Theoretical study of the application of porous membrane reactor to oxidative dehydrogenation of n-butane

This paper is the theoretical study of the oxidative dehydrogenation of n-butane in porous membrane reactors. Performance of the membrane reactors was compared with that of conventional fixed-bed reactors. The porous membrane was employed to add oxygen to the reaction side in a controlled manner so that the reaction could take place evenly.Mathematical models for the fixed-bed reactor and the membrane reactor were developed considering non-isothermal condition and both radial heat and mass dispersion. From this study, it was found that the hot spot problem was pronounced particularly near the entrance of the conventional fixed-bed reactor. In addition, the assumption of plug flow condition did not adequately represent the reaction system. The effect of radial dispersion must be taken into account in the modelling.The use of the porous membrane to control the distribution of oxygen feed to the reaction side could significantly reduce the hot spot temperature. The results also showed that there were optimum feed ratios of air/n-butane for both the fixed-bed reactors and the membrane reactors. The membrane reactor outperformed the fixed-bed reactor at high values of the ratio. In addition, there was an optimum membrane reactor size. When the reactor size was smaller than the optimum value, the increased reactor size increased the reaction and heat generation and, consequently, the conversion and the selectivity to C₄ increased. However, when the reactor size was larger than the optimum value, oxygen could not reach the reactant near the stainless steel wall. It was consumed to react with the product, C₄. As a result, the yield dropped. Finally, it was found that the increase of wall temperature increased the yield and that the feed air temperature could help control the temperature profile of the reaction bed along the reactor length.     

Scale-up analysis of autothermal operation of methane oxidative coupling with La2O3/CaO catalyst

The exothermicity of oxidative coupling of methane (OCM) renders a cooled packed-bed reactor impractical or impossible. Recently, we proposed an adiabatic autothermal reactor as a solution to this problem and reported the first results for stable autothermal operation (AO) with feed at ambient temperature. AO on the ignited branch is possible only in the region of steady-state multiplicity. High per-pass conversion and productivity requirements demand a stable ignited branch at the lowest possible feed temperature and high flow rate. To achieve OCM scale-up, many conditions must be satisfied simultaneously. Using a kinetic model for La₂O₃/CaO catalyst, we examine the impact of space time, feed methane to oxygen ratio, feed temperature, particle size, inter-phase heat and mass transfer gradients, pore-diffusion, bed scale heat/mass dispersion on the region of AO for large scale adiabatic packed-bed reactors. We show that while it is possible to achieve CH₄ conversion of about 20% and C₂ selectivity of about 80% in scaled-up reactors, these values are sensitive to the design and operating parameters.     

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     

Enhancement of Energy Yield for Ozone Production via Packed-Bed Reactors

The present work aims to enhance the energy yield of ozone production via packed-bed reactors. It has been experimentally demonstrated that ozone concentration and corresponding energy yield achieved by packed-bed reactors are significantly higher than that achieved by DBD only. The so-called packed-bed reactor is constructed by packing granular dielectric pellets within a DBD reactor. Two kinds of dielectric materials including glass beads and Al₂O₃ pellets are tested. Experimental results indicate that an ozone generator packed with Al₂O₃ pellets results in a higher ozone production compared with one packed with glass beads. The maximum ozone production takes place when Al₂O₃ pellets with diameter of 2 mm are packed. The maximum ozone concentration, ozone production rate, and energy yield achieved in this study are 61 gO₃/m³, 3.7 gO₃/hr, and 173 gO₃/kWh, respectively. The highest ozone concentration and energy yield achieved with the packed-bed reactor are about 8 and 12 times high as those with DBD reactor, respectively. Although the packed-bed reactors have a shortcoming of high temperature, it can be solved by adding a cooling system and the ozone generation can be improved thereof. As a result, the packed-bed reactor is a promising and state-of-the-art technology for ozone generation based on this study.     

