Crystallization Process Design Using Thermodynamics To Avoid Oiling Out in a Mixture of Vanillin and Water

Classical thermodynamics was used to model the vanillin and water phase diagram. Activity coefficient models, NRTL and UNIQUAC, were used to regress experimental equilibrium data. The vanillin and water mixture displays a liquid–liquid equilibrium that interacts with vanillin’s solid–liquid equilibrium, thus creating a challenge for crystallization processes. Using infrared spectroscopy we have experimentally shown that when the system entered the metastable liquid-liquid equilibrium region, by varying either composition or temperature, the phenomenon of oiling out could occur. By selecting operating points outside the metastable liquid–liquid equilibrium, oiling out was consistently avoided. Having validated the thermodynamic model’s accuracy and predictability, a robust process was designed allowing crystallized mass maximization while ensuring a direct route to crystallization. Utilizing a conceptual division of the desired portion of the phase diagram in operating zones, it was shown that a process can be qualitatively optimized with regard to process time, number of fines, and purity, concomitantly to crystallized mass maximization.