Contact Refrigeration Crystallizers

In direct-contact refrigeration crystallizers, the refrigerant is mixed with the magma circulated within the crystallizer body where it absorbs heat and is vaporized. Refrigerant vapor leaves the surface of the crystallizer similar to water vapor in a conventional evaporative crystallizer. The refrigerant vapor must be compressed, condensed, and then recirculated to the crystallizer to maintain the continuous operating conditions. Refrigerants chosen must be relatively insoluble in the solutions processed and have the necessary thermodynamic characteristics to minimize compressor horsepower. 

Examples of this are the crystallization of caustic dihydrate with Freon or propane, and the crystallization of paraxylene with liquid propane refrigerant. 

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FIG. 18-67 Direct-contact-refrigeration crystallizer (DTB type). (Swenson Process Equipment, Inc.)... 

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 5.20 Swenson direct-contact refrigeration crystallizer.

Chevron (2) A process for separating/i-xylene from its isomers by continuous crystallization, using liquid carbon dioxide in direct contact with the xylene as the refrigerant. Developed by the Chevron Research Company in 1966. 

The DirCon freeze crystallization process is a technology that can be used to purify aqueous waste streams and concentrate liquid waste by the freezing and subsequent melting of the liquid. This technology is a type of direct-contact secondary-refrigerant freeze crystallization and operates on the principle that when water freezes, the crystal structure that forms naturally exudes contaminants from its matrix. The terms freeze crystallization and freeze concentration are often used interchangeably. 

Direct freezing in which the heat of crystallization is removed by partial evaporation of the solvent, or by contacting the solution with an immiscible refrigerant. 

This compound is a severe skin irritant, hence great care should be exercised to avoid any contact. For best yields in the rearrangement step this crystallization is recommended, since I he yield of ethyl 1-naphthylacetate is reduced by about 20% if the crude product is used in the rearrangement. A sample of crystallized l-(diazoacetyl)naphthalene, m.p. 52-53°, that had been stored in a screw-top bottle in a refrigerator for about 2 weeks afforded the same yield of ethyl 1-naphthylacetate as a freshly prepared sample. 

Co.). In direct contact with the scraped surface is the process fluid which may deposit crystals upon chilling or be extremely fouling or of very high viscosity. Motors, chain drives, appropriate guards, and so on are required for the rotating element. For chilling service with a refrigerant in the outer shell, an accumulator drum is mounted on top of the unit. 

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