Modern Gasworks Practice

470 MODERN GASWORKS PRACTICE A point to bear in mind is that water-gas tar used for naphthalene washing sh.ou.ld contain a high naphtha content and preferably no naphthalene. Accordingly, the carburettor and superheater of the water-gas plant should not, if possible, be operated at temperatures greater than 750° C. (1,400° Fahr.), for up to this temperature practically no naphthalene results from the cracking of the American petroleum commonly used for emichment. In past years it was customary to operate both carburettor and superheater at temperatures of from 900° to 980° C., and whereas this may have been necessary for complete fixation of the vapours when considerable quantities of oil were used, it is no longer so now thai the amount of oil per 1,000 cubic feet of gas has been curtailed by more than half. Extraction during Benzol Recovery It is a somewhat remarkable faet that in the earlier period of the war, wh.en the “ C ” process of tar-washing was introduced, there was in many instances a marked tendency for naphthalene troubles to inerease. This may be explained by the faet that the removal of benzene and its homologues from the gas deprived the latter of constituents which have a solvent action on naphthalene, while at the same time the very faet that gas and tar came into intimate contact set up conditions which were favourable for an exchange of naphthalene from tar to gas. The recognized oil-washing process has been followed by variable results in the same direction, but so long as the correct grade of wash. oil is employed with intelligence there is no reason why it should not be instrumental in reducing the naphthalene content of the gas. The process of “ stripping ” the gas certainly extracts a portion of the low-boiling naphthas. and for this reason there is every necessity to ensure that the naphthalene is reduced to a mueh finer content than that which. would. be considered safe with an unstripped gas. The first precaution is to ensure that the temperature of the stripping vessel is little above that of the atmosphere. In otter words, both gas and wash-oil should be properly cooled before coming in contact with. each otter, otherwise if the tar or oil used contains naphthalene it will give this up to the gas. In some gasworks where benzol removal from the gas is or has been carried on it has been customary to split the main stream of gas and to strip one stream only. This practice, from th.e naphthalene standpoint, is probably to be condemned, for w ith partial serubbing the naphthalene content of the final gas may still be moderate y high, whereas the low-boiling vapours have been reduced to an extent which. render them insufficient to act as solvents for such a quantity of naphthalene. As regärds the effeet of the process of benzol stripping on the amount of naphthalene in the gas the following figures have been given by L. J-Willien 1—- 1 Proc. American Gas Inst., Dec., 1917.