Single- and dual-phase capillary membranes prepared through plastic extrusion method for oxygen permeation

Two capillary membranes, single-phase Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and dual-phase 75 wt% Ce_0.85Sm_0.15O_1.925 - 25 wt% Sm_0.6Sr_0.4Cr_0.3Fe_0.7O_3-δ (SDC-SSCF), with dense cross section, were successfully prepared through the plastic extrusion method. The dual-phase capillary membrane shows higher strength compared to the BSCF counterpart, while the two capillary membranes exhibit much higher fracture strength than those of hollow fiber membranes. The oxygen permeation fluxes of both membranes increase with the increase of temperature and flow rate of sweep gas at the ambient pressure, and can be greatly improved by applying high pressures to the feed side. The oxygen permeation flux of BSCF capillary membrane is up to 19.5 mL cm^?2 min^?1 when 0.5 MPa air was applied to the feed side at 900 °C, which is one order of magnitude higher than that of SDC-SSCF capillary membrane. Thus, both capillary membranes have their own advantages and meet applications under different operation conditions.     

Two-step sintering technique for enhancing mechanical and oxygen permeation properties of dual-phase oxygen transport membranes

The present work involved the application of a two-step sintering (TSS) technique for typical Ce_0.9Gd_0.1O_2?δ–La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (GDC–LSCF) composites and its effect on the microstructure and resultantly mechanical and O₂-permeating properties. The samples obtained by conventional sintering (CS) performed at 1100, 1250, and 1400 °C for 3 h exhibited maximum flexural strength and hardness values of 142 MPa and 8.71 GPa, respectively. However, the application of a TSS procedure produced fine-grained microstructures with improved mechanical properties. In particular, with the use of a typical 1250/1200 TSS protocol, increases of approximately 31 % and 7% in the flexural strength and hardness values, respectively, were observed compared to those obtained using the CS method. Moreover, thick-film (～ 60 μm) GDC–LSCF membrane prepared by tape casting and the TSS technique showed remarkably 1.5–2 times higher oxygen permeation flux than membrane sintered by CS method. The results are discussed and explained in detail.     

High-flux CO2-stable oxygen transport hollow fiber membranes through surface engineering

The influences of bulk diffusion and surface exchange on oxygen transport of (La_0.6Ca_0.4)(Co_0.8Fe_0.2)O_3-δ (LCCF) hollow fiber membranes were investigated. As an outcome, two strategies for increasing the oxygen permeation were pursued. First, porous LCCF hollow fibers as support were coated with a 22 μm dense LCCF separation layer through dip coating and co-sintering. The oxygen permeation of the porous fiber with dense layer reached up to 5.10 mL min^?1 cm^-2 at 1000 °C in a 50 % CO₂ atmosphere. Second, surface etching of dense LCCF hollow fibers with H₂SO₄ was applied. The surface etching of both inner and outer surfaces leads to a permeation improvement up to 86.0 %. This finding implies that the surface exchange reaction plays a key role in oxygen transport through LCCF hollow fibers. A good long-term (>250 h) stability of the asymmetric hollow fiber in a 50 % CO₂ atmosphere was found at 900 °C.     

Modeling of steam permeation through the high temperature proton-Conducting ceramic membranes

The BaCeO₃-based perovskite oxide can be applied as a steam-permeable membrane due to their noticeable mixed oxygen ion and proton conducting properties. In this article, a transient model of the steam permeation through the BaCe_0.9Y_0.1O_3-δ perovskite membrane at elevated temperatures has been developed with the Poisson–Nernst–Planck equations for analyzing the steam permeation process, in which the distribution of charged defects in the membrane is carefully considered. The effects of the operating temperature, the membrane thickness, the steam partial pressure, and the electric potential difference between two membrane sides during the steam permeation have been simulated and discussed. The modeling results indicate there exists a maximum value of the spontaneous electric potential difference with the temperature rise and the external electric potential can obviously increase the steam flux. The model is validated using the experimental results from literature under steady state operational conditions. © 2018 American Institute of Chemical Engineers AIChE J, 65: 777–782, 2019     

