Ice Crystal Formation And Growth

Ice-free aqueous solutions have to be cooled below their equilibrium freezing point before ice crystal nuclei form and freezing starts. Ice can nucleate in several ways. Heterogeneous nucleation usually predominates in ice-free solutions and rapidly occurs roughly 6°C below the initial

Figure 3. A ripening tank. SSF = scraped-surface freezer. Source: Reprinted courtesy of Niro Process Technology B.V.

Auger drive motor

Draft tube

Fine slurry inlet line

Ripening tank

Ripened slurry to next stage or wash column

Strainer

Slurry containing small crystals from SSF

__ Crystal-free liquor to SSF

Auger drive motor

Draft tube

Ripening tank

Ripened slurry to next stage or wash column

Strainer

Slurry containing small crystals from SSF

__ Crystal-free liquor to SSF

Fine slurry inlet line equilibrium freezing point of the solution. Once nucleation occurs, the solution's temperature rises and closely approaches its equilibrium freezing point at its current concentration. Ice nuclei are very small and have to be converted into much larger ice crystals for efficient separation. This can be done by bulk supercooling that causes existing crystals to grow by maintaining the solution below its equilibrium freezing point. The greater the supercooling, the greater the rate of growth. However, if the supercooling is too large, highly branched crystals from which solution does not separate cleanly will form. Further, new nuclei will continually form on cooling surfaces if temperature differences larger than 6°C are used to provide heat removal.

Therefore, it is preferable to form small ice crystals on sharply chilled, rapidly scraped surfaces and mix those crystals with larger crystals in tanks, in which the small crystals sacrificially melt and cause large-crystal growth. The process occurs because ice crystals with small radii have slightly lower equilibrium freezing points than crystals with larger radii (Fig. 2). Slurries containing mixtures of smaller and larger crystals assume a temperature between the respective equilibrium freezing temperatures of the small and large crystals. The small crystals tend to melt, and the large crystals tend to grow, a process known as Ostwald ripening (2). The temperature differences involved are very small, but the process is fairly rapid in stirred systems because the crystals provide an enormous amount of heat-transfer and mass-transfer surface.

Ripening in freeze concentration systems is frequently carried out in a large, enclosed, vertical, cylindrical tank fitted with a helical agitator and draft tube (Fig. 3). Liquid withdrawn from the mixing tank through a scraped screen at its bottom is fed to freezers and returns to the tank as

Table 1. Local Conditions in a Four-Stage Freeze-Concentration System Converting 5,000 kg/h of a 10% Solute Feed into a 50% Solute Concentrate

Stage

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