Thermal analysis of a micro tubular reactor for methanol steam reforming by optimizing the multilayer arrangement of catalyst bed for the catalytic combustion of methanolr

Packed bed tube reactors are commonly used for hydrogen production in proton exchange membrane fuel cells. However, the hydrogen production capacity of methanol steam reforming (MSR) is greatly limited by the poor heat transfer of packed catalyst bed. The hydrogen production capacity of catalyst bed can be effectively improved by optimizing the temperature distribution of reactor. In this study, four types of reactors including concentric circle methanol steam reforming reactor (MSRC), continuous catalytic combustion methanol steam reforming reactor (MSRR), hierarchical catalytic combustion methanol steam reforming reactor (MSRP) and segmented catalytic combustion reactor with fins (MSRF) are designed, modeled, compared and validated by experimental data. It was found that the maximum temperature difference of MSRC, MSRR, MSRP and MSRF reached 72.4 K, 58.6 K, 19.8 K and 11.3 K, respectively. In addition, the surface temperature inhomogeneity U_f and CO concentration of the MSRF decreased by 69.8% and 30.7%, compared with MSRC. At the same reactor volume, MSRF can achieve higher methanol conversion rate, and its effective energy absorption rate is 4.6%, 3.9% and 2.6% higher than that of MSRC, MSRR and MSRP, respectively. The MSRF could effectively avoid the influence of uneven temperature distribution on MSR compared with the other designs. In order to further improve the performance of MSRF, the influences of methanol vapor molar ratio, inlet temperature, flow rate, catalyst particle size and catalyst bed porosity on MSR were also discussed in the optimal reactor structure (MSRF).     

Thermal management of CO2 methanation with axial staging of active metal concentration in Ni-YSZ tubular catalysts

CO₂ methanation has attracted considerable interests as a promising approach to productively utilizing CO₂ and reducing emissions to realize a low-carbon society. One major difficulty with packed bed reactors for catalyzed CO₂ methanation is maintaining an optimal reactor temperature distribution. Although a high temperature increases the catalytic activity, it also leads to the formation of an inlet hotspot, which causes thermal runaway, unfavorable equilibrium products, and catalyst degradation. To address this, in this study, we proposed an approach to manage the temperature profile in CO₂ methanation reactors by increasing catalytic activity along the reactor length using different Ni composition catalysts (gradient-distributed Ni-YSZ catalyst). Ni-based tubular catalysts with different Ni compositions were prepared and stacked in order of ascending Ni content from the inlet to the outlet. The effect of gradient Ni compositions on the temperature profile was investigated based on both numerical simulations and experimental observations. The gradient-distributed Ni catalyst could successfully prevent hotspot formation at the inlet of the reactor compared to the highly active uniform catalysts. The use of the catalyst caused a small difference in the reactor temperature (of ~70 °C) and afforded a high CH₄ yield (~90%). The proposed approach using gradient-distributed catalysts could be a potential method to manage CO₂ methanation reactor temperature and to achieve high CO₂ conversion in compact reactors.     

Hydrogen production by oxidative steam reforming of methanol over anodic aluminum oxide-supported Cu-Zn catalyst

Oxidative steam reforming of methanol (OSRM), which is a convenient reaction for producing hydrogen, suffers from the hot spot formation problem when conventional particle catalysts are used. Recently, an anodic aluminum oxide (AAO)-supported Cu-Zn catalyst was proposed as an OSRM catalyst for its high thermal conductivity through the aluminum metal body. In this study, OSRM was conducted in a prototype reactor packed with the AAO plate catalyst strips. It was verified that the high thermal conductivity of the catalyst effectively suppresses the hot spot formation and makes the temperature profile smooth along the reactor. The start-up time of the reactor depended on the preheating temperature and was very short (less than 2 min) for preheating over 503 K. The methanol conversion and reactor temperature increased with increasing O₂/CH₃OH mole ratio, indicating that the mole ratio can be used as a control variable to operate the reactor at desired conditions. Further, a reactor model was developed and verified, and the simulation showed that for a given total reactor volume, an optimal reactor configuration could be achieved by shortening the reactor length while widening the cross-sectional area.     

