Reaction/separation coupled equilibrium modeling of steam methane reforming in fluidized bed membrane reactors

An equilibrium model of steam methane reforming coupled with in-situ membrane separation for hydrogen production was developed. The model employed Sievert’s Law for membrane separation and minimum Gibbs energy model for reactions. The reforming and separation processes were coupled by the mass balance. The model assumed a continuously stirred tank reactor for the fluidized bed hydrodynamics. The model predictions for a typical case were compared with those from the model of Ye et al. [15] which assumed a plug flow for bed hydrodynamics. The model predictions show satisfactory agreement with experimental data in the literatures. The influences of reactor pressure, temperature, steam to carbon ratio, and permeate side hydrogen partial pressure on solid carbon, NH_x and NO_x formation were studied using the model.     

Insights into the long-term stability of the magnesia modified H-ZSM-5 as an efficient solid acid for steam reforming of dimethyl ether

The long-term performance of the bifunctional catalyst composed of MgO-modified H-ZSM-5 and Cu/ZnO/Al₂O₃ for steam reforming of dimethyl ether (SRD) is studied under the same conditions. Although the surface chemical state and acid property of 1.55–2.47 wt% MgO modified H-ZSM-5 are almost the same, a significant impact of MgO contents on the stability of the bifunctional catalyst is observed from the 50 h SRD results. The initial dimethyl ether conversion (around 100%) and H₂ yield (∼95%) over the optimal bifunctional catalyst with 2.17 wt% MgO modified H-ZSM-5 is still kept over 90% at 50 h. Combining the characterization data of spent catalysts and SRD results, the synergetic effect between the MgO-modified H-ZSM-5 and Cu/ZnO/Al₂O₃ is rigorously revealed as the key factor in determining the stability of the bifunctional catalyst for SRD. These results demonstrate that MgO-modified H-ZSM-5 is a promising and efficient solid acid for SRD.     

Experiments on hydrogen production from methanol steam reforming in fluidized bed reactor

A novel approach for the hydrogen production which integrated methanol steam reforming and fluidized bed reactor (FBR) was proposed. The reaction was carried out over Cu/ZnO/Al₂O₃ catalysts. The critical fluidized velocities under different catalyst particle sizes and masses were obtained. The influences of the operating parameters, including that of H₂O-to-CH₃OH molar ratio, feed flow rate, reaction temperature, and catalyst mass on the performance of methanol steam reforming were investigated in FBR to obtain the optimum experimental conditions. More uniform temperature distribution, larger surface volume ratio and longer contacting time can be achieved in FBR than that in fixed bed reactor. The results indicate that the methanol conversion rate in FBR can be as high as 91.95% while the reaction temperatures is 330 °C, steam-to-carbon molar ratio is 1.3, and feed flow rate is 540 ml/h under the present experiments, which is much higher than that in the fixed bed.     

Hydrogen and/or syngas from steam reforming of glycerol. Study of platinum catalysts

In the present work, Pt catalysts prepared on different supports were evaluated in order to apply them in the steam reforming of glycerol reaction to obtain hydrogen and/or synthesis gas at temperatures lower than 450 °C. A strong support effect on the behavior of catalysts was determined.     

Zirconia supported nickel catalysts for glycerol steam reforming: Effect of zirconia structure on the catalytic performance

The effect of the zirconia structure in Ni/ZrO₂ catalysts on the glycerol steam reforming (GSR) reaction was studied. A tetragonal zirconia support was synthesized via a hydrolysis technique and loaded with 5 wt% Ni via a wet-impregnation method. Similarly, a commercial monoclinic zirconia support was also impregnated with 5 wt% Ni. Following calcination at 600 °C, physico-chemical properties of the prepared catalysts were investigated by X-Ray Diffraction (XRD), H₂-Temperature Programmed Reduction (H₂-TPR) and CO₂-Temperature Programmed Desorption (CO₂-TPD) techniques. The catalysts were then tested in the GSR reaction in the 400–700 °C range with a steam to glycerol molar ratio of 9:1 and a flow rate of 0.025 mL/min. The monoclinic catalyst exhibited a better performance giving higher hydrogen yields and glycerol conversions. This was attributed to an improved reducibility of Ni in this catalyst. Stability tests at 600 °C revealed the deactivation of the tetragonal catalyst during 6 h as a result of the formation of encapsulating coke which blocked active Ni metal sites. The monoclinic catalyst, exhibiting the formation of only filamentous coke, remained relatively stable for 24 h.     

