Fractionation of Oligomers by a Supercritical Fluid Column-Elution Method and a Simulation of Elution Behavior

Abstract

Supercritical fluids of carbon dioxide or ethylene, with tetrahydrofuran (THF) added in a stepwise manner, are used as eluents in a column-elution fractionation method to separate styrene-maleic anhydride coplymers. The column pressure is kept at 20 MPa for all experiments. The SC-C₂H₄/THF eluent is found to be superior to SC-CO₂/THF eluent for molecular weight fractionation since the solubility of oligomers in SC-C₂H₄/THF eluent decreases in the order of molecular weight. Sharper fractionation is obtained at 90°C than at 50°C. The condition of equilibrium desorption at the column outlet was investigated by using a model of mass transfer from the stationary phase; the partition coefficients K's between supercritical fluid and polymer-gel are determined. The experimental K's are correlated in terms of the separation factor σ. The elution fractions of each oligomer, calculated from the mass-transfer model by evaluating the overall mass-transfer coefficients from the diffusivity in the supercritical fluids, agree with experimental elution fractions, which will help in determining the optimum condition for oligomers separation.