Multicomponent gas separation by selective surface flow (SSF) and poly-trimethylsilylpropyne (PTMSP) membranes

A selective surface flow (SSF) membrane consisting of a thin layer of a nanoporous carbon was produced in a tubular form using a macroporous alumina support. The membrane was tested for hydrogen enrichment applications. Simulated waste gases from a petrochemical refinery and a hydrogen pressure swing adsorption unit were used as the feed gas to the membrane. Very high rejections of C₁C₃ hydrocarbons (saturated and unsaturated) and carbon dioxide over hydrogen were exhibited by the membrane at low feed gas pressures. The hydrogen enriched stream was produced at the feed gas pressure.The separation characteristics of a polymeric poly-trimethylsilylpropyne (PTMSP) membrane in a tubular form was also tested for the same applications using identical conditions of operation. This membrane also selectively rejected heavier components of the feed gas mixture over hydrogen and produced the hydrogen enriched stream at the feed gas pressure. The SSF membrane exhibited much higher hydrogen recovery and hydrocarbon rejections than the PTMSP membrane for these applications under identical conditions of operations using identical support materials.     

Multicomponent gas separation by an asymmetric permeator containing two different membranes

A numerical analysis of an asymmetric permeator containing two different kinds of membranes and capable of separating a ternary feed gas mixture into three product streams has been carried out. For negligible axial pressure drops in the gas streams, equations have been developed and calculation methods illustrated for four flow patterns: cross, parallel, countercurrent and perfectly mixed. The main parameters used in the analysis are pressure ratios, the ratio of the two membrane areas and the permeabilities. Results have been presented for a 50% H₂10% CO₂40% N₂ feed with a permeator having one cellulose acetate (CA) type membrane and one silicone membrane. For both permeate streams, countercurrent achieves highest permeate qualities and requires least membrane area amongst all four flow patterns. The presence of a silicone membrane produces a richer H₂ permeate from the CA membrane than that from a CA-membrane-alone configuration. Simultaneously, an enriched CO₂ permeate is obtained from the silicone membrane. Under the ideal condition of zero pressure ratios, the asymmetric permeator has also been evaluated against the series configuration of a permeator containing a CA membrane only followed by a permeator containing only a silicone (S) membrane. The asymmetric configuration usually performs better than this series configuration for the chosen range of parameters.     

Analysis of trace levels of impurities and hydrogen isotopes in helium purge gas using gas chromatography for tritium extraction system of an Indian lead lithium ceramic breeder test blanket module

In the fusion fuel cycle, the accurate analysis and understanding of the chemical composition of any gas mixture is of great importance for the efficient design of a tritium extraction and purification system or any tritium handling system. Methods like laser Raman spectroscopy and gas chromatography with thermal conductivity detector have been considered for hydrogen isotopes analyses in fuel cycles. Gas chromatography with a cryogenic separation column has been used for the analysis of hydrogen isotopes gas mixtures in general due to its high reliability and ease of operation. Hydrogen isotopes gas mixture analysis with cryogenic columns has been reported earlier using different column materials for percentage level composition. In the present work, trace levels of hydrogen isotopes (∼100 ppm of H₂ and D₂) have been analyzed with a Zeolite 5A and a modified γ‐Al₂O₃ column. Impurities in He gas (∼10 ppm of H₂, O₂, and N₂) have been analyzed using a Zeolite 13‐X column. Gas chromatography with discharge ionization detection has been utilized for this purpose. The results of these experiments suggest that the columns developed were able to separate ppm levels of the desired components with a small response time (<6 min) and good resolution in both cases.     

Determination of H2 and D2 content in metals and alloys using hot vacuum extraction

A hot vacuum extraction technique for the determination of hydrogen in metal and alloy samples has been standardised. After measuring the total pressure of the evolved gases, individual hydrogen and deuterium intensities are measured using an on-line quadrupole mass spectrometer. Synthetic mixtures of H₂ and D₂, in known concentrations, have been analysed by QMS and an analytical expression correlating the measured [D₂]/[HD] intensity ratio with the mole fraction of deuterium in the synthetic mixture has been arrived at. The precision and accuracy in the measurement of hydrogen is about 10% at 50 ppmw level.     

