Melt crystallization solid layer processes

The number of apphcations of melt crystallization (solid layer as well as suspension) is continuously growing. This development is supported by the fact that in the future a pardigm shift is arising in terms of the design of chemical, pharmaceutical, and food processes, which means away from single plants toward to the so-called hybrid processes. Hybrid processes mean a combination of several separation techniques, for example, distillation and crystallization, in order to enhance the throughput, the heat and mass transfer, and the reaction rates. Hybrid processes are a subset of the so-called process intensification techniques. Process intensification paves the way... 

Crystal growth rates within crystallization processes from solutions in most cases are in the range 10 -10 m/s. Growth rates in melt crystallization are quite often in the range of about 10 m/s and in extreme cases in some solid layer processes as high as 10 m/s. 

Suspension melt crystallization crystals and melt same temperature, design on degree of supersaturation, separation of crystals from melt depends on density difference in countercurrent operation. Scraped surface crystallizer. Section 4.6. The suspension methods have slower rates of crystal growth compared with the solid layer processes. 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

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. 

Among the first ones to present a complete theoretical approach are Burton, Prim, and Slitcher (1953). The authors developed a mathematical formulation of the problem for metallic systems with partial solid solubility crystallized from the melt in a suspension process. Their theory is based on a boundary layer model and does not account for the mutual dependence of heat and mass transfer but regards the influence of heat transfer as negligible. 

Finally, in some cases it can also be a disadvantage for the product to leave the apparatus in liquid form and to be solidified again. The last point as well as the third, could well be avoided someday if it becomes possible to build continuously operating processes in one plant for solid layer melt crystallization processes. 

The second group of the batch type of solid layer techniques are those with moving melts. Here again, three processes must be named the MWB-Sulzer, nowadays called Sulzer falling film (CH-PS 1967 U.S. 1985), the ICI-process (GB-PS 1964), and the BASF-process (DE-PS 1976), which is now distributed by the Kvaerner company. In all processes, the crystallization takes place on the inside of tubes, which are cooled from the outside. The melt coming from a feed tank is continuously circulated through the tubes until the crystal coat at the walls is thick enough, i.e., until... 

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). 

The cooled surface for the crystal growth of the solid layer is provided in the static plate crystallizer of Sulzer Chemtech Ltd by plates that are located in the stagnant melt. The melt feedstock is progressively crystallized by cooling the heat transfer surface. As the crystallization proceeds, the remaining melt becomes more and more impure. The crystallization process needs about 2-30 h. Subsequently, the remaining residual melt is allowed to drain by opening a valve at the bottom of the apparatus. A film of melt with residual composition, however, remains unfortunately on the crystal coat. This... 

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