Bypassing thermodynamic limitations in the Crystallization-based separation of solid solutions

Separation of solid solution forming systems into their constituents using crystallization processes is very challenging. We demonstrate the separation of L-valine and L-leucine solid solutions using a new hybrid process of evaporation and antisolvent crystallization. This system of two amino acids is characterized by alyotropic behavior. Bypassing the alyotrope via proposed new method is the main feature of the process. The change of pure component solubilities and the composition of the alyotrope with addition of antisolvent (ethanol) is quantified. Based on these solubility measurements, empirical modelling and process simulation the concept was validated in a pilot plant scale. Moreover, Powder X-Ray Diffraction (PXRD), (pseudo)-ternary and Roozeboom diagrams are exploited to deepen process understanding. Additionally, a simple graphical determination of the supersaturation using the Roozeboom diagram is proposed and demonstrated. The successful isothermal bypassing of the alyotrope at larger scale can be directly integrated into a dual counter-current crystallization setup. To achieve further improvements, this concept can be combined with a similar concept based on bypassing the double saturation point by changing temperature.