Remarkably Enhanced Gas Separation by Partial Self‐Conversion of a Laminated Membrane to Metal–Organic Frameworks

Separation methods based on 2D interlayer galleries are currently gaining widespread attention. The potential of such galleries as high‐performance gas‐separation membranes is however still rarely explored. Besides, it is well recognized that gas permeance and separation factor are often inversely correlated in membrane‐based gas separation. Therefore, breaking this trade‐off becomes highly desirable. Here, the gas‐separation performance of a 2D laminated membrane was improved by its partial self‐conversion to metal–organic frameworks. A ZIF‐8‐ZnAl‐NO₃ layered double hydroxide (LDH) composite membrane was thus successfully prepared in one step by partial conversion of the ZnAl‐NO₃ LDH membrane, ultimately leading to a remarkably enhanced H₂/CH₄ separation factor and H₂ permeance.     

Photocatalytic and antifouling properties of TiO2-based photocatalytic membranes

Photocatalysis has been extensively studied due to its potential ability to avoid the excessive use of chemical reagents and reduce the energy consumption by employing solar energy. Moreover, to alleviate the reduction in the membrane permeation selectivity, separation efficiency, and membrane service life caused by the emerging micro-pollutants and membrane fouling, membrane technology is often coupled with microbial, electrochemical, and catalytic processes. However, although physical/chemical cleaning and membrane module replacement can overcome the inherent limitations caused by membrane fouling and other membrane separation processes, high operating costs limit their practical applications. In this review, common preparation methods for TiO₂ photocatalytic membranes are described in detail, and the main approaches to enhancing their photocatalytic performance are discussed. More importantly, the mechanism of the TiO₂ photocatalytic membrane antifouling process is elucidated, and some applications of photocatalytic membranes in other areas are described. This review systematically outlines future research directions in the field of photocatalytic membrane modification, including metal and non-metal doping, fabrication of heterojunction structures, control over reaction conditions, increase in hydrophilicity, and increase in membrane porosity.     

Graphene Oxide Membranes with Heterogeneous Nanodomains for Efficient CO2 Separations

Achieving high membrane performance in terms of gas permeance and carbon dioxide selectivity is an important target in carbon capture. Aiming to manipulate the channel affinity towards CO₂ to implement efficient separations, gas separation membranes containing CO₂‐philic and non‐CO₂‐philic nanodomains in the interlayer channels of graphene oxide (GO) were formed by intercalating poly(ethylene glycol) diamines (PEGDA). PEGDA reacts with epoxy groups on the GO surface, constructing CO₂‐philic nanodomains and rendering a high sorption capacity, whereas unreacted GO surfaces give non‐CO₂‐philic nanodomains, rendering low‐friction diffusion. Owing to the orderly stacking of nanochannels through cross‐linking and the heterogeneous nanodomains with moderate CO₂ affinity, a GO‐PEGDA500 membrane exhibits a high CO₂ permeance of 175.5 GPU and a CO₂/CH₄ selectivity of 69.5, which is the highest performance reported for dry‐state GO‐stacking membranes.     

膜分离天然气脱水蒸汽技术的研究现状

天然气脱水蒸汽是天然气净化过程中的必要环节,选择合适的脱水蒸汽技术和工艺至关重要。本文首先简要概述了新兴的膜分离天然气脱水蒸汽方法的特点,然后介绍了膜分离性能的表征参数及测试、膜组件设计及操作条件的选择,重点介绍了近年来膜分离甲烷脱水蒸汽技术中的聚合物膜材料、沸石分子筛膜材料,最后展望了金属有机骨架材料的应用潜力以及天然气脱水蒸汽技术未来的发展趋势。     

A variation in fiber properties affects the performance of defect-free hollow fiber membrane modules for air separation

A flexible simulation method capable of modeling the performance of a gas separation module with variations in fiber properties is presented. Variations in fiber skin thickness, diameter and selectivity are considered. For separation of a high-pressure side product such as nitrogen from air, these variations can result in a considerable deviation from the ideal separation characteristic especially at high purities. It is shown that equal fiber quality is essential for the production of high purity nitrogen. The results presented in this publication are a part of a detailed and comprehensive sensitivity analysis on hollow fiber gas permeation modules.     

