Indirectcontact Freezing Process

In this process, heat transfer takes place through a metal wall. The process uses scraped-surface heat exchangers for both heat transfer and crystallization (see Figure 7.37.9). The feed solution is cooled in the surface crystallizers to below 32°F, and ice crystals are formed. Refrigeration is supplied by either ethylene glycol or an evaporative refrigerant, such as butane, on the shell side of the crystal-lizer.

The slurry containing ice crystals is sent to a continuous centrifuge where the ice crystals are separated from the mother liquor and sent to the melter tank after being washed. This process can obtain the heat for melting by precooling the incoming feed stream, thereby reducing the load on the refrigeration unit.

This process is used to concentrate coffee and fruit juices, but because the scraped-surface exchangers and

FIG. 7.37.9 Indirect-contact freezing process.

FIG. 7.37.10 Direct-contact freezing process with wash column.

FIG. 7.37.9 Indirect-contact freezing process.

FIG. 7.37.11 Desalination process using vacuum freezing and vapor compression.

other components are expensive, it is not used for waste-water treatment.

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