Development of an All-ceramic Module with Silica Membrane Tubes for High Temperature Hydrogen Separation

Abstract

Heat resistant hydrogen selective membranes are desired for use as membrane reactors in low-temperature hydrogen production via the steam reforming of hydrocarbons, which are usually operated over 1000 K. In addition, developing a multi-tubular type of membrane unit that can process more reactants is becoming more and more important in order to realize the practical use of membrane reactors.

In this study, an all-ceramic module consisting of 6 silica membrane tubes with a comparatively large membrane area of around 0.04 m² was fabricated by a counter-diffusion chemical vapor deposition technique. As a result, the H₂/N₂ ideal separation factor and the H₂ permeance of the module were 1300 and 1.9 × 10^?7 mol·m^?2s^?1Pa^?1 at 873 K, respectively. In a 1000-hour thermal stability test for the silica membrane module, it was found that the H₂ permeance initially decreased by about 30% and then became steady under ΔP = 0.95 MPa at 773 K.     

Steam reforming of ethanol over skeletal Ni‐based catalysts: A temperature programmed desorption and kinetic study

An investigation on reaction scheme and kinetics for ethanol steam reforming on skeletal nickel catalysts is described. Catalytic activity of skeletal nickel catalyst for low‐temperature steam reforming has been studied in detail, and the reasons for its high reactivity for H₂ production are attained by probe reactions. Higher activity of water gas shift reaction and methanation contributes to the low CO selectivity. Cu and Pt addition can promote WGSR and suppress methanation, and, thus, improve H₂ production. A reaction scheme on skeletal nickel catalyst has been proposed through temperature programmed reaction spectroscopy experiments. An Eley‐Rideal model is put forward for kinetic studies, which contains three surface reactions: ethanol decomposition, water gas shift reaction, and methane steam reforming reaction. The kinetics was studied at 300–400°C using a randomized algorithms method and a least‐squares method to solve the differential equations and fit the experimental data; the goodness of fit obtained with this model is above 0.95. The activation energies for the ethanol decomposition, methane steam reforming, and water gas shift reaction are 187.7 kJ/mol, 138.5 kJ/mol and 52.8 kJ/mol, respectively. Thus, ethanol decomposition was determined to be the rate determining reaction of ethanol steam reforming on skeletal nickel catalysts. © 2013 American Institute of Chemical Engineers AIChE J 60: 635–644, 2014     

Permeation properties of hydrogen and water vapor through porous silica membranes at high temperatures

Silica and cobalt‐doped silica membranes that showed a high permeance of 1.8 × 10^?7 mol m^?2 s^?1 Pa^?1 and a H₂/N₂ permeance ratio of ～730, with excellent hydrothermal stability under steam pressure of 300 kPa, were successfully prepared. The permeation mechanism of gas molecules, focusing particularly on hydrogen and water vapor, was investigated in the 300–500°C range and is discussed based on the activation energy of permeation and the selectivity of gaseous molecules. The activation energy of H₂ permeation correlated well with the permeance ratio of He/H₂ for porous silica membranes prepared by sol–gel processing, chemical vapor deposition (CVD), and vitreous glasses, indicating that similar amorphous silica network structures were formed. The permeance ratios of H₂/H₂O were found to range from 5 to 40, that is, hydrogen (kinetic diameter: 0.289 nm) was always more permeable than water (0.265 nm). © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Ni Catalysts Supported on Mesoporous Nanocrystalline Magnesium Silicate in Dry and Steam Reforming Reactions

Mesoporous nanocrystalline MgSiO₃ with high surface area was synthesized by a hydrothermal method and employed as support in dry and steam reforming of methane. Ni/MgSiO₃ catalysts were prepared by an impregnation method and characterized by different techniques. N₂ adsorption analysis indicated that addition of nickel shifted the pore size distributions to smaller sizes. Temperature‐programmed reduction analysis revealed that a higher nickel loading enhanced the reducibility of the catalyst. The catalytic performance was improved with increasing the nickel content. The Ni/MgSiO₃ catalyst exhibited high stability in dry reforming but methane conversion declined with time‐on‐stream in the steam reforming reaction. Temperature‐programmed oxidation profiles of spent catalysts indicated that the high amount of carbon deposited on the catalyst surface in dry and steam reforming was assigned to whisker‐type carbon.     

