Solid layer crystallization

A solid layer type of crystallization from the melt is often called progressive freezing (see, e.g., Jancic 1989), or directed crystallization (Ulrich 1988), and directed solidification (Smith 1988). All expressions describe a crystal layer growing perpendicular to a cooled wall and use the phase change as the basis for the separation of the feed mixture. This is possible due to different equilibrium concentrations of the solid and liquid phase of a mixture (see Section 7.3.). 

A further advantage of solid layer crystallization is that, besides pumps, no moving parts are needed in such processes, since only liquids are transported. A weak point of solid layer crystallization processes is the batchwise or quasicontinuous operating mode. This is different from most suspension crystallization processes that are continuous. 

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The economy of melt crystallization processes depends on the product purity, which is normally increased by an additional cleaning step. The application of gases under pressure is investigated to show possibilities of product quality improvement. Experimental devices for the determination of the freezing curve under gas pressure and for a solid layer crystallization process are shown. The influence of gas and pressure in respect to the freezing curve are explained on the basis of two binary mixtures (trioxane/water and para-/meta-dichlorobenzene) under CO2- and N2- pressure are presented. Furthermore the results of solid layer crystallization experiments with naphthalene/biphenyl and para-/meta-dichlorobenzene mixtures are shown. 

The main advantages of the solid layer crystallization techniques are as follows ... 

Figure 7.6 Flow diagram of possible process steps in solid layer crystallization.

Only moderate growth rates can be achieved as compared to the very high rates in solid layer crystallization. This is due to the isothermal growth, whereas in the layer technique, high thermal gradients can be forced on the crystals within the layer. 

Solid layer crystallization is a process in which the growth of a crystal layer takes place perpendicular to a cooled surface into the bulk of a melt (sophase change is used as the basis for the separation of the feed mixture. Such a phase separation is possible due to different equilibrium concentrations of the solid and liquid phases of the mixture (see Chapter 3). The driving force for the crystal growth is the temperature difference between the equilibrium temperature of the melt (the bulk) in front of the soUd layer and the temperature of the cooled surface (see Figure 15.2). 

Feature Solid layer crystallization Suspension crystallization... 

The dynamic operating mode of the solid layer crystallization (see Chapter 16) can be tested quite realistically with the shown cold finger equipment, too. However, several modifications of the setup are required, for instance, a circulating loop of the melt has to be installed in order to create a falling film from the top of the cold finger (see, for example. Refs [4,14]). 

The basics of melt crystallization are provided in Chapter 15. Here, the concepts of plants and/or existing and commercially available equipment are shown. As mentioned in Chapter 15, the concepts of plants can be divided into two lines of technology solid layer crystallization and suspension crystallization. Furthermore, in industrial applications these two techniques are split into continuous and batchwise as well as into static and dynamic (stagnant or flowing melt) operating modes. A detailed overview of the different techniques and apparatuses in solid layer as well as suspension crystallization is provided in the Sections 17.1.1 and 17.1.2. 

Figure 17.1 shows the most discussed technologies of solid layer crystallization from several providers (see, for example, Ref. [1]). The highlighted examples ofequipment in Figure 17.1 will be discussed in detail. 

Figure 17.1 Equipment of solid layer crystallization, (according to Ref. [1]).

Both the Hoechst Tropfapparat and the Proabd are tube bundle crystallization equipment run in the same way, as mentioned above. In principle, every plate or tube bundle heat exchanger can be used as static solid layer crystallizer however, a few special geometrical considerations have to be obeyed. 

Due to these facts, large volumes of crystallizers are required to achieve high yields with the stagnant batchwise technique. The efficiency of solid layer crystallization processes can be enhanced by mechanisms that improve the heat and mass transfer on the one hand and by continuous operating mode on the other hand. 

More detailed information on the introduced design examples of solid layer crystallization plants is, in general, industrial secret and hence not published. The authors will provide here design examples based on cold finger experiments (see Section 15.4.2) by Neumann [3]. These experiments are representative for solid layer crystallization techniques and are helpful to understand the topic concerning... 

In Table 17.1, it can be seen that the concept of the static solid layer crystallization without postcrystallization treatments leads to the desired purity. The yield, however. 

Table 17.1 Results of several concepts of solid layer crystallization plants [3].

Suspension crystallization is capable of producing very pure crystals mostly in a continuous operating mode, which is an advantage compared to the most batch solid layer crystallization processes. Another positive feature compared to solid layer crystallization is the better purification per process step and hence a less number of process steps usually with respect to crystallization. Therefore, suspension crystallization plants need in principle less energy compared to solid layer processes. Whether the investment costs of such plants are smaller as well depends on the complexity of the moving parts in suspension plant concepts compared to solid layer concepts (no moving parts, except pumps). 

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