US 4,594,131 AGranted

Process for removing ammonia and acid gases from process streams

AssigneeUnited States Steel Corporation

InventorEdward E. Maier

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Abstract

A water purification process is described for the removal of ammonia and optionally one or more acid gases from wastewaters such as coke-plant or coal conversion wastewaters. The process involves adding alkali to these wastewaters in amounts sufficient to react with fixed ammonia salts present in the wastewater and to enable substantial amounts of the ammonia to be evolved upon distillation, and subjecting the thus treated wastewater to distillation to remove substantial amounts of the ammonia and acid gases present from the wastewater. This process is achieved by two separate and successive distillations. In this process, the vapor streams being withdrawn from the first and/or second distillations are subjected to an impurity removal process to thereby produce a vapor stream substantially reduced in impurities, and useful as a stripping vapor in this process, and then reusing this vapor stream as a stripping vapor in the process. The impurity removal process may include (1) passing a vapor stream through an ammonia absorbing solution, or (2) a sequence of steps involving compression, condensation, steam stripping and vaporization.

A process for achieving substantially complete removal of acid gases and ammonia from a dilute aqueous solution thereof, said solution also containing free and fixed ammonia salts, said process comprising:

(a) subjecting said solution to a first countercurrent multi-stage continuous distillation, said distillation being conducted by heating said solution, at least in part by means of a stripping vapor, and by having a gradient of ammonia concentration decreasing towards the region of bottom liquid withdrawal which results in said bottom liquid having a pH of less than about 8.0,
(i) withdrawing from this first distillation an overhead vapor stream containing a major proportion of stripping vapor, substantially all of said acid gases from said solution, and substantially all of the ammonia from said free ammonia salts,
(ii) withdrawing from this distillation an aqueous bottom stream which contains substantially all of said fixed ammonia salts,
(b) adding alkali to said withdrawn bottom stream, said alkali being added in an amount sufficient to evolve ammonia contained in said fixed ammonia salts during subsequent distillation of said mixture,
(c) subjecting said withdrawn bottom stream to a second countercurrent multi-stage continuous distillation,
(i) withdrawing from said second distillation an overhead vapor stream said vapor steam containing a portion of the ammonia from said fixed ammonia salts,
(ii) withdrawing from said second distillation an aqueous bottom stream, the improvement comprising:
(d) compressing the overhead vapor stream being withdrawn from said first distillation,
(e) condensing the compressed overhead vapor stream from said first distillation,
(f) partially vaporizing a portion of the liquid stream produced by condensing said overhead vapor stream from said first distillation, to produce a vapor stream of ammonia and acid gases and a purified liquid stream having a reduced concentration of ammonia and acid gases, and
(g) further purifying said liquid stream produced in step (f) by stripping the liquid with steam to remove additional ammonia and acid gases, and
(h) vaporizing said purified liquid stream produced in step (g) to produce a vapor stream reduced in ammonia and acid gas content.

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Abstract

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