Batch crystallizers, applications

Batch crystallizers, applications, 102 Batch preferential crystallization, purity decrease of L-threonine crystals in optical resolution, 251-259 Bayer proce description, 329 Benzene, hi -pressure crystallization from benzene-cydohexane mixture, 281-289... 

Batch Crystallization Batch crystalhzation has been practiced longer than any other form of ciystaUization in both atmospheric tanks, which are either static or agitated, as well as in vacuum or pressure vessels. It is still widely practiced in the pharmaceutical and fine chemical industry or in those applications where the capacity is veiy small. The integrity of the batch with respect to composition and history can be maintained easily and the inventoiy management is more precise than with continuous processes. Batch ciystalhzers can be left unattended (overnight) if necessary and this is an important advantage for many small producers. 

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

Bohlin, M. and Rasmuson, A.C., 1992. Application of controlled cooling and seeding in batch crystallization. Canadian Journal of Chemical Engineering, 70, 120-126. 

Using piecewise constant control profiles and orthogonal collocation on finite elements, this approach was further developed by Renfro (Renfro, 1986 Renfro et al, 1987) to deal with much larger problems. More recent simultaneous applications that involve SQP, orthogonal collocation, and piecewise constant control profiles have been presented by Patwardhan et al (1988) for online control, and by Eaton and Rawlings (1988) for optimization of batch crystallizers. These studies have shown that simultaneous approaches can be applied successfully to small-scale applications with complex constraints. 

Example 16.5. Teflon heat transfer tubes that are thin enough to flex under the influence of circulating liquid cause a continual descaling that maintains good heat transfer consistently, 20-65 Btu/(hr)(sqft)(°F). Circulating types such as Figures (d) and (e) of ten are operated in batch mode, the former under vacuum if needed. High labor costs keep application of batch crystallizers to small or specialty production. 

Jones (1974) used the moment transformation of the population balance model to obtain a lumped parameter system representation of a batch crystallizer. This transformation facilitates the application of the continuous maximum principle to determine the cooling profile that maximizes the terminal size of the seed crystals. It was experimentally demonstrated that this strategy results in terminal seed size larger than that obtained using natural cooling or controlled cooling at constant nucleation rate. This method is limited in the sense that the objective function is restricted to some combination of the CSD moments. In addition, the moment equations do not close for cases in which the growth rate is more than linearly dependent on the crystal size or when fines destruction is... 

In this section, a discussion of model identification for batch crystallizers is given and a model-based control strategy is illustrated that conveniently handles input, output, and final-time constraints and is applicable to cases in which fines destruction is used and the growth rate is size dependent. This control scheme permits flexibility in objective function formulation and allows consideration of objective functions that take into account solid-liquid separation in subsequent processing steps. 

The total suspension density in the batch crystallizer increases rapidly with time according to the biquadratic function, Eq. (10.26). This equation is applicable to cooling crystallization or evaporative crystallization, and illustrates a generalization of more restrictive approaches employed to derive cooling and evaporation profiles in sections 10.5.1. and 10.5.2. 

Fines Destruction. In the operation of industrial crystallizers, one would usually want to avoid the fines (i.e., small crystals) since they may cause difficulties in downstream processing equipment (e.g., filtration) and affect both product quality and process economics. Excessive fines may also require a relatively long batch run time to achieve the desired final size of the product crystals. Karpinski (1981) proposed a controlled dissolution of secondary nuclei in order to improve CSD from fluidized bed crystallizers. Jones et al. (1984) first described the application of fines destruction in batch crystallization of potassium sulfate solutions. Their study demonstrated the experimental feasibility of this technology to dramatically reduce the amount of fines in the final product CSD. Their theoretical predictions, obtained from population balance models, agreed with the experimental results. 

Batch crystallization has several desirable features and advantages in laboratory and industrial applications. Industrial batch crystallizers are commonly used to manufacture a wide variety of crystalline materials with desirable product features and quality. Laboratory batch crystallizers are often used to characterize crystallization kinetics and CSDs and to determine the effects of process conditions on these kinetics and CSDs. 

The population balance concept enables the calculation of CSD to be made from basic kinetic data of crystal growth and nucleation and the development of this has been expounded by Randolph and Larson (1988), as summarized in Chapters 2 and 3. Batch operation is, of course, inherently in the unsteady-state so the dynamic form of the equations must be used. For a well-mixed batch crystallizer in which crystal breakage and agglomeration may be neglected, application of the population balance leads to the partial differential equation (Bransom and Dunning, 1949)... 

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Tailoring of the particle size of the crystals from industrial crystallizers is of significant importance for both product quality and downstream processing performance. The scientific design and operation of industrial crystallizers depends on a combination of thermodynamics - which determines whether crystals will form, particle formation kinetics - which determines how fast particle size distributions develop, and residence time distribution, which determines the capacity of the equipment used. Each of these aspects has been presented in Chapters 2, 3, 5 and 6. This chapter will show how they can be combined for application to the design and performance prediction of both batch and continuous crystallization. 

Al-Rashed, M.H. and Jones, A.G., Hannan, M. and Price, C., 1996. CFD Application on a simple geometry of batch precipitation of calcium carbonate. In Industrial Crystallization 96. Ed. B. Biscans, Progep, Toulouse, 16-19 September 1996, pp. 419M24. 

Franck, R., David, R., Villenuaux, J. and Klein, J.P., 1988. Crystallization and precipitation engineering - II. A chemical reaction engineering approach to salicylic acid precipitation Modelling of batch kinetics and application to continuous operation. Chemical Engineering Science, 43, 69-11. 

Another area of TEM application to energetic materials is the work of S.M. Kaye at PicArsn on expls and propints. He used TEM. to establish a procedure for detg the particle size distribution of LA batches of different crystal habits from various manufacturers (Ref 25. ... 

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