Internal Concentration Polarization Research Articles

Computational Fluid Dynamics Modeling of Pressure-Retarded Osmosis: Towards a Virtual Lab for Osmotic-Driven Process Simulations

Pressure-Retarded Osmosis (PRO) is an osmotically driven membrane-based process that has recently garnered significant attention from researchers due to its potential for clean energy harvesting from salinity gradients. The complex interactions between mixed-mode channel flows and osmotic fluxes in real PRO membrane modules necessitate high-fidelity modeling approaches. In this work, an efficient CFD framework is developed for the 3D simulation of osmotically driven membrane processes. This approach is based on a two-way coupling between a CFD solver, which captures external concentration polarization (ECP) effects, and an analytical representation of internal concentration polarization (ICP). Consequently, the osmotic water flux and reverse salt flux (RSF) can be accurately determined, accounting for all CP effects without any limitations on the geometrical complexity of the membrane chamber or its flow mode/regime. The proposed model is validated against experimental data, showing good agreement across various PRO case studies. Additionally, the model’s flexibility to simulate other types of osmotically driven processes such as forward osmosis (FO) is examined. Thus, the contributions of ECP and ICP effects in local osmotic pressure drop along the membrane chamber are comprehensively compared for FO and PRO modes. Finally, the capability of the CFD model to simulate a lab-scale PRO module is demonstrated across a range of Reynolds numbers from low-speed laminar up to turbulent flow regimes.

Impact of Asymmetric Microstructure on Ion Transport in Ti3C2Tx Membranes.

Consolidation or densification of low-dimensional MXene materials into membranes can result in the formation of asymmetric membrane structures. Nanostructural (short-range) and microstructural (long-range) heterogeneity can influence mass transport and separation mechanisms. Short-range structural dynamics include the presence of water confined between the 2D layers, while long-range structural properties include the formation of defects, micropores, and mesopores. Herein, it is demonstrated that structural heterogeneity in Ti3C2Tx membranes fabricated via vacuum-assisted filtration significantly affects ion transport. Higher ion permeabilities are achieved when the dense "bottom" side of the membrane, rather than the porous "top" side, faces the feed solution. Characterization of the membrane reveals distinct differences in flake alignment, surface roughness, and porosity across the membrane. The directional dependence on permeability suggests that one region of the membrane experiences stronger internal concentration polarization, potentially suppressing permeability through the porous side of the membrane.

Challenges of forward osmosis desalination processes using hydrogels as draw agents

The present study aims to investigate the properties, identification, and comprehension of the obstacles encountered in the forward osmosis process when utilizing a hydrogel drawing agent (FO-HG). Furthermore, a comparison is made between the FO-HG system and conventional forward osmosis systems employing a salt solution drawing agent. The comparison and evaluation of the swelling process of hydrogel and the kinetics of water penetration are conducted in both un-constrained and constrained states. Furthermore, the investigation and analysis are carried out to determine the presence or absence of internal concentration polarization (ICP) and external concentration polarization (ECP) phenomena. These phenomena are studied in situations with and without mixing, as well as in different orientations of the membrane (FO-mode and PRO-mode). The impact of these phenomena on the water flux of the systems FO-HG and FO-NaCl is also examined. An evaluation is conducted to determine the influence of the amount and size of hydrogel used as a draw agent in the FO-HG system on the water flux. The results of the study reveal that smaller hydrogel particles in the FO-HG system exhibit a higher flux compared to larger particles. Additionally, it is observed that the water flux in PRO-mode is unexpectedly higher when salt water is used as feed solution. This phenomenon can be ascribed to a counter-osmotic effect, originating from the FO state. Despite the high water absorption capacity of hydrogel and its potential as an ideal drawing agent in the forward osmosis process, the results demonstrate that the flux of the FO-HG system is inferior to that of the FO-NaCl system. Finally, the focus is on resolving the low flux issue by suggesting a process involving multiple cycles throughout day and night. We investigate the influence of hydrogel particle size, membrane surface, hydrogel layer thickness, as well as swelling and deswelling time in one cycle. The swelling time displays a peak at an optimal absorption duration, while the deswelling time does not show a similar optimal point, highlighting the difference between swelling and deswelling phenomena. Therefore, the hydrogels' high absorption capacity alone is insufficient for achieving desalination success. The research findings emphasize the high importance of synthesis of a membrane with minimal resistance, enabling a high water flux and suitable selectivity.

