Modern Gasworks Practice

432 MODERN GASWORKS PRACTICE are employed in continuously operated vertical retorts, has prompted Perkin and West i to develop a system by means of whicli the hydrocarbons, particularly those remaining in the tar, may, if desired, be transformed largely into the benzenoid variety. The method, it is claimed, provides a means whereby a higher yield of coal tar and of low-boiling aromatic hydrocarbons may be obtained under high, temperature conditions, with a corresponding diminution of hydrocarbons of the aliphatic series, the presence of the latter in the lower fractions of the tar adding to the difficulty of purifying the benzol and toluol. Perkin and West state thatif a gas of moderately low quality—such as “ stripped. ” coal gas—be passed into the lower end of the retort at such a rate that it is able, after traverging the incandescent coke zone, to carry the volatile matter from the partially carbonized coal through the relatively cool coal (which is just entering the hot zone of the retort) without the volatile matter becoming Condensed in the cool coal in any degree, then not only is the amount of tar distilled increased in volume, but its specific gravity is decidedly lower. In this process the stream of low quality gas is admitted beneath the coke-extractor worm; and, as the rate of supply of the subsidiary gas is of importance, it is regulated by increasing the vacuum exerted by the exhausters on the main gas outlet, or by admitting gas under pressure. The tar is found to contain. an additional proportion of benzene, toluene, xylene and phenols, with a corresponding diminution in the yield of paraffins. THE TEMPERATURE THROUGHOUT THE CHARGE The temperatures prevailing throughout the carbonizing mass vary to some extent with. the weight of the charge, and it is generally found that whereas with the light charge in the horizontal retort the hottest zone is that in contact with the bottom and sides of the retort, with complete filling the hottest portion will be that forming the surface of the coal. The pyrometric observations of Bond, recorded in the proceedings of the Institution of Gas Engineers, are of consider-able interest and throw much light on. the conditions prevailing. This investigator has shown that the solid residue tends to divide into two distinct layers, and he recorded. temperatures at the three zones (A, B, and C, Fig. 263) for varying periods of the charge. At the zone A the temperature at the commencement of the charge was 675° C. ; at the end of the first two hours, when 37 per cent, of the Fig. 263.— Temperatures at Vartovs Zones of Charge. total gas had been. evolved, it was 800° C.; whilst after four and a half hours a maximum of 1,000° C. was attained. As regards the zone B, at the centre of tlie charge, after the coal had been distilling for two hours the temperature was ap-proximately the same as at the base of the charge, but never exceeded the latter. At the commencement of carbonization tlie temperature of the core—as wotild be 1 Eng. Pat. 114937/1917.