Modern Gasworks Practice

HIGH TEMPERATURE CARBONIZATION OF COAL 423 THE EFFECT OF BENZOL STRIPPING The process of washing coal gas,1 for the recoveiy of the aromatic vapours, which was demanded by the exigencies of war has now been very largely discontinued by gas undertakings, although it is still, of course, in operation at ;coke-oven establishments. The commercial aspect of benzol recovery has been considerably altered by the modern method of selling gas on the basis of therms instead of in cubic feet. If straight coal gas is completely stripped, for instance, the aromatic vapours removed would represent approximately 1 per cent, of the total volume of the gas. Normally, 2 gallons per ton of coal are extracted, and whereas pure benzol has a calorific value of 163,000 B.Th.U. per gallon, the crude mixture removed from coal gas has an average calorific power of 130,000 B.Th.U. per gallon, and 2 gallons represents about 100 cubic feet of gas. Accordingly, the revenue from benzol sold in various forms may be approximately contrasted as follows :•— 2 gallons crude spirit @ Is. 2d. (net) = 28rf. per ton of coal carbonized. 100 cubic feet of benzol @ 5s. per 1,000 cubic feet= 6d. „ „ ,, 260 ,000 B.Th.U. @ 16<Z. per therm = 41-6<Z. „ „ „ While, therefore, it was commereially advantageous to extract benzol from the-gas while the cubic-foot basis of selling remained in force, it will be noted that, as heat units are now sold, the benzene has a far greater value if sold in. the gaseous-form. With regard to the efiect of benzol extraction on the quality of coal gas it has been shown that the loss in calorific power is approximately 4-6 per cent, when 2-1 gallons of crude spirit is extracted per ton of coal, 7 per cent, when 2-5 gallons are extracted, and 8 per cent, when 3 gallons are recovered. L. J. Willian 2 states that in America the average yield of light oils is about 3 gallons per ton of coal carbonized, but this undoubtedly refers to the gas produced from coke ovens. He states that the light oils recovered have approximately the following composition : Benzol 50 to 60 per cent., toluol 12 to 13 per cent., solvent naphtha 10 to 12 per cent., heavy naphtha and wash-oil 13 per cent. This investi-gator has carried out extensive tests in order to determine the efiect of “ stripping on the quality of gas. With a plant dealing with 100,000 cubie feet of coal gas per day, he obtained data showing a loss of 63 per cent, of the candle-power (Met. No. 2 burner) and 6-2 per cent, of the calorific power (B.Th.U. gross), whilst the naphthalene decreased 80 per cent., and the carbon bisulphide 50 per cent. Willian has also investigated the effects produced by stripping carburetted water gas, but the efiectiveness of such a procedure depends entirely upon the-quantity of oil which is employed in the carburation of the gas. The temperature prevailing in the carburettor influences the result. Temperatures between 700° and 760° C. appear to produce the maximum amount of toluene. Higher tempera- 1 For detailed particulars of the plant employed, etc., the reader is referred to Benzol, by S. E. Whitehead (Benn Bros., Ltd.). 2 American Gas Inst. News, Dec., 1917.