Concentration polarization in spacer filled reverse osmosis membrane systems

CONCENTRATION POLARIZATION IN SPACERFILLED REVERSE OSMOSIS MEMBRANE SYSTEMS MA SHENGWEI (B. Sc., Nanjing Inst. of Meteorology M. Sc., Chinese Academy of Sciences) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2005 ACKNOWLEDGEMENTS I wish to express my appreciation and gratitude to my supervisor, Associate Professor Song Lianfa for his continuous, enthusiastic and invaluable supervision and encouragement throughout the entire course of this project. I also wish to express my gratitude to National University of Singapore for providing all computing resources at Supercomputing & Visualization Unit for this project. I am deeply indebted to my parents, my wife and my daughter for their continuing support and encouragement in the years of research in NUS. i TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS ii SUMMARY .v LIST OF TABLE .vii LIST OF FIGURES . viii LIST OF SYMBOLS .xv CHAPTER INTRODUCTION 1.1 Introduction .1 1.2 Aim of the research .3 1.3 Overview of the dissertation CHAPTER LITERATURE REVIEW .4 2.1 Concentration polarization in RO systems .4 2.2 Analytical models for concentration polarization in RO membrane systems .6 2.3 Numerical models for concentration polarization in RO membrane systems .8 2.4 The impact of spacer on concentration polarization and RO membrane performance .14 2.5 Finite element method for coupled momentum transfer and solute transport problems .17 2.6 Summary .19 CHAPTER NUMERICAL MODEL .21 3.1 Introduction 21 ii 3.2 Model development 22 3.2.1 Governing equations 22 3.2.2 Penalty formulation for Navier-Stokes equations .24 3.2.3 Initial and boundary conditions 25 3. 2.4 SUPG finite element formulation and numerical strategies .26 3.3 Model validation .33 3.4 Effect of meshing scheme on accuracy .36 3.5 Summary .40 CHAPTER CONCENTRATION POLARIZATION IN SPACER-FILLED CHANNELS 42 4.1 Introduction .42 4.2 Velocity profiles in spacer-filled channels 45 4.3 Major mechanisms of concentration polarization in spacer filled channels .52 4.4 Summary .65 CHAPTER IMPACT OF FILAMENT GEOMETRY ON CONCENTRATION POLARIZATION .68 5.1 Introduction .68 5.2 Filament shape .69 5.3 Filament thickness .77 5.4 Summary .96 CHAPTER FILAMENT CONFIGURATION AND MESH LENGTH ON CONCENTRATION POLARIZATION AND MEMBRANE PERFORMANCE 98 6.1 Introduction .98 6.2 Concentration polarization patterns for different filament configurations 101 6.3 Filament configuration on membrane performance 111 iii 6.4 Impact of mesh length on membrane performance .115 6.4.1 Mesh length on permeate flux 115 6.4.2 Impact of mesh length on pressure loss 123 6.5 Summary .127 CHAPTER CONCLUSIONS AND RECOMMENDATIONS .129 7.1 Conclusions .129 7.2 Recommendations for further research .131 REFERENCES 133 APPENDIX A : LIST OF PUBLICATIONS AND CONFERENCE PRESENTATIONS 150 iv SUMMARY As a phenomenon inherently associated with membrane separations, concentration polarization has long been identified as a major problem that deteriorates the performance of RO systems. However, this phenomenon is still not well understood especially in practical spiral wound modules, where spacer is an essential part to form the feed channel. The purpose of this study was to study concentration polarization in spacer filled RO systems and to quantify the impact of feed spacer on concentration polarization and membrane performance. In this study, a fully coupled 2-D streamline upwind Petrov/Galerkin (SUPG) finite element model was developed so that it becomes possible to simultaneously simulate hydrodynamic conditions, including permeate velocity at membrane surface, and salt concentration profiles, including wall concentrations in RO membrane channels. The numerical model was compared with the available experimental data of RO systems in the literature. With this numerical model, the role of feed spacer on concentration polarization and system performance can be quantitatively investigated in realistic conditions. It was found that concentration polarization in spacer filled membrane channel was affected by two major mechanisms: concentration boundary layer disruption due to flow separation and, concentration boundary layer disruption due to the constricted flow passage. The two mechanisms may work separately or jointly dependent on spacer configurations. Filament geometry was found to have significant impact on concentration polarization although it would not change the overall concentration polarization patterns. Extremely high wall concentrations were found close to the contact point of membranes with cylindrical filaments. Increasing filament thickness v could significantly alleviate concentration polarization at cost of elevated pressure loss. It was also found that membrane performance was strongly affected by filament configurations and mesh length. In most cases, zigzag configuration provided the best permeate flux enhancement while submerged configuration resulted in the lowest peak wall concentrations. There was an optimum mesh length with cavity and zigzag configurations for maximizing permeate flux enhancement