Catalyst Bed Research Articles

Computational Fluid Dynamics of Ammonia Synthesis in Axial-Radial Bed Reactor

Ammonia synthesis by the Haber–Bosch method is a typical and effective implementation of the chemical process in the large-scale fertiliser industry. Due to the growing demand for fertilisers and food, it is desirable to study this process thoroughly using modern numerical methods to improve the operation of existing devices and facilitate the design of new devices in industrial installations. This manuscript focuses on the influence of the catalyst bed parameters on the ammonia synthesis process. Variants with different sizes of catalyst particles and modifications of the geometry of catalytic beds were considered. The axial-radial Topsoe converter with magnetite as a catalyst, commonly used in modern fertiliser industry beds, was investigated using Computational Fluid Dynamics. As a result, contours of velocity, pressure, concentration, and rate of ammonia formation were obtained. The analysis of the obtained results made it possible to determine the gradient of ammonia production rate in the catalyst bed and designate zones with negligible reaction rates. The authors also proposed possible bed geometry modifications to reduce bed volumes without affecting the converter’s performance.

Topology optimization of catalyst bed structure for a solar steam methane reforming reactor aiming at improved conversion efficiency and hydrogen yield

This study optimized the distribution of catalysts porosity for solar steam methane reforming (SMR) reactor using the topology optimization with different objective functions. The optimized structures were similar, wherein dense catalysts were filled in the high-temperature region at the front of the reactor which increased the contact area between reactants and catalysts, and sparse metal foam catalysts were filled at the rear, which was conducive to reducing the flow resistance and facilitating the collection of products. To facilitate the filling in the actual production process, the optimized structure was simplified, and the simplification structure improved the hydrogen production by 6.84%–22.33% and the methane conversion by 6.54%–11.49% under different working conditions compared with the uniform distribution structure. By adjusting the porosity distribution of the catalysts, the temperature field of optimized reactor conformed to the principle of minimum Gibbs free energy (Gmin) and provided guidance for improving the solar SMR reactor performance.

Unlocking dynamics of compact methanol reformers during on‐line catalyst activation using transient computational fluid dynamics simulation

AbstractOn‐board methanol reforming is a practical solution to supply hydrogen for fuel cell vehicles (FCVs). For commonly employed Cu‐based reforming catalysts, activation has a profound influence on subsequent reaction performance. However, tailoring of this process at the reformer level has received little research attention. Herein, we present the dynamics of compact methanol reformers with Cu/ZnO/Al2O3 catalysts during in situ H2/N2 pre‐activation as a preliminary step of online catalyst activation by computational fluid dynamics simulations. Raising inlet temperatures or hydrogen fractions is demonstrated to accelerate activation while generating a high‐temperature band within the catalyst bed, which hampers effective activation. Increasing the reductant flow rates improves the homogeneity of activation thanks to enhanced convective heat and mass transfer. Notably, we revealed that inlet reductants exceeding 453 K trigger temperature runaway that may severely damage the reformer. These new insights will enlighten optimization of operation and control of on‐board methanol reforming for FCVs.

Green synthesis and catalytic activity assessment of bespoke nano-catalyst for eco-friendly green propellant systems based on hydrogen peroxide

AbstractMuch effort has been devoted to replace pollutant, toxic, cancerogenic hydrazine-based propellants. Hydrogen peroxide could secure promising characteristics as well as high density specific impulse. Effective catalysts with high durability are required to commence H2O2 decompositions, and to inherit hypergolic nature with different hydrocarbon fuels. Silver nanoparticles (18 nm) were synthesized via hydrogen gas evolved by water electrolysis. MnO2 (20 nm) was developed in a sustainable manner via green hydrothermal processing. High crystalline, mono-dispersed particles were developed. Catalytic activity was assessed via precise measurements of liquid temperature profile (LTP) up to the boiling point of hydrogen peroxide. Whereas silver demonstrated LTP peak within 20 s; MnO2 experienced LTP within 40 s. Catalyst survivability was recorded via precise measurement of life time mass loss rate upon catalyst addition to hydrogen peroxide. While silver nano-catalyst demonstrated high performance at the reaction start; silver was found to be poisoned with crystalline phase change to silver oxide within 20 s. On the contrary, manganese oxide experienced high durable catalytic action. Consequently, MnO2 could be candidate for H2O2 monopropellant thrusters as catalyst bed. On the other hand, silver nanoparticles could be candidate for a single use in bipropellant system to inherit hypergolicity. This study presents a pioneering report on facile synthesis of catalyst nanoparticles; comprehensive catalytic activity assessment of silver to manganese oxide for H2O2 decomposition was represented. Proper catalyst assortment for practical applications was represented.

