CFD modeling for flow and mass transfer in spacer-obstructed membrane feed channels

Investigation of CO2 Absorption Rate in Gas/Liquid Membrane Contactors with Inserting 3D Printing Mini-Channel Turbulence Promoters

The CO2 absorption by Monoethanolamine (MEA) solutions as chemical absorption was conducted in the membrane gas absorption module with inserting 3D mini-channel turbulence promoters of the present work. A mathematical modeling of CO2 absorption flux was analyzed by using the chemical absorption theory based on mass-transfer resistances in series. The membrane absorption module with embedding 3D mini-channel turbulence promoters in the current study indicated that the CO2 absorption rate improvement is achieved due to the diminishing concentration polarization effect nearby the membrane surfaces. A simplified regression equation of the average Sherwood number was correlated to express the enhanced mass-transfer coefficient of the CO2 absorption. The experimental results and theoretical predictions showed that the absorption flux improvement was significantly improved with implementing 3D mini-channel turbulence promoters. The experimental results of CO2 absorption fluxes were performed in good agreement with the theoretical predictions in aqueous MEA solutions. A further absorption flux enhancement up to 30.56% was accomplished as compared to the results in the previous work, which the module was inserted the promoter without mini channels. The influences of the MEA absorbent flow rates and inlet CO2 concentrations on the absorption flux and absorption flux improvement are also illustrated under both concurrent- and countercurrent-flow operations.

Electrodialysis membrane desalination with diagonal membrane spacers: a review

Electrodialysis desalination uses ion exchange membranes, membrane spacers, and conductors to remove salt from water. Membrane spacers, made of polymeric strands, reduce concentration polarization. These spacers have properties such as porosity and filament shape that affect their performance. One important property is the spacer-bulk attack angle. This study systematically reviews the characteristics of a 45° attack angle of spacers and its effects on concentration polarization and fluid dynamics. Membrane spacers in a channel create distinct flow fields and concentration profiles. When set at a 45° attack angle, spacers provide greater turbulence and mass-heat transfer than traditional spacers. This is because both the transverse and longitudinal filaments become diagonal in relation to the bulk flow direction. A lower attack angle (<45°) results in a lower pressure drop coupled with a decline in wakes and stream disruption because when the filaments are more parallel to the primary fluid direction, the poorer their affect. This research concludes that membrane spacers with a 45° spacer-bulk attack angle function optimally compared to other angles.

Investigation on the Performance of CO2 Absorption in Ceramic Hollow-Fiber Gas/Liquid Membrane Contactors

The absorption efficiencies of CO₂ in ceramic hollow-fiber membrane contactors using monoethanolamine (MEA) absorbent under both cocurrent- and countercurrent-flow operations were investigated theoretically and experimentally; various MEA absorbent flow rates, CO₂ feed flow rates, and inlet CO₂ concentrations were used as parameters. Theoretical predictions of the CO₂ absorption flux were analyzed by developing the mathematical formulations based on Happel's free surface model in terms of mass transfer resistances in series. The experiments of the CO₂ absorption were conducted by using alumina (Al₂O₃) hollow-fiber membranes to confirm the accuracy of the theoretical predictions. The simplified expression of the Sherwood number was formulated to calculate the mass transfer coefficient of the CO₂ absorption incorporating experimental data. The data were obtained numerically using the fourth-order Runge-Kutta method to predict the concentration distribution and absorption rate enhancement under various fiber packing configurations accomplished by the CO₂/N₂ stream passing through the fiber cells. The operations of the hollow-fiber membrane contactor encapsulating N = 7 fiber cells and N = 19 fiber cells of different packing densities were fabricated in this work to examine the device performance. The accuracy derivation between experimental results and theoretical predictions for cocurrent- and countercurrent-flow operations were 1.31×10-2≤E≤4.35×10-2 and 3.90×10-3≤E≤2.43×10-2, respectively. A maximum of 965.5% CO₂ absorption rate enhancement was found in the module with embedding multiple fiber cells compared with that in the device with inserting single-fiber cell. Implementing more fiber cells offers an inexpensive method of improving the absorption efficiency, and thus the operations of the ceramic hollow-fiber membrane contactor with implementing more fiber cells propose a low-priced design to improve the absorption rate enhancement. The higher overall CO₂ absorption rate was achieved in countercurrent-flow operations than that in cocurrent-flow operations.

