Correlating the fluid phase behaviour of polydisperse polyethylene solutions using the modified Sanchez-Lacombe equation of state
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AbstractThe objective of this research was to correlate the fluid phase behaviour of polyethylene solutions using pseudocomponents to explicitly account for the molar mass distribution of the polymer. Different parts of the molar mass distribution affect different parts of the phase diagrams of polyethylene solutions. Cloud point, bubble point, and liquid - liquid bubble point experiments were carried out on a system of industrial significance: linear low-density polyethylene (LLDPE) - n-hexane - ethylene. An alternate calibration method is explained for measuring the polyethylene molar mass distribution using size exclusion chromatography (SEC). The anchored quadrature technique is introduced for transforming a continuous molar mass distribution into a discrete number of pseudocomponents by fixing one of the average molar masses and the lightest and heaviest molar masses. An algorithm is presented for performing equilibrium calculations in two or more phases involving at least one solvent and one polymer compound. The algorithm uses a damped Newton-Raphson method to perform flash and phase boundary calculations. If an equation of state is of the appropriate type, there is only one independent pseudocomponent for each solvent and two independent pseudocomponents for each polymer. The entire list of pseudocomponents for each polymer is used only in specific parts of the algorithm to reduce computational times for polymers with many pseudocomponents. These modifications are also applied to spinodal and critical point calculations. The modified Sanchez-Lacombe (MSL) equation is presented with parameters relevant for calculating the phase diagrams of polyethylene solutions. Polyethylene parameters were calculated from both polymer density data and polymer - solvent cloud point data. Dew and bubble point boundaries were correlated to obtain binary interaction parameters for solvent- solvent systems. Polyethylene - solvent binary interaction parameters were found by correlating cloud point boundaries of those mixtures. The amount of branching in the polyethylene molecule can have a significant effect on the fluid phase behaviour of a polymer - solvent mixture and this effect is incorporated into the binary interaction parameters. The MSL equation can qualitatively reproduce the phase diagrams of most polyethylene - solvent systems.
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CitationKrenz, R. A. (2005). Correlating the fluid phase behaviour of polydisperse polyethylene solutions using the modified Sanchez-Lacombe equation of state (Unpublished doctoral thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/18715
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