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     

Hydrogen Sulfide-Resistant Bifunctional Catalysts for the Steam Reforming of Methane: Activity and Structural Evolution

Results are presented from studying an iron–nickel catalyst for the steam reforming of methane, synthesized by epitaxial coating on the surface of spherical pellets of commercial γ-Al₂O₃. It is shown the catalyst is resistant to the presence of hydrogen sulfide in a steam–gas mixture. The degree of conversion of methane during reforming is close to equilibrium at a pressure of 2.0 MPa, a temperature of 800°C, a ratio of Н₂О: СН4 = 2: 1, a feedstock hourly space velocity (FHSV) of 6000 h^?1, and a H₂S concentration of 30 ppm. The structural evolution and phase state of the active components of the system are studied via X-ray diffraction analysis, transmission electron microscopy (TEM), and Mössbauer spectroscopy. The formation of paramagnetic iron oxide clusters tightly bound to the structure of the support, and of FeNi₃ iron–nickel alloy particles on the surface of the catalyst, is responsible for the polyfunctional properties of the catalyst, which displays high activity in both the steam reforming of methane and the oxidative decomposition of hydrogen sulfide to elemental sulfur.     

CO2 conversion through combined steam and CO2 reforming of methane reactions over Ni and Co catalysts

Ni‐Co bimetallic and Ni or Co monometallic catalysts prepared for CO₂ reforming of methane were tested with the stimulated biogas containing steam, CO₂, CH₄, H₂, and CO. A mix of the prepared CO₂ reforming catalyst and a commercial steam reforming catalyst was used in hopes of maximizing the CO₂ conversion. Both CO₂ reforming and steam reforming of CH₄ occurred over the prepared Ni‐Co bimetallic and Ni or Co monometallic catalysts when the feed contained steam. However, CO₂ reforming did not occur on the commercial steam reforming catalyst. There was a critical steam content limit above which the catalyst facilitated no more CO₂ conversion but net CO₂ production for steam reforming and water‐gas shift became the dominant reactions in the system. The Ni‐Co bimetallic catalyst can convert more than 70% of CO₂ in a biogas feed that contains ~33 mol% of CH₄, 21.5 mol% of CO₂, 12 mol% of H₂O, 3.5 mol% of H₂, and 30 mol% of N₂. The H₂/CO ratio of the produced syngas was in the range of 1.8‐2. X‐ray absorption spectroscopy of the spent catalysts revealed that the metallic sites of Ni‐Co bimetallic, Ni and Co monometallic catalysts after the steam reforming of methane reaction with equimolar feed (CH₄:H₂O:N₂ = 1:1:1) experienced severe oxidation, which led to the catalytic deactivation.     

Gas permeation properties of silica membranes with uniform pore sizes derived from polyhedral oligomeric silsesquioxane

The sol‐gel method was applied in the fabrication of homogenous polyhedral oligomeric silsesquioxane (HOMO‐POSS)‐derived silica membranes. Single gas permeation characteristics in a temperature range of 100–500^°C were examined to discuss the effect of silica precursor on amorphous silica networks. HOMO‐POSS‐derived membranes showed a CO₂ permeance of 1.1 × 10^?7 mol m^?2 s^?1 Pa^?1 with a CO₂/CH₄ permeance ratio of 131 at 100^°C, which is a superior CO₂/CH₄ separation performance by comparison with tetraethoxysilane (TEOS)‐derived silica membranes. Normalized Knudsen‐based permeance (NKP) was applied for quantitative evaluation of pore size. HOMO‐POSS‐derived membranes had loose amorphous silica structures compared to TEOS‐derived membranes and pore size was successfully tuned by changing the calcination temperatures. The activation energy for a HOMO‐POSS‐derived membrane fired at 550^°C with a uniform pore size of ～ 0.42 nm increased linearly with the ratio of the kinetic diameter of the gas molecule to the pore diameter, λ (=d_k/d_p), and showed a trend similar to that of DDR‐type zeolite membranes. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1733–1743, 2012     

