Patent GB-433039-A (GB)
Title: Improvements in or relating to the wet purification of gases
IPC: B01D53/50
Filed: 1933-12-28
Abstract: <PICT:0433039/III/1> In the purification of gases from oxides of sulphur and any smoke or ash present by scrubbing the gases with a recirculated aqueous washing media which is continuously replenished with lime, chalk or magnesium limestone, and removing the insoluble solids collecting in the system by purging off a part only of the recirculating liquor without settling the bulk of the liquor, the liquor after leaving the scrubber is treated so as to remove practically the whole of the supersaturation of the liquor due to calcium sulphate and sulphite before recirculation, by delaying the return of the liquor to the scrubber for a definite minimum time sufficient to enable desupersaturation to be completed. Scaling of the scrubber surfaces is thereby prevented.  The delay time is calculated for calcium sulphate, from the normal time taken for the main body of liquor leaving the bottom scrubbing elements to return to the top scrubbing elements, and for calcium sulphite, from the time taken for the main body of liquor to pass from the point of lime &c. addition to the top scrubbing elements. The delay time varies with the extent of supersaturation to be removed, inversely with the concentration of the suspended seeding crystals, inversely with the concentration of non-isomorphic material in suspension, inversely with the total ionic strength of the solution, and inversely with the temperature.  The delay times necessary under certain conditions are given.  The delay may be effected by providing a delay tank in the liquor recirculating system external to the scrubber.  This tank may take the form of a U-tube, a cylindrical tank having means for admitting liquor tangentially at the top and for withdrawing it centrally at the bottom, a tall wide tank, or a rectangular tank fitted with baffles.  The tank may have means for withdrawing liquor high in suspended solids from its base for return to the top. Preferably, the lime &c. is added to the liquor immediately it leaves the scrubber.  If however, owing to the treatment of gases having a high sulphur content, a separate oxidizer is employed, the lime must be added after the oxidizer and the increased capacity may be so arranged that part of the delay occurs between the scrubber exit and the oxidizer and the remainder after the oxidizer.  The recirculated liquor preferably contains at least 3 per cent each of suspended calcium sulphite and sulphate.  The concentration of suspended solids in the delay tank may be maintained greater than that in the circulating system as a whole.  Flue gas enters a scrubber C at A and leaves at B.  Liquor leaving the scrubber passes into a delay tank E and is returned to the system by a pump F.  Accumulated solids are removed by removing a portion of the liquor via P to a settler S. A part of the clear liquor passes from the settler to a tank T. The level in the tank T is maintained constant by the addition of make-up water through a float-operated valve N.  The rate of purge is controlled by varying the supply of returned liquor by the valve M. Part of the clear liquor passes through a line G to a lime slurry preparation tank H, the slurry being added to the circulating liquor at L.  The sludge is dewatered in a rotary vacuum filter R, the clear liquor collecting in the tank T. Specification 424,414, [Group XII], is referred to in the first Provisional Specification and Specification 358,487, [Group XII], in the third Provisional Specification  Reference has been directed by the Comptroller to Specification 420,539.