Syngas purification process

Abstract

The present invention relates to a process for purifying a syngas of the CO/H 2 or N 2 /H 2 type, which consists in removing CO 2 and possibly other gaseous impurities (water, etc.) before the gas undergoes a cryogenic process. These impurities are adsorbed by the gas stream to be purified passing over an NaLSX-type zeolite and then desorbed during a regeneration step which may be performed by raising the temperature (TSA) and/or reducing the pressure (PSA or VSA).

1 . Process for purifying syngas based on hydrogen and carbon monoxide and/or nitrogen, contaminated with carbon dioxide and one or more possible other impurities, comprising one or more cycles comprising the following successive steps: 
 a) making the gas mixture to be purified pass through an adsorption zone comprising: 
 an adsorbent capable of selectively adsorbing carbon dioxide, which comprises at least one X zeolite of the faujasite type with an Si/Al of 1, preferably ranging from 0.9 to 1.1 and advantageously ranging from 1 to 1.05, at least 70%, and preferably at least 90%, of the exchangeable sites of which are occupied by sodium ions, the rest of the cationic sites being occupied by K- or Ca-type cations or by other monovalent and/or polyvalent cations (magnesium, strontium, barium, lanthanides or rare earths, etc.),  
 one or more adsorbents, capable of selectively adsorbing each of the impurities, such as water, hydrocarbons and/or NOx,  
   the adsorbents being either intimately mixed or in the form of separate beds in successive layers;    b) desorbing the carbon dioxide and the other impurity or impurities adsorbed on the adsorbent or adsorbents described in a) by increasing the temperature and/or reducing the pressure, it being possible for this step to be supplemented with a purging phase consisting in recycling some of the purified gas; and    a) c) increasing the pressure in the said adsorption zone by introducing a flow of purified gas via the outlet of the adsorption zone and/or cooling the adsorption zone by flushing with purified cold gas.    
     
     
         2 . Process according to  claim 1  for purifying a syngas containing water and/or heavy hydrocarbons as impurities in addition to CO 2 , characterized in that the adsorbent or adsorbents capable of adsorbing water and/or the heavy hydrocarbons, preferably chosen from among alumina, silica gel or A-type or X-type zeolites, are either intimately mixed with the adsorbent capable of selectively adsorbing CO 2  or preferably are in the form of separate beds, the bed or beds of adsorbent(s) capable of selectively adsorbing water and/or the heavy hydrocarbons being placed upstream of the bed of adsorbent capable of selectively adsorbing CO 2    
     
     
         3 . Process according to  claim 1  or  2  for purifying a syngas containing one or more light hydrocarbons and/or NOx as impurities in addition to CO 2  and possibly in addition to water and/or heavy hydrocarbons, characterized in that the adsorbent or adsorbents capable of adsorbing the light hydrocarbons and/or the NOx, preferably chosen from alumina, silica gel or A-type or X-type zeolites, are either intimately mixed with the adsorbent capable of selectively adsorbing the CO 2  and possibly the adsorbent or adsorbents capable of adsorbing the water and/or heavy hydrocarbons, or preferably in the form of separate beds, the bed or beds of adsorbent(s) capable of selectively adsorbing the light hydrocarbons and/or the NOx being placed downstream of the bed of adsorbent capable of selectively adsorbing the CO 2    
     
     
         4 . Process according to any one of  claims 1  to  3  for purifying a syngas containing mercury as impurity in addition to CO 2  and possibly in addition to water and/or heavy hydrocarbons, light hydrocarbons and/or NOx, characterized in that the adsorption zone comprises a bed based on a silver-exchange zeolite.  
     
     
         5 . Process according to any one of  claims 1  to  3  for purifying a syngas containing mercury as impurity in addition to CO 2  and possibly in addition to water and/or heavy hydrocarbons, light hydrocarbons and/or NOx, characterized in that it comprises an additional step consisting in making a gas stream from which mercury has to be stripped pass, upstream or downstream of the process described in any one of  claims 1  to  4 , over active carbons impregnated with iodine or with sulphur.  
     
     
         6 . Syngas purification process according to any one of  claims 1  to  5 , characterized in that the NaLSX-type zeolite is present in agglomerated form with an agglomerating binder, the latter preferably being converted into a zeolite, which may represent from 5 to 30 parts by weight of the total weight of the agglomerate, the said agglomerates preferably having a mean diameter ranging from about  0 . 2  to about 5 mm.  
     
     
         7 . Syngas purification process according to any one of  claims 1  to  6 , characterized in that the pressure of the gas mixture to be purified during the adsorption steps a) is greater than or equal to 0.5 MPa, preferably greater than or equal to 2 MPa, and is less than or equal to 7 MPa, advantageously less than or equal to 5 MPa and more advantageously less than or equal to 3 MPa.  
     
     
         8 . Syngas purification process according to any one of  claims 1  to  7 , characterized in that the temperature of the gas stream entering the adsorption zone is between 0 and 80° C., preferably between 20 and 50° C., and in that the desorption temperature is between 100 and 300° C., preferably between 150 and 250° C.  
     
     
         9 . Syngas purification process according to any one of  claims 1  to  8 , characterized in that the CO 2  concentration of the gas mixture to be purified is less than or equal to 1,000 ppm, preferably less than or equal to 100 ppm, for adsorption pressures of around 3 MPa, and in that the CO 2  partial pressure is less than or equal to 3 Pa and preferably less than or equal to 0.3 Pa.  
     
     
         10 . Syngas purification process according to any one of  claims 1  to  9 , characterized in that it is of the PSA, VSA and/or TSA type.