Numerical modeling of the Joule heating effect on electrokinetic flow focusing

In electrokinetically driven microfluidic systems, the driving voltage applied during operation tends to induce a Joule heating effect in the buffer solution. This heat source alters the solution's characteristics and changes both the electrical potential field and the velocity field during the transport process. This study performs a series of numerical simulations to investigate the Joule heating effect and analyzes its influence on the electrokinetic focusing performance. The results indicate that the Joule heating effect causes the diffusion coefficient of the sample to increase, the potential distribution to change, and the flow velocity field to adopt a nonuniform profile. These variations are particularly pronounced under tighter focusing conditions and at higher applied electrical intensities. In numerical investigations, it is found that the focused bandwidth broadens because thermal diffusion effect is enhanced by Joule heating. The variation in the potential distribution induces a nonuniform flow field and causes the focused bandwidth to tighten and broaden alternately as a result of the convex and concave velocity flow profiles, respectively. The present results confirm that the Joule heating effect exerts a considerable influence on the electrokinetic focusing ratio.     

Polybenzimidazole (PBI) nanofiltration hollow fiber membranes applied in forward osmosis process

For the first time, the potential of polybenzimidazole (PBI) nanofiltration membrane as a forward osmosis membrane has been investigated. PBI was chosen mainly because of its unique nanofiltration characteristics, robust mechanical strength and excellent chemical stability. The MgCl₂ solutions with different concentrations and other different salt solutions were employed as draw solutions to test the water permeation flux through the PBI membrane during forward osmosis. High water permeation flux and excellent salt selectivity were achieved by using the PBI nanofiltration membrane which has a narrow pore size distribution. Effects of membrane morphology, operation conditions and flowing patterns of two feed streams within the membrane module on water transport performance have been investigated. It may conclude that PBI nanofiltration membrane is a promising candidate as a forward osmosis (FO) membrane.     

Chromium(VI) transport through the Raipore 1030 anion exchange membrane

The flat sheet Raipore R1030 anion exchange membrane has been evaluated as a sample interface in an optical sensor for Cr(VI) monitoring. The R1030 is an anion exchange membrane containing quaternary ammonium groups. The Donnan dialysis (DD) that takes place has been enhanced with facilitated transport of Cr(VI) anions by using a 1,5-diphenylcarbazide (DPC) solution as stripping phase. The DPC acts as a reducing reagent for Cr(VI), and as a complexing reagent for the generated Cr(III). The Cr(III) complex is a strongly absorbing species, and this is the basis of the optical detection. The effect of chemical parameters on Cr(VI) transport has been evaluated. Experiments with UV-VIS detection have shown that the membrane R1030-DPC system exhibits features suitable for Cr(VI) optical sensing. A simplified model based on a kinetic approach is reported describing the transport mechanism of the chemically facilitated DD process.     

Modelling the effect of operating conditions on hydrodynamics and mass transfer in a Pd–Ag membrane module for H2 purification

In this work a novel modelling approach based on Computational Fluid Dynamics (CFD) for the prediction of the gas separation process in a Pd–Ag membrane module for H₂ purification is presented. With this approach, the pressure and velocity flow fields of the gas mixture and the species concentration distribution in the selected three-dimensional domain are simultaneously and numerically computed by solving the continuity, momentum and species transport equations, including a gas-through-gas diffusion term derived from the Stefan–Maxwell formulation. As a result, the H₂ permeation calculations depend on the local determination of the mass transfer resistances offered by the gas phase and by the membrane, which is modelled as a permeable surface of known characteristics. The applicability of the model to properly predict the separation process under a wide range of pressure, feed flow rate, temperature and gas mixtures composition is assessed through a strict comparison with experimental data. The influence of inhibitor species on the module performance, that is obtained by implementing in the CFD model a suitable literature correlation, is also discussed.     

Molecular ferries: membrane carriers that promote phospholipid flip-flop and chloride transport

The facilitated transport of ionic or polar solutes through biological membranes is an essential process for cellular life, and a major technical goal of the pharmaceutical industry. Synthetic receptors with affinities for anions are shown to act as molecular ferries and facilitate the movement of chloride ions and salts across vesicle and cell membranes. A process that competes with chloride transport is phospholipid translocation or flip-flop. This has led to the development of synthetic scramblases that can alter the transmembrane distribution of phospholipids and induce biological responses such as membrane enzyme activation. The facilitated translocation of phospholipids with multiply-charged head groups, like phosphatidylserine, is a difficult supramolecular challenge that requires a complementary, multitopic receptor with appropriate amphiphilicity.     

