Measurement of phase transition, density and viscosity of supercritical carbon dioxide-Fischer-Tropsch wax mixtures

Abstract

Melting temperature, phase behaviour and densities of binary mixtures of CO₂ and three Fischer-Tropsch waxes with varying molecular weights were experimentally determined. The melting temperatures of the lower molecular weight waxes increased with CO₂ pressure, and pressure induced crystallisation of the lowest molecular weight wax occurred above 20 MPa. CO₂ solubility in the waxes decreased with increasing wax molecular weight. Trends in mixture densities with changes in temperature and pressure mimicked that of pure CO₂. The viscosity of the lowest molecular weight wax decreased with increased CO₂ concentration, and decreased with increases in temperature and pressure, with the impact of pressure minimised above the temperature inversion point. Solubility data were correlated with a modified Chrastil and the Mendez-Santiago & Teja models. The Chrastil model accurately predicted solubility of CO₂ in all three waxes to within 1 % of the measured values.