Performance And Deactivation Behavior Of Raney-Ni Catalyst In Continuous Dehvdrogenation Of Cyclohexane Under Muitiphase Reaction Conditions

The "wet-dry" multiphase reaction mode is a kind of highly efficient system for the dehydrogenation of liquid organic hydrides (LOH) with advantages of high catalyst surface temperature, faster hydrogen evolution rate and reduced occurrence of reverse reaction and catalyst deactivation. Therefore the multiphase reaction mode is regarded as a promising on-board LOH dehydrogenation system. Under "wet-dry multiphase" reaction conditions, the catalyst surface would experience wet and dry states periodically, which calls for a high stability of catalyst used and results in a different characteristic of catalyst deactivation. As a result, it will be necessary and significant to investigate the deactivation properties of Raney-Ni and to find an effective method to reduce or prevent the deactivation of the catalyst.To begin with, the deactivation kinetics of Raney-Ni was studied in the present work. The order of deactivation was observed shifting from3to1as the external mass transfer increases, which showed that the decay of Raney-Ni catalyst was parallel deactivation. An extra-low apparent activation energy of20.60kJ·mol-1indicates that the reaction is strongly diffusion-limited. And the values of the Thiele modulus reveal that the influence of the internal diffusion could be completely neglected. In the "wet-dry" multiphase reaction mode, the instant evaporation of the dropped liquid organics on the catalyst surface can produce a strong purging effect on the catalyst surface, and this purging effect will not only increase the mass transfer resistance, but also dramatically shorten the residence time of the reactant on the catalyst surface. Therefore, the dehydrogenation reaction is controlled by the external mass transfer and confined to the outer layers of catalyst pellet.Secondly, the fresh and used Raney-Ni catalysts were characterized by various analysis methods such as SEM, FTIR and XRD. The experimental results showed that the deposition of filamentous coke on the outer layers of Raney-Ni is the main reason of catalyst deactivation. Due to the strong external diffusion resistance, the coke is concentrated in the pore mouth with an amount of coke about3.25wt.%of the catalyst. The coke mainly comes from the side reactions of cyclohexane. The research also revealed that, Raney-Ni catalyst shows high mechanical strength and stability in the reaction. The high dehydrogenation temperatures wouldn’t cause noticeable changes of the sizes of Nickel crystal grain and catalyst particle.The favorable reaction conditions can enable the reaction system approach its optimal energy balance point which not only ensures the high dehydrogenation efficiency, but reduces the possibility of catalyst deactivation. In the third section of the thesis, the influence of reaction conditions on hydrogen evolution volume, hydrogen evolution rate, conversion rate of cyclohexane was investigated by the uniform design method. It can be learnt that under the conditions of salt bath temperature at390℃, feeding rate of16.75mmol-min"1and catalyst dosage of5g, the overall performance of the reaction system achieved its optimal. The highest and average hydrogen evolution rate,0.359mmol·cm-2·min-1and0.323mmol·cm-2·min-1, were obtained in the reaction. After2hours of dehydrogenation reaction, the hydrogen evolution volume could reach6132ml and the catalyst relative activity was nearly0.8.Moreover, a theoretical modeling concerning the energy balance of the dehydrogenation of cyclohexane under multiphase reaction conditions was proposed. The study revealed that cyclohexane would experience temperature rise, decomposition and vaporization before leaving the reaction system. The vaporization of reactant consumed the largest amount of energy which accounted for38.75%of the total energy needed, while the energy used for the dehydrogenation reaction was just about12.81%. The average heat flux of the catalyst during the reaction process was about27.52kW·m-2and the thermal efficiency of the whole reaction system could reach52.51%. According to the theoretical modeling proposed in this study, we can optimize the reaction conditions and improve the efficiency of dehydrogenation reaction and energy utilization.