GB436369A - Improvements in process and apparatus for the recovery of acid gases - Google Patents

Abstract

<PICT:0436369/III/1> Weakly acidic gases such as CO2, SO2, H2S, and HCN are removed and recovered from gas mixtures by treating with ammonia in the gaseous phase, dissolving the ammonia, acid gas, and ammonium salts formed in water, neutralizing the solution with a solution of NaH2PO4 or other salt of a comparatively strong acid still having more than one replaceable hydrogen atom, thereby expelling the acid gas and forming, for example, Na(NH4)HPO4 which is heated to regenerate NaH2PO4, ammonia, and residual acid gas. The two gases together with water vapour at least equal in volume to the ammonia are returned to the acid gas absorber, while the regenerated NaH2PO4 solution is concentrated, the cooled solution being recycled, and the water driven off being condensed and returned to the absorber. Waste gases from a steam boiler plant are taken through a pipe 1 to a heat exchanger 2, where they are cooled to about 90 DEG F., through SO2, soot, and dirt removers to the inlet 4 of a packed tower 6, the base 5 of which constitutes a reservoir. A stream of ammonia and water vapour enters at 17 forming ammonium bicarbonate and, preferably, some carbonate which are dissolved, together with traces of uncombined ammonia and carbon dioxide, in water descending from sprays 18. The solution in the space 5 may be recirculated to sprays 6<c>. It is then pumped via pipe 20 to a neutralizer 21 comprising a packed tower over the lower two-thirds of which the solution flows from sprays 29, and meets an upwardly sprayed, preferably 30 per cent, solution of NaH2PO4 from sprays 25 and 26. The evolved CO2 is cooled by the endothermacy of the reaction and passes over the dry grids 27 to a chamber 28 leaving at atmospheric temperature by pipe 30. The solution obtained is pumped from the tank 22 through pipe 32 to the heat exchanger 2. After further heating in exchangers 34 and 37 to just above the dissociation point, the liquor passes to a regenerator 42. This comprises a series of rings of the bubbler cap type having heating tubes 44 immersed in the liquor whereby the temperature is raised to at least 210 DEG F. and dissociation completed. The ammonia is passed by pipe 17 to the tower 6. The liquor passes by an overflow 51 to a water still 43, the upper part of which is heated by pipes 55 in series with tubes 44, and the lower by live steam pipes 55<a>. The solution is here concentrated to 30 per cent and is led by pipe 58 to the exchanger 34, through pipe 60 to coolers 61, and thence to a reservoir 64 for the neutralizer 21. The steam from the still 43 is compressed by a turbine 68 and passes through the tubes 55 and 44 where it condenses, the water passing to a well 70 whence it is pumped to the exchanger 37, through cooling coils 74, and finally to the sprays 18 of the tower 6. U.S.A. Specification 1,916,980 is referred to.