CHARACTER or THE CRUDE Gas.—At the moment of leaving the retort the crude gas contains a mixture of true gases with water vapour, and various solid and liquid hydrocarbons. The following list gives the names and formulas of the principal compounds which are present :— Of these constituents, the greater part of the solid and liquid hydrocarbons naturally separate from the gas as its temperature decreases ; but some of the more volatile bodies are partly or wholly retained by the gas, and become an integral portion of the purified product. Of these, amylene and carbon disulphide are probably wholly retained by reason of their low boiling-points, while water, benzol, toluol, and naphthalene are in part retained in the form of vapour. The heavier hydrocarbons are deposited in the hydraulic main, while those of lower boiling-point are separated by the action of the condenser. Of the gases proper, ammonia, carbonic acid, and sulphuretted hydrogen are considered as impurities, which have to be removed during the process of purification ; while the carbon disulphide has also, as a rule, to be partially eliminated from the gas supplied to London and the metropolis before it is considered sufficiently pure for consumption. The purifica tion of gas from the impurities mentioned may be said to commence at the condensers, and it is to the changes which there occur, together with those suffered by the gas during its subsequent treatment, that attention will now be directed.
GAS PURIFICATION.—Effects of Condensation.—The term " condensation " as applied to gas manufacture signifies the reduction of the temperature of the gas to about that of the surrounding atmosphere, in order to eliminate condensable substances, and to ensure the eventual product being of a permanent nature before it leaves the works. The gas enters the condensers at a temperature of about 38°-49° F.), and leaves them at a temperature of 10°-21•5° F.). During the cooling process the gas deposits a large amount of tarry matter, containing various hydrocarbons, benzol, toluol, (to.; a large amount of aqueous vapour is at the same time condensed, carrying with it ammonia, carbonic acid, and sulphuretted hydrogen, in combination as ammonium carbonate and sulphydrate. Part of the cyanogen present also combines with ammonia, to form ammonium cyanide, which, by the reaction of sulphuretted hydrogen, becomes ammonium sulpho cyanide. Small quantities of ammonium sulphide and sulphocyauide are also no doubt formed at
the same time from the mutual reaction of ammonia and carbon disulphide. The present system of condensation is by uo means perfect, for the separation of tar by a simple cooling process involves the separation from the gas of light hydrocarbons, which, if retained, would add materially to the illuminating power. The condenser tar contains as much as 7 per cent. of light naphtha, which is separated from the gas, not by the cooling process alone, but by reason of the absorbent power which is possessed by the heavy hydrocarbons over the light ones, and by which the latter are precipitated. Although the light naphthas of coal-tar have a boiling-point not far removed from that of water, ordinary gas can permanently retain a considerable quantity of such hydrocarbons in the form of vapour at ordinary temperatures. It has been proposed to restore to the gas the light hydrocarbons which have been abstracted from it by the action of the condensers, by bringing the gas leaving the condensers into contact with the tar heated to a sufficient temperature. A patent for effecting this by means of suitable apparatus has been taken out by H. Aitken (No. 2587, 24 July, 1874), and the result stated to be obtained by the use of his process is a gain of at least three candles in illuminating power. The value of the process must not, however, be estimated simply by the gain to the gas, hut by the net difference between gain to gas and loss to tar, the tar being deprived of its benzol and toluol, and being therefore of less commercial value. The process for the removal of the heavier hydrocarbons without affecting the lighter ones would be to pass the crude gas into a large vessel, where, owing to the greater area, the velocity of the gas would be so reduced that the particles of heavy tar would be naturally deposited by gravitation, the separation being completed by passing the gas through some straining medium, such as wire gauze. By such a process, tar could be separated from gas without reducing the temperature sufficiently low to cause the precipitation of light hydrocarbons.