Molecular Weight and Specific Gravity Distributions for Athabasca and Cold Lake Bitumens and Their Saturate, Aromatic, Resin, and Asphaltene Fractions

Abstract

Molecular weight and specific gravity distribution data are required for characterizing oils containing complex mixtures, and this characterization information is very essential for the computation of thermodynamic properties and phase equilibria. The accuracy of these computations will be enhanced if molecular weight and specific gravity data of fractions containing similar groups/structures or common solubility properties are used. This is because the critical properties normally correlate better for a single fraction than for the whole oil. Data in this work is relevant to the phase equilibrium calculations and predictions of asphaltene precipitation from representative Canadian bitumens. Athabasca and Cold Lake bitumen samples were used and divided into asphaltene and deasphalted oil fractions by adding 40 volumes of n-heptane. The deasphalted oils were divided into saturate, aromatic, and resin fractions using a modified ASTM D2007 procedure. The average molecular weights of these SARA fractions were measured using vapor pressure osmometry (VPO), and the molecular weight distributions of the SARA fractions were measured using gel permeation chromatography (GPC) calibrated with polystyrene standards. Results were verified using VPO measurements, and the correction factors for the GPC distributions were calculated. The specific gravities of saturate and aromatic fractions were measured using an Anton-Paar densitometer, and resin fraction values were obtained using a water pycnometer and those of asphaltenes using a helium pycnometer. Specific gravity distributions were computed using the measured data and the correlation reported in the literature.