Polypyrrolones for membrane gas separations. I. Structural comparison of gas transport and sorption properties

Despite efforts by the membrane community to develop polymeric materials with improved O₂/N₂ separation performance, limited progress has occurred for almost a decade. Molecular sieving media, which can exhibit gas separation properties superior to polymers, tend to be brittle and uneconomical to produce for large‐scale membrane separation processes. Considering this, the polymer structures investigated in this work were designed to mimic aspects of the structure of molecular sieving media such as zeolites and carbon molecular sieves while maintaining the processability associated with polymers. Significantly attractive gas separation material properties were obtained using hyper rigid polypyrrolone copolymers with controlled packing disruptions between flat, packable segments. The gas transport properties in the materials changed dramatically as a result of different average interchain spacing. Moreover, all of the polypyrrolones studied in this work exhibited performance lying on or above the existing O₂/N₂ upper bound trade‐off line between permselectivity and permeability. These results, therefore, may point the way to a new cycle of membrane materials improvements for gas separations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1235–1249, 1999     

Membrane Distillation and Related Operations—A Review

Abstract

Membrane contactors represent an emerging technology in which the membrane is used as a tool for inter phase mass transfer operations: the membrane does not act as a selective barrier, but the separation is based on the phase equilibrium. In principle, all traditional stripping, scrubbing, absorption, evaporation, distillation, crystallization, emulsification, liquid‐liquid extraction, and mass transfer catalysis processes can be carried out according to this configuration. This review, specifically addressed to membrane distillation (MD), osmotic distillation (OD), and membrane crystallization (MCr), illustrates the fundamental concepts related to heat and mass transport phenomena through microporous membranes, appropriate membrane properties, and module design criteria. The most significant applications of these novel membrane operations, concerning pure/fresh water production, wastewater treatment, concentration of agro food solutions, and concentration/crystallization of organic and biological solutions, are also presented and discussed.     

Local hydrodynamic investigation of the aeration in a submerged hollow fibre membranes cassette

The better understanding of the effective air distribution inside a membrane cassette is a particular challenge in submerged membrane bioreactor. The present study is the first one that investigates the hydrodynamics of the coarse bubbles flow inside a hollow fibre membranes cassette. The experimental investigations were carried out in a reactor equipped with commercial modules from ZENON ZeeWeed^® 500d. A bi-optical probe was used to measure the bubble size, the bubble velocity and the gas hold-up at different locations between the modules and for three different gas flow rates. These local measurements gave significant information about the lateral distribution of the air and its evolution with the height on the surface of the membrane modules, which can impact on the filtration performance and are the first step to an optimisation of the aeration system and module geometry.     

Evaluation of a mathematical model using experimental data and artificial neural network for prediction of gas separation

In recent times, membranes have found wide applications in gas separation processes. As most of the industrial membrane separation units use hollow fiber modules, having a proper model for simulating this type of membrane module is very useful in achieving guidelines for design and characterization of membrane separation units. In this study, a model based on Coker, Freeman, and Fleming＇s study was used for estimating the required membrane area. This model could simulate a multicomponent gas mixture separation by solving the governing differential mass balance equations with numerical methods. Results of the model were validated using some binary and multicomponent experimental data from the literature. Also, the artificial neural network （ANN） technique was applied to predict membrane gas separation behavior and the results of the ANN simulation were compared with the simulation results of the model and the experimental data. Good consistency between these results shows that ANN method can be successfully used for prediction of the separation behavior after suitable training of the network     

Fabrication of a Hydrogen‐Bonded Organic Framework Membrane through Solution Processing for Pressure‐Regulated Gas Separation

Ordered and flexible porous frameworks with solution processability are highly desirable to fabricate continuous and large‐scale membranes for the efficient gas separation. Herein, the first microporous hydrogen‐bonded organic framework (HOF) membrane has been fabricated by an optimized solution‐processing technique. The framework exhibits the superior stability because of the abundant hydrogen bonds and strong π–π interactions. Thanks to the flexible HOF structure, the membrane possesses the unprecedented pressure‐responsive H₂/N₂ separation performance. Furthermore, the scratched membrane can be healed by the treatment of solvent vapor, achieving the recovery of separation performance.     

Preparation and separation properties of oxalylurea‐bridged silica membranes

Two new bridged alkoxysilanes, bis(triethoxysilylalkyl)‐N,N′‐oxalylureas (alkyl = methyl or n‐propyl), bearing a highly rigid and polar oxalylurea unit in the bridges, were employed as precursors of bridged silica membranes. The gas and water separation performance of the membranes prepared from the precursors using the sol–gel process was investigated. Interestingly, the membrane properties depended on the alkyl chain length. The membrane containing methylene units (alkyl = methyl) was porous and rather hydrophilic but the other with longer propylene units (alkyl = n‐propyl) was non‐porous and more hydrophobic. High H₂/SF₆ gas permeance ratios of 3100 and 1700, and NaCl rejections of 89 and 85% for 2000 ppm aqueous NaCl were obtained using the membranes containing methyl and n‐propyl, respectively. The membrane with alkyl = methyl also showed a high CO₂/N₂ permeance ratio of 20.6 at 50°C. These results indicate the potential applications of the membranes as gas and water separation materials. Copyright © 2015 John Wiley & Sons, Ltd.     

