Millisecond methane steam reforming for hydrogen production: A computational fluid dynamics study

The potential of methane steam reforming to produce hydrogen in thermally integrated micro-chemical systems at short contact times was theoretically explored. Methane steam reforming coupled with methane catalytic combustion in microchannel reactors for hydrogen production was studied numerically. A two-dimensional computational fluid dynamics model with detailed chemistry and transport was developed. To provide guidelines for optimal design, reactor behavior was studied, and the effect of design parameters such as catalyst loading, channel height, and flow arrangement was evaluated. To understand how steam reforming can happen at millisecond contact times, the relevant process time scales were analyzed, and a heat and mass transfer analysis was performed. The importance of energy management was also discussed in order to obtain a better understanding of the mechanism responsible for efficient heat exchange between highly exothermic and endothermic reactions. The results demonstrated the feasibility of the design of millisecond reforming systems, but only under certain conditions. To achieve this goal, process intensification through miniaturization and the improvement in catalyst performance is very important, but not sufficient; very careful design and implementation of the system is also necessary to enable high thermal integration. The channel height plays an important role in determining the efficiency of heat exchange. A proper balance of the flow rates of the combustible and reforming streams is an important design criterion. Reactor performance is significantly affected by flow arrangement, and co-current operation is recommended to achieve a good energy balance within the system. The catalyst loading must be carefully designed to avoid insufficient reactant conversion or hot spots. Finally, operating windows were identified, and engineering maps for designing devices with desired power were constructed.     

Comparisons between methane and methanol steam reforming in thermally integrated microchannel reactors for hydrogen production: A computational fluid dynamics study

This paper addresses the issues related to the design and operation of steam reforming combined with catalytic combustion in thermally integrated microchannel reactors for hydrogen production. Comparisons were made between methanol and methane steam reforming, representing a low and a high temperature process respectively, under the same operating conditions to determine whether methanol-based thermally integrated systems can be more energy-efficient than methane-based ones. Computational fluid dynamics simulations were performed to gain insight into the reactor performance and thermal behavior. The effect of various design parameters was investigated to identify suitable ranges of operating conditions, and an analysis of heat and mass transfer was performed to design a highly efficient system. It was shown that steam reforming of both fuels is feasible in millisecond reactors under a variety of conditions, but very careful design is necessary. Methanol reforming can be more efficient, offering a better solution not only to simplify design but also to improve power and efficiency. The wall thermal conductivity is essential to the design and optimization of these systems, as it can significantly affect the overall energy balance. There is no significant difference in reactor performance between different channel heights at the same flow rate. The ratio of the flow rates on opposite sides of the reactor is an important design parameter and must be carefully adjusted to improve efficiency and eliminate hot spots. Finally, a simple operating strategy was proposed to achieve variable power output, and design recommendations were made.     

Design and operational considerations of packed-bed membrane reactor for distributed hydrogen production by methane steam reforming

Methane steam reforming will still account for most of hydrogen production in the coming decades. Membrane reactor can play a key role in both energy saving and process/equipment compactness, particularly for its decentralized applications. Here we design a particles-based packed-bed membrane reactor and explore the operational window and design challenges by conducting systematic study experimentally and computationally, particularly emphasizing geometrical scale of membrane reactor and catalyst activity. The results show that membrane reactor presents maximum hydrogen flux by consuming unit methane under the optimized operation conditions of GHSV (i.e., 1134 hr^?1) and steam-to-carbon ratio (i.e., 2), and computational study shows that optimal operation window is around 30 atm and 773.15 K. Moreover, the design criteria of “Catalyst activity – Membrane performance – Radial depth” is revealed quantitatively and catalyst activity is identified as the key limiting factor for further process intensification. Briefly, these results shed some lights on operation, optimal design, and further improvement of membrane reactor in methane steam reforming.     

