Surface cooling crystallization

Continuous surface cooling crystallization is not a modern process. In this method, the supersaturation is created using the surface of a heat exchangee As soon as this surface is covered by incrustations, the production output is directly affected. The 

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FIG. 18 66 Forced-circulation baffle surface-cooled crystallizer. (Swenson Process Equipment, Inc.)... 

An Oslo surface-cooled crystallizer is illustrated in Fig. 18-71. Supersaturation is developed in the circulated liquor by chilling in the cooler H. This supersaturated liquor is contacted with the suspension of ciystals in the suspension chamber at E. At the top of the suspension chamber a stream of mother hquor D can be removed to be used for fines removal and destruction. This feature can be added on either type of equipment. Fine ciystals withdrawn from the top of the suspension are destroyed, thereby reducing the overall number of ciys-tals in the system and increasing the particle size of the remaining product ciystals. 

High levels of supersaturation at the liquid surface or at the tube walls in a surface-cooled crystallizer are the dominant cause of wall salting. Although some types of crystallizers can operate for several months continuously when crystallizing KCl or (NH4)2S04, most... 

The pure fluidized bed crystallizers described later in Example 7-6, while designed for a particular purpose, have many similarities with the Oslo commercial surface-cooled crystallizer shown in Fig. 7-10. A fluidized magma in the crystallizer body (E) carries out the crystallization. Feed enters the clear overflow stream (at G), is cooled within the metastable super-saturation region in the cooler (H), and then enters as a fluidizing stream at the bottom of (E). 

Either the draft tube baffle, as shown in Figure 5.20, or the FC crystallizer may be used as a direct-contact refrigeration crystallizer in those cases where crystallization occurs at such a low temperature that it is impractical to use surface cooling or when the rapid crystallization of solids on the tube walls would foul a conventional surface-cooled crystallizer. 

Figure 5.21 shows a surface-cooled crystallizer that utilizes Teflon tubes as the heat exchanger media. The Teflon tubes... 

While the unit operation evaporation, that is, the mass transfer from the liquid phase to the vapor phase, still possesses a direct connection with vacuum techniques, the connection of today s single mass crystallization from solution with vacuum techniques is only indirect. The techniques of vacuum cooling and vacuum evaporation are only the mostly used means for inducing the crystallization process. The reason for the dominant position of vacuum crystallization over classical surface cooling crystallization is the considerably reduced inclination to form incrustations. Vacuum crystallization is used in the low vacuum field down to 1 mbar. There are also applications in the overpressure field, although with increasing pressure the number of applications is reduced. In vacuum crystallization, one can find all the classical process control options used in the more familiar vacuum evaporation processes. However, an important difference to evaporation is the fact that the separation process is not concluded with the crystallization step. The suspension formed still has to be separated into crystal mass and mother liquor. Crystallization is therefore always associated with a mechanical separation process. The better this separation, the greater the purity of the crystallized masses. 

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