Decreasing mesh length may lead to significant increase of pressure loss especially for zigzag configurations, and may lead to permeate flux decline in certain cases. The results suggest that the commonly used overall parameter of spacer (e.g., voidage) is inadequate or inappropriate to characterize spacers in a RO system. The results also imply that a universally optimized spacer design does not exist and optimization of the spacers has to be carried out particularly for different situations. Through this study, the understandings of concentration polarization in spacer-filled RO channels and the effects of spacer on concentration polarization and system performance have been significantly advanced. The numerical model developed in this study can provide a powerful tool to realistically study concentration polarization in spiral wound RO modules. The quantitative visualization and assessment of the impact of spacer on system performance would provide the technical foundation for the optimum design of RO membrane systems. vi LIST OF TABLE Table 6.1 Membrane properties and operating conditions 100 vii LIST OF FIGURES Figure 3.1 Flowchart of the solver and model organization 32 Figure 3.2 Comparison of numerical simulation results with experimental data .34 Figure 3.3 Simulated wall concentration at different values of membrane permeability (crossflow velocity: 200cm/s; other conditions: Merten et al (1964)) 34 Figure 3.4 Comparing the accuracy of simulation results due to different meshing schemes. The solution became independent of the meshing schemes when the non-uniform (exponential) elements increased to 60 and more in the channel height direction. (crossflow velocity: 20cm/s; membrane permeability: 8.29×10-6g/cm2 sec atm) 38 Figure 3.5 A successful mesh scheme (part) to capture the flow direction transition near membrane surface 39 Figure 3.6 Velocity field (part) in the flow direction transition region in an empty channel (simulation conditions: membranes at y=0 and y=H; ∆p=429psi; A=5×10-12m/s Pa; c0=9800mg/l; u0=0.01m/s; H=1mm; L=5cm) .39 Figure 4.1 Illustration of the spacer configuration .44 Figure 4.2 Illustration of the mesh adjacent to a cylindrical filament .44 Figure 4.3 Contour of flow velocity in a feed channel (part) with 0.5mm (in diameter) cylinder transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .46 Figure 4.4 Contour of flow velocity in a feed channel (part) with 0.75mm (in diameter) cylinder transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .47 Figure 4.5 Contour of x-component flow velocity in a feed channel (part) with 0.5mm (in diameter) cylinder transverse filaments (simulation conditions: viii ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .49 Figure 4.6 Contour of x-component flow velocity in a feed channel with 0.75mm (in diameter) cylinder transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .50 Figure 4.7 Velocity field near the reattachment point in a feed channel with 0.5mm (in diameter) cylinder transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .51 Figure 4.8 Salt concentration (c/c0) profiles in an empty feed channel, disproportional in height and length (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm) .54 Figure 4.9 Salt concentration (c/c0) profiles in a feed channel with 0.5mm (in diameter) cylinder transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .55 Figure 4.10 Local variation of wall concentration (cw/c0) in an empty channel and a feed channel with 0.5mm (in diameter) cylindrical transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×1012 m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) 56 Figure 4.11 Local variations of permeate flux in an empty channel and a feed channel with 0.5mm (in diameter) cylindrical transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) 57 Figure 4.12 Enlarged view of local wall concentration (cw/c0) profiles (on the membrane attached to the transverse filaments) in feed channels with 0.5mm (in diameter) cylindrical filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .59 Figure 4.13 Velocity field near the small recirculation regions in a feed channel with 0.5mm (in diameter) cylinder transverse filaments (simulation conditions: ∆p=800psi; c0=32,000mg/l; A=7.3×10-12m/s Pa; u0=0.1m/s; h=1mm; lf=4.5mm) .62 ix O.R. 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Zydney (1997), Stagnant film model for concentration polarization in membrane systems, Journal of Membrane Science, 130:275-281 149 APPENDIX A : LIST OF PUBLICATIONS AND CONFERENCE PRESENTATIONS Shengwei Ma, Lianfa Song, Say Leong Ong and Wun Jern Ng (2004), A 2-D streamline upwind Petrov/Galerkin finite element model for concentration polarization in spiral wound reverse osmosis modules, Journal of Membrane Science, 244: 129-139 Lianfa Song and Shengwei Ma (2005), Numerical studies of the impact of spacer geometry on concentration polarization in spiral wound membrane modules, Industrial & Engineering Chemistry Research, 44: 7638-7645 Shengwei Ma, Lianfa Song, Say Leong Ong and Wun Jern Ng (2003), Numerical simulation of the effects of spacers on concentration polarization in spiral wound reverse osmosis modules, presentated in the 14th Annual Meeting of North American Membrane Society (NAMS 2003), Wyoming, USA, May 2003 Shengwei Ma and Lianfa Song (2005) Numerical study of the impact of spacer filament geometry on concentration polarization in a reverse osmosis membrane channel, presented in International Congress on Membrane and Membrane Processes 2005 (ICOM2005), Korea, August 2005 150 [...]