Mass Transfer And Catalytic Conversion Characteristics In A Gas-Liquid-Solid Triple-Phase Catalytic Membrane Microreactor

Microreactor technology has undergone rapid development in recent years,which can be widely applied to chemical engineering,energy,biology,medicine and material and so on.Especially in chemical industries,microreactors can improve the heat and mass transfer and conversion,decrease energy consumption and improve the operation security.As a result,microreactors have been regarded as an important future direction.Among various microreactors,the catalytic membrane microreactor is a new type microreactor,which combines the advantages of the microreactor and membrane technology.In particular,catalytic membrane microreactors have both the microchannel and membrane structures,which can address this issue encountered in convention microreactors for gasliquid-solid triple-phase catalytic reactions,that is,high mass transfer resistance at the gas-liquid interface.As a result,the conversion can be boosted and the operating conditions can be optimized.Currently,extensive efforts on the catalytic membrane microreactors are mainly devoted to the developments of the membrane materials and catalysts.A deep understanding of the coupled mass transport and conversion in this type of microreactor is insufficient.Meanwhile,the effects of the catalyst layer,the supporting layer and the membrane microreactor configuration on the mass transport and catalytic conversion still remain unclear,which limits further development of the catalytic membrane microreactor.Aiming at these critical issues,the gas-liquid-solid triple-phase catalytic reaction of the nitrobenzene hydrogenation catalyzed by Pd is chosen as a model reaction,which has been widely used in industries.Main efforts of this thesis are paid to the mass transport and conversion characteristics of a catalytic membrane microreactor.First of all,a plate catalytic membrane microreactor with a three-layer sandwiched structure was developed,by which the mass transport and conversion characteristics was studied.Next,based on the layer-by-layer self-assembly technology,an effective catalyst layer was prepared and adopted in the plate catalytic membrane microreactor and stacked catalytic membrane microreactor.The influences of the catalyst layer and stacked structures on the conversion were studied.Then,to enhance the gas permeation and mechanical strength,a composite supporting layer consisting of PDMS and stainless steel mesh was constructed,whose transport properties and the effects on the conversion were explored.Afterwards,in order to achieve the massive production,a tube-in-tube catalytic membrane microreactor was designed and fabricated.The effect of the fill factor was studied.Finally,an integrated catalytic membrane microreactor with an ultrathin membrane was proposed.The mechanisms of the ultrathin membrane formation and the characteristics of the mass transfer coupled with the catalytic reaction were investigated.Main achievements of this thesis are summarized as follow.(1)A plate catalytic membrane microreactor with the three-layer sandwiched structure was constructed,in which the supporting layer was sandwiched between two microchannels to separate the gas and liquid reactants.The catalyst layer was prepared by the sol-gel method and coated on the side of the supporting layer facing the liquid reactant.Compared with the conventional microreactor under the same conditions,it was found that the plate catalytic membrane microreactor showed a better performance in terms of the conversion and stability.That is because the support membrane could increase the interfacial area of the gas and liquid phases,shorten the hydrogen transfer path,improve the stability of the catalyst layer.In addition,the experimental results showed that the decrease of the supporting layer thickness,the increase of the hydrogen flow rate,and the decrease of the nitrobenzene concentration and flow rate could improve the performance of the catalytic membrane microreactor.(2)Based on the layer-by-layer(LBL)self-assembly technology,a palladium nanostructured catalyst layer was prepared.As compared with the catalyst layer prepared by the sol-gel method,the catalyst layer prepared by the LBL self-assembly technology exhibited more uniform distribution of the particle size and better catalytic activity.Moreover,the catalyst layer prepared by double modifications of polydopamine and polyelectrolyte multilayers showed the best performance.For the stacked catalytic membrane microreactor with the catalyst layer prepared by this LBL self-assembly technology,as the stacked number increased,the contact area of the gas and liquid phases and catalyst loading were increased,leading to the increased hydrogen utilization.As a result,the stacked catalytic membrane microreactor could still maintain high conversion and stability under high flow rates and concentrations.(3)PDMS membranes served as the supporting layer showed a negative correlation between the gas permeability and mechanical strength.The decrease of the thickness or the degree of the crosslinking could improve the gas permeability but decrease the mechanical strength.Therefore,a composite membrane consisting of PDMS and stainless steel mesh was constructed to improve hydrogen permeability and mechanical strength simultaneously.As compared to the pure PDMS membrane,the composite membrane yielded better gas permeability and mechanical strength.Experimental results also indicated that with increasing the mesh number or decreasing the concentration of PDMS solution,the thickness of the PDMS membrane in the mesh can be reduced,decreasing the gas transport path.As a consequence,the hydrogen permeation rate and utilization and the conversion of nitrobenzene could be improved.(4)A tube-in-tube catalytic membrane microreactor was developed,which consisted of a hollow fiber membrane and a PTFE capillary.The hollow fiber membrane was inserted in the PTFE capillary and acted as the supporting layer.The catalyst layer was coated on the hollow fiber membrane by the LBL self-assembly.Experimental results showed that the conversion and stability of the tube-in-tube catalytic membrane microreactor improved with increasing the layer number of polyelectrolyte multilayers,the deposition time of the precursor solution and the length of the hollow fiber membrane.In addition,the fill factor of the tube-in-tube structure increased with decreasing the inner diameter of the PTFE capillary and increasing the number of the hollow fiber membrane.Therefore,the microreactor performance could be improved,which provides a possible way for the mass production by the catalytic membrane microreactors.(5)An integrated catalytic membrane microreactor with an ultrathin membrane was proposed by introducing two immiscible solutions with parallel laminar flow and their interfacial polycondensation reaction.Since the ultrathin membrane was in-suit formed in microchannels,the issues on the assembly and sealing of the membrane microreactor could be addressed.The ultrathin membrane performed well in the gas permeability and mechanical strength.It was shown that as the reaction time of the interfacial polycondensation reaction decreased,the ultrathin membrane got thinner,thereby enhancing the gas transport through the membrane.The results also showed that adding the pore former of PEG could result in the porous ultrathin membrane with controllable pore diameter and density,which could further enhance the gas transport and conversion.