Upgrading of solid recovered fuel (SRF) by dechlorination and catalytic pyrolysis over nanocrystalline ZSM-5 zeolite

Globally increasing concern related to municipal solid waste generation is encouraging research efforts on developing alternative routes to valorize mixed refused wastes. In this way, catalytic pyrolysis is emerging as an interesting and efficient technology due to its great flexibility in terms of feedstock. In the current work, upgrading of a Solid Recovered Fuel (SRF) has been investigated by catalytic pyrolysis over nanocrystalline ZSM-5 zeolite (n-ZSM-5), paying special attention to dechlorination effects due to the high Cl content of the raw waste. Thus, pretreatment of the SRF by water washing and mild thermal processing allows for a significant reduction of the Cl concentration. Regarding the catalytic pyrolysis step, the best conditions correspond with a temperature of 400 °C in the catalyst bed and 0.50 catalyst/SRF mass ratio, which lead to ca. 30 wt% oil yield (rich in aromatic hydrocarbons) together with about 40 wt% gas yield (rich in C3–C4 olefins). Accordingly, these products could find use as raw chemicals or for the production of advanced fuels. In addition, zeolite reutilization has been tested for several cycles, denoting a progressive modification of the products distribution because of coke deposition. However, an almost total recovery of the n-ZSM-5 zeolite catalytic performance is achieved after regeneration by air calcination, affording the production of an oil fraction with a Cl content as low as 40 ppm.

A simulation model for ruthenium-catalyzed dry reforming of methane (DRM) at different temperatures and gas compositions

Dry reforming of methane (DRM) is an environmentally friendly technology for producing syngas while consuming CO2. However, the short lifespan of catalysts due to metal sintering, coke deposition, and sulfur poisoning hinders its widespread adoption. To address such issues, perovskite catalysts, which have shown superior performance and resistance to coke formation and poisoning, were developed. In this study, a comprehensive simulation model of a DRM reactor that employs Sr0·92Y0·08Ti0·95Ru0·05O3-d (SYTRu5) using COMSOL Multiphysics software was developed. The objective of this study is to contribute to the understanding of the intricate dynamics of DRM process via utilizing simulation models. The simulation model considered five main reactions and calculated the properties of the gas mixture considering temperatures and compositions. Using the simulation model, the distribution of gas mole fractions and temperatures were obtained. According to the results, the H2 yield and CO2 conversion rate increased with increasing operating temperature. The temperature of the catalyst bed decreased rapidly owing to the endothermicity of the reaction. Additionally, the CH4 conversion rate increased with increasing proportion of CO2, whereas the reaction converged to an equilibrium state and the temperature difference in the catalyst bed decreased with decreasing gas flow rate.

Low-temperature Synthesis of Carbon Nanofibers/graphite Felt Composites Under Contactless Induction Heating

In this report induction heating was used for the catalytic chemical vapor decomposition synthesis of carbon nanofibers (CNFs) on Ni-based catalyst. The CNFs were produced with a high yield at a relatively low temperature compared to that observed for conventional heating through convection and conduction. The process also occurred in the absence of secondary toxic organic compounds, formed through side reaction in the high temperature gas-phase or through decomposition on the hot wall of reactor as encountered with traditional Joule heating mode. The improved CNFs yield under induction heating was attributed to the high reaction temperature control thanks to the high temperature regulation rate provided by the induction heating coil. The local heating of the nickel nanoparticles by the electromagnetic field could also contribute to the improvement of the CNFs yield. The results obtained indicate that inductive heating mode could be of great interest for improving the heat transfer in catalytic processes and also to reduce the problem of gradient temperature occurring inside the catalyst bed during the operating of highly exothermic or endothermic processes. It is expected that such electricity-driven heating mode could have contributed in an efficient way toward the electrification of different catalytic processes in order to reduce the associated carbon footprint.

