Degree Of Concentration Polarization Research Articles

Pressure retarded osmosis enhanced by micro-nano bubbles: Coupling effects of operation conditions and mass transfer resistance analyses

The concentration polarization is one of the bottlenecks of pressure retarded osmosis (PRO) for industrial-scale application. Micro-nano bubbles (MNBs) could form a strong repulsion and shear force on the membrane surface, thus, the degree of concentration polarization is reduced, and hence, both the water flux and power density could be increased. In this paper, the synergistic effects of MNBs with concentration, temperature, water and air flow rates, pressure, and spacers were investigated through membrane cell scale experiments. A numerical model with trans-membrane heat flux and MNBs effects is built and validated by the experiment results. The mechanism of strengthening effect of MNBs could be explained. A maximum promotion of 6.28 % was achieved for concentration combination of 0.171–1 mol/L. However, the promotion ratio of water flux was deceased significantly with higher temperature (<1 % for 45 °C) and higher pressure (nearly zero for 7 bar). The light MNBs would be pushed away from membrane under larger water flux with larger osmotic pressure difference. The variations of mass transfer resistances, and the thicknesses of boundary layers were discussed. Moreover, the key variables that have significant impacts on the metrics of PRO process were identified through partial least squares analysis.

Towards long-term operation of flow-electrode capacitive deionization (FCDI): Optimization of operating parameters and regeneration of flow-electrode

This study systematically optimized the key operating parameters and interpreted their effecting mechanisms in a flow-electrode capacitive deionization (FCDI) system. The optimal voltage, activated carbon electrode content, electrolyte concentration, feedwater flowrate, and electrode flowrate for desalinating low salinity feedwater (1.0 g L−1 NaCl) were determined to be 1.8 V, 2.0 wt%, 10.0 g L−1, 80 mL min−1, and 60 mL min−1, respectively. The variations of the above parameters can affect the system conductivity, the thickness and stability of the electric double layers, and/or the degree of concentration polarization, thereby influencing the desalination performance. Moreover, a sensitivity analysis identified the operating voltage as the dominant parameter with the most significant influence on the FCDI system. Subsequently, a long-term operation was carried out under single-pass mode. The results showed that the lab-scale FCDI system was able to constantly maintain the desalination efficiency of 1.0 g L−1 feedwater (NaCl) at 40–60 % for multiple operating cycles. Over 99.8 % of electrode material regeneration and desalination efficiency recovery was able to be obtained during a 60-h operation, demonstrating that the FCDI system showed strong stability and long-term operation potential.

Improving extraction and post-purification concentration of membrane proteins.

Membrane proteins (MPs), despite being critically important drug targets for the pharmaceutical industry, are difficult to study due to challenges in obtaining high yields of functional protein. Most current extraction efforts use specialized non-ionic detergents to solubilize and stabilize MPs, with MPs being concentrated by ultrafiltration (UF). However, many detergents are retained during the UF step, which can destabilize MPs and/or interfere with their characterization. Here, we studied the influence of detergent selection on the extraction and UF-based concentration of biomedically-relevant MPs, the light-driven sodium and chloride transporters, KR2 and halorhodopsin (pHR) which are also model proteins for more complex mammalian rhodopsins. We also designed a flat-bottomed centrifugal filter that can concentrate MPs with enhanced removal of free detergents by promoting concentration polarization (CP). We tested the performance of this new filter using four commonly employed MP detergents, octyl-β-D maltoside (OM), decyl-β-D maltoside (DM), dodecyl-β-D maltoside (DDM) and octyl-β-D glucoside (OG), over a range of detergent and salt concentrations. Detergent passage is significantly higher for the flat-bottomed filter achieving up to 2-fold greater sieving of detergent in DM-solubilized pHR system due to the high degree of CP. We observe more efficient, up to 5-fold higher extraction of KR2 in the presence of a longer 12-carbon alkyl chain detergent, DDM compared to a shorter 8-carbon detergent, OM. Assuming complete binding and elution of the extracted protein, DDM-based extraction of KR2 could lead to a potential 7-fold improvement in purification yields compared to conventional methods which yield ∼1 mg MP per liter of cell culture. However, the longer chain detergents like DDM form larger micelles that are difficult to remove by UF. Thus, there exists a trade-off between choosing a detergent that will enable efficient extraction of MP while showing easier removal during subsequent UF. The extraction efficiency and UF-based separation of detergent micelles provide insights for other applications involving detergent-mediated separation/extraction.

