Modern Gasworks Practice

THE DRY PURIFICATION OF COAL GAS 581 may probably be attributed to varying oxygen content of the gases rather than to a varying content of sulphuretted hydrogen. When air is admitted to the system it will usually be necessary to throw out the material only three or four times before it is finally spent (containing about 55 per cent, of free sulphur), which contrasts very favourably with the old method, of which. Lewis T. Wright wrote in 1895, “ Oxide can be economically sulphided and oxidized sixteen times,” this meaning that before the material was economically spent it could be thrown in and out of the purifier sixteen times. For most efficient results, so far as reviviflcation in situ is concerned, a slight excess of air above the theoretical quantity indicated by the amount of sulphuretted hydrogen present should be employed. Furthermore, the capacity of the plant should be ampie, otherwise the air will not produce its due effect. It should be borne in mind that while during the sulphiding reactions a small amount of heat is generated, the oxidation process accounts for the evolution of a rauch greater quantity of heat. These considerations mainly affect the question of moisture, for during the sulphiding process water (corresponding to the sulphuretted hydrogen formed.) is set free; and, whereas a portion of this is required for rehydration of the oxide during the oxidation stage, the excess is removed from the system. It is, therefore, important to maintain the working temperature of the system at 70° to 80° F. (21° to 27° C.) so that the moisture thrown out during sulphiding is carried forward to the oxidation zone by the warm gas. As regards the point of admission of air, opinion is divided as to whether it is preferable to admit the whole at the inlet of the system, to admit it at a later period (such, as the inlet of the second vessel), or to split it amongst the various boxes. The author has always preferred to permit the small amount of oxidation which may occur in tlie first vessel to be provided for by the oxygen actually in the gas ; admit-ting, approximately, two-thirds of the added air to the second vessel, and one-third to the third box. G. W. Anderson1 says that it is immaterial whether the air is admitted at the inlet of the first box or at the inlet of the second or third so long as the quantity added is below the theoretical figure. The nearer the latter figure is approached, however, the earlier should.be the point of addition {i.e. if the füll theoretical amount is used, admission should be at the first box), otherwise a certain amount of oxygen may be found in the gas leaving the last purifier. The Effect of Adding Air When reviviflcation, owing to admitted air, is taking place in a purifier, a rise in temperature, usually amounting to about 5° F., will be noticed between the inlet and outlet. It is unnecessary to add that no little precaution. is necessary in the regulation of thé quantity of air used, chiefly owing to its diluent effect as regards the final illuminating and calorific power of the gas, and also with regard to cur-tailing any abnormal rise in temperature. It is necessary to bear in mind that should any excessive local heating of the oxide take place, instead of a sulphide or a normal sesquisulphide being obtained, disulphide of iron is formed, or iron sulphate. 1 Gas J„ CXLVIII, p. 689.