Nickel and cobalt as active phase on supported zirconia catalysts for bio-ethanol reforming: Influence of the reaction mechanism on catalysts performance

Steam reforming of ethanol for hydrogen production was investigated on Co/ZrO₂ and Ni/ZrO₂ catalysts promoted with lanthana. Catalysts were prepared by impregnation method and characterized by XRD and TPR. TPD-R experiments were also carried out to determine the role of active phase on reaction mechanism. The results suggest that adsorbed ethanol is dehydrogenated to acetaldehyde producing hydrogen. Then, the adsorbed acetaldehyde may evolve by different mechanisms, depending on the nature of active phase. On one hand, in cobalt-based catalyst, acetaldehyde could be reformed directly. By acetaldehyde thermal decomposition, methyl and formaldehyde groups are obtained. By coupling of methyl groups, ethane can be obtained. At medium temperature range, WGS reaction contribution is noteworthy.     

A combined system of dimethyl ether (DME) steam reforming and lean NOx trap catalysts to improve NOx reduction in DME engines

This paper performs a study on a combined system of DME steam reforming (SR) and lean NOx trap (LNT) in order to improve the performance of the de-NOx catalyst in DME engines. A new concept, a combined system of SR and LNT catalysts, utilizes H₂ and CO generated from the DME SR catalyst as a reductant for the LNT catalyst. The Cu-based SR catalyst was prepared by the sol-gel method; further, the LNT catalyst was used a commercial catalyst. The parameters considered in this experiment included the particle size, dispersion, and amount of Cu loaded on the SR catalyst, the cell density of the substrate of the SR catalyst, and the amount of Zn as a promoter. The experiments revealed that the highest NOx conversion was obtained in the LNT catalyst when the concentration of DME was 1% and the lean/rich time was 55/8 s; however, we decided to supply 0.7% of DME and use 55/5 s of lean/rich time in the combined system of SR and LNT to overcome the problems of DME slip and fuel penalties. The system showed the best performance regarding NOx conversion in the combined system of SR and LNT that used the Cu29Zn1/r-Al₂O₃ catalyst with 1% of Zn as a promoter, a cell density of 600 cpsi, and a volumetric ratio of 1.3 (SR/LNT). Finally, the NOx conversion was improved by about 20% compared to the LNT catalyst used alone.     

The effect of Ni:Pt ratio on oxidative steam reforming performance of Pt–Ni/Al2O3 catalyst

Oxidative steam reforming of propane was tested over four Pt–Ni/δ-Al₂O₃ bimetallic catalysts aiming to investigate the effect of metal loadings and Ni:Pt loading ratio on catalyst performance. A trimetallic Pt–Ni–Au/δ-Al₂O₃ catalyst was additionally studied aiming to understand the effect of Au presence. Reaction temperature, carbon to oxygen ratio, and residence time were taken as the reaction parameters. The effect of C/O₂ ratio on the hydrogen production and H₂/CO selectivity was found dependent on the Pt and Ni loadings. The results underlined the importance of C/O₂ ratio as an optimization parameter for product distribution. The highest hydrogen production and H₂/CO ratio levels were obtained for the highest C/O₂ ratio tested. An optimum Ni:Pt weight ratio was found around 50 due to suppressed methanation and enhanced hydrogen production activities of these catalysts. The presence of gold in the trimetallic catalyst caused poor activity and selectivity in comparison to bimetallic catalysts.     

