Modern Gasworks Practice

580 MODERN GASWORKS PRACTICE will be depressed and rise in the pipe A, which sliould not be less than 24 inches to 30 inches in height. It should be borne in mind that most coal gas will contain a certain amount of oxygen, and the quantity varies to some considerable extent. It is important, therefore, to test the gas for oxygen before blindly admitting air, otherwise the Fig. 351.—Saibty Seal tor Air Admission. recognized limit may be considerably exceeded. In exceptional cases the gas itself contains sufficient oxygen without the mtroduction of a further quantity from the atmosphere. It is interesting to note here the remarks of one author-ity who, as a result of a great number of experiments, found that an amount of oxygen in excess of 0-3 per cent, was never absorbed from the gas in the purifiers. In view of this, it is con- tended that as from 0-2 to 0-4 per cent, of oxygen is almost sure to gain admit-tan.ce during the period for which. the retort doors are open, there is no necessity to add a further amount to the purifiers. When air is admitted at some point prior to the purifiers it must not be over-looked that there is always the possibility of its encouraging undesirable reactions in the wet purification plant. It is, however, an excellent plan to arrange for the air to be introduced on the inlet of the clean water scrubber. The merits of the “ backward rotation ” system are chiefly dependent upon the part played by the admitted air. For some time it was generally believed that the material in a purifier could not simultaneously extract sulphuretted hydrogen from the gas and undergo revivification. The assumption was not entirely accurate, as the process of sulphiding invariably takes place at a very mucli more rapid rate than does the absorption of air. In the case of “ backward rotation, however, a purifier gradually passes from the position of first “ taker,” where sulphiding mainly occurs, to the end of the series where the oxygen is permitted to carry out revivification undisturbed. In this way each purifier is accorded a period of rest in which. to prepare for taking its place once more at the head of the series. Accordingly, with alternate periods of sulphiding and revivification, tlie boxes may be operated for a lengthened time before requiring remaking, and the material may be worked up to a higher sulphur content. Dealing with the point as to whether or not sulphiding and oxidation can occur simultaneously, W. S. Curphey says that oxidation changes are no doubt solely responsible for the slight rise in temperature noticeable in the third and fourth boxes, where sulphuretted hydrogen must be considered absent, but it is a matter of uncertainty how far the oxygen is active in the first and second boxes. Experience certainly indicates that the conditions existing in the preliminary vessels must in general be extremely favourable to concurrent revivification, and the notable variations of temperature which are not infrequently recorded in the first two boxes