Mixed Gas Stream Research Articles

Synthesis and characterization of crosslinked poly(vinyl alcohol)/poly(allylamine)/2-amino-2-hydroxymethyl-1,3-propanediol/polysulfone composite membrane for CO2/N2 separation

This study reports novel CO2-selective crosslinked thin film composite poly(vinyl alcohol) (PVA)/amine blend membranes, its structural characterizations and permeation studies. The characteristic properties of the active layer were examined with Thermogravimetric analysis (TGA), Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Effects of feed pressure, temperature, and sweep side water flow rate on transport properties (CO2 and N2 flux, CO2 permeability, and CO2/N2 selectivity) were investigated with a mixed gas stream containing 20% CO2 and 80% N2 by volume. The temperature, feed pressure and sweep side water flow rate were varied from 90 to 125°C, 1.7 to 6.2atm and 0.02 to 0.075cm3/min, respectively. The good miscibility properties, obtained from the blended amines with crosslinked PVA, allowed a thin selective dense layer to be formed on self-made porous PSf support, which displays both high selectivity and permeability. The maximum CO2/N2 selectivity of 260 and a CO2 permeability of 555 Barrer were obtained for the membrane with 14µm active layer thickness. Crosslinked PVA with blended amines membrane also exhibited a good stability during a 250h test.

Competitive I2 Sorption by Cu-BTC from Humid Gas Streams

Competitive sorption of molecular iodine gas from a mixed stream containing iodine and water vapor is identified and characterized for the hydrophilic Cu-BTC metal–organic framework. By combining simulation (Grand Canonical Monte Carlo and molecular dynamics simulations) with crystallography (high-energy synchrotron-based powder X-ray diffraction data and pair distribution function analyses), we show that I2 substantially adsorbs, in preference to water vapor, into two principal areas. First, it adsorbs in the smallest cage close to the copper paddle wheel. Second, it adsorbs within the main pore with close interactions with the benzene tricarboxylate organic linker. Analysis suggests that I2 forms an effective hydrophobic barrier to minimize water sorption. The finding is relevant to mixed gas streams in nuclear energy industrial processes and accident remediation. This also represents the highest reported I2 sorption by a metal–organic framework (175 wt % I2 or 3 I/Cu).

Rapid prediction of hydrogen permeation through amorphous metal membranes: an efficient computational screening approach

Efficient purification of hydrogen from high temperature mixed gas streams using dense metal membranes can potentially play a critical role in the large-scale production of hydrogen from gasification of coal or biomaterials. We use first-principles calculations together with statistical methods to systematically predict hydrogen permeability through amorphous ternary Zr–Cu–T films (T = 17 elements) and other selected amorphous materials. These results greatly expand the range of amorphous materials that have been considered as hydrogen purification membranes. More importantly, we demonstrate that relatively simple descriptions of the site binding energies in these amorphous materials can account for the key observations from our detailed first-principles calculations. This outcome significantly reduces the computational effort required in future screening of materials in this application and also places bounds on the ultimate performance of these materials.

Hydrogen separation by dual functional cermet membranes with self-repairing capability against the damage by H2S

A Pd-YSZ cermet is synthesized to fabricate a hydrogen separation membrane that performs coupled operations of the separation of hydrogen from a mixed-gas stream containing H2S and the simultaneous production of hydrogen by water-splitting. The reason for the loss of hydrogen permeation function of Pd-based membranes in H2S environment has been the formation of Pd4S by the reaction of H2S with Pd. In the present study, the high thermal and chemical stability of YSZ in H2S environment helps operation at higher temperature to attain a high hydrogen permeation flux and, also, the Pd-YSZ membrane shows better tolerance against H2S because of its self-repairing capacity against the damage by H2S. The self-repairing capacity of Pd-YSZ membrane originates mainly from the conversion of Pd4S back into metallic Pd and SO2 by ambipolar-diffused oxygen obtained from water-splitting. Also, the operation at higher temperatures helps in reducing the effective concentration of H2S in the mixed-gas stream by converting a part of H2S into SO2 and H2 by ambipolar-diffused oxygen assisted reactions. The Pd-YSZ cermet membrane devised in the present work gives a comprehensive solution to many issues related with the hydrogen separation membranes.

A three-dimensional microvascular gas exchange unit for carbon dioxide capture.