Zirconia supported catalysts for bioethanol steam reforming: Effect of active phase and zirconia structure

Three new catalysts have been prepared in order to study the active phase influence in ethanol steam reforming reaction. Nickel, cobalt and copper were the active phases selected and were supported on zirconia with monoclinic and tetragonal structure, respectively. To characterize the behaviour of the catalysts in reaction conditions a study of catalytic activity with temperature was performed. The highest activity values were obtained at 973 K where nickel and cobalt based catalysts achieved an ethanol conversion of 100% and a selectivity to hydrogen close to 70%. Nickel supported on tetragonal zirconia exhibited the highest hydrogen production efficiency, higher than 4.5 mol H₂/mol EtOH fed. The influence of steam/carbon (S/C) ratio on product distribution was another parameter studied between the range 3.2–6.5. Nickel supported on tetragonal zirconia at S/C = 3.2 operated at 973 K without by-product production such as ethylene or acetaldehyde. In order to consider a further application in an ethanol processor, a long-term reaction experiment was performed at 973 K, S/C = 3.2 and atmospheric pressure. After 60 h, nickel supported on tetragonal zirconia exhibited high stability and selectivity to hydrogen production.     

Coke deactivation of Ni and Co catalysts in ethanol steam reforming at mild temperatures in a fluidized bed reactor

The deactivation by coke deposition of Ni and Co catalysts in the steam reforming of ethanol has been studied in a fluidized bed reactor under the following conditions: 500 and 700 °C; steam/ethanol molar ratio, 6; space time, 0.14 g_catalyst h/g_ethanol, partial pressure of ethanol in the feed, 0.11 bar, and time on stream up to 20 h. The decrease in activity depends mainly on the nature of the coke deposited on the catalysts, as well as on the physical–chemical properties (BET surface area, pore volume, metal surface area) of the catalysts. At 500 °C (suitable temperature for enhancing the WGS reaction, decreasing energy requirements and avoiding Ni sintering), the main cause of deactivation is the encapsulating coke fraction (monoatomic and polymeric carbon) that blocks metallic sites, whereas the fibrous coke fraction (filamentous carbon) coats catalyst particles and increases their size with time on stream with a low effect on deactivation, especially for catalysts with high surface area. The catalyst with 10 wt% Ni supported on SiO₂ strikes a suitable balance between reforming activity and stability, given that both the capability of Ni for dehydrogenation and C–C breakage and the porous structure of SiO₂ support enhance the formation of filamentous coke with low deactivation. This catalyst is suitable for use at 500 °C in a fluidized bed, in which the collision among particles causes the removal of the external filamentous coke, thus minimizing the pore blockage of the SiO₂. At 700 °C, the coke content in the catalyst is low, with the coke being of filamentous nature and with a highly graphitic structure.     

Effect of ceria addition on NiRu/CeO2Al2O3 catalysts in steam reforming of acetic acid

NiRu bimetallic catalysts with different amount of CeO₂ loaded on the γ-Al₂O₃ support were prepared. The properties of catalysts were characterized by means of N₂ adsorption-desorption, XRD, H₂-TPR and XPS techniques. Catalytic activities for the steam reforming of acetic acid over these catalysts were investigated at the temperature range from 650 °C to 750 °C. The addition of CeO₂ dramatically improved the activity and stability of the catalyst. Among these catalysts, the NiRu/10CeAl catalyst showed the highest catalytic activity as well as a good stability owing to the abundant Ce³⁺ on the surface of catalyst. The existence of Ce³⁺ promoted the formation of CO₂ from CO because of the mobilizable oxygen, which was favorable for the formation of hydrogen. The coke amount and species deposited on the catalysts after the activity tests were analyzed by DTG. As expected, the NiRu/10CeAl catalyst showed the best resistance to carbon formation. The temperature stepwise steam decoking experiment of the spent catalysts was conducted to elucidate the relationship between the existence of Ce³⁺ and the decoking abilities of various catalysts. It was verified that the existence of Ce³⁺ significantly promoted the decoking abilities of the catalysts.     

Enhancement of membrane hydrogen separation on glycerol steam reforming in a fluidized bed reactor

Membrane hydrogen separation can effectively promote fuel conversion and hydrogen yield by means of altering chemical equilibrium of reforming reactions. In this work, the enhancing process of glycerol steam reforming via a fluidized bed membrane reactor is numerically investigated. Under the framework of the Euler-Euler method, chemical kinetic model is implemented and the reforming performance with and without membrane separation is compared. The effect of densified zones caused by membrane separation is examined. Meanwhile, the impacts of operating parameters including hydrogen partial pressure on the permeate side and fuel gas velocity on densified zones and hydrogen yield are evaluated. The results demonstrate that the excessive reduction of hydrogen partial pressure on the permeate side and the increase of feed gas velocity are detrimental to fuel conversion and hydrogen yield.     

