Two-phase Flow And Mass Transfer Characteristics Coupled With Catalytic Conversion In A Gas-liquid-solid Microreactor And Its Performance Enhancement

With the rapid development of society,it becomes more important to explore economical and environmentally friendly reaction ways because of the increasingly serious energy consumption,safety and environmental issues in the traditional chemical industry.Micro-chemical technology has thus emerged and become a new research direction.As the core of micro-chemical technology,microreactor has been considered to be of great significance to realize the sustainable development of chemical industry as a result of its intrinsic advantages of excellent heat and mass transfer performance,high safety,convenient and continuous operation,and easy design,etc.These features make microreactors to receive ever-increasing attention all over the world.Complex two-phase flow,mass transfer and catalytic conversion occur in microreactors.The coupling mechanism between the multiphase flow and mass transport and conversion as well as the property of the catalyst layer in the microreactor play key roles in the microreactor performance.Although microreactors have been widely investigated,past main efforts are devoted to the catalytic layer preparation in microreactors,and the flow behavior and mass transfer characteristics in microreactors without or with bulk reactions.The characteristics of two-phase flow and mass transfer coupled with the catalytic reaction in microreactors remain unclear.Aiming at these critical issues,in this thesis,the nitrobenzene hydrogenation to aniline was used as a reference multi-phase catalytic reaction.The catalyst layer preparation and the gas-liquid flow behavior as well as the mass transfer characteristics were investigated.Firstly,based on the electroless deposition,a novel preparation method of the catalyst layer with additional hydrogen reduction was proposed,which could enhance the utilization efficiency of adsorbed precursor ions and intensify the catalytic performance.Based on the microreactor fabricated by this method,the gas-liquid flow behavior,mass transfer and conversion characteristics in the microreactor were explored through visual experiment.Then,considering the unavoidable catalyst deactivation in the actual process,interaction between the gas-liquid flow behavior and catalytic activity in the microreactor was studied under long-term operation,and the catalyst deactivation was also analyzed.To relieve the nanocatalyst deactivation,inspired by the shell-core structure,polydopamine encapsulated catalyst layer structure was proposed,which could effectively inhibit the nanocatalyst agglomeration and intensify the dispersion of the deposited nanocatalysts.The Taylor flow behavior coupled with the catalytic reaction in the microreactor under long-term operation was also discussed.Finally,a microreactor with excellent catalytic activity and durability was proposed based on the configuration regulation and pH control.The two-phase flow behavior and mass transfer characteristics coupled with the catalytic reaction were also studied.The main outcomes are summarized as follows:(1)Compared with the catalytic layer prepared by conventional electroless deposition method,the proposed method could significantly improve the catalyst utilization efficiency and further improve the stability of the prepared catalyst layer.On the basis of the mass transfer and conversion characteristics,excess ratio was proposed to characterize the difference between the actual minimum gas consumption and the theoretical value,which was caused by the mass transfer resistance between the phases.The experimental results showed that the increase of both the liquid flow rate and inlet nitrobenzene concentration could enhance the mass transfer in the microreactor and the reactants utilization,resulting in the gradually decreased excess ratio.(2)The gas-liquid two-phase flow behavior in the microreactor was significantly different from that in the conventional gas-liquid two-phase flow.Because of the hydrogen consumption,there existed a clear gas-liquid interface retraction behavior.This phenomenon became increasingly obvious with the decrease of the liquid flow rate and the increase of the initial nitrobenzene concentration.In addition,the gradually consumed hydrogen also lowered the gas holdup in the microreactor,which could then increase the residence time of the gas and liquid reactants in the microreactor.(3)To over come the capillary threshold pressure,there exist two stages of inburst stage and waiting stage for the gas bubble formation.During the reation process,the catalytic reaction mainly occurred at the waiting stage,whereas almost no hydrogen consumption was observed at the inburst stage.Along the flow direction,the hydrogen consumption rate gradually decreased with the increase of the gas flow rate and the inlet nitrobenzene concentration,while it became more unobvious with the increase of liquid flow rate.(4)The evolution of the gas slug length along the microreactor is closely related to the catalytic activity during the long-term operation.It was found that in the primary stage,the gas slug length was first declined rapidly due to hydrogen consumption and then almost unchanged along the flow direction.After operating with a few hours,the variation of the gas slug length became obscure at the upstream due to the nanocatalyst deactivation but obvious at the downstream.Finally,the gas slug length was nearly unchanged.Besides,the physico-chemical properties of the catalysts before and after the long-term operation showed that the nanocatalysts agglomeration was the main reason leading to the catalysts deactivation.(5)The polydopamine-encapsulated catalyst layer structure has superior catalytic performance.Similar to the shell-core structure,the polydopamine-encapsulated structure could inhibit the catalyst agglomeration during the reduction and operation process,thereby enhancing the catalyst dispersion and increasing the catalyst utilization.Experimental results indicated that the stability of the catalytic layer could be significantly improved with the nitrobenzene conversion of nearly 100%.In addition,visual results also showed that although the “shell” structure could reduce the catalytic reaction rate,the intensified dispersion and inhibited agglomeration could also significantly improve the long-term operation stability of the microreactor.(6)A novel multilayered palladium catalyst layer with nano-bulge structure was proposed.This structure could provide more surface area and more accessible sites for precursor adsorption,which could enhance the nanocatalysts dispersion.Experimental results showed that the proposed multilayer Pd catalyst significantly outperformed previous study,yielding the intensified reaction rate wiht an order of magnitude longer durability for the nitrobenzene hydrogenation.In addition,pH controlled precursor ions deposition in the microreactor were also investigated to clarify the synergistic effect of the electrostatic force and ion transfer on the nanocatalysts deposition and dispersion.And the results also indicated that the both the particle size distribution and the dispersity as well as the palladium loading were inherently associated with the synergy of the electrostatic interaction and the ion transfer.Such feature influenced both the catalytic activity and the durability for the nitrobenzene hydrogenation.