Membrane-based Gas Separation Research Articles

Optimization model for the design and feasibility analysis of membrane-based gas separation systems for CO2 enhanced coal bed methane (CO2-ECBM) applications

The purpose of this study is the development of an optimization model to design a coal bed methane (CBM) gas separation system using a multi stage membrane process. The proposed model is formulated using a nonlinear programming (NLP) technique and includes maximizing the total profit as the objective function, along with several constraints to fulfill the design specification under various scenarios. Using the proposed model, we can (i) identify the optimal system configuration and operation strategies such as the required membrane area, the number of stages, recycle strategies, and utility consumption; and (ii) analyze the cost contribution and major cost drivers that have effects on the system configuration and economics. We also investigate the sensitivity of major parameters, including the composition of the raw CBM feed and CH4 market price on the process configurations and operation strategies that affect the overall economics of the CBM production system.

Accelerating Membrane-based CO2 Separation by Soluble Nanoporous Polymer Networks Produced by Mechanochemical Oxidative Coupling.

Achieving homogeneous dispersion of nanoporous fillers within membrane architectures remains a great challenge for mixed-matrix membrane (MMMs) technology. Imparting solution processability of nanoporous materials would help advance the development of MMMs for membrane-based gas separations. A mechanochemically assisted oxidative coupling polymerization strategy was used to create a new family of soluble nanoporous polymer networks. The solid-state ball-milling method affords inherent molecular weight control over polymer growth and therefore provides unexpected solubility for the resulting nanoporous frameworks. MMM-based CO2 /CH4 separation performance was significantly accelerated by these new soluble fillers. We anticipate this facile method will facilitate new possibilities for the rational design and synthesis of soluble nanoporous polymer networks and promote their applications in membrane-based gas separations.

Gas Separation through Bilayer Silica, the Thinnest Possible Silica Membrane.

Membrane-based gas separation processes can address key challenges in energy and environment, but for many applications the permeance and selectivity of bulk membranes is insufficient for economical use. Theory and experiment indicate that permeance and selectivity can be increased by using two-dimensional materials with subnanometer pores as membranes. Motivated by experiments showing selective permeation of H2/CO mixtures through amorphous silica bilayers, here we perform a theoretical study of gas separation through silica bilayers. Using density functional theory calculations, we obtain geometries of crystalline free-standing silica bilayers (comprised of six-membered rings), as well as the seven-, eight-, and nine-membered rings that are observed in glassy silica bilayers, which arise due to Stone-Wales defects and vacancies. We then compute the potential energy barriers for gas passage through these various pore types for He, Ne, Ar, Kr, H2, N2, CO, and CO2 gases, and use the data to assess their capability for selective gas separation. Our calculations indicate that crystalline bilayer silica, which is less than a nanometer thick, can be a high-selectivity and high-permeance membrane material for 3He/4He, He/natural gas, and H2/CO separations.

Polysulfone mixed-matrix membrane incorporating talc clay particles for gas separation

Mixed-matrix membranes (MMMs) have showed advantages in membrane-based gas separation in recent years. However, there is still intensive demand for the development of membranes incorporated by proper fillers for further enhancement of gas separation performance. In this study, mixed-matrix membrane (MMMs) has been synthesized by incorporating of different content of talc into polysulfone (PSf) polymer specifically upon the separation of carbon dioxide from methane. The influence of filler content on the gas separation performance of the fabricated membranes was studied. The prepared MMMs were analyzed by thermal gravimetric analysis (TGA), X-ray diffraction (XRD), derivative thermo-gravimetric (DTG), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and pure gas permeation testing. XRD analysis suggested that talc layers crystalline structure was changed slightly due to blending and just limited PSf chain intercalation occurred. The results showed that the rate of gas permeation increased with increasing talc content while the selectivity declined at 0.5 wt% due to filler agglomeration as confirmed by FESEM. The best results were obtained at 0.1 wt% talc compared to neat PSf with superior gas separation behavior.

