Chapter 8, Problem 8.75P

Freeze concentration is used to produce a fruit-juice concentrate. A stream of fresh juice containing

12 wt% soluble solids in water at 20°C is combined with a recycle stream to form a preconcentrate, which is fed to a crystallizer. The mixture is cooled in the crystallizer to —7°C, thereby crystallizing 20,000 kg/h of ice. A slurry leaves the crystallizer containing 10wt% ice and is fed to a filter. The filtrate, which contains 45 wt% dissolved solids, is removed as the process product. The remaining slurry, which contains all the ice and some concentrate (also containing 45% dissolved solids), is sent to a separator that cleanly removes all of the ice. The residual liquid is the recycle stream that combines with the fresh feed to form the preconcentrate.

1. Determine the rates (kg/h) at which fresh fruit juice is fed and concentrate is produced, and the mass flow rate (kg/h) and solids concentration of the preconcentrate.

Calculate the cooling requirement (kW) for the freezer, assuming that the temperature of the recycle stream is 0°C and the heat capacity of all solutions is 4.0kJ/(kg·°C).

An alternative to freeze concentration is to evaporate some of the water from the juice at atmospheric pressure, producing a concentrate with 45% dissolved solids. There are two reasons this process may be less attractive: one is applicable to any solute, and the second applies particularly to a food product. Speculate on what they are.

A second alternative would also involve vaporization, but at low pressure. Why might this alternative be preferable to the one in Part (c), and what would determine whether it would be preferable to freeze concentration?