Low Temperature Heat Capacity Measurement and Prediction as a Probe for Molecular Structure and Phase Transitions in Resids and Asphaltenes

Nasser Sallamie and J. M. Shaw

Department of Chemical and Materials Engineering

University of Alberta

Edmonton, Canada

Diverse average molecular structures have been proposed for asphaltenes and other ill-defined petroleum fractions obtained in some cases from the same sources and analyzed in a broadly similar manner. Proposed average structures range from pendant core models, characterized by a large central polynuclear aromatic + naphthenic sheet with alkane chains connected to it, to archipelago models, characterized by multiple but small polynuclear aromatic + naphthenic sheets interconnected with short alkane chains. A central and unresolved question in this literature is which of these two structural models more closely approaches the actual average structure for these ill-defined petroleum fractions. Further, the phase behavior of these fractions is a key building block for the development of processes for both production and refining. Heat capacity at either constant pressure or volume is a potentially powerful tool for probing molecular structures and phase transitions associated with ill-defined petroleum fractions because of the close relation between molecular structure, vibration spectra, and heat capacity - particularly for solids. For example, low temperature heat capacities can define a reference state for the calculation of phase transition enthalpies in these complex mixtures. In addition, recently reported low temperature heat capacity measurements for asphaltenes and resids, and asymptotic calculations suggest that the heat capacity of the more flexible archipelago molecular prototype to be significantly greater than the heat capacity of the pendant core molecular prototype. In this contribution, the heat capacity of the two molecular prototypes is predicted using standard statistical thermodynamic techniques based on vibration spectra computed using density functional theory. The computational methods and preliminary results concerning the suitability of these two molecular prototypes for both phase transition evaluation and the development of structural models for asphaltenes and resids are discussed.