Pure H2 production through hollow fiber hydrogen‐selective MFI zeolite membranes using steam as sweep gas

Hollow fiber MFI zeolite membranes were modified by catalytic cracking deposition of methyldiethoxysilane to enhance their H₂/CO₂ separation performance and further used in high temperature water gas shift membrane reactor. Steam was used as the sweep gas in the MR for the production of pure H₂. Extensive investigations were conducted on MR performance by variations of temperature, feed pressure, sweep steam flow rate, and steam‐to‐CO ratio. CO conversion was obviously enhanced in the MR as compared with conventional packed‐bed reactor (PBR) due to the coupled effects of H₂ removal as well as counter‐diffusion of sweep steam. Significant increment in CO conversion for MR vs. PBR was obtained at relatively low temperature and steam‐to‐CO ratio. A high H₂ permeate purity of 98.2% could be achieved in the MR swept by steam. Moreover, the MR exhibited an excellent long‐term operating stability for 100 h in despite of the membrane quality. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3459–3469, 2015     

Upgrading of Simulated Syngas by Using a Nanoporous Silica Membrane Reactor

The permeance properties of a nanoporous silica membrane were first evaluated in a laboratory‐scale porous silica membrane reactor (MR). The results indicated that CO, CO₂, and N₂ inhibited H₂ permeation. Increased H₂ permeability and selectivity were obtained when gas was transferred from the lumen side to the shell side. This was therefore selected as a suitable permeation direction. On this basis, upgrading of simulated syngas was experimentally investigated as a function of temperature (150 – 300 °C), feed pressure (up to 0.4 MPa), and gas hourly space velocity (GHSV), by using a nanoporous silica MR in the presence of a Cu/ZnO/Al₂O₃ catalyst. The CO conversion obtained with the MR was significantly higher than that with a packed‐bed reactor (PBR) and broke the thermodynamic equilibrium of a PBR at 275 – 300 °C and a GHSV of 2665 h^–1. The use of a low GHSV and high feed pressure improved the CO conversion and led to the recovery of more H₂.     

Conceptual feasibility studies of a CO<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript>-free hydrogen production from ammonia decomposition in a membrane reactor for PEM fuel cells

CO_X-free hydrogen production from ammonia decomposition in a membrane reactor (MR) for PEM fuel cells was studied using a commercial chemical process simulator, Aspen HYSYS®. With process simulation models validated by previously reported kinetics and experimental data, the effect of key operating parameters such as H₂ permeance, He sweep gas flow, and operating temperature was investigated to compare the performance of an MR and a conventional packed-bed reactor (PBR). Higher ammonia conversions and H₂ yields were obtained in an MR than ones in a PBR. It was also found that He sweep gas flow was favorable for X_NH3 enhancement in an MR with a critical value (5 kmol h^-1), above which no further effect was observed. A higher H₂ permeance led to an increased H₂ yield and H₂ yield enhancement in an MR with the reverse effect of operating temperature on the enhancement. In addition, lower operating temperature resulted in higher X_NH3 enhancement and H₂ yield enhancement as well as NG cost savings in a MR compared to a conventional PBR.     

Carbon-carbon dioxide reaction: Kinetics at low pressures and hydrogen inhibition

The gasification of a very high purity natural graphite was studied at temperatures between 960 and 1120°C and at CO₂ pressures below 108 millitorr. For CO₂ depletion up to at least 90%, gasification rates were first order in CO₂ pressure, that is with no inhibition by CO observed. The activation energy for the rate constant for the oxygen transfer step (103.5 ± 5.8 kcal/mole) agreed within experimental error with that found from kinetic studies at intermediate CO₂ pressures where CO does inhibit the reaction. The rate of the oxygen transfer reaction is markedly inhibited by the presence of low pressures of H₂. As H₂ pressure is increased up to 3 millitorr, the gasification rate in CO₂ at 1100°C monotonically decreases. Further increase in H₂ pressure, has a negligible effect on rate. From measurements of hydrogen uptake at reaction temperature, it is clear that inhibition is caused by dissociative chemisorption of hydrogen on to active sites. Inhibition by hydrogen is even more marked for the Graphon-CO₂ reaction and is attributed not only to its chemisorbing on carbon sites but also on to impurity catalyst sites. It is doubtful if true rate constants for the C-CO₂ reaction, uninhibited by hydrogen, have ever been reported.     

