THE PRELIMINARY PURIFICATION OF COAL GAS 549
The Koppers system of semi-direct reeovery is shown diagrammatically in Fig. 344. The hot gas coming direct from the ovens enteis the coolers or condensers, where it is reduced in temperature to about 25° C. The gas then passes to the exhauster and is taken through a tar extractor of some efficient type. After the tar has been removed, the gas is led through the re-heater, where, by means of exhaust steam, its temperature is raised to about 55° C. The heated gas then passes to the saturator, where the ammonia is recovered from it by absorption in sulphuric acid. The sulphate as it is formed falls to the bottom of the vessel and is continuously removed by means ■of a steam or compressed air ejector. Finally, the sulphate is delivered to a ■centrifugal drying machine, in which. the mother liquor is removed from it. The condensate thrown down in the coolers, also the overflow products from the tar extractor, are run to the separa-ting tank, which is so arranged that tar and liquor can separate, owing to the •difference in tlieir specific gravities. The two products then flow into their re-■spective storage tanks. The virgin liquor is finally pumped. up into the ammonia still, where it is heat ed with steam and lime in the usual way. The ammonia, ■sulpliuretted hydrogen, and carbon ■dioxide, resulting from the distillation of the liquor, are then admitted to the ■crude gas main at a point just preceding the saturator.
Fig. 345 illustrates a Wilton plant, from the outlet of the hydraulic main to the inlet of the dry purifiers, for the ■condensation of coal gas and the removal of tar and tar-fog, as well as for the reeovery of sulphate of ammonia.
Fie. 345.—Wilton Plant rot’. Sbmi-Dikect Recoveey.