Mathematical modeling of osmotic membrane bioreactor process for oily wastewater treatment

ABSTRACT To evaluate the disposal effluent from the Aldura refinery in Iraq, which comprises oily wastewater, a mathematical model has been developed for both forward osmosis (FO) and osmotic membrane bioreactor (OsMBR). The procedure is explained mathematically, accounting for both the concentration and polarization aspects. As a result of mathematical modeling, the water flux was determined by the osmotic pressure, the concentration, and the polarization of the feed and draw solutions. Based on traditional methods of predicting water flux using external and internal concentration polarizations, it is determined that water flux will occur in the first model (Model-1). To increase the accuracy of Model-1, the resistivity (K) of the solute has been modified to be independent of the diffusivity of the solute. The old model (Model-1) and the updated model (Model-2) overestimated water flux by 17 and 25%, respectively. It was possible to make a valid comparison between the experiment and theory based on the results of both experiments.

Optimising the effectiveness of osmotic desalination process by using graphene-based nanomaterials

This work examines how graphene-based nanoparticles can be integrated into membranes to improve the effectiveness of water treatment in osmotic desalination processes. This is important since sustainable practices can help address the world's water scarcity. Water treatment, desalination, and resource recovery are areas where osmotic desalination shows great potential. However, membrane performance constraints frequently impede its efficacy. High mechanical strength, superior hydrophilicity, and the ability to lessen internal concentration polarisation are just a few of the remarkable qualities that make graphene-based nanoparticles stand out. In order to increase the membranes' overall functionality, these nanoparticles were created and added to them. Comparing the study to conventional membranes, the main goals were to increase water flux rates and salt ion rejection capacities. It was shown by experimental results that the membranes strengthened with graphene-based nanoparticles performed better. They outperformed conventional membranes in terms of water flow growth and salt ion rejection rates improvement. In order to advance osmotic desalination technologies towards more effective and sustainable water treatment options, this study highlights the revolutionary potential of graphene-based nanoparticles. Graphene-based nanoparticles provide an attractive option for tackling major water issues worldwide by improving membrane characteristics that are essential for osmotic desalination, such as permeability and selectivity. Water management techniques that are environmentally sustainable are supported by their integration into membranes, which also enhances performance metrics. This study opens the door for creative approaches to resource recovery and water treatment by providing important insights into the creation of cutting-edge materials specifically designed for osmotic desalination applications.

Pilot‐Scale Evaluation of Spiral‐Wound Modules in Osmotically Assisted Reverse Osmosis

AbstractThe osmotically assisted reverse osmosis (OARO) process is gaining attention for cost‐effective aqueous solution concentration. However, there is a lack of pilot‐scale studies. This research used two spiral‐wound modules in a pilot‐scale plant. The first one, an adapted commercial reverse osmosis module, showed no positive results, likely due to excessive membrane thickness and high internal concentration polarization. In contrast, the second one consisting of a novel forward osmosis prototype demonstrated promising outcomes, achieving water fluxes exceeding 2 L m−2 h−1 even at high salt concentrations (50 g L−1) and relatively low applied pressures (above 12 bar). The study highlights OARO potential, underscores limitations, and emphasizes the need for dedicated module design.

Viability of Total Ammoniacal Nitrogen Recovery Using a Polymeric Thin-Film Composite Forward Osmosis Membrane: Determination of Ammonia Permeability Coefficient.

Total ammoniacal nitrogen (TAN) occurs in various wastewaters and its recovery is vital for environmental reasons. Forward osmosis (FO), an energy-efficient technology, extracts water from a feed solution (FS) and into a draw solution (DS). Asymmetric FO membranes consist of an active layer and a support layer, leading to internal concentration polarization (ICP). In this study, we assessed TAN recovery using a polymeric thin-film composite FO membrane by determining the permeability coefficients of NH4+ and NH3. Calculations employed the solution-diffusion model, Nernst-Planck equation, and film theory, applying the acid-base equilibrium for bulk concentration corrections. Initially, model parameters were estimated using sodium salt solutions as the DS and deionized water as the FS. The NH4+ permeability coefficient was 0.45 µm/s for NH4Cl and 0.013 µm/s for (NH4)2SO4 at pH < 7. Meanwhile, the NH3 permeability coefficient was 6.18 µm/s at pH > 9 for both ammonium salts. Polymeric FO membranes can simultaneously recover ammonia and water, achieving 15% and 35% recovery at pH 11.5, respectively.

