Study On The Liquid Phase Polymerization Characteristics And Regulation In Microreactors

As a typical representative of chemical process chemical process intensification,micro-chemical technology is the hotspot in the research of chemical engineering.The microreactor has many unique characteristics,such as high effective surface-to-volume ratio,enhanced heat-and mass-transfer rates,low energy consumption and excellent process safety.In recent years,microreactors have been widely used in polymerization processes.The optimization of the molecular weight and molecular weight distribution can be easily realized due to the rapid mixing and excellent mass transfer performance in microreactors.With the aid of microfluidic technology,the polymeric molecular structure can be flexibly designed with well controlled morphology.In particular,the polymerization process is more complex than the small molecule reaction process,the properties of the reaction liquid(eg.,viscosity,density,etc.)change with the generation of polymers.The investigation on the polymerization characteristics in microscale is significant to design of the reactor,optimization of polymerization process and control over the product's properties.This thesis focused on the mixing,mass transfer and flow characteristics during the polymerization in microreactors.The flow,mixing and mass transport during the polymerization were characterized and analyzed by dimensionless parameters.Moreover,based on the understanding of process characteristics,the continuous and controllable preparations of polyaniline and PPy/Mn O₂ composite were achieved in microreactors.This thesis deepened the understanding of the influence of mixing and mass transfer enhancement on the polymerization process,enriched the theory of chemical reaction engineering,and provided a new method for the regulation of polymerization process in microscale.The detailed contents of each part are listed as follows:(1)Nonliving free radical polymerization of acrylamide was chosen as a model reaction to investigate the effect of mixing performance on the polymerization in capillary microreactors.The average characteristic mixing time of the fluid in the capillary microreactor was deduced according to the calculation of the diffusion coefficients of monomers and polymers.The calculation results of Bodenstein number(Bo)indicated that the fluid in the microreactor deviated obviously from plug flow with the polymerization processing.The calculation results of Taylor diffusion coefficient indicated that the axial convective mixing aggravated the heterogeneity of the reactant concentrations in the capillary microreactor,especially for the microreactor with a larger inner diameter.Moreover,it was found that the addition of a pre-mixing stage minimized the effect of insufficient mixing between the initiators and the monomers on the polymerization,and benefited for obtaining the polymer with higher Mn and lower PDI.(2)Process characteristics of nonliving free radical polymerization of acrylamide in capillary microreactors were investigated at high temperatures(70?90°C).Poly(acrylamide)with Mn of 3.4×10⁵g/mol and polydispersity index(PDI)of 2.6 could be obtained with 1.61 min residence time at 70°C.It was found that the viscosity of the reaction mixture at the outlet of the microreactor decreased with the increase of the shearing stress,which indicated the reaction mixture changed from Newtonian fluid to non-Newtonian fluid with the polymerization processing.Furthermore,the rheological test indicated that hydrogels were produced in this polymerization process.Low values of Re and Bo indicated strictly laminar flow and obvious deviations from plug flow in capillary microreactors during the polymerization.Moreover,the polymerization performance in the microreactor was compared with that in the batch reactor.The results indicated that the product obtained from the microreactor had high Mn and lower PDI.(3)The hydrodynamics and mass transfer performance in the chemical oxidative polymerization of aniline in capillary microreactors within the liquid-liquid slug flow regime were investigated.Internal recirculation in the dispersed phase slugs was captured by high-speed camera.The slug coalescence became significant with increasing Reynolds number,thus reducing the mass transfer rate.The calculation results of Hatta number verified the experimental phenomena that the polymerization of aniline in the microreactor mainly occurred in the dispersed slugs,which was different from the interfacial polymerization in the conventional batch reactor.By calculating the efficiency factor(E),it was proved that the polymerization could improve the mass transfer efficiency,and polyaniline can also achieve a high yield in a short time due to the mass transfer enhancement in the microreactor.(4)Based on the study of the characteristics of the liquid-liquid two-phase chemical oxidation polymerization of aniline in the microreactor,the effect of the dispersed phase composition on the morphology of polyaniline was investigated.Hansen's solubility parameters and Flory-Huggins model were introduced to explicate the microstructure variation of PANI particles synthesized with different media.The characteristic dimension of PANI particles tended to be smaller with the increase of the Flory parameter of the reaction medium.Electrochemical measurement indicated that the specific capacitance of PANI could be improved by adding an organic co-solvent into the reaction medium.The maximum specific capacitance of 426 F/g was achieved at the current density of 1 A/g with the voltage window ranging from-0.2 to+0.8 V(vs.Ag/Ag Cl)in 1 mol/L H₂SO₄ solution.The maximum energy density reached 59.2 Wh/kg with the power density of 500 W/kg.Moreover,this continuous synthesis method can be used for the preparation of other conducting conjugated polymers,such as polypyrrole,polythiophene,etc.(5)Furthermore,the liquid-liquid two-phase flow method was applied to the chemical oxidation polymerization of pyrrole and the synthesis of Mn O₂.A slug-based microreactor system was constructed to prepare PPy/Mn O₂ composite with high controllability by multi-feeding strategy.The size distribution variation of Mn O₂particles was elaborated by the crystallization theory.It was found that the morphology of the PPy/Mn O₂ composite was significantly affected by the molar ratio among KMn O₄,Mn SO₄ and pyrrole and the residence time.A probable mechanism for the formation of the PPy(shell)/Mn O₂(core)composite was proposed.The maximum specific capacitance of 303 F/g was achieved at the scan rate of 20 mv/s with the voltage window ranging from-0.3 to 0.7 V(vs.Ag/Ag Cl)in 1 mol/L Na₂SO₄ solution using a three-electrode test system.Moreover,compared with previous reports with the use of droplet-based microfluidics or microreactors,this work developed a flexible and efficient continuous-flow strategy for the preparation of particles with core-shell structures in slug-based microreactors.In addition,this slug-based microreactor had a flux with one or two orders of magnitude higher compared with the droplet-based microreactor for preparing materials with core-shell structures.