A lattice Boltzmann simulation of mass transport through composite membranes

Composite membranes with a porous support layer and a dense skin layer have been extensively used in gas separation processes. A new approach, a mesoscale Lattice Boltzmann Simulation approach, is proposed and used to model the pore‐scale gas flow and mass transfer in the inhomogeneous membrane matrixes studied. Only physical forces are considered. Chemical forces are equivalently converted to physical forces through the relaxation time. Selective permeation of moisture through a composite membrane is modeled. The overall permeability is evaluated. It is found that mass transfer inhomogeneity exists not only in the porous media but also in the seemingly uniform dense skin layer. Increasing the diffusivity in the skin layer is more effective than decreasing the skin layer thickness in optimizing the overall membrane performance. The new approach gives more detailed insights into the directions for future design of composite membranes for gas separations like air dehumidification. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3925–3938, 2014     

Stofftrennung durch Membranen

Separation by membranes . The paper reviews membrane processes in general, excluding the electro-membrane processes. The authors' aim is to discuss the aspects common to all membrane processes as well as the essential differences between them. The following processes will be examined: Reverse osmosis, ultrafiltration, gas permeation through porous or diffusion type membranes, pervaporation and liquid membrane techniques. In liquid systems, mass transport at the membrane surface proves to be the limiting factor. Conditions on the permeate side tend to influence flux and quality of the permeate only when a considerable pressure drop occurs. A typical example is that of hollow-fiber modules. It is shown how the permeability and dimensions of the membrane require selective adjustment in order to establish maximum flux.     

Characterization of gas permeation in the pores of Zn(II)-based metal organic framework (MOF)/polymer composite membranes

ABSTRACT

The separation of greenhouse gases from industrial processes is an ongoing focus for research aimed at mitigating environmental impacts. As a result, it is important to develop experimental techniques for the characterization of the transport of gases through porous crystalline materials with potential applications in gas separations. We report on the fabrication and characterization of gas transport in supported Zn(II)-based MOF membranes. The MOF membranes were used to develop an approach to study membrane quality and determine the transport mechanism through the pores of the crystalline membrane. Membranes were synthesized via a solvothermal method with structural defects sealed by a low-permeability polymer coating, allowing for the measurement of permeation in materials that do not form uniform, defect-free films. Membrane permeation was proportional to the inverse square root of the molecular weight of the permeant gases, indicating that diffusion occurs via Knudsen diffusion. Membrane quality was studied via selectivity measurements as a function of temperature. A study of the gas permeation through a polymer coated sparse MOF membrane, was used to confirm that gas transport occurs through the pores of the MOF, rather than through pinholes or defects in the structure.     

Highly selective polymeric membranes for gas separation

Polymeric membranes have gained an important place in chemical technology and are used in a broad range of applications. The key property that is exploited is the ability of a membrane to control the permeation rate of a chemical species through the membrane. The goal is to allow one component of a mixture to permeate the membrane freely, while hindering permeation of other component. To accomplish this, we proposed a novel concept of a (universal) ‘organic molecular sieve’ and experimentally proved its possibility by showing that organic polymer molecules at the interface between the permeable phase and the impermeable phase play the role of molecular sieves. This resulted in a significantly improved selectivity in gas separation, in fact going over the so-called ‘upper-bound’ sought for the past 30 years by many researchers but without much success. Since, this is not size selective like an inorganic molecular sieve but diffusion selective (the compatibilizer works like a molecular sieve to separate one gas molecules from the other), it can be used for the preparation of polymeric membranes for separation of any gas molecules pair. Because of polymer processability, this method is quite promising for the continuous mass production of polymeric membranes for real applications, especially when the polymers are insoluble to common solvents so that solution based techniques are hard to apply. This strategy can be applicable to various separation processes of many chemicals and gases.     

Polyamide–imide membranes with surface immobilized cyclodextrin for butanol isomer separation via pervaporation

A novel cyclodextrin (CD) derivative, m‐xylenediamine‐β‐cyclodextrin (m‐XDA‐β‐CD), has been synthesized and used to graft β‐CD on membrane surface for the pervaporation separation of butanol isomers. The reaction mechanisms for the m‐XDA‐β‐CD synthesis and the membrane surface grafting are confirmed by FTIR and TGA. The as‐fabricated novel CD‐grafted polyamide‐imide (PAI) membranes show homogeneous morphology and significant improved separation performance as compared to the unmodified PAI membranes and PAI/CD mixed matrix membranes made of physical blends. The effects of chemical modification time and dope concentration on the asymmetric membrane have been studied. The optimal separation performance can be found with the CD‐grafted PAI membrane cast from a 22 wt % dope concentration, which exhibits a total butanol flux of 15 g/m²/h and a separation factor of 2.03. This newly developed membrane with surface‐immobilized CD may open new perspective for the development of next‐generation high‐performance pervaporation membranes for liquid separations. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Ultrafast water-selective permeation through graphene oxide membrane with water transport promoters