Minimization of hot spot in a microchannel reactor for steam reforming of methane with the stripe combustion catalyst layer

Hot spot formation is inevitable in a heat exchanger microchannel reactor used for steam reforming of methane because of the local imbalance between the generated and absorbed heat. A stripe configuration of the combustion catalyst layer was suggested to make the catalytic combustion rate uniform in order to minimize the hot spot near the inlet. The stripe configuration was optimized by response surface methodology with computational fluid dynamics. With the optimal catalyst layer, the hot spot was not observed near the inlet and the maximum temperature decreased by 130 K from that of the uniform catalyst layer without any conversion loss. The maximum relative particle diameters of the uniform and the optimal stripe catalyst layer were about 3.68 and 2.51, respectively, and the surface-averaged particle diameter of the optimal stripe catalyst layer was 7.64% less than that of the uniform stripe catalyst layer.     

Reforming of methanol with steam in a micro-reactor with Cu–SiO2 porous catalyst

In the present work, we report the results of a series of experiments for the hydrogen production via steam reforming of methanol with Cu–SiO₂ porous catalyst coated on the internal walls of a micro-reactor with parallel micro-passages. The catalyst was prepared by coating copper and silica nanoparticles on the internal surface of the microchannel via convective flow boiling heat transfer, followed by a calcination procedure at 973 K and therefore, the catalyst does not require any supportive material, which in turn reduced the complexity and cost of the preparation. The experiments were conducted at reactant flow rates of 0.1–0.9 lit/min, operating temperatures of 523–673 K, catalyst loading of 0.25 gr to 1.25 gr and at heat flux value of 500 kW/m². Results of the experiments showed that the methanol conversion can reach 97% at catalyst loading of 1.25 gr. It was also found that with an increase in the gas hourly space velocity (GHSV) of the reactants, the methanol conversion decreases, which was attributed to the decrease in the residence time, the suppression in diffusion of reactants into the pores of the catalyst, and also the decrease in the average film temperature of the reactor. The highest methanol conversion was obtained at gas hourly space velocity of 24,000 ml/(gr.hr) and T = 773 K and for molar ratio of methanol to water of 0.1. The molar ratio of methanol to water also influenced the thermal response of the reactor such that the surface temperature profile of the micro-reactor was more decreased at low methanol/water molar ratios.     

Dynamic optimization of a novel radial-flow,spherical-bed methanol synthesis reactor in the presence of catalyst deactivation using Differential Evolution (DE) algorithm

In this work, a novel radial-flow spherical-bed methanol synthesis reactor has been optimized using Differential Evolution (DE) algorithm. This reactor's configuration visualizes the concentration and temperature distribution inside a radial-flow packed bed with a novel design for improving reactor performance with lower pressure drop. The dynamic simulation of spherical multi-stage reactors has been studied in the presence of long-term catalyst deactivation. A theoretical investigation has been performed in order to evaluate the optimal operating conditions and enhancement of methanol production in radial-flow spherical-bed methanol synthesis reactor. The simulation results have been shown that there are optimum values of the reactor inlet temperatures, profiles of temperatures along the reactors and reactor radius ratio to maximize the overall methanol production. The optimization methods have enhanced additional yield throughout 4 years of catalyst lifetime, respectively.     

Optimal monolithic configuration for heat integrated ethanol steam reformer

A design concept for optimal design of monolith catalyst is presented through modeling of transport–kinetic interactions in a monolith catalyst. We argue that reactors employing monolithic catalysts should be based on its optimal choice of geometry. In line with that argument, we present a thorough analysis of the geometrical parameters influencing the performance of non-isothermal reactor operation. In this study, an optimal monolith configuration is estimated to be a combination (d_h, t_w) of (0.9 mm, 0.2 mm) for a compact ethanol reformer to produce hydrogen for portable applications where maximum volumetric reactor activity exists. A three-dimensional modeling framework is developed for the resulting optimal monolithic catalyst design that couples the reforming section with a suitable heat source in a recuperative way. As a result, greater ethanol conversion is obtained from the monolith channels near the periphery of the block. The coupling with combustion could predict the formation of cold and hot spots inside the reactor, their nature being dependent on the flow configuration. Further, the effect of altering the feed inlet operating conditions over the variation of ethanol conversion and temperature inside the reactor is also analyzed. The increase in reforming inlet velocity decreases the outlet conversion and shifts the cold spot, forward and deeper in co-flow configuration. The decreasing inlet feed temperature enhances the transfer of heat, eliminating the cold spot.     