Multiscale Analysis of Permeable and Impermeable Wall Models for Seawater Reverse Osmosis Desalination

In recent years, high permeability membranes (HPMs) have attracted wide attention in seawater reverse osmosis (SWRO) desalination. However, the limitation of hydrodynamics and mass transfer characteristics for conventional spiral wound modules defeats the advantage of HPMs. Feed spacer design is one of the effective ways to improve module performance by enhancing permeation flux and mitigating membrane fouling. Herein, we propose a multiscale modeling framework that integrates a three-dimensional multi-physics model with a permeable wall and an impermeable wall, respectively, at a sub-millimeter scale and a system-level model at a meter scale. Using the proposed solution framework, a thorough quantitative analysis at different scales is conducted and it indicates that the average errors of the friction coefficient and the Sherwood number using the impermeable wall model are less than 2% and 9%, respectively, for commercial SWRO membrane (water permeability 1 L m−2 h−1 bar−1) and HPMs (3 L m−2 h−1 bar−1, 5 L m−2 h−1 bar−1 and 10 L m−2 h−1 bar−1) systems, compared to the predictions using the permeable wall model. Using both the permeable and impermeable wall models, the system-level simulations, e.g., specific energy consumption, average permeation flux, and the maximum concentration polarization factor at the system inlet are basically the same (error < 2%), while the impermeable wall model has a significant advantage in computational efficiency. The multiscale framework coupling the impermeable wall model can be used to guide the efficient and accurate optimal spacer design and system design for HPMs using, e.g., a machine learning approach.

Electrodialysis desalination: The impact of solution flowrate (or Reynolds number) on fluid dynamics throughout membrane spacers

The incorporation of a spacer among membranes has a major influence on fluid dynamics and performance metrics. Spacers create feed channels and operate as turbulence promoters to increase mixing and reduce concentration/temperature polarization effects. However, spacer geometry remains unoptimized, and studies continue to investigate a wide range of commercial and custom-made spacer designs. The in-depth discussion of the present systematic review seeks to discover the influence of Reynolds number or solution flowrate on flow hydrodynamics throughout a spacer-filled channel. A fast-flowing solution sweeping one membrane's surface first, then the neighboring membrane's surface produces good mixing action, which does not happen commonly at laminar solution flowrates. A sufficient flowrate can suppress the polarization layer, which may normally require the utilization of a simple feed channel rather than complex spacer configurations. When a recirculation eddy occurs, it disrupts the continuous flow and effectively curves the linear fluid courses. The higher the flowrate, the better the membrane performance, the higher the critical flux (or recovery rate), and the lower the inherent limitations of spacer design, spacer shadow effect, poor channel hydrodynamics, and high concentration polarization. In fact, critical flow achieves an acceptable balance between improving flow dynamics and reducing the related trade-offs, such as pressure losses and the occurrence of concentration polarization throughout the cell. If the necessary technical flowrate is not used, the real concentration potential for transport is relatively limited at low velocities than would be predicted based on bulk concentrations. Electrodialysis stack therefore may suffer from the dissociation of water molecules. Next studies should consider that applying a higher flowrate results in greater process efficiency, increased mass transfer potential at the membrane interface, and reduced stack thermal and electrical resistance, where pressure drop should always be indicated as a consequence of the spacer and circumstances used, rather than a problem.

