THE PRELIMINARY PURIFICATION OF COAL GAS 535
any variations or contingencies that may arise. As to the design and arrangement of the plant this must embody the following essentials :—
(1) Adequate contact should be obtained between the coal gas and the re-circulated gaseous ammonia, in. order to allow of the formation of the soluble ammonium compounds which. are to be absorbed in the ammoniacal solutions and water being circu-lated. As the reactions involved necessitate time, as also does the absorption of the soluble ammonium compounds, the size of the vessels and the free space of the packing medium (assuming packed serubbers are used) should be sufficiently large to allow of these desiderata being secured.
(2) The gas to be purified should be freed as effeetually as possible from tar and hydrocar-bons, which are likely to accumu-late and eventually prevent the satisfactory and efficient working of the packing medium used.
(3) The relative merits and demerits should be carefully con-sidered of (a) vertical serubbers provided with suitable filling medium, which necessitate the installation of a series of bottom circulating tanks and pumps, and (6) liorizontally disposed. mechan-ical washer-serubbers arranged in Cascade form, in which. a counter-current is established between the gas and re-circulated gaseous ammonia and the absorbing liquors and water.
A simple type of plant as suggested by Parrish is ilhistrated in Fig. 338.
Fig. 338.— Lay-out of Simple Liquor Purification Plant.