Explicit expression for water permeation flux in forward osmosis desalination process

Forward osmosis (FO) is a membrane process of water separation and purification. FO uses the osmotic pressure difference across a semipermeable membrane. The effective osmotic pressure at the membrane–solution interface on both the feed and permeate sides of the membrane is the main driving force for the generation of the water flux. The major hindrance to the permeation of the water flux is the prevalence of concentration polarization on both sides of the membrane. Concentration polarization inhibits permeate flux by increasing the osmotic pressure at the membrane active layer interface on the feed side of the membrane. This work focused on the development of a mathematical model for water flux in the FO process. Combined film theory model and diffusion transport through the membrane were utilized. The effect of internal concentration polarization and external concentration polarization on the flux was studied. Both internal and external concentration polarization were taken into consideration in both membrane orientations, i.e., active layer facing the feed solution and active layer facing the draw solution. The obtained explicit expression for water permeation flux in forward osmosis desalination process shows excellent agreement with the literature data.

Feasibility and challenges of high-pressure pressure retarded osmosis applications utilizing seawater and hypersaline water sources

Pressure retarded osmosis (PRO) harnesses salinity gradient energy through the mixing of freshwater and saltwater, addressing the demand for sustainable energy sources. PRO typically utilizes river water or secondary wastewater as the feed solution, paired with seawater reverse osmosis (SWRO) brine as the draw solution. However, the limited availability of low-saline water presents a significant obstacle to energy generation. Therefore, the feasibility of a high-pressure PRO process utilizing seawater as the feed solution and hypersaline water as the draw solution was assessed to generate sustainable blue energy. Seawater has the potential to achieve the maximum extractable Gibbs-free energy through high-pressure PRO by maximizing the feed/draw ratio. The performance of the high-pressure PRO was theoretically evaluated, resulting in a tenfold increase in specific energy production compared to conventional SWRO-PRO due to the higher feed/draw ratio. However, high hydraulic pressure increased the membrane structural parameter and further reduced water flux and power density due to significant internal concentration polarization. Nevertheless, the high-pressure PRO process can achieve a power density exceeding 84 W/m2 when the structural parameter remains below 100 μm. The implications of the high-pressure PRO were further discussed to advance the concept of a blue circular economy, enhance environmental resilience, and promote sustainability.

Predicting forward osmosis performance with synthesized polyamide-based membrane: An integrated machine learning (MATLAB and ANN) and economic analysis framework

Forward Osmosis (FO) has emerged as a highly promising membrane-based water treatment process owing to its intrinsic advantages, such as low energy requirements and the potential for high water recovery. The lab-prepared highly efficient thin film composite (TFC) FO membranes that were used in this research work. The resultant polyamide-based FO membrane showed exceptional performance, notably achieving a high-water flux of 60.94 ± 3.5 L/m2.h and a constrained solute flux of 1.52 ± 0.08 g/m2.h. Machine learning methodologies are strategically employed in this study to construct predictive models for FO membrane performance. Notably, the accuracy of the van't Hoff equation's linear assumption is significantly enhanced by introducing an osmotic pressure calculation based on water activity, considering the nuanced effects of external and internal concentration polarization. The research employs MATLAB software alongside an artificial neural network (ANN) to develop predictive models. MATLAB facilitates the creation of a comprehensive theoretical model, offering insights into forecasting water flux through FO membranes by systematically analyzing diverse membrane parameters and their impact on performance. Concurrently, the ANN model was employed to predict the permeate flux for the laboratory-scale FO membrane system, utilizing the input parameters within the network's training range (R2 > 0.91). An economic assessment is also conducted to estimate the projected membrane cost, a pivotal determinant of system viability. Collectively, these investigations yield valuable insights into the intricacies of FO membrane design and optimization, elucidating their performance dynamics in water treatment and allied applications. This research significantly contributes to the ongoing quest to develop more efficient and precise membrane processes.