Simulation of binary gas separation in hollow fiber asymmetric membranes by orthogonal collocation

Modeling of hollow fiber asymmetric membrane modules can provide useful guidelines to achieve desirable separations of gas mixtures. In this work the performance of a countercurrent flow separator was analyzed through a parametric study of the most important system variables as functions of basic design and operational parameters. Results refer to CO₂–N₂ separation from power station flue gases as a typical, potential process. The appropriate model equations were solved by orthogonal collocation to approximate differential equations, and to solve the resulting system of non-linear algebraic equations by the Brown method. This technique compared to other applied computational procedures minimized the computational time and effort and improved solution stability. This is very important if the pressure and concentration profiles along the permeator, both in the residue and the permeate streams, need to be determined. These profiles influence strongly the permeator performance and, under certain conditions such as moderate and high feed pressure, they may result in lower than expected permeate purity. The simulation results also indicate that the role of the basic design parameters may be of equal if not higher importance to membrane selectivity. Thus industrial permeator performance, as it is expressed by stage cut and permeate purity, is not very sensitive to membrane permselectivity beyond a modest value of 40–50, especially at moderate and high (15–20 bar) feed pressures. A desirable gas separation may then be achievable with a reasonably permeable, albeit not very selective membrane, provided that design and operating variables are selected appropriately.     

Dihydrogen carrier gases in gas-solid chromatography. Study of hydrogen weight and nuclear spin isomer adsorption

Summary A new version of gas-solid chromatography (GSC) of hydrogen isotopes and spin isomers which makes use of the adsorbable dihydrogen carrier gases (H₂, HD and D₂) is described. Major advantages of the new technique in comparison with conventional GSC with inert carrier gases are discussed. The new version of GSC was found to be a powerful tool for acquisition of the data necessary to design adsorption systems to separate hydrogen isotopes.     

A comparison of some recycle permeators for gas separations

The performances of several different recycle permeators are compared by calculating the enrichment, extend of separation, and area requirements for different overall stage cuts and recycle ratios, using the separation of air as an example. Membrane modules investigated included a two-unit series type permeator with permeate recycle, a continuous membrane column with high-pressure feed, and two single-unit countercurrent permeators with permeate recycle (both high-pressure and low-pressure feed). The use of Rony's extent of separation in the analysis greatly aids in the visualization of limiting permeator performance at different recycle ratios. The composition of O₂ in the high-pressure product (reject) stream becomes zero at specific values of the stage cut which depend on recycle ratio for all but the low-pressure feed module. This would require an infinite permeator area, hence stage cuts greater than this limiting value are not possible. For the case of the low-pressure feed unit, the reject stream composition approaches a minimum finite value that depends on pressure ratio in the permeator. This limiting minimum composition would also be reached at specific values of stage cut that depend on recycle ratio and hence a stage cut greater than this limiting value cannot be obtained. Other permeator types (co-current, perfect mixed models) do not behave in this manner. It is concluded that the two-unit series type permeator gives the best separation and requires smaller membrane areas at high recycle ratios. Performance of the four permeators is not greatly different at lower recycle ratios.     

Hydrogen Isotope Exchange Reactions in an Atmospheric Pressure Discharge Utilizing Water as Carrier Gas

Allowing water/hydrogen or water/hydrogen/He gas mixture to flow through micro- hollow type of electrodes and applying 60 Hz AC power between the electrodes made it possible to sustain large area and atmospheric pressure discharge. The electrode assembly was constructed by sandwiching a dielectric spacer with two thin metal sheets and boring an array of micro holes through them. Another variation of the assembly was prepared by stacking thin metallic sheets so that the stack functions as an electrode through which the gas mixture flows for generating dielectric barrier discharge. A large volume of the gas mixture, while producing plasma, underwent instantaneous hydrogen isotope exchange reactions between H₂O and D₂O or between D₂O and H₂ gas molecules. The efficiency of the atmospheric pressure discharge was assessed by measuring the extent of the exchange reactions at a given flow rate of the gas mixture.     

