Dynamic modeling and simulations of the behavior of a fixed‐bed reactor‐exchanger used for CO2 methanation

A multidimensional heterogeneous and dynamic model of a fixed‐bed heat exchanger reactor used for CO₂ methanation has been developed in this work that is based on mass, energy and momentum balances in the gas phase and mass and energy balances for the catalyst phase. The dynamic behavior of this reactor is simulated for transient variations in inlet gas temperature, cooling temperature, gas inlet flow rate, and outlet pressure. Simulation results showed that wrong‐way behaviors can occur for any abrupt temperature changes. Conversely, temperature ramp changes enable to attenuate and even fade the wrong‐way behavior. Traveling hot spots appear only when the change of an operating condition shifts the reactor from an ignited steady state to a non‐ignited one. Inlet gas flow rate variations reveal overshoots and undershoots of the reactor maximum temperature. © 2017 American Institute of Chemical Engineers AIChE J, 64: 468–480, 2018     

Performance evaluation of fixed‐bed,millistructured, and metallic foam reactor channels for CO2 methanation

Power‐to‐gas technologies, combining hydrogen produced by water electrolysis with carbon dioxide to produce substitute natural gas (SNG), can support the increased penetration of renewable electricity sources. However, the technical and economic feasibility of these technologies requires the conversion efficiency of the whole process, including the methanation step. This paper provides an experimental performance comparison of three catalytic methanation reactor concepts, a fixed‐bed reactor, a millistructured reactor, and a metallic foam reactor with the same nickel‐alumina catalyst. The response of each reactor was analyzed in light of five performance criteria, representing the methane yield, the reactor compactness, and the maximum temperature elevation. The millistructured reactor channel showed a higher methane space‐time yield and volumetric productivity than the other reactors, but a significant catalyst temperature elevation. The metallic foam reactor showed a much lower space‐time yield and volumetric productivity, but very good thermal management.

     

Transient Effects during Dynamic Operation of a Wall‐Cooled Fixed‐Bed Reactor for CO2 Methanation

The power‐to‐gas process is an option to transform fluctuating renewable electric energy into methane via water electrolysis and subsequent conversion of H₂ by methanation with CO₂. The dynamic behavior of the methanation reactor may then be a critical aspect. The kinetics of CO₂ methanation on a Ni‐catalyst were determined under isothermal and stationary conditions. Transient isothermal kinetic experiments showed a fast response of the rate on step changes of the concentrations of H₂, CO₂; in case of H₂O, the response was delayed. Non‐isothermal experiments were conducted in a wall‐cooled fixed‐bed reactor. Temperature profiles were measured and the effect of a changing volumetric flow was studied. The experimental data were compared with simulations by a transient reactor model.     

Multidimensional modeling of a microfibrous entrapped cobalt catalyst Fischer‐Tropsch reactor bed

Thermal management of highly exothermic Fischer‐Tropsch synthesis (FTS) has been a challenging bottleneck limiting the radial dimension of the packed‐bed (PB) reactor tube to 1.5 in. ID. A computational demonstration of a novel microfibrous entrapped cobalt catalyst (MFECC) in mitigating hot spot formation has been evaluated. Specifically, a two‐dimensional (2‐D) model was developed in COMSOL^®, validated with experimental data and subsequently employed to demonstrate scale‐up of the FTS bed from 0.59 to 4 in. ID. Significant hot spot of 102.39 K in PB was reduced to 9.4 K in MFECC bed under gas phase at 528.15 K and 2 MPa. Improvement in heat transfer within the MFECC bed facilitates higher productivities at low space velocities (≥1000 h^?1) corresponding to high CO conversion (≥90%). Additionally, the MFECC reactor provides an eightfold increase in the reactor ID at hot spots ≤ 30 K with CO% conversions ≥ 90%. This model was developed for a typical FTS cobalt‐based catalyst where CO₂ production is negligible. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1723–1731, 2018     

Oxidative Dehydrogenation of a C4 Raffinate‐2 towards 1,3‐Butadiene in a Two‐Zone Fluidized Bed

The oxidative dehydrogenation of a C4 raffinate‐2 consisting of n‐butane, 1‐butene, and 2‐butene was conducted in a two‐zone fluidized bed reactor using a Mo‐V‐MgO catalyst. This study reports the influence of the operating conditions temperature, hydrocarbon inlet height, and oxygen/hydrocarbon molar ratio on the product distribution, in particular on the formation of 1,3‐butadiene. Axial concentration profiles were measured to elucidate the reaction sequence in the fluidized bed.     