Performance enhancement of methanol reforming reactor through finned surfaces and diffused entry for on-board hydrogen generation

Hydrogen-rich combustion in engines helps in reducing pollutants significantly. But hydrogen usage on a moving vehicle is not getting large-scale user acceptance mainly due to its poor energy storage density resulting in shorter driving ranges. This storage issue led to the hunt for mediums that can efficiently produce on-board hydrogen. Methanol proves to be an efficient alcohol fuel for producing hydrogen through steam reforming reaction. The heat energy required for such endothermic reaction is obtained through exhaust engine waste energy and this process is collectively known as thermochemical recuperation. However, the conventional reactor used for this process faces a lot of problems in terms of efficiency and methanol conversion. In this study, an attempt has been made to improve the design of the reactor for on-board hydrogen generation using engine exhaust heat for addressing the challenges related to performance and hydrogen yield. For enhancing the heat transfer, a finned surface (straight & wavy) was introduced in the reactor which resulted in an increment in methanol conversion significantly. It was found that wavy fin improved the methanol conversion up to 96.8% at an exhaust inlet temperature of 673 K. Also, a diffusing inlet section was introduced to increase the residence time of reactant gases while passing through the catalyst zone. Under given inlet conditions, the methanol conversion for 6° diffuse inlet reactor goes up to 87.9% as compared to 75.4% for the conventional reactor.     

Autothermal reforming of n‐hexadecane over Rh catalyst to produce syngas in microchannel reactor using finite element method

Numerical study on the autothermal reforming of n‐hexadecane, which can be used in proton exchange membrane fuel cell for automotive applications, in microchannels is necessary. A 2D computational fluid dynamics (CFD) model, with combustion and reforming channels thermally coupled and separated by a metal medium wall, is developed and studied in terms of hydrogen production and catalyst activity. Rh supported on CeO₂ is used as a catalyst and applied to the inner surface of the channels, where the catalytic endothermic and exothermic reactions occur. CFD analysis shows considerable results in terms of reactor performance. Along the reactor channel length, the mole percentage of hydrogen is 86% after over 2 hours of catalyst activity. The corresponding fuel conversion in respective channels is 85% on the catalytic surface of the reactor. The predicted hydrogen production from the CFD model is 59% higher than that as equilibrium conditions. Heat conduction through the medium solid wall depends on the thermal conductivity of a material. In this model, a metal solid wall with thermal conductivity of 40 W/m K, which transfers heat from the combustion channel within milliseconds, is used. The calculated model operating temperature in the reforming channel ranges from 660 to 850 K.     

Robust Mesh-type structured CuFeMg/γ-Al2O3/Al catalyst for methanol steam reforming in microreactors

Utilizing a compact, efficient and fast-response reactor for on-site reforming of liquid methanol is an effective method to solve the storage and transportation problems of hydrogen. In this paper, a mesh-type structured CuFeMg/γ-Al₂O₃/Al catalyst with strong bonding force was prepared by anodic oxidation method, and its intrinsic catalytic activity, hydrogen production capacity and start-up performance were compared with commercial granular catalyst in a plate microreactor. The results showed that although the mesh-type structured catalyst displayed lower intrinsic activity, it exhibited higher methanol conversion, which was because of the enhanced mass transfer ability. Overall, for the mesh-type structured catalyst, 27.1% higher hydrogen production capacity per unit volume was achieved when methanol conversion was 90%, and the reactor start-up time was reduced by 16.1% owing to the high thermal conductivity of the aluminum substrate. Moreover, the mesh-type structured catalyst also showed excellent stability in 160 h test.     