Mass and momentum transport in extra-luminal flow (ELF) membrane devices for blood oxygenation

In this paper, we report on the characterisation of transport in membrane modules for blood oxygenation where blood is circulated outside hollow fibre membranes arranged in double layer cross-laid mats at an angle with respect to the main direction of blood flow. The effect of design and operating variables on module performance was investigated with respect to oxygen transfer into water, as gaseous oxygen and water are circulated counter-currently, respectively inside the membrane lumen and through the membrane assembly.Increasing water flow rates and membrane angles enhanced oxygen transfer across the membrane and resulted in robust operation but also in high pressure drops.Module pressure drop and oxygen transfer rate were correlated to module geometry, fibre packing density, water flow rate and membrane angle with respect to the main direction of the liquid flow in non-dimensional equations that can be used by membrane module manufacturers for the design of optimal ELF blood oxygenators. The results suggest that an optimum membrane angle exists, beyond which module operation is not convenient in terms of energy.     

Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement

New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.     

Preparation of chelating resin filled composite membranes and selective adsorption of Cu(II)

In this paper, chelating composite membranes were prepared with polysulfone (PS) and D418 chelating resin. The effects of process parameters such as the content of PS and D418 resin, concentration and temperature of the casting solution, and evaporation time on the structure and properties of the membranes were studied in detail. The performance of the static adsorption and the dynamic facilitated transport process were described. During the static experiments, the influences of membrane thickness, average resin particle size and the chelating reaction conditions on the adsorption rate and adsorption capacity of the membranes were discussed, respectively. The experiments of simultaneous extraction and stripping of Cu(II) were carried out in the course of the dynamic facilitated transport process.     

A review on hollow fiber membrane module towards high separation efficiency: Process modeling in fouling perspective

Hollow fiber microfiltration (MF) and ultrafiltration (UF) membrane processes have been extensively used in water purification and biotechnology. However, complicated filtration hydrodynamics wield a negative influence on fouling mitigation and stability of hollow fiber MF/UF membrane processes. Thus, establishing a mathematical model to understand the membrane processes is essential to guide the optimization of module configurations and to alleviate membrane fouling. Here, we present a comprehensive overview of the hollow fiber MF/UF membrane filtration models developed from different theories. The existing models primarily focus on membrane fouling but rarely on the interactions between the membrane fouling and local filtration hydrodynamics. Therefore, more simplified conceptual models and integrated reduced models need to be built to represent the real filtration behaviors of hollow fiber membranes. Future analyses considering practical requirements including complicated local hydrodynamics and nonuniform membrane properties are suggested to meet the accurate prediction of membrane filtration performance in practical application. This review will inspire the development of high-efficiency hollow fiber membrane modules.     

用于C1化学具有选择吸附—脱附特性的气体分离膜

本工作研究了吸附剂、催化剂等固体添加物混入微孔聚砜膜中对C_1化学用原料气-H_2和CO的选择透气性的影响,普遍的结果是显著地提高了膜对气体的分离因子,且与吸附剂、催化剂的化学、物理性质有一定关系。因此,气体的渗透过程除了有诺森流和粘性流外,还可能生成吸附态过渡络合物,影响气体的吸附-脱附能力而提高了分离因子,提出了过渡吸附态的气体促进输送机理。     

考虑催化层中气液态水影响的PEMFC微观模型

在考虑气、液两相水影响的条件下基于微观格点催化层模型对聚合物膜燃料电池(PEMFC)性能进行了模拟. 通过氧浓度分布和反应速率分布的比较, 说明了同时考虑催化层中气、液两相水影响的必要性. 模拟分析了液态水体积分数、氧气浓度及氧还原反应速率等在阴极催化层中的分布情况和影响因素. 考察了不同程度‘水淹’情况下的电池性能以及催化层孔隙率对水传递和电池性能的影响. 结果显示, 催化层中‘水淹’程度对电池性能有显著影响. 催化层中较大的孔隙率便于其中水的排出, 从而有利于提高电池性能.     

