A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.     

Performance simulation of a wall-type reactor in which exothermic and endothermic reactions proceed simultaneously, comparing with that of a fixed-bed reactor

By combining endothermic and exothermic reactions in one reactor, a mutual utilization of thermal energy involved in reactions is expected to produce a saving energy and a cost-down for running in industrial reaction process. In this case, a wall-type reaction system is thought to be suitable because such reaction system is good at exchangeability of thermal energy by conductive heat transfer. This study supposed a wall-type reaction system consisting of endothermic and exothermic reaction channels stacked up and a fixed-bed reaction system of the same configuration, and compared them by numerical simulation in the case where endothermic and exothermic reactions progress simultaneously.

In the fixed-bed reaction system, heat transfer in the catalyst bed takes place by convection, and this transfer becomes the rate-limiting process. Accordingly, occurrence of hot spot in the exothermic channel and shortage of thermal energy in the endothermic channel were predicted. This trend became distinct by making the feed gas directions flowing in the two channels countercurrent and by stacking the channels in multiple tiers. In the wall-type reaction system, however, the temperature distributions in the exothermic and endothermic channels almost conformed to the set temperatures, and the temperature difference between channels was small. Even if the feed gases flowed in countercurrent and even if the channels were stacked several deep, this trend did not change. In the wall-type reaction system, the exchange of thermal energy would take place efficiently by conductive heat transfer between the endothermic and exothermic channels. Furthermore, it was inferred that the wall-type reaction system would provide a stable operation in mutual utilization of thermal energy.     

Effect of catalytic combustion of hydrogen on the dehydrogenation processes in a membrane reactor. I. Mathematical model of the process

Mathematical modeling of a catalytic membrane reactor was performed for thermodynamically coupled processes using as an example the endothermic dehydrogenation of propane and the exothermic combustion (oxidation) of hydrogen. Benefits of using the membrane reactor to increase the yield of target products by shifting equilibrium was demonstrated theoretically. The effect of hydrogen combustion on the main characteristics of the endothermic dehydrogenation process was studied. The hydrogen combustion reaction makes it possible to further increase the conversion of propane and compensate for the energy consumption in the endothermic dehydrogenation process.     

Coupling exothermic and endothermic reactions in adiabatic reactors

The steady state and the dynamic behavior of coupling exothermic and endothermic reactions in directly coupled adiabatic packed bed reactors (DCAR) are analyzed using one-dimensional pseudo-homogeneous plug flow model. Two different configurations of DCAR (simultaneous DCAR—SIMDCAR and sequential DCAR—SEQDCAR) are investigated. In SIMDCAR, the catalyst bed favors both exothermic and endothermic reactions and both reactions occur simultaneously. SEQDCAR has alternating layers of catalyst beds for exothermic and endothermic reactions and hence the exothermic and endothermic reactions occur in a sequential fashion. The performance of both reactors, in terms of conversion achieved and manifested hot spot behavior, is compared with that of the co-current heat exchanger type reactor. Various possible operational regimes in SIMDCAR have been classified and the conditions for the existence of hot spots or cold spots in SIMDCAR are obtained analytically for the first order reactions with equal activation energies. The reactor behavior for the reactions with non-equal activation energies is also presented. The preliminary criteria for the selection of suitable reactor type and the general bounds on the reaction parameters to obtain the desired conversion for endothermic reaction with minimal temperature rise are proposed. The dynamic behavior of these reactors is important for control applications and we have reported some of the transient behavior.     

Modelling propane dehydrogenation in a rotating monolith reactor

The catalytic dehydrogenation of propane is equilibrium limited, strongly endothermic and normally carried out at high temperatures. The catalyst deactivates due to the laydown of carbonaceous species on the surface. This is conventionally countered by subjecting the catalyst to periodic regeneration. In commercially available processes, the catalyst time on line for a given cycle is in the order of 10–10,000 min.

In this study, the catalyst has been observed to exhibit very high activity and selectivity in the short period after regeneration. Conceptual and model development of a reactor with structured catalyst to capitalise on this beneficial early activity is presented.

The preferred reactor comprises a cylindrical block of honeycomb monolith that rotates past various feed zones, subjecting the catalyst successively to propane and regenerating gas. The exothermic nature of the regeneration reactions is used at least in part to provide heat to the endothermic dehydrogenation reaction via the regenerative heat transfer facilitated by the movement of the solid monolith. Specifically, it is noted that an oxidisible catalyst provides operating advantage due to the additional exotherms associated with the regeneration stage.