Graphene oxide (GO), as a representative two-dimensional material, has shown great prospect in developing high-performance separation membranes via forming ordered and tunable nanochannels. However, for aqueous molecular separations, the implementation of an excellent separation performance remains a critical challenge due to the membrane swelling phenomenon and the trade-off effect between permeation flux and separation factor. Herein, a facile and tunable approach is presented for introducing water transport promoters into GO interlayer channels to construct water transport highways. The combination of covalently cross-linked channel structure, facilitated water-selective sorption, and expedited water-preferential diffusion overcome the trade-off effect, achieving a superior performance from an ultrathin GO membrane with a flux of 5.94 kg/m²∙h and a water/butanol separation factor of 3,965, which exceeds the performance of state-of-the-art membranes for water/butanol separation. The strategy proposed here is straightforward, holding great potential to produce high-efficiency GO and other two-dimensional (2D)-material membranes for precise aqueous molecular separations.     

Targeting the development of membranes for gas separation

This paper presents the new concept of ‘Targeting’ the properties of new membrane materials. Cost parameters are defined in terms of effective selectivity and cost permeability. These new parameters, obtained from the analysis of membrane plant suppliers' bids, are used in a procedure which determines the optimal degree of separation and recovery. The use of the targeting methods is illustrated here in the development of new membranes for the separation of CO from an H₂/CO syngas for the large-scale manufacture of acetic acid. The targeting approach can be extended to include other gas separation technologies and will aid the development of a generic methodology for the design of gas separation processes.     

Gas-Trennung mit Membranen

Gas Separation with membranes . Gas separation with membranes has already been tested in numerous fields of application, e. g. uranium enrichment or H₂ separation. In many of these processes the mass transfer units, so-called permeators, have to be connected in tandem in order to achieve high concentrations. A most economical operating method provides for each case an optimization of the cascades with regard to the membrane materials, construction and design of module. By utilization of the concentration gradient along the membrane a new process development has been accomplished – the continuously operating membrane rectification unit. Investment and operating costs can be reduced considerably for a number of separating processes by combining a membrane rectification unit with a conventional recycling cascade. However, the new procedure requires that the specifications for the module construction, flow design, and membrane properties be reconsidered.     

膜分离过程中原料气流量的控制方法

介绍了气体膜分离原理;提出了理想膜过程的新概念;归纳了气体膜分离装置的运行特点;比较分析了气体膜分离过程中原料气流量控制的各种方法;提出通过在尾气侧安装调节阀来定量调节原料气流量的控制方法;该方法成功应用于金陵石化加氢裂化低分气中提取高浓氢的工艺中,验证了这种控制策略的正确性和有效性;整体的控制方案又进一步提高了膜分离装置的操作柔性及适应能力;该方法可同时保证渗透气快气浓度及回收率的性能指标,适用于所有气体膜分离过程中对处理气量的自动控制.     

A study of permeation of organic solvents through polymeric membranes based on polymeric alloys of polyphosphonates and acetyl cellulose. II. Separation of benzene,cyclohexene, and cyclohexane

Membranes prepared from polymeric alloys of polyphosphonates and acetyl cellulose are highly permeable to benzene and the cyclohexene, but practically impermeable to the aliphatic hydrocarbons cyclohexane and decalin. Pervaporation and osmotic distillation techniques were used in order to achieve separation of the benzene–cyclohexane mixtures. The flux of the permeate increases sharply with increasing temperature, concentration of benzene in the feed solution, and fraction of polyphosphonate in the membrane. The increase of permeate flux is accompanied by a slight decrease of the separation factors. The permeation characteristics of the membranes were compared with those predicted from the results of sorption experiments. The agreement between the observed and the predicted fluxes and separation factors indicates that the permeation mechanism can be described in terms of molecular diffusion. The high selectivity of the membranes makes possible the development of a novel “osmotic distillation” technique. Such a technique, which combines osmotic permeation of organic liquids with conventional distillation, may be advantageous in the separations of azeotropic mixtures.     

State-of-the-art membrane based CO2 separation using mixed matrix membranes (MMMs): An overview on current status and future directions

The main purpose of research in membrane gas separation is to develop membranes with high permeability and selectivity. Historically, the gas separation performance of polymeric membranes has been constrained to an upper performance limit. Hence, different methods have been investigated to prepare membranes that can exceed this limitation including the incorporation of inorganic materials into polymer matrices. Membranes formed by this method are called mixed matrix membranes (MMMs). The major challenge is to prepare a defect-free polymer/inorganic nanoparticles interfaces with enhanced separation performance and mechanical and thermal stability. For this purpose, various types of nanoparticles have been proposed and examined experimentally. This review is especially devoted to summarize the fundamental concepts that have to be considered to prepare various types of MMMs, including considerations for the design novel MMMs that will eventually surpass the Robeson's trade-off upper bound. In addition, it provides the pros and cons of various factors that affect the MMM preparation especially for CO₂ separation processes.     

