Two-phase Flow Transfer And Conversion Characteristics In A Structured Nickel-based Triple-phase Microreactor And Its Performance Enhancement

Micro-chemical technology can promote the realization of efficient,green and safe production of chemicals,and has an important contribution to alleviate energy and environmental problems facing our planet.As the key of the micro-chemical technology,microreactors are able to greatly enhance heat and mass transfer due to its small characteristic scale and large specific surface area,showing promising prospect in the fields of chemistry,chemical engineering,energy and biology.In a microreactor,the catalyst is usually coated on the inner wall of the microchannel,which can effectively avoid the issues of the catalysts separation and recycle.However,limited by the small scale of the reactor,the area for loading the catalysts is very small.Although the micro packed-bed reactor can significantly increase the area for loading the catalysts,it may suffer from flow blockage and larger pressure drop in the reactor.Fortunately,structured microreactor combine the advantages of structured catalysts and microreactors,it can not only significantly increase the loading area,but also possess small pressure drop,which is regarded as a promising microreactor structure type.In the structured microreactor,the gas/liquid reactants flow through the pore space of the structured catalyst accompanying with the catalytic reactions.Under such a circumstance,there are complex and coupled processes involved with multiphase flow,mass transfer and catalytic conversion.These complicated processes are closely related to the activity of the structured catalyst,gas-liquid two-phase flow,phase distribution and reactor structure.Further understanding of the relevant mechanisms and laws will help to improve the reactor performance.This thesis is devoted to studying the two-phase flow transfer and conversion characteristics in a structured nickel-based triple-phase microreactor and its performance enhancement,based on the nickle foam as the structured catalyst substrate.First of all,a plate structured microreactor was constructed,which used the nitrobenzene hydrogenation reactions as the experimental object to study the mass transfer and conversion characteristics in the reactor.Then,from two aspects which include the build of the high-efficient and low-cost structured catalyst,and construction of high-efficient structured microreactor has been carried out for in-depth study.A preparation method of high performance structured catalyst was proposed;the gas-liquid two-phase flow and phase distribution characteristics in a structured microreactor were obtained.The influence of gas-liquid two-phase flow under different structure parameters and operating conditions on the mass transfer and conversion characteristics was discussed.A method of controlling gas-liquid flow and enhancing the interface mass transfer was proposed to improve the performance of the microreactor.Main outcomes are presented as follows.(1)In order to improve the loading area of the catalyst,the nickel foam was used as the catalyst support,the biomimetic poly-dopamine coating was proposed for the Ni foam based structured catalysts.And then a flat-plate structured microreactor was constructed by filling it in the microchannel.The performance of structured microreactor and conventional microreactor were compared under the same experimental conditions.The results showed that the conversion and stability of nitrobenzene in the structured microreactor were improved greatly because the nickel foam increased the loading area of the palladium catalyst and the interlinked pore structure enhanced the mass transfer of reactants.In addition,increasing the length of nickel foam and the gas flow rate,and decreasing the liquid concentration or flow rate all contributed to the improvement of the reactor performance.(2)In order to further improve the dispersion of nanocatalysts,Ni nanostructures were synthesized on the surface of Ni foam by hydrothermal method.It was found that the hydrothermal temperature,hydrothermal time and structure-guiding agent had important effects on the formation of the Ni nanostructure.When the hydrothermal temperature was 160°C and the hydrothermal time was 8 h,a"snowflake"like Ni nanostructure could be formed on the Ni foam surface in assistance with the structure-guiding agent,which could increase the specific surface area of Ni foam from1 m~2/g to 33 m~2/g.Then,a one-step chemical displacement method was used to prepare Pd particles on the Ni nanostructures.The results showed that the high specific surface area of Ni nanostructures could not only significantly improve the activity surface area of Pd,but also guarantee a strong interaction between the Pd catalyst and support,endowing the structured catalyst with higher catalytic performance and better stability.(3)Finally,in order to reduce the reactor cost,a structured Ni-B catalyst was proposed.The results showed that element B and element Ni formed amorphous alloy Ni–B,producing unsaturated coordination site of Ni.At the same time,element B donated the electrons to element Ni and made the Ni electron-rich,which enhanced the adsorption of reactants and promoted the catalytic hydrogenation of nitrobenzene.When the B/Ni molar ratio was 2,this structured catalyst had the largest Ni active surface area,thus yielding the best catalytic performance.(4)In order to solve the problem of uneven phase distribution in structured microreactors,a segmented structured microreactor was proposed to control the reactant flow,by which the phase distribution in the whole microreactor could be regulated.Besides,a uniformity evaluation factor was proposed to quantitatively evaluate the phase distribution in the porous structure.The results showed that the segmented arrangement of structured catalysts could significantly improve the phase distribution and liquid holdup,and a more uniform phase distribution could be obtained to increase the gas-liquid contact area,which avoided the problem of low catalyst utilization and improved the reactor performance.(5)Aiming at the instability of gas-liquid interface in structured microreactors,a structured microreactor with independent gas and liquid flow channel was proposed.Appropriate design of the pore density of the Ni foam and the gas and liquid flow rates could guarantee a stable phase contact,realizing the controllable interface.It was also found that the increase of the Ni foam thickness could limit the mass transfer of gas phase reactants to inner catalyst surface,and thus limit the reactor performance.(6)To further increase gas-liquid contact area in structured microreactors,a structured microreactor using porous titanium to generate microbubbles was designed and fabricated to increase gas-liquid contact area.The results showed that the pore size of porous titanium and the gas flow rate significantly affected the number,size and velocity of microbubbles.Porous titanium with smaller pore size could produce more microbubbles with more uniform bubble size,which could enhance the mass transfer and reactor performance.When the pore size of porous titanium was too large,the increase of gas phase flow rate was mainly manifested as the increase of the bubble size and velocity,which could not effectively improve the reactor performance.