Transfer And Reactive Performances Of Metallic Based Monolithic Catalysts And Reactors

Process intensification refers to the development of novel apparatuses and techniques that are expected to bring dramatic improvements in manufacturing and processing compared to those commonly used today. As one of the process intensification techniques, monolith catalyst (structured catalyst) has a lot of advantages over conventional pellet catalyst in transport characteristics and macro-kinetics of heterogeneous catalytic reactions. Conventional fixed-bed reactor randomly packed with the pellets of catalyst has a poor heat transfer that can induce hot spots in exothermic reactions. The hot spots will bring side reactions and reduce the selectivity of the desired product. The monolith catalytic reactor whose catalyst support is monolith structure has much higher surface area per unit volume of the bed, which guarantees a good heat transfer throughout the reactor. Besides, it also offers a much lower pressure drop in comparison with the pellet packed-bed reactor. So, monolith catalyst has received much attention from academia and industrial practice since it came up in the 1990's late period, and more and more researches for the performance of monolith catalytic reactor are coming up.Recently, metal monolith catalyst with high thermal conductivity is considerably attractive. Accordingly, a series of investigations for the metal monolith catalyst and reactor, such as the preparations, characterizations and reactive performance evaluation are carried out in our research group. Other than experimental research, mathematical modeling and simulation have been powerful tools to offer theoretical support in the study and development of chemical processes, simultaneously saving large amounts of labors and resources. In this paper, the detailed numerical simulations are implemented to predict the transport and reaction performances of metallic monolith catalytic reactor. Moreover, A metallic monolith reactor for directly coupling steam reforming with catalytic combustion of methane is proposed, the metallic monolith is used as a co-current heat exchanger and the catalysts are deposited on channel walls of the monolith. This monolith catalytic reactor is proved to be a promising alternative to the steam reforming system of conventional industry in which the heat needed by the reforming reaction is supplied by flame combustion, causing low heat efficiency, large size equipment and high NO_X emission. Actually, the reaction performances of monolith catalyst considerably depend on transport characteristics of its support, which are rarely reported in researches. Therefore, firstly, modeling and simulation based on computational hydrodynamics and heat transfer for metallic structured packed bed that is only filled with metal monolith supports without catalyst are carried out to predict the flow field and temperature field, and to evaluate its performance in transport aspect. In order to show the advantages of metal structured packed bed, the comparison between the simulation results for the metallic structured packed bed and the experimental heat transfer performance as well as pressure drop for the conventional pellet packed bed is made, which quantitatively validates that transport performance of the metallic structured packed bed is much better. Furthermore, the effects of geometric parameters and the property of solid phase on heat transfer of the metallic structured packed bed are discussed. By analyzing the effects of these parameters on heat transfer, it can be concluded that the heat transfer is controlled by the convection in low Re number and increasing the specific surface area of the bed can effectively intensify it. But when Re number is high, the static conduction dominates. In this case, heat transfer can be improved by means of decreasing the voidage or taking advantage of materials with high intrinsic conductivity of the support. Besides, some other useful methods are proposed to enhance heat transfer, for instance, under turbulent flow, choosing an appropriate height of the rough layer between the gas phase and solid phase is also proved to be really helpful.Subsequently, the study for the performance of monolith catalytic reactor is indispensable to gain insight in reactor behavior and improve reactor performance. Monolith catalytic reactor contains hundreds of parallel channels, and the catalyst is dispersed within a washcoat that is coated onto the surface of the channels, where catalytic reaction occurs. Monolith catalytic reactor plays an important role in catalytic combustion such as combustion chambers for gas turbines used in power generation where methane is the main reactant. Compared to conventional gas-phase combustion, catalytic combustion permits lower operation temperature, greater range of fuel-air ratios and near zero pollution emission because of lower activation energies involved in the catalytic surface reactions. Here the modeling for the catalytic combustion of CHVair in monolith reactor is constituted to describe the heterogeneous reaction at the channel wall in a single channel as well as at the channel walls in the whole reactor. The simulations for a single channel and the whole reactor are all verified to be valid. However, the simulation based on the whole reactor model is proved to be more reasonable than that of the single channel model, especially when the property of solid phase and the heat loss are considered. So, for further research and development of monolith reactor, the whole reactor should be simulated taking account of the heat loss and the property of solid in order to get a better understanding for the reactor behavior. Besides, it is found that the performance of the monolith reactor, at least under the conditions considered here, is more sensitive to the inlet temperature, concentration and gaseous velocity than to the geometry and the catalyst loading.Many of industrial chemical processes such as hydrocarbon cracking, dehydrogenation, and reforming are highly endothermic. The natural gas flame combustion is one of the main sources of thermal energy supply for these processes, leading to substantial heat loss, large size equipment and high NO_X emission. Autothermal multifunctional reactors offer an attractive solution for implementing high-temperature reactions by using catalytic combustion to provide the heat required. Owing to their high thermal efficiency, the autothermal reactors, which offer the intimate coupling of endothermic reaction and catalytic combustion, have been a subject of vital research and development. A jacket metal monolith reactor for directly coupling methane steam reforming with catalytic combustion is proposed in this paper. A model based on the whole reactor, in which the oxidation and reforming reactions take place at different parts, is developed to explore heat transfer and reaction performances of the reactor. The results show that effects of the operating parameters on the performances of the reactor are obvious, so the suitable adjustment of operating conditions should be carried out to improve the reactor performance and facilitate energy supply. Moreover, The proper channel arrangement or catalyst axial distribution, such as more specific surface area of the reforming part or non-uniform catalyst distribution of the combustion side, can alleviate the excessive radial temperature gradients in both sides, and increase the methane conversion of reforming side considerably.In the above research for the jacket metallic monolith reactor, it has been found that combustion reaction couldn't supply sufficient heat for the reforming reaction when the feed concentration of methane under explosion limit is adopted at the entrance of the combustion side. Therefore, a novel reactor configuration is designed to supply more heat for the reforming side by inputting methane of combustion side not only at the entrance also at different axial positions of the reactor, simultaneously avoiding the methane concentration over the explosion limit. The transport and reaction performances of the axial distributed-feeding reactor are studied utilizing heterogeneous model based on the whole reactor. The distributed-feeding reactor is verified to have a lot of advantages over the non- distributed feeding and industry reactors. Several modes of feeding distributions of methane volume flux at different axial positions in the combustion side are discussed, of which one mode is proved to enhance heat transfer and increase the reactant conversion. At the same time, it is found that the changes of operating conditions affect not only the methane conversion of reforming side, but also the thermodynamic limit of reforming reaction. Besides, The conceivable configuration of the reactor is also proposed to realize the distributed feeding and avoid too large gas flowrate caused by the feeding at the more axial positions when the reactor is lengthened in future application.Finally, the experiment for coupling CO₂ reforming and catalytic combustion of methane in the jacket monolith reactor was carried out to verify the feasibility of this intimate coupling.