For the capture of CO(2) from mixed gas streams, materials for increased gas exchange are necessary. Efficient gas exchange systems already exist in the form of vascularized lung-tissue. Herein we report a fabrication technique for the synthesis of three-dimensional microvascular gas exchange units capable of removing CO(2) from flowing gas created using the recently reported Vaporization of a Sacrificial Component (VaSC) technique. We demonstrate the spatiotemporal pattern of CO(2) reactivity in the microvascular gas exchange unit using colorimetric, pH sensitive dyes. Control over three-dimensional placement of channels is shown to increase capture efficiencies. A computational finite element model validates and explains the experimental observations.

Humidified Gas Stream Separations at High Temperatures Using Matrimid 5218

Most industrially relevant high temperature gas separations (≥150°C) of either carbon dioxide (flue gas) or hydrogen (syn-gas) must be performed in the presence of water vapor. At ambient temperatures, water vapor can permeate easily through most polymeric membranes and can influence the permeation of other gases through interaction with the polymer, such as swelling and clustering. At higher temperatures, water vapor can be destructive to polymer membranes by changing the polymer structure that can result in diminished gas separation performance. Little data has been reported on the influence of water vapor in gas separations at >100°C because most polymers are not stable at temperature. Many high performance (HP) polymers are able to endure high temperatures and aggressive chemical conditions. For example, polyimides are promising HP polymers that effectively separate permanent gases at temperatures higher than 150°C under dry conditions. In this report, the analysis of selected HP polymers in humidified gas streams (2–4 vol% water) shows that they can perform modest separations at ambient temperatures. In general, it was observed that water vapor permeability is greater than other tested gases. Additionally, the permeabilities of the analyte gases were somewhat influenced by the presence of humidity and their selectivities were significantly lower, as compared to corresponding experiments performed in the absence of water. To elucidate the role of water vapor in gas transport, energy of activation of permeation (Ep) values were obtained for Matrimid 5218 from 30–200°C in a humidified mixed gas stream, and it was found that the selectivities are nearly identical to dry gas streams at 150°C. This data suggests that water vapor functions as a gas and only slightly decreases selectivity of the other gases at elevated temperatures. As a result, economic wet gas separations may be possible using these materials if the gas stream is kept at higher temperature (≥150°C), which is assisted by the inherent stability of the membranes.

Identifying Metal Alloys with High Hydrogen Permeability Using High Throughput Theory and Experimental Testing

Dense metal membranes are one useful approach for purifying H2 from mixed gas streams. A longstanding challenge in the development of metal membranes has been to find film compositions that give high permeability for H2 relative to well-known materials such as elemental Pd. We used first-principles calculations to predict the H2 permeability of all disordered alloys of composition Pd96M4. These calculations not only identify the Pd-based alloys that are already known to have favorable membrane properties but also identify as promising a number of materials that have not been previously examined. We tested our predictions by fabricating and testing PdTm films, which, in agreement with our calculations, were found to have good permeability properties for pure H2.

Sulfur-Resistant NO Decomposition Catalysts Derived from Co−Ca/Ti−Al Hydrotalcite-like Compounds

Co1.5M1.5/Al1−xTix hydrotalcite-like compounds (where M = Co, Ca and x = 0, 0.1) were synthesized by a constant-pH coprecipitation. The derived oxides from hydrotalcites upon calcination at 800 °C for 4 h were all of spinel phase without crystalline TiO2 phase being detected. Substitution of partial Al for Ti significantly enhanced NO direct decomposition activity of these catalysts. In particular, catalyst Co3.0/Al0.9Ti0.1O (CATO) showed the highest NO direct decomposition percentage, up to 86% at 300 °C with GHSV of 30 000 h−1 (800 ppm of NO and 8% O2 in N2 stream). CATO also showed the highest resistance to SO2 poisoning to NO direct decomposition, with the activity being only reduced by 16% in the presence of 64 ppm of SO2 in the mixed gas stream at 300 °C. The in-situ FT-IR spectra indicate different adsorption species over the catalysts, revealing NO surface storage/decomposition involves different adsorption reactions that determine the NO decomposition activity and resistance to SO2 poisoning.

