Fixed‐Bed Multi‐Tubular Reactors for Oxidative Dehydrogenation in Ethylene Process

An industrial‐scale reactor for ethylene production was modeled using the oxidative dehydrogenation of ethane (ODHE) in a multi‐tubular reactor system, examining a variety of parameters affecting reactor performance. The model showed that a double‐bed multi‐tubular reactor with intermediate air injection scheme was superior to a single‐bed design, due to the increased ethylene selectivity while operating under lower oxygen partial pressures. The optimized reactor length for 100 % oxygen conversion was theoretically determined for both reactor designs. The use of a distributed oxygen feed with a limited number of injection points indicated a significant improvement on the reactor performance in terms of ethane conversion and ethylene selectivity. This concept also overcame the reactor runaway temperature problem and enabled operations over a wider range of conditions to obtain enhanced ethylene production.     

Optimal oxygen concentration strategy through an isothermal oxidative coupling of methane plug flow reactor to obtain a high yield of C2 hydrocarbons

An optimal oxygen concentration trajectory in an isothermal OCM plug flow reactor for maximizing C₂ production was determined by the algorithm of piecewise linear continuous optimal control by iterative dynamic programming (PLCOCIDP). The best performance of the reactor was obtained at 1,085 K with a yield of 53.9%; while, at its maximum value, it only reached 12.7% in case of having no control on the oxygen concentration along the reactor. Also, the effects of different parameters such as reactor temperature, contact time, and dilution ratio (N₂/CH₄) on the yield of C₂ hydrocarbons and corresponding optimal profile of oxygen concentration were studied. The results showed an improvement of C₂ production at higher contact times or lower dilution ratios. Furthermore, in the process of oxidative coupling of methane, controlling oxygen concentration along the reactor was more important than controlling the reactor temperature. In addition, oxygen feeding strategy had almost no effect on the optimum temperature of the reactor. Finally, using the optimal oxygen strategy along the reactor has more effect on ethylene selectivity compared to ethane.     

Numerical simulation of particle/monolithic two-stage catalyst bed reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane

A three-dimensional geometry model of the particle/monolithic two-stage reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane (OCM) was set up. The improved Stansch kinetic model adapting different operating temperatures was established to calculate the OCM reactor performance using computational fluid dynamics (CFD) and FLUENT software. The results showed that the calculated values matched well with the experimental values of the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO₂, CO) in the OCM reactor. The distributed dioxygen feeding with the percentage of 5–20% based oxygen flow rate of top inlet promoted the OCM reaction in monolithic catalyst bed and led to the conversion of CH₄ and the selectivity and yield of C₂ (C₂H₆ and C₂H₄) increase obviously. The distributed dioxygen feeding was 15%, the conversion of CH₄, the selectivity and the yield of C₂ reached 34.1%, 68.2% and 23.3%, respectively.     

Conversion of syngas to hydrocarbons in a tube-wall reactor using CoFe plasma-sprayed catalyst: experimental and modeling studies

Catalyst activity and product selectivity studies of the conversion of synthesis gas to various hydrocarbon fractions were performed in a single-tube tube-wall reactor (TWR) using a CoFe plasma-sprayed catalyst with the operating conditions: temperature 250–275°C, pressure 0.1–1.03 MPa, exposure velocity 139–722 μms⁻¹, and a H₂:CO ratio of 2.0. The catalyst activity in terms of CO conversion was highest (98.5% m/m) at an exposure velocity of 139 μms⁻¹, temperature of 275°C, and in the pressure range 0.69–1.03 MPa. The selectivity to hydrocarbons was 43–50% (m/m) in the pressure range 0.69–1.03 MPa whereas the selectivity to C₅ + hydrocarbons was over 40% of the total hydrocarbons produced. The production of propylene was higher than ethylene under similar process conditions. The performance of the TWR was predicted by a numerical model. The model is based on the complete two-dimensional transport equations and reaction rate equations, developed for the CoFe catalyst. Predictions are made for the temperature along the axis of the reactor, for CO and H₂ conversions as functions of the reactor length and the exposure velocity, and the axial H₂O and CO₂ concentrations.     

