Modern Gasworks Practice

tn CO MODERN GASWORKS PRACTICE The Claus Process The Claus process is of particular interest, in that it was tried on a fairly large practical scale at the Belfast gasworks, but was ultimately abandoned as unsuit-able. Simply explained, the process consists of the following :— 1. The crude liquor (i.e. saturated with sulphuretted hydrogen and carbon dioxide) from the washers is passed through a series of towers, where it is submitted to the action of carbon dioxide gas. Sulphide of ammonia is, tJierefore, converted into the carbonate whilst sulphuretted hydrogen is liberated. 2. The sulphuretted hydrogen gas is then burned in a Claus kiln and yields sulphur. 3. The crude liquor, now consisting chiefly of ammonium carbonate, is passed into heating towers, in which its temperature is raised to from 85° to 95° C. In. this way the CO2 is expelled. from the liquor to the extent of from 60 to 75 per cent, of the quantity in which it is present. This CO2, or rather a portion, of it, pro-vides the CO2 gas current mentioned under (1). 4. The partly purified liquor from which. the CO2 has been liberated is tlien, distilled so that ammonia gas and the remaining ammonium carbonate are evolved. 5. The ammonia gas and ammonium carbonate vapour are then passed through cooling towers, wh.en the carbonate sublimes to the solid state, leaving s compar-atively pure ammonia gas. 6. Lastly, the ammonia is admitted to the stream of crude coal gas before the latter enters the washers. In this way the original ammonia which has combined with CO2 and H2S in the crude gas is recovered in the gaseous form and caused to react once more with the acids. A surplus of ammonia is, of course, obtained, and may be converted into sulphate of ammonia in the ordinary way. Although. theoretically the expulsion of the acid gases is comparatively simple, in practice considerable loss of ammonia occurs unless th.e plant is designed with skiil. The earlier investigators undoubtedly failed through lack of experi-ence in plant design and operation, for under present day conditions it is pos-sible to operate with. a loss of 1 per cent. In practice, however, it has been shown. that for complete removal of the sulphuretted hydrogen and carbonic acid from the gas so much as from four to six times the quantity of ammonia yielded by a ton of coal must be employed. Consequently, the working loss is multiplied by this extent. Considerable light has been thrown. upon the theory and practice of liquor purification by P. Parrish,1 wh.o states that, theoretically, 5-077 times the quantity of ammonia in the gas is the minimum quantity required for satisfactory working of the process. The same autliorify also states tliat as the volumc and coni position of the gas vary from day to day—even from horn to hour t'hese variations can only be satisfactorily met by maintaining in stock in the serubbers or circulating tanks an excess quantity of caustic ammonia ; so as to provide against 2 Proc. Southern Assccn. Gas Eng., Nov., 1918.