... capable of modeling concentration polarization in more realistic conditions in the spiral wound RO modules under various operating conditions and with different spacer configurations; 2 identifying and assessing the major mechanisms of concentration polarization in spacer filled RO channels; 3 investigating the effects of filament geometry on concentration polarization; 4 investigating the effects... polarization and membrane fouling are the most important twin problems in most practical RO membrane systems In all RO membrane separation systems, concentration polarization is an inherent phenomenon When water continuously passes through the membrane as permeate, part of the rejected solutes and colloids will accumulate near the membrane 1 surface and form a concentration layer with concentration substantially... domain and/or boundary with complex geometry; therefore, it is almost impossible for the impact of the spacer on concentration polarization or membrane performance to be simulated by these types of models although they may better our understanding of the role of certain parameters in concentration polarization 2.3 Numerical models for concentration polarization in RO membrane systems Concentration polarization. .. than that in the bulk This phenomenon is known as concentration polarization Concentration polarization is less pronounced in the crossflow systems than dead-end systems because the rejected contaminants are continuously carried away from the membrane surface by the cross flow However, even in the cross flow RO system, concentration polarization is inevitable and an important factor for membrane performance... upwind Petrov/Galerkin (SUPG) finite element model for concentration polarization in spiral wound RO modules Chapter 4 studies the concentration polarization patterns and major mechanisms in spacer- filled RO channels Chapter 5 studies the effects of filament geometry on concentration polarization Chapter 6 studies the impact of filament configuration and mesh length on concentration polarization and membrane. .. of wall concentration and permeate velocity is essential for concentration polarization minimization and membrane system optimization However, because concentration polarization occurs in a very thin layer close to membrane surface and concentration gradient in this layer is very sharp, it is still a challenge to capture the details of concentration profiles and permeate flux in concentration polarization. .. numerical methods in computational fluid dynamics to deal with complex computing domains like spacerfilled channels Streamline upwind Petrov-Galerkin (SUPG) finite element has been widely reported in solving convection dominated problems such as solving NavierStokes equations and convection-diffusion equation The recent advances of finite element method in mass and momentum problems are reviewed in Section... hindered the progress in spacer design and optimization to alleviate concentration polarization in spiral wound RO modules 2 1.2 Aim of the research The aim of this research is to study the concentration polarization in spacerfilled RO membrane channels and to quantify the impact of feed spacer on concentration polarization and membrane performance This is to be achieved through: 1 developing a numerical... factors for concentration polarization became intangible and the settings for the study of effect of spacers were unduly oversimplified Early numerical studies of concentration polarization were mainly focused on empty channels (without spacers) For example, Sherwood et al (1965) developed concentration polarization models in empty channels for both turbulent and laminar flow conditions in both cylindrical... sections and membrane sections could relax concentration polarization and therefore increase membrane productivity noticeably This implies that the impermeable spacer filaments, which invariably block some membrane areas, may relax concentration polarization noticeably even if the mixing is not enhanced significantly by the spacer 14 Early studies on the effects of spacers usually focused on obtaining some . models for concentration polarization in RO membrane systems 6 2.3 Numerical models for concentration polarization in RO membrane systems 8 2.4 The impact of spacer on concentration polarization. polarization and membrane fouling are the most important twin problems in most practical RO membrane systems. In all RO membrane separation systems, concentration polarization is an inherent phenomenon CONCENTRATION POLARIZATION IN SPACER- FILLED REVERSE OSMOSIS MEMBRANE SYSTEMS MA SHENGWEI (B. Sc., Nanjing Inst. of Meteorology M. Sc., Chinese Academy of Sciences)