Modern Gasworks Practice

NAPHTHALENE IN COAL GAS 465 MEANS OF DEALING WITH NAPHTHALENE The methods which have been. introduced for the prevention of naphthalene deposition may be summarized as follows:— (a) The employment to the best advantage of the process of condensation; including shock condensation, or abrupt cooling, involving a wide temperature drop. (&) Treatment of the gas by means of special solvent® employed in a was her. scrubber, or mechanically-operated apparatus. (c) Carburation of the gas with suitable oils in such a way that they act as so-called “ carriers ” of naphthalene. (<Z) The passage of the gas through a special porous medium which will absorb naphthalene. (e) Deposition of naphthalene with the aid of an electrical discharge (see page 456). In addition to the above it may be pointed out, as a matter of historical interest, that one of the early investigators, Bremond, found that naphthalene is set free whenever there is condensation of the aqueous vapour in the gas, that its deposition is preceded by the deposition of water, and that gas deprived as far as possiblé of aqueous vapour does not deposit naphthalene. Bremond therefore dried bis gas by passing it through a vessel containing a desiccating material. It is said that a good deal of success attended this method; but the experiments were only carried out on a comparatively small scale. As a present-day parallel, however, may be quoted the custom of some engineers to cover the water in gasholders and station meters with a surface film of oil so as to prevent further Saturation of the gas with water vapour at these points. It is claimed that condensation in the district mains (probably attended by the deposition of naphthalene) is thus curtailed. Such a possibility is indicated by the lower Saturation naphthalene content of moist gas compared with dry gas at temperatures below 40° C. as determined by Thomas. Condensation as a Factor So far as the process of condensation affects the problem of naphthalene it must be borne in mind that this portion of the plant merely plays a preliminary part and must not be expected to effect entire elimination of the product from the gas. This is particularly the case with the gas from horizontal and inclined retorts, for in those instances where the whole of the gas produced is derived from continuous vertical retorts it should be possible to reduce the naphthalene content to a safe figure without recourse to any plant other than the condensers. It is necessary to recall that warm crude coal gas as it is found at the inlp.f, of the condensers contains not only naphthalene but a mixture of low-boiling vapours which may be classed as naph.th.as, the boiling-points of many of which very closely approach the boiling-point of naphthalene (218° C.). Ac-cordmgly, when the temperature of the gas is reduced by the condensers, these vapours are deposited in addition to the naphthalene. The light naphthas are, moreover, solvents for naphthalene; thus some portion of the product is carried H H