Theoretical analysis of oxidative coupling of methane and Fischer Tropsch synthesis in two consecutive reactors: Comparison of fixed bed and membrane reactor

In this paper, theoretical performance of Fischer Tropsch (FT) synthesis is analyzed where its feed comes from an oxidative coupling of methane (OCM) reactor. In this model based analysis, two consecutive reactors are intended that first reactor is OCM and second reactor is FT and FT reactor performance is compared in two conditions of fixed bed and membrane reactor (MR). The parameters concerned, were CH₄/O₂ ratio, contact time, temperature, and amount of N₂ in OCM feed. High CH₄/O₂ ratio gave low yield of C₂₊ in OCM due to insufficient oxygen, but favored FT reaction due to more yield of C₅₊ and other products. Therefore, it was concluded that production and yield of C₅₊ could be more by use of these configurations.     

Modelling and optimization of catalytic-dielectric barrier discharge plasma reactor for methane and carbon dioxide conversion using hybrid artificial neural network—genetic algorithm technique

A hybrid artificial neural network-genetic algorithm (ANN-GA) was developed to model, simulate and optimize the catalytic-dielectric barrier discharge plasma reactor. Effects of CH₄/CO₂ feed ratio, total feed flow rate, discharge voltage and reactor wall temperature on the performance of the reactor was investigated by the ANN-based model simulation. Pareto optimal solutions and the corresponding optimal operating parameter range based on multi-objectives can be suggested for two cases, i.e., simultaneous maximization of CH₄ conversion and C₂₊ selectivity (Case 1), and H₂ selectivity and H₂/CO ratio (Case 2). It can be concluded that the hybrid catalytic-dielectric barrier discharge plasma reactor is potential for co-generation of synthesis gas and higher hydrocarbons from methane and carbon dioxide and performed better than the conventional fixed-bed reactor with respect to CH₄ conversion, C₂₊ yield and H₂ selectivity.     

Optimization of ODHE membrane reactor based on mixed ionic electronic conductor using soft computing techniques

This work presents the optimization of the operating conditions of a membrane reactor for the oxidative dehydrogenation of ethane. The catalytic membrane reactor is based on a mixed ionic–electronic conducting material, i.e. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_δ−3, which presents high oxygen flux above 750 °C under sufficient chemical potential gradient. Specifically, diluted ethane is fed into the reactor chamber and air (or diluted air) is flushed to the other side of the membrane. A framework based on Soft Computing techniques has been used to maximize the ethylene yield by simultaneously varying five operation variables: nominal reactor temperature (Temp); gas flow in the reaction compartment (QHC); gas flow in the oxygen-rich compartment (QAir); ethane concentration in the reaction compartment (%C₂H₆); and oxygen concentration in oxygen-rich compartment (%O₂). The optimization tool combines a genetic algorithm guided by a neural network model. This shows how the neural network model for this particular problem is obtained and the analysis of its behavior along the optimization process. The optimization process is analyzed in terms of: (1) catalytic figures of merit, i.e., evolution of yield and selectivity towards different products and (2) framework behavior and variable significance. The two experimental areas maximizing the ethylene yield are explored and analyzed. The highest yield reached in the optimization process exceeded 87%.     

Pure H2 production through hollow fiber hydrogen‐selective MFI zeolite membranes using steam as sweep gas

Hollow fiber MFI zeolite membranes were modified by catalytic cracking deposition of methyldiethoxysilane to enhance their H₂/CO₂ separation performance and further used in high temperature water gas shift membrane reactor. Steam was used as the sweep gas in the MR for the production of pure H₂. Extensive investigations were conducted on MR performance by variations of temperature, feed pressure, sweep steam flow rate, and steam‐to‐CO ratio. CO conversion was obviously enhanced in the MR as compared with conventional packed‐bed reactor (PBR) due to the coupled effects of H₂ removal as well as counter‐diffusion of sweep steam. Significant increment in CO conversion for MR vs. PBR was obtained at relatively low temperature and steam‐to‐CO ratio. A high H₂ permeate purity of 98.2% could be achieved in the MR swept by steam. Moreover, the MR exhibited an excellent long‐term operating stability for 100 h in despite of the membrane quality. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3459–3469, 2015     