Zeolite membranes: microstructure characterization and permeation mechanisms

Since their first synthesis in the 1940s, zeolites have found wide applications in catalysis, ion-exchange, and adsorption. Although the uniform, molecular-size pores of zeolites and their excellent thermal and chemical stability suggest that zeolites could be an ideal membrane material, continuous polycrystalline zeolite layers for separations were first prepared in the 1990s. Initial attempts to grow continuous zeolite layers on porous supports by in situ hydrothermal synthesis have resulted in membranes with the potential to separate molecules based on differences in molecular size and adsorption strength. Since then, further synthesis efforts have led to the preparation of many types of zeolite membranes and better quality membranes. However, the microstructure features of these membranes, such as defect size, number, and distribution as well as structure flexibility were poorly understood, and the fundamental mechanisms of permeation (adsorption and diffusion), especially for mixtures, were not clear. These gaps in understanding have hindered the design and control of separation processes using zeolite membranes. In this Account, we describe our efforts to characterize microstructures of zeolite membranes and to understand the fundamental adsorption and diffusion behavior of permeating solutes. This Account will focus on the MFI membranes which have been the most widely used but will also present results on other types of zeolite membranes. Using permeation, x-ray diffraction, and optical measurements, we found that the zeolite membrane structures are flexible. The size of defects changed due to adsorption and with variations in temperature. These changes in defect sizes can significantly affect the permeation properties of the membranes. We designed methods to measure mixture adsorption in zeolite crystals from the liquid phase, pure component adsorption in zeolite membranes, and diffusion through zeolite membranes. We hope that better understanding can lead to improved zeolite membranes and eventually facilitate the large-scale application of zeolite membranes to industrial separations.     

Atomically detailed models of gas mixture diffusion through CuBTC membranes

Metal–organic frameworks are intriguing crystalline nanoporous materials that have potential applications in adsorption-based and membrane-based gas separations. We describe atomically detailed simulations of gas adsorption and diffusion in CuBTC that have been used to predict the performance of CuBTC membranes for separation of H₂/CH₄, CO₂/CH₄ and CO₂/H₂ mixtures. CuBTC membranes are predicted to have higher selectivities for all three mixtures than MOF-5 membranes, the only other metal–organic framework material for which detailed predictions of membrane selectivities have been made. Our results give insight into the physical properties that will be desirable in tuning the pore structure of MOFs for specific membrane-based separations.     

Synthesis and characterization of polyacrylamide‐grafted sodium alginate copolymeric membranes and their use in pervaporation separation of water and tetrahydrofuran mixtures

Polyacrylamide‐grafted sodium alginate (PAAm‐g‐Na‐Alg) copolymeric membranes have been prepared, characterized, and used in the pervaporation separation of 10–80 mass % water‐containing tetrahydrofuran mixtures. Totally three membranes were prepared: (1) neat Na‐Alg with 10 mass % of polyethylene glycol (PEG) and 5 mass % of polyvinyl alcohol (PVA), (2) 46 % grafted PAAm‐g‐Na‐Alg membrane containing 10 mass % of PEG and 5 mass % of PVA, and (3) 93 % grafted PAAm‐g‐Na‐Alg membrane containing 10 mass % of PEG and 5 mass % of PVA. Using the transport data, important parameters like permeation flux, selectivity, pervaporation separation index, swelling index, and diffusion coefficient have been calculated at 30°C. Diffusion coefficients were also calculated from sorption gravimetric data of water–tetrahydrofuran mixtures using Fick's equation. Arrhenius activation parameters for the transport processes were calculated for 10 mass % of water in the feed mixture using flux and diffusion data obtained at 30, 35, and 40°C. The separation selectivity of the membranes ranged between 216 and 591. The highest permeation flux of 0.677 kg/m² h was observed for 93% grafted membrane at 80 mass % of water in the feed mixture. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 272–281, 2002     

用于水质净化的氧化石墨烯膜研究进展

氧化石墨烯膜具有超高的水通量、可控的层间距以及卓越的分离性能,这些优异的特性使氧化石墨烯膜有望成为新一代膜材料并用于水环境中物质的精确分离。对氧化石墨烯膜的研究已经取得了许多重要的成果,本文系统地阐述了用于水质净化的氧化石墨烯膜的结构特性和构效关系,总结了氧化石墨烯膜典型的制备方法,重点介绍了氧化石墨烯膜的改性方式,概述了氧化石墨烯膜在多种水环境中的应用,总结并展望了氧化石墨烯膜的发展方向,为设计和合成高性能氧化石墨烯膜用于水质净化提供新的思路。     

