Modern Gasworks Practice

506 MODERN GASWORKS PRACTICE Distribution of Sulphur in Gas Liquor Sulphur combined with free ammonia as sulphides . . . 55 to 75 per cent, of total. „ „ „ fixed ammonia as sulphocyanide . 15 to 25 „ „ „ „ „ „ „ as sulphate . 1 to 5 „ „ „ „ „ „ „ as thiosulphate , . 5 to 15 ,, „ „ „ in other forms (polysulphide, etc.) . I to 5 „ „ The ammonia present in coal gas has provided tlie gas engineer with a simple means of eliminating the chief impurities in the gas, but there is a limit to the extent of tlie purification. which can be carried out through, its agency, for the amount of the alkali present in crude gas is only in the neighbourhood of 1'5 per cent, by volume. Of this, about one-fourth to one-fifth is combined as fixed salts, thus the surplus available for the removal of sulphuretted hydrogen and carbon dioxide amounts to about 1’2 per cent. Together the H2S and CO2 in the crude gas (excluding any C02 which may be drawn. in from the furnace) total about 2| per cent, by volume, thus the ammonia available is theoretically sufficient for removing only about one-half of these impurities. At the outlet of the condensers the impurities in coal gas will be present in approximately the following proportions:— Impukities in Coal Gas at Outlet of Condensebs Grains per 100 cubic feet. Per cent, by volume (average) Ammonia ...... . 200 to 300 0-75 Sulphuretted hydrogen . 500 to 900 10 Carbon dioxide ..... . 900 to 1,400 1-5 Sulphur compounds .... 35 to 45 Hydrocyanic acid ..... 60 to 80 Naphthalene ..... 20 to 25 The Absorption of Carbon Disulphide As previously pointed out, water absorbs only about one-thousandth part of its own volume of carbon disulphide ; and, in the usual way, from 2 to 4 grains of this impurity are removed from the gas during its passage through the washers and scrubbers. The reduction. of carbon disulphide by treatment with liquor has for long beeil a subject of Investigation, and the extent to which the compound may be absorbed, also the actual Chemical männer in which its removal is effected, have given rise to a good deal of discussion. In the first place, it is by no means certain (as is so often stated) that CS2 is directly absorbed by ammonium sulphide with the formation of ammonium thiocarbonate, according to the equation— (NH4)2S +CS2= (NH4)2CS3. Tilden states that an interaction takes place between carbon disulphide and ammonia whereby a sulphocyanide and sulphide are formed, but heat is necessary for the reaction—4 NH3 + CS2= NH4SCN + (NH4)2S. The same authority states that combination of the two gases may give rise to ammonium sulphocyanide and thiocarbonate—4 NH3 + 2 CS2 = NH4SCN + (NHJaCSs- The actual manner in which the CS2 is removed is, however, not at all clear, and whilst it appears to unite with ammonium sulphide on the lines of the first of the reactions shown above it seems essential that polysulphides or free sulphur should be present; for the two substances do not combine when in the pure state. The carbon dioxide