Modern Gasworks Practice

NAPHTHALENE IN COAL GAS 467 early stage in the foul main, for if allowed to remain these tars will subsequently bring down with themselves the greater part of the light oil fog and thus preclude it from exerting its full influence on naphthalene. The lieavier constituents of the tar, moreover, are moderately active absorbents of aromatic hydrocarbons, so that the longer they are permitted to remain the more pronounced will tlieir “ Stripping ” action be. Inadequate Condenser Plant Some engineers at the present day are still of the opinion that “ shock ” or abrupt condensation provides one of the best solutions of the naphthalene question. The author’s experience bas prompted him to form the opinion that drastic cooling and sudden abnormal temperature drops may—except, perhaps, in the case of the largest works—prove remedies which are in the main too drastic. The effects arising from such treatment will be fully appreciated from what has previously been said; but a policy of extremes in either direction must be avoided and inadequate cooling is to be deprecated far more than is excessive cooling. In fact, insufficiency of condenser capacity is probably responsible for the majority of naphthalene troubles. The condenser, as will be appreciated, is a piece of apparatus considerably influenced by climatic conditions, more especially if it be of the atmospheric type. When the plant is put. down in the first place its capacity is in variably computed on the maximum day’s production of gas. Normally, this occurs in the winter time when the weather conditions are more effective from the condensation standpoint; but it must be remembered that the yearly load curve is tending more and more to level up, so that the capacity which may be ample for a maximum winter day may fall measurably short of that essential for a maximum summer day. Accordingly, during the summer months the proportion of naphthalene in the gas is frequently at its highest; although, owing to the higher average temperatures prevailing, no greater inconvenience may then be suffered. There is no question, however, that many of the stoppages occurring during the winter months are largely due to the indiscretions committed during the summer months. Although, therefore, the policy of rating condenser capacity on the basis of so many superficial feet of cooling area per unit make of gas (see page 440) is sound within limits, such tlieoretical rules should always be checked when the plant is in operation by ensuring that at all periods of the year the condenser outlet temperature is capable of being maintained at a reasonably low figure, say 20° to 25° C. as a maximum. Treatment of the Gas with Solvents Secondary treatment of the gas for the removal of naphthalene is not by any means universally carried out, but it is to be recommended in the majority of cases where the gas is produced from horizontal or inclined retorts. A question which is still at issue is as to whether it is preferable to extract the naphthalene by some process such as washing with. solvents at the works, or whether it is more effective to admit special “ carriers ” to the district mains. If it is a question of starting de novo there can be no doubt that the best policy is to attack the problem at the fountain-head and ensure that no gas containing naphthalene enters the district