Supersaturation control in membrane distillation crystallisation

Membrane distillation crystallisation (MDC) has emerged as a potential alternative to conventional industrial crystallisers. MDC provides controlled hydrodynamics and uniform supersaturation conditions for crystallisation, thereby enhancing scalability, which is desperately lacking in conventional crystallisation systems. Unfortunately, crystallisation near the membrane surface is also associated with inorganic fouling (scaling), which can ultimately lead to process failure. As such, the viability of the technology is dependent on scale-free bulk crystallisation. To date however, scaling mitigation strategies have been based on empirical observations with contradictory postulations regarding the governing crystallisation mechanism(s). In this work, the distinct mechanisms of scaling and bulk crystallisation have been elucidated for the first time. The application of novel inline and online experimental techniques facilitated the development of a mechanistic framework which is able to predict the likelihood of scaling in addition to mediating bulk nucleation kinetics. As such, scale-free operation was achieved at temperatures and hydrodynamic conditions which were previously associated with scaling. This study has therefore broadened the perceived range of kinetic trajectories achievable with MDC and evidenced its applicability to multi- component systems, polymorph selection, and a variety of product specifications. Furthermore, the use of hydrodynamics to decouple nucleation and growth kinetics revealed the potential of MDC to minimise the usual trade-off between product quality and yield in crystallisation systems. While existing scaling mitigation strategies are largely hydrodynamic and thermodynamic in nature, this study has shown that the contribution of crystallisation kinetics (supersaturation rate) to scaling propensity cannot underestimated. Hence, application of the kinetic framework developed could provide more targeted strategies for scaling prevention in various applications such as heat exchangers and reverse osmosis/nanofiltration (RO/NF), where polarisation phenomena are prevalent