Modern Gasworks Practice

THE DRY PURIFICATION OF COAL GAS ÜI <1 The Influence of Temperature In oxide purification the temperature of the material plays an important part, and there can be little doubt that more consistent results would be obtained from the process if fluctuations in temperature were avoided. In many gasworks the purifiers are not housed, with the result that they are readily influenced by the vagaries of atmospheric conditions. In general, the working temperature should lie between 21° and 27° C. (70° and 80° F.). Where air is being admifrted with the gas the outlet temperature will be some 6° to 10° F. higher than the miet temperature. A rise of this kind in temperature is desirable in that oxidation of the sulphides is more pronounced at the higher temperature. W. S. Curphey 1 states that (following the semi-direct process) temperatures much exceeding the limits mentioned above are found, and that such, temperatures are consistent with efficiency. It should be borae in mind, however, that, if the temperature is permitted to exceed the limits named,. the material will tend to become caked (thus giving rise to heavy back-pressure) owing to the moisture being carried forward to succeeding boxes. The backward rotation system is undoubtedly beneficial in the direction of distributing the moisture equally throughout the system, for each box is in its turn brought into the reaction zone, this assisting in the maintenance of a uniform temperature throughout and the equalization of moisture conditions. At the present day it is common to find a supply of live steam arranged for in the oxide purifiers. Some precaution is necessary in the use of steam in this way, as excessive moisture may give rise to abnormal back-pressure, resulting in the remaking of the box. The chief advantage of the steam is the faet that it accounts for some rise in temperature of the gas, whereby the oxide is maintained in a warm condition, which is conducive to its efi'ective working. For this reason it is found that the efficiency of a purifier may be enhanced by arranging for a steam coil in the gas inlet or in the base of the box—no live steam coming actually in contact with. the gas. Raising the temperature of the inlet gas by 10° to 20° F. will frequently be found to get over any difficulty arising from sluggish material. In this respect it will be noticed that when purifiers are partly buried the average temperature of the gas will be somewhat higher than when the vessels are above ground in open sheds and exposed to the weather. Live steam, however, when used in moderation, will frequently overcome inac-tivity or back-pressure resulting from caking. G. W. Anderson 2 suggests that for such purposes the quantity of steam employed should be 0’25 Ib. per 1,000 cubic feet of gas. It is a common works experience to find that a series of purifiers will absorb all but the merest trace of sulphuretted hydrogen, the lead papers being practically, but not perfeetly clean. The condition may prevail for some time, but without any tendency to become more aggravated. Weyman suggests that the remnant of sulphuretted hydrogen left is the result of the secondary reaction. 3 S + 2 H2O = 2 H2S + SO2. 1 Alkali Report, 1916. 2 Gas J„ CXLV1II, p. 688. P P