A two-dimensional mechanistic model for scaling in spiral wound membrane systems

• 2-d Model was developed for mineral precipitation in membrane feed channels.
• The model suggests membrane-spacer contact points are prone to scaling.
• Local concentration of solutes and flow conditions are important for precipitation.
• Thermodynamic and kinetic effects govern gypsum precipitation in membrane channels.
• General trends for performance decline are well described by the numerical results.

A two-dimensional mathematical model integrating fluid flow and solutes mass transport with a particle-based approach for crystal nucleation and growth was developed to study gypsum (CaSO4⋅2H2O) scaling in membrane feed channels. The model is able to describe the generally observed trends for performance decline due to scaling: permeate flux deterioration when constant pressure is applied and trans-membrane pressure increase when a constant permeate flux is set.Model results for precipitation in the feed channel show a gradual increase of precipitated gypsum in the axial direction. The zones around spacer-membrane contacts are most prone to scaling even when the feed water is under-saturated. Simulations highlight the importance of both thermodynamic and kinetic effects on gypsum precipitation for various salinities. Despite having the same degree of saturation of the feed water, variable feed stream compositions result in different amounts of precipitate formed. The numerical results suggest that the degree of saturation of feed water (DSin) may not be a reliable indicator for precipitate formation in membrane feed channels.Results support the need for considering the dynamics of local flow and solute patterns, combined with ionic activity calculations for understanding mineral precipitation in spiral-wound membrane modules used in desalination.

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