Kinetic of myristic acid esterification with methanol in the presence of triglycerides over sulfated zirconia

The esterification of myristic acid with methanol in presence of triglycerides using sulfated zirconia prepared by solvent-free method as the heterogeneous catalyst was studied. The effects of reaction temperature (393–443 K), catalyst loading (1–3 %wt) and molar ratio of oil to methanol (1:4–1:20) on the conversion of myristic acid were investigated. The experimental data was interpreted with a second order pseudo-homogeneous kinetic model. The kinetic parameters were obtained. A good agreement between the experimental data and the model was observed. Sulfated zirconia prepared by solvent-free method exhibited high catalytic activity for this reaction. Low activation energy of 22.51 kJ mol^?1 was obtained in the range of temperature of 393–443 K.     

Renewable hydrogen production by steam reforming of butanol over multiwalled carbon nanotube-supported catalysts

The conversion of bio-oxygenates into hydrogen (H₂) via catalytic steam reforming is a green approach for H₂ generation. In the present work, butanol was chosen as renewable feedstock for producing H₂. Two catalysts supported on multiwalled carbon nanotubes, Ni/CNT and Co/CNT, were synthesized by the wetness impregnation method and used for butanol reforming. Trials were performed in a fixed-bed reactor in the 623–773 K range using S/C ratio equal to 33.3 mol/mol (here, S/C denotes steam to carbon ratio). The Ni/CNT catalyst exhibited higher reforming activity. The best catalytic performance for Ni/CNT was observed at T = 773 K. At this temperature, high values of butanol conversion (87.3%) and H₂ yield (0.75 mol/mol) were observed at W/F_A0 = 16.7 g h/mol (here, W is the catalyst mass and F_A0 is the molar flow rate of butanol at the inlet). The performance of Ni/CNT catalyst for steam reforming of synthetic bio-butanol was also investigated at T = 773 K and H₂ yield of 0.65 mol/mol was achieved.     

Experimental investigation on hydrogen production by methanol steam reforming in a novel multichannel micro packed bed reformer

A novel multichannel micro packed bed reactor with bifurcation inlet manifold and rectangular outlet manifold was developed to improve the methanol steam reforming performance in this study. The commercial CuO/ZnO/Al₂O₃ catalyst particles were directly packed in the reactor. The flow distribution uniformity in the reactor was optimized numerically. Experiments were conducted to study the influences of steam to carbon molar ratio (S/C), weight hourly space velocity (WHSV), reactor operating temperature (T) and catalyst particle size on the methanol conversion rate, H₂ production rate, CO concentration in the reformate, and CO₂ selectivity. The results show that increase of the S/C and T, as well as decrease of the WHSV and catalyst particle size, both enhance the methanol conversion. The CO concentration decreases as the S/C and WHSV increase as well as the T and catalyst particle size decrease. Moreover, T plays a more important role on the methanol steam reforming performance than WHSV and S/C. The impacts on CO concentration become insignificant when the S/C is higher than 1.3, WHSV is larger than 1.34 h⁻¹ and T is lower than 275 °C. A long term stability test of this reactor was also performed for 36 h and achieved high methanol conversion rate above 94.04% and low CO concentration less than 1.05% under specific operating conditions.     

Optimization of Liquid Organic Hydrogen Carrier (LOHC) dehydrogenation system

In this paper, the perhydrodibenzyltoluene dehydrogenation flowsheet has been simulated. Modelling of the dehydrogenation reactor has been performed using the 1-D model. External and internal mass transfer resistances are also considered. Non-isothermal pellet condition has been considered for simulating the dehydrogenation reactor. The flowsheet simulation has been carried out in DW-Sim v 6.5.2 integrated with the reactor model coded in Python. NET. The dehydrogenation reactor is operated at a feed temperature between 523 K ?613 K, a wall temperature of 623 K and 653 K, and a reactor pressure maintained at 1.2 atm. The amount of catalyst required for the perhydrodibenzyltoluene (PDBT) dehydrogenation reactor is evaluated such that the conversion reaches 99%. The process flowsheet has been simulated to produce 10 Nm³/hr of industrial-grade hydrogen. The effects of feed temperature, wall temperature, and hydrogen burner efficiency on various system requirements, including catalyst weight, energy supplied to the dehydrogenation reactor, areas of the heat exchanger, and hydrogen production from the reactor, have been discussed. Preliminary cost optimization based on the heat exchangers and catalyst at various feed temperatures, reactor wall temperature, and hydrogen burner efficiency has been carried out.     