Dry and steam reforming of biomass pyrolysis gas for rich hydrogen gas

Biomass pyrolysis gas (including H₂, CO, CH₄, CO₂, C₂H₄, C₂H₆ and etc.) reforming for hydrogen production over Ni/Fe/Ce/Al₂O₃ catalysts was presented in this study. This study investigated how the operating conditions, such as the calcinations temperature of catalysts, the reaction temperature, the gas hourly space velocity (GHSV) and the ratio of H₂O/C, affect the conversion of CH₄ and CO₂ and the selectivity of hydrogen from dry and steam reforming of pyrolysis gas. The experimental results indicated that, under the conditions: the reaction temperature of 600 °C, the GHSV of 900 h⁻¹ and H₂O/C of 0.92, the reaction efficiency is the optimal. Especially, the concentration of H₂, CO, CH₄, CO₂, and C₂H_n (C₂H₄ and C₂H₆) were 36.80%, 10.48%, 9.61%, 42.62%, 0.49% respectively. The conversion of CH₄ and CO₂ reached 45.9% and 51.09%, respectively. There were all kinds of reactions during the processing of reforming of pyrolysis gas. And the main reactions changed with the operation condition. It was due to the promoting or inhibiting interaction among different constituents in the pyrolysis gas and the different activity of catalysts in the different operation condition.     

Hydrogen production by coupled catalytic partial oxidation and steam methane reforming at elevated pressure and temperature

Hydrogen production by coupled catalytic partial oxidation (CPO) and steam methane reforming of methane (OSMR) at industrial conditions (high temperatures and pressures) have been studied over supported 1 wt.% NiB catalysts. Mixture of air/CH₄/H₂O was applied as the feed. The effects of O₂:CH₄ ratio, H₂O:CH₄ ratio and the gas hourly space velocity (GHSV) on oxy-steam reforming (OSRM) were also studied. Results indicate that CH₄ conversion increases significantly with increasing O₂:CH₄ or H₂O:CH₄ ratio. However, the hydrogen mole fraction goes through a maximum, depending on reaction conditions, e.g., pressure, temperature and the feed gases ratios. Carbon deposition on the catalysts has been greatly decreased after steam addition. The supported 1 wt.% NiB catalysts exhibit high stability with 85% methane conversion at 15 bar and 800 °C during 70 h time-on-stream reaction (CH₄:O₂:H₂O:N₂ = 1:0.5:1:1.887). The thermal efficiency was increased from 35.8% by CPO (without steam) to 55.6%. The presented data would be useful references for further design of enlarged scale hydrogen production system.     

Exergy analysis of hydrogen production via steam methane reforming

The performance of hydrogen production via steam methane reforming (SMR) is evaluated using exergy analysis, with emphasis on exergy flows, destruction, waste, and efficiencies. A steam methane reformer model was developed using a chemical equilibrium model with detailed heat integration. A base-case system was evaluated using operating parameters from published literature. Reformer operating parameters were varied to illustrate their influence on system performance. The calculated thermal and exergy efficiencies of the base-case system are lower than those reported in literature. The majority of the exergy destruction occurs due to the high irreversibility of chemical reactions and heat transfer. A significant amount of exergy is wasted in the exhaust stream. The variation of reformer operating parameters illustrated an inverse relationship between hydrogen yield and the amount of methane required by the system. The results of this investigation demonstrate the utility of exergy analysis and provide guidance for where research and development in hydrogen production via SMR should be focused.     

Methanol and ethanol steam reforming in membrane reactors: An experimental study

In this work a comparison between methanol steam reforming (MSR) reaction and ethanol steam reforming (ESR) reaction to produce hydrogen in membrane reactors (MRs) is discussed from an experimental point of view.     