Membrane Design Principles for Ion-Selective Electrodialysis: An Analysis for Li/Mg Separation.

Selective electrodialysis (ED) is a promising membrane-based process to separate Li+ from Mg2+, which is the most critical step for Li extraction from brine lakes. This study theoretically compares the ED-based Li/Mg separation performance of different monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes at the coupon scale using a unified mass transport model, i.e., a solution-friction model. We demonstrated that monovalent selective CEMs with a dense surface thin film like a polyamide film are more effective in enhancing the Li/Mg separation performance than those with a loose but highly charged thin film. Polyamide film-coated CEMs when used in ED have a performance similar to that of polyamide-based NF membranes when used in NF. NF membranes, when expected to replace monovalent selective CEMs in ED for Li/Mg separation, will require a thin support layer with low tortuosity and high porosity to reduce the internal concentration polarization. The coupon-scale performance analysis and comparison provide new insights into the design of composite membranes used for ED-based selective ion-ion separation.

An Updated Model Using a Reflection Coefficient for Predicting Performance of Pressure-Retarded Osmosis

Mathematical model was used to predict the performance of membrane process. In this study, an updatedmathematical model has been developed to study the forward osmosis (FO) and pressure-retarded osmosis (PRO) processes. This model has been advanced by taking into consideration the reflection coefficient, internal and external concentration polarizations, and structural membrane parameters, as well as the equations of applied pressure and mass transport for feed and draw solutions together. The model was validated through prediction of the water and salt fluxes using the membrane performance data published in other articles. The estimated values of the reflection coefficient ranged from 0.868 to 0.9 for FO and 0.9 to 0.95 for PRO. For this reason, it is not possible to consider the reflection coefficient to be equal to one as normally assumed in membrane modeling study. Other findings have showed that the varying salt fluxes at different concentrations and cross-flow velocities adversely impacted the FO and PRO modules. Internal and external concentration polarizations have been studied and compared with their effect on the effectiveness of the FO/PRO process. The model has demonstrated its ability to predict flux in the PRO greater than FO because of the model primary reliance on the applied pressure to drive the osmotic process. The ratio of must be effectively managed to correspond with operating limits and the value must also be kept to a minimum in order to avert the drastical drop in the flux, the fouling, and the membrane damage.

CFD simulation of hydrodynamics and concentration polarization in osmotically assisted reverse osmosis membrane systems

Osmotically assisted reverse osmosis (OARO) has been recently suggested as an alternative to improve water recovery of reverse osmosis (RO) for applications in which RO has reached its limit. To elucidate the physics, a computational fluid dynamics (CFD) methodology is developed that describes all important physical phenomena occurring in the feed, porous and draw sides of OARO. The CFD model shows good agreement with the reported experimental data and predicts the water flux better than a simplified analytical model. This paper reveals that external concentration polarization (ECP) at the feed side is more important than internal concentration polarization (ICP) within the porous support layer, especially for a system with a high transmembrane pressure, Δp (≥40 bar). In contrast to conventional RO, where concentration polarization (CP) at the permeate side is negligible, OARO experiences a non-negligible ECP at the draw (permeate) side, particularly in cases with high Δp. This analysis also found that both counter-current and co-current configurations show similar flux performance at module scale.

Support-free interfacial polymerized polyamide membrane on a macroporous substrate to reduce internal concentration polarization and increase water flux in forward osmosis

