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Title of Thesis

Pressure And Concentration Gradients In Membrane Feed Channels Numerical And Experimental Investigations

Author(s)

Muhammad Shakaib

Institute/University/Department Details
Department of Mechanical Engineering / NED University of Engineering & Technology, Karachi
Session
2009
Subject
Mechanical Engineering
Number of Pages
271
Keywords (Extracted from title, table of contents and abstract of thesis)
Fouling, Permeate, Back, Mixing, Fluent, Correlations, Membrane, CFD, TDS, Turbulence, Literature, Methods, Stagnation, Rejection, Spacers, Geometries

Abstract
Spiral wound module is the most common type of membrane configuration used for water purification applications due to high packing density, cost-effectiveness and its availability from several manufacturers. A significant problem in spiral wound membrane is the occurrence of concentration polarization and fouling which reduces the flux through the membrane, increases the passage of solutes and increases module pressure drop. Various methods are employed to prevent concentration polarization and fouling and to improve the membrane performance. The methods include modification of leaf geometry and control of operating conditions such as feed composition, inlet pressure and permeate recovery. A common technique to enhance the back-mixing of the accumulated particles is the use of net-type spacers between two adjacent membrane layers. These spacers not only define the channel height and provide passage for the feed flow but also promote mixing by continuously disrupting the concentration polarization layer. This in-turn increases the product water quality and quantity and the membrane life. On the other hand, the spacers increase pressure drop and create dead zones where the fouling phenomenon can increase.
A number of experimental and numerical studies appeared in literature in recent years due to the importance of feed spacer in the spiral wound element. An in-depth understanding, of the flow and concentration patterns in membrane feed channels, however, was not developed experimentally due to difficulties associated in applying flow visualization and measurement techniques. The detailed and systematic CFD investigations were limited to two-dimensional studies and the effect of different parameters of spacer geometry on flow behavior and spatial distribution of shear stress and mass transfer at membrane surfaces was not examined.
In this thesis three-dimensional modeling is performed using commercial CFD package FLUENT. The work carried out in this thesis is a thorough and systematic analysis of the effect of feed spacer geometry which include parameters such as filament spacing, thickness and flow attack angle on the membrane process. The fluid dynamics modeling in these channels reveal complex flow behavior and shows considerable effect of spacer geometry on the shear stress distribution. The values of critical Reynolds number at which flow leaves the laminar regime and becomes transient are also indicated. These values are found to vary between 75–300 depending on the spacer geometry. Another important contribution is the identification of appropriate turbulence models for simulating flows in spacer-filled channels. It is also shown that the improper
selection of turbulence models can affect the results both qualitatively and quantitatively. An over-prediction of upto 70 % in pressure drop values may result with some models when compared to the direct numerical simulation (DNS) results. Furthermore, the flow situations in which the two-dimensional approximation is valid for these narrow channels have also been indicated. In mass transfer modeling the concentration profiles are predicted and the spacer performances are determined in terms of mass transfer coefficient / pressure drop ratios. The spacers which involve zigzag flow have been found to be suitable for spiral wound modules since they result in higher values of spacer performance with minimal stagnation regions. The transient simulations show the movement of vortices and variation of shear stress and mass transfer coefficient with time. Results related to the entrance region in membrane feed channels are included as well.
The effect of feed spacer is also investigated experimentally. The pressure drop, product flow rate and concentration are measured for spacers of different heights, spacings and flow attack angles. The effect of operating conditions like feed pressure and total dissolved solids (TDS) on product flow rate and salt rejection is studied. The effect on product flow is more significant as it reduces approximately 30 % when feed TDS is doubled while the salt rejection only decreases 3 %. The CFD results are compared with experiments and a reasonable agreement is noticed between the two results. The comparison of shear stress pattern (from CFD) with the fouling pattern (from experiments) indicates that high shear stress region coincides with the low fouling area and vice versa. The effect of spacer on membrane fouling is determined by operating the experimental unit for longer periods. The measurement of permeate flux and salt passage showed declination of 45 % and 20 % respectively with the passage of time. The correlations are also developed for friction factor, Sherwood number and product flux to predict fouling rate. The uncertainty analyses for measurements shows that mass transfer coefficient, pressure drop and salt rejection are sufficiently accurate to compare different spacer geometries and to validate the numerical results.

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S. No. Chapter Title of the Chapters Page Size (KB)
1 0 CONTENTS

 

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2

1

INTRODUCTION

1.1 Membrane processes for water treatment

1.2 Concentration polarization and fouling in membrane systems
1.3 Models for predicting flux and concentration polarization in membranes
1.4 Configurations for membrane modules
1.5 Factors affecting the membrane modules and performance parameters
1.6 Modifying hydrodynamics in membrane modules to control concentration polarization and fouling
1.7 Outline of the thesis
1.8 Summary of contributions from this research

1
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3 2 LITERATURE REVIEW

2.1 Studies to predict concentration polarization and mass transfer in membrane processes

2.2 Simulation of spiral wound membrane modules
2.3 CFD modeling of flow and mass transfer in plane membrane channels
2.4 Effect of feed spacer on performance of spiral wound modules
2.5 Objectives of this research work

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4 3 MODELING HYDRODYNAMICS IN SPIRAL WOUND MODULES

3.1 Simulation of fluid flow in membrane feed channels

3.2 Spacer types and considered geometries

3.3 Modeling procedure
3.4 Flow simulations in diamond spacers
3.5 Flow simulations in parallel spacers
3.6 Comparison of unit-cell and four-cells approach
3.7 Flow instabilities and unsteady shear stress patterns in diamond spacers using unit-cell approach
3.8 Simulations using turbulence models
3.9 Comparison of spacer-filled flat and curved channel

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5 4 MASS TRANSFER SIMULATIONS IN SPACER-OBSTRUCTED CHANNELS

4.1 Concentration profiles in membrane modules

4.2 Spacer types and its geometric characteristics
4.3 Numerical details and approach for mass transfer modeling
4.4 Simulations in diamond spacers
4.5 Simulations in parallel spacers
4.6 Comparison of spacers in terms of mass transfer coefficients and pressure drops
4.7 Modeling fluid flow and mass transfer at high Reynolds number
4.8 Simulations using turbulence model

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6 5 EXPERIMENTAL INVESTIGATIONS AND VALIDATION OF CFD RESULTS

5.1 Description of experimental set-up

5.2 Geometric parameters of spacers
5.3 Effect of feed pressure and concentration on membrane performance
5.4 Effect of spacer geometry on pressure drops and mass transfer coefficients

5.5 Comparison of CFD simulations with experimental results
5.6 Comparison of numerical results with other experimental data
5.7 Effect of feed spacer on fouling of reverse osmosis membranes
5.8 Accuracy of experiments and uncertainty analysis

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7 6 CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

6.2 Recommendations for future work

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8 7 REFERENCES AND APPENDIX

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