Regeneration of alkanolamine absorbing solution in gas sweetening processes

Abstract

A catalytic amount of ammonia or ammonium compounds are utilized to prevent deterioration of alkanolamines employed in gas sweetening processes. The ammonia is added either to the sour gas or the absorbing solution and acts to suppress thiocyanate .[.formation.]. .Iadd.accumulation .Iaddend.during absorption. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to gas purification and more particularly to gas purification by the removal of acid gases from industrial gas in an absorption/desorption process. 2. Prior Art The term "gas sweetening" pertains to gas purification processes in which acid gases, particularly sulfurous gases are removed from industrial gases. Gas sweetening processes commonly utilize aqueous absorbing solutions into which the acid gases are scrubbed (absorbed) to be later stripped (desorbed) for possible further treatment. Desorbing is accomplished by heating the spent solution to expel the acid gases. The process steps generally include concurrent reutilization of the absorbing solution. Examples of the industrial gases which are sweetened by absorption/desorption processes include coke oven gases, producer gas, natural gas and synthesis gas. The described process is also applicable to synthetic natural gas and the products of coal gasification. The absorbing solution, which is normally alkaline, is often an alkanolamine such as monoethanolamine. The acid gas contaminants removed from the industrial gases include such gases as hydrogen cyanide, and sulfurous gases such as sulfur dioxide, hydrogen sulfide and mercaptans. In the absorption of acid gases containing both sulfurous compounds and hydrogen cyanide by alkanolamines it has been found that, due to the formation of thermally stable amine thiocyanates such as monoethanolamine thiocyanate from the alkanolamine and the acid gases, the alkanolamine solution gradually becomes "poisoned" or less efficient. Proposed prior art solutions for avoiding the regeneration problems engendered by the presence of thermally stable thiocyanates in spent alkanolamine absorbing solutions are known, but have failed to be practical. One proposal, disclosed in U.S. Pat. No. 2,137,602, for example, is merely an expedient in which two stage washing is utilized to conserve absorbent. The first scrubbing solution is discarded after use, the industrial gas is rescrubbed and only the second wash solution is regenerated. U.S. Pat. No. 2,399,142 proposes to solve the problem of .[.theemally.]. .Iadd.thermally .Iaddend.stable, non-decomposable intermediates by stripping the spent absorbent under pressure to force decomposition. This, however, is a costly procedure in large scale operations. A third proposal, disclosed in U.S. Pat. No. Re. .[.18,959.]. .Iadd.18,958.Iaddend., preferentially substitutes aliphatic and cyclo paraffin amines such as hexylamine for the lower boiling mono and diethanolamine in order to try to achieve a more easily decomposible intermediate. None of the prior art proposals have succeeded in providing a truly successful means of regenerating spent alkanolamine absorbing solutions. SUMMARY OF THE INVENTION We have discovered a method of regenerating spent alkanolamine absorbing solutions utilized in acid gas absorption/desorption processes which overcomes the aforementioned prior art problems. Briefly, the method of this invention provides a catalytic amount of ammonia in ionizable form for use in the alkanolamine absorbent during acid gas absorption. The ammonia may be, for example, in the form of ammonia (NH 3 ) itself, aqueous ammonium solutions, or an ammonium salt of a weak acid. The manner in which the ammonia is provided to the absorbing process may vary in that the ammonia may be combined with the sour gas, i.e. the acid gas contaminated industrial gas, or the ammonia may be combined with the absorbing solution prior to the absorbing process. During the subsequent desorbing step spent absorbent now becomes readily regenerable. Thermally stable thiocyanates no longer remain in the absorbent and the regenerated absorbent may be continually recycled to the stripper and reused. .Iadd.The presence of a catalytic amount of ammonia in ionizable form thus acts as a deterrent with respect to thermally stable thiocyanates remaining in the absorbent. .Iaddend. While we do not wish to be restricted to a theory of how our process works, we postulate that the presence of a catalytic amount of ammonia in the absorbing step causes the formation of thiourea from thiocyanates present in the solution according to the equation: NH.sub.4.sup.(+) +CNS.sup.(-) →(NH.sub.2).sub.2 CS (1) thiourea is thermally unstable due to its reaction with water in the basic media according to the equation: (NH.sub.2).sub.2 CS + 2 H.sub.2 O → 2 NH.sub.3 + CO.sub.2 + H.sub.2 S (2) the reaction of equation (2) will regenerate the ammonia catalyst, and generate carbon dioxide and hydrogen sulfide which can exit from the system as part of the foul gas stream. .Iadd.The accumulation of thermally stable alkanolamine thiocyanates in the absorbing solution is thus prevented and the absorbing solution does not become poisoned. .Iaddend. It is therefore a primary object of this invention to provide an easily regenerable alkanolamine absorbing solution for use in industrial gas sweetening processes. It is another object of this invention to solve the long standing problem of spent alkanolamine absorbent regeneration by a method of simplicity and economy. These and other objects will be readily apparent to one skilled in the art from a consideration of the accompanying drawings, description and detailed exemplary embodiments.