Microporous Polyamide Membranes for Molecular Sieving of Nitrogen from Volatile Organic Compounds

Microporous polymer membranes continue to receive tremendous attention for energy-efficient gas separation processes owing to their high separation performances. A new network microporous polyamide membrane with good molecular-sieving performance for the separation of N₂ from a volatile organic compound (VOC) mixture is described. Triple-substituted triptycene was used as the main monomer to form a fisherman's net-shaped polymer, which readily forms a composite membrane by solution casting. This membrane exhibited outstanding separation performance and good stability for the molecular-sieving separation of N₂ over VOCs such as cyclohexane. The rejection rate of the membrane reached 99.2 % with 2098 Barrer N₂ permeability at 24 °C under 4 kPa. This approach promotes development of microporous membranes for separation of condensable gases.     

Modular Customization and Regulation of Metal–Organic Frameworks for Efficient Membrane Separations

Metal–organic frameworks (MOFs) are considered ideal membrane candidates for energy-efficient separations. However, the MOF membrane amount to date is only a drop in the bucket compared to the material collections. The fabrication of an arbitrary MOF membrane exhibiting inherent separation capacity of the material remains a long-standing challenge. Herein, we report a MOF modular customization strategy by employing four MOFs with diverse structures and physicochemical properties and achieving innovative defect-free membranes for efficient separation validation. Each membrane fully displays the separation potential according to the MOF pore/channel microenvironment, and consequently, an intriguing H₂/CO₂ separation performance sequence is achieved (separation factor of 1656–5.4, H₂ permeance of 964–2745 gas permeation unit). Taking advantage of this strategy, separation performance can be manipulated by a non-destructive modification separately towards the MOF module. This work establishes a universal full-chain demonstration for membrane fabrication-separation validation-microstructure modification and opens an avenue for exclusive customization of membranes for important separations.     

Cost Comparisons of Some Separation Processes

Abstract

There are many separation methods in use today in the chenical inaustriesl. In the last 50 years or there about, the common and dominant separation prooesses have been the conventional processes of distillation, liquid-liquid extraction, gas absorption, leaching, crystallization, evaporation, e.t.c. This is so because of advantages of using these processes among which are the technical know-how on the design, construction, control and operation of these processes. Naturally, manfacturers are usually reluctant to venture into new areas. But in recent years, attention is being focussed on newer separation processes which are sometimes referred to as the “Novel Separaation Processes (NSP)”. These are usually seen as alternatives to conventional processes. Example of such novel processes include the membrane processes (like the Ultrafiltration (UF), Electrodialysis (ED), Reverse Osmosis (RO), etc.), adsorption, ion exchang and gas or liquid chromatography.     

Two‐Dimensional Metal–Organic Framework Nanosheets for Membrane‐Based Gas Separation

Metal–organic framework (MOF) nanosheets could serve as ideal building blocks of molecular sieve membranes owing to their structural diversity and minimized mass‐transfer barrier. To date, discovery of appropriate MOF nanosheets and facile fabrication of high performance MOF nanosheet‐based membranes remain as great challenges. A modified soft‐physical exfoliation method was used to disintegrate a lamellar amphiprotic MOF into nanosheets with a high aspect ratio. Consequently sub‐10 nm‐thick ultrathin membranes were successfully prepared, and these demonstrated a remarkable H₂/CO₂ separation performance, with a separation factor of up to 166 and H₂ permeance of up to 8×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at elevated testing temperatures owing to a well‐defined size‐exclusion effect. This nanosheet‐based membrane holds great promise as the next generation of ultrapermeable gas separation membrane.     

Engineering of the Filler/Polymer Interface in Metal–Organic Framework‐Based Mixed‐Matrix Membranes to Enhance Gas Separation

Traditional films cannot fully adapt to industrial applications and to intensified processes. Advanced mixed‐matrix membranes comprising metal–organic frameworks (MOF) embedded in a polymer matrix have been developed with the goal of breaking the trade‐off effect of traditional polymer membranes and achieving separation performance beyond Robeson's upper limit. The key challenges in the fabrication of MOF‐based mixed‐matrix membranes are an enhancement in compatibility between the inorganic filler and the polymer matrix, elimination of the irregular morphology and non‐selective interfacial defects, and further improvement in the gas‐separation performance. This review summarizes the recent advances in protocols and strategies in terms of designing interfacial interactions to enhance the MOF/polymer interface compatibility. This review aims at providing some meaningful insights into preparing MOF‐based mixed‐matrix membranes targeting ideal interfacial morphology and leading to excellent gas‐separation performance.     

