Emulsion crystallization of phenanthrene by supercritical fluid extraction of emulsions

Abstract

The emulsion crystallization of phenanthrene has been investigated by solvent extraction from oil in water emulsions through dilution with water and through supercritical fluid extraction of emulsions (SFEE). In order to contribute to an improved understanding of this process, different physical mechanisms involved in particle formation have been explored by observation of crystallizing emulsion samples progressing towards equilibrium. Phenanthrene crystals are formed either by self-nucleation of single emulsion droplets, or through contact of emulsion droplets with already existing crystals. On the other hand, emulsion droplets are mainly consumed by the growth of phenanthrene crystals, involving mass transfer through the continuous water phase. Furthermore, the influence of various process parameters on the crystallization of phenanthrene has been investigated experimentally. When increasing the temperature, the average median x50 of the particle size distribution increased from 15μm at 20°C to 103μm at 60°C. Other parameters investigated were the stirring rate and the suspension density, which could be increased through solvent removal by SFEE. The latter method yielded the smallest particles, exhibiting a median x50 of 3.0μm. Emulsion crystallization enables the formation of crystals through the liquid–liquid two phase region instead of avoiding it, and may ultimately lead to the development of crystallization processes that can be carried out faster and at higher solid densities, thus at a more intensive level. The present study demonstrates that this process exhibits good reproducibility and enables controlling the particle size in a rather wide range, thus further underlining the potential of this technique for applications both in research and in the pharmaceutical industry.