Unusual retrograde condensation and asphaltene precipitation in a model heavy oil system.

A partial phase diagram for the system athabasca bitumen vacuum bottoms (ABVB) (24.6 wt. % / 2 mole %) + dodecane (73.8 wt. % / 47 mole %)+ hydrogen (1.6 wt % /51 mole %) was constructed in the temperature range 425 K to 725 K and the pressure range 2 MPa to 7 MPa using an X-ray view cell apparatus. This fluid system is shown to exhibit two phase L1V and three phase L1L2V phase behaviour over parts of this P-T region. The shape of the low temperature boundary between the L1V and L1L2V zones is characteristic of dilute asymmetric mixtures where a heavy liquid phase, L-2, appears then disappears within the light liquid phase, L-1, on isothermal compression. Such phase behaviour is referred to as unusual retrograde condensation and is of both practical and theoretical interest. Transitions between the multiphase regions were found to be reversible at temperatures less than 660 K, in all cases. At higher temperatures irreversible "asphaltene precipitation" arose within the L-2 phase. "Asphaltene precipitation" did not arise in the absence of the L-2 phase, i.e.: within the L1V region, even at temperatures in excess of 700 K. These data provide a strong link between "asphaltene precipitation" and multiphase behaviour, and demonstrate a physical rather than kinetic basis for asphaltene precipitation at elevated temperatures.