Continuous‐Flow Di‐N‐Alkylation of 1H‐Benzimidazole in a Fixed‐Bed Reactor

A new process for a continuous‐flow di‐N‐alkylation of 1H‐benzimidazole to 1H‐benzimidazole‐3‐ium iodide by methylene iodide in the presence of potassium carbonate in a fixed‐bed reactor is presented. The synthesis was transferred from batch to continuous operation with similar yields and conversion rates. Moreover, the influence of temperature and residence time in the continuous flow setup was characterized; optimized conditions led to a doubling of yield. In addition, the continuous flow allowed for a better control of the two‐step reaction by adding an additional tube reactor after the fixed bed that further enhanced the overall performance. With this, the continuous‐flow system presented itself as superior due to higher available temperatures and a better controllability.     

In‐situ‐Reaktionsverfolgung mittels ATR‐IR‐Spektroskopie bei der CO2‐Hydrierung zu Ameisensäure

In the hydrogenation of carbon dioxide to formic acid, the analysis is a challenge due to the unfavorable position of equilibrium and the complex reaction medium. Low yields, despite the use of a Ru/γ‐Al₂O₃ catalyst, the formation of formic acid‐amine adducts, and subsequent reactions with the solvent complicate the analytics. The coupling of the reactor to the ATR‐IR spectroscopy allows precise in situ monitoring of the reaction. Another advantage is the easy accessibility of the initial kinetics. That way, a deviation from the Arrhenius behavior was found.     

Advantages of Forced Non‐steady Operated Trickle‐Bed Reactors

Trickle‐bed reactors are usually operated in the steady state trickle flow regime. Uneven liquid distribution and the formation of hot spots are the most serious problems experienced during trickle flow operation. In this paper, we advocate the use of non‐steady state operation of trickle‐bed reactors. Based on a square‐wave cycled liquid feed, several operation modes are developed that involve the artificial induction of natural pulses and control of the catalyst wetting efficiency over longer times. The operation modes aim at increasing the mass transfer rate of the limiting reactant and simultaneous prevention of flow maldistribution and hot spot formation. The operation modes are distinguished by a relatively fast and slow cycling of the liquid feed. The potential advantages of the developed feed strategies on reactor performance are evaluated.     

Simulation and Analysis of a Tubular Fixed‐Bed Fischer‐Tropsch Synthesis Reactor with Co‐Based Catalyst

A two‐dimensional pseudohomogeneous reactor model is proposed to simulate the performance of fixed‐bed Fischer‐Tropsch synthesis (FTS) reactors by lumped thought. A CO consumption kinetics equation and a carbon chain growth probability model were incorporated into the reactor model. The model equations discretized by a two‐dimensional orthogonal collocation method were solved by the Broyden method. Concentration and temperature profiles were obtained. The validity of the reactor model against the pilot plant test data was investigated. Satisfactory agreements between model prediction values and experiment results were obtained. Further simulations were carried out to investigate the effect of operating conditions on the reaction behavior of the fixed‐bed FTS reactor.     

Mathematical modeling of a moving bed reactor for post‐combustion CO2 capture

A mathematical model for a moving bed reactor with embedded heat exchanger has been developed for application to solid sorbent‐based capture of carbon dioxide from flue gas emitted by coal‐fired power plants. The reactor model is one‐dimensional, non‐isothermal, and pressure‐driven. The two‐phase (gas and solids) model includes rigorous kinetics and heat and mass transfer between the two phases. Flow characteristics of the gas and solids in the moving bed are obtained by analogy with correlations for fixed and fluidized bed systems. From the steady‐state perspective, this work presents the impact of key design variables that can be used for optimization. From the dynamic perspective, the article shows transient profiles of key outputs that should be taken into account while designing an effective control system. In addition, the article also presents performance of a model predictive controller for the moving bed regenerator under process constraints. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3899–3914, 2016     

Syngas chemical looping process: Dynamic modeling of a moving‐bed reducer

The syngas chemical looping process coproduces hydrogen and electricity with iron oxide based oxygen carriers in a circulating moving bed system. In this article, a one‐dimensional (1‐D) dynamic model is developed to simulate the countercurrent gas–solid reactive flow in the moving‐bed reducer. This model is validated by TGA and bench‐scale experiments. Both the steady state and dynamic composition profiles are obtained to help understand the reaction and reactor behaviors. Numerical simulation on the effects of reactor length is conducted to optimize the moving‐bed reducer design. It is also found that minor variations in the feed rate ratio near a critical point that is represented by the reaction equilibrium could yield a significant difference in the time required for the reactions to reach a steady‐state operation. Such a difference has an important practical implication in that the moving‐bed reducer should be designed and operated to circumvent the critical point. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3432–3443, 2013     

