Pulsed-Power Electromagnetic Field Effects On Crystallization During Desalination

Reverse osmosis (RO) membrane desalination is a well-known pressure-driven water purification process which removes inorganic electrolytes (salts) from sea water or brackish water effectively. A natural consequence of desalination is the flux decline due to crystallization of sparingly soluble salts on the membrane surface which decreases the water recovery rate and also reduces membrane lifetime. Studies have shown that electromagnetic fields can affect crystallization kinetics and morphology of various electrolytes. Incorporation of a pulsed-power electromagnetic (PP-EM) field in a hybrid RO-crystallization system could be an approach to prevent membrane scaling. Herein, a study of the influence of the PP-EM field (produced by a commercial device) on salt crystallization was carried out in a bench-scale RO system.

A bench-scale RO system including a commercially available PP-EM system was designed and assembled. An experimental design was conducted with a highly concentrated calcium sulfate solution. The solution residence time in the PP-EM field and its dewatering rate (increase in supersaturation) were varied by adjusting the applied transmembrane pressure and flow velocity. Control experiments with the PP-EM field off were included. Post-mortem analysis including the conductivity and visual observation of the supersaturated retentate solution, the gravimetric and area coverage of the membrane surface, and the morphology of the crystals on the membrane surface were conducted. Clear evidence showed that PP-EM field enhances precipitation of calcium sulfate in the bulk solution versus on the membrane surface.

These results reinforce earlier pilot scale studies, with more complex feed streams, which indicated discernible changes in both membrane flux decline and crystal growth on the membrane surface during desalination, when the PP-EM field was applied. We will present a summary of these studies as well.

In addition, a non-invasive particle size measurement system, acoustic spectroscopy, was developed to characterize real-time crystallization kinetics. We will describe our results with it as a tool to measure the precipitation kinetics of CaCO₃ from CaCl₂ and Na₂CO₃ mixtures.