Molten metal capillary reactor for the high-temperature pyrolysis of methane

In this work, the further development of the molten metal capillary reactor in slug-flow regime is presented. The preliminary results from the high-temperature pyrolysis of methane at 1300 °C and 9 Nml/min are presented with a calculated conversion of 80%, and, a mean residence time of 1.36 s. Due to carbon deposition, difficult gas separation and unstable slug-flow, it was deemed necessary to redesign the system. For that, several alloys were tested looking for improved wettability and more favorable hydrodynamics. The modified experimental set-up is described, which led to improvements in gas separation, but not enough stability in the slug-flow. Finally, the current experimental set-up is introduced. There, a characterization of the hydrodynamics is performed using a low temperature alloy of gallium, indium and tin, GaInSn, and, a stable regular slug-flow is established for various gas and liquid flows. The presence of a film in the slug-flow remains subject to question and the conclusions on the direction of the project are drawn towards an alternative reactor system or further hydrodynamic studies.     

Numerical modeling of methane pyrolysis in a bubble column of molten catalysts for clean hydrogen production

Methane pyrolysis using molten catalysts in a bubble column reactor (BCR) has recently been proposed to produce hydrogen with separable carbon particles as byproducts. In this study, a numerical model of the BCR of molten catalysts for methane pyrolysis was developed and validated using experimental data. Based on a non-isothermal 1-D simplification, continuous liquid and discrete bubble phases were considered by incorporating submodels for bubble behaviors, catalytic and homogeneous reactions, heat/mass transfer, and a submerged orifice for methane supply. The initial bubble diameter was predicted using the correlation derived from measurements. When applied to experiments with Ni₍₂₇₎Bi₍₇₃₎ and mixtures of KCl–MnCl₂, the model accurately reproduced the methane conversion at different temperatures and column heights. Furthermore, detailed information on the key phenomena was acquired, including the profiles of the bubble diameter, rise velocity, reaction rates, temperature, and gas composition. A sensitivity analysis confirmed that the uncertainties regarding the physical properties of molten catalysts had a negligible impact. A comparison of the performances of Ni₍₂₇₎Bi₍₇₃₎ and KCl₍₅₀₎MnCl₂₍₅₀₎ under the same reaction conditions revealed a favorable influence of the catalyst density on methane conversion because of the increased pressure. The proposed model would be useful in reactor optimization and scale-up with high hydrogen productivity.     

Novel structured catalysts configuration for intensification of steam reforming of methane

Structured catalysts, using highly conductive carriers, can improve the heat transfer along the catalytic bed, affording high performance with a flattened radial temperature gradient. The effect of thermal conductivity of structured carriers on highly endothermic Steam Reforming reaction is investigated. The performance of the structured catalysts, obtained on Cordierite and Silicon Carbide (SiC) monoliths, demonstrates the direct correlation between the thermal conductivity of the carrier, the methane conversion and the hydrogen productivity. The evaluation of the monolith configuration shows that the SiC “wall flow” guarantees a better axial and radial thermal distribution, with respect to the SiC “flow through”, resulting in better catalytic activity up to a temperature reaction of 750 °C. The comparison among the performance of the structured catalysts and the commercial 57-4MQ, provided by Katalco-JM, highlights the choice of structured catalysts, which require a lower temperature outside of the reactor, increasing the process efficiency.     

Steam reforming of methane in a tapered membrane - Assisted fluidized - Bed reactor: Modeling and simulation

A compartment model was developed to describe the flow pattern of gas within the dense zone of a tapered membrane-assisted fluidized-bed reactor (TMAFBR), in the bubbling mode of operation for steam reforming of methane under wall heat flux. The parameters of the developed model (i.e., number of compartments for the bubble and emulsion phases) were determined using the experimental data reported elsewhere [Adris AM, Lim CJ, Grace JR. The fluidized bed membrane reactor system: a pilot scale experimental study. Chem Eng Sci 1994; 49:5833-43.] and good agreements were obtained between model predictions and corresponding experimental data. The developed model was then utilized to predict the behavior of TMAFBR under various operating and design conditions. Moreover, the influences of tapered angle, bed operating temperature and pressure, and feed temperature on the methane conversion and the total yield of hydrogen were carefully investigated. Furthermore, the performance capability of the TMAFBR was compared with that of a columnar one under identical operating conditions.     