Oxygen permeation of various archetypes of oxygen membranes based on BSCF

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ tubes, capillaries, capillary modules, and asymmetric membranes were prepared and tested for oxygen permeation in a dead‐end vacuum operation mode at temperatures up to 850^°C. The capillary module was built up by reactive air brazing using seven capillaries and a supply tube. Two machined discs were used as an end cap and as a connector plate. The oxygen permeation behaves according to Wagner at small driving forces, but significant negative deviations were observed for asymmetric membranes and single capillaries at higher ones. This is caused by pressure drops at the vacuum side for single capillaries. The highest oxygen flux was revealed for the capillary module with 175.5 mL(STP)/min at a low‐vacuum pressure of 0.042 bar at 850^°C, but the asymmetric membrane showing a little bit higher flux at moderate vacuum pressures above 0.07 bar. © 2012 American Institute of Chemical Engineers AIChE J, 58: 3195–3202, 2012     

Gas permeation and isobutane dehydrogenation over very thin Pd/ceramic membranes

An attempt was made in this study to prepare very thin palladium membranes. Pd was electrolessly deposited on the inner side surface of a mesoporous ceramic tube using a peristaltic pump to circulate the electroless solution through the tube. To obtain defect-free Pd membranes, a two-step deposition procedure from a dilute Pd²⁺ bath and a concentrated Pd²⁺ bath was practised with great care to avoid the difficulties associated with the mismatch of thermal expansion coefficients between Pd and the ceramic substrate. The various membranes produced were characterized through a study of their permeation behaviors. A dense Pd membrane about 2 μm thick was permselective to hydrogen. It was found that in this very thin Pd membrane case, the hydrogen permeation flux was better correlated to hydrogen pressure using a power higher than 0.5, showing deviation from Sieverts' law. To evaluate the efficiency of the prepared thin Pd/ceramic membranes, iso-butane dehydrogenation was used as a model reaction. Enhanced dehydrogenation yields were obtained in the membrane reactor due to the separation of hydrogen from the reaction medium.     

Oxyfuel combustion using a catalytic ceramic membrane reactor

Membrane catalytic combustion (MCC) is an environmentally friendly technique for heat and power generation from methane. This work demonstrates the performances of a MCC perovskite hollow fibre membrane reactor for the catalytic combustion of methane. The ionic–electronic La_0.6Sr_0.4Co_0.2Fe_0.8O₃₋ (LSCF6428) mixed conductor, in the form of an oxygen-permeable hollow fibre membrane, has been prepared successfully by means of a phase-inversion spinning/sintering technique. For this process polyethersulfone (PESf) was used as a binder, N-methyl-2-pyrrollidone (NMP) as solvent and polyvinylpyrrolidone (PVP, K16-18) as an additive. With the prepared LSCF6428 hollow fibre membranes packed with catalyst, hollow fibre membrane reactors (HFMRs) have been assembled to perform the catalytic combustion of methane. A simple mathematical model that combines the local oxygen permeation rate with approximate catalytic reaction kinetics has been developed and can be used to predict the performance of the HFMRs for methane combustion. The effects of operating temperature and methane and air feed flow rates on the performance of the HFMR have been investigated both experimentally and theoretically. Both the methane conversion and oxygen permeation rate can be improved by means of coating platinum on the air side of the hollow fibre membranes.     

Preparation of thermosensitive cellophane-graft-N-isopropylacrylamide copolymer membranes and permeation of solutes through the membranes

Thermosensitive membranes with high mechanical strength were prepared by heterogeneous graft copolymerization of N-isopropylacrylamide (NIPAAm) onto cellophane in a nitric acid solution using cerium ammonium nitrate as an initiator, and the permeation behavior of solutes such as lithium chloride and poly(ethylene glycol)s (PEGs) through the membranes at various temperatures was investigated. The degree of graft copolymerization of NIPAAm on cellophane depended on temperature, time, initiator concentration, and so on. The copolymer membranes having a high content of the NIPAAm moiety could be obtained at 25°C for 24 h. The permeation of Li⁺ through the membranes was affected by temperature, i.e., the permeation rate of Li⁺ increased with increasing temperature up to 32°C and then decreased rapidly above 35°C. The permeation rate of Li⁺ through the copolymer membranes at 40°C decreased considerably, but that at 20°C decreased slightly with an increasing amount of the NIPAAm moiety in the membranes. The permeation rate of PEGs with a molecular weight more than 1000 through the cellophane-g-NIPPAm copolymer membranes was considerably suppressed and only the permeation rate of PEG300 increased with increasing temperature up to 35°C and then decreased at 40°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 209–216, 1997     

