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.]. accumulation during absorption. thiourea is thermally unstable due to its reaction with water in the basic media according to the equation: 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.]. thermally 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.]. 18,958, 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. 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.sub.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. 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. 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: 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. The accumulation of thermally stable alkanolamine thiocyanates in the absorbing solution is thus prevented and the absorbing solution does not become poisoned. 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.

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