Modeling the oxidative coupling of methane using artificial neural network and optimizing of its operational conditions using genetic algorithm

The effect of some operating conditions such as temperature, gas hourly space velocity (GHSV), CH₄/O₂ ratio and diluents gas (mol% N₂) on ethylene production by oxidative coupling of methane (OCM) in a fixed bed reactor at atmospheric pressure was studied over Mn/Na₂WO₄/SiO₂ catalyst. Based on the properties of neural networks, an artificial neural network was used for model developing from experimental data. To prevent network complexity and effective data input to the network, principal component analysis method was used and the number of output parameters was reduced from 4 to 2. A feed-forward back-propagation network was used for simulating the relations between process operating conditions and those aspects of catalytic performance including conversion of methane, C₂ products selectivity, C2 yielding and C₂H₄/C₂H₆ ratio. Levenberg-Marquardt method is presented to train the network. For the first output, an optimum network with 4-9-1 topology and for the second output, an optimum network with 4-6-1 topology was prepared. After simulating the process as well as using ANNs, the operating conditions were optimized and a genetic algorithm based on maximum yield of C₂ was used. The average error in comparing the experimental and simulated values for methane conversion, C₂ products selectivity, yield of C₂ and C₂H₄/C₂H₆ ratio, was estimated as 2.73%, 10.66%, 5.48% and 10.28%, respectively.     

Simulation of CO2 hydrogenation with CH3OH removal in a zeolite membrane reactor

A thermodynamic analysis of the CO₂ hydrogenation to methanol where competitive reactions take place is presented for a membrane reactor (MR) where methanol was selectively removed. A non-isothermal mathematical model was written to simulate a micro-porous MR. Zeolite membranes with different values of the CH₃OH and H₂O permeances were considered in the MR modelling. The effect of temperature, pressure and species permeation on the conversion, selectivity and yield was analysed. A higher CO₂ conversion and CH₃OH selectivity can be reached by the use of an MR. An increased CH₃OH yield allows to reduce the consumption of reactant and also to operate at lower pressures and higher temperatures, a fact, which favours the kinetics reducing the residence time and the reactor volume. The MR with the highest CH₃OH/H₂O permeance ratio resulted in better selectivity and yield of CH₃OH with respect to the other MR characterised by a higher conversion.     

Slurry-phase CO2 hydrogenation to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst: Investigation of reaction conditions

The hydrogenation of CO₂ to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst was studied in a slurry reactor for the first time. Reducibility of the catalyst and effect of reaction variables (temperature, pressure and H₂/CO₂ ratio of the feed gas) on the catalytic reaction performance were investigated. The reaction results indicated that the Fe-Cu-Al/K catalyst showed a good CO₂ hydrogenation performance at a relatively low temperature (533 K). With the increase of reaction temperature CO₂ conversion and olefin to paraffin (O/P) ratio in C₂-C₄ hydrocarbons as well as the selectivity to C₂-C₄ fraction increased, while CO and CH₄ selectivity showed a reverse trend. With the increase in reaction pressure, CO₂ conversion and the selectivity to hydrocarbons increased, while the CO selectivity and O/P ratio of C₂-C₄ hydrocarbons decreased. The investigation of H2/CO₂ ratio revealed that CO₂ conversion and CH₄ selectivity increased while CO selectivity and O/P ratio of C₂-C₄ decreased with increasing H₂/CO₂ ratio.     

