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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