Modeling and assessment of the transfer effectiveness in integrated bioreactor with membrane separation

Integrating a reaction process with membrane separation allows for effective product removal, favorable shifting of the reaction equilibrium, overcoming eventual inhibitory or toxic effects of the products and has the advantage of being energy and space saving. It has found a range of applications in innovative biotechnologies, generating value-added products (exopolysaccharides, antioxidants, carboxylic acids) with high potential for separation/ concentration of thermosensitive bioactive compounds, preserving their biological activity and reducing the amount of solvents and the energy for solvent recovery. Evaluating the effectiveness of such integrated systems is based on fluid dynamics and mass transfer knowledge of flowing matter close to the membrane surface – shear deformation rates and shear stress at the membrane interface, mass transfer coefficients. A Computational Fluid Dynamics (CFD)-based approach for assessing the effectiveness of integrated stirred tank bioreactor with submerged membrane module is compiled. It is related to the hydrodynamic optimization of the selected reactor configuration in two-phase flow, as well as to the concentration profiles and analysis of the reactor conditions in terms of reaction kinetics and mass transfer.

Theoretical and Experimental Studies of CO2 Absorption in Double-Unit Flat-Plate Membrane Contactors

Theoretical predictions of carbon dioxide absorption flux were analyzed by developing one-dimensional mathematical modeling using the chemical absorption theory based on mass-transfer resistances in series. The CO2 absorption into monoethanolamine (MEA) solutions was treated as chemical absorption, accompanied by a large equilibrium constant. The experimental work of the CO2 absorption flux using MEA solution was conducted in double-unit flat-plate membrane contactors with embedded 3D turbulence promoters under various absorbent flow rates, CO2 feed flow rates, and inlet CO2 concentrations in the gas feed stream for both concurrent and countercurrent flow operations. A more compact double-unit module with embedded 3D turbulence promoters could increase the membrane stability to prevent flow-induced vibration and enhance the CO2 absorption rate by overwhelming the concentration polarization on the membrane surfaces. The measured absorption fluxes with a near pseudo-first-order reaction were in good agreement with the theoretical predictions for the CO2 absorption efficiency in aqueous MEA solutions, which was shown to be substantially larger than the physical absorption in water. By embedding 3D turbulence promoters in the MEA feed channel, the new design accomplishes a considerable CO2 absorption flux compared with an empty channel as well as the single unit module. This demonstrates the value and originality of the present study regarding the technical feasibility. The absorption flux enhancement for the double-unit module with embedded 3D turbulence promoters could provide a maximum relative increase of up to 40% due to the diminution in the concentration polarization effect. The correlated equation of the average Sherwood number was obtained numerically using the fourth Runge–Kutta method in a generalized and simplified expression to calculate the mass transfer coefficient of the CO2 absorption in the double-unit flat-plate membrane contactor with turbulence promoter channels.

Combined Membrane Dehumidification with Heat Exchangers Optimized Using CFD for High Efficiency HVAC Systems

Traditional air conditioning systems use a significant amount of energy on dehumidification by condensing water vapor out from the air. Membrane-based air conditioning systems help overcome this problem by avoiding condensation and treating the sensible and latent loads separately, using membranes that allow water vapor transport, but not air (nitrogen and oxygen). In this work, a computational fluid dynamics (CFD) model has been developed to predict the heat and mass transfer and concentration polarization performance of a novel active membrane-based energy exchanger (AMX). The novel design is the first of its kind to integrate both vapor removal via membranes and air cooling into one device. The heat transfer results from the CFD simulations are compared with common empirical correlations for similar geometries. The performance of the AMX is studied over a broad range of operating conditions using the compared CFD model. The results show that strong tradeoffs result in optimal values for the channel length (0.6–0.8 m) and the ratio of coil diameter to channel height (~0.5). Water vapor transport is best if the flow is just past the turbulence transition around 3000–5000 Reynolds number. These trends hold over a range of conditions and dimensions.