Advanced Materials for Membrane–Based Gas Separation

The development of advanced materials for membrane gas separation has received much attention in the recent years. There are many important aspects involved in the development of new and advanced membrane materials including the type of material, transport mechanism, the relationship between membrane structure and performance and membrane preparation techniques. There are numerous materials being widely investigated as suitable candidates using advanced materials for membrane gas separation. Therefore, this paper discusses the progress in the development of an advanced membrane material and methods to prepare membranes for gas separation. The recent development and future direction in membrane material research are also presented.

Effects of zinc salts on the microstructure and performance of zeolitic-imidazolate framework ZIF-8 membranes for propylene/propane separation

Zeolitic-imidazolate frameworks (ZIFs), a sub-class of metal-organic frameworks (MOFs), show a lot of promise for membrane-based gas separation applications. In particular, a prototypical ZIF-8 has attracted tremendous research interests as a promising membrane material for the industrially important propylene/propane separation. Of the different synthesis techniques developed for polycrystalline ZIF membranes, the microwave-assisted seeding followed by the secondary growth has been shown effective to obtain high quality polycrystalline ZIF-8 membranes by achieving strong attachment of seed crystals to ceramic supports. However, as with any polycrystalline films, the performance of membranes synthesized by this method is determined by membrane microstructures (i.e., grain boundary structure, the size/shape/orientation of grains, etc.) which, in turn, are affected by processing conditions. Here, we report the effects of the nature of zinc salts on the microstructures of ZIF-8 seed layers prepared under microwave irradiation as well as subsequent secondarily-grown ZIF-8 membranes. Significant differences in the microstructures of both seed layers and membranes, consequently binary propylene/propane separation performance of membranes, were observed with different zinc salts as metal sources. It was found that likely due to its fast nucleation as compared to other zinc salts, zinc nitrate was most effective as a zinc source in both the microwave-assisted seeding and the secondary growth step, resulting in both seed layers and membranes with better microstructures. The resulting ZIF-8 membranes showed the excellent propylene/propane separation factor of ∼105.

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.

Ionic Liquid/Metal-Organic Framework Composites: From Synthesis to Applications.

Metal-organic frameworks (MOFs) have been widely studied for different applications owing to their fascinating properties such as large surface areas, high porosities, tunable pore sizes, and acceptable thermal and chemical stabilities. Ionic liquids (ILs) have been recently incorporated into the pores of MOFs as cavity occupants to change the physicochemical properties and gas affinities of MOFs. Several recent studies have shown that IL/MOF composites show superior performances compared with pristine MOFs in various fields, such as gas storage, adsorption and membrane-based gas separation, catalysis, and ionic conductivity. In this review, we address the recent advances in syntheses of IL/MOF composites and provide a comprehensive overview of their applications. Opportunities and challenges of using IL/MOF composites in many applications are reviewed and the requirements for the utilization of these composite materials in real industrial processes are discussed to define the future directions in this field.

Review of Membrane Oxygen Enrichment for Efficient Combustion

Oxygen enrichment from air is a simple way of increasing the efficiency of combustion process, as in oxy-combustion. Oxy-combustion has become one of the most attracting combustion technologies because of its potential to address both pollutant reduction and CO2 capture. In oxy-combustion, the fuel and recycled flue gas are combusted with oxygen enriched air (OEA). By using OEA, many benefits can be obtained, such as increasing available heat, improving ignition characteristics, flue gas reduction, increasing productivity, energy efficiency, turndown ratio, and flame stability. Membrane-based gas separation for OEA production becomes an attractive technology over the conventional technology due to the some advantages, including low capital cost, low energy consumption, compact size, and modularity. A single pass through membrane usually can enrich O2 concentration in the air up to 35% and a 50% concentration can be achieved with a double pass of membrane. The use of OEA in the combustion process eliminates the presence of nitrogen in the flue gas. Hence, the flue gas is mainly composed of CO2 and condensable water that can be easily separated. This paper gives an overview of oxy-combustion with membrane technology for oxygen enrichment process. Special attention is given to OEA production and the effect of OEA to the efficiency of combustion.

