Solid-liquid axial dispersion performance of a mesoscale continuous oscillatory flow crystalliser with smooth periodic constrictions using a non-invasive dual backlit imaging technique

A dual backlit imaging technique has been developed for liquid and solid phase residence time distribution (RTD) measurements in a mesoscale (millilitre) continuous oscillatory flow crystalliser with smooth periodic constrictions (herein known as the SPC mesoscale crystalliser). The pixel-based technique enables reliable determination and direct comparison of experimentally determined hydrodynamic parameters for the liquid and solid phase without concern for errors that may be introduced by utilising different measurement techniques for each phase. Additionally, the non-invasive technique offers benefits over traditional intrusive methods as demonstrated herein. Results of solid-liquid RTD experiments reveal that for a set of oscillatory flow conditions, particles do not experience the same degree of axial dispersion and mean residence time as the continuous liquid phase. The SPC mesoscale crystalliser, however, provides solid-liquid plug flow at low net flow rates. Findings in this work emphasise the importance of characterising solid phase axial dispersion for active pharmaceutical ingredient (API) systems during continuous crystallisation development to identify minimum dispersion operating conditions essential for solid-liquid plug flow crystallisation.