Ethane‐selective carbon composites CPDA@A‐ACs with high uptake and its enhanced ethane/ethylene adsorption selectivity

Novel composites (CPDA@A‐ACs) of carbonized polydopamine (CPDA) and asphalt‐based activated carbons (A‐ACs) were successfully synthesized, and characterized for adsorption separation of ethane/ethylene. The resulting CPDA@A‐ACs exhibited high Brunauer–Emmett–Teller surface area of 1971 m²/g. The O and N contents on CPDA@A‐ACs are higher than those on A‐ACs due to the introduction of CPDA. Interestingly, CPDA@A‐ACs exhibited great preferential adsorption of ethane over ethylene. Its ethane capacity reached as high as 7.12 mmol/g at 100 kPa and 25°C, and its ethane/ethylene adsorption selectivity became higher compared to A‐ACs, reaching as high as 3.0～20.6 below 100 kPa, significantly superior to the reported ethane‐selective adsorbents. Simulation results revealed the mechanism of enhanced selectivity toward C₂H₆/C₂H₄, and suggested that the surface oxygen functionalities of the composites play predominant role in enhancing ethane/ethylene adsorption selectivity. Fixed‐bed experiments showed that C₂H₆/C₂H₄ mixtures can be well separated at room temperature, suggesting great potential for industrial C₂H₆/C₂H₄ separation. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3390–3399, 2018     

Conceptual feasibility studies of a CO<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript>-free hydrogen production from ammonia decomposition in a membrane reactor for PEM fuel cells

CO_X-free hydrogen production from ammonia decomposition in a membrane reactor (MR) for PEM fuel cells was studied using a commercial chemical process simulator, Aspen HYSYS®. With process simulation models validated by previously reported kinetics and experimental data, the effect of key operating parameters such as H₂ permeance, He sweep gas flow, and operating temperature was investigated to compare the performance of an MR and a conventional packed-bed reactor (PBR). Higher ammonia conversions and H₂ yields were obtained in an MR than ones in a PBR. It was also found that He sweep gas flow was favorable for X_NH3 enhancement in an MR with a critical value (5 kmol h^-1), above which no further effect was observed. A higher H₂ permeance led to an increased H₂ yield and H₂ yield enhancement in an MR with the reverse effect of operating temperature on the enhancement. In addition, lower operating temperature resulted in higher X_NH3 enhancement and H₂ yield enhancement as well as NG cost savings in a MR compared to a conventional PBR.     

Enhanced Production of C2–C4 Olefins Directly from Synthesis Gas

An attempt made for the selective production of C₂–C₄ olefins directly from the synthesis gas (CO + H₂) has led to the development of a dual catalyst system having a Fischer–Tropsch (K/Fe–Cu/AlOx) catalyst and cracking (H-ZSM-5) catalyst operate in consecutive dual reactors. The flow rate (space velocity) and H₂/CO molar ratio of the feed have been optimized for achieving higher CO conversions and olefin selectivities. The selectivity to C₂–C₄ olefins is further enhanced by optimizing the reaction temperature in the second reactor (cracking), where the product exhibited 51% selectivity to C₂–C₄ hydrocarbons rich in olefins (77%) with a stable time-on-stream performance in a studied period of 100 h.     

Selective hydrogenation of acetylene through a short contact time reactor

A novel approach to the selective hydrogenation of acetylene involves a short contact time reactor. A thin Pd/γ-Al₂O₃ catalytic membrane (about 5 μm thick) was observed to be void of minor defects. Forcing a dilute C₂H₂/H₂/Ar mixture through the layer produced high conversions coupled with high selectivity at high temperatures. The selectivity was observed to increase with temperature. A gas-dispersion model with derived kinetics was employed to explain the results. A solution to the corresponding coupled nonlinear differential equations resulted in a Peclet number of 64.5 and a contact time of 9.5 × 10⁻³ s through the reactor at 100°C. A critical thickness is predicted to exist for a maximum in ethylene concentration. A value of 2.5 μm at 200°C is representative. Lastly, permeability experiments agree with modeling results to show that Knudsen diffusion is absent despite a narrow pore-size distribution.     