Enhancing Absorption Performance of CO2 by Amine Solution through the Spiral Wired Channel in Concentric Circular Membrane Contactors

The CO₂ absorption rate by using a Monoethanolamide (MEA) solution through the spiral wired channel in concentric circular membrane contactors under both concurrent-flow and countercurrent-flow operations was investigated experimentally and theoretically. The one-dimensional mathematical modeling equation developed for predicting the absorption rate and concentration distributions was solved numerically using the fourth Runge-Kutta method under various absorbent flow rate, CO₂ feed flow rate and inlet CO₂ concentration in the gas feed. An economical viewpoint of the spiral wired module was examined by assessing both absorption flux improvement and power consumption increment. Meanwhile, the correlated average Sherwood number to predict the mass-transfer coefficient of the CO₂ absorption mechanisms in a concentric circular membrane contactor with the spiral wired annulus channel is also obtained in a generalized and simplified expression. The theoretical predictions of absorption flux improvement were validated by experimental results in good agreements. The amine solution flowing through the annulus of a concentric circular tube, which was inserted in a tight-fitting spiral wire in a small annular spacing, could enhance the CO₂ absorption flux improvement due to reduction of the concentration polarization effect. A larger concentration polarization coefficient (CPC) was achieved in the countercurrent-flow operations than that in concurrent-flow operations for various operations conditions and spiral-wire pitches. The absorption flux improvement for inserting spiral wire in the concentric circular module could provide the maximum relative increment up to 46.45%.

Fouling mitigation in reverse osmosis processes with 3D printed sinusoidal spacers

Feed spacers are an essential part of spiral wound modules for reverse osmosis (RO). They create flow channels between membrane sheets and manipulate hydrodynamic conditions to control membrane fouling. In this work, additive manufacturing (Polyjet) was used to print novel sinusoidal spacers with wavy axial filaments connected by perpendicular (ST) or slanted (SL) transverse filaments. When tested with 2 g/L NaCl solution, conventional and SL spacers had similar flux while the ST spacer had about 5-7% lower flux. The pressure losses for ST and SL spacers increased by up to 3 folds depending on the flow condition. In the colloidal silica fouling and biofouling tests, the sinusoidal spacers showed lower membrane permeability decrease of 46% for ST, 41% for SL vs 56% for conventional and 26% for ST, 22% for SL vs 33% for conventional, respectively. Optical coherence tomography images from colloidal silica fouling and confocal images from biofouling tests revealed that fouling patterns were closely associated with the local hydrodynamic conditions. Overall, sinusoidal spacers showed promising results in controlling membrane fouling, but there is potential for further optimizations to reduce channel pressure loss.

Roles and performance enhancement of feed spacer in spiral wound membrane modules for water treatment: A 20-year review on research evolvement

Membrane technologies have been widely applied in water treatment, wastewater reclamation and seawater desalination. Feed spacer present in spiral wound membrane (SWM) modules plays a pivotal role in creating flow channels, promoting fluid mixing and enhancing mass transfer. However, it induces the increase of feed channel pressure (FCP) drop and localized stagnant zones that provokes membrane fouling. For the first time, we comprehensively review the research evolvement on feed spacer in SWM modules for water treatment over the last 20 years, to reveal the impacts of feed spacer on the hydrodynamics and biofouling in the spacer-filled channel, and to discuss the potential approaches and current limitations for the modification of feed spacer. The research process can be divided into three phases, with research focus shifting from hydrodynamics in Phase Ⅰ (the year of 2001-2008), to biofouling in Phase Ⅱ (the year of 2009-2015), and then to novel spacer designs in Phase Ⅲ (the year of 2016-2020). The spacer configuration has a momentous impact on the hydraulic performance regarding flow velocity field, shear stress, mass transfer and FCP drop. Biofouling initially occurs on feed spacer, especially around spacer filaments and the contact zones with membrane surface, and ultimately degrades the overall membrane performance indicating the importance of controlling spacer biofouling. The modification of feed spacer is mainly achieved by altering surface chemistry or introducing novel configurations. However, the stability of spacer coating and the economy and practicality of 3D-printed spacer remain a predicament to be tackled. Future studies are suggested to focus on the standardization of testing conditions for spacer evaluation, the effect of hydrodynamics on membrane fouling control, the design and fabrication of novel feed spacer adaptable for SWM modules, the application of feed spacer for drinking water production, organic fouling control in spacer-filled channel and the role of permeate spacer on membrane performance.

