A global investigation of phase equilibria using the Perturbed-Chain Statistical-Associating-Fluid-Theory (PC-SAFT) approach

摘要

The recently developed Perturbed-Chain Statistical Associating Fluid Theory(PC-SAFT) is investigated for a wide range of model parameters including theparameter m representing the chain length and the thermodynamic temperature Tand pressure p. This approach is based upon the first-order thermodynamicperturbation theory for chain molecules developed by Wertheim and Chapman etal. and includes dispersion interactions via the second-order perturbationtheory of Barker and Henderson. We systematically study a hierarchy of modelswhich are based on the PC-SAFT approach using analytical model calculations andMonte Carlo simulations. For one-component systems we find that the analyticalmodel in contrast to the simulation results exhibits two phase-separationregions in addition to the common gas-liquid coexistence region: One phaseseparation occurs at high density and low temperature. The second demixingtakes place at low density and high temperature where usually the ideal gasphase is expected in the phase diagram. These phenomena, which are referred toas "liquid-liquid" and "gas-gas" equilibria, give rise to multiple criticalpoints in one-component systems, as well as to critical end points (CEP) andequilibria of three fluid phases, which can usually be found in multicomponentmixtures only. Furthermore, it is shown that the "liquid-liquid" demixing inthis model is not a consequence of a "softened" repulsive interaction asassumed in the theoretical derivation of the model. Experimental data for themelt density of polybutadiene with molecular mass Mw=45000g/mol are correlatedhere using the PC-SAFT equation. It is shown that the discrepancies in modelingthe polymer density at ambient temperature and high pressure can be traced backto ...