Polymer Composite Membrane with Penetrating ZIF‐7 Sheets Displays High Hydrogen Permselectivity

Oriented and penetrating molecular sieving membranes display enhanced separation performance. A polyimide (PI) solution containing highly dispersed ZIF‐7(III) sheets in CHCl₃ was deposited on a glass side and subjected to flat‐scraping with a membrane fabricator. In this way we developed a novel oriented and penetrating ZIF‐7@PI mixed matrix membrane (MMM) with 50 wt. % ZIF‐7 loading. Because the height of the ZIF‐7 sheets (5 μm) is higher than the film thickness, every ZIF‐7 sheet penetrates both surfaces of the polyimide film. Since the ZIF‐7 channels are the dominant pathway for gas permeation, the ZIF‐7@PI MMM displays a high molecular sieve performance for the separation of H₂ (0.29 nm) from larger gas molecules. At 100 °C and 2 bar, the mixture separation factors of H₂/CO₂ and H₂/CH₄ are 91.5 and 128.4, with a high H₂ permeance of about 3.0×10^?7 mol m^?2 s^?1 Pa^?1, which is promising for hydrogen separation by molecular sieving.     

Cost effective,robust, and reliable coupled separation techniques for the identification and quantification of phospholipids in complex biological matrices: Application to insects

The quantification of phospholipid classes and the determination of their molecular structures are crucial in physiological and medical studies. This paper's target analytes are cell membrane phospholipids, which play an important role in the seasonal acclimation processes of poikilothermic organisms. We introduce a set of simple and cost‐effective analytical methods that enable efficient characterization and quantification of particular phospholipid classes and the identification and relative distribution of the individual phospholipid species. The analytical approach involves solid‐phase extraction and high‐performance thin‐layer chromatography, which facilitate the separation of particular lipid classes. The obtained fractions are further transesterified to fatty acid methyl esters and subjected to gas chromatography coupled to flame ionization detection, which enables the determination of the position of double bonds. Phospholipid species separation is achieved by high‐performance liquid chromatography with mass spectrometry, which gives information about the headgroup moiety and attached fatty acids. The total content of each phospholipids class is assessed by phosphorus determination by UV spectrophotometry. The simultaneous analysis of phosphorus, fatty acid residues, and phospholipid species provides detailed information about phospholipid composition. Evaluation of these coupled methods was achieved by application to an insect model, Pyrrhocoris apterus. High correlation was observed between fatty acid compositions as determined by gas chromatography and high‐performance liquid chromatography analysis.     

Molecular‐Sieving Membrane by Partitioning the Channels in Ultrafiltration Membrane by In Situ Polymerization

Commercial ultrafiltration membranes have proliferated globally for water treatment. However, their pore sizes are too large to sieve gases. Conjugated microporous polymers (CMPs) feature well‐developed microporosity yet are difficult to be fabricated into membranes. Herein, we report a strategy to prepare molecular‐sieving membranes by partitioning the mesoscopic channels in water ultrafiltration membrane (PSU) into ultra‐micropores by space‐confined polymerization of multi‐functionalized rigid building units. Nine CMP@PSU membranes were obtained, and their separation performance for H₂/CO₂, H₂/N₂, and H₂/CH₄ pairs surpass the Robeson upper bound and rival against the best of those reported membranes. Furthermore, highly crosslinked skeletons inside the channels result in the structural robustness and transfer into the excellent aging resistance of the CMP@PSU. This strategy may shed light on the design and fabrication of high‐performance polymeric gas separation membranes.     

A parametric study of CO2/N2 gas separation membrane processes for post-combustion capture

Capture of CO₂ from flue gases produced by the combustion of fossil fuels and biomass in air is referred to as post-combustion capture. Chemisorbent processes are considered to be the most feasible method and are already at an advanced stage of development, but gas separation membranes are attracting more and more attention as a possible alternative. This paper describes a detailed parametric study of mass and energy balances for a simulated single membrane process. Typical operating conditions (CO₂ concentration in the flue gas, pressure and temperature, etc.) together with the influence of the membrane quality (permeability, selectivity) and membrane area on membrane performance (CO₂ separation degree and CO₂ purity) are simulated over a wide range of parameters.