CO2 desorption using high-pressure bipolar membrane electrodialysis

The electrodialysis of gas evolving solutions may prove to be an important technology for many gas-separation applications, including CO2 and SO2 separation from mixed-gas streams. Progress on the use of electrodialysis for gas separation has been hampered by the increased resistance caused by gas bubbles on the surface of the electrodialysis membranes. This effect reduces the effective membrane surface area, causing increased voltages and reduced membrane lifetimes due to localized “hot spots” of high current density. To overcome this problem, we designed, constructed, and tested a bipolar membrane electrodialysis (BPMED) system designed to operate up to pressures as high as 20 atm. For given process conditions, operation at a sufficiently high pressure keeps all gas dissolved in solution, eliminating the problems caused by gas bubbles on the membrane surfaces. We performed CO2 desorption from aqueous bicarbonate solutions, demonstrating that high pressures decrease the resistance, voltage, and energy of the desorption process. Our results demonstrate that at high current densities (139 mA cm−2), the CO2 desorption energy from aqueous bicarbonate solutions under high-pressure operation can be 29% lower than under ambient-pressure operation.

Palladium–gold membranes in mixed gas streams with hydrogen sulfide: Effect of alloy content and fabrication technique

Self-supported Pd–Au membranes were produced by magnetron sputtering and cold working with compositions between 7 and 20 wt% Au. Permeation tests were performed in synthetic water-gas shift reaction mixtures with up to 50 ppm H 2S. Membranes with higher gold content showed less flux inhibition by either carbon or sulfur containing species, regardless of fabrication technique or thickness. A 20 wt% Au alloy had the same permeability in pure hydrogen as it did in a sulfur-free WGS mixture, and only lost 40% of its permeability in a 20 ppm H 2S mixture. However, membranes produced by sputtering experienced irreversible loss of hydrogen selectivity when exposed to mixtures containing sulfur, caused by a significant decrease in membrane thickness. No equivalent decrease in thickness or selectivity was observed for cold-worked membranes. This metal loss is explained by a corrosive mechanism in which palladium sulfides flake off the feed surface and are entrained. Although sulfidation also occurs in cold-worked membranes only sputtered membranes display this corrosion, due to a different microstructure in which grains are oriented perpendicular to the surface.

Selective separation of low concentration CO2 using hydrogel immobilized CA enzyme based hollow fiber membrane reactors

This paper reported the results of developing a novel hollow fiber membrane reactor contained immobilized enzyme for selective separation of low concentration CO2 from mixed gas streams. In the reactor, two bundles of poly(vinylidene fluoride) (PVDF) hollow fiber membranes were aligned staggered parallel in a tube module, and lab-made poly(acrylic acid-co-acrylamide)/hydrotalcite (PAA-AAm/HT) nanocomposite hydrogel was filled between fibers, in which carbonic anhydrase (CA enzyme) was immobilized. The effects of CA concentration, buffer concentration, the flow rate of sweep gas, operational temperature, and CO2 concentration on separation performance were investigated in detail. The results showed that the transport resistance was mainly from the hydrogel layer, and decreased greatly with immobilization of carbonic anhydrase in hydrogel. Moreover, immobilized CA could retain over 76% enzymatic activity and thermal stability was also improved. The data showed that this enzyme-based membrane reactor could effectively separate CO2 at low concentration from mixed gas streams. For the feed with 0.1% (v/v) of CO2, the selectivity of CO2/N2 was 820, CO2/O2 was 330, and CO2 permeance was 1.65×10−8 mol/m2 s Pa. Prolonged runs lasting 30 h showed that separation performances of the membrane reactor were quite stable.

The Modulation of Optical Property and its Correlation with Microstructures of ZnO Nanowires

ZnO nanowires with both good crystallinity and oxygen vacancies defects were synthesized by thermal oxidation of Zn substrate pretreated in concentrated sulfuric acid under the air atmosphere, Ar- and air-mixed gas stream. The photoluminescence spectra reveal that only near-band-edge (NBE) emission peak was observed for the sample grown in the air atmosphere; the broad blue–green and the red-shifted NBE emission peaks were observed for the sample grown in the mixed gas stream, indicating that the sample grown in the mixed gas stream has a defective structure and its optical properties can be modulated by controlling its structure. The high-resolution transmission electron microscope and the corresponding structural simulation confirm that the oxygen vacancies exist in the crystal of the nanowires grown in the mixed gas stream. The ZnO nanowires with oxygen vacancies defects exhibit better photocatalytic activity than the nanowires with good crystallinity. The photocatalytic process obeys the rules of first-order kinetic reaction, and the rate constants were calculated.

