Modern Gasworks Practice

THE DRY PURIFICATION OF COAL GAS 617 of cliarcoal utilized, therefore, is limited to that amount which would just be saturated with sulphur compounds and naphthalene. In practice it has been found that the sulphur compounds and naphthalene may be removed by employing 70 Ib. of charcoal per ton of coal carbonized. Once the initial outlay has been met the process should certainly prove economical, for the saturated charcoal by being heated to about 300° C., tlirows ofE the absorbed vapours, and is thus regenerated for use again as a pwifying medium. The process possesses some of the characteristics of that introduced by Soddy 1 for the purpose of stripping and recovering the illuminants from coal gas, and for the isolation of ethylene. Soddy found that at ordinary temperatures 1 ton of charcoal serves to remove the ethylene from 10,000 cubic feet of average coal gas, or 1 ton will absorb approximately 300 cubic feet of ethylene. The process, of course, differs from that of Adam, in that the latter necessarily has in mind the retention in the gas of the hydrocarbons, while Soddy seeks to recover them for otlier industrial purposes. It is interesting, however, to follow up Soddy’s process from the faet that it migfrt have proved a reasonable alternative to the oil-stripping treatment of coal gas which was so largely adopted to meet the exigencies of war.2 In carrying out the process coal gas, preferably purified, is passed through chai-coal at ordinary temperature, and the charcoal is subsequently heated to expel the absorbed constituents. The invention also includes a method by which the process may be made continuous, and which depends in part on the known principle under-lying methods of scnibbing generally—namely, that continued passage of the gas causes displacement of the more volatile constituents first absorbed by the less volatile constituents as these accumulate in the absorptive agent. The process also depends on the well-known faet that the expulsion of a dissolved or absorbed gas by heat is aided by diminishing the partial pressure of the gas in the atmosphere suTTOunding the solvent or absorbent. Whatever the mode of treating the coal gas with. charcoal to absorb the illuminants, carbon bisulphide and sulphuretted hydrogen should first be removed. The sulphur compounds are absorbed by the charcoal effectively ; but the charcoal deteriorates in absorbing power when unpurified gas is used. It is also of advantage that carbon dioxide and water vapour should be removed, as representing useless diluents subtracting from the efficiency of the cliarcoal as an absorbent of illuminants. Hence the process is best applied to gas which has undergone the usual purification process for the removal of these impurities. But still further purification is desirable when the separation of the more volatile illuminants—particularly ethylene—is the main object of the treatment. As already explained, the ethylene is displaced from the charcoal as benzene accumulates ; and the charcoal is so efficient in removing ben-zene that, even though the gas has already been subjected to a process for stripping the benzene from it, there should be a further treatment with charcoal for completing this purpose. Carbon disulphide may be removed by utilizing certain compounds such as aniline, when sulphuretted hydrogen is evolved and a somewhat complex organic 1 B.P. 7698/18. 2 See Chapter XII, page 423.