Modern Gasworks Practice

610 MODERN GASWORKS PRACTICE THE REDUCTION OF SULPHUR COMPOUNDS It has already been pointed out that tlie sulphur compounds remaining in coal gas after the extraction of sulphuretted hydrogen consist for the most part of carbon disulphide, and are present to the extent of about 0-02 per cent, by volume, or from 25 to 50 grains per 100 cubic feet of gas. The carbon, disulphide represents about 75 to 80 per cent, of the impurities classed as “ sulphur compounds,” the Constitution of the remaining 7 to 10 grains being still uncertain. No doubt, from 60 to 70 per cent, of the remainder consists of thiophen, whilst it seems probable that a portion is accounted for by ethyl and methyl sulphides and certain. mercaptans. During recent years the process of purification lias received a good deal of attention from the scientific standpoint, with the result that many new processes for the reduction of sulphur compounds have been successfully developed on a semi-technical scale. From the commercial standpoint, however, it must be borne in mind that the sulphur compounds other than. sulphuretted hydrogen are present in quantities so small (more especially where a mixture of coal gas and water gas is distributed) as to render them more or less irmocuous. Thus, with. the notable exception of the South Metropolitan Gas Company, no serious attempt has been made to remove them. Of the modern methods for CS2 purification those of chief importance at the moment depend upon the faet that under certain conditions the folio wing reaction will take place— CS2 +2H2 = 2H2S +C. The reaction occuts readily at high temperatures and with a moderately high concentration of CS 2 in the gas mixture ; but owing to the extremely small quantities (0-02 per cent.) in which carbon disulphide is present in coal gas it is essential that the reaction, if it is to be effeetive, should be of extremely high, velocity. The development of hot purification methods is largely due to the inventors of the Oregon process, who, after numerous experiments with. plant which proved un-suitable, evolved apparatus very similar in appearance to the carburettor of a modern water-gas plant. These investigators found that when the gas was passed through a vessel containing broken firebricks at a temperature of about 870° C., the CS2 was decomposed into sulphuretted hydrogen and free carbon. The plant employed consisted of two cylindrical vessels lined with. fireclay and yacked with fire-bricks laid chequerwise. The vessels were heated intemally by the combustion of producer gas, and whilst one was being heated, the second was purifying the gas, and conversely. That is to say, when the vessel which was purifying was cooled down by the inflowing gas to a temperature at which its efficiency was low, the operations were reversed, the gas being diverted to the second vessel, which had been undergoing a spell of reheating. In a series of experiments, extending through out 1909, it was found that the average reduction in the quantity of sulphur compounds amounted to 71 per cent. In a process of this description, where no catalyst is employed, temperature must necessarily play an important part. The inventors found that at 340° C. the percentage reduction averaged only 20-7 per cent., whereas at 700° C.