Effect of external mass transfer in a packed bed reactor system for a reversible enzyme reaction

A mathematical model was developed to describe the effect of external mass transfer for a packed-bed enzyme reactor in which a reversible, one-substrate, two-intermediate enzyme reaction took place. The model equation was applied to the analysis of an immobilized glucose isomerase reactor system. A Colburn-type mass transfer correlation was obtained from the Colburn j-factor versus Reynolds number plot: i.e., j_D = 0.045N_Re^−0.48. The values of mass transfer coefficient for the system under study ranged from 0.01 to 0.1 cm h⁻¹ depending on the substrate flow rate. Very good agreements were observed between the computer simulation using a plug flow reactor model with the derived mass transfer correlation and the experimental results obtained from the packed-bed reactor operation.     

Methanol synthesis from CO2 and H2 in multi-tubular fixed-bed reactor and multi-tubular reactor filled with monoliths

This work investigates the impact of catalyst structuring into particles or monoliths on methanol production from only CO₂ and H₂ at a large scale. Methanol synthesis in multi-tubular reactors is evaluated using packed-bed and monolithic reactors by modeling heat and mass transfer in each reactor. The obtained simulation results show that, at low gas hourly space velocity (GHSV = 10,000 h⁻¹), the performances of both reactor technologies are similar. In this case, the packed-bed reactor technology is the most appropriate technology due to its simplicity of installation and operation. At high GHSV (25,000 h⁻¹), the packed-bed reactor technology is limited by a considerable pressure drop that causes an important loss in productivity due to thermodynamic equilibrium, whereas the monolithic reactors exhibit negligible pressure drop and achieve far better performances.     

Effect of Reaction and Permeation Rates on the Performance of a Catalytic Membrane Reactor for Methylcyclohexane Dehydrogenation

Abstract

The effect of the relative rates of reaction and H₂ permeation through palladium-silver (Pd-Ag) membranes upon the performance of a catalytic membrane reactor (CMR) for methylcyclohexane dehydrogenation has been investigated. Mathematical models have been used to identify the conditions at which a membrane reactor gives yields of toluene (TOL) and H₂ significantly in excess of equilibrium values at throughputs of industrial interest. The simulation shows that a catalyst with no product TOL inhibition performs exceptionally well in a CMR, giving conversions considerably above the equilibrium values at favorable operating conditions. Using a membrane unit between two conventional packed-bed reactors to separate the H₂ ex-situ gives significant improvement in performance over the shell-and-tube type CMR, resulting in conversions substantially higher than equilibrium at 633 K, 1.5 MPa, and liquid hourly space velocities of 3–10 volume feed/h/catalyst volumes.

     

A computational method for determination of the mass-transfer coefficient in packed-bed immobilized enzyme reactors

A computational method was developed that determined the mass-transfer coefficient k_L or the volumetric mass-transfer coefficient k_La in packed-bed immobilized enzyme (IME) reactors. To study the performance of this method, two experimental systems were considered where an enzyme was immobilized on a non-porous support surface (surface-IME system) or within a porous support (pore-IME system). The values of k_L and k_La determined in these packed-bed IME reactor systems were successfully expressed in terms of the substrate concentration at the reactor inlet and the liquid flow rate. Furthermore, the correlations obtained for k_L and k_La were used to calculate the unconverted fractions of substrate at the reactor outlet. Comparison showed that the calculated results were in satisfactory agreement with the experimental values.     

Temperature profiles of the monolith catalyst in CO<Subscript>2</Subscript> reforming of methane with<Emphasis Type="Italic">in-situ</Emphasis> combustion of methane and ethane

The temperature profiles in a monolith reactor were measured in CO₂ reforming of CH₄ within-situ combustion of methane and ethane in order to find out in what sequence the reactions are occurring. The reaction gas flowed both upward and downward. A hot spot was observed at low furnace temperatures, and it tended to disappear as the furnace temperature increased. This is due to natural extinguishment of the flame caused by the endothermic reforming reactions occurring. The hot spot disappeared at a lower temperature with the up-flow when compared with the down-flow. When the hot spot appears, H₂O and CO₂ are produced by complete oxidation and accordingly the steam reforming and the CO₂ reforming occur competitively in the rear part of the monolith. If the hot spot does not appear, it is considered that the partial oxidation of methane occurs predominantly over the complete oxidation, resulting in more efficient CO₂ removal.     

