Modern Gasworks Practice

502 MODERN GASWORKS PRACTICE means of renioving this impurity. Water also possesses some affinity for carbon dioxide and sulphuretted hydrogen, but its capacity in this direction is but slight, one volume absorbing about three and a quarter volumes of sulphuretted hydrogen, or one volume of carbon dioxide. Carbon disulphide gas is also soluble in water, but to a very small extent, amounting to about one-thousandth part of the volume of water. Advantage is taken of the faet that the ammonia solution formed, being of an alkaline nature, is capable of ready reaction with. the acid impurities in the gas. Thus carbon dioxide, the strongest acid present, and sulphuretted hydrogen, are absorbed, with the formation of certain definite salts which are themselves soluble in the water. In the ordinary sense ammoniacal liquor consists of water containing in solution a certain proportion of these salts. It is necessary to emphasize that the quantity of ammonia which water can absorb is not entirely dependent upon temperature, for solubility also vanes in direct proportion to the pressure of the ammonia gas. If pure ammonia gas is in contact with water at normal temperature and pressure, then. the water is capable of absorbing an amount equal to 780 times its own volume. In erude gas, however, the proportion of ammonia to the whole volume is trifling, hence the pressure it will exert is only in proportion. to the extent of its presence. For instance, if ammonia is present to the extent of one per cent, of the whole, then the pressure it exerts will amount to merely 0'01 atmospheres, at which pressure very little affinity exists between it and the water. It is this faet which explains the necessity for using a comparatively large quantity of water per ton of coal carbonized (see page 508), and shows why the question of time contact is one which must be considered in the design of washers and serubbers. Ammonia in erude coal gas is usually found to the extent of about 1 5 per cent, by volume, and of this amount from a third to one-half will have been washed out by the aqueous vapour and will have been deposited by the time the gas has reaclied the outlet of the condensers. In this respect the figures of Hunt, who found the distribution of ammonia as follows, are of interest:— Ammonia removed by condensation . . . .42-7 per cent, of total. „ „ first serubber . . . . 43-3 „ „ „ „ „ seoond „ 14'0 „ „ „ W. S. Curphey 1 has more recently stated that of the total ammonia at least 50 per cent., and probably 60 per cent., is found in the virgin liquor. When gas containing sulphuretted hydrogen and carbon dioxide comes in. contact with a solution of ammonia, the amount of the acid gases absorbed may be sufficient to give rise to a neutral or normal salt, or absorption may continue until the ammonia solution contains about 20 per cent, more of the acids than is sufficient to give the neutral salts. Hence, both acid and normal salts result as follows:— Ammonia and Carbon Dioxide— (1) 2 NH40H + CO2= (NH4)2CO3 (ammonium carbonate) + H2O. Normal. (2) NH40H + CO2= NH4,HCO3 (ammonium bicarbonate). Acid. Ammonia and Sulphuretted Hydrogen— (1) 2 NH40H + H2S = (NH4)2S (ammonium sulphide) + 2H2O. Normal. (2) NH4OH + H2S .== NH4,HS (ammonium hydrosulphide). + H2O. Acid. 2 Alkali Report, 1916, p. 29.