Modern Gasworks Practice

594 MODERN GASWORKS PRACTICE The total amount of sulphur compounds (other than H2S) in crude coal gas varies in normal cases from 35 to 50 grains per 100 cubic feet, although in exceptional instances the figure may rise to 60 grains. Chemical Reactions in CS2 Removal The exact nature of the Chemical reactions occurring during the removal of carbon disulphide is still open to question. It should be stated at once, however, that although the active medium consists primarily of calcium sulphide, and although this compound in the moist state is capable of absorbing CS2, the reaction is certainly not represented by the following simple equation— CaS + CS2 = CaCS3. The most enlightening work on the subject has been carried out by Veley (1885),. whose theory is generally accepted as correct. This investigator concluded. that the compound which absorbs the CS2 is not calcium hydrosulphide (as has sometimes been supposed), but calcium hydroxyhydrosulphide (CaS,H2O, or CaOH,SH). The hydroxyhydrosulphide, it is understood, is formed in the following männer (1) Calcium hydrosulphide results in the first place— Ca(OH)2 + 2 H2S = Ca(SH)2 + 2 H2O. This compound Ca(SH)2 has no action on the CS2. (2) By the addition of water or oxygen the hydrosulphide is then converted into the hydroxyhydrosulphide— (a) Ca(SH)2 4~ H20 = CaOH,SH 4~ H2S or (b) 2 Ca(SH)2 + O2 = 2 CaOH,SH + 28. The former reaction is the one most likely to occur in practice, for oxygen may not be present at this stage. (3) The CS 2 and calcium hydroxyhydrosulphide intereact as follows:— (a) 2 CaOH.SH + CS2 = H2S + Ca(OH)2,CaCS3 or (6) 3 CaOH,SH + CS2 + H2O = 2 H2S + 2 Ca(OH)2,CaCS3. In either case it will be seen that sulpliuretted hydrogen is thrown off and is absorbed in the final oxide boxes. Derangements with lime purification are only too common, owing to the difficulty of applying in practice a series of somewhat delicate Chemical operations. Occasion -ally the sulphided boxes will actually throw off carbon disulphide, and thus increase instead of reduce the sulphur compounds in the gas. If adequate precautions are taken, however, this trouble should not occur. The chief necessity is that of ensur-ing that no CO2 should be allowed to travel forward to the CS2 boxes. The CO2 will instantly displace the carbon disulphide, which must then travel forward with the gas. An important point to bear in mind is that the sulphiding action should never be carried out at a low temperature, otherwise the activity of the material may be impaired. The temperature of the lime during this operation should, if