Enhanced process for the purification of anhydrous hydrogen chloride gas

Abstract

The present invention relates to a process for purifying anhydrous hydrogen chloride gas (“aHCl”), and preferably the anhydrous hydrogen chloride gas recovered from an isocyanate production process. In the process of the present invention, the content of chlorinated organics may be reduced from up to 1000 ppm by volume to below 10 ppb by volume levels. Generally, the process of the invention allows for chlorinated organic levels to be reduced to from 1 to 100 ppb, rendering the treated hydrogen chloride gas usable in a catalytic oxychlorination process or a Deacon process. The treated gas is also suitable for absorption in water or dilute hydrochloric acid.

1 . A cooling and distillation process to remove contaminants having boiling points higher than hydrogen chloride from a hydrogen chloride-containing gas comprising: 
 a) compressing said hydrogen chloride-containing gas,    b) cooling the resultant compressed gas in a first heat exchanger resulting in a first condensate stream and a first gas stream, wherein said compressed gas is cooled to a temperature low enough to partially condense said contaminants and at a rate sufficiently low that fog formation is prevented,    c) feeding said first gas stream from said first heat exchanger to a distillation column having a top portion and a bottom portion to a point between said top portion and said bottom portion, to cause mass transfer between liquid and gas and to thereby concentrate the contaminants in the bottom portion of said column and hydrogen chloride gas in the top portion of said column,    d) feeding said hydrogen chloride gas from said top portion to a second heat exchanger whereby the hydrogen chloride gas is partially condensed to form a second condensate stream and a second gas stream,    e) feeding said second condensate stream to said top portion of said column to provide reflux to said column,    f) feeding said first condensate stream to said distillation column below the point where said first gas stream is fed,    g) feeding said second gas stream from step d) to said first heat exchanger as cooling medium,    h) recovering purified hydrogen chloride gas from said first heat exchanger, and    i) feeding said contaminants from the bottom potion of said column to a collection vessel.    
   
   
       2 . The process of  claim 1 , wherein the contaminants contained in the gas stream are chlorinated aromatic compounds.  
   
   
       3 . The process of  claim 2 , wherein the gas stream also contains a contaminant with an intermediate boiling range between the hydrogen chloride boiling point and the chlorinated aromatic compound boiling point.  
   
   
       4 . The process of  claim 3  wherein said intermediate is phosgene and said intermediate is removed from said distillation column.  
   
   
       5 . The process of  claim 1  wherein the temperature of the compressed gas is reduced to a temperature of from +10 to −25° C. in said first heat exchanger.  
   
   
       6 . The process of  claim 1  wherein in step a), the gas is compressed to a pressure of from 5 to 30 bars absolute.  
   
   
       7 . The process of  claim 1 , wherein step f) comprises 
 f1) feeding said first condensate to a separation vessel (or vessels) to trap solids and wherein a solids stream and an overflow condensate stream are formed,    f2) feeding said overflow condensate stream to said distillation column at a point below the point where the first gas stream is fed.    
   
   
       8 . The process of  claim 1 , wherein step e) comprises: 
 e1) feeding said second condensate stream to one or more separation vessels used to trap any solids present and to form a solids stream and a third condensate stream,    e2) feeding said third condensate stream to said top portion of said column to provide reflux to said column.    
   
   
       9 . The process of  claim 1  wherein step i) comprises 
 i1) feeding liquid from the bottom portion of said column to a reboiler to generate stripping vapors for the bottom portion of the column, and wherein the reboiler heats the said liquid at low heat flux so as to prevent foaming action and    i2) removing any remaining liquid from the reboiler to a collection vessel for disposal.    
   
   
       10 . The process of  claim 9 , wherein the reboiler is designed to prevent the formation of foam and has a heat flux of from 500 to 20,000 BTU/hr/ft 2  as a lower limit and from 3,000 to 30,000 BTU/hr/ft 2  as a higher limit.  
   
   
       11 . The process of  claim 9 , wherein a portion of the liquid removed from the reboiler comprises hydrogen chloride and contaminants and is sprayed into the gas stream being fed to the first heat exchanger.  
   
   
       12 . The process of  claim 9 , wherein from 5 to 95% by weight the liquid fed to said reboiler is evaporated.  
   
   
       13 . The process of  claim 1 , wherein the purified hydrogen chloride gas from the first heat exchanger is further purified by treatment with activated charcoal.  
   
   
       14 . A cooling and distillation process to remove contaminants having boiling points higher than hydrogen chloride from a hydrogen chloride-containing gas comprising: 
 a) compressing said hydrogen chloride-containing gas,    b) cooling the resultant compressed gas in a first heat exchanger resulting in a first condensate stream and a first gas stream, wherein said compressed gas is cooled to a temperature low enough to partially condense said contaminants and at a rate sufficiently low that fog formation is prevented,    c) feeding said first gas stream from said first heat exchanger to a distillation column having a top portion and a bottom portion to a point between said top portion and said bottom portion, to cause mass transfer between liquid and gas and to thereby concentrate the contaminants in the bottom portion of said column and hydrogen chloride gas in the top portion of said column,    d) feeding said hydrogen chloride gas from said top portion to one side of a third heat exchanger flasher and feeding said contaminants from the bottom portion of said column to the other side of said third heat exchanger to flash against and cool the hydrogen chloride gas passing through said third heat exchanger, whereby the following streams are formed: 
 1) a second gas stream containing contaminants,  
 2) a contaminant stream,  
 3) a third cooled gas stream, and  
 4) a second condensate stream,  
   e) feeding said third gas stream to a second heat exchanger whereby the hydrogen chloride gas is partially condensed to form a third condensate stream and a fourth gas stream,    f) combining said second condensation stream and said third condensation stream and feeding the resulting combined stream to said top portion of said column to provide reflux to said column,    g) feeding said first condensate stream to said distillation column below the point where said first gas stream is fed,    h) feeding said fourth gas stream from step d) to said first heat exchanger as cooling medium, and    i) recovering purified hydrogen chloride gas from said first heat exchanger.