Numerical investigation of ethylbenzene dehydrogenation and nitrobenzene hydrogenation in a membrane reactor: Effect of operating conditions

This paper is a numerical study about ethylbenzene (EB) dehydrogenation and nitrobenzene (NB) hydrogenation in a membrane reactor. Both sides of the membrane reactor were filled with an appropriate catalyst. Effect of different parameters in the dehydrogenation side including inlet temperature, pressure, catalyst porosity, and initial ethylbenzene concentration was investigated on temperature distribution and ethylbenzene and nitrobenzene conversion in the membrane reactor. Generally, the results showed that an increase in all parameters except the catalyst porosity can improve ethylbenzene and nitrobenzene conversion. The temperature was firstly decreased in the dehydrogenation side as the reactions were endothermic but there was an increase in temperature after a shorter distance from the entrance because of transferring heat from hydrogenation side into dehydrogenation side. Change in pressure has a considerable effect on the hydrogen transferring from dehydrogenation side into hydrogenation side. The styrene yield was not improved by increasing the initial ethylbenzene concentration from 5 to 20 mol/s but it has a positive influence on the yields of benzene and toluene. It was possible to achieve 0.97 of ethylbenzene conversion at 950 K.     

Methanol steam reforming in a planar wash coated microreactor integrated with a micro-combustor

A numerical simulation of methanol steam reforming in a microreactor integrated with a methanol micro-combustor is presented. Typical Cu/ZnO/Al₂O₃ and Pt catalysts are considered for the steam reforming and combustor channels respectively. The channel widths are considered at 700 μm in the baseline case, and the reactor length is taken at 20 mm. Effects of Cu/ZnO catalyst thickness, gas hourly space velocities of both steam reforming and combustion channels, reactor geometry, separating substrate properties, as well as inlet composition of the steam reforming channel are investigated. Results indicate that increasing catalyst thickness will enhance hydrogen production by about 68% when the catalyst thickness is increased from 10 μm to 100 μm. Gas space velocity of the steam reforming channel shows an optimum value of 3000 h⁻¹ for hydrogen yield, and the optimum value for the space velocity of the combustor channel is calculated at 24,000 h⁻¹. Effects of inlet steam to carbon ratio on hydrogen yield, methanol conversion, and CO generation are also examined. In addition, effects of the separating substrate thickness and material are examined. Higher methanol conversion and hydrogen yield are obtained by choosing a thinner substrate, while no significant change is seen by changing the substrate material from steel to aluminum with considerably different thermal conductivities. The produced hydrogen from an assembly of such microreactor at optimal conditions will be sufficient to operate a low-power, portable fuel cell.     

Fractal channel design in a micro methanol steam reformer

The aim of the present article is to study the fractal channel pattern design and the gradient catalyst layer in relation to their effects on the performance of a micro methanol steam reformer. A three-dimensional simulation model is established for the purpose of predicting the effects of bio-channel design on the performance of a micro-reformer. The CO concentration in the production gases, which is necessary to avoid the poisoned catalyst layers of low temperature fuel cells, is also investigated. In addition, the distributions of velocity and gas concentrations are predicted, and the methanol conversion ratios are also evaluated. Due to further decreases of the CO in product gases, a gradient catalyst layer arrangement is proposed to delay the timing of hydrogen generation and thus avoid the presence of hydrogen in the catalyst layer too long. This catalyst arrangement can effectively decrease the possibility of a reverse water gas shift reaction to reduce CO generation. Results showed that the fractal channel design increases the conversion ratio, decrease CO as well as decrease the pressure drop in the channels. Relative to a parallel channel design, the CO and methanol conversion ratio of this fractal channel design pattern with uniform catalyst layer can be decreased and increased by 17% and 8%, respectively, based on a 0.3 cc/min flow rate, respectively. Meanwhile, the pressure drops in the parallel channel design and in the fractal channel design were found to be 254 Pa and 51 Pa, respectively. From an energy consumption point of view, a low pressure drop also implies low input pumping power. Furthermore, compared to the fractal design with a uniform catalyst layer, the gradient catalyst layer was demonstrated to effectively increase the conversion ratio by 8.5% and decrease CO by 11% when the inlet liquid flow rate was fixed at 1.0 cc/min.     

