Modern Gasworks Practice

592 MODERN GASWORKS PRACTICE When working in this manner it would seem essential that “ catch-boxes ” should be used, owing to the tendency of the final purifier to throw off a certain amount of snlphuretted hydrogen. In fact, it may be said that in all cases the intro-duction of “ catch-boxes,” although theoretically unnecessary, is a wise prccaution against sudden eventualities. LIME PURIFICATION Lime in the hydrated or slaked condition is an active absorbent of snlphuretted hydrogen and carbon dioxide, and—under certain conditions—is capable of removing a proportion of the remaining sulphur compounds. On the smallest of works, where the amount of sulphur recoverable would (owing to cost of carriage and other con-siderations) not materially afiect the receipts, lime is used exclusively. On the larger works its use has been largely discontimied, owing to the general removal of statutory restrictions relating to sulphur compounds—other than snlphuretted hydrogen.—in the district gas. The admixture with coal gas of water gas, which contains but a small proportion of sulphur compounds, has tended to curtail considerably the total quantity of these compounds in the mixed gases. In the case of some of those concerns where coal gas alone is manufactmed, the removal of a portion of the carbon disulphide is still occasionally practised. In some instances it is effected by methods known as “ hot ” purification, but more generally with the aid of lime. The sulphur compounds remaining in the gas after the snlphuretted hydrogen has been removed consist mainly of carbon disulphide (which is present to the extent of about 0-02 per cent.). Of the remainder, thiophene is the cliief constituent, but so far the other compounds have not been definitely determined. In studying purification by lime it is, primarily, necessary to bear in mind that of the three acid gases involved, carbon dioxide has the strongest acidic properties, snlphuretted hydrogen following next, with carbon disulphide last in order. Thus when the cru.de gas meets an active base such, as slaked lime there is a tendency for the most vigorous acid to be absorbed first. The combination between lime and CO 2 takes place on simple lines, as follows— Ca(0H)2 + C02 = CaC03 + H2O. Snlphuretted hydrogen reacts with lime in the following männer— Ca(OH)2 + H2S = CaS + 2 H2O. Thus, as before explained, the whole of the purification can, by employing a suffi-ciency of material, be carried out with lime. The great drawback to such, methods, however, is the loss of the sulphur, which. cannot be recovered in useful form, as is the case where oxide of iron is employed. As explained above, when the gas from the wet purification plant enters the first (lime) purifier, the CO2 is absorbed to the greatest extent, although some snlphuretted hydrogen is also removed. As the process proceeds, this sulphuretted hydrogen is displaced by the strenger acidic nature of the CO2, and the sperrt material consists almost wholly of the carbonate. The männer in which the sulphide is displaced is shown as follows— CaS + CO2 + H2O = CaCO3 + H2S.