Correlations for Concentration Polarization and Pressure Drop in Spacer-Filled RO Membrane Modules Based on CFD Simulations

Empirical correlations for mass transfer coefficient and friction factor are often used in process models for reverse osmosis (RO) membrane systems. These usually involve four dimensionless groups, namely Reynolds number (Re), Sherwood number (Sh), friction factor (f), and Schmidt number (Sc), with the associated coefficients and exponents being obtained by fitting to experimental data. However, the range of geometric and operating conditions covered by the experiments is often limited. In this study, new dimensionless correlations for concentration polarization (CP) modulus and friction factor are presented, which are obtained by dimensional analysis and using simulation data from computational fluid dynamics (CFD). Two-dimensional CFD simulations are performed on three configurations of spacer-filled channels with 76 combinations of operating and geometric conditions for each configuration, covering a broad range of conditions encountered in RO membrane systems. Results obtained with the new correlations are compared with those from existing correlations in the literature. There is good consistency in the predicted CP with mean discrepancies less than 6%, but larger discrepancies for pressure gradient are found among the various friction factor correlations. Furthermore, the new correlations are implemented in a process model with six spiral wound modules in series and the predicted recovery, pressure drop, and specific energy consumption are compared with a reference case obtained by ROSA (Reverse Osmosis System Analysis, The Dow Chemical Company). Differences in predicted recovery and pressure drop are up to 5% and 83%, respectively, highlighting the need for careful selection of correlations when using predictive models in process design. Compared to existing mass transfer correlations, a distinct advantage of our correlations for CP modulus is that they can be directly used to estimate the impact of permeate flux on CP at a membrane surface without having to resort to the film theory.

Augmenting CO2 Absorption Flux through a Gas-Liquid Membrane Module by Inserting Carbon-Fiber Spacers

We investigated the insertion of eddy promoters into a parallel-plate gas–liquid polytetrafluoroethylene (PTFE) membrane contactor to effectively enhance carbon dioxide absorption through aqueous amine solutions (monoethanolamide—MEA). In this study, a theoretical model was established and experimental work was performed to predict and to compare carbon dioxide absorption efficiency under concurrent- and countercurrent-flow operations for various MEA feed flow rates, inlet CO₂ concentrations, and channel design conditions. A Sherwood number’s correlated expression was formulated, incorporating experimental data to estimate the mass transfer coefficient of the CO₂ absorption in MEA flowing through a PTFE membrane. Theoretical predictions were calculated and validated through experimental data for the augmented CO₂ absorption efficiency by inserting carbon-fiber spacers as an eddy promoter to reduce the concentration polarization effect. The study determined that a higher MEA feed rate, a lower feed CO₂ concentration, and wider carbon-fiber spacers resulted in a higher CO₂ absorption rate for concurrent- and countercurrent-flow operations. A maximum of 80% CO₂ absorption efficiency enhancement was found in the device by inserting carbon-fiber spacers, as compared to that in the empty channel device. The overall CO₂ absorption rate was higher for countercurrent operation than that for concurrent operation. We evaluated the effectiveness of power utilization in augmenting the CO₂ absorption rate by inserting carbon-fiber spacers in the MEA feed channel and concluded that the higher the flow rate, the lower the power utilization’s effectiveness. Therefore, to increase the CO₂ absorption flux, widening carbon-fiber spacers was determined to be more effective than increasing the MEA feed flow rate.

