Modern Gasworks Practice

NAPHTHALENE IN COAL GAS 469 With the object of avoiding the partial absorption of the aromatic hydrocarbons from the gas it is customary in some instances to pre-benzolize the wash-oil by adding from 4 to 5 per cent, of benzol to it. In this way equilibrium is bro light about between the vapour pressure of the benzene in the gas and the vapour pressure of the benzene in the oil. The South Metropolitan Gas Company have installed a very complete system for naphthalene removal by scrubbing all the gas with. anthracene oil in horizontal washer-scrubbers before it leaves for the district. The oil used shows normally the following test on distillation :— Drop point........................... 280° C. 280° to 325° C. . . . . . . . .32 per cent, by voIume. 325° to 360° C. ....................49 Residue ......... 19 „ „ Originally the oil, which may take up so rauch as 8 per cent, of aromatic hydro-carbons in cold weather, was pre-benzolized before use; but the Stripping effect was not found to be material once a condition of Saturation had been obtained, hence the addition of benzol is no longer carried on. The wash-oil is practically free from naphthalene when first admitted to the washer, and the quantity of fresh oil added is determined by the test for naphthalene on the putlet gas. The process is remark-ably effective, showing an efficiency of 100 per cent., with but the barest trace of naphthalene in the finished gas, and frequently no trace whatever. The spent oil running away from the plant is heated and treated meanwhile with steam. The process amounts to revivification, the naphthalene and lower-boiling constituents being removed in this way, and the oil then being ready for further service. The spent oil from the washer-scrubbers normally contains about 5 per cent, of benzene and light naphthas. The quantity of oil used varies with the season and the efficiency of the washer, but it approximates to 25 gallons per million cubic feet of gas. The autlior has for some years employed a light oil from carburetted water-gas tar as a washing medium, the treated gas being practically free from naphthalene. In this case the crude gas is dealt with, a scrubber of ordinary construction and filled with boards being interposed for the purpose just prior to the final ammonia scrubber. This position is, perhaps, the most advisable owing to the faet that the gas issuing from the naphthalene scrubber holds in mechanical suspension a certain amount of light oil fog which. is eliminated by the water scrubber, and is not there-fore cårried forward to the dry purifiers. The light tar used is in reality an oil which separates in the tar storage tank from the heavier tar and rises to the surface. The distillation test of the tar oil used is as follows:— Water-Gas Tak Oil (Freed from Wateb) Per cent, by volume. Up to 200° C........................................................nil 200° to 300° C......................................................48 300° to 360° C......................................................23 Residue ............................................................29 Specific gravity, 0-947. Free carbon, trace. Naphthalene, nil.