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Single-stage fractionation of poly(ethylene-co-vinyl acetate) in supercritical ethylene with saft

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Abstract

Using the discrete thermodynamics approach, the single-stage fractionation of a polydisperse poly(ethylene-co-vinyl acetate) in supercritical ethylene and ethylene-vinyl acetate mixtures is modeled with an EOS rooted in statistical associating fluid theory (SAFT). The simulation results are compared to new high-pressure coexistence data and size-exclusion chromatography data obtained on a few selected extracts. The polymer molecular-weight distribution is optimally represented by ten nearly-monodisperse pseudocomponents, determined by a nonuniform lumping method. SAFT quantitatively captures the effect of pressure, temperature and solvent composition on the solvent capacity and the extract yield over a broad range of conditions. The ethylene capacity monotonically increases with increasing pressure between 200 bar and the cloud point pressure, with increasing VA concentration in the solvent mixture, and with increasing temperature above ∼ 480 bar. At intermediate pressure (200–480 bar), SAFT predicts that the ethylene capacity behaves nonmonotonically with temperature. The extract and raffinate molecular weight and polydispersity are predicted also from SAFT as a function of pressure and solvent composition for a typical bubble- and dew-point-type fractionation, thereby illustrating the underlying differences between those two phase disengagement mechanisms.

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