Impacts of non-uniform filament feed spacers characteristics on the hydraulic and anti-fouling performances in the spacer-filled membrane channels: Experiment and numerical simulation

Feed spacer is universally used in spiral-wound nanofiltration (NF) and reverse osmosis (RO) membrane modules. It can separate membrane sheets, create flow channels, promote turbulence and enhance mass transfer. However, it also induces increased pressure drop across the flow channel, and generates dead zones for biofilm growth at specific locations. Optimization of feed spacer geometries is highly desirable for energy saving and biofouling control. In this study, four kinds of commercial feed spacers featured with non-uniform filaments were compared in terms of hydraulic and anti-fouling performances. Computational fluid dynamics (CFD) simulations were launched to give insights into the impacts of feed spacer characteristics on the flow field. Results show that the hydraulic performance was substantially affected by the number of filament layers (single or dual layer), the non-uniformity of filament diameter and the width of thinning zones. The design of single layer feed spacer of non-uniform filaments was not recommended due to high flow resistance and poor anti-fouling performance. The feed spacer structure of alternating filament diameter contributed to reducing dead zones and alleviating membrane fouling. The thinning zones located adjacent to the filament junctions achieved better anti-fouling performance, as it disturbed the dead zones and partially washed away the deposited foulants. This study demonstrates for the first time that the characteristics of non-uniform filament feed spacer had a crucial impact on the hydraulic and anti-fouling performances, and suggests that more emphasis should be laid on number of filament layers, variation of filament diameter and width and positioning of thinning zones for the optimization of feed spacer geometries in the future.

Minimal and zero liquid discharge with reverse osmosis using low-salt-rejection membranes

Minimal and zero liquid discharge (MLD/ZLD) are wastewater management strategies that are attracting heightened attention worldwide. While conventional reverse osmosis (RO) has been proposed as a promising technology in desalination and MLD/ZLD processes, its application is limited by the maximum hydraulic pressures that current RO membranes and modules can withstand. In this study, we develop low-salt-rejection RO (LSRRO), a novel staged RO process, that employs low-salt-rejection membranes to desalinate or concentrate highly saline feed streams, requiring only moderate hydraulic pressures. Based on process modeling, we demonstrate that LSRRO can overcome the hydraulic pressure limitations of conventional RO, achieving hypersaline brine salinities (>4.0 M NaCl or 234 g L^-1 NaCl) that are required for MLD/ZLD applications, without using excessively high hydraulic pressures (≤70 bar). In addition, we show that the energy efficiency of LSSRO is substantially higher than traditional thermally-driven phase-change-based technologies, such as mechanical vapor compressor (MVC). For example, to concentrate a saline feed stream from 0.1 to 1.0 M NaCl, the specific energy consumption (SEC) of 4-stage LSRRO ranges from 2.4 to 8.0 kWh of electrical energy per m³ of feedwater treated, around four times less than that of MVC, which requires 20-25 kWh_e m^-3. Furthermore, compared to osmotically mediated RO technologies that require bilateral countercurrent stages to treat hypersaline brines, LSRRO is eminently more practical as it can be readily implemented by using 'loose' RO or nanofiltration membranes in conventional RO. Our study highlights LSRRO's potential for energy efficient brine concentration using moderate hydraulic pressures, which would drastically improve the energetic and economic performance of MLD/ZLD processes.

Modelling Forward Osmosis Treatment of Automobile Wastewaters

Forward osmosis (FO) has rarely been investigated as a treatment technology for industrial wastewaters. Within this study, common FO model equations were applied to simulate forward osmosis treatment of industrial wastewaters from the automobile industry. Three different models from literature were used and compared. Permeate and reverse solute flux modelling was implemented using MS Excel with a Generalized Reduced Gradient (GRG) Nonlinear Solver. For the industrial effluents, the unknown diffusion coefficients were calibrated and the influences of the membrane parameters were investigated. Experimental data was used to evaluate the models. It could be proven that common model equations can describe FO treatment of industrial effluents from the automobile industry. Even with few known solution properties, it was possible to determine permeate fluxes and draw conclusions about mass transport. However, the membrane parameters, which are apparently not solution independent and seem to differ for each industrial effluent, are critical values. Fouling was not included in the model equations although it is a crucial point in FO treatment of industrial wastewaters. But precisely for this reason, modelling is a good complement to laboratory experiments since the difference between the results allows conclusions to be drawn about fouling.