Nature of flow on sweeping gas membrane distillation

The process of sweeping gas membrane distillation (SGMD), with the liquid feed and the sweeping gas counterflowing in a plate and frame membrane module, has been studied. A theoretical model, which was presented in a previous paper and permitted to obtain the temperature profiles inside the fluid phases, has been developed in order to analyse the physical nature of the transmembrane water flux. Two porous hydrophobic membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as the inlet and outlet temperatures or the circulation velocities of the fluids, has been studied. The experimental results have been analysed according to the model and the conclusion is that the water transport takes place, apparently, via a combined Knudsen and molecular diffusive flow mechanism. From the temperature profiles, a local temperature polarisation coefficient may be defined. From this local value, an overall one for the whole system is then defined. The new theoretical predictions have been applied to the obtained results and the accordance may be considered good.     

Solvent extraction in cross-flow multipass parallel-plate membrane modules of fixed configuration

The effect of multipass arrangement on membrane extraction through a cross-flow rectangular module has been investigated both theoretically and experimentally. In contrast to a single-pass device, improvement in mass transfer is obtained if cross-flow membrane extraction is operated in a multipass device of same configuration. It is concluded that multipass effect which provides the increase of fluid velocity, can enhance mass transfer, especially for concentrated solution with high distribution coefficient where the liquid-phase mass-transfer resistances are more extremely predominant.     

Reversible Oxygen Binding to the Polymeric Cobalt Tetraazaporphyrin Complex and Oxygen‐Facilitated Transport through Its Membrane

Summary: A membrane of a cobalt tetraazaporphyrin polymer complex was prepared with a nanometer thickness and used as an oxygen‐facilitated transport membrane. Rapid and reversible oxygen binding to the cobalt tetraazaporphyrin complex with a polymeric imidazole ligand was observed at low temperature. Oxygen transport through the membrane was facilitated and a high (oxygen/nitrogen) permselectivity of 28 was obtained.

Oxygen‐facilitated transport through a cobalt tetraazaporphyrin complex‐polymer membrane of nanometer thickness.     

Study of the casting of sulfonated polyimide ionomer membranes: structural evolution and influence on transport properties

A casting process has been studied for charged polymers: the sulfonated polyimide ionomer membrane. The formation of the membrane has been followed by X-ray reflectivity as a function of temperature. The effect of equivalent weight has been also investigated. The thickness loss presents two regimes: the first one is linear vs time indicating that the models developed for noncharged polymer may be suitable for ionomers in the early period of drying. The second one corresponds to the loss of X-ray reflectivity signal. Moreover, the X-ray reflectivity signal seems to be correlated to the characteristic time of the sample drying. In complement, we have studied the influence of casting on the properties of the dried ionomer membranes. The transport coefficients of N(CH(3))(4)(+) ions confined in two kinds of membranes that were differently cast were measured. The results show that shearing the ionomer solution during casting may lead to an enhancement of the anisotropy of structure and of transport. Moreover, we have studied the effect of both interfaces on the ion transport properties through the dried membranes.     

Voltammetric study on ion transport across a bilayer lipid membrane in the presence of a hydrophobic ion or an ionophore

This review describes voltammetric studies on ion transport from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) containing a hydrophobic ion, valinomycin (Val) or gramicidin A (GA). In particular, the ion transport mechanisms are discussed in terms of the distribution of a pair of ions between aqueous and BLM phases. By addition of a small amount of hydrophobic ion into W1 and/or W2 containing a hydrophilic salt as a supporting electrolyte, the hydrophobic ion was distributed into the BLM with the counter ion to maintain electroneutrality within the BLM phase. It was found that the counter ion was transferred between W1 and W2 across the BLM by applying a membrane potential. Facilitated transport of alkali ions across a BLM containing Val as an ion carrier compound, could be interpreted by considering not only the formation of the alkali metal ion–Val complex but also the distribution of both the objective cation and the counter ion. In the case of addition of GA as a channel-forming compound into the BLM, the facilitated transport of alkali ions across the BLM depended on the ionic species of the counter ions. It was discovered that the influence of the counter ion on the facilitated transport of alkali ions across the BLM could be explained in terms of the hydrophobicity and the ionic radius of the counter ion.     