Hydrogen Permeation Properties and Hydrothermal Stability of Sol–Gel‐Derived Amorphous Silica Membranes Fabricated at High Temperatures

The sol–gel method was applied to the fabrication of amorphous silica membranes for use in hydrogen separation at high temperatures. The effects of fabrication temperature on the hydrogen permeation properties and the hydrothermal stability of amorphous silica membranes were evaluated. A thin continuous silica separation layer (thickness = <300 nm) was successfully formed on the top of a deposited colloidal silica layer in a porous glass support. After heat treatment at 800°C for an amorphous silica membrane fabricated at 550°C, however, it was quite difficult to distinguish the active separation layer from the deposited colloidal silica layer in a porous glass support, due to the adhesion of colloidal silica caused by sintering at high temperatures. The amorphous silica membranes fabricated at 700°C were relatively stable under steam atmosphere (500°C, steam = 70 kPa), and showed steady He and H₂ permeance values of 4.0 × 10^?7 and 1.0 × 10^?7 mol·m^?2·s^?1·Pa^?1 with H₂/CH₄ and H₂/H₂O permeance ratios of ~110 and 22, respectively. The permeance ratios of H₂/H₂O for membranes fired at 700°C increased drastically over the range of He/H₂ permeance ratios by factors of ~3–4, and showed a value of ~30, which was higher than those fired at 500°C. Less permeation of water vapor through amorphous silica membranes fabricated at high temperatures can be ascribed to the dense amorphous silica structure caused by the condensation reaction of silanol groups.     

Acid-base properties and the directions of oxidative transformation of methane over nickel-based catalysts

The reactions of dry (CO₂) reforming and partial oxidation of methane have been investigated in a membrane reactor. The membrane is composed of a dense thin silica (SiO₂) film supported on porous Vycor tubes and was synthesized by chemical vapor deposition. The hydrogen permeance of the membrane was 0.2–0.3 cm³/(cm² min atm) at 600°C combined with a H₂/N₂ selectivity of 200–300. Significant increases in methane conversion were attained in both reactions at 500–750°C, albeit at very low space velocities. The membrane permeance declined by 50% after exposure to feeds containing H₂O, but otherwise exhibited excellent stability under reaction conditions.     

Hybrid H2-Selective Silica Membranes Prepared by Chemical Vapor Deposition

Hybrid organic-inorganic H₂-selective membranes consisting of single-layer or dual-layers of silica incorporating aromatic groups are deposited on a porous alumina support by chemical vapor deposition (CVD) in an inert atmosphere at high temperature. The single-layer silica membranes, which are made by the simultaneous decomposition of phenyltriethoxysilane (PTES) and tetraethylorthosilicate (TEOS), have good hydrothermal stability at high temperature and a high permeance for hydrogen in the order of 10^?7 mol m^?2 s^?1 Pa^?1 at 873 K, while preventing the passage of other larger molecular gases such as CH₄ and CO₂. The dual-layer silica membranes, which are obtained from the sequential decomposition of PTES and TEOS, exhibit an extremely high permeance for hydrogen of 3.6 × 10^?6 mol m^?2 s^?1 Pa^?1 at 873 K with a permselectivity of hydrogen over methane of 30. A normalized Knudsen based permeance method is applied to measure the pore size of PTES-derived silica membrane on the dual-layer silica membrane before treatment with TEOS. The method indicates that the pore size of the silica network is approximately in the range of 0.50–0.85 nm, which is higher than the characteristic length of pure silica membranes of 0.3 nm, accounting for the high permeance of the hybrid membranes.     

