Study On Dispersion Of High-viscosity Fluid And Diffusion Characteristics Of Volatiles In High-Speed Disperser

Removal of volatiles(devolatilization)is an important step in the process of polymer production and processing.The removal ratio of volatile monomers,solvents and other impurities in the polymers has an important impact on the quality,performance and environmental friendliness of the product.High speed disperser is a kind of process intensification device that can be used for the devolatilization of high viscosity fluid.Under the huge centrifugal force and shear of rotor,high-viscosity fluid can be dispersed into liquid filaments,which can significantly increase the mass transfer area and surface renewal rate,so as to enhance the mass transfer in the devolatilization process.In this thesis,the diffusion coefficient of volatiles in the desorption process,the surface renewal of liquid filament in the high-speed disperser,the bubble growth in the process of foaming devolatilization were studied by experiment,numerical simulation and modelling.The behaviors of volatiles in each key stage of devolatilization process were described.The influence of environmental conditions(such as temperature)and material properties(such as volatile concentration,polymer viscosity,degree of polymerization,etc.)on diffusion coefficient for normal alkanes in PDMS by weight analysis,and we proposed an improved numerical analytical method in the calculation of instantaneous diffusion coefficient,and a model for the instantaneous diffusion coefficient was built.Molecular simulation was used to accurately calculate the self-diffusion coefficient of volatile molecules in the polymer,and it was found that PCFF field,Ewald non-bonding method and hyperfine precision can be good methods to describe the volatiles molecules diffusion in polymer.Experimental results and molecular simulation both show that the relationship between diffusion coefficient and temperature follows the laws of Arrhenius.Average diffusion coefficient of alkane volatile in PDMS increases with the increase of temperature,concentration of volatile,and decreases when increasing the polymer polymerization degree and viscosity.The smaller molecular weight is conducive to the migration and spread of the volatiles.Molecular polarity and intermolecular forces in homologue can also affect diffusion coefficient.Henry's coefficient is an important parameter in the process of mass transfer,which can indicate the distribution capacity and the migration direction of volatiles in the gas-liquid phase in devolatilization.Henry's coefficient of n-pentane,n-hexane,n-heptane,n-octane in PDMS polymer were studied by experiment,and then the influence of temperature,polymer viscosity,volatile species on the Henry's coefficient of volatiles was studied.Results show that the relation between Henry's coefficient and temperature is consistent with Arrhenius law,and Henry's coefficient of n-alkanes in polymer solution decreases with the increase of the molecular weight of volatiles,while polymer viscosity has little effect on the equilibrium partial pressure of volatiles.High-speed camera technology was used to capture the free-falling film process of high viscous fluid through hole structure and slot structure.The effect of physical properties,operating conditions and structure parameters were concerned on the the flow and membrane performance for high viscous fluid.A corresponding model of film area was built.This work provides a method for numerical simulation of dispersion and flow characteristics of liquid filament in high speed disperser.Based on the experiment and numerical simulation of falling film with static structure,the formation and stretching process of liquid filament in the high-speed disperser were studied by fluid visualization and VOF simulation,including the influence of rotor speed,fluid viscosity,surface tension on the width,residence time and velocity of liquid filament.The results show that the trajectory of liquid filament conforms to Archimedes spiral,and the width of liquid filament is inversely proportional to its viscosity.Based on the assumption that there is no backmixing during the stretching of liquid filament,the surface renewal-stretching(SRS)model of liquid filament is established by Lagrange method.Foam devolatilization is also an important stage in the process of volatiles removal.Volatiles in polymer solution enter bubbles through diffusion under in the vaccum environment,then the bubbles go through nucleation,growth,coalescence,and finally break on the gas-liquid interface.In this work,high speed camera was used to study the growth rate and residence time of single bubble after nucleation in the viscous liquid with different viscosity and under various pressures.The results show that the lower the absolute pressure of the system is,the faster the bubble grows.When the absolute pressure was less than 1 0kPa,the bubble growth rate increased significantly.The growth rate of bubble in the system with higher viscosity is slow.The growth rate of bubble in the solution at 5kPa was much higher than that of all the solutions at 15kPa.Based on the results of gravimetric experiment,fluid visualization experiment,molecular simulation and CFD numerical simulation,three models in the process of devolatilization in a high speed disperser were built,which are dynamic model of diffusion coefficient for volatiles in polymer,surface renewal model of liquid filament in high speed disperser,and concentration distribution model around the bubble in the liquid phase in the foaming devolatilization.These three models connect the basic data such as diffusion coefficient and Henry's coefficient and describe the process of mass-transfer by diffusion,surface renewal and bubble growth in the high-speed disperser,which can provide guidance for the structural design and optimization of devolatilization device for high-viscosity polymer solution.