We claim: 
     
       1. In combination with an aqueous alkanolamine absorption/desorption process wherein industrial gases are sweetened by removal of acid gases including hydrogen cyanide and sulfide gases, a process for preventing irreversible deterioration of the alkanolamine solution comprising: maintaining sufficient ammonium ions in the alkanolamine solution during absorption to insure a continued presence of said ammonium ions in the alkanolamine solution during the absorption process, said ammonium acting as a deterrent with respect to the .[.formation.]. .Iadd.accumulation .Iaddend.of thiocyanates whereby irreversible deterioration of said alkanolamine solution by said thiocyanates is suppressed.   
     
     
       2. The process of claim 1 in which the industrial gas is coke oven gas. 
     
     
       3. The process of claim 1 in which the ionizable form of ammonia supplied is substantially ammonia. 
     
     
       4. The process of claim 1 in which the ionizable form of ammonia is substantially an aqueous solution of an ammonium salt of a weak acid. 
     
     
       5. The process of claim 2 in which the ionizable form of ammonia supplied is substantially ammonia. 
     
     
       6. The process of claim 2 in which the ionizable form of ammonia is substantially an aqueous solution of an ammonium salt of a weak acid. 
     
     
       7. The process of claim 2 in which the ionizable form of ammonia is ammonia in an amount of at least about 0.2 mole percent of the industrial gas. 
     
     
       8. The process of claim 2 in which the ionizable form of ammonia is an aqueous solution of an ammonium salt of a weak acid in the amount of about 0.2 to 1 weight percent of the absorbing solution. 
     
     
       9. The process of claim 7 in which the alkanolamine is substantially monoethanolamine. 
     
     
       10. The process of claim 8 in which the alkanolamine is substantially monoethanolamine. 
     
     
       11. The process of claim 1 in which at least part of the ionizable ammonia is supplied by recirculating desorbed solution. 
     
     
       12. An improved gas sweetening process for removing said gases from industrial gases containing hydrogen cyanide and sulfide components by means of aqueous alkanolamine absorbing solutions without irreversible deterioration of the absorption capacity of the alkanolamine solution due to combination of the alkanolamine with thiocyanates comprising: a. absorbing the acid gases into the alkanolamine solution in the presence of ammonium ions,   b. desorbing the acid gases from the alkanolamine to regenerate the solution and recirculating the regenerated and undeteriorated alkanolamine solution back to the absorbing step.   
     
     
       13. An improved method of gas sweetening according to claim 12 wherein ammonium ions are maintained in both the absorption and desorption steps of the gas sweetening process. 
     
     
       14. An improved method of gas sweetening according to claim 13 additionally comprising: c. recovering ammonia from the foul gases leaving the desorption step of (b) and recycling said ammonia to the absorption step of (a).   
     
     
       15. An improved method of gas sweetening according to claim 13 wherein the ammonia is recycled to the absorption step of (a) along with the recirculated alkanolamine solution from the desorption step of (b). 
     
     
       16. An improved method of gas sweetening according to claim 12 wherein the ammonia is provided in excess of the molar amount which would theoretically be necessary to form thiourea from thiocyanates. 
     
     
       17. An improved method of gas sweetening according to claim 16 in which solution of the elevated temperature in the desorption step is sufficient to decompose thiourea if present, to gaseous carbon dioxide and hydrogen sulfide. 
     
     
       18. In combination with an .[.alkaholamine.]. .Iadd.alkanolamine .Iaddend.absorption/desorption process wherein industrial gases are sweetened by removal of acid gases including hydrogen cyanide and sulfide gases, a process for preventing irreversible deterioration of the alkanolamine solution comprising: a. providing prior to the absorbing process, an aqueous alkanolamine containing solution for the absorption of acid gases therein;   b. providing ammonia in ionizable form;   c. absorbing acid gases into said alkanolamine solution in the presence of said ammonia whereby thiourea is formed;   d. desorbing said solution including reacting said thiorurea with water to generate gaseous carbon dioxide and hydrogen sulfide and removing desorbed gases to purify said solution. .Iadd.   
     
     
       19.  In combination with an aqueous alkanolamine absorption/desorption process wherein industrial gases are sweetened by removal of acid gases including hydrogen cyanide and sulfide gases, a process for preventing irreversible deterioration of the alkanolamine solution comprising: maintaining sufficient ammonium ions in the alkanolamine solution during absorption to insure a continued presence of said ammonium ions in the alkanolamine solution during the absorption process, said ammonium acting as a deterrent with respect to thiocyanates remaining in the absorbent whereby irreversible deterioration of said alkanolamine solution by said thiocyanates is suppressed. .Iaddend. .Iadd.   
     
     
       20.  In combination with an aqueous alkanolamine absorption/desorption process wherein industrial gases are sweetened by removal of acid gases including hydrogen cyanide and sulfide gases, a process for preventing irreversible deterioration of the alkanolamine solution comprising: maintaining sufficient ammonium ions in the alkanolamine solution during absorption to insure a continued presence of said ammonium ions in the alkanolamine solution during the absorption process, said ammonium acting as a deterrent with respect to the formation of thermally stable alkanolammonium thiocyanate salts whereby irreversible deterioration of said alkanolamine solution by said alkanolammonium thiocyanates is suppressed..Iaddend.