Ultrapurification Chromatography Definition

Crystallization from solution is a separation technique where a solid phase is separated from a mother liquor. In contrast to other separation processes, however, the dispersed phase consisting of numerous solid particles also forms the final product, that has to meet the required product specifications. Crystallization can thus also be seen as a technique to obtain solid products, where the crystallization process has to be carefully controlled in order to meet the ever-increasing demands of the customer on particle properties like particle size distribution, crystal shape, degree of agglomeration, caking behaviour and purity. Since the particles must also be easily separated from the mother liquor, additional demands on filterability and washability can be formulated.

Because of the mostly rigid structure of the solid phase, the formation of solid particles is a rather slow process, and to reach an acceptable production rate large vessels are generally needed. This rigid structure on the other hand impedes the incorporation of foreign substances or solvent molecules, and in only one separation step a pure solid product is obtained.

Crystallization is often used as a generic term for evaporative or cooling crystallization, precipitation and melt crystallization. There are, however, considerable differences between the three types of crystallization as far as the processing method and the corresponding equipment are concerned. In precipitation the drop-out of the solid phase is achieved by mixing two feed streams that are either two reactants or a solvent containing the solute and an antisolvent. The hydrodynamics of the process therefore play a predominant role in precipitation with regard to the properties of the obtained product.

In melt crystallization the potential of crystallization to produce a pure product is mainly utilized, and the solid phase is remolten to obtain the final product. The applications are mainly in the ultrapurification of organic compounds or to produce pure water as a concentration technique.

An upcoming technique in crystallization is supercritical crystallization, mostly with condensed CO2, because of its benign properties compared to organic solvents. Condensed CO2 can be used either as a solvent or as an antisolvent, and specifically adapted processes and equipment have been developed for these high pressure crystallization techniques.

Also the crystallization of proteins requires its own dedicated approach, because large, sometimes easily degradable molecules require carefully designed processes.

Because 70% of the products sold by the process industry and the pharmaceutical industry - as bulk products, intermediates, fine chemicals, biochemicals, food additives and pharmaceutical products - are solids, crystallization in its widest definition is the largest separation process after distillation.

Although this chapter will primarily focus on evaporative and cooling crystallization, the energy, mass and population balances treated here as well as the kinetic rate expressions for the physical processes such as nucleation, growth and agglomeration and the characterization of the particles can equally be applied to the other types of crystallization.

Several books on the diverse aspects of crystallization have been published over the last 10 years. These books that can be recommended for a wide overview in this field contain an abundance of references. The authors of these books are Mersmann (1995), Mullin (1993), Randolph and Larsen (1987), Myerson (1993), Nyvlt (1992), Tavare (1995) and Hurle (1993). Sohnel and Garside (1992) have written a book on precipitation and Arkenbout (1995) a book on melt crystallization.

This article reviews industrial evaporation and cooling crystallization processes. A basic modelling approach is presented which enables the analysis and design of industrial crystallization processes, either by analytical calculations or by making use of modern computational tools.

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