| The objective of the research described in this thesis is to provide fundamental data for the development of supercritical fluid assisted polymer technology, especially the devolatilization of polymers, by detailed investigation on the phase behaviour and mass transfer in the systems of styrene/polystyrene/CO2. This thesis is composed by the literature review, experimental investigation, and theoretical development, etc.In the literature review, starting from the description of the characteristics and advantages of supercritical fluid (SCF) and related processes, the progresses in SCF technology were described. The concerns in the research on thermodynamics (phase equilibrium) and mass transfer were pointed out. The experimental and modeling methods in phase equilibrium and mass transfer for the solute (volatile)/SC-CO2/polymer ternary system were also analyzed and evaluated.In the experimental section, the phase behaviour of a solute (volatile)/SC-CO2/polymer ternary system and the distribution coefficients of solute between supercritical fluid phase and polymer phase were investigated. A semi-static method is used to determine the composition of supercritical fluid phase for a styrene/CO2/polystyrene ternary system at 318 K and 328 K over the pressure range of 12 to 20 MPa. The Sanchez-Lacombe model is used to correlate the experimental results, and the model parameters were obtained by optimizing the experimental data. Therefore, the distribution coefficients of styrene between two phases were calculated.Furthermore, the Sanchez-Lacombe was used to predict the phase behaviour for a solute/CO2/polymer ternary system at wide range of temperatures, pressures and concentrations. The effects of temperature, pressure and the model parameters on the triangle phase diagram of the system and the distribution coefficients of solute were analyzed by computational simulation, which provides the thermodynamics data for the design of devolatilization of polymers using supercritical fluid.As an inspiring spark in the section for theoretical development, a novel method was originated to describe the mass transfer of small molecules in swelling polymers by proposing a new coordinate system, which enables the analysis of mass transfer in swelling polymer-SCF systems to be quantitatively. The molecules of SCF have strong permeability and powerful swelling ability, and the polymer will be swollen with time in SCF atmosphere, so that the volume of the polymer will change with time. Furthermore, the diffusion coefficients of small molecules in polymers strongly depend on their concentration, which makes it very difficult to model the diffusion of small molecules in such systems. To overcome those difficulties, the size evolution of the pure polymer involved was taken into account, and the new swelling coordinate system was constructed. Under this coordinate system, the model of diffusion for solute and supercritical CO2 in polymer particles was derived as follows:The new definition of the polymer dimension solved the problem of convection term in diffusion equation caused by the swelling of polymer. The effects of the saturation concentration of CO2 in polymer and the ratio of diffusion coefficient of CO2 to that of solute when the concentration is infinite (D02/A01) on the concentration profiles inpolymer and the integral curves were also analyzed by computational simulation, which provide us the mass transfer information in devolatilization process.The devolatilization rate in diffusion mechanism is controlled by two factors. One is the rate of solute diffusion, and the other one is phase equilibrium. In general, the diffusion coefficient of solute in SCF is larger than that in the polymer. So the barrier of the mass transfer is mostly in polymer phase. By combining the continuity equation of solute in devolatilization cellwith the equation of diffusion in polymer particle, the model for devolatilization process has been obtained. The effects of the flow rate of SCF on the solute c...
|
PDF Full Text Request