Modern Gasworks Practice

THE PRELIMINARY PURIF1CATI0N OF COAL GAS 541 from the right-hand side, and is passed through the left-hand side of the plant» where the oxide (previously fouled) has been undergoing revivification ; .whilst the former purifying side is now submitted. to a heated air-blast, so thai; this oxide may be revivified. Revivification. is carried out by passing a powerful air-blast through the spent oxide, and the compounds of sulphur and iron are violently oxidized so that sulphurous acid is produced. The air-blast (laden with the acid) then passes to the washer, where it meets with. a stream of neutral liquor produced in the otter side of the plant—this absorbing the acid gases and running out as an acid lye from the bottom of the washer. This acid liquor is then pumped over to the opposite side of the plant, and is utilized for absorbing ammonia as before explained. A definite volume of liquor is thus kept in circulation, and becomes gradually charged with salt until the Saturation point is reached, when the salt is deposited at the bottom of the neutral lye tank as ammonium sulphite and sulphate, the reaction heilig :—H2O +SO2 + 2NH3= (NH4)2SO3(sulphite). Thesulphiteisthenoxidized into sulphate, taking up another atom of oxygen from an air-blast. For this purpose the apparatus shown in Fig. 341 is used, the sulphite being heated by a steam jacket and sublimed tliere-by. The cylinder in which the worm conveyer works is jacketed for the first half of its length with cold. water, and for the second half with. steam. The sulphite, on reaching the heated portion, is volatilized and driven back by a biast of air entering at the outlet end, this air converting the sulphite into sulphate, which is then deposited. The principle of the apparatus is de-pendent upon the faet that ammonium sulphite is sublimed at temperatures below 100° C., whereas the sulphate is not. The salt tlirown. down in the liquor tank con-sists of about 67 per cent, sulphite, and the remainder sulphate. In a process of this kind there is some liability to accident owing to over-heating during the revivification of the oxide. This, however, is guarded against by arrang-ing a system of water-cooling for the oxide, also by ensuring the presence of water vapour in the biast. In practice, there is little fear of danger unless a temperature of 300° C. is exceeded ; and in the Burkheiser plant the temperature rarely rises above 140° C. Moreover, as a safeguard, an electric alarm thermometer is fitted to the purifler undergoing regeneration. In a process of this kind there is, of course, an excess of sulphur beyond that necessary to neutralize the whole of the ammonia. This surplus may be recovered for sale in various ways. The salt obtained by the process is of a somewhat inferior quality, owing to the incomplete nature of the oxidizing process, and, consequently, the presence of ammonium sulphite. Fig. 341.— Bukkheiseb’s Oxidation Apparatus.