Feed spacer of spiral wound membrane module for nanofiltration and reverse osmosis: modeling, simulation and design

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Date
2007
Authors
Lau, Kok Keong
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Abstract
Since 1970s, the demand for spiral wound membrane (SWM) has been rapidly increasing in both local and worldwide market. Current market available SWM possess lifespan between one to three years depends on the applications. In order to extend SWM lifespan, the most influencing factor is the design of optimal SWM feed spacer to overcome fouling problem. Since fouling problem in SWM starts with the formation of concentration polarization and different feed spacers generates different degree of energy loss, an optimal feed spacer was designed based on the concentration polarization and energy loss using Computational Fluid Dynamics (CFD) approach. With the integration of permeation properties, commercial CFD code Fluent 6 was employed to simulate the hydrodynamics in the empty SWM feed channel. The integrated CFD model was validated experimentally in terms of permeation properties. Based on the results of the study, it proved that the membrane interface should be modeled as permeable wall with varying permeate flux. Besides that, the effect of feed Reynolds number, transmembrane pressure and solutes on concentration polarization development was studied. In the spacer filled SWM feed channel simulation, permeation properties were successfully incorporated in the solution of governing equations and validated experimentally. Based on the unsteady hydrodynamics analysis, the emergence of unsteady hydrodynamics in the spacer filled SWM feed channel can be detected at low feed Reynolds number (Ref 100-300) at certain transition length from the channel entrance. Different spacer designs were found to produce different magnitude of unsteady hydrodynamics. Under current study, energy loss in the spacer filled SWM feed channel was determined using specific power consumption (λ). Different spacer designs were found to generate different degree of λ. Based on the experimental and simulated results, the unsteady hydrodynamics in the spacer filled SWM feed channel can significantly disrupt the development of concentration polarization. Feed spacer design parameters which consisted of spacer filament geometry (FG), mesh length ratio (ML), mesh angles (α and β) and filament ratio (SFR) were optimized based on the minimum effective concentration polarization factor, Ψ which further validated by wall shear stress analysis, contour plot profile and localized concentration factor. Based on current study, equal cylindrical filaments with mesh length ratio 3 and mesh angle (α= 120° and β=30°) was the optimum spacer design parameters. The optimum spacer model was validated experimentally in term of hydrodynamics and permeation properties. Based on the experimental performance comparison with others spacers, optimum spacer generated the highest flux enhancement which was more than 100% as compared to empty membrane channel. Optimum spacer generated higher flux enhancement (approximately 6%-11%) as compared to spacers with identical mesh length ratio (ML=3) and mesh angles (α=120° and β=30°). Based on the observed rejection comparison, optimum spacer yielded the highest observed rejection as compared to the spacers with identical mesh length ratio or mesh angles.
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PhD
Keywords
Science biology , spiral wound membrane module , nanofiltration , reverse osmosis
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