Hydraulics characteristics of forward osmosis membrane module boundary based on FBG sensing technology: Hydraulic properties and operating condition optimization

To obtain more information on the hydraulic properties of membrane interface, the fiber Bragg grating (FBG) sensing technology was imported to investigate the effect of feed solution (FS) flow rate, draw solution (DS) flow rate and cross-flow direction on the membrane flux and membrane shear-force distribution of forward osmosis (FO) process. Results from experimental work demonstrated that a non-uniform spatial variation of the shear-force distribution exists along the membrane, and higher shear force is distributed in the middle position which resulted in higher diffusion load on the particular location of the membrane rind. Besides, increasing the inlet flow simply to a certain value didn't result in a higher shear force and lower the effect of concentration polarization (CP). Compared to co-current mode, counter-current mode showed the better hydraulic characteristics of higher shear-force, faster scouring frequency and consistent shear-force distribution, which will enhance the utilization of membrane and exhibit higher flux by increasing the inlet flow. Moreover, with the increase of FS and DS flow, the stress distribution showed more uniformed. Higher FS flow is more beneficial to FO process which will reduce ECP and improve flux in comparison to increasing DS flow which will produce adverse influence on ICP and diminish flux.

Modeling Dynamics of Colloidal Fouling of RO/NF Membranes with A Novel Collision-Attachment Approach

We report a novel collision-attachment approach for modeling the dynamics of colloidal fouling. The model treats fouling as a two-step process: colloidal particles colliding with a membrane surface followed by their attachment onto the surface. An attachment coefficient is adopted to describe the probability of successful foulant attachment for any given collision event, the value of which can be determined by the classical Boltzmann distribution. Our model shows excellent agreement with experimental data in terms of both the kinetics of flux decline and foulant mass deposition. Modeling results reveal the critical roles of water flux and energy barrier in governing colloidal fouling. Greater water flux or lower energy barrier can lead to a collision-controlled condition, where severe fouling occurs and nearly all collision events lead to successful foulant attachment. On the contrary, fouling is increasingly controlled by the probability of successful attachment at lower water flux and/or greater energy barrier. Our model provides deep insights into the various mechanisms governing the dynamics of colloidal fouling (i.e., concentration polarization, collision, and attachment) and the self-limiting fouling behavior under constant-pressure mode.

Synthesis of graphene oxide membranes and their behavior in water and isopropanol

Graphene oxide (GO) membrane has been synthesized on commercial polysulfone ultrafiltration membranes (Pore size: 17 nm) using the drop casting method followed by baking at 90 C for 24 h. Baking resulted in the reduction of GO and removal of bulk water intercalated in the GO sheets. Deposited GO film showed high stability under shear stress variation. This work shows that water adsorption on the GO membrane determines its permeation performance. Despite the higher viscosity of isopropyl alcohol (IPA), its permeability was 7 times higher than water through the baked ("dry") GO membranes, which were never contacted with water. However, IPA permeability of GO membranes dropped to 44% (of deionized water) when contacted with water ("hydrated" or "wet" GO membranes). Extensive size exclusion (rejection) studies with various dye and dendrimer molecules showed pore size reduced from 3.3 nm in the "dry" state to 1.3 nm in the "wet" state of GO membranes. FT-IR characterization of GO membrane suggested adsorption of water on the nanochannels of the active layer. Also, significant decay in flux was observed for water (82% of its initial flux) as compared to IPA (38% of its initial flux) for initially dry GO membranes.