Ammonia decomposition in catalytic membrane reactors: Simulation and experimental studies

Catalytic decomposition of NH₃ with H₂‐selective microporous silica membranes for CO_x‐free hydrogen production was studied theoretically and experimentally. The simulation study shows that NH₃ conversion, H₂ yield and H₂ purity increase with the Damköhler number (Da), and their improvement is affected by the effect of H₂ extraction as well as NH₃ and N₂ permeation through the membranes. The experimental study of NH₃ decomposition was carried out in a bimodal catalytic membrane reactor (BCMR), consisting of a bimodal catalytic support and a H₂‐selective silica layer. Catalytic membranes showed H₂ permeances of 6.2–9.8 × 10^?7 mol m^?2 s^?1 Pa^?1, with H₂/NH₃ and H₂/N₂ permeance ratios of 110–200 and 200–700, respectively, at 773 K. The effect of operating conditions on membrane reactor performance with respect to NH₃ conversion, H₂ yield and H₂ purity was investigated, and the results were in agreement with those calculated by the proposed simulation model. © 2012 American Institute of Chemical Engineers AIChE J, 59: 168–179, 2013     

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     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

GAS PERMEANCE BEHAVIOR AT ELEVATED TEMPERATURE IN MESOPOROUS ANODIC OXIDIZED ALUMINA SYNTHESIZED BY PULSE-SEQUENTIAL VOLTAGE METHOD

Mesoporous anodic oxidized alumina (MAOA) capillary tubes with and without a barrier layer have been synthesized by applying a pulse-sequential voltage. The single gas permeances at an elevated temperature and the thermal and hydrothermal stabilities of MAOA were investigated. A highly oriented radial mesopore channel with pore sizes from 40 to 4 nm was formed in the MAOA tubes. Micropores with sizes from 0.4 to 0.8 nm were formed in the barrier layer. The H₂ permeance of MAOA with a barrier layer (barrier type) was approximately 540 times lower than that of MAOA without a barrier layer (block type) at 773 K. The H₂/N₂ permselectivity of the barrier type in the temperature range from 333 to 673 K was 3.4; those of the barrier type at 773 and 823 K were 4.4 and 11, respectively. On the other hand, the H₂/N₂ permselectivities of the block type were from 3.1 to 3.6 in the temperature range from 333 to 773 K. The H₂ permeance and the H₂/N₂ permselectivity of the amorphous silica membrane on the block type were 1.1 × 10^?7 mol/m² · s · Pa and 40 at 773 K, respectively. MAOA synthesized by the pulse-sequential voltage method can be applied to the mesoporous support of the gas separation membrane at elevated temperatures.     

Pore Structure and Permeance of Amorphous Si-C-O Membranes with High Durability at Elevated Temperature

Abstract

A porous α-alumina tube of 2.4 mm o.d. with an average pore size of ca. 150 nm was coated with p-xylene solutions of polycarbosilane (PC) and polystyrene (PS). The weight fraction of PS to total solid mass (PC + PS) was varied as 0, 1, 3 and 5 wt% (denoted as PC-PSx for a solution of x wt%). Then the film was cured in air at 473 K and pyrolyzed in argon at 1223 K. The procedure of dipping, drying, and pyrolysis was repeated three or four times in total. An Si-C-O membrane that was prepared with the PC-PS1 solution exhibited an H₂ permeance of 4 × 10^?8mol·m^?2·s^?1·Pa^?1 and an H₂/N₂ selectivity of 20 at 773 K. The PS was concentrated into ellipsoidal domains due to phase separation during the curing step, and it was depolymerized and dissipated during the pyrolysis step. All membranes were resistant to heat treatment at 1223 K in argon for 20 hours. Except for the PC-PS5 membrane, voids that were formed by the decomposition of PS did not perforate the top layer. When the PC-PS1 membrane was exposed to a mixture of steam and helium at 773 K, the permeance was decreased to half in the first day, but was not greatly changed during the next 2 days.     