Separation performance of foils from Pd—In(6%)—Ru(0.5%), Pd—Ru(6%), and Pd—Ru(10%) alloys and influence of CO<Subscript>2</Subscript>, CH<Subscript>4</Subscript>, and water vapor on the H<Subscript>2</Subscript> flow rate through the test membranes

The H₂ flow rate through the 30-μm thick foil from Pd—Ru(6%) and Pd—Ru(10%) alloys at 673 and 773 K was found to be controlled by the diffusion of H atoms in the foil bulk. The interrelation between hydrogen permeability through the Pd—In(6%)—Ru(0.5%), Pd—Ru(10%), Pd—Ru(6%), and Pd—Ag(23%) membranes and the permeability pre-exponential factors in the Sieverts equation in the 573—773 K temperature interval indicated that the hydrogen permeability depended on the structural characteristics of palladium alloys. The influence of the CO₂, CH₄, and water vapor impurities on the H₂ flow rate through the studied membranes depended on the driving force nature (the sweep gas or transmembrane pressure) used for the development of the partial hydrogen pressure difference across the membrane. The negative influence of CH₄ and CO₂ was observed only when using a transmembrane pressure and at the impurity content of 20% or more. This effect increased with increasing temperature in the 573—773 K range, with the influence of CO₂ being more pronounced due to its reaction with hydrogen leading to the formation of CO. The influence of water vapor was studied at its 11—23% content in hydrogen and at 573 and 773 K of temperature. The negative influence of water vapor was found to subside as its content in the hydrogen mixture decreased and the temperature increased. It was shown that water vapor can be used as a sweep gas and at T = 773 K its influence on the H₂ flow rate through the membrane was almost the same as that of N₂.     

Tubular ultrafiltration flux prediction for oil-in-water emulsions: analysis of series resistances

Using the resistance-in-series (RIS) approach to permeate flux modeling, a general relationship between permeate flux, transmembrane pressure, cross-flow velocity, and feed kinematic viscosity was developed for the tubular ultrafiltration (UF) of synthetic oil-in-water emulsions. The fouling layer resistance, R_f, was 63% of the total membrane resistance, R_m′; however, concentration polarization was the predominant factor controlling resistance in the tubular UF system. An explicit form of the resistance index, Φ, was postulated based on the observed interactions between Φ, cross-flow velocity and feed kinematic viscosity and the RIS model was modified to further describe the interactions between permeate flux and operational parameters. The modified model adequately predicted flux–pressure data over the range of experimental variables examined in this study. Additionally, a set point operating pressure was determined as a function of cross-flow velocity and feed viscosity to achieve a balance between polarization and total membrane resistance.     

Carbon dioxide separation from hydrogen and nitrogen by fixed facilitated transport in swollen chitosan membranes

The separation of carbon dioxide from hydrogen and nitrogen at high temperatures would be valuable to fuel cell, flue gas purification, and ammonia processes. A feed gas mixture of carbon dioxide, hydrogen and nitrogen (10% CO₂, 10% H₂, and 80% N₂) was used to evaluate water-swollen chitosan as a facilitated transport membrane for these applications. The amino group of the chitosan repeating unit could be the fixed carrier that facilitates carbon dioxide transport in the presence of water.     

In Situ Observation of Hydrogen‐Induced Surface Faceting for Palladium–Copper Nanocrystals at Atmospheric Pressure

Nanocrystal (NC) morphology, which decides the number of active sites and catalytic efficiency, is strongly determined by the gases involved in synthesis, treatment, and reaction. Myriad investigations have been performed to understand the morphological response to the involved gases. However, most prior work is limited to low pressures, which is far beyond realistic conditions. A dynamic morphological evolution of palladium–copper (PdCu) NC within a nanoreactor is reported, with atmospheric pressure hydrogen at the atomic scale. In situ transmission electron microscopy (TEM) videos reveal that spherical PdCu particles transform into truncated cubes at high hydrogen pressure. First principles calculations demonstrate that the surface energies decline with hydrogen pressure, with a new order of γ_H‐001<γ_H‐110<γ_H‐111 at 1 bar. A comprehensive Wulff construction based on the corrected surface energies is perfectly consistent with the experiments. The work provides a microscopic insight into NC behaviors at realistic gas pressure and is promising for the shaping of nanocatalysts by gas‐assisted treatments.     