Methane pyrolysis in a molten gallium bubble column reactor for sustainable hydrogen production: Proof of concept & techno-economic assessment

Nowadays, nearly 50% of the hydrogen produced worldwide comes from Steam Methane Reforming (SMR) at an environmental burden of 10.5 t_CO2,eq/t_H2, accelerating the consequences of global warming. One way to produce clean hydrogen is via methane pyrolysis using melts of metals and salts. Compared to SMR, significant less CO₂ is produced due to conversion of methane into hydrogen and carbon, making this route more sustainable to generate hydrogen. Hydrogen is produced with high purity, and solid carbon is segregated and deposited on the molten bath. Carbon may be sold as valuable co-product, making industrial scale promising. In this work, methane pyrolysis was performed in a quartz bubble column using molten gallium as heat transfer agent and catalyst. A maximum conversion of 91% was achieved at 1119 °C and ambient pressure, with a residence time of the bubbles in the liquid of 0.5 s. Based on in-depth analysis of the carbon, it can be characterized as carbon black. Techno-economic and sensitivity analyses of the industrial concept were done for different scenarios. The results showed that, if co-product carbon is saleable and a CO₂ tax of 50 euro per tonne is imposed to the processes, the molten metal technology can be competitive with SMR.     

Demonstration of a scaled-down autothermal membrane methane reformer for hydrogen generation

A novel concept for hydrogen generation by methane steam reforming in a thermally coupled catalytic fixed bed membrane reformer is experimentally demonstrated. The reactor, built from three concentric compartments, indirectly couples the endothermic methane steam reforming with the exothermic methane oxidation, while hydrogen is separated by a permselective Pd membrane. The study focuses on the determination of the key operation parameters and understanding their influence on the reactor performance. It has been shown that the reactor performance is mainly defined by the dimensionless ratio of the methane steam reforming feed flow rate to the hydrogen maximal membrane flow rate and by the ratio of the oxidation and steam reforming methane feed flow rates.     

Membrane reformer module with Ni-foam catalyst for pure hydrogen production from methane: Experimental demonstration and modeling

Results of experiments and modeling of a compact (800 cm³) membrane reformer module for the production of 0.25–0.30 Nm³/h hydrogen by methane steam reforming are reported. The module consists of a two-sided composite membrane disc with a 50 μm PdAg layer and two adjacent 4 mm thick Ni foam discs (60 ppi). A nickel catalyst and a porous support were deposited on the foam discs to give the final composition of 10%Ni/10%MgO/Ni-foam. Membrane permeability by pure hydrogen was investigated, and coefficients of transverse hydrogen transport across the Ni foam to the membrane in the case of inlet binary N₂H₂ mixture were refined in order to account for concentration polarization effect into the model. Activity of the catalytic discs was measured in a differential laboratory scale reactor at a pressure of 1 bar and temperature of 400–600 °C. Modules were tested at a 8–13 bar pressure of the mixture in the reforming zone and at 1 bar of pure hydrogen under the membrane, H₂O/C = 2.5–3 and a module temperature of 550–680 °C (with and without hydrogen removal). Two modifications of the module were tested: consecutive (I-type) and parallel (II-type) flow of the reaction mixture around two sides of the membrane disc. In order to optimize construction of the module, calculations were made for revealing the effect of thickness of the PdAg membrane layer (5–50 μm), thickness of the Ni foam discs (0.5–8 mm) and temperature (600–700 °C) on the hydrogen output of the module. A comparison of the values obtained in our experiments (>1 MW/m³ and >0.7 kg(H₂)/h/m²) with the literature data reported by other authors showed that the developed modules are promising for practical application as components of a fuel processor section for mobile applications.     

