GB761877A - Improvements in or relating to crystallizing processes and apparatus - Google Patents

Abstract

<PICT:0761877/III/1> In the production of ammonium sulphate by the absorption of ammonia-containing gas in a dilute sulphuric acid solution saturated with ammonium sulphate so as to produce a supersaturated solution in the metastable field (the concentration being below that at which new crystals are formed in the presence of seed crystals), passing the supersaturated solution upward through a crystallization stage, and returning the desupersaturated solution to the absorption stage, the crystallization stage consists of a lower high velocity zone and an upper low velocity zone, the velocity in the low velocity zone being 5-15 in./min. and 6-7 times smaller than that in the high velocity zone, most of the solution returned to the absorption stage being withdrawn from the lower zone, the remainder being withdrawn from the upper zone, and thereafter admixed with make up acid and heated directly with steam before return to the absorption stage. As shown solution passes from the absorption vessel 120 via pipe 150 provided with deflectors 190 to crystallization bowl 100 wherein crystals are withdrawn at suction head 700. The bulk of the solution is withdrawn through pipe 610, provided with throttling valve 612, and returned to the absorption vessel via pump P1, pipe 630 provided with throttling valve 632, and a ring of sprays 450, while the remainder is withdrawn through pipe 750, is admixed with make-up acid in pipe 754, is injected tangentially into a direct steam heater 756 and is then returned to the absorption vessel via pipe 758, valve 786, pump P1 of pipe 630 and sprays 450. Direct steam heater 756 may be replaced by a combined direct and indirect heater (Fig. 3, not shown). A portion of the solution recycled from the crystallization stage may be returned direct to the solution in vessel 120 via pipe 640, valve 650 of indirect steam heater 788. An overflow pipe 540 recycles solution in the absorption vessel via a tar skimmer, a mother liquor storage tank, pump P2 and spray 520. Instead of withdrawing solution from the absorption vessel by means of overflow 540 a third stream of solution may be withdrawn from the crystallization bowl (Fig. 4, not shown) and passed through the tar skimmer &c. The ammonia containing gas enters the absorption vessel at inlet 260, passes through an annular space, around an inner liquid separation cylinder 200, wherein it is contacted with solution from sprays 450, passes up a vertical passage 280 wherein it is contacted with solution from spray 520 and then passes tangentially into the upper part of cylinder 200 wherein entrained liquid is separated, whereafter it passes out of outlet pipe 230.