Colloidal structures of vacuum residua and their thermal stability in terms of saturate, aromatic, resin and asphaltene composition

The physical structure of vacuum residua on the micron scale was unveiled through the freeze-fracture replication transmission electron microscopy. It was confirmed that vacuum residua assume colloid structures, in which asphaltenes co-aggregate with the heavier resins of higher polarity and form mixed micelle, and the remaining fractions including the lighter resins, aromatics and saturates act as the dispersing media. Based on the equilibrium distribution of resins on the mixed micelle and in the dispersing media, a stability function characterizing vacuum residuum colloids was derived. When applying this function to correlate the onset points of the neophase identified by the optical microscopy and reaffirmed by the transmission electron microscopy of the real-time samples from the thermal reaction systems of vacuum residua, an expression representing the colloidal stability of vacuum residua and their SARA (the acronym of saturates, aromatics, resins and asphaltenes) compositions was developed. This expression quantitatively elucidated the respective roles played by the SARA constituents in keeping the colloidal stability of vacuum residua.