Reversed thermo-switchable molecular sieving membranes composed of two-dimensional metal-organic nanosheets for gas separation

It is highly desirable to reduce the membrane thickness in order to maximize the throughput and break the trade-off limitation for membrane-based gas separation. Two-dimensional membranes composed of atomic-thick graphene or graphene oxide nanosheets have gas transport pathways that are at least three orders of magnitude higher than the membrane thickness, leading to reduced gas permeation flux and impaired separation throughput. Here we present nm-thick molecular sieving membranes composed of porous two-dimensional metal-organic nanosheets. These membranes possess pore openings parallel to gas concentration gradient allowing high gas permeation flux and high selectivity, which are proven by both experiment and molecular dynamics simulation. Furthermore, the gas transport pathways of these membranes exhibit a reversed thermo-switchable feature, which is attributed to the molecular flexibility of the building metal-organic nanosheets.

On the optimal design of membrane-based gas separation processes

Gas-separation processes are of paramount importance for several industrial applications. In this context, membrane-based gas separation has a great innovation potential in term of limiting the energy consumption and simplifying the process operation and control. Much of the research in this field focused on parametric analyses or optimization procedures aimed at the investigation of some performance indicators, such as separation performance, energy consumption, membrane area. This work aims at developing a comprehensive method which provides criteria for the optimal design and operation of membrane gas separation processes. First, a parametric analysis is carried out, resulting in the definition of attainable regions for different process configurations and product specifications. Subsequently, a multi-objective optimization is carried out by employing a genetic algorithm that minimizes the compression energy and the required membrane area by selecting the optimal process layout, operating variables, and membrane materials.

Origins and Evolution of Inorganic-Based and MOF-Based Mixed-Matrix Membranes for Gas Separations

Gas separation for industrial, energy, and environmental applications requires low energy consumption and small footprint technology to minimize operating and capital costs for the processing of large volumes of gases. Among the separation methods currently being used, like distillation, amine scrubbing, and pressure and temperature swing adsorption, membrane-based gas separation has the potential to meet these demands. The key component, the membrane, must then be engineered to allow for high gas flux, high selectivity, and chemical and mechanical stability at the operating conditions of feed composition, pressure, and temperature. Among the new type of membranes studied that show promising results are the inorganic-based and the metal-organic framework-based mixed-matrix membranes (MOF-MMMs). A MOF is a unique material that offers the possibility of tuning the porosity of a membrane by introducing diffusional channels and forming a compatible interface with the polymer. This review details the origins of these membranes and their evolution since the first inorganic/polymer and MOF/polymer MMMs were reported in the open literature. The most significant advancements made in terms of materials, properties, and testing conditions are described in a chronological fashion.

Preparation, structural characterization, and gas separation properties of functionalized zinc oxide particle filled poly(ether-amide) nanocomposite films

In this study, a new diacid monomer containing amide, imide, and sulfone functional groups was successfully used to synthesize a new poly(ether-amide) for membrane-based gas separation applications. The synthesized poly(ether-amide) was soluble in organic solvents, has high thermal stability (up to 460℃ under nitrogen atmosphere, for 10% weight loss), and high glass transition temperature (Tg = 274℃). This poly(ether-amide) was combined with different amounts of surface-modified zinc oxide nanoparticles to provide organic–inorganic nanocomposites. The optically transparent and flexible membranes of these hybrid nanocomposites were prepared. The obtained materials were characterized by Fourier transform-infrared spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, X-ray powder diffraction, field emission-scanning electron microscopy, and transmission electron microscopy techniques. Transmission electron microscopy of the nanocomposite film with 15 wt% zinc oxide confirms that the nanoparticles are well dispersed in the polymer matrix. Thermal gravimetric analysis data indicated that the hybrid materials had better thermal behavior with increasing surface-modified zinc oxide nanoparticles nanoparticle content. The poly(ether-amide)/surface-modified zinc oxide nanoparticles nanocomposite film mechanical properties improved with increasing surface-modified zinc oxide nanoparticles content. The permeability and selectivity of the poly(ether-amide)/zinc oxide membranes as a function of the surface-modified zinc oxide nanoparticles weight percentage were studied, and the results indicated that the CO2 and CH4 permeability increased with increasing zinc oxide content. In general, the membranes prepared from these polymers showed very good permeability and permselectivity for a pair of gases.

