Slow Crystal Growth Rate

This technique has a high operating safety due to the simple assembly (no moving parts) and no additional solid/liquid separation. However, the space-time yield of this technique is relatively small. The reason is that the heat and mass transfer are forced just by conduction and diffusion, respectively, and are supported only by natural convection. Hence, only very slow crystal growth rates lead to high purities. 

Diffusive crystal growth at a fixed temperature would not result in a constant crystal growth rate (see below). However, under some specific conditions, such as continuous slow cooling, or in the presence of convection with diffusion across the boundary layer, time-independent growth rate may be achieved. Similarly, time-independent dissolution rate may also be achieved. 

Reaction Time. The rate of formation of to at reflux temperature is illustrated in Figure 2. The rate was estimated by taking samples from the reaction mixture at various times and examining them by x-ray diffraction. The crystallization is characterized by an unusually long induction period of about 13 hr, which is followed by a relatively slow crystal growth. No alteration of the zeolite was observed during an extra 50 hr reaction time after crystallization had been completed. 

The general recommendation is to keep supersaturation levels in the low to medium range to prevent additional nucleation events. This strategy has an additional beneficial effect on purity owing to slow crystal growth, the purity of the final crystals is usually higher compared to that in the case of fast growth rates. Finally, the addition of seeds efficiently avoids the formation of amorphous material, which can later recrystallize in an uncontrollable fashion. 

Eutectic forming mixtures provide a theoretical opportunity to achieve one component by crystallization that has 100% purity. This is of course only possible if, on the one hand, the crystal growth rates are so extremely slow that perfect crystals are created and, on the other hand, the necessary solid-liquid separation after the growth process is also perfect. 

The crystallization rate of PHBV compared with most polymers used in industrial processes is very slow, due to the low nucleation that is a result of its high purity and stereoregularity and the slow nuclear growth rate, resulting from the HV units inclusion in the PHB crystalline cell (Reinsch and Kelley 1996). 

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