The process modelling shows the design to be feasible in terms of matching the heats of reactions and achieving high conversions, but questions are raised over its practicability from mechanical design and process stability viewpoints.     

Efficient reactor concepts for coupling of endothermic and exothermic reactions

Multifunctional autothermal reactors are a novel concept in process integration and intensification. They can be implemented as a countercurrent or reverse-flow reactor. A promising field of application is the coupling of endothermic and exothermic reactions. Methane steam reforming coupled with methane combustion is considered as a particular example. Several novel reactor configurations with co- and countercurrent flow in the reaction zone will be discussed by numerical simulation and an example for experimental verification will be presented.     

A comparative study of two different configurations for exothermic–endothermic heat exchanger reactor

The coupling of the energy intensive endothermic reaction systems with appropriate exothermic reactions reduces the size of the reactors and can improve the thermal efficiency of processes. One type of a suitable reactor for such a kind of coupling is the heat exchanger reactor. In this study, the catalytic methanol synthesis is coupled with the catalytic dehydrogenation of cyclohexane to benzene in an integrated reactor formed from two fixed beds separated by a wall where heat is transferred across the surface of the tube. A steady-state heterogeneous model of the two fixed beds predicts the performance of the two different configurations of the thermally coupled reactor. The co-current mode is investigated and the simulation results are compared with the corresponding predictions for the industrial methanol fixed bed reactor operating in the same feed conditions. The results of the study reveal that should the exothermic and endothermic reactions be located in the shell side and tube side, respectively, the methanol production rate will increase in comparison with the conventional methanol synthesis reactor as well as the case where the exothermic reaction is located in the tube side and endothermic reaction in the shell side.     

Recuperative coupling of exothermic and endothermic reactions

Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improves the thermal efficiency of processes and reduces the size of the reactors. One type of reactor suitable for such a type of coupling is the heat exchanger reactor. In this work, a one-dimensional pseudo-homogeneous plug flow model is used to analyze and compare the performance of co-current and counter-current heat exchanger reactors. A parametric analysis is carried out to address the vital issues, such as the exit conversion of the endothermic reaction, the temperature peak (hot spot) of the exothermic reaction and the reactor volumetric productivity. The measures to reduce the hot spot by different catalyst profiling techniques are also addressed. Some features of the dynamic behavior exhibited by these reactors, which are important from design, operational and control point of view, are presented.     

Coupling of highly exothermic and endothermic reactions in a metallic monolith catalyst reactor: A preliminary experimental study

An LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl metallic monolith catalyst for the exothermic catalytic combustion of methane and an Ni/SBA-15/Al₂O₃/FeCrAl metallic monolith catalyst for the endothermic reforming of methane with CO₂ have been prepared. A laboratory-scale tubular jacket reactor with the Ni/SBA-15/Al₂O₃/FeCrAl catalyst packed into its outer jacket and the LaFe_0.5Mg_0.5O₃/Al₂O₃/FeCrAl catalyst packed into its inner tube was devised and constructed. The reactor allows a coupling of the exothermic and endothermic reactions by virtue of their thermal matching. An experimental study in which the temperature difference between the chamber of the external electric furnace and the metallic monolith catalyst bed in the jacket was kept very small, by adjusting the power supply to the furnace, confirmed that the heat absorbed in the reforming reaction does indeed partly come from that evolved in the catalytic combustion of methane, and that the direct thermal coupling of the two reactions in the reactor can be realized in practice. When the temperature of the electric furnace chamber was 1088 K, and the gas hourly space velocities (GHSVs) of the reactant mixtures passed through the inner tube and the jacket were 382 h⁻¹ and 40 h⁻¹, respectively, the conversions of methane and CO₂ in the reforming reaction were 93.6% and 91.7%, respectively, and the heat efficiency reached 81.9%. Stability tests showed that neither catalyst underwent deactivation during 150 h on stream.     

Optimization of the active component distribution through the catalyst bed for the case of adiabatic reactor

This article is the next one in the previous series of publications concerning the optimal active component distribution. The aim of the present work is to extend the optimization problem taking into account the energy balance. The variational problem on searching for the optimal active component distribution profile along the catalyst bed was formulated for the case of an adiabatic reactor at an arbitrary form of reaction rate expression. The Euler differential equation was derived and the existence conditions for the solution of variational problem were obtained. A numerical algorithm was suggested and the optimal profiles of the active component distribution were calculated for the first-order reaction. Under isothermal conditions with linear dependence of the reaction rate on reactant concentration at a constant mass transfer coefficient, the uniform distribution is optimal. As opposed to the case of an isothermal reactor with a first-order catalytic reaction, in an adiabatic reactor considerable economy in the active component loading might be achieved due to optimization of its distribution. It was shown that the optimal active component distribution profiles were axially decreasing for the first-order exothermic reactions and increasing for endothermic reactions. The optimal active component distribution profile was calculated for the case of methane combustion on catalytic monoliths.     