On the energy efficiency of hydrogen production processes via steam reforming using membrane reactors

The novel paradigm of distributed energy production foresees the production of hydrogen from methane and biomasses in small plants, which may take advantage from membrane-based processes. By means of a modeling approach, this paper compares the energy efficiency of two membrane-based processes to produce H₂ from methane steam reforming. The two-step process (TS) envisages a high temperature classical reactor and a following WGS stage in a membrane reactor, while the alternative process uses a simple packed-bed membrane reactor (MR). Both processes show a general increase of H₂ production and energy efficiency with the pressure and a maximum energy efficiency for S/C of 4, while the increase of the space velocity reduces the performances of the MR. The results show that the TS process performs better than the studied MR and that the maximum energy efficiency of both processes is between 30 and 40%. A comparison with the literature shows that the TS process may achieve similar performances respect to an intensified MR.     

Efficient operation of autothermal microchannel reactors for the production of hydrogen by steam methane reforming

Efficient conversion of methane to hydrogen has emerged as a significant challenge to realizing fuel cell-based energy systems. Autothermal microchannel reactors, coupling of exothermic and endothermic reactions in parallel channels, have become one of the most promising technologies in the field of hydrogen production. Such reactors were utilized as an intensified design for conducting the endothermic steam methane reforming reaction. The energy required by the endothermic process is supplied directly through the separating plates of the reactor structure from the exothermic process occurring on the opposing side. Optimal design problems associated with transport phenomena in such an autothermal system were analyzed. Various methods for designing and operating autothermal reactors employed in steam methane reforming were discussed. Computational fluid dynamics simulations were performed to identify the underlying principles of process intensification, and to delineate several design and operational features of the intensified reforming process. The results indicated that the autothermal reactor is preferable to be thermally conductive to ensure its structural integrity and maximum operating regime. However, the thermal properties of the reactor structure are not essential due to efficient heat transfer existing between endothermic and exothermic process streams. A reactor design which minimizes the mass transfer resistance is highly required, and the channel dimension is of critical importance. Furthermore, the challenges presented by the efficient operation of the autothermal system were identified, along with demonstrating the implementation of transport management in order to improve overall reactor performance and to mitigate extreme temperature excursions.     

ANOVA analysis of an integrated membrane reactor for hydrogen production by methane steam reforming

Steam methane reforming is an endothermic reaction and it used to produce hydrogen and syngas. In this research, a factorial design is developed for an integrated Pd-based membrane reactor, producing hydrogen by methane steam reaction. In literature, no analogous works are present, because a simple sensitivity analysis is carried out without finding significant factors for the process. The reactor is modelled in MATLAB software using the Numaguchi kinetic. The reactor does not use conventional catalysts, but a Ni(10)/CeLaZr catalyst supported on SSiC ceramic foam. In ANOVA analysis, inlet temperature (550 K-815 K), methane flow rate in the feed (0.1 kmol/h-1 kmol/h), hydrogen permeability (1000 m³μmm⁻²hrbar^0.5–3600 m³μmm⁻²hrbar^0.5), the thickness of membrane (0.003 m-0.02 m) are the chosen factors. The analyzed responses are: hydrogen yield, carbon dioxide conversion and methane conversion. Results show that only inlet temperature, methane flow rate, their interaction and the thickens of membrane are significant. Also, the optimal operating conditions are obtained with inlet temperature, methane flow rate, hydrogen permeability and thickness of membrane equal to 550 K, 0.1 kmol/h, 3600 m³μmm⁻²hrbar^0.5 and 0.003 m.     

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.     

RuRh based catalysts for hydrogen production via methanol steam reforming in conventional and membrane reactors

Results of hydrogen production study in methanol steam reforming (MSR) process with the use of Ru_0.5–Rh_0.5 catalysts supported on different carbon materials: synthetic graphite-like material Sibunit, carbon black Ketjenblack EC600DJ, detonation nanodiamonds (DND) and ZrO₂-based material with fluorite structure, doped with ceria, have been described. The samples have been tested in conventional flow reactor and membrane (MR) reactor, containing Pd-based membranes with different composition, thickness and surface architecture. It has been shown that the catalytic activity of the composites depends on the support nature. The RuRh/DND catalyst exhibits the highest activity, whereas RuRh/Ce_0.1Zr_0.9O_2–δ is the most selective. The use of PdAg (23%) foil with the surface modified by palladium black showed great advantages comparing to the smooth dense membrane. The use of the MR with the PdAg membrane improves the MSR reaction and provides almost 50% increase in the hydrogen yield. The hydrogen produced with the use of the MR is ultra pure.     

