Investigation of batch cooling crystallization in a liquid-liquid separating system by PAT

Crystallization of butyl paraben from water-ethanol mixtures has been investigated. The liquid-liquid phase separation and the solid-liquid solubility have been determined from 1 to 50 °C. Cooling crystallizations have been performed at different starting compositions, and the processes have been recorded by in-situ infrared spectroscopy, focused beam reflectance measurement, and particle video microscopy. In pure water the butyl paraben solubility is below 1 mg/g, while in pure ethanol the solubility is more than 3 orders of magnitude higher. While the solution saturated with butyl paraben is homogeneous at 1 °C, at the higher temperatures butyl paraben induces a liquid-liquid phase separation of the ethanol-water mixture, and the ternary phase diagram contains up to five different regions. The size of the liquid-liquid phase separation region increases with increasing temperature. At 50 °C, even the binary butyl paraben water system separates into two different liquid phases. In the cooling crystallizations, the resulting product crystals and the behavior of the process are quite different, depending on the starting composition. The largest crystals and the least agglomeration were obtained in that experiment where liquid-liquid phase separation was not occurring. In all of the other experiments the crystals were smaller and more agglomerated, and the particle size distribution was wider or more irregular. The work illustrates how Process Analytical Technology (PAT) can be used to increase the understanding of complex crystallizations.