Oxidative dehydrogenation of butenes over Bi‐Mo and Mo‐V based catalysts in a two‐zone fluidized bed reactor

The oxidative dehydrogenation of a 1‐butene/trans‐butene (1:1) mixture to 1,3‐butadiene was carried out in a two‐zone fluidized bed reactor using a Mo‐V‐MgO and a γ‐Bi₂MoO₆ catalyst. The significant operating conditions temperature, oxygen/butene molar ratio, butene inlet height, and flow velocity were varied to gain high 1,3‐butadiene selectivity and yield. Furthermore, axial concentration profiles were measured inside the fluidized bed to gain insight into the reaction network in the two zones. For optimized conditions and with a suitable catalyst, the two‐zone fluidized bed reactor makes catalyst regeneration and catalytic reaction possible in a single vessel. In the lower part of the fluidized bed, the oxidation of coke deposits on the catalyst as well as the filling of oxygen vacancies in the lattice can occur. The oxidative dehydrogenation reaction takes place in the upper zone. Thorough particle mixing inside fluidized beds causes permanent particle exchange between both zones. © 2016 American Institute of Chemical Engineers AIChE J, 63: 43–50, 2017     

Entwicklung des Katalysatorbetts in einem Flachbettreaktor zur CO2-Methanisierung

This work presents a development step for a novel flatbed reactor for CO₂ methanation as well as a CFD-based method that can be used for the development and optimization of reactors. The development step comprises the construction of the catalyst bed in the flat bed reactor into which a pure stoichiometric reactive gas is fed. The results show that a high CO₂ conversion (92.9 %) and a small rise in temperature in the catalyst bed (12.4 °C) can be achieved with an arrangement of the catalyst bed. The heat flow reaches 1924 W m⁻² in the hot spot zone.     

Analysis of solar‐driven gasification of biochar trickling through an interconnected porous structure

The efficient transfer of high‐temperature solar heat to the reaction site is crucial for the yield and selectivity of the solar‐driven gasification of biomass. The performance of a gas‐solid trickle‐bed reactor constructed from a high thermal conductivity porous ceramic packing has been investigated. Beech char particles were used as the model feedstock. A two‐dimensional finite‐volume model coupling chemical reaction with conduction, convection, and radiation of heat within the packing was developed and tested against measured temperatures and gasification rates. The sensitivity of the gasification rate and reactor temperatures to variations of the packing's pore diameter, porosity, thermal conductivity, and particle loading was numerically studied. A numerical comparison with a moving bed projected a more uniform temperature distribution and higher gasification rates due to the increased heat transfer via combined radiation and conduction through the trickle bed. © 2014 American Institute of Chemical Engineers AIChE J, 61: 867–879, 2015     

Influence of ash agglomerating fluidized bed reactor scale‐up on coal gasification characteristics

To study the influence of fluidized‐bed reactor scale‐up on coal gasification characteristics, a model of the ash agglomerating fluidized‐bed reactor has been developed using an equivalent reactor network method. With the reactor network model, the scale‐up effects of a gasifier were studied in terms of the characteristics of the chemical reactions in the jet zone, the annulus dense‐phase zone and the freeboard zone. Results showed that the changes occurred in the inequality proportion of the volume of the jet zone during the reactor scale‐up. Taking into consideration the utilization of a portion of the backflow gas, the expansion of the jet zone volume and the coal particle residence time, the temperature of the jet zone was increased from 1592 to 1662 K. Also, both the annulus dense‐phase zone temperature and the freeboard zone temperature decreased, causing subsequent decrease in the carbon conversion efficiency. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1821–1829, 2014     

Entwicklung eines optisch zugänglichen Reaktors zur Thermographiemessung in einer Katalysatorschüttung

To measure the microkinetics in heterogeneously catalyzed strongly exothermic gas phase reaction heat and mass transfer limitations should be excluded. Also an unequal distribution of temperature in the catalyst fixed bed, like a hot spot, distorts the results of microkinetic investigations. A promising approach to measure the temperature contactless is thermography. For this work, an optically accessible reactor is developed to measure the temperature in a catalytically powder fixed bed. In the scope of this work, this technique is used to investigate the temperature development and distribution during the exothermal CO₂ methanation depending on the bed and gas dilution and the reactor performance.     

