Generation of Supersaturation in Batch Crystallizations

In the majority of batch crystallizations, three ways to generate supersaturation are used. These will be discussed in order of their usage in some detail below. 

Cooling crystallization is the preferred option for batch crystallizations as the temperature profile in a reactor can be easily controlled giving perfect control of the supersaturation profile. This curve gives information at which temperature the process has to be started in order to have a reasonable solute/solvent ratio and avoid unnecessary high dilution on the other hand, the solubility at low temperatures fixes the yield that can be achieved with this process. 

Typical cooling rates for organic moieties are on the order of 0.1-0.2 K/min. A rule of thumb classifies the cooling regimes as summarized in Table 10.2. 

The only tricky point to keep in mind is the temperature at the inner crystallizer wall. Depending on the cooling rate, this temperature is somewhere between internal temperature and jacket temperature. If the jacket is too cold in order to reach high cooling rates, local supersaturation at the wall can become so high that nucleation occurs on the wall and thus the formation of crusts or in the solution dose to the wall, which can broaden the particle size distribution obtained. 

The maximum yield Yof a cooling crystallization is given by the starting concentration Co and the equilibrium solubility at the final temperature, (Equation 10.1)  

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