Steam reforming of heptane in a fluidized bed membrane reactor

n-Heptane served as a model compound to study steam reforming of naphtha as an alternative feedstock to natural gas for production of pure hydrogen in a fluidized bed membrane reactor. Selective removal of hydrogen using Pd₇₇Ag₂₃ membrane panels shifted the equilibrium-limited reactions to greater conversion of the hydrocarbons and lower yields of methane, an intermediate product. Experiments were conducted with no membranes, with one membrane panel, and with six panels along the height of the reactor to understand the performance improvement due to hydrogen removal in a reactor where catalyst particles were fluidized. Results indicate that a fluidized bed membrane reactor (FBMR) can provide a compact reformer for pure hydrogen production from a liquid hydrocarbon feedstock at moderate temperatures (475-550 °C). Under the experimental conditions investigated, the maximum achieved yield of pure hydrogen was 14.7 moles of pure hydrogen per mole of heptane fed.     

Effects of synthesis methods on performance of CuZn/MCM-41 catalysts in methanol steam reforming

CuZn-based catalysts are active in production of hydrogen by methanol steam reforming. However, there is a need to have further insight on their physico-chemical properties to improve selectivity to hydrogen. Therefore, a series of CuZn/MCM-41 catalysts was synthesized by four different routes; one pot hydrothermal synthesis (OPMCM), co-impregnation (COMCM), serial impregnation (SRMCM) and copper impregnated on Zn-MCM-41 (ZNMCM). Samples of catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), inductively coupled plasma (ICP) emission spectrometry, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). XRD revealed disruption in the ordered pore network typical in MCM-41 for all catalysts synthesized and also showed that the one pot synthesis catalyst had wide spread dispersion of Cu and Zn. SEM micrographs captured irregularly shaped particles of different sizes. While XPS showed that different Cu and Zn species were formed within the catalyst matrix. XPS also confirmed that there was wide spread dispersion and interaction of Cu and Zn with MCM-41 matrix in the OPMCM catalyst. COMCM and OPMCM demonstrated the highest activity with 88 and 65% methanol conversion with corresponding H₂ selectivity of 91 and 86% respectively. They are better than SRMCM and ZNMCM which had average H₂ selectivity of 19% and 31% respectively. CO selectivity was less than 1.8% for the COMCM and OPMCM catalysts. While SRMCM and ZNMCM had CO selectivity's as high as 8.9% and 7.2% respectively. The data generated shows that catalytic activity is largely affected by the nature of Cu species within the catalyst matrix.     

Ni and Co-based catalysts supported on ITQ-6 zeolite for hydrogen production by steam reforming of ethanol

Ni and Co catalysts supported on ITQ-6 zeolite have been synthesized and evaluated in the steam reforming of ethanol (SRE). Catalysts were also characterized by means of N₂ adsorption-desorption, XRD, H₂-TPR, and H₂-chemisorption. ITQ-6 containing Co (Co/ITQ-6) presented a higher conversion of ethanol and production of hydrogen than ITQ-6 containing Ni (Ni/ITQ-6). The lower size of the metallic cobalt particles shown in Co/ITQ-6 seems to be the major responsible of its higher catalytic performance. Regarding the reaction by-products (CO, CH₄, C₂H₄O and CO₂), Co/ITQ-6 showed the lowest selectivity at medium and high temperatures (773 and 873 K). At low reaction temperatures (673 K) the dehydrogenation reaction predominates in the Co/ITQ-6, what it is supported by the high concentration of acetaldehyde detected at this temperature. In the case of the Ni/ITQ-6 the main side reaction at 673 K seems to be the methanation reaction since large concentrations of methane are detected. Stability studies were also carried out showing lower deactivation of Co/ITQ-6 at large reaction times (24 h). Characterization of the exhausted catalysts after reaction showed the presence of coke in both catalysts. Nevertheless, Co/ITQ-6 presented the lowest coke deposition. In addition, Co/ITQ-6 exhibited the lowest metal sinterization, what could be also account for the lower deactivation exhibited by this sample. This fact could be related to the higher interaction between the cobalt metallic particles and the ITQ-6 support as the H₂-TPR studies demonstrate.     

