The basic components in most freeze concentration systems are freezers, ripening tanks, and separators (2). Originally, single-stage systems,—systems containing one ripening tank or, in some cases, a bulk-growth tank—were
used and are still used when little concentration is required. Countercurrent multistage systems that use wash columns as separators are used to provide high degree of concentration. Compared to single-stage operation, countercurrent multistage operation provides faster, cleaner separation; greater thermodynamic efficiency (reduced energy consumption); faster ripening and crystal growth; and higher concentrations. Figure 4 shows a modern three-stage countercurrent freeze-concentration system. Systems containing up to five stages are used. Each stage consists of a ripening tank served by several scraped-surface freezers operating in parallel (3). Clear liquid, drawn out of the tank through a scraped strainer, is pumped through the associated freezers and returns to the tank as an ice-containing slurry. Although high scraping rates and strong chilling are used, the liquid passes through the freezers so quickly that only 7.5 to 8% of the stream is converted into ice, and 5- to 10-//m diameter crystals are produced.
Flow within the system is arranged so that the liquid being concentrated and the ice produced move countercurrent with respect to each other in an overall sense. Part of the liquid drawn through the ice strainer at the bottom of a ripening tank is sent to the next tank in terms of concentrate flow, that is, to the tank with the next higher concentration. Ice-concentrate mix continuously transfers out of each stage. Most of the concentrate in the mix is mechanically expelled; much of what remains is displaced by more dilute concentrate from the stage the ice is about to enter. The ice enters the next stage in the direction of ice flow, which is the next-more-dilute stage. Ice-crystal size progressively increases as ice moves from the most concentrated stage to the most dilute stage.
Strained, ice-free product concentrate discharges from the most concentrated stage. Ice is removed in slurry form at the opposite end, the dilute end of the system, where separation is easiest. The discharged mixture of ice and dilute stage liquor is sent to a wash column, where it is propelled upward and slightly mechanically compressed. The compression expels most of the accompanying liquor. The ice is melted near the top of the column, forming melted water that displaces remaining solute-containing liquor from the pores in the advancing bed of ice. If the rate of ice advance is suitably slow, very clean displacement is obtained, and, as previously mentioned, less than 0.01% solute is lost. If it is too fast, very large amounts of solute are lost. The maximum rate of ice advance that can be safely used is roughly proportional to the ice crystal diameter squared and inversely proportional to the difference between viscosity of the displaced liquor and the viscosity of the water carrying out the displacement. Concentrations, temperatures, ice-formation rates, and mean ice crystal diameters in the stages of a four-stage freeze concentration system processing 5,000 kg/h of liquid feed are listed in Table 1.
The total rate of ice production, 4,800 kg/h, exceeds the net rate of ice removal, 4,000 kg/h, because some ice melts because of stirring-induced friction in the system. Roughly two hours of ice-holdup time in a multistage, freeze-concentration system may be required to achieve adequate ice crystal size growth. The average solute holdup is usually roughly twice as long. Less than a few minutes of holdup time are used in juice-concentration evaporators.
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