Modern Gasworks Practice

THE PRELIMINARY PURIFICATION OF COAL GAS 505 From the foregoing remarks, it will be gathered that the final composition of ammoniacal liquor depends upon — (a) The dass of coal carbonized. (b) The amount of salt in the coal. (c) The temperature of distillation. (d) The means employed for separating liquor and gas. (e) The temperature to which the liquor is subjected. (f) The amount of oxygen in the gas, the amount of air drawn. in on the inlet side of the exhauster, and the extent to which. the liquor is exposed to the atmosphere. Item (d) is one which is frequently overlooked, and may have come considerable influence on the amount of ammonia present in the crude gas. For instance, wlien tar towers are in use the ammonia in the gas at the outlet of the condensers is frequently lower than 200 grains per 100 cubic feet, in contrast witli about 300 grains when the system of a separate draw-off to each hydraulic is installed. The reason would appear to be that in the case of tar towers the maintenance of the seal is ensured by running liquor into the tower alone. When. weir valves or other individual means are in use the liquor is usually laid on to each hydraulic, and by giving rise to splashing sets free ammonia, which is accordingly carried forward by the gas. In the ordinary way the hydraulic main liquor contains about 50 per cent, of its ammonia in the fixed form, chloride and sulphocyanide predominating. At the present day, however, it is a growing practice to circulate the liquor repeatedly through the hydraulics, and in such, cases the fixed content will be considerably increased, so that it may amount to as much as 80 per cent.' of the ammonia present. The proportion of ammonia contained in gas liquor is in reality quite small. In normal cases the total ammonia varies from 1 ’5 to 2 per cent., being appor-tioned as follows :— Free ammonia....................................1-2 to 1-6 per cent. Fixed ammonia . . . . . . . . 0-3 to 0-4 „ „ As regards CO2 and H2S the folio wing figures may be taken as illnst,rating average conditions :— CO 2 in gas liquor..............................1-3 to 2 per cent. SH2 „ ................................0-1 to 0-6 „ „ Sulphur „ ...................................0-5 to 0-7 „ „ It is difficult to lay down any hard and fast figures in Connection with. the manner in which the ammonia is distributed, but in general it will be accounted for somewhat as follows. The manner in which the sulphur is combined is also given. Distbibution of Ammonia in Gas Liquor “Free” [Ammonium sulphides .... 6 per cent, of total ammonia. I „ carbonates .... 74 „ ,, „ { „ chloride ..... 15 „ „ „ „ sulphocyanide . . . . 1-75 „ „ thiosulphate, sulphite, etc. 3 „ „ „ „ sulphate . .... 0-25 „ „ „