Batch crystallization external seeding

There are two basic practical approaches that can help control CSD in batch cooling crystallization. The first is concerned with the use of seed crystals (external seeding), and the second is concerned with limiting the occurrence of spontaneous nucleation only to the initial stage of the process. Both require the control of the cooling rate via the control of the solution temperature in the crystallizer and the manipulations of the flow rate of cooling agent. 

The initial population density n L, 0) for a batch crystallizer is not well defined. For a crystallizer seeded externally, n(L, 0) may be denoted by an initial seed distribution function hs L). However, in an unseeded system, initial nucleation can occur by several mechanisms, and one cannot realistically use a zero initial condition for the size distribution. To overcome this difficulty, Baliga (1970) suggested the use of the size distribution of crystals in suspension at the time of the first appearance of crystals as the initial population density. 

As discussed earlier, the analytical solutions for the CSD for a batch or semibatch crystallizer are difficult to obtain unless both the initial condition for the CSD and appropriate kinetic models for nucleation and growth are known. An example of such an analytical solution—simple yet not overly restrictive—was given by Nyvlt (1991). It is assumed that the process, in which both external seeding and nucleation take place, occurs at constant supersaturation (G = constant, Bq = constant) in an ideally mixed crystallizer. An additional assumption of size-independent growth allows one to rewrite the time-dependent moments, Eqs. (10.12)-(I0.15), in terms of the physical properties such as the total number (A), length (L), surface area A), and mass of... 

The quality, productivity, and batch-to-batch consistency of the final crystal product can be affected by the conditions of the batch crystallizer. Several factors considered here include batch cycle time, supersaturation profile, external seeding, fouling control, CSD control, growth rate dispersions, and mixing. 

As mentioned before, in an ideal case, the supersaturation level during the externally seeded batch growth process stays below the critical (maximum) supersaturation so that the number of uniformly sized seeds introduced at the onset of the batch crystallization remains unchanged throughout the growth. As a result, a fines-free product of uniform size can be obtained. 

Larson (1978) considered the operation of a batch, externally seeded evaporative crystallizer with a constant growth rate, (J, in which no spontaneous nucleation was allowed. Applying the population balance Eq. (10.1) to this system gave the following equations for the solvent profile... 

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