Compatibilized Immiscible Polymer Blends for Gas Separations.

Membrane-based gas separation has attracted a great deal of attention recently due to the requirement for high purity gasses in industrial applications like fuel cells, and because of environment concerns, such as global warming. The current methods of cryogenic distillation and pressure swing adsorption are energy intensive and costly. Therefore, polymer membranes have emerged as a less energy intensive and cost effective candidate to separate gas mixtures. However, the use of polymeric membranes has a drawback known as the permeability-selectivity tradeoff. Many approaches have been used to overcome this limitation including the use of polymer blends. Polymer blending technology synergistically combines the favorable properties of different polymers like high gas permeability and high selectivity, which are difficult to attain with a single polymer. During polymer mixing, polymers tend to uncontrollably phase separate due to unfavorable thermodynamics, which limits the number of completely miscible polymer combinations for gas separations. Therefore, compatibilizers are used to control the phase separation and to obtain stable membrane morphologies, while improving the mechanical properties. In this review, we focus on immiscible polymer blends and the use of compatibilizers for gas separation applications.

Effect of coagulant bath on the gas permeation properties of cellulose acetate asymmetric membrane

Membrane based gas separation process technology has been recognized as one of the most efficient and advanced unit operation for gas separation. One of the problems in membrane gas separation is membrane performance. This paper explores the application of cellulose acetate (CA) membrane for natural gas purification and separation by improving its permeability and selectivity. The main interest in this research is to study the effect of quench medium on the gas separation performance towards its physical characteristics and gas separation performance of CA membrane. Cellulose acetate polymer was dissolved in n- methyl-2-pyrrolidone solvent and casted onto a glass plate using a pneumatically controlled casting system with fixed shear rate and solvent evaporation times. The parameter varied was the non-solvent used as quench medium during membrane post treatment that were methanol and n-hexane. The different quench media as post treatment affected the O2 and N2 gas permeation and O2/N2 selectivity as well as the tensile strength of the flat sheet asymmetric membrane. Combination of methanol and n-hexane as quench media gave the best result than the other steps. This solvent exchange step influenced the morphology by producing thin skin layer and thus gives better gas separation performance than other steps

Mathematical modelling of membrane gas separation using the finite difference method

Membrane gas separation has been recognized as the main technology that is used in chemical engineering for hydrogen recovery, air separation, natural gas sweetening, helium recovery, natural gas dehydration, and so on. Membrane-based gas separation processes have great potential for these industrial applications because of their environmental friendliness, energy efficiency and ease of scale up. Mathematical modelling of the membrane-based gas separation process can be useful to predict the performance of such separation processes. An improved mathematical model has been implemented in this research for the separation of a binary gas mixture using a membrane separator. The finite difference method (FDM) is applied to predict the membrane gas separation behaviour. The method is helpful as it involves the least effort and computational time because algebraic equations are used instead of differential equations. Different configurations, such as single stage and double stage, are used in this study. The results of the FDM simulation are compared with the simulation results of the model and the experimental data of several membrane systems.

A novel intrinsically microporous ladder polymer and copolymers derived from 1,1′,2,2′-tetrahydroxy-tetraphenylethylene for membrane-based gas separation

A novel intrinsically microporous polymer was synthesized by polycondensation reaction of 1,1′,2,2′-tetrahydroxy-tetraphenylethylene (TPE) and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN).

Sorption Behavior of Compressed CO2 and CH4 on Ultrathin Hybrid Poly(POSS-imide) Layers.

