Batch crystallization supersaturation

It was shown in Chapter 3 that supersaturation, or concentration driving force, is essential for any crystallization. In a batch crystallizer supersaturation can be generated in several ways, either solely or in combination ... 

In a batch crystallizer supersaturation can be achieved by cooling, evaporation of solvent, or both combined if Ac /AT) > 0. If orrly one component crystalUzes, the mass balance for this component is (suspension derrsity nij = p - [Pg.436]

Crystallization batches range from 30,000 to 60,000 Hters for each pan. Continuous centrifugals are typically used for second, third, and affination steps continuous vacuum pans are less common but are used in the U.S. for intermediate strikes. Most horizontal batch crystallizers have been replaced by continuous units, and all are designed for controlled cooling of the massecuite to maintain supersaturation. 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

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. 

In all such laboratory studies, plant conditions and compositions should be employed as far as possible. Agglomeration rates tend to increase with the level of supersaturation, suspension density and particle size (each of which will, of course, be related but the effects may exhibit maxima). Thus, agglomeration may often be reduced by operation at low levels of supersaturation e.g. by controlled operation of a batch crystallization or precipitation, and the prudent use of seeding. Agglomeration is generally more predominant in precipitation in which supersaturation levels are often very high rather than in crystallization in which the supersaturation levels are comparatively low. 

A theoretical analysis of an idealized seeded batch crystallization by McCabe (1929a) lead to what is now known as the AL law . The analysis was based on the following assumptions (a) all crystals have the same shape (b) they grown invariantly, i.e. the growth rate is independent of crystal size (c) supersaturation is constant throughout the crystallizer (d) no nucleation occurs (e) no size classification occurs and (f) the relative velocity between crystals and liquor remains constant. 

Although cooling crystallization is the most common method of inducing supersaturation in batch crystallization processes, other methods can be used, as discussed in Chapter 10. For example, evaporation can be used, in which case the profile of the rate of evaporation through the batch can also be optimized7. Indeed, the profiles of both temperature and rate of evaporation can be controlled simultaneously to obtain greater control over the level of supersaturation as the batch proceeds7. However, it should be noted that there is often reluctance to use evaporation in the production of fine, specialty and pharmaceutical products, as evaporation can concentrate any impurities and increase the level of contamination of the final product. 

Tavare and Garside ( ) developed a method to employ the time evolution of the CSD in a seeded isothermal batch crystallizer to estimate both growth and nucleation kinetics. In this method, a distinction is made between the seed (S) crystals and those which have nucleated (N crystals). The moment transformation of the population balance model is used to represent the N crystals. A supersaturation balance is written in terms of both the N and S crystals. Experimental size distribution data is used along with a parameter estimation technique to obtain the kinetic constants. The parameter estimation involves a Laplace transform of the experimentally determined size distribution data followed a linear least square analysis. Depending on the form of the nucleation equation employed four, six or eight parameters will be estimated. A nonlinear method of parameter estimation employing desupersaturation curve data has been developed by Witkowki et al (S5). 

Phenomena, methods of operation, etc. have been studied extensively for the use of crystallization in separation processes. Although much remains to be learned about such processes, relatively little attention has been given to the other functions and the purpose of this work was to examine the role of various process variables in determining the purity of crystals recovered from a batch crystallizer. The system studied experimentally was a model system of amino acids, and the key variables were the composition of the liquor from which a key amino acid was crystallized, the rate at which supersaturation was generated by addition of an acid solution to reduce solubility, and the degree of mixing within the batch unit. 

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... 

The key factors controlling the purity of L-Ile recovered from batch crystallizers are shown in the study to be the composition of the solution from which the crystals are recovered, agitation, and the rate at which supersaturation is generated. Also, the molecular form of the recovered amino acid determines which of the impurities investigated, L-Leu and L-Val, is the more plentiful impurity in the recovered crystals. 

In this section, a brief description of the necessary experiments to identify the kinetic parameters of a seeded naphthalene-toluene batch crystallization system is presented. Details about the experimental apparatus and procedure are given by Witkowski (12). Operating conditions are selected so that the supersaturation level is kept within the metastable region to prevent homogeneous nucleation. To enhance the probability of secondary nucleation, sieved naphthalene seed particles are introduced into the system at time zero. 

Batch crystallization studies of D-fructose from aqueous ethanolic solutions demonstrate that crystal growth rate is dependent on supersaturation (possibly to the 1.25 power), ethanol content and temperature. It appears that solution viscosity also has an effect. Growth rates of up to 1 pm/mln were measured. 

The Investigation was carried out using a seeded, batch crystallization In the absence of nucleatlon. Supersaturated solutions were prepared, seeded and maintained at a constant temperature while crystallization proceeded. Samples were taken periodically to give a solution for analysis and crystals for size analysis and crystal content determination. 

Nucleation rates were obtained for varying seed densities, seed sizes, temperature and supersaturation in the batch crystallizer. 

For any fixed batch crystallization temperature, the effective nucleation rate passes through a maximum even at high seed densities. It is suggested that the induction period r uired to activate the seed surfaces may be responsible for the lower initial nucleation rate observed when the supersaturation was higher. It is also suggested that agglomeration may have caused the observed phenomenon. 

The formation of small particles is favoured when solids formation is maintained via primary nucleation throughout the batch crystallization. A widely used relationship between the nucleation rate and supersaturation is usually given as ... 

The above equations can be applied to any batch crystallization process, regardless of the mode by which supersaturation is generated. For example, suppose a model is needed to guide the operation of a seeded batch crystallizer so that solvent is evaporated at a rate that gives... 

It is clear that stringent control of batch crystallizers is critical to obtaining a desired crystal size distribution. It is also obvious that the development of a strategy for generating supersaturation can be aided by the types of modeling illustrated above. However, the initial conditions in the models were based on properties of seed crystals added to the crystallizer. In operations without seeding, initial conditions are determined from a model of primary nucleation. 

Control of a batch crystallizer is almost always the most difficult part and very often is not practiced except to permit homogeneous nucleation to take place when the system becomes supersaturated. If control is practiced, it is necessary to have some means for determining when the initial solution is supersaturated so that seed of the appropriate size, quantity, and habit may be introduced into the batch. Following seeding, it is necessary to limit the cooling or evaporation in... 

Batch crystallization process control using the first-principles and direct design approaches were discussed. The first-principles approach utilizes crystallization process models, which require the associated determination of crystallization kinetics. The optimal seed characteristics and/or supersaturation profile to obtain the desired product characteristics are then computed. The direct design approach involves feedback control of a state measurement, in this case the solution... 

Gron, H. Borissova, A. Roberts, K.J. In-process ATR-FTIR spectroscopy for closed-loop supersaturation control of a batch crystallization producing monosodium glutamate crystals of defined size. Ind. Eng. Chem. Res. 2003, 42 (1), 198-206. 

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