Aspergillus oryzae Lipase-Catalyzed Synthesis of Dioleoyl; Palmitoyl-Rich Triacylglycerols in Two Reactors

Dioleoyl; palmitoyl‐rich triacylglycerols (OPO‐rich TAG) were synthesized through Aspergillus oryzae lipase (AOL)‐catalyzed acidolysis of palm stearin with commercial oleic acid by a one‐step process in a stirred tank reactor and continuous packed bed reactor to evaluate the feasibility of using immobilized AOL. AOL was found to be valuable for the synthesis of OPO‐rich TAG when compared with commercial lipase from Thermomyces lanuginose (Lipozyme^® TL IM; Novozymes A/S, Bagsvaerd, Denmark). The C52 (triglycerides with a carbon number of 52, stands for OPO, OPL, LPL and their isomers) content of AOL was higher (45.65 %), and the intensity of treatment (IOT: lipase amount × reaction time/TAG amount) of AOL was just 6.25 % of that of Lipozyme^® TL IM under similar reaction conditions in the stirred tank reactor. Response surface methodology were used to optimize the reaction conditions of the AOL‐catalyzed acidolysis is reaction in the packed bed reactor. The optimal point for the set of experimental conditions generated were as follows: residence time 3.0 h; temperature 62.09 °C; substrate molar ratio 7.13 mol/mol. The highest C52 content obtained was 48.60 ± 2.36 %, with 57.46 ± 1.72 % total palmitic acid at the sn‐2 position and 74.21 ± 2.45 % oleic acid at the sn‐1,3 positions. The half‐life of AOL was 24 h in the stirred tank reactor and 140 h in the packed bed reactor. The immobilized AOL achieved similar conversion and selectivity to commercial lipases for the catalyzed synthesis of OPO‐rich TAG and may offer a cheaper alternative.     

Application of chitosan‐entrapped β‐galactosidase in a packed‐bed reactor system

The model enzyme β‐galactosidase was entrapped in chitosan gel beads and tested for hydrolytic activity and its potential for application in a packed‐bed reactor. The chitosan beads had an enzyme entrapment efficiency of 59% and retained 56% of the enzyme activity of the free enzyme. The Michaelis constant (K_m) was 0.0086 and 0.011 μmol/mL for the free and immobilized enzymes, respectively. The maximum velocity of the reaction (V_max) was 285.7 and 55.25 μmol mL^?1 min^?1 for the free and immobilized enzymes, respectively. In pH stability tests, the immobilized enzyme exhibited a greater range of pH stability and shifted to include a more acidic pH optimum, compared to that of the free enzyme. A 2.54 × 16.51‐cm tubular reactor was constructed to hold 300 mL of chitosan‐immobilized enzyme. A full‐factorial test design was implemented to test the effect of substrate flow (20 and 100 mL/min), concentration (0.0015 and 0.003M), and repeated use of the test bed on efficiency of the system. Parameters were analyzed using repeated‐measures analysis of variance. Flow (p < 0.05) and concentration (p < 0.05) significantly affected substrate conversion, as did the interaction progressing from Run 1 to Run 2 on a bed (p < 0.05). Reactor stability tests indicated that the packed‐bed reactor continued to convert substrate for more than 12 h with a minimal reduction in conversion efficiency. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1294–1299, 2004     

Synthesis of the Biofuel Additive 1,1‐Diethoxybutane in a Fixed‐Bed Column with Amberlyst‐15 Wet

The synthesis of 1,1‐diethoxybutane (DEB) through the acetalization reaction between ethanol and butyraldehyde was studied in a fixed‐bed adsorptive reactor packed with Amberlyst‐15 wet. The miscibilities of reactants and water were evaluated and breakthrough experiments with nonreactive pairs of ethanol‐water and ethanol‐DEB were performed. The parameters of the isotherms were fitted by a Langmuir competitive model. Synthesis of the acetal was carried out with mixtures of ethanol and butyraldehyde at different molar ratios. The dynamic behavior of the fixed‐bed adsorptive reactor was described by a mathematical model developed taking into account the reaction kinetics, adsorption mechanisms, mass transfer resistances, and velocity variations.     

Fluidized‐bed reactor modeling for polyethylene production

Gas‐phase technology for polyethylene production has been widely used by industries around the world. A good model for the reactor fluid dynamics is essential to properly set the operating conditions of the fluidized‐bed reactor. The fluidized‐bed model developed in this work is based on a steady‐state model, incorporating interactions between separate bubble, emulsion gas phase, and emulsion solid polymer particles. The model is capable not only of computing temperature and concentration gradients for bubble and emulsion phases, calculating polymer particle mean diameter throughout the bed and polyethylene production rate, but also of pinpointing the appearance of hot spots and polymer meltdown. The model differs from conventional well‐mixed fluidized‐bed models by assuming that the particles segregate within the bed according to size and weight differences. The model was validated using literature and patent data, presenting good representation of the behavior of the fluidized‐bed reactor used in ethylene polymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 321–332, 2001