Microfibrous entrapment of Ni/Al2O3 for dry reforming of methane: Heat/mass transfer enhancement towards carbon resistance and conversion promotion

An 8-μm-copper microfibrous entrapped Ni/Al₂O₃ (Cu-MFE-Ni/AlO) composite catalyst was developed for demonstrating the process intensification effectiveness of the novel microfibrous entrapment technology on dry reforming of methane (DRM), which is highly regarded for CH₄ utilizing and CO₂ chemical cycling. Computational fluid dynamics (CFD) calculation was employed to illustrate the significant enhancement of the heat transfer of the microfibrous structured bed at steady working state. The results indicated that the average bed temperature of Cu-MFE-Ni/AlO was 1039 K, 75 K higher than that of packed bed with Ni/AlO (PB-Ni/AlO), when the wall temperature was set at 1073 K. As a result, carbon resistance of the catalyst bed was significantly improved by a thermodynamic way along with visible conversion promotion. For instance, at temperature of 1073 K, more than 4-fold reduction of average carbon deposition rate was achieved in the Cu-MFE-Ni/AlO composite bed compared to the PB-Ni/AlO, while the CH₄ conversion was promoted from 84% on the PB-Ni/AlO to 89% on our Cu-MFE-Ni/AlO composite bed with a gas hourly space velocity (GHSV) of 20,000 mL g_cat⁻¹ h⁻¹. Moreover, such microfibrous entrapment technology also provided a unique combination of small catalyst particle size (0.15–0.18 mm) and entirely open structure with large void volume (71.3 vol%) thereby leading to enhanced mass transfer and high permeability (low pressure drop).     

Methane steam reforming in a Pd-Ag membrane reformer: An experimental study on reaction pressure influence at middle temperature

In this experimental work, methane steam reforming (MSR) reaction is performed in a dense Pd-Ag membrane reactor and the influence of pressure on methane conversion, CO_x-free hydrogen recovery and CO_x-free hydrogen production is investigated. The reaction is conducted at 450 °C by supplying nitrogen as a sweep gas in co-current flow configuration with respect to the reactants. Three experimental campaigns are realized in the MR packed with Ni-ZrO catalyst, which showed better performances than Ni-Al₂O₃ used in a previous paper dealing with the same MR system. The first one is directed to keep constant the total pressure in both retentate and permeate sides of the membrane reactor. In the second case study, the total retentate pressure is kept constant at 9.0 bar, while the total permeate pressure is varied between 5.0 and 9.0 bar. As the best result of this work, at 450 °C and 4.0 bar of total pressure difference between retentate and permeate sides, around 65% methane conversion and 1.2 l/h of CO_x-free hydrogen are reached, further recovering 80% CO_x-free hydrogen over the total hydrogen produced during the reaction. Moreover, a study on the influence of hydrogen-rich gas mixtures on the hydrogen permeation through the Pd-Ag membrane is also performed and discussed.     

CFD study of heat and mass transfer in ethanol steam reforming in a catalytic membrane reactor

This work shows the analysis of ethanol steam reforming process within a catalytic membrane reactor. A 2-D non-isothermal CFD model was developed using Comsol Multiphysics, based on previous experimentally validated isothermal model. A comprehensive heat and mass transfer study was carried out utilizing the model. Operating conditions such as liquid hourly space velocity (LHSV) (3.77–37.7 h^?1), temperature (673–823 K), reaction side pressure (4–10 bar) and permeate side sweep gas flow pattern were discussed. A temperature gradient along the reactor was observed from the model and a “cold spot” was seen at the reactor entrance area, which is unfavorable for the highly endothermic ethanol steam reforming process. By changing the sweep gas pattern to counter-current, the “cold spot” appears to be smaller with a reduced temperature drop. By studying the individual reaction rates, reverse methane steam reforming (methanation) was observed, caused by the low temperature in the “cold spot”. Optimal operating conditions were found to be under LHSV = 37.7 h^?1 and counter-current sweep gas conditions.     

