536
MODERN GASWORKS PRACTICE
Davidson’s Process
Waste Liquor Outlet
Fig. 339.—Davidson’s Ammonia Still.
A recent process for the self-purification of crude gas is that introduced by Dr. W. B. Davidson. The principle of the process is on tlie same lines as those attempted by earlier experimenters, but it is operated in a more scientific männer. The process of separating the acid gases from the ammonia is carried on in. a ver-tical still of the usual pattern, to the top of whicli the liquor is fed. The still is heated by steam introduced at the base, and wliilst the ammonia is taken off at a point half-way up the column, the CO2 and H2S are expelled at the top. This separation of the ammonia and acid gases is made possible by graduating the temperature and pressure within the still—the pressure being so regulated that a comparatively high, pressure is obtained in the portion, of the still from whicli the ammonia is collected. After leaving the still, the gaseous ammonia is passed through absorbers to remove any traces of the acid gases, and is then re-mixed with the crude coal gas at any convenient point, such as the condensers or washers. From the washers, serubbers, and condensers, the ammoniacal liquor is collected and returned to the still, whilst the surplus ammonia, being recovered in an extremely pure form, provides a valuable by-product. The working pressures in the still amount, as a minimum, to approximately 5 Ib. per square inch at the point where the steam is applied ; 4 Ib. per square incli where tiie ammonia is withdrawn; and 3 Ib. per square inch. at the zone where the acid gases are removed. Higher pressures tlian these (worked in. the same ratio) will, however, give more satisfactory results. Though not essential to the process, Dr. Davidson prefers to treat the gas whicli has passed the ordinary washers and serubbers witli a sludge of ferrous hydrate, prepared from iron chloride or sulpliate and ammoniacal
liquor. In this way the whole of the sulphuretted hydrogen may be removed. The process then. resolves itself into the foHowing:—
1. Tar extractor.—Tar in crude gas reduced to less than 2 grains per 100 cubic feet.
2. Cy anogen washer.—Iron sulphide sludge employed for remo ving cyanögen :— FeS + 6 HCN + 4 NH3 = (NH4)4FeC!iN'6 + H2S. Ammonium ferrocyanide is thereby formed, with the liberation of sulphuretted hydrogen.
3. First ammonia washer.—To whicli the ammonia gas liberated from the