Fluidizedbed agitation
As described above, the Oslo-Krystal unit is a fluidized-bed agitated crystal-lizer in which the gentle action minimizes secondary nucleation and allows large crystals to grow. Oslo-Krystal vacuum crystallizers can be of the 'open' (Figure 8.51) or 'closed' (Figure 8.45) types. In the former the crystallization zone is at atmospheric pressure. In the latter all parts of the equipment are under reduced pressure.
Vapour
Figure 8.50. Escher-Wyss Tsukishima double propeller (DP) crystallizer
M Clear
^ liquor
Baffle Draft tube
Elutriation leg
Product crystals
Figure 8.50. Escher-Wyss Tsukishima double propeller (DP) crystallizer p—Vapour p—Vapour
Vaporiser
Barometric leg
Fine salt separator
Vaporiser
Barometric leg
Fine salt separator
r^Vapour
Solution manifold r^Vapour
^Overflow
■Butterfly valve
Circulating pump
Suspension container
Steam
Steam
^Overflow
■Butterfly valve
Circulating pump
Suspension container
Figure 8.51. An Oslo-Krystal vacuum crystallizer showing two different methods of operation: (a) classified suspension, (b) mixed suspension. (After Saeman, 1956)
Two different methods of operation are shown in Figure 8.51 with (a) a classified suspension (circulating liquor) and (b) a mixed suspension (circulating magma). Classified operation, while capable of producing large regular crystals, limits productivity because both the liquor velocity and the mass of crystals in suspension have to be restricted to keep the fines level below the pump inlet. Modification to magma circulation can improve the productivity considerably, because higher circulation rates and magma densities can be employed. Furthermore, the suspension volume is increased because magma circulates through the vaporizer and downcomer. In this type of operation, however, the bulk classifying action is lost, and it is necessary to provide a secondary elutriation zone in the suspension to permit segregation and removal of excess nuclei. Fines can be redissolved with live steam and the resulting solution fed to the vaporizer.
One of the major factors in the successful operation of any controlled suspension crystallizer is the incorporation of a suitable fines trap. The earlier the excess nuclei are collected and destroyed the more efficient will be the process. In practice, fines are most economically removed when they reach 5 to 10% of the average product size (Saeman, 1956).
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