Gas transport in vertically-aligned carbon nanotube/parylene composite membranes

Vertically-aligned carbon nanotube (VACNT) composite membranes were fabricated by impregnating carbon nanotube (CNT) forests with poly-para-xylylene (parylene-C) through room-temperature chemical vapor deposition (CVD). Transport properties of diverse gases through the CNT/parylene membranes were investigated. The gas permeances scaled inversely with their molecular weights in accordance with Knudsen model and the value of permeance was about 30 times higher than that predicted by the Knudsen diffusion kinetics, which was attributed to the atomically smooth interiors of CNTs. In addition, the gas permeance values in this work were higher than those reported for other VACNT membranes, due to the smaller membrane thickness and good crystallinity of the CNTs.     

Synthesis and performances of bio-sourced nanostructured carbon membranes elaborated by hydrothermal conversion of beer industry wastes

Hydrothermal carbonization (HTC) process of beer wastes (Almaza Brewery) yields a biochar and homogeneous carbon-based nanoparticles (NPs). The NPs have been used to prepare carbon membrane on commercial alumina support. Water filtration experiments evidenced the quasi-dense behavior of the membrane with no measurable water flux below an applied nitrogen pressure of 6 bar. Gas permeation tests were conducted and gave remarkable results, namely (1) the existence of a limit temperature of utilization of the membrane, which was below 100°C in our experimental conditions, (2) an evolution of the microstructure of the carbon membrane with the operating temperature that yielded to improved performances in gas separation, (3) the temperature-dependent gas permeance should follow a Knudsen diffusion mechanism, and (4) He permeance was increasing with the applied pressure, whereas N₂ and CO₂ permeances remained stable in the same conditions. These results yielded an enhancement of both the He/N₂ and He/CO₂ permselectivities with the applied pressure. These promising results made biomass-sourced HTC-processed carbon membranes encouraging candidates as ultralow-cost and sustainable membranes for gas separation applications.     

Silicalite-1 zeolite composite membranes

This review paper discusses the preparation of silicalite-1 zeolite membranes, the experimental procedures used for gas separation measurements and the results of single gas and gas mixture experiments. Silicalite-alumina composite membranes were prepared by an in-situ zeolite synthesis method using an alumina membrane tube with a 5-nm-pore-diameter, γ-alumina layer as a substrate. Single gas permeances of H₂, Ar, n-C₄H₁₀, i-C₄H₁₀ and SF₆ were measured and mixtures of H₂/i-C₄H₁₀ and H₂/SF₆ were separated to characterize the silicalite membrane. These measurements were made from 300 to 737 K. Transport through the silicalite membrane appeared to be controlled by molecular size and adsorption properties. Permeances of all components studied were activated, and activation energies ranged from 8.5 to 16.2 kJ/mol. The ratio of single gas permeances was as high as 136 for H₂/SF₆ and 1100 for H₂/i-C₄H₁₀ at 298 K. Separation selectivities at elevated temperatures were significantly above Knudsen diffusion selectivity and were larger than ratios of pure gas permeances at the same temperature. The largest permeance ratio for the separation of mixtures was 12.8 for H₂/SF₆ at 583 K. Separation selectivities were higher when a pressure drop was maintained across the membrane than when an inert sweep gas was used because of counter diffusion of the sweep gas.     

Effects of Thermal Treatment on Gas Transport Through Porous Silica Membranes

Abstract

The effects of thermal treatment from 180°C to 1150°C on the gas transport properties of porous silica membranes were systematically studied for various gases. The permeance of all gases, except for CO₂, has a maximum at 800°C. The CO₂ permeance was constant from 180°C to 600°C and then decreased monotonically. Membranes thermally treated at 1150°C did not exhibit any gas permeation because of pore collapse. The gas transport behavior follows a combination of Knudsen diffusion and surface diffusion for all gases tested except for carbon dioxide. The permeation of carbon dioxide is strongly affected by capillary condensation. We propose a new transport model composed of two components; that is, the Knudsen diffusion factor, α, and the surface diffusion factor, β. A transition was observed for α and β at around 800–900°C, which is close to the strain point of the membrane. This transition treatment temperature can be correlated with the changes in gas permeance. The model allows qualitative evaluation of gas transport through porous membranes regardless of their actual microporous structures.     

