Melt crystallization column processes

Separation of a chemical species from a mixture of similar compounds can be achieved by crystallization. The mode of crystallization may fall in the realm of what is known as melt crystallization. In such processes, the mother liquor largely is comprised of the melt of the crystallizing species, and, subsequent to its crystallization, crystals formed from the mother liquor are remelted to produce the product from the crystallizer. Para(p)-xylene can be crystallized from a mixture that includes ortho and meta isomers in a vertical column that causes crystals and mother liquor to move countercurrently. Heat is added at the bottom of the column to melt the p-xylene crystals a portion of the melt is removed from the crystallizer as product and the remainder flows up the column to contact the downward-flowing crystals. Effluent mother liquor, consisting almost entirely of the ortho and meta isomers of xylene, is removed from the top of the column. 

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Figure ll-12a presents room temperature adsorption isotherms for some representative activated carbon samples on Technical Grade DMSO, showing the difficulty of reducing UV275 to the desired level of <0.10 without additional purification. Figure ll-12a is a schematic of the melt crystallization-carbon column recycle system which was employed to get around this problem. The higher concentration of impurities in the unfrozen melt altered the equilibrium concentration on the activated carbon. In the steady state (Fig. 1 l-12b) a reasonably sized carbon column could produce effluent suitable for further freeze CrystalUzation, and the yield of the total process was close to 100%. 

One type of suspension processes using a scraped crystallizer to create the feed suspension is the Phillips process (see McKay 1967). The crystallizer consists of a vessel with filters in the wall of the upper section. There is a heater at the bottom to remelt the crystals. Some of the melt is taken as product, some is used as reflux. The reflux serves mainly as washing liquid for the crystals which are forced downward by a reciprocating piston, or with pulses from a pulsator pump. The Phillips crystallizer column is shown in Figure 7.14. 

A process for the fractional crystallization of a melt in a countercurrent column was first patented by P. M. Arnold in 1951. In 1961, H. Schildkneeht utilized a spiral conveyor to transport the solids up or down the column. These techniques, together with other later refinements, may now be seen in several commercial-scale melt crystallizers. 

For an earlier introduction to column crystallization vis-a-vis the variety of equipment and patents, see Albertins et al. (1967). Ulrich (1993) provides a more modern version of melt crystallization devices, plants and processes. For mathematical modeling and optimization of multistage crystallization processes, see Gilbert (1991). A more Involved model of continuous countercurrent contacting with axial dispersion in melt crystallization has been illustrated in Hemy and Moyers (1984). 

As an alternative to multistage batch crystallization processes with their attendant problems of material handling and losses, several types of continuous column crystallizers have been developed, in which the product crystals are washed with their own melts in countercurrent flow. Those illustrated in Figures 16.14-16.17 will be described. Capacities of column purifiers as high as 500gal/(hr) (sqft) have been reported but they can be less than one-tenth as much. Lengths of laboratory size purifiers usually are less than three feet. 

Separation of Ice from Brine. Brine must be rather completely separated from the mass of small ice crystals formed in the freezer before the ice can be melted to produce fresh water. In the Carrier process this is accomplished by countercurrent washing with fresh water in a vertical moving bed called the wash-separation column. In this column the ice-brine slurry is introduced at the bottom, where much of the brine is removed by filtration. The remaining bed of ice crystals is pushed vertically upward through the column by hydraulic forces and is freed of its salt content by the countercurrent stream of fresh water. The ice is continuously harvested at the top of the column. Figure 6 shows a schematic view of the wash column in operation. 

Fig. 5 shows a process sequence for the initial stages of making electronic-grade Si. The unit operations in this sequence resemble those typical of conventional chemical processing, and include furnaces, fluidized bed reactors, and distillation columns. The product semiconductor emerging from this process must however be purified further than what distillation allows, and must also be converted into single-crystal form. Invariably, the latter process involves melting the... 

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