Research progress of graphene oxide membrane for water purification

Graphene oxide membranes have ultra-high water flux, controllable interlayer spacing and excellent separation properties. These outstanding characteristics make graphene oxide membranes promising to be a new generation of membrane materials and used for the precise separation of substances in the water environment. At present, researchers have performed numerous studies on graphene oxide membranes and achieved breakthrough results, including the transfer behavior of water in the membrane, the separation mechanism of the membrane and the preparation methods of the membrane, etc. However, there is still a lack of comprehensive understanding of graphene oxide membranes. This review systematically described the structural properties and structure-effect relationships of graphene oxide membrane, and summarized the typical preparation methods. In terms of the challenges faced by graphene oxide membrane in practical application, we focused on the existing modification methods of graphene oxide membrane. The applications of graphene oxide membrane in various water environments were also discussed. Finally, the future development of graphene oxide membrane was summarized and prospected. The aim of this paper is to provide novel ideas for the design and synthesis of high-performance graphene oxide membranes for water purification.     

Effect of Temperature on the Transport Properties and Morphology of Polymeric Asymmetric Membranes

Abstract

Polymeric membranes have not been practical for application to high temperature processes, due to the thermal instability of most polymeers. New materials are being developed with higher glass transition temperatures and a greater degree of thermal stability. Use of these polymers to perform gas separations at higher temperatures is promising; however, the performance of the membrane as the process approaches the polymer's glass transition temperature is unknown. This study was conducted to explore this issue. Gas flux and helium/nitrogen ideal selectivities through heat treated integrally-skinned asymmetric polysulfone membranes were measured. Membranne morphology was also evaluated through bubble point tests and SEM micrographs. Experiments demonstrated that as the polymer is exposed to temperatures approaching the polymer glass transition temperature, internal pores begin to collapse, causing both the gas flux and selectivity to decrease.     

全球气体膜分离技术的研究和应用趋势——基于近20年SCI论文和专利的分析

为把握整体气体膜分离产业研究和创新状况,在Web of Knowledge(WOK)平台的Web of Science®数据库和Derwent(德温特)专利数据库检索了有关气体膜分离技术的文献,并采用文献计量学的方法进行分析。结果表明,1995-2014年全球相关论文共2972篇,专利4266项。气体膜分离技术现处于研究的成长期,已形成核心作者群。气体膜分离材料的研究热点为混合基质膜、沸石膜和炭膜,但工业化应用以传统有机高分子材料为主,气体膜分离技术主要的研究和应用领域为氢回收、空分和脱碳。美国和日本的研究和应用优势较明显,我国气体膜分离的研发主体为高校和科研院所,尽管发文量位居世界第二,但科研质量和国际影响力仍需提高,科研成果转化率不高。预计未来气体膜分离的研究重点会在沸石膜和炭膜等新型膜材料的空间结构的设计合成和碳捕获的技术应用上,渗透汽化膜的应用和挥发性有机物(VOCs)分离也是未来的研究方向。     

沸石咪唑骨架膜的制备及其应用进展

近年来,将金属-有机骨架材料(MOFs)和膜基材料结合,制备新型MOFs分离膜成为膜领域研究的热点之一。由于MOFs具有类似分子筛结构和空间拓扑结构,在分离、催化等方面具有潜在的应用前景。沸石咪唑框架材料(ZIFs)作为MOFs中重要分支之一,因其具有优异的热稳定性和化学稳定性被应用于膜分离。本工作重点阐述了原位生长、界面反扩散、逐层组装、二次生长、气相沉积和微流体处理等方法制备ZIFs多晶膜和杂化膜,并系统介绍了ZIFs复合膜在染料与重金属离子去除、气体分离、天然气净化、生物医药和电化学传感中的应用。最后,总结了ZIFs复合膜制备过程中存在的问题和挑战,并对ZIFs复合膜未来研究的方向提出了展望。     

Diffusion in porous functional materials: Zeolite gas separation membranes, proton exchange membrane fuel cells, dye sensitized solar cells

The function of numerous technical apparatus and processes is diffusion controlled. In three cases studies, the diffusion of molecules, ions and electrons in gas separation membranes, fuel cell membranes and dye sensitized solar cells is discussed. In novel functional materials often an overlap of transport due to a concentration and/or a potential gradient takes place. The transport parameters measured in materials evaluation such as impedance spectroscopy can reflect a physical situation which is different from that of the working device. A detailed fundamental knowledge of various factors is necessary to fully understand the nature of transport as a basis to optimise the corresponding functional materials.