Here, r denotes the specific density of the suspension or solution, g the acceleration due to gravity, H the delivery head, h the efficiency, and Vcryst the filled volume of the crystallizer. In designing the circulation pump, the impeller diameter D and the number of revolutions n of the impeller per unit time can be varied at constant power N:
As a result, the tip speed and, hence, the secondary nucleation rate B° can be altered at a constant rate of dissipated energy e. Industrial-scale experience indicates that:
If crystallization is used as a simple precipitation process only, the production process may require only crystals that can be separated easily. Such crystal size can average about 0.2 mm, and this allows impeller pumps that operate at higher speeds. This is applied to FC-type crystallizers (fig. 4, left).
For coarser crystals the need for reduced energy input is achieved in the so-called draft-tube-baffled (DTB) crystallizers (fig. 4, centre), which are equipped with an impeller pump located within a draft tube. These are circulators with large diameters which operate at low tip speeds that lead to low secondary nucleation. These impellers have diameters up to more than 1 m, resulting in far lower tip speed than conventional pumps located in an external circulation loop. Additionally, in the case of evaporative crystallization, the DTB has a further advantage over the FC type. In the DTB crystallizers the heat exchanger which is responsible for most of the pressure loss (and thus the higher energy input) is not part of the slurry circulation. Thus, the central impeller pump consumes only about one third of energy than an equivalent FC unit, and thus provides improved crystal growth environment. The heat exchanger is installed in a second circuit operated only with clear liquor. The operation with clear liquor has an additional beneficial effect on crystal size. The DTB has an integral settling section (the Baffle), from which clear liquor can be with-drawn for the heater circuit. The clarification is such that the withdrawn liquid still contains the smaller crystal particle fractions. These fines are typically destroyed in the heater cir-cuit (heating increases the saturation point of the liquid), and this fines destruction reduces the number of small crystals in the crystallizer. In DTB type crystallizers, therefore, average crystal sizes between 0.5 up to 1.5 mm can be achieved.
For production of very coarse crystals the circulation of the suspension is avoided alto-gether (to minimize the crystal attrition due to the pump impeller). In fluidized-bed crystal-lizers only clear liquor is circulated (fig. 4, right) and therefore the secondary nucleation rate is drastically reduced. Such crystallizers can produce crystals of 5-6 mm.
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