The Process

When the system is first started, the unit is filled with product without ice. The product is first cooled to form a slurry of approximately 30% ice. Then the wash column is started, and water is removed from the system. This discharged water is replaced with fresh feed containing the solute(s) being concentrated. As water is removed from the system in this manner the solute concentration of the liquid in the system will increase. When the desired solute concentration, or the maximum allowed by product characteristics, is reached, concentrate is removed from the filtrate line of the recrystallizer. The solids removal rate (product output) is controlled to maintain a constant solute concentration in the final stage.

In practice this is quite simple. A system will be set to produce (and remove) a fixed amount of ice, and the product flow can be adjusted according to the equilibrium temperature in the recrystallizer. A product has a certain freezing point at each solute concentration. While operating as a two-phase mixture in the recrystallizer the system has a given equilibrium temperature for that solution. This means that as the solute concentration of the liquid phase increases, its equilibrium temperature will decrease. This signal can be used to control the flow of product from the system. Therefore, for a given solution, the ice production and product temperature (concentration) are set so that the system will react automatically to changes in feed concentration. Higher feed concentrations under the previous conditions will result in higher production rates and vice versa. This is the basic form for a single-stage FC system.

Washwater

Washwater moves downward due to the pressure difference between the top and bottom of the ice bed.

Water recrystallizes onto the surface of the incoming ice crystals in the form of dendrites. This increases the flow resistance of the washed part of the bed.

Since the washed part now has a greater flow resistance water will flow along path "A" to maintain a horizontal washfront.

The water that has recrystallized is now carried back to the top of the ice bed and discharged. This provides for an efficient washing action without recycle of the wash liquid.

Concentrate

Figure 6 Wash water flow in a packed bed wash column. Wash water moves downward due to the pressure difference between the top and bottom of the ice bed (P, > P2). Water recrystallizes onto the surface of the incoming ice crystals in the form of dendrites. This increases the flow resistance of the washed part of the bed. Since the washed part now has a greater flow resistance, water will flow along path A to maintain a horizontal wash front. The water that has recrystallized is now carried back to the top of the ice bed and discharged. This provides for an efficient washing action without recycling the wash liquid.

The single-stage system must operate at the highest viscosity and lowest equilibrium temperature for a particular product. This implies that the ice crystals must grow and the wash column must remove these crystals under the least favorable conditions [3], The high product viscosity reduces the crystal growth rate, increases pumping and mixing energy requirements, reduces heat transfer coefficients, and limits the wash column capacity. The refrigeration system must remove all the heat at the lowest temperature, increasing its size and reducing efficiency. These have the effect of reducing the overall capacity and total efficiency of the process. Multistage systems with up to six separate crystallization sections and only one separation section (which may consist of a number of wash columns) overcome these limitations by dividing the ice production over a range of product concentrations. Figure 7 shows an example illustrating the increased capacity provided by a multistage system.

In Figure 7 you see a typical single-stage system with a water removal capacity of 250 kg/hr (550 lb/hr) and in comparison a three-stage system with a water removal capacity of 1500 kg/hr (3300 lb/hr). This is twice the sum of the individual units. Each stage is a complete crystallization and recrystallization section. The ice crystals formed in one stage are pumped to the next. This ice is replaced with concentrate from the previous stage, resulting in a counter current flow of ice and soluble solids. Multistaging increases the capacity of a freeze concentration system by separating the ice production section from the separation section. This allows the wash column(s) to operate with a liquid much lower in concentration and viscosity. Multistaging reduces energy consumption by dividing the ice production over the stages so that more ice is made at higher freezing temperatures and lower viscosities, which improves the energy efficiency of the scraped surface heat exchangers (SSHEs) and reduces the overall energy needed for pumping and mixing. The average crystal growth rate is increased by reducing the residence time needed per stage. The crystals produced in stage 1 are transported to stage 2, where they

Feed 250 kg/h 50%TS

Feed 250 kg/h 50%TS

Water

Water

Figure 7 Example illustrating capacity increase through multistaging. (a) In a single-stage system, separation capacity is limited by the high viscosity of the product in the wash column, (b) Multistaging increases the separation efficiency of the wash column by removing the ice at the lowest viscosity. Average crystal growth rates increase as the ice moves from high to low product concentrations.

Figure 7 Example illustrating capacity increase through multistaging. (a) In a single-stage system, separation capacity is limited by the high viscosity of the product in the wash column, (b) Multistaging increases the separation efficiency of the wash column by removing the ice at the lowest viscosity. Average crystal growth rates increase as the ice moves from high to low product concentrations.

grow larger, and so on. These crystals are also exposed to the more favorable growth conditions of lower solute concentration and lower viscosity. Multistage systems also provide for a wider capacity range since the components are modular and can be added as greater capacity is needed.

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