气体透过碳膜的非平衡动力学模拟研究

The permeation of various pure gas （H2, He, Ne, CH4 and At） through carbon membranes is investigated using a dual control volume grand canonical molecular dynamics method. A two-dimensional slit pore is employed instead of the one-dimensional pore. Compared with the experiments, simulation results show that the improvement of pore model is very necessary. The effects of membrane thickness, pore width and temperature on gas permeance and ideal separation factor are also discussed. Results show that gas permeates through membrane according to Knudsen diffusion in large pore, while Knudsen diffusion is accompanied by molecular sieving in small pore. Moreover, methane is easily adsorbed on the membrane surface due to strong attractive interactions of membrane and shows higher permeance than that of Knudsen flow. In addition, it is noted that when membrane thickness is thin enough the permeance of gas does not decrease with the increase of membrane thickness due to the strong adsorption until membrane resistance becomes dominant.     

Gas permeation characteristics of silica/alumina composite membrane prepared by chemical vapor deposition

Amorphous silica membranes were deposited by thermal decomposition of tetraethoxysilane at 600–650 ‡C on a porous α-alumina tube with pore size of 110–180 nm or γ-alumina coated α-alumina tube with pore size of 6–8 nm.The forced cross-flow through the porous wall of the support was very effective in plugging macropores. The membranes formed on γ-alumina coated oc-alumina tube showed H₂ permeances much higher than the SiO₂ membranes formed on the α-alumina tube. This indicated that the γ-alumina film was effective in improving the H₂ permeance and H₂/N₂ selectivity. The permeation tests with CO₂, N₂, CH₄, C₃H₈ and i-C₄H₁₀ showed that a very small number of mesopores remained unplugged by the CVD. Permeation of hydrogen was explained by activated diffusion, and that of the other gases by Knudsen diffusion through the unplugged pores. Thus, the total permeance was composed of permeances due to the activated and Knudsen diffusion mechanisms. The contribution of Knudsen diffusion pores decreased to 0.02 when the γ-alumina film was modified at 650 ‡C until P_fe=50 Pa.     

Permeation Characteristics of Light Hydrocarbons Through Poly(amide-6-β-ethylene oxide) Multilayer Composite Membranes

In this paper, poly(amide-6-β-ethylene oxide) (PEBA1657) copolymer was used to prepare multilayer polyetherimide (PEI)/polydimethylsiloxane (PDMS)/PEBA1657/PDMS composite membranes by dip-coating method. Permeation behaviors of ethylene, ethane, propylene, propane, n-butane, methane and nitrogen through the multilayer composite membranes were investigated over a range of operating temperature and pressure. The permeances of light hydrocarbons through PEI/PDMS/PEBA1657/PDMS composite membranes increase with their increasing condensability, and the olefins are more permeable than their corresponding paraffins. For light hydrocarbons, the gas permeances increase significantly as temperature increasing. When the transmembrane pressure difference increases, the gas permeance increases moderately due to plasticization effect, while their apparent activation energies for permeation decrease.     

Diffusive transport through mesoporous silica membranes

The results of a detailed study of the permeation of several light gases through unmodified and modified mesoporous silica membranes are reported. The base membranes which were synthesized by dip-coating multiple layers of a templated silica onto a macroporous alumina support showed relatively high permeances with evidence of both support resistance and a contribution from viscous flow, in addition to Knudsen diffusion, through the active layer. The behavior of the modified membranes, which were prepared by silanation of the original membranes with octadecyldimethylchlorosilane, was more interesting. Permeances were lower and there was no evidence of support resistance or viscous flow. Permeation through the active layer appeared to occur by a “Knudsen-like” process. Permeance and permeance ratios measured in both single component and binary systems showed the characteristic inverse dependence on the square root of the molecular weight (or the molecular weight ratio) but the temperature dependence was much stronger than expected for classical Knudsen flow. The behavior of CO₂ was somewhat anomalous yielding permeances that were about 25% larger than those for propane which has the same molecular weight.     

Modified gas‐translation model for prediction of gas permeation through microporous organosilica membranes

A modified gas‐translation (GT) model was applied for the theoretical analysis of gas permeation through microporous organosilica membranes derived from bis(triethoxysilyl)ethane (BTESE) via a sol–gel method using different water/alkoxide molar ratios. The pore sizes of BTESE‐derived membranes were quantitatively determined by normalized Knudsen‐based permeance analysis, which was based on a modified‐GT model, using experimentally obtained permeances of He, H₂, N₂, C₃H₈, and SF₆. The pore sizes of BTESE‐derived membranes were successfully controlled from 0.65 to 0.46 nm by increasing the H₂O/BTESE ratio from 6 to 240. Furthermore, theoretical correlations of all possible pairs of permeance ratios were calculated based on the modified‐GT model. The experimental data were in good agreement with the theoretical correlation curves, indicating that the modified‐GT model can clearly explain gas permeation mechanisms through microporous membranes, and, thus, can be used to predict the gas permeation properties for these membranes. © 2014 American Institute of Chemical Engineers AIChE J 60: 4199–4210, 2014     