Macroporous substrates have great potential in the development of high performance thin film composite forward osmosis (TFC FO) membranes. This paper reports on the preparation of TFC FO membrane via support-free interfacial polymerization (SFIP) technology on a macroporous substrate. SFIP technology can effectively break the bottleneck of conventional interfacial polymerization (IP) technology, which makes it difficult to form a complete polyamide (PA) film on the macroporous substrate. Briefly, the complete PA film is formed at the free water-organic phase interface and then attached to the macroporous substrate by vacuum filtration. The monomer concentration and filtration pressure were adjusted to form the optimal SFIP-PA film. Meanwhile, we chose a macroporous polyvinylidene fluoride (PVDF) microfiltration substrate and screened the pore size to reduce the structural parameters and then the internal concentration polarization, which caused the improvement of water flux. The performance of SFIP-PVDF membranes was investigated and discussed. Under the AL-FS mode with DI water and 1.0 M NaCl solution as the feed solution and draw solution, respectively, the SFIP-PVDF membrane, which used a macroporous PVDF substrate with a pore size of 0.45 μm exhibited a water flux (20 L/m2·h) and a reverse salt flux (2.5 g/m2·h). Besides, the membrane showed good stability in the 72 h test. The specific salt flux (Js/Jw) of SFIP-PVDF membranes was much lower than that of IP-PVDF membranes prepared via IP technology, which were incomplete and defective, suggesting that SFIP technology was superior to IP technology. This work effectively exploits the potential of macroporous substrates for the development of high performance TFC FO membranes and provides an insight into the high performance FO membrane design.

Modeling study of the effects of intrinsic membrane parameters on dilutive external concentration polarization occurring during forward and pressure-retarded osmosis

In membrane-based desalination systems, the mass transfer coefficient is a critical determinant when estimating changes within the boundary layers. However, it is pivotal to recognize the substantial role of intrinsic membrane parameters that can significantly influence the formation of boundary layers by modulating the water and salt fluxes. In this context, we investigated the effects of intrinsic membrane parameters on dilutive external concentration polarization (dECP) during forward osmosis (FO) and pressure-retarded osmosis (PRO). In particular, the Péclet number is utilized to evaluate the degree of dECP. The results revealed that the effects of the membrane parameters on dECP differ completely in accordance with the membrane orientations or magnitude of hydraulic pressure. Moreover, this study emphasizes that the reduction of solute permeability, i.e., improving the selectivity, should be the priority of FO/PRO membrane optimization because solute permeability is the only parameter that simultaneously suppresses both the internal concentration polarization and dECP. Hence, the results of the current study are expected to serve as valuable guidelines for future FO/PRO membrane optimization research.

Chemical modification of reduced graphene oxide membranes: Enhanced desalination performance and structural properties for forward osmosis

Graphene oxide (GO) nanosheets can offer a breakthrough into the development of novel and efficient laminated forward osmosis membranes, with high hydrophilicity and outstanding desalination performance. However, fabrication of desirable GO-based nanocomposite membrane which can overcome the on-going trade-off between forward osmosis (FO) performance and chemical/mechanical stability, remains to be challenging in practical application. In this study, polyethyleneimine (PEI) was used to reduce GO (rGO) and crosslink and surface modify membrane's GO sheets for the performance enhancement. The crosslinked rGO was then deposited on a hydrophilic nylon support layer (modified by polydopamine, pDA, for structural stability) using vacuum filtration method. The resulting PEI:rGO/pDA membrane showed excellent water flux (25.5 LMH), high sodium chloride (NaCl) salt rejection (98.9%), and low reverse solute flux (0.91 gMH) in lab-scale tests. The combination of crosslinking GO with PEI and modifying the support layer with polydopamine (pDA) enhanced the membranes hydrophilicity and structural stability. The incorporation of PEI into GO resulted in compacted nanochannels, improving molecular/ions separation. Moreover, the pDA coating enhanced desalination performance and reduced internal concentration polarisation effects, further improving the membrane durability and integrity through the formation of numerous binding sites for firmly anchoring with the PEI:rGO nanocomposite structure.