Membrane bioprocesses for the denitrification of drinking water supplies

A membrane bioreactor (MBR) system, consisting of a bioreactor coupled to a hollow fiber ultrafiltration membrane module, has been developed at the Centre International de Recherche sur l'Eau et l'Environnement (CIRSEE). This process combines the advantages of biological conversion of nitrate to nitrogen with that of hollow fiber ultrafiltration technology to produce high quality drinking water. The influence of several parameters, both biological and hydraulic, on the overall performance of the process has been investigated. With adequate membrane backwashing frequency an crossflow velocity, we were able to obtain, and to maintain for more than two months, a net permeate flow rate of over 100 1-hr⁻¹-m⁻². It has also been found that such a high permeate flow rate was not detrimental to the overall denitrification process, since permeate nitrate and nitrite concentration remained below 20 and 0.1 mg-l⁻¹, respectively, for a nitrate volumetric loading rate of 2.8 kg-m⁻³-day⁻¹ at a hydraulic retention time of either 60 or 30 minutes.     

Parameters influencing transport across conducting electroactive polymer membranes

The parameters which influence electrochemically facilitated transport of electroinactive ions across conducting electroactive polymer membranes have been investigated. The design of membranes and the materials used as well as transport cells and systems have been addressed to improve selectivity and flux. Polypyrrole-para-toluenesulfonate (PPy-pTS) was compared with the copolymer of pyrrole with 3-carboxy-4-methylpyrrole (PPy/PCMP-pTS) and their different chemistries resulted in different membrane selectivities for ions. Platinum mesh was found to be the most suitable auxiliary electrode material and its placement in the cell chamber(s) facilitated ion incorporation/expulsion at the membrane working electrode. This enhanced the flux of ion transport. The flux can also further enhanced by narrowing the distance between the membrane working electrode and the platinum mesh auxiliary electrode(s), and/or by stirring to improve the hydrodynamics. An alternative cell design, namely a dual membrane flow through cell, also proved to be more efficient for ion transport. Good connection geometry to the membrane as well as the application of a square wave pulsed potential waveform to the membrane was found to be essential for achieving high and sustainable flux in ion transport.     

The permeation of binary gas mixtures through support structures of composite membranes

The dusty gas model (DGM) is used to describe transport of binary gas mixtures through porous membrane supports to quantify the resistance towards permeation. The model equations account for three different transport mechanisms for the permeating components: conventional viscous pore flow, Knudsen diffusion, and binary diffusion. Experimental data obtained with the uncoated membrane supports are used to determine the morphological parameters needed in the DGM equations. Flat sheet and hollow fiber membrane supports are characterized by the permeation of a TCE/nitrogen vapor. The DGM shows an excellent fit to experimental data when the asymmetric structure of the membrane supports is taken into account, but the morphological parameters cannot necessarily be related to precise physical structure parameters such as pore size, porosity, and tortuosity. The DGM works well even when the membrane supports are modeled as a single homogenous structure. The membrane supports exhibit different resistances towards the various transport mechanisms that occur within the porous support and the resistances vary with process conditions so that support optimization is not straightforward. With the analysis presented in this paper and transport equations specific to the dense coating and module geometries, the influence of the support layer on gas or vapor separation can be quantified.     

Physicochemical Characterization of Transport in Nanosized Membrane Structures

The understanding of polymer–solvent interactions is highly important for the development of tailored membrane manufacturing procedures and for the prediction of membrane performance from transport mechanisms. This study examines the permeation performance of organic solvents through state‐of‐the‐art polyimide membranes (STARMEM, Membrane Extraction Technology Ltd.). Solvents are systematically selected to allow investigation of the effects of key physicochemical transport parameters by keeping constant all other parameters thought to be relevant. The effect of the solubility parameter, polarity (dielectric constant), surface tension, and viscosity are studied in detail. Dead‐end permeation experiments are carried out at 20 bar with STARMEM 122 and STARMEM 240 membranes. Results for the selected solvents show higher permeation rates for ketones over alcohols and aromatics as well as for acids. It is suggested that the viscosity and polarity have a greater influence than the other parameters. The effect of solvent molar volume is also investigated. Transport of solvents with high molar volume, independent of their polarity and compatibility with the membrane material, is slower in all cases than for solvents with smaller molar volume. The solubility parameter does not show any significant effect on transport phenomena.