Energy Balance of the Continuously Operated Crystallizer

In Fig. 8.3-4, a schematic of an evaporative crystallization stage is depicted. During cooling crystallization the heat flow gout removed, during evaporative crystallization the heat flow is supplied. 

The mass flow AZp of the evaporated solvent with a corresponding enthalpy flow leaves the crystallizer. 

In addition energy can be supplied by the circulation device and, for nonadiabatic operation, heat can be exchanged with the environment. If the crystallizer is operated at steady-state conditions, the following energy balance for the crystallizer is obtained  

The heat of crystallization is the heat that has to be supplied or removed during crystallization at constant temperature. It is equal to the negative value of the heat of solution during the dissolution of crystals in an almost saturated solution. The heat of crystallization is accounted for in the enthalpy values. Processes in crystallizers can easily be tracked, if an enthalpy concentration diagram is available for the respective system. The pure component s enthalpy is zero at reference temperature, not the enthalpy of real mixtures however. In such diagrams, the lever rule is applicable. This is shown for the system calcium chloride/water in Fig. 8.3-5, where the specific enthalpy is plotted vs. the mass fractions. 

During cooling crystallization (1-2) heat is removed and the enthalpy decreases from point 1 to point 2. Point 2 lies in a two-phase region, in which solution and hexahydrate are in equilibrium. The line segments (2-2 ) and (2 -2) denote the solids content. The process of evaporative crystallization in vacuum at 0.5 bar can also be seen in the diagram. During heating of the feed solution (x = 0. 45, 

---