SE362234

1356653 Sulphur trioxide BAYER AG 25 May 1971 [2 June 1970] 16930/71 Heading C1A SO 3 is produced by the catalytic oxidation of SO 2 with air in a plurality of catalyst stages wherein an air stream, all or part of which has been passed through oleum, is mixed with an SO 2 -containing gas (e.g. a sulphur combustion gas; a roasting or cleavage gas) so that the gas which is fed to the first catalyst stage contains from 1À82 to 10% by volume SO 3 and wherein the reacted gases are subjected to an absorption treatment to remove part of the sulphur trioxide at a catalyst stage prior to the last and after a SO 2 conversion of from 80 to 95%. The catalyst stages are subdivided by heat exchangers or equivalent devices and the air is passed through an exhausting tower connected in parallel to an oleum tower to charge it with SO 3 .