Modern Gasworks Practice

THE PRELIMINARY PURIFICATION OF COAL GAS 547 cautions are observed. The semi-direct plant to-day is mainly employed on coke-oven works, alth.ou.gh a small number of gasworks now have the process in operation. Briefly stated, the advantages of the semi-direct method as compared with the ordinary separate distillation plant for recovering sulphate of ammonia are as follows:— (a) No washers and scrubbers are required. (ö) Liquor storage wells may in many cases be dispensed with. (c) Small steam consumption. (d) Considerable saving in ground space and working costs. On the other hånd, experience shows that unless the plant is carefully operated and supervised the quantity of ammonia recovered may fall measurably short of the normal amount obtained under more general methods, while serious derangement1 of the dry purifiers may result. The best method for operating plants of this kind lias been the subject of a special inquiry undertaken by W. S. Curphey,2 who says that exact control of ammonia in the direct process is always a matter of difficulty. The ammonia content in the crude gas entering the bubbler is govemed by the volume and temperature of the virgin liquor that separates in the hydraulic main and condensers, and when a long main is in use between the bubbler and purifier, condensation accompanied by absorption of ammonia takes place to a very notable extent. Control by test, therefore, is the only satisfactory procedure. Curphey states that tlie most serious sources of loss undoubtedly occur at the still, and at the bubbler, when crude gas is by-passed in quantity. Loss at the still is mainly due to deficiency of lime. The following tests are representative of the results obtained at one works :— Ammonia, per 100 parts of Spent Liquor. Calculated loss of sulphate, Ibs. per ton of coal. Free. Fixed. Total. 1. (a) Lime feed defective •007 •653 •660 5 (6) Lime feed defective •009 •440 •449 31 (c) Better feed ; reduced flow of liquor. •030 ■005 •035 •3 2. Lime feed repaired ■0025 •0003 ■0028 •02 The loss due to by-passing of gas is more difficult to control, and in the winter months. with. wet fresh oxide in use, must be serious. Thirteen grams ammonia per 100 cubic feet gas correspond to about 1 Ib. sulphate per ton of coal carbonized. In the summer months 2 to 3 grains ammonia appear to be ampie. In the winter months, hitherto 15 to 20 grains ammonia have been required. The loss at the still is clearly preventable, if suitable qualitative tests are regularly applied, as they should be. The loss at the purifiers is in part preventable, 1 See page 578. 2 Alkali Works Report, 1916.