Enhancement of pure hydrogen production through the use of a membrane reactor

Pure hydrogen production is of great interest as it is an energy carrier which can be used in PEM fuel cells for power production. Methane Steam Reforming (MSR) is commonly used for hydrogen production although the produced hydrogen is not free of other components. Membrane Reactors (MR) enable a pure hydrogen product stream and allows the reaction to take place at significantly lower temperatures (lower than 550 °C) than in conventional reactors (greater than 800 °C) with comparable methane conversion. This is achieved by hydrogen removal through a permselective Pd–Ag based membrane that cause a favorable shift in chemical equilibrium towards hydrogen production. In the present study, a two-dimensional, nonlinear, and pseudo-homogeneous mathematical model of a catalytic fixed-bed membrane reactor for methane steam reforming over a nickel-based foam supported catalyst is presented. Simulated results referring to the distribution of species, methane conversion, temperature and hydrogen flowrate along the reactor for different radial positions are obtained and analyzed. The performance of structured catalyst and catalyst supported on foam configurations under the same operating conditions is also studied. Experimental results for the membrane facilitate the identification of suitable operating conditions.     

Performance of microwave reactor system in decomposition of ammonia using nickel based catalysts with different supports

In this study, the ammonia decomposition reaction to produce COx-free hydrogen is investigated in a microwave reactor system using nickel-based catalysts supported by different materials. Unlike the activated carbon supported catalyst (Ni@AC), the alumina supported catalyst (Ni@Alumina) is mixed with carbon in a 1:1 ratio to reach the necessary reaction temperature in the microwave reactor. Ni@Alumina gives an overall hydrogen production rate of 73 mmol/min.g_cat with 99% conversion at 400 °C under pure ammonia flow (60 ml/min). Ni@Alumina outperforms Ni@AC under microwave reactor conditions, but underperforms Ni@AC under the conventional testing, which is done for comparison. It is suggested that selective heating of nickel species in Ni@Alumina enables better performance in the microwave reactor in comparison to Ni@AC. On the other hand, high surface area and small nickel particles present in the Ni@AC structure in comparison to the Ni@Alumina structure, causes higher activity in the conventional reactor at temperatures over 550 °C. Between 400 and 550 °C, both Ni@Alumina and Ni@AC have substantially lower activity under conventional heating than microwave heating when compared at the same temperatures. Hot spot formation and microwave selective catalytic effect are considered as possible reasons for the improved performance of microwave reactor system.     

Reforming of methanol to produce hydrogen over the Au/ZnO catalyst

Gold particle with an average size of d_Au ~ 4 nm was dispersed on ZnO by the deposition precipitation method. The fabricated Au/ZnO catalyst was used to produce hydrogen from reforming of methanol. Four reforming reactions, i.e., decomposition of methanol (DM), steam reforming of methanol (SRM), partial oxidation of methanol (POM) and oxidative steam reforming of methanol (OSRM), were evaluated in a fixed bed reactor. A reaction temperature of T_R > 623 K was required for catalyzing reactions of DM and SRM. Interestingly, high methanol conversion (C_MeOH > 90%) was found from reforming reactions of POM and OSRM at an amazing low temperature of T_R < 473 K. Besides, a presentable hydrogen yield (R_H2 ~ 2.4) and a low selectivity of CO (S_CO ~ 1%) were simultaneously attained from the reaction of OSRM. Therefore, the low temperature OSRM reaction over the Au/ZnO catalyst is suggested as a friendly reforming process for on-board production of hydrogen.