Molecular simulation of methane steam reforming reaction for hydrogen production

By means of advanced techniques of molecular simulations, we have studied the chemical equilibrium of methane steam reforming reaction. We have computed the conversion of CH₄, yield and selectivity of H₂, etc. in the gas phase by reactive canonical Monte Carlo (RCMC) method and compared with those from Gibbs energy of formation method. The consistency of the two methods encourages us to use the RCMC method to optimize the operating conditions. We found that under low pressure 0.1 MPa, high temperature 1073 K and high water-gas ratio H₂O/CH₄ = 5, the CH₄ conversion, H₂ yield and selectivity were the highest, with the values of 99.93%, 3.51 mol/molCH₄ and 99.98%, respectively. In addition, the pore size of activated carbon significantly affects the chemical equilibrium composition in the pores. Since low pressure and high temperature are not conducive to the adsorption of reactive components by activated carbon, the chemical balance in the pores cannot be improved. At 773 K, 3.0 MPa and pore width is less than 2 nm, the pores are mainly occupied by CH₄ and H₂O reactant molecules. Further increasing the temperature can increase the H₂ content in the pores, but the adsorption capacity in the pores will decrease. We use activated carbon to adsorb and separate CO and H₂ (CO:H₂ = 1:3), the main components after the gas phase reaction reaches equilibrium. At 298 K, 7.5 MPa and the optimal pore width of 0.76 nm, the CO/H₂ selectivity is 28.3 and the CO adsorption capacity is 8.45 mmol/cm³.     

Pure hydrogen production in a Pd-Ag multi-membranes module by methane steam reforming

An experimental test campaign has been carried out in order to investigate the performances in terms of pure hydrogen production of a multi-membrane module coupled with a methane reforming fixed bed reactor. The effect of operating parameters such as the temperature, the pressure, the water/methane feed flow rates and the feed molar ratio has been studied. The hydrogen produced into the traditional reformer has been recovered in the shell side of the membrane module by vacuum pumping. The membrane module consists of 19 Pd/Ag permeator tubes of wall thickness 150 μm, diameter 10 mm and length 250 mm: these dense permeators permitted to separate ultra-pure hydrogen.The experiments have been carried out with the reaction pressure of 100-490 kPa, the temperature of the reformer of 570-720 °C and the temperature of the Pd/Ag membranes module of 300-400 °C. A water/methane stream of molar ratio of 4/1 and 5/1 has been fed into the methane reformer at GSHV of 1547.6 and 1796.1 L_(STP) kg⁻¹ h⁻¹. Hydrogen yield value of about 3 has been measured at reaction pressure of 350 kPa, temperature reformer of 720 °C and methane feed flow rate of 6.445 × 10⁻⁴ mol s⁻¹.     

Hydrogen production from methane and natural gas steam reforming in conventional and microreactor reaction systems

Ni-based (over MgO and Al₂O₃) and noble metal-based (Pd and Pt over Al₂O₃) catalysts were prepared by wet impregnation method and thereafter impregnated in microreactors. The catalytic activity was measured at several temperatures, atmospheric pressure and different steam to carbon, S/C, ratios. These conditions were the same for conventional, fixed bed reactor system, and microreactors. Weight hourly space velocity, WHSV, was maintained equal in order to compare the activity results from both reaction systems. For microreactor systems, similar activities of Ni-based catalyst were measured in the steam methane reforming (SMR) activity tests, but not in the case of natural gas steam reforming tests. When noble metal-based catalysts were used in the conventional reaction system no significant activity was measured but all catalysts showed some activity when they were tested in the microreactor systems. The analysis by SEM and TEM revealed a carbon-free surface for Ni-based catalyst as well as carbon filaments growth in case of noble metal-based catalysts.     