Reactor Structure for the Decomposition of ADN-Based Monopropellant

Ammonia dinitramide (ADN)-based liquid monopropellants are considered to be environmentally friendly alternatives to the toxic and carcinogenic hydrazine-based propellants. Hence, Space Solutions Co., Ltd. is developing a 1N ADN-based liquid monopropellant thruster by conducting a combustion performance in different types of reactors. Various parameters, such as preheating temperature and the size of thermal and catalyst beds, were examined. The results showed that the decomposition of the propellant in a Pt-LHA catalyst bed, which was used in the Type-1 reactor, resulted in insufficient combustion at low preheating temperatures. Furthermore, increasing the preheating temperature led to partial reaction of the propellant, but resulted in low combustion efficiency due to disintegration of the catalyst. However, when a thermal bed (STS ball) was used in addition to the catalyst bed (Pt-LHA) in the Type-2 and Type-3 reactors, the combustion efficiency was improved, with a minimal pressure drop of 0.2 bar. It was also confirmed that the catalyst was not damaged even after repeated operations. In conclusion, this study suggests that the propellant needs to vaporize before decomposing on the catalyst bed to achieve optimal combustion efficiency.

Synergistic enhancement of reaction and separation for a solar membrane reactor by topology optimization of catalyst bed

Enhancing separation and reaction in solar membrane reactors is critical for achieving temperature reduction and efficiency improvement. Based on the topology optimization method, this study investigates the optimal catalyst bed porosity distribution in solar methane steam reforming membrane reactor to maximize separation and reaction, while revealing the performance enhancement mechanism. The final optimized distribution exhibits a uniform high porosity distribution in the front and numerous inclined radial paths in the middle and rear, which achieves enhanced radial bidirectional transport. This bidirectional transport enhancement is the intrinsic mechanism for the synergistic effect of reactions and separations in reactor, thus enhancing comprehensive performances. The optimized porosity distributions with various original reaction levels and reactor lengths tend to be similar in topology optimization. They always improve solar membrane reactor performances, especially at lower reaction levels and longer reactors. At the original methane conversion of 0.6, the optimized solar membrane reactor has the maximum improvement with 28.9%, 42.8% and 32.0% in methane conversion, fuel efficiency and hydrogen recovery, respectively, which reaches 92.3% of the ideal performance. Moreover, higher relative improvements in overall performance of 30.9–59.0% are reached at reactor length of 3 m.

Simultaneous Observation of Gas Properties’ Effect on Emulsion Phase Voidage and Bubble Size in Fluidized Catalyst Beds

Simultaneous Observation of Gas Properties’ Effect on Emulsion Phase Voidage and Bubble Size in Fluidized Catalyst Beds

Probing the Pyrolysis Process of Rice Straw over a “Dual-Catalyst Bed” for the Production of Fuel Gases and Value-Added Chemicals