Inkjet Printing of Microporous Silica Gas Separation Membranes

Inkjet printing was applied to manufacture silica‐based gas separation membranes, which were coated on a pore‐graduated alumina substrate with a mesoporous γ‐alumina interlayer. A silica sol diluted by 1‐propanol was used to print the membrane layer followed by thermal treatment in a rapid thermal processing furnace. The membrane thickness was varied between 30 and 110 nm by conducting one, two, and three coating steps. In the latter case, H₂ permeance in the range of 2.0 × 10^?8–3.3 × 10^?8 mol/s·m²·Pa combined with H₂/CO₂ selectivities in the range of 15–25 were achieved, proving the concept that inorganic gas separation membranes can be successfully processed by inkjet printing.     

Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 prepared with nickel citrate precursor

The reaction of combination of CO₂ reforming and partial oxidation of methane to produce syngas (CRPOM) was tested over Ni/SiO₂ catalysts which were prepared via incipient-wetness impregnation using precursors of nickel citrate and nickel nitrate. The catalysts were characterized by X-ray powder diffraction analysis (XRD) and H₂-temperature-programmed reduction (H₂-TPR) techniques. It was shown that the nickel citrate precursor strengthened interaction between NiO and support to form nickel silicate like species which could be reduced to produce small crystallites of metallic nickel at high temperatures. The Ni/SiO₂ prepared with the nickel citrate precursor exhibited good catalytic performances for its highly dispersed metallic nickel derived from the nickel silicate species.     

Mathematical Modeling and Optimization of Combined Steam and Dry Reforming of Methane Process in Catalytic Fluidized Bed Membrane Reactor

Mathematical modeling of the methane-combined reforming process (steam methane reforming–dry reforming methane) was performed in a fluidized bed membrane reactor. The model characterizes multiple phases and regions considering low-density phase, high-density phase, membrane, and free board regions that allow study of reactor performance. It is demonstrated that the combined effect of membrane and reaction coupling provides opportunities to overcome equilibrium limits and helps to achieve higher conversion. Additionally, the influence of key parameters on reactor performance including reactor temperature, reactor pressure, steam to methane feed ratio (S/C), and carbon dioxide to methane feed ratio (CO₂/C) were investigated in the multi-objective genetic algorithm to find the optimal operating conditions. Finally, the process of steam reforming was simulated in selected optimal conditions and the results are compared to those of the combined reforming process. Comparison reveals the superiority of the combined reforming process in terms of methane conversion, catalyst activity, and outlet H₂/CO ratio in the syngas product in being close to unity.     

Adsorption of Off‐Gases from Steam Methane Reforming (H2, CO2, CH4, CO and N2) on Activated Carbon

Abstract

Hydrogen is the energy carrier of the future and could be employed in stationary sources for energy production. Commercial sources of hydrogen are actually operating employing the steam reforming of hydrocarbons, normally methane. Separation of hydrogen from other gases is performed by Pressure Swing Adsorption (PSA) units where recovery of high‐purity hydrogen does not exceed 80%.

In this work we report adsorption equilibrium and kinetics of five pure gases present in off‐gases from steam reforming of methane for hydrogen production (H₂, CO₂, CH₄, CO and N₂). Adsorption equilibrium data were collected in activated carbon at 303, 323, and 343 K between 0‐22 bar and was fitted to a Virial isotherm model. Carbon dioxide is the most adsorbed gas followed by methane, carbon monoxide, nitrogen, and hydrogen. This adsorbent is suitable for selective removal of CO₂ and CH₄. Diffusion of all the gases studied was controlled by micropore resistances. Binary (H₂‐CO₂) and ternary (H₂‐CO₂‐CH₄) breakthrough curves are also reported to describe the behavior of the mixtures in a fixed‐bed column. With the data reported it is possible to completely design a PSA unit for hydrogen purification from steam reforming natural gas in a wide range of pressures.