Mixed oxygen ionic-carbonate ionic conductor membrane reactor for coupling CO2 capture with in situ methanation

CO₂ methanation is one of the vital reactions to utilize CO₂ and realize power to gas process. To decrease the CO₂ capture cost and alleviate the hot spots during the strong exothermic methanation reaction, here, we report a coupling of CO₂ capture process with in situ CO₂ methanation process through a ceramic-molten carbonate (MC) dual phase membrane reactor over the Ni-based catalyst. The performance of the membrane reactor was systematically investigated and compared with the traditional fixed-bed reactor. The results show that the performance of the membrane reactor is higher than that of the fixed-bed reactor, since the produced steam through the methanation process can be partially removed through the dual-phase membrane, which promotes the reaction shift to right side. A stability test shows no obvious degradation within 32 h. These results indicate that the membrane reactor is promising for coupling CO₂ capture with in situ methanation process.     

A novel slurry bubble column membrane reactor concept for Fischer–Tropsch synthesis in GTL technology

Fischer–Tropsch synthesis (FTS) plays an important role in the production of ultra-clean transportation fuels, chemicals, and other hydrocarbon products. In this work, a novel combination of fixed-bed and slurry bubble column membrane reactor for Fischer–Tropsch synthesis has been proposed. In the first catalyst bed, the synthesis gas is partially converted to hydrocarbons in a water-cooled reactor which is fixed bed. In the second bed which is a membrane assisted slurry bubble column reactor, the heat of reaction is used to preheat the feed synthesis gas to the first reactor. Due to the decrease of H₂/CO to values far from optimum reactants ratio, the membrane concept is suggested to control hydrogen addition. A one-dimensional packed-bed model has been used for modeling of fixed-bed reactor. Also a one-dimensional model with plug flow pattern for gas phase and an axial dispersion pattern for liquid-solid suspension have been developed for modeling of slurry bubble column reactor. Proficiency of a membrane FTS reactor (MR) and a conventional FTS reactor (CR) at identical process conditions has been used as a basis for comparison in terms of temperature, gasoline yield, H₂ and CO conversion as well as selectivity. Results show a favorable temperature profile along the proposed concept, an enhancement in the gasoline yield and, thus a main decrease in undesirable product formation. The results suggest that utilizing this type of reactor could be feasible and beneficial. Experimental proof of concept is needed to establish the validity and safe operation of the proposed reactor.     

Contribution to emission reduction of CO2 by a fluidized-bed membrane dual-type reactor in methanol synthesis process

In this work, a fluidized-bed membrane dual-type reactor was evaluated for CO₂ removal in methanol synthesis process. The feed synthesis gas is preheated in the tubes of the gas-cooled reactor and flowing in a counter-current mode with reacting gas mixture in the shell side. Due to the hydrogen partial pressure driving force, hydrogen can penetrate from feed synthesis gas into the reaction side through the membrane. The outlet synthesis gas from this reactor is fed to tubes of the water-cooled packed-bed reactor and the chemical reaction is initiated by the catalyst. The methanol-containing gas leaving this reactor is directed into the shell of the gas-cooled reactor and the reactions are completed in this fluidized-bed side. A two-phase dynamic model in bubbling regime of fluidization was developed in the presence of long-term catalyst deactivation. This model is used to compare the removal of CO₂ in a FBMDMR with a conventional dual-type methanol synthesis reactor (CDMR) and a membrane dual-type methanol synthesis reactor (MDMR). The simulation results show a considerable enhancement in the CO₂ conversion due to have a favourable profile of temperature and activity along the fluidized-bed membrane dual-type reactor relative to membrane and conventional dual-type reactor systems.     

The use of OCM reactors for ignition enhancement of natural gas combustion for practical applications: Reactor design aspects

The major goal of the study is the improvement of the ignition characteristics of lean, premixed natural gas (NG) combustion under engine-like conditions. A new process is investigated involving the oxidative coupling of methane reaction (OCM) for the in situ production of C₂ hydrocarbons to be used as ignition enhancers during lean combustion of methane in internal combustion (IC) engines. Addition of the OCM product mixture enhances the ignition characteristics of lean methane/air mixtures, the beneficial effect resulting from the C₂H₄/C₂H₆ components. OCM in both conventional plug flow and packed-bed membrane reactors is modelled and optimised with respect to reactor conditions and ignition characteristics. An additional ignition technique that includes flowing a jet of a fuel/air stream against a hot inert surface has been implemented for quantifying the effect of the proposed ignition enhancers.     

