Modern Gasworks Practice

 THE DRY PURIFICATION OF COAL GAS 593 If a small proportion of air is admitted to the purifier, a certain amount of the calcium sulphide will be converted into free sulphur, and will thus escape ultimate decom-position by CO2. In preparing ordinary dry lime for use in purifiers, the material should be well slaked with water, and as much as possible of the “ core ” (i.e. inert matter, such as chalk, stones, etc.) should be removed by screening. In addition to the water actually required for slaking, the lime should contain a further 30 per cent, of moisture. Excessive watering will, however, give rise to caking. Caking, and consequent loss in power of absorption, may also result from too much sulphuretted hydrogen having been passed through the lime, this occasionally occurring in those boxes which are set aside for the removal of CS2. So far as the thickness of the layers of lime is concerned, it may be pointed out that a depth of about 8 inches is preferable to the thinner layers of 4 and 5 inches which were often employed in the past. What-ever the type of purifying material, a thin layer is likely to “ blow ” and thus permit a certain amount of gas to escape untreated. When gas manufactured. at the larger works is to be freed from the whole of its impurities, it is usual to instal a system in which oxide of iron and lime are used in conjimction. In this way the greater proportion of the sulphuretted hydrogen may be recovered and not wasted. Many methods of operating the purifiers for this purpose have been employed, but the whole principle may be readily grasped by a consideration of the following system. It consists in the main of four sets of purifiers operated in rotation :— First set. Contains lime. Second set. Contains oxide of iron. Third set. Contains lime. Fourth set. Contains oxide of iron. These remove the whole of the CO2 and some H2S, which is after-wards displaced by CO2. Some CS2 is also absorbed. These remove the whole of the H2S, inclading any which may be displaced from the lime in the previous set. This lime has been previously “ sulphided ” and removes the bulk of the CS2. It will not remove the whole of the sulphur compounds. These vessels act as “ catch-boxes ” and remove any HaS which is disengaged from the third set. In order to understand the method of working, it must be borne in mind that ordinary slaked lime—per se—lias no action upon carbon disulphide. The Chemical reactions involved in the elimination of this impurit.y are somewhat complex and are set out below. First, it will be seen that the calcium hydrate must be “ sulphided ” by passing through it a stream of sulphuretted hydrogen. For this purpose the crude gas, after travelling through the first set of purifiers, is led direct into the third set, the oxide vessels comprising the second series being by-passed for the time. Working in this way is continued until sulphuretted hydrogen is noticed at the outlet of the lime boxes forming the third set. At this juncture the two oxide vessels (second set) are put into action. The lime vessels forming the third set are then in a condition to absorb carbon disulphide and purification proceeds as outlined above. Of-the total CS2 present, up to 20 per cent, may be removed by the first lime vessels, 55 per cent, by the sulphided boxes, whilst about 25 per cent, will remain in the gas.