Upgrading of a syngas mixture for pure hydrogen production in a Pd-Ag membrane reactor

Water gas shift (WGS) is a thermodynamics limited reaction and CO equilibrium conversion of a traditional reactor is furthermore reduced owing to the presence of H₂ (ca. 50%) in the feed stream coming from a reformer.The upgrading of a simulated reformate stream was experimentally investigated as a function of temperature (280-320 °C), feed pressure (up to 600 kPa), gas hourly space velocity (GHSV), etc. using a Pd-alloy membrane reactor (MR) packed with a commercial catalyst CuO/CeO₂/Al₂O₃; no sweep gas was used. The MR performance was also evaluated using new parameters such as conversion index, H₂ recovery and extraction index, etc., which evidence the advantages with respect to a traditional reactor.A Pd-based MR operated successfully overcoming the thermodynamic constraints of a traditional reactor and, specifically, the drawback introduced by the hydrogen presence. In fact, a CO conversion of 90% significantly exceeded (three times) the thermodynamics upper limit (<36%) of a traditional reactor owing to ca. 80% of hydrogen permeated through the membrane.The overall process performance was significantly improved by the presence of the Pd-based membrane and, thus, by the high reaction pressure which allowed and drove the hydrogen permeation.     

Water Gas Shift in Microreactors at Elevated Pressure: Platinum-Based Catalyst Systems and Pressure Effects

A new lab-scale microstructured reactor was used for investigations on enhancing the H₂/CO ratio in synthesis gas from biomass feedstocks via the water gas shift reaction. A model mixture of carbon monoxide, carbon dioxide, water, and hydrogen was used to reproduce the typical synthesis gas composition from dry biomass gasification. Catalyst layers were prepared and characterized; a combined incipient wetness impregnation and sol–gel technology was applied. The catalytic activities of Pt/CeO₂ and Pt/CeO₂/Al₂O₃ films were determined at temperatures of 400–600 °C and pressures of up to 45 bars. Increased pressure leads to higher values of CO conversion and to increased formation of hydrocarbons (CH₄, C₂H₆, etc.) and coke. Methane is the main by-product, and coke formation was attributed to the catalytic activity of peripheral reactor components.     

Integrated membrane system for pure hydrogen production: A Pd-Ag membrane reactor and a PEMFC

In this work, an integrated system consisting of single stage hydrogen production and a commercial PEMFC was investigated experimentally. The CO-free hydrogen fed to the PEMFC was produced in a Pd-Ag membrane reactor (MR), upgrading a syngas stream with a composition similar to that coming out of a reformer (CO 45%; H₂ 50%; CO₂ 4%; N₂ balance, on dry basis). The performance of the MR was evaluated in terms of CO conversion and H₂ recovery as a function of the feed pressure (up to 600 kPa) and space velocity; no sweep gas was used for promoting the H₂ permeation, since this role was assigned exclusively to the feed pressure.Special attention was paid to the analysis of the integrated system, focusing on the influence of the Pd-Ag MR operating conditions on the electrical performance of the PEMFC. The PEMFC internal crossover was also considered to have an effect on the electrical performance and this was taken into account estimating the PEMFC actual efficiency. Furthermore, the chemical efficiency of the integrated membrane plant was evaluated, considering the H₂ converted into electricity with respect to the total amount of H₂ contained in the feed mixture. An interesting performance was shown by the integrated system since the PEMFC performance was close to the power nominal value.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

Simulation of CO2 hydrogenation with CH3OH removal in a zeolite membrane reactor

A thermodynamic analysis of the CO₂ hydrogenation to methanol where competitive reactions take place is presented for a membrane reactor (MR) where methanol was selectively removed. A non-isothermal mathematical model was written to simulate a micro-porous MR. Zeolite membranes with different values of the CH₃OH and H₂O permeances were considered in the MR modelling. The effect of temperature, pressure and species permeation on the conversion, selectivity and yield was analysed. A higher CO₂ conversion and CH₃OH selectivity can be reached by the use of an MR. An increased CH₃OH yield allows to reduce the consumption of reactant and also to operate at lower pressures and higher temperatures, a fact, which favours the kinetics reducing the residence time and the reactor volume. The MR with the highest CH₃OH/H₂O permeance ratio resulted in better selectivity and yield of CH₃OH with respect to the other MR characterised by a higher conversion.     

