Batch crystallization supersaturation cooling curve

In conventional batch-cooling-crystallization a saturated solution is cooled from an initial temperature at which the solute has a high solubility to a final lower operating temperature (with lower solubility) along an optimal cooling curve. This cooling is used to maintain a reasonably constant level of supersaturation, and a constant crystal growth-rate. 

An early attempt to establish an optimum cooling curve for batch cooling crystallization (Mullin and Nyvlt, 1971) considered a controlled operation at constant supersaturation, and used a calculation method based on a supersaturation balance in which nuclei were generated in a sequence of discrete time steps. 

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. 

In the direct design approach, a desired supersaturation profile that falls between the solubility curve and the metastable limit of the system is followed based on feedback control of the concentration measurement. This is in contrast to the traditional first-principles approach, where a desired temperature profile or antisolvent addition rate profile is followed over time such as shown in Fig. 14. For a cooling crystallization, the direct design approach follows a setpoint profile that is solution concentration vs. temperature (or solvent-antisolvent ratio) as opposed to temperature (or addition rate) vs. time. Because the desired crystallizer temperature is determined from an in-situ solution concentration measurement, the batch time is not fixed. 

The supersaturation before the addition of seeds should be adjusted according to the solubility curve and the supersolubility curve (cf. Figure 10.2). Typically, seeding at 4—5 K below saturation temperature is fine. Of course, the metastable zone width has to be considered here and the seeding point should be closer to the solubility curve than to the supersolubility curve. It should be kept in mind that the metastable zone width is not thermodynamically determined, but strongly depends on plant properties and process parameters, such as cooling rate. If the metastable zone width is very narrow, for the sake of process robustness temperature control has to be improved or even an inline measurement of the supersaturation (e.g., by NIR) may have to be used to detect supersaturation close to the solubility curve and to avoid spontaneous nudeation or unwanted dissolving of the seed crystals. In such cases, special care has to be taken that no crystals are present in the crystallizer from the previous batch. 

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