Thermodynamic analysis of propane dry and steam reforming for synthesis gas or hydrogen production

Thermodynamics was applied to investigate propane dry reforming (DR) and steam reforming (SR). Equilibrium calculations employing the Gibbs free energy minimization were performed upon a wide range of pressure (1–5 atm), temperature (700–1100 K), carbon dioxide to propane ratio (CPR, 1–12) and water to propane ratio (WPR, 1–18). From a thermodynamic perspective, it is demonstrated that DR is promising for production of synthesis gas with low hydrogen content, as opposite to SR which favours generation of synthesis gas with high hydrogen content. Complete conversion of propane was obtained for the range of pressure, temperature, CPR and WPR considered in this study. Atmospheric pressure is shown to be preferable for both DR and SR. Approximately 10 mol of synthesis gas can be produced per mole of propane at a temperature greater than 1000 K from DR when CPR is higher than 6. The optimum conditions for synthesis gas production from DR are found to be 975 K (CPR = 3) for a H₂/CO ratio of 1 and 1100 K (CPR = 1) for a H₂/CO ratio of 2. The greatest CO₂ conversion (95%) can be obtained also at 1100 K and CPR = 1. Preferential conditions for hydrogen production from SR are achieved with the temperatures between 925 and 975 K and WPRs of 12–18. The maximum number of moles of hydrogen produced is 9.1 (925 K and WPR = 18). Under conditions that favour hydrogen production, methane and carbon formation can be eliminated to negligible level.     

Hydrogen production in steam reforming of glycerol by conventional and membrane reactors

The aim of this study is to produce hydrogen through the glycerol steam reforming process. The reaction is carried out in a traditional reactor and an electrolessly plated Pd/Ag alloy membrane reactor, with varying reaction temperature, weight hourly specific velocity (WHSV) and water glycerol molar ratio (WGMR). The non-catalytic test was also employed for comparative purposes. The results show that the reaction is highly depending on temperature, and the maximum glycerol conversion achieved to 96.24% at 800 °C with a hydrogen yield of 5.82 mol-H₂/mol-C₃H₈O₃. It also found that the Pd/Ag membrane can effectively separate hydrogen from the reaction side and subsequently enhance the reaction rate in the membrane reactor. TGA measurements were employed to quantify the amounts of deposited carbon and the results also confirmed that the CeO₂ modified catalyst can improve the carbon resistance as well as activity and stability.     

Compartment model for steam reforming of methane in a membrane-assisted bubbling fluidized-bed reactor

A compartment model was developed to describe the flow pattern of gas within the dense zone of a membrane-assisted fluidized-bed reactor (MAFBR), in the bubbling mode of operation for steam reforming of methane both with (adiabatic) and without (isothermal) entering oxygen. Considering such a flow pattern and using the experimental data reported elsewhere [Roy S, Pruden BB, Adris AM, Grace JR, Lim CJ. Fluidized-bed steam methane reforming with oxygen input. Chem Eng Sci 1999; 54:2095–2102.], the parameters of the developed model (i.e., number of compartments for the bubble and emulsion phases) were determined and fair agreements were obtained between model predictions and experimental data. The developed model was utilized to describe the behavior of an industrial scale adiabatic and isothermal MAFBR. Moreover, the influences of various operating and design parameters such as steam-to-methane ratio (SMR), oxygen-to-methane ratio (OMR), operating temperature and pressure, and the number of hydrogen membrane tubes on the performance capability of the MAFBR were investigated. Furthermore, the performance capability of the MAFBR was optimized subject to the various operating and design constraints, including 1 ≤ SMR ≤ 4 and 500 ≤ T ≤ 1250 K, in the bubbling regime.