Membrane Concentration Polarization Research Articles

Small particle size activated carbon enhanced flow electrode capacitive deionization desalination performances by reducing the interfacial concentration difference

Flow electrode capacitive deionization (FCDI) has great advantages in high salinity wastewater desalination. However, due to the limited adsorption capacity of electrode material, membrane concentration polarization would form and increase energy consumption, and affect the continuity of desalination. In this work, milling technology was used to modify original activated carbon (AC), the obtained ball milled activated carbon (BAC) was used as FCDI electrode material. The milling process modified multiple physical and electrochemical characteristics of original AC. The highest average salt removal rate (ASRR) of BAC was 2.2 times of that of AC, which was attributed to the modified surface properties including more oxygenated functional group, larger electrostatic gravitation characteristics and better hydrophilicity. The reduced concentration polarization during the desalination was responsible for the increased adsorption capacity, which was due to larger electrolyte contact area, enhanced ions capacity by more suitable pore structures, and 5 times higher energy normalized removal salt (ENRS) of BAC flow electrode than that of AC. Because of its simple operation and remarkable modification effect, ball milling technology shows great potential in large scale modification of electrode materials.

Mathematical Modeling of the Effect of Pulsed Electric Field on the Specific Permselectivity of Ion-Exchange Membranes.

The application of pulsed electric field (PEF) in electrodialysis has been proven to be efficient for a number of effects: increasing mass transfer rate, mitigation of scaling and fouling, reducing water splitting. Recently, the improvement of the membrane permselectivity for specific counterions was discovered experimentally by the group of Laurent Bazinet (N. Lemay et al. J. Memb. Sci. 604, 117878 (2020)). To better understanding the effect of PEF in electrodialysis, simulations were performed using a non-stationary mathematical model based on the Nernst–Planck and Poisson equations. For the first time, it was not only the condition used when the current density is specified but also the condition when the voltage is set. A membrane and two adjacent diffusion layers are considered. It is shown that when applying the regime used by Lemay et al. (the same current density in conventional continuous current (CC) mode and during the pulses in PEF mode), there is a significant gain in specific permselectivity. It is explained by a reduction in the membrane concentration polarization in PEF mode. In the CC mode of electrodialysis, increasing current density leads to a loss in specific permselectivity: concentration profiles in the diffusion layers and membrane are formed in such a way that ion diffusion reduces the migration flux of the preferentially transferred ion and increases that of the poorly transferred ion. In PEF mode, the concentration profiles are partially restored during the pauses when the current is zero. However, if a different condition is used than the condition applied by Lemay et al., that is, when the same average current density is applied in both the PEF and CC modes, there is no gain in specific permeability. It is shown that within the framework of the applied mathematical model, the specific selectivity depends only on the average current density and does not depend on the mode of its application (CC or PEF mode).

Transport properties of lithium-ion in green nonaqueous solutions and polymer ionic exchange membranes: an attempt to elaboration of ecological Li-ion battery

Organic solutions are considered as good electrolytes yielding higher lithium-ion (Li-ion or Li+) transference numbers for lithium-ion batteries during charge and discharge processes. However, owing to ecological reasons, the transference numbers of this metal ion in green viscous solvent such as glycerol at various concentrations and temperatures have been measured by means of the concentration cell and Bruce–Vincent techniques. An average value of the transport number less than 0.5 was obtained at 298.15 K, suggesting that the solvated Li+ carries less electric charges than the larger size chloride anion. The results were then compared to a recent study utilizing molecular dynamics simulation where it was confirmed the smallness values of the Li+ transference number. Besides, Hittorf’s method using the polymer cation exchange membrane (nafion 117) has been also investigated and shows the highest transport, suggesting its advantageous utilization in ecological lithium-ion batteries. Furthermore, the concentration polarization of membrane equilibrated with the electrolyte solution of lithium was carried out by means of voltamperometry that allowed recording the current–voltage characteristic. It yielded, however, low limiting current density, indicating that the polarization is more pronounced with such light metal, in contrast to larger diffusion constant and average transport number.

Modulation of transmembrane pressure in manufacturing scale tangential flow filtration N-1 perfusion seed culture.

Mammalian cells were grown to high density in a 3,000 L culture using perfusion with hollow fibers operated in a tangential flow filtration mode. The high-density culture was used to inoculate the production stage of a biomanufacturing process. At constant permeate flux operation, increased transmembrane pressures (TMPs) were observed on the final day of the manufacturing batches. Small scale studies suggested that the filters were not irreversibly fouled, but rather exposed to membrane concentration polarization that could be relieved by tangential sweeping of the hollow fibers. Studies were undertaken to analyze parameters that influence the hydrodynamic profile within hollow fibers; including filter area, cell density, recirculation flow rate, and permeate flow rate. Results indicated that permeate flow rate had the greatest influence on modulating TMP. Further evaluation showed a significant decrease in TMP when permeate flow was reduced, and this occurred without any negative effect on cell growth or viability. Hence, a 30% reduction of permeate flow rate was implemented at manufacturing scale. A stable operation was achieved as TMP was successfully reduced by 75% while preserving all critical factors for performance in the perfusion bioreactor.

