Liquid membrane transport: a survey

The literature pertaining to facilitated transport and liquid membrane separations is reviewed and summarized, especially work reported since 1977. Liquid membranes of all geometries are discussed, including immobilized liquid membranes and liquid surfactant or emulsion liquid membranes. Emphasis is placed on facilitated, or carrier-mediated transport in both configurations although other mechanisms such as coupled-transport and transport due to solubility differences are discussed. Mathematical modeling and analytical solutions for facilitated transport models are summarized. The possibility of industrial application of liquid membrane technology is mentioned and the most important experimental techniques for liquid membrane research are discussed. Also, directions for future research are recommended.     

离子液体支撑液膜分离CO2

支撑液膜是一种在湿法冶金、生物技术以及气体分离等多个领域都有应用的重要膜分离技术。本文回顾了支撑液膜技术分离CO2的研究进展,按照液膜相的不同,分类介绍了常规载体支撑液膜和离子液体支撑液膜,指出了常规载体支撑液膜分离CO2的局限性,重点介绍了离子液体支撑液膜分离CO2的发展,分析了气体在离子液体支撑液膜中的传质机理以及常规离子液体结构、含量和支撑膜材料等对分离效果的影响;讨论了离子液体的功能化方法以及功能化离子液体支撑液膜分离CO2的渗透率、选择性和液膜稳定性;介绍了两种新的离子液体支撑液膜改进方法:聚离子液体膜与凝胶化离子液体支撑液膜。最后指出了今后用于CO2分离的离子液体支撑液膜的发展方向。     

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.     

A review on the latest development of carbon membranes for gas separation

Inorganic membranes have been developed before 1945. The earlier application of inorganic membranes was primarily concentrate on military purpose. Carbon membrane is one type of porous inorganic membrane. Although the concept of carbon membrane for gas separation has been found in the early 1970, the interest to develop carbon membrane only increased, since Koresh and Soffer successfully prepared apparently crack-free molecular sieving hollow fiber carbon membranes. Nowadays, plenty of researchers have used different polymeric materials; including polyimides, to prepare carbon membranes by using pyrolysis. In general, carbon membranes can be divided into four major configurations: flat sheet, membrane supported on tube, capillary, and hollow fiber. Permeation properties of carbon membranes have been improved greatly in these 20 years. Carbon membranes offer advantages over polymeric membranes especially in terms of selectivity as well as thermal and chemical stability. More attention will be paid to carbon membranes in this century. This paper will review the development of carbon membranes in the last 30 years and give a clear future direction in research for carbon membrane.     

A simple instrument to characterize convective and diffusive transport of single hollow fibers of short length

The use hollow fiber membranes for cell encapsulation is being developed as an experimental transplantation technology in which a permselective membrane physically isolates grafted cells from directly interacting with host cells or tissue. Although laboratory characterization of commercial ultrafiltration and dialysis membranes using multi-fiber test modules is well established, a simple apparatus for characterizing the transport properties of small individual segments of hollow fiber membrane has not been described. In the current study, we describe an instrument for evaluating the diffusive and convective transport characteristics of individual cell encapsulation membranes at size scales typically used in animal and human clinical studies. The performance capabilities of the instrument are described, as well as methods for determining hydropermeability, diffusive permeability, and solute rejection.     

Micelles in Separations: Practical and Theoretical Review

This paper presents a brief overview of improved Liquid membrane (LM) separation techniques. including modified bulk, supported and emulsion liquid membranes as well as hollow fibre contained liquid membranes, electrostatic pseudo liquid membranes (ESPLIM) reverse micelle and recently developed hybrid (HLM) and other LM configurations. The discussion also includes design of ion specific carriers, analytical importance, aspects of stability and modelling of LMs and their applications in the separation/removal of metal captions from a range of diverse matrices. In general, an attempt has been made to review the literature published from 1990 to 1997 in order to focus on the present status of different liquid membrane configurations. The LM studies dealing with separation and removal of organic compounds and gases are not included in this article owing to limitations of space.     

Membrane-based microextraction techniques in analytical chemistry: A review

The use of membrane-based sample preparation techniques in analytical chemistry has gained growing attention from the scientific community since the development of miniaturized sample preparation procedures in the 1990s. The use of membranes makes the microextraction procedures more stable, allowing the determination of analytes in complex and “dirty” samples. This review describes some characteristics of classical membrane-based microextraction techniques (membrane-protected solid-phase microextraction, hollow-fiber liquid-phase microextraction and hollow-fiber renewal liquid membrane) as well as some alternative configurations (thin film and electromembrane extraction) used successfully for the determination of different analytes in a large variety of matrices, some critical points regarding each technique are highlighted.     