NANOSTRUCTURED ALUMINA CERAMIC MEMBRANES FOR GAS SEPARATION

Nanostructured alumina ceramic templates have been fabricated by anodizing annealed high-purity aluminium foil. Pore diameter, pore separation and thickness in these alumina ceramics can be controlled using a range of acid electrolytes and anodizing voltage profiles. Thermal development of the structure of these robust and optically clear templates have been compared using XRD, thermal analysis and 27 Al MAS NMR techniques, showing that species substituted in the alumina lattice from decomposition of the acid electrolyte play a major role in determining the chemical and physical stability of the ceramic template at elevated temperatures. Deposition of ultrathin palladium films on the surface of these alumina templates creates robust membranes that enable hydrogen separation from mixed gas streams at elevated temperatures. Gas permeability measurements through these membranes as a function of temperature have demonstrated their very high selectivity for hydrogen.

The Effect of SO2 on CO2 Absorption in an Aqueous Potassium Carbonate Solvent

In solvent absorption systems for carbon dioxide capture, potassium carbonate shows particular promise as a solvent. One of the advantages of this solvent is that it is more manageable by way of separation and/or cost effectiveness in the presence of SO2. This paper examines the feasibility of using the same absorber system for simultaneous capture of both CO2 and SO2 from post combustion flue gases. Using a dynamic equilibrium apparatus, the changes in the vapour pressure were measured upon the introduction of sulphur dioxide to a CO2/ N2 mixed gas stream. It has been shown that there is a small change in the resultant vapour pressure. The overall affect of SO2 on the efficiency of CO2 capture is discussed.

Carbon dioxide separation from hydrogen and nitrogen: Facilitated transport in arginine salt–chitosan membranes

CO 2-selective membranes that obtain high CO 2 permeabilities accompanied with high CO 2/H 2 and CO 2/N 2 separation factors at industrial temperatures and pressures are applicable to fuel cell operations and flue gas purification. This paper describes the separation of carbon dioxide from a mixed gas stream of hydrogen and nitrogen by a chitosan membrane containing 40 wt% sodium arginate. Continuous membrane separations were done for a feed gas with 10% carbon dioxide, feed gas total pressures of 152 and 507 pa (1.5 and 5 atm), and temperatures ranging from 20 to 150 °C. The addition of arginine salts increases the number of amino groups for facilitated transport of CO 2 and increases the water levels in the arginine salt–chitosan membranes compared to swollen chitosan membranes at the same humidification conditions. At 152 pa (1.5 atm) feed pressure and 110 °C, there are maxima in the carbon dioxide permeabilities (1500 barrers), and the separation factors for CO 2/N 2 (852) and CO 2/H 2 (144). At higher pressure (507 pa (5 atm)), there were no maxima in the carbon dioxide permeabilities or separation factors, and there was less bound water in the membrane.

Using first-principles calculations to accelerate materials discovery for hydrogen purification membranes by modeling amorphous metals

Efficient purification of hydrogen from high temperature mixed gas streams can potentially play a critical role in the large-scale production of hydrogen from gasification of coal or biomass. Dense metal membranes have many favorable properties for this kind of purification, but existing membranes based on crystalline metal alloys have a number of limitations. The use of amorphous metal films as membranes has potential to overcome at least some of the disadvantages of crystalline metal membranes. We present new modeling methods that make it possible for the first time to quantitatively predict the performance of amorphous metal films as hydrogen purification membranes. These methods are introduced by examining amorphous Fe3B, a material where comparisons can be made to a crystalline material with the same composition. A membrane made from the amorphous material is predicted to have a hydrogen permeability 1.5–2 orders of magnitude higher than a crystalline membrane. The methods we introduce here will be useful in accelerating the development of amorphous membranes for practical applications.

