A new approach for performing phase equilibrium studies with opaque hydrocarbon mixtures at elevated temperatures and pressures has been developed and tested. Using x-ray radiographic imaging, density differences as small as 40 ks/m$\sp3$ were distinguishable between two liquid phases in a hydrocarbon mixture. The phase behaviour of a series of Athabasca bitumen vacuum bottoms (ABVB) + n-dodecane + hydrogen mixtures were investigated. Complex phase behaviour and phase transitions were observed with some of the mixtures. Preliminary pressure-temperature phase diagrams for four mixtures of ABVB + n-dodecane + hydrogen were constructed along with a pressure-composition phase diagram at 674 K. Complex phase haviour arose with mixtures of the ABVB + n-dodecane + hydrogen and processed crude systems that had asphaltene contents greater than 12 wt.% and 797 K+ fractions greater than 30 wt.%
Samples of the "solid" phase which existed at low temperatures and which appeared at temperatures $>$650 K for one of the ABVB + n-dodecane + hydrogen mixtures were analyzed and it was found that the "solids" had H/C ratios of 1.3 and 0.85, aromatic contents of 57 and 47 with solubilities in toluene of 100 wt.% and 95 wt.%, respectively. Enthalpies of fusion, determined from the slopes of the pressure-temperature trajectories and a fugacity model, for the "solids" present at low temperatures were approximately 8 J/g based on a molecular weight of 5000 g/mol.
With the view cell apparatus it was also possible to make absolute fluid density measurements by rating the relationship between fluid density and intensity. Using these relationships, densities of the heavy oil-bitumen mixtures were determined at elevated temperatures.
These new findings of complex phase behaviour with heavy oil-bitumen mixtures were compared with current industrial heavy oil upgrading processes. These phase diagrams explain non operating practices and problems arising with these processes. For example, zones exhibiting complex phase behaviour are largely being avoided by many of the operating, low conversions and plugging of transfer lines in pressure let-down systems can be explained by the presence of complex phase behaviour.