Separation characteristics of air by polysulfone hollow fiber membranes in series

Characteristics of air separation are determined in a serial configuration of hollow fiber polysulfone membranes. One, two, and three separation cells in series are used in the measurements. All systems are operated in the counter-current flow mode and effects of the reject flow rate and feed pressure are considered in the measurements. The plug flow model is used to simulate and analyze the system. Results include variations in species permeance, stage cut, permeate enrichment, reject depletion, and recovery of oxygen and nitrogen gases. Most of the plug flow model predictions are found to closely match the measured data, with deviations less than 10%. However, deviations in N₂ recoveries are found to be larger than other system parameters, with deviations close to 30%. Increase of the number of separation cells results in higher stage cuts and in turn to higher species recovery in the permeate stream. Simultaneously, the purity of the reject is increased and that of the permeate stream is decreased. At constant reject flow rate, the highest permeate enrichment is found in the permeate stream of the first cell in the two- and three-cell systems. This is caused by the increase in the feed flow rate, which results in reduction of the gas residence time and in turn the gas permeation is highly selective and is dominated by the fast permeating species O₂.     

Treatment Variable Effects on Supercritical Gasification of High-Diversity Grassland Perennials

Low-input high-diversity (LIHD) mixtures of native grassland perennials were subjected to a supercritical treatment process with the aim of obtaining hydrogen-rich gases. The process was studied based on the following treatment variables: reaction temperature (374 °C to 575 °C, corresponding to a pressure range of 22.1 to 40 MPa), residence time (10 to 30 min), biomass content in the feed, and catalysts (0% to 4% NaOH and solid alkali CaO–ZrO₂). The gaseous phase produced from gasification of LIHD primarily consisted of hydrogen (H₂), with a mixture of carbon monoxide (CO), methane (CH₄), and carbon dioxide (CO₂). The statistical significance of treatment variables was evaluated using analysis of variance (ANOVA). It showed that at the level of P?<?0.05, temperature, catalysts, and biomass content in the feed significantly affected gas yields, while residence time was not significant.     

Molecular Filtration of Hydrocarbon Isomers Through Polymer/[Liquid Crystal] Composite Membranes

Ultrathin membranes of a polymer/(liquid crystal) mixture were prepared by spreading a single drop of a casting solution on the water surface. The thickness and the aggregation state of the water-cast membrane can be controlled by the kind of solvent and the concentration of the solution. In the case of a liquid crystalline state above the crystal-nematic phase transition temperature, T _KN, the polymer (liquid crystal) composite membrane follows Henry's law for the sorption isotherm of hydrocarbon gases and, also, Fickian sorption for the sorption-desorption kinetics. These results indicate that hydrocarbon gases permeate through a homogeneous medium composed of liquid crystalline molecules. Therefore, the permeability coefficients of hydrocarbon gases can be controlled by the dimensions of the channels through which the gas molecules diffuse. The channel for diffusion is generated by thermal or fluctuating molecular motion which opens up the intermolecular distance between liquid crystalline molecules. In the case of a self-supported liquid crystalline membrane, the channel dimension can be controlled in the range of several Å by both the intermolecular distance and the degree of thermal molecular motion of the liquid crystalline molecules. Separation of hydrocarbon isomers was investigated by use of composite membranes composed of a polymer matrix and self-supported liquid crystalline molecules.     

Preparation and characterization of a composite PDMS membrane on CA support

Polydimethylsiloxane (PDMS) is the most commonly used membrane material for the separation of condensable vapors from lighter gases. In this study, a composite PDMS membrane was prepared and its gas permeation properties were investigated at various upstream pressures. A microporous cellulose acetate (CA) support was initially prepared and characterized. Then, PDMS solution, containing crosslinker and catalyst, was cast over the support. Sorption and permeation of C₃H₈, CO₂, CH₄, and H₂ in the prepared composite membrane were measured. Using sorption and permeation data of gases, diffusion coefficients were calculated based on solution‐diffusion mechanism. Similar to other rubbery membranes, the prepared PDMS membrane advantageously exhibited less resistance to permeation of heavier gases, such as C₃H₈, compared to the lighter ones, such as CO₂, CH₄, and H₂. This result was attributed to the very high solubility of larger gas molecules in the hydrocarbon‐based PDMS membrane in spite of their lower diffusion coefficients relative to smaller molecules. Increasing feed pressure increased permeability, solubility, and diffusion coefficients of the heavier gases while decreased those of the lighter ones. At constant temperature (25°C), empirical linear relations were proposed for permeability, solubility, and diffusion coefficients as a function of transmembrane pressure. C₃H₈/gas solubility, diffusivity, and overall selectivities were found to increase with increasing feed pressure. Ideal selectivity values of 9, 30, and 82 for C₃H₈ over CO₂, CH₄, and H₂, respectively, at an upstream pressure of 8 atm, confirmed the outstanding separation performance of the prepared membrane. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of operating variables on the flux and selectivity in sweep gas membrane distillation for dilute aqueous isopropanol