Preparation of poly(vinyl alcohol)-graft-N-isopropylacrylamide copolymer membranes with triphenylmethane leucocyanide and permeation of solutes through the membranes

Thermo- and photosensitive hydrogel membranes were prepared by graft copolymerization of an N-isopropylacrylamide (NIPAAm) and triphenylmethane leucocyanide (LeCN) monomer onto poly(vinyl alcohol) (PVA). The yield of the graft copolymerization as well as the content of the poly(NIPAAm) segments grafted onto PVA was depressed by copolymerization of the LeCN monomer. The change in the permeation rate of poly(ethylene glycol)s through the hydrogel membranes was induced photochemically by dissociation of LeCN moieties in the membranes. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1931–1937, 1998     

Characteristics of ammonia permeation through porous silica membranes

A sol–gel method was applied for the preparation of silica membranes with different average pore sizes. Ammonia (NH₃) permeation/separation characteristics of the silica membranes were examined in a wide temperature range (50–400°C) by measurement of both single and binary component separation. The order of gas permeance through the silica membranes, which was independent of membrane average pore size, was as follows: He > H₂ > NH₃ > N₂. These results suggest that, for permeation through silica membranes, the molecular size of NH₃ is larger than that of H₂, despite previous reports that the kinetic diameter of NH₃ is smaller than that of H₂. At high temperatures, there was no effect of NH₃ adsorption on H₂ permeation characteristics, and silica membranes were highly stable in NH₃ at 400°C (i.e., gas permeance remained unchanged). On the other hand, at 50°C NH₃ molecules adsorbed on the silica improved NH₃‐permselectivity by blocking permeation of H₂ molecules without decreasing NH₃ permeance. The maximal NH₃/H₂ permeance ratio obtained during binary component separation was ～30 with an NH₃ permeance of ～10^?7 mol m^?2 s^?1 Pa^?1 at an H₂ permeation activation energy of ～6 kJ mol^?1. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Perovskite foams used in combination with dense ceramic membranes for oxygen transport membrane applications

The future industrial implementation of membranes for oxygen transport requires new designs to increase oxygen semi-permeation, together with robust performance under operating conditions. This work describes the development of innovative membrane designs based on perovskite foams with a very large porosity (above 90%) supporting thin dense perovskite membranes (thickness of 50–100?µm). The preparation of such foam-supported membranes is described in this paper. The performances in terms of oxygen semi-permeation are also measured and compared with the best results reported in the literature up to date.     

Fabrication of BSCF-based mixed ionic-electronic conducting membrane by electrophoretic deposition for oxygen separation application

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), which exhibits a high mixed oxide ionic-electronic conduction, was used for the fabrication of an oxygen separation membrane. An asymmetric structure, which was a thin and dense BSCF membrane layer supported on a porous BSCF substrate, was fabricated by the electrophoretic deposition method (EPD). Porous BSCF supports were prepared by the uniaxial pressing method using a powder mixture with BSCF and starch as the pore-forming agent (0–50 wt.%). The sintering behaviors of the porous support and the thin layer were separately characterized by dilatometry to determine the co-fired temperature at which cracking did not occur. A crack-free and thin dense membrane layer, which had about a 15 μm thickness and >95% relative density, was obtained after optimizing the processes of EPD and sintering. The dense/porous interface was well-bonded and the oxygen permeation flux was 2.5 ml (STP) min⁻¹ cm^-2 at 850 °C.     

