【作者】 陈刚；

【导师】 朱恂； 陈蓉；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2020， 博士

【摘要】 微化工技术有助于实现化工过程的高效、绿色、安全生产,对缓解当今世界能源与环境问题具有重要的实际意义。微化工技术的核心是微反应器,由于其特征尺度小、比表面积大,因而具有极高的热质传递速率,在化学、化工、能源、生物等领域具有广阔的应用前景。在微反应器中,催化剂负载在通道壁面的方式可以避免催化剂分离和回收问题,但受限于反应器尺寸,催化剂负载面积极小。填充床微反应器虽然能显著增加催化剂负载面积,但会导致反应器流动阻塞及压降过大。结构化微反应器融合了结构化催化剂与微反应器的双重优势,具有催化剂负载面积大、流动压降小的特点,是具有前景的微反应器结构型式。在结构化微反应器中,气、液两相反应物在结构化催化剂的孔隙中流动,其内部存在复杂且相互耦合的多相流动和物质传递与转化过程,而这些过程与结构化催化剂的活性、气-液两相流动、相界面分布以及反应器结构等密切相关,对相关机理及规律的深入认识将有助于提升反应器性能。本文以泡沫镍作为结构化催化剂载体,开展了结构化镍基气-液-固三相微反应器内两相流动传输和转化特性及性能强化的研究。首先,通过构建平板式结构化微反应器,选用常见的硝基苯加氢反应为实验对象,对微反应器内物质传输及转化规律进行了研究。接着,分别从高效低成本结构化催化剂构建、新型高效结构化微反应器构型两方面开展了深入研究,提出了高性能结构化催化剂的制备方法,获得了结构化微反应器内气液两相流动及相分布特性,探讨了不同结构参数和运行条件下气液两相流动对微反应器内物质传输及转化特性的影响规律,提出了气-液流动可控且界面传输强化的方法,实现了微反应器性能的提升。主要的研究工作及成果如下:(1)为了提升催化剂负载面积,以泡沫镍作为催化剂载体,采用聚多巴胺对其表面进行改性,制备了结构化催化剂,然后将结构化催化剂填充于微通道内部,构建了平板式结构化微反应器。在相同实验条件下,对比了结构化微反应器与常规微反应器的性能。结果表明:由于泡沫镍载体增加了钯催化剂的负载面积,同时其相互连通的孔隙结构强化了反应物的传输,结构化微反应器的硝基苯转化率和稳定性均大幅提升。此外,增加泡沫镍载体长度、增加气相流量、减小液相浓度或流量均有助于反应器性能的提高。(2)为进一步提升催化剂的分散性,采用水热法在泡沫镍表面合成了镍纳米结构。研究发现水热温度、水热时间以及结构导向剂对镍纳米结构的形成具有重要的影响。当水热温度为160°C,水热时间为8 h时,在结构导向剂CTAB的作用下,泡沫镍表面可形成“雪花”状镍纳米结构,该纳米结构可使泡沫镍的比表面积从1m~2/g增加到33 m~2/g;进而采用一步化学置换法在镍纳米结构上制备了钯颗粒,结果表明,高比表面积的镍纳米结构不仅能够显著提升钯的活性表面积,同时使Pd催化剂与载体之间具有强相互作用,保证了该结构化催化剂具有更高的催化性能和更好的稳定性。(3)为降低结构化催化剂成本,提出了一种基于镍硼(Ni–B)非贵金属的结构化催化剂。结果表明:元素B与元素Ni形成了非晶态合金Ni–B,产生了不饱和的Ni配位位点;同时,元素B将部分电子转移给元素Ni,使Ni富电子从而增强了其对反应物的吸附,促进了硝基苯催化加氢反应。当B/Ni摩尔比例为2时,所制备的催化剂具有最大的Ni活性表面积,表现出最好的催化性能。(4)针对结构化微反应器内气液相分布不均问题,提出分段式结构化微反应器,通过对反应物流动的控制,进而实现对反应器整个多孔床层的相分布调控。提出了均匀度评价因子来定量评价多孔结构内的相分布。结果表明:分段式布置结构化催化剂能够显著改善微反应器内相分布和含液率,更均匀的相分布能够增加气液接触面积,同时避免催化剂利用率低的问题,提升反应器性能。(5)针对结构化微反应器中气液相流动不可控问题,提出气、液独立流动通道设计的结构化微反应器,通过合理设计泡沫镍孔密度和气、液相流量,可保证气-液反应物流动的可控。研究还发现泡沫镍厚度增加会限制气相反应物的传输,不利于催化反应,限制反应器的性能。(6)针对结构化微反应器中气液相接触面积小的问题,构建了一种利用多孔钛产生微小气泡以增加气液接触面积的结构化微反应器。结果表明:多孔钛孔隙和气相流量能够显著影响微气泡的数量、大小和运动速度。较小的多孔钛孔隙能够产生数量更多、分布更均匀的微小气泡,从而强化物质传输,提升反应器性能。多孔钛孔隙过大时,气相流量的增加主要表现为气泡尺寸和气泡运动速度的增加,不能有效提升反应器性能。更多还原

【Abstract】 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.更多还原