Effect of hydrogen on carbon deposition from carbon monoxide on nickel catalyst

The role of hydrogen in carbon deposition on Ni has been studied at H₂/CO < 1 and 698 K by determining the respective rates of the carbon-forming reactions: (1) CO + H₂ -C + H₂O and (2) 2CO C + CO₂. The steady-state rate of reaction (1) increases in proportion to H₂ pressure. On the other hand, reaction (2) is facilitated by the addition of an extremely small amount of H₂, so that the rate becomes about eight times that for pure CO but hardly varies as more H₂ is added. Similarly, there is a great difference in catalytic activity for ethylene hydrogenation between spent catalysts obtained in the deposition with and without H₂. These findings suggest that hydrogen, even in a small amount, makes free Ni surface area larger than for pure CO.     

A CuO-CeO2 Mixed-Oxide Catalyst for CO Clean-Up by Selective Oxidation in Hydrogen-Rich Mixtures

A CuO-CeO₂ mixed-oxide catalyst was shown experimentally to be highly active and selective for the oxidation of CO in hydrogen-rich mixtures, and an attractive alternative to the noble metal catalysts presently used for CO clean-up in hydrogen mixtures for proton-exchange membrane fuel cells (PEMFC). Although the presence of H₂O and CO₂ in the feed decreased the activity and increased the reaction temperature considerably to achieve a given CO conversion with a reactor, the selectivity profile with respect to the conversion remained virtually the same. The effect of H₂O and CO₂ on the reaction was found to increase the required energy for reduction of the active copper species in the redox cycles undergone during the reaction. The catalyst showed a slow, reversible deactivation, but the activity was restored on heating the catalyst at 300 °C, even under an inert flow. At space velocities above 42 g h m^-3, the catalyst reduced the CO content to less than 10 ppm in the temperature range 166-176 °C for a feed of 1% CO, 1% O₂, 50% H₂, 20% H₂O, 13.5% CO₂ and balance He. Hence, with this catalyst it is feasible to clean up the CO in a single-stage reactor with relatively small excess oxygen, which is in contrast to the typical multistage reactor systems using noble metal catalysts.     

Hydroformylation of Cyclohexene with Carbon Dioxide and Hydrogen Using Ruthenium Carbonyl Catalyst: Influence of Pressures of Gaseous Components

Hydroformylation of cyclohexene was studied with a catalyst system of Ru₃(CO)₁₂ and LiCl using H₂ and CO₂ instead of CO in NMP. The influence of H₂ and CO₂ pressures on the total conversion and the product distribution was examined. It was shown that increasing total pressure of H₂ and CO₂ promoted the reverse water gas shift reaction and increased the yield of cyclohexanecarboxaldehyde. Its hydrogenation to cyclohexanemethanol was promoted with increasing H₂ pressure but suppressed with increasing CO₂ pressure. Cyclohexane was also formed along with those products and this direct hydrogenation was suppressed with increasing CO₂ pressure. The roles of CO₂ as a promoter as well as a reactant were further examined by phase behavior observations and high pressure FTIR measurements.     

Autothermal Reforming of Methane with Integrated CO2 Capture in a Novel Fluidized Bed Membrane Reactor. Part 1: Experimental Demonstration

Two fluidized bed membrane reactor concepts for hydrogen production via autothermal reforming of methane with integrated CO₂ capture are proposed. Ultra-pure hydrogen is obtained via hydrogen perm-selective Pd-based membranes, while the required reaction energy is supplied by oxidizing part of the CH₄ in situ in the methane combustion configuration or by combusting part of the permeated H₂ in the hydrogen combustion configuration (oxidative sweeping). In this first part, the technical feasibility of the two concepts has been studied experimentally, investigating the reactor performance (CH₄ conversion, CO selectivity, H₂ production and H₂ yield) at different operating conditions. A more detailed comparison of the performance of the two proposed reactor concepts is carried out with a simulation study and is presented in the second part of this work.     