Threshold flux and limiting flux for micellar enhanced ultrafiltration as affected by feed water: experimental and modeling studies

Micellar enhanced ultrafiltration is a promising technology for removing trace amounts of organic pollutants in wastewater, but membrane fouling caused by Gemini micelles is a significant problem restricting its industrial application. In this study, a flat sheet ultrafiltration membrane with four types of Gemini surfactant systems and various feed concentrations was used to treat phenol wastewater, while the flux behavior and fouling mechanism were investigated. The optimal permeate flux of phenol decreased in the sequence as follows: CG12 > CG16 > CG12 − Brij35 > CG16 − Brij35. In order to study the rate of membrane fouling, a developed liner regression method was utilized to help determine the threshold flux, as well the degree of concentration polarization and membrane fouling. Furthermore, limiting flux was investigated to avoid extensive cake layer formation. Threshold flux and limiting flux strongly depended on the feed concentration and the type of micelles. The threshold flux reduced gradually with feed concentration, due to the increased fouling rate and presence of irreversible fouling. Owing to the effect of charge density and hydrophobicity of micelles, CG12 exhibited the highest threshold flux and limiting flux. Flux-TMP relation was divided into three regions for analyzing the corresponding changes of permeate flux, filtration resistance and conceptual particle deposition model. Besides, fouling mechanism of MEUF was discussed. This study provides a guideline for analyzing membrane fouling and seeking an optimum operation strategy for controlling fouling in membrane water treatment.

Effect of concentration polarization on electrodialytic concentrating of dilute NaCl and NH4NO3 solutions

Electrodialytic concentrating of dilute sodium chloride and ammonium nitrate solutions is studied in a wide range of current densities. Using the model of limiting electrodialytic concentrating, based on the experimental data, the transport characteristics of MK-40/MA-40 membrane pair in the above electrolytes and the hydration numbers of these electrolytes were determined. It is found that a degree of concentration polarization has an effect on the main parameters of limiting electrodialytic concentrating: the current efficiency, the water transfer number across the test membrane pair, and the concentration of solution in the concentrating compartment. These parameters determine the effectiveness of concentrating. It is found that the electrodialytic concentrating is most effective at the limiting current due to the lowest electroosmotic permeability of membrane pair and the highest current efficiency.

Numerical analysis of spacer impacts on forward osmosis membrane process using concentration polarization index

Understanding the mass transfer in membrane separation processes plays a crucial role in improving the process performance. In this study, we numerically scrutinized the impacts of spacers—a structure enhancing the mass transfer in the vicinity of the membrane—on the concentration polarization (CP) in a forward osmosis (FO) process. To quantitatively measure the extent of CP, we introduce a concentration polarization index (CPI), which is proportional to the degree of CP and inversely proportional to the permeate flux. Based on the CPI, we simulated the impact of spacers on FO performance at various concentration conditions and found that the FO spacers generally reduce the extent of the CP; subsequently, this shows that FO spacers are important factors for the advancement of FO technology. In FO processes it was found that the boundary layer compression is more efficient for diminishing the CP than the boundary layer disruption, whereas the disruption mechanism was more successful in alleviating the CP in reverse osmosis (RO) processes. This contradiction was subsequently attributed to the intrinsic characteristics of the boundary layer disruption, which inevitably caused a dead zone near the spacer attachment and thereby boosted the CP in the FO process. Moreover, it was observed that spacers in feed channels increase the CP even when the concentration of the feed solution is zero, unless the membrane completely rejects salt, because any solute penetrating through the active layer of the membrane by a reverse diffusion mechanism accumulates near the spacer attachment. Hence, it is crucial to minimize the adverse impacts of FO spacers to maximize the beneficial effects of spacers to decrease the CP and improve the performance of FO processes.