Rice straw is an agricultural byproduct primarily produced in Asian regions. It is crucial to discover an effective method for converting this waste into chemicals that can be utilized to substitute goods derived from fossil fuels. Pyrolysis serves as an interesting procedure to obtain bio-oil from this rice straw. The composition of the bio-oil obtained after the pyrolysis procedure contains a small quantity of value-added chemicals in addition to various gas components in the gas product. Therefore, the development of catalytic systems that improve this pyrolytic reaction is mandatory. Herein, the design of a dual catalyst bed (CEM/ZSM-5) that catalyzes the volatiles that it releases has been developed. The highest output of 42.1 wt.% of bio-oil, 29.9 wt.% of gases and 28.0 wt.% of bio-char was obtained. Nevertheless, the inclusion of single zeolites to biomass yields biofuel outputs of 42.8 wt.%, gas yields of 27.7 wt.%, and a bio-char yielding of 29.5 wt.%. Additionally, the addition of cement to biomass results in a bio-oil yield of 40.4 wt.% and 30.5 wt.% of gas, along with 29.1 wt.% of char. Regarding pyrolysis gas products, the H2 yield in the produced biogas was increased from 35.9 mL/g to 45.7 mL/g, and the CH4 output was increased from 21.1 mL/g to 27.4 mL/g. The bioenergy output was evaluated employing GC-FID and GC-MS (gas and biofuel). The dual catalytic bed had a significant impact on the contents of the generated biofuel, increasing the quantity of hydrocarbons and other value-added compounds.

Warm plasma catalytic coreforming of dilute bioethanol and methane for hydrogen production

AbstractToward hydrogen production from renewable sources, coreforming of dilute bioethanol (5 mol% ethanol) and methane using gliding arc discharge warm plasma is reported. For warm plasma alone, increases in methane to ethanol ratio and specific energy input lead to improving hydrogen yield, but low energy efficiency (<60%). The warm plasma coupled with Ni/CeO2/Al2O3 catalyst in a heat‐insulation reactor achieves an energy efficiency of 80%, but the large axial‐temperature drop of the catalyst bed causes low conversions. Therefore, additional heating is provided to maintain the catalyst bed temperature. Under optimal conditions, total carbon conversion of 97% and hydrogen yield of 78% are achieved with an energy cost of 1.16 kWh/Nm3 and energy efficiency of 85%.

Numerical Studies on Hydrogen Production from Ammonia Thermal Cracking with Catalysts

To explore and optimize the process of hydrogen production from plasma-assisted ammonia-cracking, a tubular ammonia-cracking on-site hydrogen production device with plasma-assisted ammonia combustion flue gas as the heat source was developed. Using the Temkin–Pyzhev kinetic model and the local thermal equilibrium (LTE) hypothesis, the effects of operating conditions, such as combustion flue gas temperature and ammonia flow rates, on ammonia-cracking efficiency were investigated. The numerical results are quantitatively consistent with the experiment. Ammonia cracking efficiency is notably influenced by the initial combustion gas temperature. When the gas velocity of the cracking system is less than or equal to 0.03 m/s, the cracking rate increases by 63% when the inlet temperature of the heat pipe changes from 700 K to 800 K. The cracking rate of ammonia decreased with the increase of ammonia flow rate, and this trend reached the maximum and began to weaken when the flow rate was 0.3 m/s. Longer catalyst bed length does not always mean higher cracking efficiency; the length of the cracking tube over 0.6 m shows little effect on cracking efficiency. Response surface methodology was used to conduct multi-factor analysis of the three main factors affecting the cracking rate of the cracker, namely, the temperature of the heating tube, the flow rate of flue gas in the heating process, and the inlet flow rate of the catalytic bed. It was found that the flow rate of the catalytic bed was the most significant factor affecting the cracking rate, which could be used as the main control method. The numerical results would provide technical guidance for industrial applications of on-site hydrogen production devices from ammonia decomposition.

Performance analysis of methanol steam micro-reformers for enhanced hydrogen production using CFD

Present study shows the performance analysis of methanol steam reforming considering different miniature-scale reformer geometries. Three different types of 3D models of steam reformers have been considered: straight annular, spiral, and serpentine-type reactors. Each reformer is packed with a copper-based catalyst to enhance the reaction between steam and methanol. Methanol steam reforming is an endothermic reaction with a two-step mechanism, whose reaction kinetic models were mathematically implemented. The study shows different hydrodynamic aspects of the flow fields through the catalyst bed, the conversion of methanol, and production of hydrogen for each type of reactors. Simulations were carried out at different temperatures and inlet velocities to study the methanol conversion and identify the most optimized reformer design. It was found that in the spiral and serpentine-type reformer, the methanol conversion was more due to the bending in the tubular reactor, which disturbs the flow field with an enhancement of mixing, causing more conversion. Spiral reformer exhibited conversion up to 91.16% at 650 K and producing 0.15 mass fraction of hydrogen, which was found to be 3% more than the conversion achieved in serpentine type and almost 8% more than the straight annular type geometry when the inlet velocity was kept at 0.1 m/s for all the cases.

