Modern Gasworks Practice

472 MODERN GAS WORKS PRACTICE increasing the naphthalene solvents in the gas. A point to bear in mind is that the best liquid is not necessarily that which has the lowest boiling-point. In faet, the higher the boiling-point, within reasonable limits, the more permanent will the effeet be. As explained above, two alternative methods of adding solvents present themselves, namely:— (a) Addition by spraying, or tliorough atomization of the solvent. (b) Addition by carburation, or evaporation of the solvent into the gas. It is essential, of course, that the liquid employed should be as free as possible from naphthalene; and, although various coal-tar and carburetted water-gas tar ■distillates are quite suitable, preference is usually shown nowadays for a light paraffin oil or kerosene. From the point of view of their naphthalene solvent power ordinary coal-tar naphthas are preferable to paraffin, but the latter is more readily ■obtainable at a reasonable cost and possesses the great advantage of containing no naphthalene whatever. Wheii the solvents are injected into the main gas stream care should be exer-cised to ensure that the spray is placed at the centre of the main and not at its periphery, and from the point of injection onwards there should be as long a length. of straight main as possible. Sharp bends or abrupt changes of direction occurring adj acent to tlie point of spraying may be responsible for deposition of the oil mist before it has had time to become thoroughly dispersed throughout the gas volume. The most suitable place for the insertion of the spray is at some convenient point in the district main just prior to its leaving the works, although in aggravated cases or straggling districts it may be necessary to arrange for local sprays at some distance from the works. On the Continent it has been customary in some instances to atomize a light tar oil into the erude gas as it issnes from the condensers. The oil employed consists of 60 per cent, distilling over between 100° and 180° C., and 35 to 40 per cent, between 180° and 230° C. This can be readily atomized at ordinary temperatures, th.e mist being removed prior to the wet purification by means of P. and A. tar extractors. It is claimed thai naphthalene may be entirely eliminated by this process. When a solvent is introduced into the district mains the first essential is to ■ensure that it will carry effectively throughout the whole system. Paraffin, and many of the otter common solvents, should be atomized and not vaporized, for they have a comparatively low vapour pressure, and, if vaporized, will not remain in the gas for any length of time. Although, in the author’s opinion, the solvents are preferably admitted to the district main, it may be pointed out that in some instances the spray is fitted at a point'between the station meters and the gasholders. Of the special spraying methods introduced, may be mentioned that of Botley, who injects about 4 gallons of thoroughly atomized paraffin oil per million cubic feet of gas. This investigator found that the oils most suited for enriching purposes are not the best for naphthalene washing ; thus petroleum is more efficient than a light «pirit or benzol. When the system is in use, a dark green oil is pumped from the Street syphons, this being recovered and made use of on the works for lubrication, •or, in some cases, recarburetting. The recovered portion amounts to about 25 per