Sorption of compressed gases into thin polymeric films is essential for applications including gas sensors and membrane based gas separation. For glassy polymers, the sorption behavior is dependent on the nonequilibrium status of the polymer. The uptake of molecules by a polymer is generally accompanied by dilation, or swelling, of the polymer material. In turn, this dilation can result in penetrant induced plasticization and physical aging that affect the nonequilibrium status of the polymer. Here, we investigate the dilation and sorption behavior of ultrathin membrane layers of a hybrid inorganic-organic network material that consists of alternating polyhedral oligomeric silsesquioxane and imide groups, upon exposure to compressed carbon dioxide and methane. The imide precursor contains fluoroalkene groups that provide affinity toward carbon dioxide, while the octa-functionalized silsesquioxane provides a high degree of cross-linking. This combination allows for extremely high sorption capacities, while structural rearrangements of the network are hindered. We study the simultaneous uptake of gases and dilation of the thin films at high pressures using spectroscopic ellipsometry measurements. Ellipsometry provides the changes in both the refractive index and the film thickness, and allows for accurate quantification of sorption and swelling. In contrast, gravimetric and volumetric measurements only provide a single parameter; this does not allow an accurate correction for, for instance, the changes in buoyancy because of the extensive geometrical changes of highly swelling films. The sorption behavior of the ultrathin hybrid layers depends on the fluoroalkene group content. At low pressure, the apparent molar volume of the gases is low compared to the liquid molar volume of carbon dioxide and methane, respectively. At high gas concentrations in the polymer film, the apparent molar volume of carbon dioxide and methane exceeds that of the liquid molar volume, and approaches that of the gas phase. The high sorption capacity and reversible dilation characteristics of the presented materials provide new directions for applications including gas sensors and gas separation membranes.

Fine-Tuned Intrinsically Ultramicroporous Polymers Redefine the Permeability/Selectivity Upper Bounds of Membrane-Based Air and Hydrogen Separations.

Intrinsically ultramicroporous (<7 Å) polymers represent a new paradigm in materials development for membrane-based gas separation. In particular, they demonstrate that uniting intrachain "rigidity", the traditional design metric of highly permeable polymers of intrinsic microporosity (PIMs), with gas-sieving ultramicroporosity yields high-performance gas separation membranes. Highly ultramicroporous PIMs have redefined the state-of-the-art in large-scale air (e.g., O2/N2) and hydrogen recovery (e.g., H2/N2, H2/CH4) applications with unprecedented molecular sieving gas transport properties. Accordingly, presented herein are new 2015 permeability/selectivity "upper bounds" for large-scale commercial membrane-based air and hydrogen applications that accommodate the substantial performance enhancements of recent PIMs over preceding polymers. A subtle balance between intrachain rigidity and interchain spacing has been achieved in the amorphous microstructures of PIMs, fine-tuned using unique bridged-bicyclic building blocks (i.e., triptycene, ethanoanthracene and Tröger's base) in both ladder and semiladder (e.g., polyimide) structures.

Bendable Zeolite Membranes: Synthesis and Improved Gas Separation Performance.

Separation and sequestration of CO2 emitted from fossil energy fueled electric generating units and industrial facilities will help in reducing anthropogenic CO2, thereby mitigating its adverse climate change effects. Membrane-based gas separation has the potential to meet the technical challenges of CO2 separation if high selectivity and permeance with low costs for large-scale manufacture are realized. Inorganic zeolite membranes in principle can have selectivity and permeance considerably higher than polymers. This paper presents a strategy for zeolite growth within the pores of a polymer support, with crystallization time of an hour. With a thin coating of 200-300 nm polydimethylsiloxane (PDMS) on the zeolite-polymer composite, transport data for CO2/N2 separation indicate separation factors of 35-45, with CO2 permeance between 1600 and 2200 GPU (1 GPU = 3.35 × 10(-10) mol/(m(2) s Pa)) using dry synthetic mixtures of CO2 and N2 at 25 °C. The synthesis process results in membranes that are highly reproducible toward transport measurements and exhibit long-term stability (3 days). Most importantly, these membranes because of the zeolite growth within the polymer support, as contrasted to conventional zeolite growth on top of a support, are mechanically flexible.