Modeling of propane dehydrogenation combined with chemical looping combustion of hydrogen in a fixed bed reactor

A redox process combining propane dehydrogenation(PDH) with selective hydrogen combustion(SHC) is proposed, modeled, simulated, and optimized. In this process, PDH and SHC catalysts are physically mixed in a fixed-bed reactor, so that the two reactions proceed simultaneously. The redox process can be up to 177.0% higher in propylene yield than the conventional process where only PDH catalysts are packed in the reactor. The reason is twofold: firstly, SHC reaction consumes hydrogen and then shift...     

Effect of catalytic combustion of hydrogen on the dehydration processes in a membrane reactor. III. calculation of the industrial reactor

This paper describes the mathematical simulation of an industrial membrane reactor for propane dehydrogenation in the thermodynamic coupling with hydrogen combustion (oxidation). Due to the effective removal of hydrogen through a membrane and the heat release as a result of an exothermic reaction, the temperature of the reaction stream at the input could be reduced to 500?C. The fact that the process is carried out on an industrial-level membrane reactor makes it possible to reach a propane conversion of 75% with a propylene selectivity of 97%, which exceeds the figures obtained per pass in existing industrial devices at higher temperatures.     

Temperature profile in a two-stage fixed bed reactor for catalytic partial oxidation of methane to syngas

Detailed axial temperature distribution has been studied in a two-stage process for catalytic partial oxidation of methane to syngas, which consists of two consecutive fixed bed reactors with oxygen or air separately introduced. The first stage of the reactor, packed with a combustion catalyst, is used for catalytic combustion of methane at low initial temperature. While the second stage, filled with a partial oxidation catalyst, is used for the partial oxidation of methane to syngas. A pilot-scale reactor packed with up to 80 g combustion catalyst and 80 g partial oxidation catalyst was employed. The effects of oxygen distribution in the two sections, and gas hourly space velocity (GHSV) on the catalyst bed temperature profile, as well as conversion of methane and selectivities to syngas were investigated under atmospheric pressure. It is found that both oxygen splitting ratio and GHSV have significant influence on the temperature profile in the reactor, which can be explained by the synergetic effects of the fast exothermic oxidation reactions and the slow endothermic (steam and CO₂) reforming reactions. Almost no change in activity and selectivity was observed after a stability experiment for 300 h.     

Modelling endothermic reactions in a compound membrane reactor

This paper models the performance of a membrane reactor. The membrane, a composite alumina-based one, is packed with a catalyst and allows low molecular weight gases to diffuse through it at a faster rate than gases with a higher molecular weight. This allows a greater conversion to be achieved in one pass through the reactor. The reaction that is specifically considered in this paper is the dehydrogenation of methyl-cyclohexane to toluene with the production of hydrogen. This latter species is preferentially removed by the membrane. Data for the performance of the membrane have been estimated from previous experiments using single gases and the mechanisms considered are Knudsen and bulk flow. Surface flow is not considered in the model as it is possibly not important as the endothermic reaction is carried out at a high temperature. A standard kinetic model is also incorporated in the calculations. The correlations of maximum effective length of membrane reactors and maximum percentage conversion as functions of the feed velocity and the membrane diameter are demonstrated in this paper. This paper also considers the behaviour of a compound reactor in which the first section is a straightforward ‘plug flow’ reactor where the catalyst is confined in an impermeable tube with the same internal diameter as the membrane. This is followed by a section containing the membrane. The reason for considering this configuration is to avoid unnecessary leakage of methyl-cyclohexane feed in the initial stages of the reaction. This innovation leads to predicted increases in the overall conversion of the process.     