Fluorescence imaging of macromolecule transport in high molecular weight cut-off microdialysis

When microdialysis (MD) membrane exceeds molecular weight cut-off (MWCO) of 100 kDa, the fluid mechanics are in the ultrafiltration regime. Consequently, fluidic mass transport of macromolecules in the perfusate over the membrane may reduce the biological relevance of the sampling and cause an inflammatory response in the test subject. Therefore, a method to investigate the molecular transport of high MWCO MD is presented. An in vitro test chamber was fabricated to facilitate the fluorescent imaging of the MD sampling process, using fluoresceinylisothiocyanate (FITC) dextran and fluorescence microscopy. Qualitative studies on dextran behavior inside and outside the membrane were performed. Semiquantitative results showed clear dextran leakage from both 40 and 250 kDa dextran when 100 kDa MWCO membranes were used. Dextran 40 kDa leaked out with an order of magnitude higher concentration and the leakage pattern resembled more of a convective flow pattern compared with dextran 250 kDa, where the leakage pattern was more diffusion based. No leakage was observed when dextran 500 kDa was used as a colloid osmotic agent. The results in this study suggest that fluorescence imaging could be used as a method for qualitative and semiquantitative molecular transport and fluid dynamics studies of MD membranes and other hollow fiber catheter membranes. Graphical Abstract

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氯化锂为添加剂制得的疏水聚偏氟乙烯不对称微孔膜的形态结构及其膜蒸馏性能

结合膜的形态结构研究了以 LiCl为添加剂制得的疏水 PVDF膜的膜蒸馏性能。与来用水溶液高分子添加剂制得的PVDF微孔膜相比,膜蒸馏性能有了较大提高,尤其具有更高的截留率。制得的微孔膜的蒸馏通量已接近商品膜的膜蒸馏通量,表明以LiCl为添加剂制得的PVDF疏水微孔膜是一种适用于膜蒸馏的较理想的疏水微孔膜。     

Assessing ionic transport during apple juice electro-acidification: influence on system efficiency

The purpose of this work was to assess ionic transport, during electro-acidification of apple juice, and its influence on the acidification rate and energy usage. In order to fulfill this objective, advanced current–voltage characterization of monopolar (cationic and anionic) and bipolar membranes was carried out. Experiments were conducted using two electrodialysis configurations (bipolar-cationic membranes and bipolar-anionic membranes) with two different systems (KCl–KCl and KCl–juice). For the bipolar-anionic configuration, the system HCl–juice was also considered. The characteristic values of the transmembrane potential and of the estimated membrane resistance were correlated to the ionic transport through the membranes, and to the energy usage of the systems. Influence of the membrane boundary layers on the transmembrane potential was also investigated by working at different feed flow rates. It was shown that for the present operating conditions, the boundary layers do not affect the transmembrane potential. Furthermore, use of HCl with the bipolar-anionic configuration enables to benefit from the advantages of each configuration: a small anionic membrane resistance due to the transport of H⁺, as for the bipolar-cationic configuration, and a high acidification rate of the apple juice due the neutralization of OH⁻ by HCl. It was for this system that the acidification rate was the fastest.     

Pore Engineering for Covalent Organic Framework Membranes

Membrane technology is of particular significance for the sustainable development of society owing to its potential capacity to tackle the energy shortage and environmental pollution. Membrane materials are the core part of membrane technology. Researchers have always been pursuing predictable structures of advanced membrane materials, which provides a possibility to fully unlock the potential of membranes. Covalent organic frameworks(COFs), with the advantage of controllable pore microenvironment, are considered to be promising candidates to achieve this design concept. The customizable function of COF membranes through pore engineering does well in the enhancement of selective permeability performance, which offers COF membranes with great application potentials in separation and transportation fields. In this context, COF-based membranes have been developed rapidly in recent years. Herein, we present a brief overview on the strategies developed for pore engineering of COF membranes in recent years, including skeleton engineering, pore surface engineering, host-guest chemistry and membrane fabrication. Moreover, the features of transmission or separation of molecules/ions based on COF membranes and corresponding applications are also introduced. In the last part, the challenges and prospects of the development of COF membranes are discussed.     