Pathways for CO2 Formation and Conversion During Fischer–Tropsch Synthesis on Iron-Based Catalysts

CO₂ reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of ¹³C-labeled and unlabeled CO₂ to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H₂O and with adsorbed CO to form CO₂. The H₂O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H₂O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO₂ with increasing reactor residence time. The forward rate of CO₂ formation increases with increasing residence time as H₂O concentration increases, but the net CO₂ formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO₂ addition to synthesis gas does not influence CO₂ forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO₂ could lead to the minimization of CO₂ formation during FTS and to the preferential removal of oxygen as H₂O. This, in turn, leads to lower average H_2/CO ratios throughout the catalyst bed and to higher olefin content and C₅₊ selectivity among reaction products. The addition of ¹³CO₂ to H₂/¹²CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO₂ is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO₂ to form hydrocarbons during reactions of H₂/CO/CO₂ mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO₂ as the chemical interconversion of CO and CO₂ becomes rapid at WGS reaction equilibrium.     

The positive effect of hydrogen on the reaction of nitric oxide with carbon monoxide over platinum and rhodium catalysts

The effect of adding 330–4930 ppm hydrogen to a reaction mixture of NO and CO (2000 ppm each) over platinum and rhodium catalysts has been investigated at temperatures around 200–250°C. Hydrogen causes large increases in the conversion of NO and, surprisingly, also of CO. Oxygen atoms from the additional NO converted are eventually combined with CO to give CO₂ rather than react with hydrogen to form water. This reaction is described by CO + NO +3/2H₂ CO₂ + NH₃ and accounts for 50–100% of the CO₂ formed with Pt/Al₂O₃ and 20–50% with Rh/Al₂O₃. With the latter catalyst a substantial amount of NO converted produces nitrous oxide. Comparison with a known study of unsupported noble metals suggests that isocyanic acid (HNCO) might be an important intermediate in a reaction system with NO, CO and H₂ present.     

The use of NiO as an oxygen carrier in chemical-looping combustion

The feasibility of using NiO as an oxygen carrier during chemical-looping combustion has been investigated. A thermodynamic analysis with CH₄ as fuel showed that the yield of CH₄ to CO₂ and H₂O was between 97.7 and 99.8% in the temperature range 700–1200 °C, with the yield decreasing as the temperature increases. Carbon deposition is not expected as long as sufficient metal oxide is supplied to the fuel reactor. Hydrogen sulfide, H₂S, in the fuel gas will be converted partially to SO₂ in the gas phase, with the degree of conversion increasing with temperature, but decreasing as a function of pressure. There is the possibility of sulfide formation as Ni₃S₂ at higher partial pressures of H₂S+SO₂ in the reactor. The reactivity of freeze granulated particles of NiO with NiAl₂O_4, MgAl₂O₄, TiO₂ and ZrO₂ sintered at different temperatures was investigated in a small fluidized bed reactor by exposing them cyclically to 50% CH₄/50% H₂O and 5% O₂ at 950 °C. During the reducing period, the NiO initially reacted with the CH₄ to form CO₂ and H₂O. However, there were always minor amounts of CO from the outlet of the reactor even at high concentrations of CO₂, which was due to the thermodynamic limitations. Here, the ratio CO/(CO₂+CO+CH₄) was between 1.5 and 2.5% at 950 °C for the oxygen carriers with alumina based inert. A small amount of CH₄ was released from the reactor at high degrees of oxidation of the NiAl₂O₄ and MgAl₂O₄-based carriers. As the time under reducing conditions increased, steam reforming of CH₄ to CO and H₂ became considerable, with Ni catalyzing this reaction. Whereas the ZrO₂ particles showed similar behavior as the alumina-based carriers, the TiO₂-based particles showed a markedly different reaction behavior, likely due to the complex interaction between NiO and TiO₂.     