CFD simulation of an industrial steam methane reformer: Effect of burner fuel distribution on hydrogen production

Simulation of the steam methane reformer serves as a launching pad to investigate the optimal amount of hydrogen production. In this study, a 3D-CFD simulation is implemented to model transport phenomena for a turbulent fluid flow with chemical reactions within the reformer. Accordingly, the realizable k-ɛ turbulence model and discrete ordinates (DO) radiation model have been employed to simulate the kinetic mechanism of the SMR process and hydrogen/methane combustion. In addition, the REV-scale approach is considered to model the flow in the porous catalyst bed. Numerical results show that 3D-CFD simulation can accurately model such complex flows in comparison to 2D simulation. In the following, 3D numerical simulations have been performed to investigate the effect of fuel distribution between burners on the amount of heat provided, which affects the amount of hydrogen production and the tube wall temperature. Results indicate that the burner feed flow without purge gas and vent gas streams has the highest efficiency in hydrogen production and leads to a ca. 10% increase in hydrogen production compared to industrial case. Although feeding the burner with purge gas and without vent gas increases hydrogen production by about 7% compared to industrial data, it may be a more suitable option due to lower natural gas consumption and economic savings.

Application of TiO2 nanotubes as bulky supports of highly active CO2 methanation catalysts prepared via the polygonal barrel-sputtering method

Herein, TiO2 nanotubes (TiO2-NTs) were used as bulky supports for the preparation of highly active CO2 methanation catalysts to maintain the stability of the reaction at high gas flow rates. The nanotube shapes of the prepared TiO2-NTs remained unchanged at a calcination temperature of 300 °C. The CO2 methanation reaction on Ru-sputtered catalysts indicated that at high gas flow rates of 10% CO2/Ar and H2, some crack-like gaps were formed in the catalyst bed using raw anatase TiO2 particle supports, whereas no gaps appeared in the case where TiO2-NTs were used as the support, leading to a stable reaction. This result is attributed to the formation of a sparse catalyst bed owing to the bulky TiO2-NT supports.

Impact of Hydrogen Peroxide Concentration on Manganese Oxide and Platinum Catalyst Bed Performance

This study investigates the use of MnxOy/Al2O3 and Pt/Al2O3 catalysts for the decomposition of hydrogen peroxide in thrusters. It describes the purpose, procedures, performance, and conclusions coming from the test campaign of the catalyst lifetimes. In particular, eight different propellant samples with two different catalysts were tested twice (in order to exclude uncertainty). Similar operating and starting conditions were applied. All hot tests were performed in a thruster-like catalyst bed configuration with a propellant injector and outlet nozzle. Each bed was filled with the same mass of catalyst (for the same type of catalyst). The results show that platinum is a more effective catalyst than manganese oxides for the decomposition of hydrogen peroxide. The findings have important implications for the development of catalysts for “green” propellants.

Highly effective and energy-saving adsorption-microwave catalytic decomposition of NO with novel high-adsorption-capacity adsorbent in a two-stage fixed-bed reactor

