Facile supersaturation control strategies for regulating nucleation and crystal growth in membrane crystallisation

Abstract

The integration of crystallisation into membrane distillation has been proposed for brine mining as a method to effectively control primary nucleation which uses the excess supersaturation to create a new crystal phase. However, primary nucleation desaturates the solvent, subsequently introducing competition between crystal growth and nucleation mechanisms, which must be regulated to achieve a high yield and good product quality. This study therefore investigated supersaturation control strategies that can regulate nucleation and crystal growth mechanisms in membrane distillation crystallisation (MDC) following induction. Membrane area was used to adjust supersaturation, as this can modify kinetics without introducing changes to mass and heat transfer within the boundary layer. An increase in concentration rate shortened induction time and raised supersaturation at induction. This broadened the metastable zone width, and reduced scaling, due to the increased supersaturation driving force which favours a homogeneous primary nucleation pathway. Modulating supersaturation also repositioned the system within specific regions of the metastable zone that can favour crystal growth versus primary nucleation. Scaling was further mitigated using in-line filtration to ensure crystal retention within the crystalliser to reduce deposition. This permitted a consistent supersaturation rate to be sustained, enabling a longer hold-up time following induction. Population balance confirmed a reduction in nucleation rate with longer hold-up times, due to the desaturation of the solvent caused by crystal growth which resulted in larger crystal sizes. Through segregating the crystal phase into the bulk solution, growth can be more closely controlled to improve habit, shape and purity, independent of nucleation, due to the development of two discrete regions of supersaturation. The supersaturation control strategies described herein are unique to MDC and address the acknowledged challenge of supersaturation control within existing industrial evaporative crystalliser design.