Effect of support layer on gas permeation properties of composite polymeric membranes

PES/Pebax and PEI/Pebax composite membranes were prepared by coating the porous PES and PEI substrate membranes with Pebax-1657. The morphology and performance of the prepared membranes were investigated by SEM and CO₂ and CH₄ permeation tests. The CO₂ permeances of 28 and 52 GPU were achieved for PES/Pebax and PEI/Pebax composite membranes, respectively, with CO₂/CH₄ selectivities almost equal to that of Pebax (26). The experimental data were further subjected to a theoretical analysis using the resistance model. It was found that the porosity and the thickness of the dense section of PES substrate were an order of magnitude higher than those of PEI substitute. The porosity/thickness ratio of PEI substrate was, however, higher than PES, explaining the higher permeance of PEI/Pebax composite membrane. Substrates with porosities much higher than the Henis-Tripodi gas separation membrane were used in this work, aiming to achieve the selectivity of Pebax, rather than those of the substrate membrane materials.     

Effects of pressure and temperature on fixed-site carrier membrane for CO2 separation from natural gas

In this paper, the effect of testing temperature on the performance of fixed carrier membrane for CO₂ separation were studied. The blend composite membranes were developed respectively with a blend of PEI-PVA (polyetheleneimine-polyvinyl alcohol) as separation layer and PS (polysulfone) ultrafiltration membranes as the substrates. The permselectivity of the membranes was measured with CO₂/CH₄ mixed gas. The effect of testing temperature on membrane separation performance was investigated. The results showed that both the permeances of CO₂ and CH₄ decreased with the increase of temperature, and the permeances decreased more quickly under low pressure than those under high pressure. At the feed pressure of 0.11 MPa, the CO₂/ CH₄ selectivity of PEI-PVA/PS blend composite membrane reduced along with temperature increment. Under the feed pressure of 0.21 MPa, as well as 1.11 MPa, the selectivity decreased with the increase of temperature.     

Effects of pressure and temperature on fixed-site carrier membrane for CO<Subscript>2</Subscript> separation from natural gas

In this paper, the effect of testing temperature on the performance of fixed carrier membrane for CO₂ separation were studied. The blend composite membranes were developed respectively with a blend of PEI-PVA (polyetheleneimine-polyvinyl alcohol) as separation layer and PS (polysulfone) ultrafiltration membranes as the substrates. The permselectivity of the membranes was measured with CO₂/CH₄ mixed gas. The effect of testing temperature on membrane separation performance was investigated. The results showed that both the permeances of CO₂ and CH₄ decreased with the increase of temperature, and the permeances decreased more quickly under low pressure than those under high pressure. At the feed pressure of 0.11 MPa, the CO₂/ CH₄ selectivity of PEI-PVA/PS blend composite membrane reduced along with temperature increment. Under the feed pressure of 0.21 MPa, as well as 1.11 MPa, the selectivity decreased with the increase of temperature.     

Evaluation and fabrication of pore‐size‐tuned silica membranes with tetraethoxydimethyl disiloxane for gas separation

Organic/inorganic hybrid silica membranes were prepared from 1,1,3,3‐tetraethoxy‐1,3‐dimethyl disiloxane (TEDMDS) by the sol‐gel technique with firing at 300–550°C in N₂. TEDMDS‐derived silica membranes showed high H₂ permeance (0.3–1.1 × 10^?6 mol m^?2 s^?1 Pa^?1) with low H₂/N₂ (～10) and high H₂/SF₆ (～1200) perm‐selectivity, confirming successful tuning of micropore sizes larger than TEOS‐derived silica membranes. TEDMDS‐derived silica membranes prepared at 550°C in N₂ increased gas permeances as well as pore sizes after air exposure at 450°C. TEDMDS had an advantage in tuning pore size by the “template” and “spacer” techniques, due to the pyrolysis of methyl groups in air and Si? O? Si bonding, respectively. For pore size evaluation of microporous membranes, normalized Knudsen‐based permeance, which was proposed based on the gas translation model and verified with permeance of zeolite membranes, reveals that pore sizes of TEDMDS membranes were successfully tuned in the range of 0.6–1.0 nm. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Gas separation using sol-gel derived microporous zirconia membranes with high hydrothermal stability☆