Preparation and oxygen permeation of U‐shaped perovskite hollow‐fiber membranes

The oxygen permeation of dense U‐shaped perovskite hollow‐fiber membranes based on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ prepared by a phase inversion spinning process is reported. The perovskite hollow fibers with totally dense wall were obtained with the outer diameter of 1.147 mm and the inner diameter of 0.691 mm. The dependences of the oxygen permeation on the air flow rate on the shell side, the helium flow rate on the core side, the oxygen partial pressures, and the operating temperatures were experimentally investigated. According to the Wagner theory, it follows that the oxygen transport through the U‐shaped hollow‐fiber membrane is controlled by both surface reaction and bulk diffusion at the temperature ranges of 750–950°C. High oxygen permeation flux of 3.0 ml/(min cm²) was kept for about 250 h at 950°C under the conditions of the air feed flow rate of 150 ml/min and the helium flow rate of 50 ml/min. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Preparation and gas permeation of immobilized fullerene membranes

Fullerene‐dispersed membranes were homogeneously prepared under the conditions in which a 10 wt % polystyrene solution containing 1 wt % fullerene was dried under a reduced pressure of 50 cmHg at room temperature. The fullerene membranes prepared with 1,2‐dichlorobenzene were found to have the darkest color, and showed no evidence of fullerene crystals in their photomicrographs. UV‐visible and infrared absorption spectra of the fullerene membranes showed fullerene bands, which indicated that the fullerene was homogeneously dispersed in the membranes. The permeability coefficients of pure nitrogen, oxygen, carbon dioxide, ethane, and ethylene were found to increase significantly in the fullerene membranes compared to those in the polystyrene membranes, although the ideal separation factors for oxygen/nitrogen and ethylene/ethane in the fullerene membranes (i.e., 4.3 and 1.7, respectively) were slightly less than the separation factors in the polystyrene membranes. The permeability increase originated from the increase in diffusion coefficients in the fullerene membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 529–537, 2000     

Influence of oxygen partial pressure on the oxygen diffusion and surface exchange coefficients in mixed conductors

The performances of mixed ionic and electronic conductors is used in many applications, such as oxygen transport membranes or electrodes in solid oxide fuel cells. The performances of these systems depend mainly on two fundamental parameters including oxygen diffusion (D_O) and the oxygen exchange coefficient (k). This work focuses on the impact of the oxygen partial pressure on oxygen diffusion and surface exchange coefficients of mixed conducting materials, as reported studies are scarce in the literature. In this way, two different mixed conducting materials are studied, namely, La_0.6Sr_0.4Fe_0.6Ga_0.4O_3-δ, a perovskite-type material, and the nickelate La₂NiO_4+δ. The D_O and k coefficients are determined by a specific oxygen permeation measurement and by the isotopic exchange depth profile method.     

Enhanced oxygen permeation through perovskite hollow fibre membranes by methane activation

Hollow fibre membranes of mixed conducting perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) were prepared via the combined phase inversion and sintering technique. The fibres were tested for air separation with a home-made reactor under the oxygen partial pressure gradient generated by the air/He streams. Some fibres were in situ activated by introducing methane in the He sweeping gas at high temperatures. The activated membranes with new morphology were created by transforming the inner densified surface layer to a porous structure. Compared to the original membranes, the activated gave appreciable higher oxygen fluxes. At 800 °C, the oxygen fluxes were increased by a factor of 10 after activation was carried out at 1000 °C for 1 h.     

Effect of metal‐support interface on hydrogen permeation through palladium membranes

Thin palladium membranes of different thicknesses were prepared on sol‐gel derived mesoporous γ‐alumina/α‐alumina and yttria‐stabilized zirconia/α‐alumina supports by a method combining sputter deposition and electroless plating. The effect of metal‐support interface on hydrogen transport permeation properties was investigated by comparing hydrogen permeation data for these membranes measured under different conditions. Hydrogen permeation fluxes for the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes are significantly smaller than those for the Pd/YSZ/α‐Al₂O₃ membranes under similar conditions. As the palladium membrane thickness increases, the difference in permeation fluxes between these two groups of membranes decreases and the pressure exponent for permeation flux approaches 0.5 from 1. Analysis of the permeation data with a permeation model shows that both groups of membranes have similar hydrogen permeability for bulk diffusion, but the Pd/γ‐Al₂O₃/α‐Al₂O₃ membranes exhibit a much lower surface reaction rate constant with higher activation energy, due possibly to the formation of Pd‐Al alloy, than the Pd/YSZ/α‐Al₂O₃ membranes. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Enhanced permeation performance of polyether-polyamide block copolymer membranes through incorporating ZIF-8 nanocrystals

