Continuous crystallizers processes

It should therefore not be surprising that for relatively small-scale operations involving solids handling within the fine and intermediate chemicals industry, batch operation is preferred. Similarly, continuous processes that involve precipitation or crystallization, a common unit operation in fine chemicals, are rare. Small-scale examples are known, for instance, a continuous crystallization process was used by Bristol-Myres Squibb in order to improve dissolution rates and bioavailability of the product [12]. The above does indicate that not all process or parts thereof are suited for conversion from B2C, given the current technology. 

Both experimental and theoretical work has demonstrated that growth rate dispersion exists, and has a measurable effect on the CSD in both batch and continuous crystallization processes. Further understanding of this phenomenon on a fundamental level will be required to develop methods to make use of or control growth rate dispersion and make it a tool in control of particle size and shape. 

The observed transients of the crystal size distribution (CSD) of industrial crystallizers are either caused by process disturbances or by instabilities in the crystallization process itself (1 ). Due to the introduction of an on-line CSD measurement technique (2), the control of CSD s in crystallization processes comes into sight. Another requirement to reach this goal is a dynamic model for the CSD in Industrial crystallizers. The dynamic model for a continuous crystallization process consists of a nonlinear partial difference equation coupled to one or two ordinary differential equations (2..iU and is completed by a set of algebraic relations for the growth and nucleatlon kinetics. The kinetic relations are empirical and contain a number of parameters which have to be estimated from the experimental data. Simulation of the experimental data in combination with a nonlinear parameter estimation is a powerful 1 technique to determine the kinetic parameters from the experimental... 

A continuous crystallization process ultimately reaches a steady state, in which the rates of nucleation and growth are constant with time. For a given set of operating conditions, crystal size distribution depends considerably upon the degree to which product classification is practiced. Figures 23 and 24 illustrate schematically the possible extremes between... 

Synthetic zeolites are used in a variety of catalyst and adsorbent applications. Most zeolites are synthesized in batch processes, but U.S. 6,773,694 (to UOP) describes a continuous crystallization process for zeolite formation. The resulting crystals can be dried and formulated into catalysts, adsorbents, and other products. Estimate the costs of producing zeolite X and Mordenite by this method. 

Although methods to avoid this could have been implemented, this option was not pursued because of development time pressures. It is discussed here as an illustration of the key importance of surface area. This factor in a continuous crystallization process is also discussed in Example 11-6. 

The analysis of batch crystallization processes is generally more difficult than that of continuous crystallization processes. This is mainly due to the complexity of problems encountered in the batch systems the mass and surface area of the crystals increase during the run, and the supersaturation varies in a complex way as a function of time. Thus, in the development of a descriptive model, one needs to consider the time-dependent batch conservation... 

Generally speaking, the median crystal size of a crystalline product can be increased through a decrease in the number of nuclei or an increase in the growth period of the crystals present in a crystallizer operated at an optimum level of supersaturation. Of these, the most practical way to alter the particle size distribution in a continuous crystallization process is to adjust the residence time of the crystals. 

Adachi, T. Ohnuki, M. Yoshida, N. Sonobe, T. Kawamura, W. Takeishi, H. Gunji, K. Kimura, T. Suzuki, T. Nakahara, Y. Muromura, T. Kobayashi, Y. Okashita, H. Yamamoto, T. (1990). Dissolution Study of Spend PWR Fuel Dissolution Behavior and Chemical Properties of Insoluble Residues. Journal of Nuclear Materials, Vol. 174, No. 1, (November 1990), pp. 60-71, ISSN 0022-3115 Anderson, H. H. (1949). Alkali Plutonium(IV) Nitrates, In The Transuranium Elements, National Nuclear Energy Series IV, Vol. 14B, G. T. Seaborg, J. J. Katz, W. M. Manning, (Eds.), pp. 964-967, McGraw-Hill Book Co. Inc., New York, USA Ebert, K Henrich, E. Stahl, R. Bauder, U. (1989). A Continuous Crystallization Process for Uranium and Plutonium Refinement, Proceedings of 2nd International Conference on Separation Science Technology, pp. 346-352, Paper No. S5b, Hamilton, Ontario, Canada, October 1-4,1989... 

For the operation of agitators in a continuous crystallization process, this means that the draw-off point for the particle suspension has to be chosen carefully. To reach conditions dose to an ideal mixed tank reactor, the combination of agitation and draw-off has to guarantee a constant solids concentration and together with that a stable particle size distribution. 

I 76 Examples of Realized Continuous Crystallization Processes Typical situation ... 

Usually, batch crystallization is used when a relatively low production capacity is required, e.g., below 50 t of product per day. When batch crystallization is equipped with the proper temperature control and seeding system, the crystallization conditions can be adjusted in such a way that the residence times of the crystals, of various sizes, can be kept about the same. Therefore, the CSD can be narrower in batch crystallization than in continuous crystallization, which is one of the significant differences between batch and continuous crystallization without fine removal or a classification method for the product. In practice, industrial continuous crystallization processes contain fines-removal or classification units, such as hydrocyclones, in order to produce crystals of a narrow CSD. 

Rohani, S., M. Haeri, and H. C. Wood, Modeling and Control of a Continuous Crystallization Process Part 1. Linear and Non-Linear Modeling, Comput. Chem. Eng, 23,263 (1999). 

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