A microporous zirconia membrane with hydrogen permeance about 5 × 10?8 mol·m?2·s?1·Pa?1, H2/CO2 permselectivity of ca. 14, and excellent hydrothermal stability under steam pressure of 100 kPa was fabricated via polymeric sol–gel process. The effect of calcination temperature on single gas permeance of sol–gel derived zirconia membranes was investigated. Zirconia membranes calcined at 350 °C and 400 °C showed similar single gas permeance, with permselectivities of hydrogen towards other gases, such as oxygen, nitrogen, methane, and sulfur hexafluoride, around Knudsen values. A much lower CO2 permeance (3.7 × 10?9 mol·m?2·s?1·Pa?1) was observed due to the interaction between CO2 molecules and pore wall of membrane. Higher calcination tem-perature, 500 °C, led to the formation of mesoporous structure and, hence, the membrane lost its molecular siev-ing property towards hydrogen and carbon dioxide. The stability of zirconia membrane in the presence of hot steam was also investigated. Exposed to 100 kPa steam for 400 h, the membrane performance kept unchanged in comparison with freshly prepared one, with hydrogen and carbon dioxide permeances of 4.7 × 10?8 and~3 × 10?9 mol·m?2·s?1·Pa?1, respectively. Both H2 and CO2 permeances of the zirconia membrane de-creased with exposure time to 100 kPa steam. With a total exposure time of 1250 h, the membrane presented hydrogen permeance of 2.4 × 10?8 mol·m?2·s?1·Pa?1 and H2/CO2 permselectivity of 28, indicating that the membrane retains its microporous structure.     

Separation of VOCs from N2 using poly(ether block amide) membranes

This work deals with the separation of volatile organic compounds (VOCs) from nitrogen streams for organic vapour emission control by poly(ether block amide) membranes. As representative air pollutant VOCs, n‐pentane, n‐hexane, cyclohexane, n‐heptane, methanol, ethanol, n‐propanol, n‐butanol, acetone, dimethyl carbonate, and methyl tert‐butyl ether were used in this study. The separation of both binary VOC/N₂ and multicomponent VOCs/N₂ gas mixtures was carried out, and the membranes exhibited good separation performance. A VOC concentration of more than 90 mol% was achieved at a feed VOC concentration of 5 mol%. It was found that the permeances of the VOCs were mainly dominated by their solubilities in the membrane, whereas the permeance of N₂ was affected by the presence of the VOCs. The permeance of N₂ in the VOC/N₂ mixtures was shown to be higher than pure N₂ permeance due to membrane swelling induced by the VOCs dissolved in the membrane. Nevertheless, theVOC/N₂ selectivity increased with an increase in the feed VOC concentration. Among the VOCs studied, the membrane showed a higher permeance to alcohol VOCs than paraffin VOCs. The effects of feed VOC concentration, temperature, stage cut, and permeate pressure on the separation performance were investigated.     

钯复合膜的制备及其在氢气氮气分离中的应用

Thin palladium composite membranes were prepared by modified electroless plating method on a-alumina supports and a dense Pd/α-Al2O3 composite membrane with high hydrogen flux, good selectivity for hydrogen was obtained. It was tested in a single gas permeation system for hydrogen permeance and hydrogen selectivity over mtrogen. The hydrogen permeance of the corresponding membrane was ashigh as 2.45×10^-6mol·m^-2·s^-1.Pa^-1 and H2/N2 selectivityover700 at 623K and a pressure difference of 0.1MPa. The-main resistance of the composite membrane to H2 permeation lies in the aluminum ceramic support rather than the thin Pd layer.     

Evaluation of hydrogen permselective separation from “synthesis gas” components based on single gas permeability measurements on anodic alumina membranes

Anodic alumina membranes were prepared by anodizing aluminum followed by chemical and hydrothermal treatments. Anodization was performed on both tube surfaces. External anodization was restricted to selected spots leaving an aluminum mechanically strong frame. Nitrogen sorption data analysed with the CPSM method (Corrugated Pore Structure Model) detected a mesopore structure (i.e. D_mean;15–20) and surface areas of 2–20 m²/g. SEM microscopy images revealed a regular pore structure of independent pores with densities of N_pore ~ 5.6 × 10¹⁴ pores/m². Single gas permeances (Π) for X: H₂, CH₄, CO and CO₂ were measured on a Wicke–Kallenbach apparatus at varying mean transmembrane pressure P_m by the “dead-end” method. The observed slight dependence of Π on Ρ_m is indicative of strong Knudsen diffusion and weak viscous flow contributions. By correlating the experimental data with a linear Π vs Ρ_m relationship, Knudsen contribution evaluation was enabled, and found to vary in the range K_C ≈ 0.7–1.0. The Knudsen number criterion for flow regime discrimination is critically discussed and a realistic dual Knudsen number approach is proposed. Experimental permselectivities α_Η2Χ = Π_H2/Π_X (X ≠ H₂) approach by 70–100% their Knudsen selectivity counterparts. Anodic alumina membranes exhibit pore structure and gas permeability characteristics useful in designing integrated gas separation and catalytic membrane reactor systems.     