Brown coal conversion under the action of supercritical water

A test bench was developed and the conversion of the organic matter of coal (OMC) in supercritical water (SCW) was studied under conditions of a continuous supply of a water-coal suspension to a vertical flow reactor at 390–760°C and a pressure of 30 MPa. From 44 to 63% OMC was released as liquid and gaseous products from coal particles (from the water-coal supension) during the time of fall to the reactor. This stage was referred to as the dynamic conversion of coal. The particles passed through the stage of the dynamic conversion of coal did not agglomerate in the reactor in the subsequent process of batch conversion in a coal layer at T = 550–760°C. The volatile products of the overall process of the dynamic and batch conversion of coal included saturated hydrocarbons (CH₄ and C₂H₆), aromatic hydrocarbons (C₆H₆, C₇H₈, and C₈H₁₀), synthesis gas (H₂ and CO), and CO₂. At T < 600°C, CO₂ and CO were the degradation products of oxygen-containing OMC fragments, whereas they also resulted from the decomposition of water molecules at higher temperatures in accordance with the reaction (C) + H₂O = CO + H₂. The mechanisms were considered, and the parameters responsible for the dynamic conversion of coal were calculated.     

Promotion of CO<Subscript>2</Subscript> hydrogénation to hydrocarbons in three-phase catalytic (Fe-Cu-K-Al) slurry reactors

A three-phase slurry reactor has been employed to increase the CO₂ conversion and decrease the selectivity of CO in the direct hydrogénation of CO₂ to hydrocarbons, as it is beneficial for removal of the heat generated due to highly exothermic nature of the reaction. Experiments were conducted over iron-based catalysts (Fe-Cu-K-Al, d_p,=45-75 Μm) in a slurry reactor. It was found that the slurry reactor is preferable to the fixed bed reactor. The productivity and selectivity of hydrocarbons in the slurry reactor appeared to be better than that in the fixed bed reactor for the hydrogénation of CO₂. The CO₂ conversion was increased with increasing reaction temperature (275-300 ‡C), pressure (1-2.5 MPa) or H₂/CO₂ ratio (2-5) in the three-phase slurry reactor. The CO₂ conversion was increased with increasing the amount of CO₂ fed.     

Beneficial effects of the use of a nickel membrane reactor for the dry reforming of methane: Comparison with thermodynamic predictions

The development of a nickel composite membrane with acceptable hydrogen permselectivity at high temperature in a membrane reactor for the highly endothermic dry reforming of methane reaction was the purpose of this work. A thin, catalytically inactive nickel layer, deposited by electroless plating on asymmetric porous alumina, behaved simply as a selective hydrogen extractor, shifting the equilibrium in the direction of a higher hydrogen production and methane conversion. The main advantage of such a nickel/ceramic membrane reactor is the elimination or limitation of the side reverse water gas shift reaction. For a Ni/Al₂O₃ catalyst, containing free Ni particles, normally sensitive to coking, the use of the membrane reactor allowed an important reduction of carbon deposition (nanotubes) due to restriction of the Boudouard reaction. For a Ni–Co/Al₂O₃ catalyst, where the metallic nickel phase was stabilized by the alumina, the selective removal of the hydrogen significantly enhanced both methane conversion (+67% at 450 °C, +22% at 500 °C and +18% at 550 °C) and hydrogen production (+42% at 450 °C, +32% at 500 °C and +22% at 550 °C) compared to the results obtained for a packed-bed reactor. The hydrogen selectivity during the catalytic tests at 550 °C, maintained with constant separation factors (7 for H₂/CH₄, 8 for H₂/CO and 10 for H₂/CO₂), higher than Knudsen values, attested to the high thermal stability of the nickel composite membrane.     

Autothermal Reforming of Methane with Integrated CO2 Capture in a Novel Fluidized Bed Membrane Reactor. Part 2 Comparison of Reactor Configurations

The reactor performance of two novel fluidized bed membrane reactor configurations for hydrogen production with integrated CO₂ capture by autothermal reforming of methane (experimentally investigated in Part 1) have been compared using a phenomenological reactor model over a wide range of operating conditions (temperature, pressure, H₂O/CH₄ ratio and membrane area). It was found that the methane combustion configuration (where part of the CH₄ is combusted in situ with pure O₂) largely outperforms the hydrogen combustion concept (oxidative sweeping combusting part of the permeated H₂) at low H₂O/CH₄ ratios (<2) due to in situ steam production, but gives a slightly lower hydrogen production rate at higher H₂O/CH₄ ratios due to dilution with combustion products. The CO selectivity was always much lower with the methane combustion configuration. Whether the methane combustion or hydrogen combustion configuration is preferred depends strongly on the economics associated with the H₂O/CH₄ ratio.