Usually, a crystallization system is not complete at the crystallization
step alone. The suspension in the crystallizer still has to be separated, the
crystals have to be dried and packed. Vapors leaving the crystallizer need to
be condensed, and the non-condensable gases need to be removed by vacuum pumps.
Fig. 8 shows a simplified flow sheet of such a complete, vacuum evaporation
crystallization plant.
Figure 8 Simplified flowsheet
In the illustration of Fig (8), one might envision any type of crystallizer
both as a single unit or a multiple-effect. Heating methods alternative to live
steam, such as mechanical or thermal vapor recompression can be utilized. In
this example the vapors from the (last) crystallizer are condensed in a surface
condenser. A direct-contact (barometric) con-denser could be used instead. The
suspension (in the example) is not thick enough to feed the centrifuge:
typically, FC units operate at 15 to 25%wt crystals, whereas centrifuges
require feed suspension in the 40-60%wt range. Therefore, the suspension is
pre-concentrated in a gravity thickener (shown) or with hydrocyclones. The
clear liquor over-flow is recycled. Purge liquor to remove system impurities
may be discharged from the system at this point. The thickener underflow is fed
to the centrifuge. Depending on the CSD there is the choice among 4-5 types of
separation devices: a filter for very small par-ticles, the decanter or peeler
centrifuges for the small crystals, and the sieve-worm or the pusher centrifuge
for larger crystals. The final drying of the product in most of the cases is
achieved in rotary or vibrating fluidized-bed dryers. Cooling of the dry
crystals can be done in separate cooler units or in an integral part of the
dryer.
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