Membrane-based CO2 separation is a promising alternative in terms of energy and environmental issues to other conventional techniques.Polyether-polyamide block copolymer (Pebax) membranes are promising for CO2 separation because of their excellent selectivity,but limited by their moderate gas permeability.In this study,fresh-prepared zeolitic imidazolate framework-8 (ZIF-8) nanocrystals were integrated into the Pebax(R)1657 matrices to form mixed matrix membranes.The resulting membrane exhibits significantly improved CO2 permeability (as high as 300％ increase),without the sacrifice of the selectivity,to the pristine polymer membrane.Several physical characterization techniques were employed to confirm the good interfacial interaction between ZIF-8 fillers and Pebax matrices.The effect of added ZIF-8 fillers on the transport mechanism through MMMs is also explored.Mixed-gas permeation for both CO2/N2 and CO2/CH4 was also evaluated.The separation performance for CO2/CH4 mixtures on the ZIF-8/Pebax MMMs is very close to the Roberson upper bound,and thus is technologically attractive for purification of natural gas.     

Effects of Al content on the oxygen permeability through dual-phase membrane 60Ce0.9Pr0.1O2-δ-40Pr0.6Sr0.4Fe1-xAlxO3-δ

Ceramic dual-phase oxygen transport membranes with the composition of 60 wt% Ce_0.9Pr_0.1O_2-δ-40 wt%Pr_0.6Sr_0.4Fe_1-xAl_xO_3-δ (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0) (60CPO-40PSF_1-xA_xO) based on 60Ce_0.9Pr_0.1O_2-δ-40Pr_0.6Sr_0.4FeO_3-δ doped Al was successfully synthesized through a modified Pechini method. Crystal structure, surface microtopography and oxygen permeability are investigated systematically. The cell parameters of perovskite phase first increased and then decreased with the increase of Al content, which is related to the radius of the Al³⁺ and the formation of impurity phase. As x ranges from 0.1 to 0.8, the oxygen permeability of the materials first increases and then decreases, and the maximum value of oxygen permeation rate for 60CPO-40PSF_1-xA_xO membranes with 0.4 mm thickness at 1000 °C is 1.12 mL min⁻¹ cm⁻² when x = 0.4. XRD measurements revealed high temperature stability and CO₂-tolerant property of the dual-phase composites. The partial replacement of Fe³⁺/Fe⁴⁺ by Al³⁺ causes the material not only to exhibit good stability, but also to increase the oxygen permeability of the membranes.     

Modelling and simulation of hydrogen permeation through supported Pd-alloy membranes with a multicomponent approach

Hydrogen transport in Pd-based supported membranes was described by means of a model considering several elementary steps of the permeation process, improving what done by Ward and Dao [1999. Model of hydrogen permeation behavior in palladium membranes. Journal of Membrane Science 153 (2), 211-231] for self-supported membranes. The model includes the external mass transfer in the multicomponent gaseous phases on both sides of the membrane, described by the Stefan-Maxwell equations. The transport of the multicomponent mixture in the multilayered porous support was also considered and described by means of the dusty gas model, which takes into account Knudsen, Poiseuille and ordinary diffusion. The diffusion in the Pd-alloy layer is modeled by the irreversible thermodynamics theory, taking the hydrogen chemical potential as the driving force of the diffusion in the metallic bulk. The interfacial phenomena (adsorption, desorption, transition from Pd-based surface to Pd-based bulk and vice-versa) were described by the same expressions used by Ward and Dao (1999). Thicknesses of 1 and are considered for the Pd-alloy layer. The asymmetric support consists of five layers, each one characterized by a specific thickness and mean pore diameter. The model separates the permeation steps and consequently their influence, quantifying the relative resistances offered by each of them. Comparison with some experimental data in several conditions in the literature shows a good agreement. The developed tool is able to describe hydrogen transport through a supported Pd-based membrane, recognizing the rate-determining steps (e.g., diffusion in the metallic bulk or in the porous support) involved in the permeation.