Optimizing the catalyst distribution for countercurrent methane steam reforming in plate reactors

Microscale autothermal reactors remain one of the most promising technologies for efficient hydrogen generation. The typical reactor design alternates microchannels where reforming and catalytic combustion of methane occur, so that exothermic and endothermic reactions take place in close proximity. The influence of flow arrangement on the autothermal coupling of methane steam reforming and methane catalytic combustion in catalytic plate reactors is investigated. The reactor thermal behavior and performance for cocurrent and countercurrent are simulated and compared. A partial overlapping of the catalyst zones in adjacent exothermic and endothermic channels is shown to avoid both severe temperature excursions and reactor extinction. Using an innovative, optimization‐based approach for determining the catalyst zone overlap, a solution is provided to the problem of determining the maximum reactor conversion within specified temperature bounds, designed to preserve reactor integrity and operational safety. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Analysis of a novel reverse-flow reactor concept for autothermal methane steam reforming

One of the main shortcomings of existing multifunctional reactor concepts for the autothermal coupling of endothermic and exothermic reactions is inefficient heat integration leading to excessive maximum temperatures or poor reactor performance. For the asymmetric operation of a reverse-flow steam reforming reactor, conditions under which these shortcomings can be overcome are proposed. The asymmetric process is based on the formation of travelling reaction zones. The features of these transient phenomena are analysed by means of a simplified model.During the endothermic semicycle, the heat consumption forms a temperature wave with an expansive low-temperature and a compressive high-temperature part. During the exothermic semicycle a proper axial distribution of the heat supply is necessary in order to maintain a favourable temperature profile in the cyclic operation mode.The results obtained with the simplified model are verified by direct dynamic simulation.     

Comparison of the performance of a direct-contact bubble reactor and an indirectly heated tubular reactor for solar-aided methane dry reforming employing molten salt

A theoretical approach is presented for the comparison of two different atmospheric pressure reactors—a direct-contact bubble reactor (DCBR) and an indirectly heated tubular reactor (IHTR)—to evaluate the reactor performance in terms of heat transfer and available catalytic active surface area. The model considers the catalytic endothermic reactions of methane dry reforming that proceeds in both reactors by employing molten salts at elevated temperatures (700–900 °C) in the absence of catalyst deactivation effects. The methane conversion process is simulated for a single reactor using both a reaction kinetics model and a heat transfer model. A well-tested reaction kinetics model, which showed an acceptable agreement with the empirical observations, was implemented to describe the methane dry reforming. In DCBR, the heat is internally transferred by direct contact with the three phases of the system: the reactant gas bubbles, the heat carrier molten salts and the solid catalyst (Ni-Al₂O₃). In contrast, the supplied heat in the conventional shell-and-tube heat exchanger of the IHTR is transferred across an intervening wall. The results suggest a combination system of DCBR and IHTR would be a suitable configuration for process intensification associated with higher thermal efficiency and cost reduction.     

Adsorptive reactor technology for VOC abatement

The use of the monolith as an adsorptive reactor (M_AR) is proposed as a viable and novel alternative for VOC disposal. The M_AR combines adsorptive separation and catalytic combustion of the VOC in a single reactor unit and is thought to make effective utilisation of energy due to efficient heat integration. Theoretical studies on the feasibility and application of the adsorptive reactor concept for VOC oxidation is presented in this paper. Thus unlike previous work, present studies focus on an exothermic reaction system and the ability of the M_AR to control thermal runaway. A two dimensional mathematical model accounting for non isothermal adsorption and reaction, mass transfer limited adsorption kinetics and non linear (Tóth) adsorption equilibria, has been developed. The process is operated cyclically in two steps: adsorption and desorption/reaction. The VOC is fed into the reactor in the adsorption step and captured to produce a pure carrier gas effluent. Concentration and thermal swing is induced in the second step by means of an air feed. The most outstanding feature of the M_AR is its ability to prevent thermal runaway whilst maintaining a high VOC conversion. Simulation results indicate that the careful selection of step times for adsorption and desorption, feed temperatures and inlet velocities lead to stability and energy requirements which outperform equivalent conventional designs. The M_AR is thermally more stable due to the controlled release of the reactant from the adsorbed phase into the reaction zone, and also the heat integration of endothermic desorption and exothermic reaction.     

板翅式反应器中甲醇水蒸气重整制氢

研制了一种高效板翅式反应器,其特点是体积相对较小,便于放置,便于扩大规模;集预热、气化、重整、催化燃烧于一体;板翅式反应器内部热量利用合理,放热反应与吸热反应、气化与冷却之间实现了较好的热量耦合;可实现完全自供热.在反应器中进行了一系列甲醇水蒸气重整的实验,考察了不同条件对甲醇重整制氢过程的影响、对反应器床层温度分布的影响,及反应器的稳定性.另外,由于板翅式结构的良好传热性,甲醇水蒸气重整在获得较高转化率的同时重整气中CO浓度较低,且反应器的稳定性良好.