Unseeded cooling crystallization

An experiment was performed to examine the supposition that increases in supersaturation lead to greater impurity content in crystals. In a batch unseeded cooling crystallizer supersaturation is expected to be high at the point of nucleation, diminish rapidly after nucleation and then approach zero as the batch is... 

Unseeded Cooling Crystallization. In a similar fashion, a cooling profile can be derived for the unseeded case in which spontaneous nucleation and growth are allowed to occur at constant rates. The actual solutions to the resultant third-order differential equations found in the literature differ due to the different sets of the four initial conditions used by various authors (Karpinski et al. 1980b Nyvlt 1991 Randolph and Larson 1988). Understandably, all of them result in a cooling profile of the general form... 

Figure 10.18 Cooling profiles for crystallization of MgSO4 -7H20 in a 21-L batch cooling crystallizer, as reported by Karpinski et al. (1980b). Saturation temperature T = 35 °C. Seeded crystallization (1) Cooling profile calculated from Eq. (10.54), mean seed size i, = 0.4075 mm. Unseeded crystallization (2) Cooling profile calculated from Eq. (10.56), (3) Linear cooling profile, (4) Natural cooling profile.

Based on the two cases discussed so far, one can consider yet another case of interest, in which nucleation is allowed to proceed at a constant rate from t = 0 until a certain time /], after which no new nuclei are generated. For sueh a cooling crystallization, a biquadratic cooling profile applies until t = t, and Eq. (10.54) for the time t> t. This is the preferred method of praetical unseeded batch cooling crystallization. 

Batch Crystallization. Crystal size distributions obtained from batch crystallizers are affected by the mode used to generate supersaturation and the rate at which supersaturation is generated. For example, in a cooling mode there are several avenues that can be followed in reducing the temperature of the batch system, and the same can be said for the generation of supersaturation by evaporation or by addition of a nonsolvent or precipitant. The complexity of a batch operation can be ihustrated by considering the summaries of seeded and unseeded operations shown in Figure 19. 

Carefully selected seed crystals are sometimes added to a crystalliser to control the final product crystal size. The rapid cooling of an unseeded solution is shown in Figure 15.20a in which the solution cools at constant concentration until the limit of the metastable zone is reached, where nucleation occurs. The temperature increases slightly due to the release of latent heat of crystallisation, but on cooling more nucleation occurs. The temperature and concentration subsequently fall and, in such a process, nucleation and growth cannot... 

Figure 2. Diagram of furnace used for unseeded crystal growth by slow cooling of high temperature solutions. (From Ref. 5.)...

Nucleation temperatures in the presence of crystalline materials can be determined by a procedure similar to that for the measurement of unseeded data by introducing two small crystals ( 2mm in size) into the flask when the solution has cooled to its predetermined saturation temperature. 

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