N2优先渗透ZIF-8复合膜的制备及其CO2捕集

为了获得经济节能的烟道气CO₂回收方法,制备了一种新型的N₂优先渗透ZIF-8复合膜。以柔性聚砜(PSf)多孔膜为支撑层,采用Zn²⁺与壳聚糖的交联溶液对聚砜支撑层表面改性,使Zn²⁺固定在PSf膜表面;然后与2-甲基咪唑(Hmim)配位得到ZIF-8晶种层;最后通过界面聚合法二次生长制得ZIF-8复合膜。采用FTIR、XRD及SEM对ZIF-8复合膜的形貌结构进行表征,结果显示成功制备了致密的ZIF-8复合膜。在进料气为纯气条件下,探究了二次生长时间、Zn²⁺溶液的浓度、测试时间及测试压力对ZIF-8复合膜N₂/CO₂分离性能的影响,阐明其N₂优先渗透机理;并进一步考察了混合气分离性能。结果表明：在25℃和0.1 MPa下,最优ZIF-8复合膜的N₂渗透性为523 GPU,N₂/CO₂选择性为19;同条件下混合气的N₂渗透性和N₂/CO₂选择性分别为517 GPU和18。所制备的ZIF-8复合膜可以使N₂优先渗透,实现烟道气中高浓度N₂渗透,低浓度CO₂截留在膜的上游侧。原因主要是ZIF-8复合膜含有较多的CO₂强吸附位点,使CO₂被吸附在膜内不易从膜的下游侧脱附,渗透性小,而N₂优先渗透,这为N₂优先渗透膜的制备提供了一种新思路。     

Characterization of composite membranes by their non-equilibrium thermodynamic transport parameters

Composite reverse osmosis membranes were prepared by interfacially polymerizing aromatic polyamide discriminating layers on the inside surface of microporous polyethersulfone hollow fibers and on the surface of flat sheet polysulfone ultrafilters. The salt rejection and flux of these membranes were measured at various feed pressures. From these measurements, the membrane reflection coefficients, salt permeances and hydraulic permeances were estimated. Neither the polysulfone ultrafilters nor the microporous polyethersulfone hollow fibers possessed any inherent salt rejecting capability. Both had a hydraulic permeance at least two orders of magnitude greater than that of the respective composite membranes. Consequently, it was concluded that the estimated transport parameters for both composite membranes were characteristics exclusively of thier polyamide discriminating layers. Comparison of these transport parameters generated insight into structural and functional aspects of the membrane that could not be visualized by scanning electron microscopy.     

High‐Selectivity Y2O3‐Doped SiO2 Nanocomposite Membranes for Gas Separation in Steam at High Temperatures

Asymmetric structures were fabricated by depositing Y₂O₃‐doped SiO₂ (Si/Y) membranes onto γ‐Al₂O₃ supported by tubular α‐Al₂O₃. The thickness of the Y₂O₃‐doped SiO₂ deposits was approximately 100 nm. The deposits/membranes have micropores with a pore diameter ~ <0.40–0.55 nm. Pore size distribution measurements were conducted directly on the membranes before and after hydrothermal treatment with a nano‐permporometer. The gas permeance properties of the membranes were measured in the temperature range 100°C–500°C. The Y‐doped SiO₂ membrane (Si/Y = 3/1) was found to exhibit asymptotically stable permeances of 2.39 × 10^?7 mol/m²/s/Pa for He and 6.19 × 10^?10 mol/m²/s/Pa for CO₂, with a high selectivity of 386 (He/CO₂) at 500°C for 20 h in the presence of steam. The Y‐doped silica membranes exhibit very high gas permeances for molecules with smaller kinetic diameters. The apparent activation energies of the H₂ permeance at 400°C were 24.2 ± 0.2 and 21.3 ± 0.7 kJ/mol for SiO₂ and Si/Y, respectively.