Imaging of membrane concentration polarization by NaCl using 23Na nuclear magnetic resonance

Forward osmosis (FO) and reverse osmosis (RO) membrane processes differ in their driving forces: osmotic pressure versus hydraulic pressure. Concentration polarization (CP) can adversely affect both performance and lifetime in such membrane systems. In order to mitigate against CP, the extent and severity of it need to be predicted more accurately through advanced online monitoring methodologies. Whilst a variety of monitoring techniques have been used to study the CP mechanism, there is still a pressing need to develop and apply non-invasive, in situ techniques able to produce quantitative, spatially resolved measurements of heterogeneous solute concentration in, and adjacent to, the membrane assembly as caused by the CP mechanism. To this end, 23Na magnetic resonance imaging (MRI) is used to image the sodium ion concentration within, and near to, both FO and RO composite membranes for the first time; this is also coupled with 1H MRI mapping of the corresponding water distribution. As such, it is possible to directly image salt accumulation due to CP processes during desalination. This was consistent with literature expectations and serves to confirm the suitability of 23Na MRI as a novel non-invasive technique for CP studies.

An optimization of water transport through polyurethane silica-nanocomposite membrane

Analysis of water transport through polyurethane silica nano-composite membranes were performed by numerical simulation and experiment. Simulation work was performed using a finite element analysis (FEA) software, COMSOL Multiphysics. Experimental work was performed to validate the simulation model. Through simulation we suggest the minimum membrane-to-membrane distance for stacked membranes to be 2-mm to avoid the membrane concentration polarization effect. The method used to create a concentration gradient between the membrane feed side (humid air) and the membrane permeate side (connected to vacuum) involved with creating a pressure gradient. Water vapor pressure gradient creates a concentration gradient which drives water vapor molecules to diffuse from the membrane feed side to the membrane permeate side. Different membrane configurations including rectangular parallel membranes, perforated cylinder with circular membranes, perforated group of cylinders with circular membranes, and perforated cylinder with curved rectangular membranes were compared in terms of water vapor removal from humid air while connecting all membrane configurations to 47-mm vacuum port. The parallel 22 membranes exhibited maximum water removal. Based on the total membrane area, water vapor removal was maximized at the maximum membrane effective area in contact with humid area. The maximum total water removal was obtained for the 11 cassettes with 22 rectangular membranes with a value of 142 g/(m2·s) while the relative humidity dropped from 60% at the duct inlet to 25.6% at the duct outlet.

Permeate recirculation impact on concentration polarization and fouling on RO purification of olive mill wastewater

Analysis of permeate recirculation impact on membrane concentration polarization and fouling was studied upon well-controlled reverse osmosis (RO) experiments for final purification of secondary-treated olive mill wastewater (OMW). Two commercial RO membranes presenting different characteristics, that is, a thin-film composite (TFC) and a low-pressure one, were selected for comparison purposes. Incrementing the permeate recirculation ratio to the feed tank (10–30%) reduced the concentration polarization and fouling build-up, that is 15.8–42.1% for the TFC membrane whereas 22.9–55.2% for the low-pressure membrane. This fact is especially relevant for the low-pressure membrane on which fouling triggers rapidly causing severe flux loss, but tends to stabilize upon 30% permeate recirculation. Moreover, irreversible fouling was satisfactorily minimized, reducing scaling formation and permitting restoration of the initial permeability. The fouling index can be minimized by 57.1% for the TFC membrane by recirculating 30% of the permeate stream (measured at 15°C), and also 47.1% reduction of the fouling index was observed for the low-pressure membrane upon the same recirculation ratio. The data obtained in this study highlight permeate recirculation as an important operating variable for membrane fouling control in the final RO purification of OMW.

Numerical simulation of the electrodeionization (EDI) process accounting for water dissociation

The electrodeionization process (EDI) is usually operated at overlimiting current density, and is thus characterized by water dissociation and concentration polarization. We attempt to study the useful and harmful effects of water dissociation on the EDI process. A numerical steady state model was established to simulate the process of EDI, accounting for the effects of water dissociation. The differences in concentration polarization of membranes were investigated to study the effects of water dissociation on cation and anion membranes. Protons produced by water dissociation caused the resin to transform into the H-form. The H-form resin, which has high conductivity and high transport number, depletes protons in the interstitial solution. This explains the experimentally detected phenomenon that at high current densities, the pH value of the effluency of the dilute compartment (DC) stops decreasing when current increases. We suggest that the useful role of water dissociation in EDI is due to the H-form resin bringing more salt cations of the interstitial solution into the resin phase, thus producing a high conductivity channel for the electro-migration of the salt cations. This mechanism avoids the decrease in salt ion conductivity brought about by concentration polarization. The disadvantageous effect of concentration polarization on the transportation of salt ions in interstitial solution is thus lessened. An intermediate point between the useful and harmful effects of water dissociation was determined by the dependence of current efficiency and removal rate for both cations and anions as a function of current density.

Concentration polarization and specific selectivity of membranes in pulse mode

Nonstationary concentration polarization of membranes was analyzed theoretically. Desalination and transport of single- and double-charged ions through MK-40 cation-exchange membrane was studied in the stationary and pulse modes with the fixed voltage. Main generalities in the changes of the pH and conductivity of desalinated solution as well as concentrations of single- and double-charged ions as the functions of solution flow velocity, voltage, frequency and off-duty ratio of pulses were established.

Mass transfer in the membrane concentration polarization layer under turbulent cross flow : I. Critical literature review and adaptation of existing sherwood correlations to membrane operations

In its first part, this work contains: (a) a critical review on the mass transfer correlations under turbulent duct flow, as they appeared in the literature ( 1934-1984); (b) a discussion on the factors influencing mass transfer during membrane operations (reverse osmosis and ultrafiltration), like porosity and roughness of the membrane wall and change of viscosity and diffusion coefficient due to the strong concentration gradient; (c) a proposal for a modified mass transfer equation, including the term ( Sc/Sc w) 0.11 where Scw is the Schmidt wall number with D w, and v w both functions of the concentration at the membrane wall ( C w), as well as a correlation factor accounting for the effect of roughness and permeate flux. The second part of this paper will contain an application of the modified Sh equation in the characterization of UF membranes.