Modern Gasworks Practice

578 MODERN GASWORKS PRACTICE Thus the whole of the original sulphuretted hydrogen is being removed, but prevailing conditions are favourable for secondary formation of this impurity as shown above. The trouble can probably be traced in most instances to temperature, and wlien it occurs the temperature of the final 'boxes should be lowered. The Admission of Ammonia It is now generally accepted that the operation of dry purification is facilitated by the admission of a small proportion of ammonia to the purifying vessels. It is customary, therefore, to permit from 0'5 to 1 grain of ammonia per 100 cubic feet of gas to travel forward’with. the gas issuing from the clean water scrubber of the wet purification plant. Experience sliows that while purifiers will undoubtedly work effectively under conditions which result in the formation of soluble salts of iron (ferrous sulphate), the activity of the material is distinctly higher when sufficient ammonia is present in the crude gas to maintain the oxide in a neutral or slightly alkaline condition. The effect of ammonia so far as cyanogen Compounds are con-cerned has already been discussed,1 but the actual part it plays in gas purification is not as yet perfectly clear. W. S. Curphey2 has directed attention to the determination of the minimum amount of ammonia which is necessary, in his opinion, to maintain the purifiers at maximum efficiency. Results as yet are difficult to Interpret, but there is reason to believe that the amount of ammonia required bears a direct relationship to the quantity of ferrous sulphate formed, and that the formation of ferrous sulphate is favoured by excessive moisture and possibly by low temperature. Ferrous sulphate is an objectionable constituent in that it will not undergo revivification, or at least only in the presence of alkalis or carbonates in the mass, and then only slowly. Moreover it reduces the rate of revivification of the whole material, and may give rise to the well-known effect of “ souring ” of the oxide. The quantity of ferrous sulphate actually present at any given moment appears to be small, probably only a fraction of 1 per cent, in normal working, but in relation to the actual reacting weights of sulphuretted hydrogen and ferric hydrate the amount cannot be regarded as negligible. This is owing to the fact that it is the surface of the oxide particles which react, and if the particles are coated with a skin of ferrous sulphate the activity of the mass may be immensely reduced. Curphey makes a suggestion with regard to the point of admission of the ammonia to the purifiers which merits the attention of gas engineers in general. In ordinary practice it is customary to admit all the ammonia with the crude gas entering the first purifying vessel. It is suggested, however, that a certain proportion of the ammonia should be admitted to the second vessel as well, instead of supplying an excess to the first box in the hope that some of it may escape absorption and travel farther along the series. The Admission of Air Oxide of iron is generally used in the purifiers in layers varying from 8 indies to 10 inches in depth, although in some cases it is still customary to adhere to deeper 1 See previous chapter. 2 Alkali Report, 1917.