The Crystallization Section

The crystallization process is built up of three components as shown schematically in Figure 3, starting with a set of scraped surface heat exchangers (SSHEs) that remove the necessary heat from the system to provide for ice crystal formation. The SSHEs are connected to the recrys-tallizer by a recirculation stream through a scraped filter in the recrystallizer. This provides a continuous crystal free stream over the SSHEs where small ice crystals (nuclei) are formed and pumped into the recrystallizer. Of these small crystals, some survive and grow while the majority melt, absorbing energy from the surroundings and causing water to crystallize on the surviving ice crystals. This growth process is known as ripening and can be described by the Gibbs-Thomson formula depicted in Figure 4 for an aqueous sugar solution.

Small crystals have a slightly lower equilibrium temperature, the temperature at which they neither grow nor melt, than larger crystals. When small and large crystals are mixed in an adi-abatic vessel, the temperature of the bulk solution will reach a value somewhere between the equilibrium temperatures of the large and small crystals [7]. This puts the larger crystals in an environment below their equilibrium temperature so they will not melt, whereas the small crystals are in an environment above their equilibrium temperature so they tend to melt, which removes energy from the bulk solution that can be exactly balanced by the recrystallization of

Heat Removal

The small ice crystals form in the SSHE

The ice crystals travel in the product flow to the recrystallizer, where the ripening process begins. Most of the small cryatals melt, but a few survive and start growing.

A crystal-free liquid stream of product is continuously recirculated over the SSHE providing a constant supply of small crystals to fuel the ripening process.

When enough ice crystals have formed, a slurry stream is diverted from the recrystallizer to the washcolumn. The ice is separated from the concentrate and discharges after being melted.

Feed

Feed

New product from the feed tank replaces the discharged water so that the solute concentration in the system increases to the desired production value when we can withdraw part of the recirculation stream as product.

Figure 3 Schematic buildup of an externally cooled crystallizer with separation section, (a) The small ice crystals form in the SSHE. (b) The ice crystals travel in the product flow to the recrystallizer, where the ripening process begins. Most of the small crystals melt, but a few survive and start growing, (c) A crystal-free liquid stream of product is continuously recirculated over the SSHE, providing a constant supply of small crystals to fuel the ripening process, (d) When enough ice crystals have formed, a slurry stream is diverted from the recrystallizer to the wash column. The ice is separated from the concentrate and discharges after being melted, (e) New product from the feed tank replaces the discharged water so that the solute concentration in the system increases to the desired production value when we withdraw part of the recirculation stream as product.

270.54 i 01

Diameter (microns) 100

Temperature (K)

270.50

270.45j

270.50

270.45j

Figure 4 Schematic representation of ripening for ice in a 30% sucrose solution.

water on the larger crystals. Therefore, the small crystals melt and disappear while the larger crystals grow larger. By providing a continuous supply of small crystals to an existing slurry of larger crystals we can force the growth of crystals large enough to be cleanly separated from the concentrate. This recrystallization carried out in a mixed vessel provides for very pure spherical crystals. The round crystal shape is clearly an advantage in the following separation section.

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