Methane steam reforming using a membrane reactor equipped with a Pd-based composite membrane for effective hydrogen production

Herein, a methane steam reforming (MSR) reaction was carried out using a Pd composite membrane reactor packed with a commercial Ru/Al₂O₃ catalyst under mild operating conditions, to produce hydrogen with CO₂ capture. The Pd composite membrane was fabricated on a tubular stainless steel support by the electroless plating (ELP) method. The membrane exhibited a hydrogen permeance of 2.26 × 10^?3 mol m² s^?1 Pa^?0.5, H₂/N₂ selectivity of 145 at 773 K, and pressure difference of 20.3 kPa. The MSR reaction, which was carried out at steam to carbon ratio (S/C) = 3.0, gas hourly space velocity (GHSV) = 1700 h^?1, and 773 K, showed that methane conversion increased with the pressure difference and reached 79.5% at ΔP = 506 kPa. This value was ～1.9 time higher than the equilibrium value at 773 K and 101 kPa. Comparing with the previous studies which introduced sweeping gas for low hydrogen partial pressure in the permeate stream, very high pressure difference (2500–2900 kPa) for increase of hydrogen recovery and very low GHSV (<150) for increase hydraulic retention time (HRT), our result was worthy of notice. The gas composition monitored during the long-term stability test showed that the permeate side was composed of 97.8 vol% H₂, and the retentate side contained 67.8 vol% CO₂ with 22.2 vol% CH₄. When energy was recovered by CH₄ combustion in the retentate streams, pre-combustion carbon capture was accomplished using the Pd-based composite membrane reactor.     

Pd–Ag membrane reactor for steam reforming reactions: A comparison between different fuels

The simulation of a dense Pd-based membrane reactor for carrying out the methane, the methanol and the ethanol steam reforming (SR) reactions for pure hydrogen production is performed. The same simulation is also performed in a traditional reactor.     

Co-production of hydrogen and carbon black from solar thermal methane splitting in a tubular reactor prototype

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen and carbon black (CB) as a high-value nano-material with the bonus of zero CO₂ emission. The work focused on the development of a medium-scale solar reactor (10 kW) based on the indirect heating concept. The solar reactor is composed of a cubic cavity receiver (20 cm-side), which absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results in the temperature range 1740-2070 K are presented: acetylene (C₂H₂) was the most important by-product with a mole fraction of up to about 7%, depending on the gas residence time. C₂H₂ content in the off-gas affects drastically the carbon yield of the process. The effects of temperature and residence time are analyzed. A preliminary process study concerning a 55 MW solar chemical plant is proposed on the basis of a process flow sheet. Results show that 1.7 t/h of hydrogen and 5 t/h of CB could be produced with an hydrogen cost competitive to conventional steam methane reforming.     

Double-walled reformer tubes using high-temperature thermal storage of molten-salt/MgO composite for solar cavity-type reformer

Composite materials with alkali carbonate and magnesia have been examined for high-temperature thermal storage in solar tubular reformers. The concept of a double-walled reactor tube involves packing a molten-salt/ceramic composite material into the annular region between internal catalyst tube and exterior solar-absorber wall. In this paper, the shape and interior structure of the reactor tube are newly designed for use in solar cavity-type reformers using straight reactor tubes. Na₂CO₃, K₂CO₃, and Li₂CO₃ composite materials with magnesia were tested as thermal storage media for CO₂ reforming of methane during cooling mode of the reactor tube at a laboratory scale. The efficiency of Na₂CO₃/MgO composite with various MgO contents was also estimated. Composite materials of Na₂CO₃ 80–90 wt% and MgO 20–10 wt% were successfully delayed the cooling of the catalyst bed and sustained methane conversion at >90%. A solar cavity-type reformer consisting of multiple straight reactor tubes is expected to enable stable operation of the solar reforming process under fluctuating solar insolation during cloud passage.