Mixed-Conducting Membranes for Hydrogen Production and Separation

AbstractMixed-conducting oxides, possessing both ionic and electronic charge carriers, have found wide application in recent years in solid-state electrochemical devices that operate at high temperatures, e.g., solid-oxide fuel cells, batteries, and sensors. These materials also hold promise as dense ceramic membranes that separate gases such as oxygen and hydrogen from mixed-gas streams. We are developing Sr-Fe-Co oxide (SFC) as a membrane that selectively transports oxygen during partial oxidation of methane to syngas (mixture of CO and H2) because of SFC's high combined electronic and ionic conductivities. We have evaluated extruded tubes of SFC for conversion of methane to syngas in a reactor that was operated at ≈900°C. Methane conversion efficiencies were >90%, and some of the reactor tubes were operated for >1000 h. We are also developing dense proton-conducting oxides to separate pure hydrogen from product streams that are generated during methane reforming and coal gasification. Hydrogen selectivity in these membranes is nearly 100%, because they are free of interconnected porosity. Although most studies of hydrogen separation membranes have focused on proton-conducting oxides by themselves, we have developed cermet (i.e., ceramic-metal composite) membranes in which metal powder is mixed with these oxides in order to increase their hydrogen permeability. Using several feed gas mixtures, we measured the nongalvanic hydrogen permeation rate, or flux, for the cermet membranes in the temperature range of 500-900°C. This rate varied linearly with the inverse of membrane thickness. The highest rate, ≈32 cm3(STP)/min-cm2, was measured at 900°C for an ≈15-μm-thick membrane on a porous support structure when 100% H2 at ambient pressure was used as the feed gas.

Micromixing of Miscible Liquids in Segmented Gas−Liquid Flow

We present an integrated microfluidic system that achieves efficient mixing between two miscible liquid streams by introducing a gas phase, forming a segmented gas-liquid (slug) flow, and completely separating the mixed liquid and gas streams in a planar capillary separator. The recirculation motion associated with segmented flow enhances advection in straight microchannels without requiring additional fabrication steps. Instantaneous velocity fields are quantified by microscopic particle image velocimetry (muPIV). Velocities in the direction normal to the channel amount to approximately 30% of the bulk liquid velocity inside a liquid segment. This value depends only weakly on the length of a liquid segment. Spatial concentration fields and the extent of mixing (EOM) are obtained from pulsed-laser fluorescence microscopy and confocal scanning microscopy measurements. The mixing length is reduced 2-3-fold in comparison with previously reported chaotic micromixers that use three-dimensional microchannel networks or patterned walls. Segmented gas-liquid microflows allow mixing times to be varied over several orders of magnitude between milliseconds and second time scales.

A Novel Amperometric O2 Gas Sensor Based on Supported Room‐Temperature Ionic Liquid Porous Polyethylene Membrane‐Coated Electrodes

AbstractA novel solid‐state amperometric O2 gas sensor based on the supported 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EMIBF4) porous polyethylene membrane‐coated electrodes has been proposed with its characterization. By electrochemical impedance technique, the ionic conductivity of the supported EMIBF4 membrane was estimated to be ca. 0.6 S m−1, indicating that the supported EMIBF4 membrane (the thickness: 50 μm) can be used as a solid state ionic conductor at room temperature. The cyclic voltammograms obtained for the one‐electron redox reaction of O2/O2−.(O2−.: superoxide ion) couple at high scan rates (>100 mV s−1) showed a couple of usual redox peaks, while at low scan rates (<30 mV s−1) S‐shaped steady‐state voltammograms similar to those obtained by rotating disk voltammetry were obtained. These results were explained on the basis of the mass transport of O2 at the supported EMIBF4 membrane‐coated electrode system. The transient and steady‐state reduction currents for the reduction of O2 to O2−.as well as the transient oxidation current for the reoxidation of O2−.to O2, which were obtained by potential‐step chronoamperometry, could be used to measure the change of O2 concentration in O2‐N2 mixed gas stream. The present O2 gas sensor demonstrated a wide detection range, a high sensitivity and an excellent reproducibility.

Highly CO2‐Permeable and Selective Polymer Nanocomposite Membranes

As technologies turn to more efficient routesto remove CO2from mixed gas streams for H2or air purification, new cost‐effective multifunctional materials are needed for the separation processes. Here, hybrid poly(ethylene glycol)/fumed silica nanocomposite membranes exhibiting high CO2/H2selectivity (> 11) and CO2permeability (∼ 83 Barrer), as well as robust mechanical properties and optical clarity, are reported (see Figure).