High-performance support-free molecular layer-by-layer assembled forward osmosis membrane incorporated with graphene oxide

Forward osmosis (FO) membranes generally have a thin-film composite (TFC) structure comprising an ultrafiltration (UF)-grade support layer and a polyamide (PA) active layer. However, the lack of high-performance membranes limits FO. To improve the FO performance, internal concentration polarization (ICP) should be controlled to reduce the water flux within the support layer. In this study, a novel support-free molecular layer-by-layer (SF-mLbL) technique using a microfiltration (MF)-grade support layer for minimum ICP was applied to produce a robust and uniform active layer, even on large pores of the support layer. Based on the location of graphene oxide (GO) nanoparticles, thin-film nanocomposite (TFN) and thin-film nanocomposite-interlayer (TFNi) membranes were fabricated. Among these, the TFNi membrane with the highest performance contained 0.7 wt% GO nanoparticles and was stacked in 15 cycles. The 0.7 wt%-15 cycles TFNi membrane showed high water flux (87.18 ± 0.15 LMH) and low reverse salt flux (5.06 ± 0.11 gMH) when deionized (DI) water and 0.5 M NaCl solution were used as feed and draw solutions, respectively, in FO mode. This study demonstrates that the SF-mLbL technique is suitable for manufacturing high-performance FO membranes.

Influence of porous support structure and the possible presence of active layer defects on FO membrane behaviour

In forward osmosis, defects on the selective active layer and changes in the porous structure of the support layer can be detrimental factors affecting the membrane performance. This study focuses on the impacts of (i) the possible presence of defects and (ii) the changes in pore structures on the water flux and membrane selectivity via computational fluid dynamics analyses. Results suggest that diffusion of the draw solute through the support layer, i.e. internal concentration polarization, can be strongly enhanced or reduced by widening or narrowing the shape of the pore, respectively, while no significant effect on water permeation can be associated with the change in draw solution cross-flow velocity or in the loss of draw solute during filtration. Interestingly, defects within the active layer may affect the water flux exponentially as function of the defect size, suggesting the presence of a threshold below which the convective passage of contaminated water flux through the defect is not affecting the membrane productivity. Moreover, the presence of defects may not be a detrimental factor for membrane operating with high nominal rejection (>90%) and low percentage of defected area (<1%).

Membrane properties overview in integrated forward osmosis/osmotically assisted reverse osmosis systems

Integration of forward osmosis (FO) and osmotically assisted reverse osmosis (OARO) provides low energy consumption and controlled fouling for brine treatment. This study explains the membrane properties required for cost–optimal OARO-based systems. A detailed model accounting for the relationship between burst pressure, permeate spacer gap, and structural parameters of OARO membranes was implemented with woven and non-woven spacers. Results indicate that the tradeoff between internal concentration polarization (ICP) and permeate–side pressure drop in OARO stages is a crucial consideration for optimal membrane element design. Due to the lower ICP of no-woven spacers, a system using non-woven spacers yielded lower levelized cost of water (LCOW) and specific energy consumption (5.83 $/m3 and 11.65 kWh/m3, respectively) as compared to the system with woven spacers (7.31 $/m3 and 9.90 kWh/m3, respectively) for dewatering a stream of 75 g/L salinity with 70 % water recovery. Sensitivity analysis shows that increasing the maximum pressure in OARO membranes requires a higher membrane structural parameter, aggravating ICP; however, the elevated driving force mitigates adverse effects of ICP. Moreover, a 15 % reduction in LCOW can be achieved if the membrane replacement factor of RO and OARO is reduced from 15 % to 5 %, which accounts for mitigated fouling.

Quantifying the potential of pressure retarded osmosis advanced spacers for reducing specific energy consumption in hybrid desalination

A hypothetical PRO advanced spacer that delivers a 50 % mass transfer enhancement (i.e., 50 % higher Sherwood number) is simulated for a range of feed conditions and membrane properties, to shed insights into the effect of improved PRO spacer on the overall specific energy consumption (SEC) of RO-PRO hybrid desalination. Results show that a large increase in pressure drop in the PRO module has negligible impact on power density (PD) and SEC for RO-PRO. The analysis revealed that the PRO advanced spacer has marginal impact on SEC for a typical current PRO membrane. Even so, the PRO advanced spacer has an important impact in terms of PD, which can increase by 10 %, especially under severe external concentration polarization. The sensitivity analysis demonstrates that the extent of SEC reduction or power density enhancement related to the advanced spacer is most sensitive to the structural parameter. This is because internal concentration polarization is the major cause for osmotic pressure loss in PRO, which limits the potential PRO performance improvements from advanced spacers. Nevertheless, the benefits of PRO advanced spacers can be further exploited through the continuous development of new materials for novel membranes with a reduced structural parameter.