GB815462A - Improvements relating to the treatment of aqueous solutions containing ammonia and weak acids - Google Patents

Abstract

<PICT:0815462/III/1> <PICT:0815462/III/2> Aqueous solutions which contain ammonia and weak acids, e.g. hydrogen sulphide, carbon dioxide, or hydrocyanic acid, obtained e.g. as washing liquors and condensates, during the production of coke-oven gas and other fuel conversion gases are worked up by the employment of cation exchangers in an ion-exchanging system consisting of at least two exchanger containers connected in series, through which the aqueous solution flows and which is used for the separation of ammonia from the solution, the enriched exchange mass is regenerated after the flow of the solution has been discontinued, and the absorbed ammonia is removed. The aqueous solution from pipe 1 is fed to first container 4 of ion-exchanging system 2 through one or more layers 5, 6 of ion-exchange mass. The solution, free of most of the NH3, and some H2S, from 4 passes through gas separator 13 and pipe 7 to second container 8 similar to 4. From 8 the solution passes through gas separator 13 and pipe 9 to washer 10 where it is contacted with a gas of high CO2 content added through 11 to remove the remainder of the H2S and HCN if present. The remaining liquor containing, in the case of coke oven gas liquors, CO2, H2SO4, HCl, phenols, and pyridines is drawn off at 12 for further processing. The entrained gases, CO2 and some H2S, separated in separators 13 are drawn off through pipe 14 to dry purification plant 16 where H2S and HCN, if present, are absorbed, and CO2 is withdrawn at 17 Part of the CO2 gas is diverted to washer 10 and after enrichment with H2S is returned to pipe 14 at a point before the dry purifier 16. When 4 is sufficiently charged with NH3, 4 and 8 are changed over, and solution to be treated is conveyed direct to 8 through 20, back by pipe 22 to 4, and through 23 to washer 10. When 8 is also charged with NH3, the solution from 1 is diverted to second system 3 similar to 2. The residual solution in 4, 8 is, preferably, with drawn through pipes 26 to the main stream to be treated. The regeneration of the ion-exchanger may be effected with H2SO4 of such concentration that saturated (HN4)2SO4 solution is formed, or by circulating 10 per cent H2SO4 until it contains 1 per cent free acid, or by HCl and the NH3 is expelled from the resulting volatile ammonium chloride by means of a base, e.g. lime, or by indirect heating to recover NH3, or by steam and CO2 under increased pressure and the NH3 distilled from the condensate. As shown in Fig. 2 containers 4, 8 are washed with, preferably softened, water through pipe 27 and the wash liquor is withdrawn through 28 to waste 29 or to acid container 31. Acid is then passed into 4, 8 from top to bottom, and, if desired, in reverse, and is returned to tank 31. After sufficient enrichment with NH3, the solution is withdrawn from 31 and coarse crystals of (NH4)2SO4 may be obtained by evaporation and crystallization, or by precipitation with methanol and centrifuging. Heating coil 33a may be fitted in one container 8 to drive out NH3 vapours, which pass through 34 to pipe 26, and may be passed through pipe 35 into the saturator or crystallizer in which the sulphate precipitates. After the acid treatment, the containers are again flushed with fresh water which drains to waste 29 or to acid tank 31. The solution to be treated is then diverted back to system 2, and system 3 is regenerated; common acid supply tank washer, purifier, &c., are used for systems 2, 3. Alternatively, in Fig. 3 (not shown), crude gas passes through a sulphur washer where it is sprayed with aqueous ammonia solution, and an ammonia washer. The discharged solution from the sulphur washer passes through two series-connected exchangers where NH3 is absorbed and the H2S is precipitated as S on the exchange mass. A dry purifier is provided as in Fig. 1 to absorb the remainder of the H2S and pure CO2 is withdrawn therefrom. Steam heating coils in the exchangers expel NH3 vapour, as necessary, which is absorbed in water and used in the sulphur washer. Alternatively, NH3 may be liberated from the ion-exchanger by direct heating or by electrolytic action. The sulphur is extracted from the ion-exchanger by a solvent, e.g. ammonium sulphide solution, when the ion-exchanger is fully charged with NH3, but it is not necessary to remove sulphur until NH3 has been repeatedly expelled. The sulphur solvent may be used repeatedly until sufficiently enriched. In Fig. 4 (not shown), liquor containing NH3, H2S, and CO2 has an oxygen carrier, preferably air, added before it is passed through series-connected exchangers to oxidize the H2S. Extraction of S and NH3 is effected as described above. The liquor may also contain high-boiling hydrocarbon and tar contaminants, which are deposited on the ion exchange mass; and these should be removed by flushing with an organic solvent, e.g. dichlorethane, before S and NH3 are removed. Ion-exchange masses referred to are those based on vinyl ester.