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.
Description
PhD
Keywords
Science biology , spiral wound membrane module , nanofiltration , reverse osmosis