Reverse osmosis at elevated temperatures: influence of temperature on degree of concentration polarization and transmembrane flux

A model describing the influence of temperature on longitudinal distribution of the degree of concentration polarization (CP) and transmembrane flux in reverse osmosis systems was developed. The model is based on the following assumptions: (1) membrane morphology is temperature independent; (2) transport characteristics of membranes are invariant with their coordinate; (3) specific water permeability of membranes is based on exponential dependence of viscosity vs. temperature; (4) temperature dependence of membrane rejection is assumed to be linear. The proposed model gives quantitative correlations between the longitudinal mass flow and the CP degree at different operating temperatures. The model enables analyzing the influence of temperature, mass flow, bulk concentration, axial velocity, channel geometry, physical properties and membrane rejection on longitudinal distribution of the degree of CP. This model can be hybridized with sub-models describing driving force, gel resistance, etc., and segmented into a more advanced algorithm for modeling longitudinal variation of characteristics. The model does not contain digital integration and can be easily incorporated into optimization algorithms.

Modeling the influence of temperature on degree of concentration polarization in reverse osmosis systems

A model describing the influence of temperature on degree of concentration polarization is proposed. The model is based on the following assumptions: (1) membrane morphology is temperature-independent; (2) transport characteristics of membrane are invariant with coordinate; (3) specific water permeability of the membrane is based on exponential dependence of viscosity vs. temperature; (4) temperature-dependence of membrane rejection is assumed to be linear. Proposed models permit quantitative correlations of longitudinal mass flow and degree of concentration polarization at different operating temperatures. It enables analysis of the influence of temperature, mass flow, bulk concentration, axial velocity, channel geometry, physical properties and membrane rejection on longitudinal distribution of concentration polarization degree. The submitted model does not contain digital integration and can be segmented into optimization algorithms.

Modeling Concentration Polarization Phenomena for Shell-Side Flow in Ultrafiltration Process

A correlation for the longitudinal mass flow of a dissolved component in terms of the degree of concentration polarization has been developed. The solution developed is applicable to shell-side flow and is based on the assumptions that the flow is to be isothermal, laminar, incompressible, and steady-state. Balance equations for viscous and diffusion boundary layers are the core of the model. Integration of these equations over the control volume of a hexagonal configuration has been carried out. This model permits analysis of the influence of mass flow, bulk concentration, velocity, and channel geometry on the degree of concentration polarization. In addition, it can give the mass flow by using the degree of concentration polarization as an input variable. The solution presented can be segmented and built into a complicated algorithm of a membrane-based process. The solution does not contain digital integration and permits simulation and analysis of the influence of various factors. Calculated data are included.

Modelling concentration polarization phenomena for membrane channel with cylindrical geometry in an ultrafiltration process

A correlation for longitudinal mass flow of dissolved component against degree of concentration polarization has been developed. The developed solution is applicable to tube-side flow and based on the assumptions that the flow is to be isothermal, laminar, incompressible and steady state. Balance equations for viscous and diffusion boundary layers are the core of the model. Integration of these equations over the control volume of the cylindrical configuration has been carried out. This model permits the analysis of the influence of mass flow, bulk concentration, and velocity on the degree of concentration polarization. It can also give mass flow using the degree of concentration polarization as an input variable. The presented solution can be segmented and built into a complicated algorithm of a membrane-based process. The solution does not contain digital integration and permits simulation and analysis of the influence of various factors. The calculated data are included.

Effect of Convective Boundary Layer on the Current Efficiency of a Membrane Bearing Nonuniformly Distributed Fixed Charges

The current efficiency η of a planar membrane bearing nonuniformly distributed fixed charges is analyzed theoretically. In particular, the effect of the convective boundary layer near the outer boundary of the membrane is discussed. Two types of fixed charge distribution are considered: a class of linear distribution and a class of exponential distribution. We show that, if the flow velocity in the membrane phase is small, increasing the inhomogeneity of fixed charge distribution is capable of raising η. However, if it is appreciable, the reverse is true. If a boundary is present, η decreases with the increase in current density; the reverse is true if it is absent. The former is due to the fact that the degree of concentration polarization in the boundary layer increases with current density. Although the existence of a boundary layer has the effect of reducing η, the convective flow of fluid perpendicular to membrane has a positive effect on η. If a boundary layer is absent and the fixed charges are di...