Study On Phase Equilibrium Of CO₂-1,3-PDO-PTT Ternary System

Poly（trimethylene terephthalate）（PTT）as a new polymer material with high functionality and wide application,however in the production process,a variety of small molecules volatile impurities residues in the PTT inevitably,such as monomers,solvents and by-products,these volatiles are not only bring harms to the environment and users,and it has a certain influence on the performance and molecular weight of the polymer.Conventionally,the volatiles are removed from polymers by vacuum removal that it’s relies on latent heat,this method has high energy consumption and easy to cause side reactions,and in the later stage of devolatilization,the devolatilization efficiency is low and the polycondensation products contain more residues because of the system viscosity is higher,so it can not to meet the increasingly stringent requirements of health and hygiene.Devolatilization of polymer system based on supercritical fluid（SCF）technology can effectively solve the above problems.However,in order to design and optimize a polymer devolatilization process with supercritical fluid,it is necessary to study the phase equilibrium for SCF-solute-polymer ternary system.In the process of supercritical carbon dioxide（SCCO₂）enhanced polymer devolatilization,the distribution coefficients of phase equilibrium is the basis of the seperation for the polymerization system with SCCO₂,thus,it is very important to obtain the distribution coefficients based on experimental method for removing volatile by SCCO₂ in the process of polymer devolatilization.In this study,the supercritical high viscosity melt polycondensation/devolatilization apparatus based on a static-analytic method was used to measure the phase equilibrium of carbon dioxide（CO₂）-1,3-Propanediol（1,3-PDO）-Poly（trimethylene terephthalate）（PTT）ternary system at temperatures between（313.15 and353.15）K and pressure from 7 to 19 MPa,and the composition data of fluid phaseequilibrium in the ternary system were determined,the distribution coefficients of1,3-PDO between the CO₂ and PTT phases were calculated.In addition,the effects of temperature and pressure on phase equilibrium were investigated,the experimental results show:（1）When the pressure is a constant,the distribution coefficient increases with the increase of temperature;（2）When T=313.15 K,the distribution coefficient decreases with the increase of pressure;When T>313.15 K,the distribution coefficient increases first and then decreases with the increase of pressure.Finally,the Sanchez-Lacombe（S-L）equation of state is used to optimize the experimental results and obtained the unknown parameters of the model,thus the distribution coefficient of 1,3-PDO in the two phases were calculated.The results of correlation analysis show that the calculated values of the S-L state equation are in good agreement with the experimental results.