Modern Gasworks Practice

 THE DRY PURIFICATION OF COAL GAS 611 it might be so much as 76 per cent. The cost of the process, including all charges, was given at that time as 0-5d. per 1,000 cubic feet of gas treated. The Carpenter-Evans Process The Carpenter-Evans system of hot purification is in many respects similar to that described above, but difEers in one very important particular, and that is the working temperature ©mployed. The Chemical reaction occurring is precisely similar to that taking place in the Oregon process, but the velocity of reaction is greatly increased by the adoption of a suitable catalyst. Consequently, extremes of temperature are not needed. The process is carried out on the following lines: The gas is first heated up to about 410° C. by passage through tubular heat-inter-changers, and afterwards through tubes placed within the furnace. It then passes into chambers containing a catalysing agent, wherein the carbon disulphide is con-verted into sulphuretted hydrogen and free carbon— CS2 + 2 H2 = 2 H2S + C. The catalytic agent employed consists of fireclay balis about 1 inch in diameter impregnated with nickel reduced from the chloride, the apparatus being rnaintained at a temperature of about 430° C. The heated gas, ou leaving the catalyser, passes through a special heat interchanger in which it gives up part of its heat to the inflowing cool gas. The purified gas is then reduced to atmospheric temperature, and, finally, the sulphuretted hydrogen from the above reaction is removed by oxide of iron in the usual manner. The free carbon is left behind in the catalysing appar-atus, and after about thirty days’ run it is necessary to remove it. This is done by the process known as “aeration.” In order to regenerate the material of any distinet unit the temperature is per-mitted to drop, and air is pumped at a definite rate through the tubes containing the catalyst. This process usually occupies about a week, and consists in burning out the deposit of free carbon, which travels away as CO2. During the first four days of aeration the air is deprived of the whole of its oxygen. When once oxygen makes its appearance in the effluent products the temperature of the chamber is raised to 430° C., and this temperature is rnaintained until carbon dioxide ceases to be evolved, when the process is complete. Some precatrtion is necessary when preparing for aeration, or when recommencing purifying after aeration has taken place. The tubes will contain either gas or air as the case may be, and the formation of an explosive mixture is prevented by passing in a current of inert waste gases collected from the furnace chimney. In this way the gas or air is displaced by nitrogen and carbon dioxide. The original activity of the catalyst is immediately restored by the aeration process. E. V. Evans 1 has pointed out that in a system of this kind it is necessary, for efl'ective working, to bear in mind the following salient points :— 1. The temperature of the gas should be raised to that of the reaction before coming in contact with the catalyst. 1 J. Soc. Chem. Ind., XXXIV, 1915.