New Formula Of Binary Interaction Parameter And Its Application In Vinyl Chloride/Carbon Dioxide System

The complicated post-treatment processes of vinyl chloride (VC) suspension polymerization resulted in the pollution of waste air and water, while VC bulk polymerization possesses the disadvantage of high viscosity, difficult agitation, difficult heat-removing and inconvenient control of resin qualities. Aiming to overcome the above problems, VC polymerization using supercritical carbon dioxide (SC-CO2) as the reaction medium was proposed in this thesis. New theoretical formulas for the binary interaction parameters of cubic equation of state (EoS) were deduced and used to predict the phase behavior of VC/CO2 mixture, and the laws of VC polymerization in SC-CO2 were investigated.The complex fluids, such as CO2/VC binary system, are not in accord with two parameters corresponding states principle. According to Mie theory and London dispersion forces theory, and with the introduction of the molecular shape factors (θ,(?)) basing on Joffee-Stewart's pseudocritical rule and keeping ionization potentialterm, the general formula of binary interaction parameter for the attraction force parameter of cubic EoS was obtained as follows:Mie potential constant (n) was set to 3 empirically and the general formula could be simplified as follows:When θ_ij and (?)_ij were respectively considered the arithmetical mean of θ and (?),the simplified formula can be used to evaluate the binary interaction parameters of the attraction force parameter of cubic EoS without the back-calculation. When thesimplified formula was used in the Peng-Robinson EoS to calculate the critical pressure of binary mixture, the average absolute deviation was 3.385%.Based on Coutinho's deduction and with the introduction of Leland's molecular shape factors, the new formula for binary interaction parameters of repulsive term of SRK EOS was obtained as follows:+bJWhen the potential constant n was set to be 6 according to Lennard-Jones potential theory, a new formula for binary interaction parameters of the attractive term of SRK EoS was obtained as follows:The satisfied results were achieved as the binary interaction parameter formulas of SRK EoS were used to calculate the critical pressures of mixtures. The most advantage of binary interaction parameter formulas of SRK EoS is that they can be used to evaluate the binary interaction parameters of the attractive term and therepulsive term without the back-calculation.A new method was proposed to measure the bubble point, dew point and criticalpoint of mixture by using the constant-volume vessel with a window. The proposed measurement method was applied to CO2/VC binary system at 22—97°C and with CO2 molar fraction from 0.377 to 0.964. The PR EoS and SRK EoS with their relevant formulas for the binary interaction parameters were used to fit the experimental critical pressures. The average absolute deviations of critical pressure was 1.905% and 1.471%, respectively.During the polymerization of VC in SC-CO2, the polymerization mixture wasinitially homogeneous, composed of CO2, monomer and initiator. Shortly after the starting of polymerization, polyvinyl chloride (PVC) chains exceed the critical length for dissolution, the phase separation occurred. CC^-rich phase and PVC-rich phase were existed till the end of polymerization.Assuming that the polymerization system pressure is constant, the volume variation during the polymerization could be depicted as follows:dv/ -/ / dAVy AVydt /dt+ /dt+ /dtAccording to the phase behavior and the pressure variation, the polymerization was divided into four stages. The first stage was the induction period and pressure was kept at a constant. In the second stage, pressure began to the pressure dropV (r -\)dxF / mainly resulted from m v ' m'/, < 0. During the third stage, the pressure startedto go up to a pressure plateau after a short period of pressure drop. During the last stage, the pressure decreased till the end of polymerization and the monomer in CO2-rich continue phase was totally consumed.Aiming to obtain PVC resins with high conversions, the effect of process conditions, such as the feeding way of the initiator, system pressure and agitation on the polymerization conversion was studied. The results showed that the polymerization rate, conversion and molecular weight decreased with the increase of pressure. The polymerization conversion substantially decreased in the presence of efficient stirring. The supplement of initiator during polymerization can significantly increase the conversion of the precipitation polymerization of VC in SC-CO2. The PVC powders prepared in SC-CO2 were composed by the aggregates of primary particles with no skin, which was similar to PVC resin prepared by the bulk polymerization. The above results offered the base for the industrial production ofPVC resin using SC-CO2 as the reaction medium.