Modern Gasworks Practice

THE DRY PURIFICATION OF COAL GAS 615 It will be noted that whereas in the former process an. objectionable solid (carbon) remains as a residue and chokes the catalyst, in the newer method the residue is in gaseous form. Herein lies the advance, for it is the necessity for burning off the deposited carbon from the surface of the catalyst which demands intermitten! operation in the Carpenter-Evans process, while Rideal and Taylor effect contirmity of action by reason of the faet that the carbon passes out of the plant in the form of a gas. In the Carpenter-Evans process the catalyst employed is costly, namely, nickel reduced from the chloride, while in the newer process oxide of iron isused, which is activated by the addition of a small proportion of oxide of chromium. The water vapour for the reaction is provided by the admission of a small quantity of steam, passed in with the erude coal gas at the inlet of a preheater which is situated just prior to the catalyst vessel. As regards the quantity of steam admitted, it has been found that it is necessary to add only that amount which is required to prevent the reduction to the metallic state of the catalyst by the reducing action of tlie gas. For example, at a temperature of 350° C. over 90 per cent, of the sulphur impurities may be removed by the addition of 7 per cent, by volume of steam. As in the Carpenter-Evans process, the sulpliuretted hydrogen formed by the reaction is subsequently removed in the ordinary way by means of oxide of iron. The temperature of Saturation with water vapour and the temperature of the catalyst are depen-dent upon the composition of the gas, its sulphur content, and the nature of the catalyst employed. W. G. Leamon1 has introduced a process in America whereby the gas to be purified is subjected, in the presence of oxygen, to the action of heat ata temperature suffi-ciently high to produce effeetive dissociation of the contained hydrogen sulphide (about 280° to 300° C. is the optimum) by the aid of porous contact-material, which may carry a more actively catalytic substance, under conditions such that the hydrogen of the hydrogen sulphide is oxidized to water by reaction with oxygen contained in, or added to, the gas, while the sulphur passes on through the purifying system to be mechaiiically removed as elemental sulphur by some suitable method such as washing. Alternatively, it is also oxidized to a greater or less extent and removed by absorption in a suitable absorbing medium. F. W. Berk & Co.2 have patented a process which is on somewhat similar line«, to the Carpenter-Evans method. Coal gas is brought into contact with granulated alumina, preferably prepared from the trihydrate of alumina. In place of pure alumina, however, high-grade bauxite may be employed, or ignited magnesite. The material is raised to a temperature varying between 200° and 600° C. and the gas is then passed through it. In this way the carbon disulphide is dissociated with the formation of sulphuretted hydrogen. As in the Carpenter-Evans process the deposited carbon eventually impairs the activity of the material, and the procedure of burning-ofE is then resorted to. 1 B.P. 15568/18. 2 B.P. 5079/19.