It is a great challenge to achieve high efficiency and low energy consumption denitrification without adding reducing agent at low temperature. Herein, we developed MgCo2O4-BaCO3/AC carbon-based composites as novel and cheap adsorbents with high adsorption capacity to fill the first section of two-stage fixed-bed reactor to form adsorption bed, and MgCo2O4-40%BaCO3 as catalyst was filled in the second section as catalyst bed. Significantly, under the condition of excess O2, the adsorption capacity of 20%MC-0.5Ba/AC for NO was as high as 40.2 mg/g, and the conversion of adsorbed NO was as high as 99.7% under microwave irradiation at 250 °C. The denitrification efficiency of the two-stage fixed bed reactor was much higher than that of the one-stage fixed bed reactor. The energy efficiency of adsorption-decomposition method was 14 times higher than that of direct decomposition method. Interestingly, the activated carbon was not consumed during the whole reaction process, so no secondary pollution such as CO and CO2 was produced. More importantly, despite the presence of SO2, CO2 or H2O in the inlet gas, 20%MC-0.5Ba/AC still showed excellent adsorption capacity for NO, and the absorbed NO could be successfully decomposed, demonstrating excellent resistance to SO2, CO2 and H2O. This work presents a new route for denitrification technology with high efficiency, low energy consumption and no secondary pollution, and provides guidance for rational design of adsorbents with high adsorption capacity for NO.

Evaluation of the relevant mass and heat transfer phenomena in a packed bed membrane reactor for the direct conversion of CO2 to dimethyl ether

This study investigates the relevant heat and mass transfer phenomena occurring at the different scales in a packed bed (membrane) reactor for the direct conversion of CO2 to dimethyl ether (DME) via the implementation of 2D heterogeneous reactor models. Intra-particle diffusion limitations were found to be relevant for particle diameters larger than 1 mm and temperature above 220 ⁰C, such that the catalyst efficiency drops down to 50% and 5% for the Cu/ZnO/Al2O3 and the HZSM-5, respectively, in the most critical conditions (i.e., 270 ⁰C and Dp of 10 mm). A component-specific Thiele modulus-efficiency correlation was developed based on the results of the rigorous particle model to account for pore-diffusion limitations without having to solve a complex heterogeneous reactor model. This correlation shows the typical behavior reported in literature for power law kinetics and accurately predicts the reaction performance with deviation of less than 5% for values of the Thiele modulus lower than 2. In the packed bed membrane reactor (PBMR), the concentration polarization (CP) also showed to affect the reactor performance. The concentration of water at the surface of the membrane selective layer was found to be up to 64% lower than the concentration in the bulk phase, hindering the effectiveness of the membrane separation. To account for this phenomenon via a simplified approach, a Sherwood-type correlation was developed to determine a CP mass transfer coefficient, based on the results obtained via the rigorous 2D PBMR model. Such correlation showed to predict with high accuracy (i.e., errors lower than 5%) the effect of the CP on the PBMR performance. Differently from the pore diffusion and CP phenomena, the intra-particle heat transfer, the particle–fluid mass and heat transfer as well as the axial dispersion were found to have a negligible effect on the reactor behavior. Finally, given the relevant mass/heat transfer phenomena, this study proposes further reactor optimization strategies, such as the reduction of the zeolite loading in the bifunctional catalyst bed by ca. 90% with respect to what is reported in literature.

Sunlight Powered Continuous Flow Reverse Water Gas Shift Process Using a Plasmonic Au/TiO2 Nanocatalyst.

The continuous flow reverse water gas shift (rWGS) process was efficiently catalyzed by a plasmonic Au/TiO2 nanocatalyst using sunlight as sole and sustainable energy source. The influence of the catalyst bed thickness on the CO production rate was studied, and three different catalytic regimes were identified as direct plasmon catalysis (DPC), shielded plasmon catalysis (SPC) and unused plasmon catalysis (UPC). The CO2:H2 ratio was optimized to 4:1 and a maximum CO production rate of 7420 mmol·m-2·h-1 was achieved under mild reaction conditions (p = 3.5 bar, no external heating, Ee = 14.0 kW·m-2), corresponding to an aparent quantum efficiency of 4.15%. The stability of the Au/TiO2 catalyst was studied for 110 h continuous operation, maintaining more than 82% of the initial CO production rate. On/off experiments mimicking discontinuous sunlight powered processing furthermore showed that the Au/TiO2 catalyst was stable for 8 consecutive runs.