ,  414 , and  424 . 
         [0050]    Combustion gas is injected into the compressor  102  via line  101 , the compressor  102  compresses the combustion gas and injects the combustion gas into an inlet side of the separation membrane  104  via line  103 . Carbon dioxide of the combustion gas that is injected into the inlet side of the separation membrane  104  flows into a permeate side of the separation membrane  104 . The vacuum pump  109  applies vacuum to the permeate side of the separation membrane  104  via line  108  to smoothly flow the combustion gas that is injected into the inlet side of the separation membrane  104  into the permeate side of the separation membrane  104 . Residue gas that does not pass through the separation membrane  104  is injected from a residue side of the separation membrane  104  into an inlet side of the separation membrane  402  via line  105 . 
         [0051]    Permeate gas of the separation membrane  402  is supplied to the compressor  102  via line  404 . Residue gas of separation membrane  402  is injected into an inlet side of the separation membrane  405  via line  406 . Permeate gas of the separation membrane  405  is supplied to the compressor  410  via line  408 , and the permeate gas compressed by the compressor  410  is injected into the inlet side of the separation membrane  402  via line  412 . Residue gas of the separation membrane  405  is injected into an inlet side of the separation membrane  414  via line  416 , the permeate gas of the separation membrane  405  is supplied to the compressor  420  via line  418 , and the permeate gas supplied to the compressor  420  is injected into the inlet side of the separation membrane  405  via line  422 . In other words, in the separation membranes  104 ,  402 ,  405 , and  414  connected in series, permeate gases of the separation membrane  402 ,  405 , and  414  are injected into the inlet sides of separation membranes  104 ,  402 , and  405  of previous stages. The separation membrane  414  has carbon dioxide removal rate of approximately equal to or greater than about 80% and discharges residue gas having carbon dioxide concentration of approximately equal to or less than about 3% via line  417 . 
         [0052]    Permeate gas of the separation membrane  104  is suctioned by the vacuum pump  109  and supplied to the compressor  111  via line  110 , and compressed by the compressor  111  and injected into an inlet side of a separation membrane  424  via line  112 . Permeate gas of the separation membrane  424  is suctioned by the vacuum pump  116  via line  115  and supplied to a carbon dioxide capture apparatus (not shown in the drawing). A part of the permeate gas of the separation membrane  424  is supplied to the compressor  111  via line  118  and injected into the inlet side of the separation membrane  424 . The permeate gas of the separation membrane  424  is injected into the inlet side of the separation membrane  424  and therefore carbon dioxide selectivity of the separation membrane  424  is improved by the self-recycle loop. Residue gas of the separation membrane  424  is injected into the inlet side of the separation membrane  104 . 
         [0053]    The permeate gas of the separation membrane  402  and the residue gas of the separation membrane  424  are injected into the inlet side of the separation membrane  104  and therefore carbon dioxide selectivity of the separation membrane  104  is improved by increasing carbon dioxide concentration of inlet gas of the separation membrane  104 . Carbon dioxide selectivity of the separation membrane  402  is improved since the permeate gas of the separation membrane  405  is injected into the inlet side of the separation membrane  402 . Carbon dioxide selectivity of the separation membrane  405  is improved since the permeate gas of the separation membrane  414  is injected into the inlet side of the separation membrane  405 . Carbon dioxide selectivity of the separation membrane  424  is improved by the self-recycle loop in which permeate gas is injected into an inlet side. As a result, carbon dioxide purity of the permeate gas of the separation membrane  424  is improved equal to or greater than 80%. 
         [0054]      FIG. 5  is a block diagram showing a carbon dioxide separation system  500  according to a fifth embodiment of the present invention. As shown in  FIG. 5 , the carbon dioxide separation system  500  includes two separation membranes  502  and  506 . A compressor  102  may be provided on an inlet side of a separation membrane  502 , and vacuum pumps  109  and  512  may be selectively provided on permeate sides of the separation membranes  502  and  506 . 
         [0055]    Combustion gas is injected into the compressor  102  via line  101  and the compressor  102  compresses the combustion gas and injects the combustion gas into the inlet side of the separation membrane  502  via line  103 . Carbon dioxide of the combustion gas that is injected into the inlet side of the separation membrane  502  flows into a permeate side of the separation membrane  502 . The vacuum pump  109  applies a vacuum to the permeate side of the separation membrane  502  via line  108  to smoothly flow the combustion gas that is injected into the inlet side of the separation membrane  502  into the permeate side of the separation membrane  502 . A part of the permeate gas of the separation membrane  502  is injected into the inlet side of the separation membrane  502  by dividing the permeate gas at a branch point  518 , and another part is supplied to a carbon dioxide capture apparatus (not shown in the drawing). Residue gas that does not pass through the separation membrane  502  is injected from a residue side of the separation membrane  502  into an inlet side of the separation membrane  506  via line  504 . 
         [0056]    Permeate gas of the separation membrane  506  is suctioned by the vacuum pump  512  and supplied to the compressor  102 . The separation membrane  506  has carbon dioxide removal rate of approximately equal to or greater than about 80% and discharges residue gas having carbon dioxide concentration of approximately equal to or less than about 3% via line  508 . 
         [0057]    Carbon dioxide selectivity of the separation membrane  502  is improved since permeate gases of the separation membranes  502  and  506  are injected into the inlet side of the separation membrane  502 . As a result, carbon dioxide purity of the permeate gas of the separation membrane  502  is improved by equal to or greater than 80%. 
         [0058]    Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.