Dynamics of CO2 adsorption on sodium oxide promoted alumina in a packed-bed reactor

CO₂ adsorption in packed-bed reactors has potential applications in flue gas CO₂ capture and adsorption enhanced reaction processes. This work focuses on CO₂ adsorption dynamics on sodium oxide promoted alumina in a packed-bed reactor. A comprehensive model is developed to describe the coupled transport phenomena and is solved using orthogonal collocation on finite elements. The model predicted breakthrough curve matches very well with experimental data obtained from a pilot-scale packed-bed reactor. Several dimensionless parameters are also derived to explain the shape of the breakthrough curve.     

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.     

Investigation of CO2 Reaction with CaO and an Acid Washed Lime in a Packed-Bed Reactor

In this work, a mathematical model was developed for the prediction of packed-bed reactor behavior for CaO+CO₂ reaction based on the random pore model. A natural limestone and a modified sorbent using acetic acid washing were used for the experiments. The performances of these sorbents were initially determined using a thermogravimeter analyzer. Then, the reaction was accomplished in a packed-bed reactor for obtaining CO₂ breakthrough curves and investigation of model predictions. This model was able to successfully predict the effect of process conditions and solid texture on the breakthrough curves of the packed-bed reactor.     

Selective Oxidation of Methanol to Green Oxygenates – Feasibility Study of Fixed-Bed and Membrane Reactors

A feasibility study of a conventional fixed-bed reactor and a packed-bed membrane reactor (PBMR) in distributor configuration is carried out for the selective oxidation of methanol to the oxygenates dimethoxymethane and methyl formate on a VO_x/TiO₂ catalyst. Kinetic experiments provided the reaction network and a reliable kinetic model of six main reactions involved. The PBMR offers significant advantages and potential for the formation of both target products due to an effective kinetic coupling of the oxidative dehydrogenation of methanol and the consecutive reaction steps to dimethoxymethane and methyl formate in a broad range of operation conditions.     

Methanation of carbon dioxide by hydrogen reduction using the Sabatier process in microchannel reactors

This paper describes the development of a microchannel-based Sabatier reactor for applications such as propellant production on Mars or space habitat air revitalization. Microchannel designs offer advantages for a compact reactor with excellent thermal control. This paper discusses the development of a Ru-TiO₂-based catalyst using powdered form and its application and testing in a microchannel reactor. The resultant catalyst and microchannel reactor demonstrates good conversion, selectivity, and longevity in a compact device. A chemically reacting flow model is used to assist experimental interpretation and to suggest microchannel design approaches. A kinetic rate expression for the global Sabatier reaction is developed and validated using computational models to interpret packed-bed experiments with catalysts in powder form. The resulting global reaction is then incorporated into a reactive plug-flow model that represents a microchannel reactor.     

Effect of kinetic models on hot spot temperature prediction for phthalic anhydride production in a multitubular packed bed reactor

Although there are several kinetic models for the production of phthalic anhydride from the partial oxidation of o‐xylene, only few studies have compared the effect of the kinetic model on the prediction of the hot‐spot temperature. In this work, the predicted temperature profile for the partial oxidation of o‐xylene to phthalic anhydride in a multitubular packed bed reactor was obtained for different kinetic mechanisms using one‐dimensional pseudo‐homogeneous and heterogeneous models. The predicted temperature profile using the one‐dimensional heterogeneous model with the kinetic model of Calderbank et al. but with the adjusted kinetic and transport parameters proposed by Anastasov presented a good correlation with regard to experimental data. Nevertheless, in the hot‐spot zone deviations, up to 30 K were presented. In conclusion, the temperature performance in the production of phthalic anhydride is suitably predicted by the one‐dimensional heterogeneous model and the Calderbank et al.'s kinetic model. Though, prediction using bidimensional models should be done to establish the best correlation with experimental data.