Modern Gasworks Practice

576 MODERN GASWORKS PRACTICE and ferric hydrate to which wood dust is added in order to light en it. It is thcn used for the purification of gas from sulphuretted hydrogen and hydrocyanic acid. The Extent of Hydration The extent to which oxide of iron should be hydrated is a point of considerable importance, the gas engineer generally preferring to work with a moisture content varying between 40 and 60 per cent. It would seem, however, that such high proportions of water are liable to have a harmful influence on activity, particularly if the active particles are so surrounded with water that the gas has to pass through the latter before any reaction can occur. The result is that excess water impedes rathcr than promotes reaction, and the same applies in the process of revivification. Weyman speaks of the fact that, since the solubility of oxygen in water is very much smaller than that of sulphuretted hydrogen, excessive moisture will practically bring oxidation of the sulphides to a standstill. The slower rate of oxidation as compared with. Sulphidation can be observed in practice, for whereas revivification may occupy the whole time during which the gas passes through, säy, four böxes, sulphiding takes place in the first two. In the operation of the semi-direct process 1 for the recovery of sulphate of ammonia it has been found that periods of purifier disorganiza-tion have been most frequent with a very moist material, and less frequent when the material has been dry. Very much must depend, however, on the männer in which the process of purification is carried out, for the different conditions prevailing (e.g. upward or downward flow of gas, temperature, etc.) give rise to entirely different effects, so that it is extremely difficult to lay down any definite rule for moisture content. Moreover, comparisoii between normal purification methods and purification following the semi-direct ammonia process is likely to be misleading owing to the fact that much more moisture is in general Condensed from the gas when it reaches the oxide after passing the acid bubbler employed in the latter process. A factor of importance is that of the admission of air for revivification purposes, since where any considerable amount of air is passed in with the gas there is a tendency for moisture to be extracted. W. S. Curphey2 speaks of the fact that the higher temperature of the gas which. has to be dealt with when the semi-direct process is in use aecounts for the evaporation of the moisture in the leading vessels, which. moisture is eventually deposited in the later vessels of the series, thus accounting for excessive water content in the material in the final boxes. It is recommended, therefore, that instead of the usual 40 per cent, or so of water, the hydration of the material should be no more than 5 to 10 per cent., and that the quantity of water used should be gauged from the ferric hydrate content of the oxide rather than from its total bulk. For purification as carried out when the normal process of preliminary wet purification is employed it may be said that, so far as our knowledge of prevailing conditions goes at present, it is preferable to strike a balance between excessive water content and abnormally dry material, and to regulate the moisture to something approaching, but not exceeding, 30 per cent. 1 See Chap. XVI, p. 546. 2 Alkali Report, 1917.