Upgrading of Simulated Syngas by Using a Nanoporous Silica Membrane Reactor

The permeance properties of a nanoporous silica membrane were first evaluated in a laboratory‐scale porous silica membrane reactor (MR). The results indicated that CO, CO₂, and N₂ inhibited H₂ permeation. Increased H₂ permeability and selectivity were obtained when gas was transferred from the lumen side to the shell side. This was therefore selected as a suitable permeation direction. On this basis, upgrading of simulated syngas was experimentally investigated as a function of temperature (150 – 300 °C), feed pressure (up to 0.4 MPa), and gas hourly space velocity (GHSV), by using a nanoporous silica MR in the presence of a Cu/ZnO/Al₂O₃ catalyst. The CO conversion obtained with the MR was significantly higher than that with a packed‐bed reactor (PBR) and broke the thermodynamic equilibrium of a PBR at 275 – 300 °C and a GHSV of 2665 h^–1. The use of a low GHSV and high feed pressure improved the CO conversion and led to the recovery of more H₂.     

Selective catalytic transformations of methanol to C2-C3 hydrocarbons over the new zeolite type LZ 132

The small pore zeolite HLZ 132 exhibits, in comparison with other zeolites, an increased selectivity for the transformation of methanol to ethylene in the reaction temperature range 350–500 °C: the weight ratio of C₂H₄ to C₃H₆ in the products ranges between 1 and 4 at WHSV=2 h^–1. Besides the effect of the reactant shape selectivity this fact may be interpreted by the participation of the asymmetrical methoxy groups in the surface as well as by proton-donor centres of lower acidity which do not catalyze the oligomerization of ethylene but which do the more basic molecule of propylene, thereby generating polyene-type coke.     

Catalytic partial oxidation of ethane to ethylene and syngas over Rh and Pt coated monoliths: Spatial profiles of temperature and composition

The catalytic partial oxidation of C₂H₆ over Pt and Rh coated monolithic supports (4.7 wt% M/α-Al₂O₃ 45 PPI) was investigated with a capillary sampling technique for a range of C₂H₆/air ratios at constant inlet flow (～8 ms contact time), with and without H₂ addition. Effluent data clearly indicate the differences in product distribution between catalysts and equilibrium. Rh effectively converts the reactant mixtures to syngas with ～80% selectivity, whereas Pt produces C₂H₄ with ～55% C-atom selectivity, while neither produces thermodynamically favored C. Spatially resolved measurements provide direct evidence of the multi-zone nature of the reactors. With Rh, complete conversion of O₂ occurs to produce mostly CO, H₂ and H₂O within the first 3 mm of catalyst, followed by a reforming zone to produce additional syngas. Pt consumes O₂ more slowly, which results in a steady increase in temperature along the reactor. Ethylene formation correlates to reactor temperatures >750 °C, regardless of C/O, in line with the onset of homogeneous reactions. Hydrogen addition tests (C₂H₆/O₂/H₂=2/1/2) clearly exhibit preferential oxidation of H₂ with O₂ over Pt, which shifts the maximum in temperature upstream while preserving a portion of the C₂H₆ for C₂H₄ production. H₂ addition modifies the concentration and temperature profiles minimally on Rh. The main differences between catalysts are the high reforming and O₂ consumption activity with Rh compared to Pt, which are likely responsible for differences in C₂H₄ yields.     

Comparative study of the two-zone fluidized-bed reactor and the fluidized-bed reactor for oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst

In this work, oxidative coupling of methane over Mn/Na₂WO₄/SiO₂ catalyst is studied in a two-zone fluidized-bed reactor (TZFBR) and its performance is compared with a fluidized-bed reactor (FBR). Diluted oxygen in argon was introduced into the bottom of the TZFBR through a quartz ferrite and methane was entered at higher positions along the fluidized bed. The catalyst circulated between the oxygen-rich and methane-rich zones in the TZFBR reactor. The effects of the main operating variables including bed temperature, the methane/oxygen ratio (R_mo), and the height at which methane was introduced into the reactor (H_m) were investigated. It is found that under some operating conditions the TZFBR gives a higher C₂ selectivity than that obtained in the FBR reactor. Reaction of methane with lattice oxygen of the Mn/Na₂WO₄/SiO₂ redox catalyst in the methane-rich zone may have led to the higher selectivity.