Multistage Countercurrent Crystallization for the Separation of Solid Solutions

While one crystallization step can be sufficient to provide pure components in case of eutectic systems, multistage operation is required to resolve solid solutions. A recently published equilibrium stage model was adopted here and generalized to simplify the description of the corresponding solid‐liquid equilibrium (SLE). The equilibria of the system potassium sulfate/ammonium sulfate/water were studied at different temperatures. The existence of a controversially discussed miscibility gap below 25 °C could not be confirmed. Using the equilibrium stage model and the studied specific SLE data, process variants considering different evaporation strategies were simulated. The influence of supersaturation by evaporation on a countercurrent crystallization cascade separating the system K2SO4/(NH4)2SO4/H2O was studied by simulation. While cascade‐wide constant evaporation factors tend to generate low‐efficient separation stages, stage‐independent evaporation was found to be very promising for improved process design.