Sweep gas membrane distillation was examined as a possible technique for isopropanol (IPA)–water separation using PTFE hollow fiber membrane module. The composition and flux of the permeate were monitored when feed concentration, operating temperature and flow rate were varied. The upper feed concentration tested was 10 wt.% IPA. Within the feed temperature range of about 20–50°C, IPA selectivity of 10–25 was achieved. Since the concentration near the surface on the membrane increased by the selective adsorption of IPA on the hydrophobic membrane, the selectivity increases. The permeate flux and IPA selectivity increase as feed temperature increase. The flux and selectivity increase at higher flow rates is mainly due to the reduced effects of concentration and temperature polarization. The effect of salt addition to the feed mixture was also examined.     

氢分离与CO催化甲烷化双功能型钯复合膜

以多孔Al₂O₃陶瓷为基体材料, 采用浸渍法担载NiO后用2B铅笔修饰NiO/Al₂O₃表面, 通过化学镀法沉积约5 μm厚的金属钯, 还原后成功制得Pd/Pencil/Ni/Al₂O₃膜. 为进行对比, 还制备了未担载镍的Pd/Pencil/Al₂O₃膜. 膜的表面和断面形貌分别采用扫描电镜和金相显微镜观测, 膜的透氢动力学通过H₂/N₂单气体法测试, 并以成分为H₂ 77.8%, CO 5.2%, CO₂ 13.5%和CH₄ 3.5%的原料氢测定了膜的氢分离效果. 结果表明, 未载镍的Pd/Pencil/Al₂O₃膜只具有氢分离作用, 而Pd/Pencil/Ni/Al₂O₃膜还可以有效地将钯膜泄漏的CO和CO₂转化为甲烷, 因而成为双功能型钯膜. 这种双功能膜尤其适用于面向质子交换膜燃料电池(PEMFC)的氢气分离, 既有效解决了PEMFC对氢燃料中CO格外敏感的难题, 又提高了对钯膜缺陷的容忍度, 因而延长了钯膜的使用寿命.     

Transport of binary water–ethanol mixtures through a multilayer hydrophobic zeolite membrane

Transport of water–ethanol mixtures through a hydrophobic tubular ZSM-5 (Si/Al = 300) zeolite membrane during pervaporation was studied experimentally and theoretically. The zeolite membrane was deposited on a support made of pure titania coated with three intermediate ceramic titania layers. The influence of feed concentration, feed temperature and permeate pressure on permeate fluxes and permeate concentrations was investigated in a wide range. Dusty gas model parameters of the support and all ceramic intermediate layers were calculated on the basis of gas permeation data. Mass transfer resistances and pressure drops in the different membrane layers during pervaporation were calculated for several process conditions. In particular the influence of the undesired but unavoidable pressure drop in the support and the intermediate layers on the effective driving force for pervaporation was evaluated and found to be relevant for predicting the overall process performance. The membrane prepared was found to be suitable for the recovery of highly concentrated ethanol from feed mixtures of relatively low ethanol concentrations at relatively low feed temperatures.     

Liquid membranes for the production of oxygen-enriched air: II. Facilitated-transport membranes

We have identified a class of facilitated-transport membranes for the production of oxygen-enriched air. The membranes consist of a transition-metal complex dissolved in a suitable solvent and immobilized within the pores of a microporous membrane. Under certain conditions, these membranes exhibited oxygen/nitrogen selectivities in excess of 20, and permeate streams with oxygen purity in the range of 80–90 mole% were achieved using a l-atm feed stream and a low-pressure permeate stream. An oxygen permeability (P_o2 = 26 x 10⁻⁹ cm³ (STP)-cm/cm²-sec-cmHg) about one-half that of silicone rubber was observed. Membrane life is measured in weeks.