Dimethyl ether synthesis on clinoptilolite zeolite and HZSM5-based hybrid catalysts in a fixed-bed reactor

In this study 4 different acid catalysts were prepared and mixed with commercial CZA catalysts and investigated in direct DME synthesis. Some of the used acid catalysts were not investigated in the literature therefore the work involves novelty. In a fixed-bed reactor, dimethyl ether (DME) was synthesized from the synthesis gas on two catalysts from, natural clinoptilolite and zeolite catalysts. The clinoptilolite (HK and DK) and two (HZSM5(117) and HZSM5(360)) catalysts mixed with commercial CuO/ZnO/Al₂O₃ (CZN) catalysts. The catalysts were also characterized by analytical chemistry techniques such as XRD, BET, TGA, and FTIR. Four different catalysts (HK, DK, HZSM5(117) and HZSM5(360)) and CZA catalysts were mixed at a ratio of 3/1, respectively, and studies were carried out in a fixed-bed reactor. Four different catalyst composition activity tests were made at temperatures 250, 275, and 300 °C. At the same time, the pressure was 30 and 40 bar and four different times (30, 60, 90, and 120 min). The composition of the gases fed to the system for DME was adjusted to N₂/CO₂/CO/H₂ = 36/10/18/36 by volume. DME selectivity (S_DME) and total carbon (X_C) conversion were calculated for each condition. The experimental results showed that the highest DME selectivity of 96.50% was observed in the reaction of the DK + CZA catalyst mixture at 250 °C and 30 min at 40 bar. In addition, high DME selectivity was obtained in all reactions of DK + CZA and HK + CZA catalyst compositions at three different temperatures. The highest DME selectivity obtained is 89.69% for the reaction of the HK + CZA catalyst mixture at 300 °C and 60 min at 30 bar. Experimental results gave insights into Dimethyl ether synthesis from syngas on clinoptilolite zeolite and HZSM5-based hybrid catalysts in a fixed-bed reactor.     

Experimental and DFT studies on catalytic reforming of acetic acid for hydrogen production over B-doped Co/Al2O3 catalysts

Steam reforming of bio-oil for hydrogen production is a promising green technology. Acetic acid was used as the bio-oil model compound. Experimental and density functional theory calculations were carried out to study the performance of Co/Al₂O₃ catalysts doped with boron (B) with a 1 wt.%–5 wt.% content. Catalyst characterization by BET, XRD, XPS, NH₃-TPD, H₂-TPR, TEM, and TG-DTG was performed. We found that the catalyst performance improved significantly by B doping. Under the reaction conditions of T = 500 °C, steam-to-carbon ratio (S/C) = 5, and liquid hourly space velocity (LHSV) = 4.3 h^?1, the catalyst with a B doping ratio of 1 wt.% had the highest hydrogen yield of 85% and a maximum acetic acid conversion rate of 95%. The corresponding hydrogen productivity was 0.8 mmol/min. The stability of this catalyst exceeded 29 h. Density functional theory calculations showed that the interactions between the reaction intermediates and the surface were strengthened with B addition.     

Experiments on hydrogen production from methanol steam reforming in the microchannel reactor

The entire experiments were conducted for microchannel methanol steam reforming, by which, the selection of catalyst, the operating parameters and the configuration of microchannels were discussed thoroughly. It was found that the higher the Cu concentration is, the more the corresponding active surface area of Cu will be, thereby improving the catalytic activity. The Cu-to-Zn ratio in Cu/ZnO/Al₂O₃ catalyst should be set at 1:1. The impacts of reaction temperature, feed flow rate, mixture temperature, and H₂O-to-CH₃OH molar ratio on the methanol conversion rate were also revealed and discussed. Characteristics of micro-reactors with various microchannels, including that 20 mm and 50 mm in length, as well as non-parallel microchannels, were investigated. It was found that the increase of microchannel length can improve the methanol conversion rate significantly. Besides, non-parallel microchannels help to maintain flow and temperature distribution uniformity, which can improve the performance of micro-reactor. In the present experiments, the presence of CO was under the condition that the methanol conversion rate was above 70%.