Carbon nanotube enhanced membrane distillation for online preconcentration of trace pharmaceuticals in polar solvents

Carbon nanotube enhanced membrane distillation (MD) is presented as a novel, online analytical preconcentration method for removing polar solvents thereby concentrating the analytes, making this technique an alternate to conventional thermal evaporation. In a carbon nanotube immobilized membrane (CNIM), the CNTs serve as sorbent sites and provide additional pathways for enhanced solvent vapor transport, thus enhancing preconcentration. Enrichment using CNIM doubled compared to membranes without CNTs, while the methanol flux and mass transfer coefficients increased by 61% and 519% respectively. The carbon nanotube enhanced MD process showed excellent precision (RSD of 3-5%), linearity, and the detection limits were in the range of 0.001 to 0.009 mg L(-1) by HPLC analysis.     

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.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic-Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion-exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent-induced phase separation (NIPS) and spin-coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m⁻² under a 50-fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.     

Unconfined membranes: Transfer performance and module design

The transfer performance and power requirements of unconfined sealed end hollow fiber membranes were studied. Membrane modules, operated in the sealed end mode, were pressurized with oxygen and placed within a submerged rectangular jet discharge. This design is especially well suited for use in waters containing high solids concentrations. The membranes have a greater degree of freedom for movement and are, therefore, less likely to become fouled due to solids being lodged within the fiber bundle. Furthermore, because the membranes are not enclosed within a tubular shell, head losses across the membranes are reduced. In a related study, Johnson et al. [1] developed a mass-transfer correlation for unconfined membranes similarly situated within a circular jet discharge. In this study, the mass-transfer correlation developed for circular jets is verified experimentally for larger rectangular module designs better suited for use in large scale applications. The headlosses across the modules are characterized in detail as a function of water flowrate and membrane spacing. These data are used to estimate the power requirements for different designs under a variety of operating conditions. The mass-transfer rates and corresponding power requirements for the unconfined configuration are compared with other membrane configurations.     

Mechanical properties of surfactant bilayer membranes from atomistic and coarse-grained molecular dynamics simulations

We use simulations to predict the stability and mechanical properties of two amphiphilic bilayer membranes. We carry out atomistic MD simulations and investigate whether it is possible to use an existing coarse-grained (CG) surfactant model to map the membrane properties. We find that certain membranes can be represented well by the CG model, whereas others cannot. Atomistic MD simulations of the erucate membrane yield a headgroup area per surfactant a(0) of 0.26 nm(2), an elastic modulus K(A) of 1.7 N/m, and a bending rigidity kappa of 5 k(B)T. We find that the CG model, with the right choice for the size and potential well depth of the head, correctly reproduces a(0), kappa, as well as the fluctuation spectrum over the whole range of q values. Atomistic MD simulations of EHAC, on the other hand, suggest that this membrane is unstable. This is indicated by the fact that kappa is of the order of k(B)T, which means that the interface is extremely flexible and diffuse, and K(A) is close to zero, which means that the surface tension is zero. We argue that the CG model can be used if the headgroups are uncharged, dipolar, or effectively dipolar due to headgroup charge screening induced by counterion condensation.     

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.     

Conjugate polymer-based membranes for gas separation applications: current status and future prospects

Conjugate polymers provide the possibility of exploiting both the chemical and physical attributes of the polymers for membrane-based gas separation. The presence of delocalized π electrons provides high chain stiffness with low packing density, thus making the membrane a rigid structure that favors facilitated transport. Historically, the polymeric membranes were constrained by the tradeoff relationship between gas permeability and gas selectivity. So, different methods were investigated to prepare the membranes that can overcome the limitation. In recent years, electroconductive polymeric membranes have gained attention with their enhanced transportation properties combining the separation behavior depending on both molecular size discrimination as well as the facilitated transport. They offer better selectivity toward polar gases such as CO₂ because of the increased solubility. This review is aimed to provide a literature survey on gas separation using conjugate polymers such as polyaniline, polypyrrole, and some derivatives of polythiophenes. It contains various methods used by different researchers to enhance the gas separation properties of the membranes with improved mechanical and thermal stability such as changing the morphology and membrane preparation methods. In addition, it provides the pros and cons of various factors affecting the conjugate polymer membrane performance. The major challenges and future work that can be done in improving the transportation properties through the membrane to achieve viable membranes are also discussed so that they can be used for commercial and practical applications in the future.