Modern Gasworks Practice

508 MODERN GASWORKS PRACTICE as this contains a large proportion of uncombined ammonia, available for the removal of CO 2 and H2S. Considerable care should be taken in order to prevent the passage forward of tar to the ammonia washers. At the inlet to the washing plant this impurity should be reduced to as low as 2 grains per 100 cubic feet of gas. Davidson has said that the crude gas in the hydraulic main contains as much as 8,000 grains of tar per 100 cubic feet. The effect of submitting the gas to clean water in the final scrubber is to remove practically the last traces of ammonia from the gas, whilst the water is converted into a weak liquor (from 1 to 2 oz. strength), which, mixed with that flowing from the condensers and hydraulic mains, is passed down, the previous portions of apparatus and finally worked up to th.e desired strength. Herring says that the weak liquor has a slightly greater affinity for the sulphuretted hydrogen, but as its strength. in-creases the carbon dioxide exerts its superior affinity, actively combines with the free ammonia, and breaks up some of the already formed sulphide, giving, instead, ammonium carbonate. Tlius in certain of the washing vessels evolution of sul-phuretted hydrogen may occur. As regards the amoimt of water (other than that formed from the distillation of the coal) which is necessary in ordet to ellect the wet purification of gas, the äuthor fmds that with Durham coal the flow through the final scrubber amounts to from 10 to 13 gallons per ton of coal carbonized. With this quantity the ammonia in the outlet gas is reduced to 0’5 grains per 100 cubic feet. Some years ago it was con-sidered that the water required for complete removal of ammonia should be 3 gallons per ton. of coal used ; but much must depend upon the nature of the coal, the systems of carbonization and condensation, and the type of scrubber in use. WASHING AND SCRUBBING APPARATUS The arrangement of the wet purification plant conforms to no regulär rules, so that the apparatus and tlie method of working it vary considerably, in. accordance with. the tastes of the engineer. Formerly, the removal of ammonia was eftected by the use of no other energy than the pressure of the gas itself; but, recently, mechan-ical devices, necessitating the use of external driving power, have come into favour. The latter apparatus is commonly known as the washer-scrubber, owing to its ability —due to increasecl efficiency—to perform the greater portion, of the work of these two vessels. Washing plant may be taken to embrace those portions of the apparatus in which the gas is actually caused to bubble through seals or to pass through weirs of liquor. In scrubbing plant, on. the other hånd, the gas comes in contact with a wetted surface of coke, boards, or other suitable material. In tlie design of the washer, the depth of the seals employed, or the resistance offered (and, accordingly, the effectiveness of the machine for removing ammonia) must be balanced against the back-pressure thrown, and in no case should the latter exceed 3 inch.es for a single machine. Washers are invariably operated with liquor of the maximum strength, whereas in tlie serubbers, liquor of a weaker quality is circulated.