Patent GB-1470529-A (GB)
Title: Process for producing sulphur from sulphur oxide
IPC: B01D53/86
Filed: 1974-05-10
Abstract: 1470529 Sulphur oxide reduction EXXON RESEARCH & ENG CO 10 May 1974 [29 May 1973 22 April 1974] 20749/74 Heading C1A A gaseous sulphur oxide is reducable to elemental sulphur and/or hydrogen sulphide by contacting with gaseous ammonia in a high temperature reaction zone. The reaction can be effected thermally or catalytically at temperatures in the range 500-3000 F. In a preferred embodiment ammonia is used as an absorbent for the sulphur oxide and ammonium salts formed, e.g. sulphite, bisulphite, sulphate or bisulphate, and the salts decomposed to gaseous ammonia and sulphur oxide which are reacted to yield sulphur and/or hydrogen sulphide. In an example of the preferred embodiment (illustrated by the Figure) a flue gas stream 10, containing sulphur oxides (SO 2  and SO 3 ) at a temperature of approximately 350 F. is passed through a fly ash removal and quench unit 11. The quenched flue gas stream 13 is passed to scrubber 14. The flue gas is first countercurrently contacted with an aqueous ammonia solution in SO x  removal section 16 and then with water, admitted via inlet 17, in ammonia removal section 18. The aqueous medium admitted via line 15 is preferably a solution but may be a slurry containing dissolved as well as undissolved ammonium salts. Gaseous ammonia may also be injected via line 15. Scrubbed flue gas leaves the top of scrubber 14 via overhead line 19, and may then be reheated in a conventional reheater 20 and discharged to the atmosphere through stack 21. This flue gas contains only a small fraction, e.g. 10% or less, of its original SO x  content, and will ordinarily also have an NO x  content lower than that of the entering flue gas. The overhead gas stream 19 is essentially free of SO 3  and ammonia. The aqueous scrubber effluent containing ammonium sulphite, bisulphate and optionally the sulphate is separated from the flue gas stream and removed via line 22; this aqueous scrubber effluent medium is fed to the heated kiln 23. The ammonium salts are decomposed in the latter portion of kiln 23 at temperatures ranging from 225 to 800 F., preferably 250-700 F., to yield a vapour mixture containing sulphur dioxide, ammonia, and water vapour. Sulphur trioxide may also be present in small amounts due to the presence of ammonium sulphate. Ammonium sulphite and ammonium sulphate are essentially decomposed completely into gaseous products, i.e., ammonia, the corresponding sulphur oxide, and water vapour. The gaseous products are removed from kiln 23 via line 25, preheated in preheater 26 to a temperature of approximately 1200 F., and then passed into the high temperature reactor 27. Preferably the reactor contains a catalyst, e.g. a transition metal or oxide, activated bauxite, activated carbon or silica based catalyst. An oxidationreduction reaction will take place in the reactor with the formation of a reaction product mixture containing elemental sulphur, hydrogen sulphide, unreacted sulphur oxide, nitrogen, and water vapour. This reaction is exothermic. The reaction product mixture may then be cooled in cooler 28 to a suitable Claus reactor inlet temperature, e.g. 500-600 F., and then may be passed through one or more Claus reactors 29 containing a suitable catalyst such as activated bauxite. Hydrogen sulphide and sulphur dioxide react in a 2/1 mol ratio in the Claus converters 29 to form further quantitites of elemental sulphur. The Claus reactor effluent stream 30 is cooled in cooler 31 and passed to separator drum 32. Liquid elemental sulphur 33 and the Claus unit tail gas stream 34 are taken off separately from drum 32. This tail gas stream 34 may be returned to the scrubber 14 if desired in order to scrub out SO x  as well as H 2 S prior to discharging into the atmosphere.