Hydrodynamics (Reynolds number) determine scaling, nucleation and crystal growth kinetics in membrane distillation crystallisation

Reynolds number (Re) has been previously related to several scaling mitigation and crystallisation strategies that offer distinct hypotheses for how Re may regulate the kinetics of nucleation and crystal growth in membrane crystallisation. Such ambiguity has arisen from the present inability to discretely characterise induction time in membrane systems. This study therefore introduces techniques for the detection of induction time, with mea-surements used to develop a modified power-law relation between nucleation rate and supersaturation to establish how Re can be used to adjust nucleation kinetics. Increasing Re enhanced mass and heat transfer processes which raised permeate flux. The interfacial supersaturation set by the increase in flux, also modified the supersaturation rate at induction for crystals formed in the bulk solution, providing the first direct evidence that it is the supersaturation level set within the boundary layer which controls primary nucleation in the bulk solution. Bulk nucleation rate can therefore be adjusted in proportion to Re. While the extent of scaling was also determined by the interfacial supersaturation set by Re, its formation was shown to be more dependent on the interfacial diffusion coefficient which regulates solute backtransport and the activation energy for nucleation. Through this work we suggest that the nucleation mechanisms underlying scale formation and bulk crystal-lisation are distinct. The regulation of nucleation rate in the bulk solution by Re is described analytically through classical nucleation theory, while scaling can be mitigated through operation below a critical threshold super -saturation value that determines the rate and type of scaling that prevails. These seemingly distinct strategies can be combined through modifications to T and dT with Re to suppress scaling and offer refined control over the kinetics of nucleation and crystal growth.

Automated summary

The study introduces techniques for detecting induction time and develops a modified power-law relation between nucleation rate and supersaturation to show how Reynolds number (Re) can regulate nucleation kinetics. Increasing Re enhances mass and heat transfer processes, therefore increasing permeate flux. The interfacial supersaturation set by the increase in flux modifies the supersaturation rate at induction for crystals formed in the bulk solution, providing evidence that it is the supersaturation level within the boundary layer that controls primary nucleation. The formation of scaling is found to depend more on the interfacial diffusion coefficient and activation energy for nucleation.