Visbreaking of Vacuum Residue Deasphalted Oil: New Asphaltenes Formation

The formation of new heavier material during thermal processing has long been known, and under typical visbreaking conditions, vacuum conversion of deasphalted oil is described using a first-order kinetic reaction. Using the equivalent residence time (ERT) assumption, the residence time and reaction temperature are interchangeable variables to achieve the same conversion, where conversion is defined as the decrease of the vacuum residue through its conversion to lighter boiling fractions. With the combination of these observations about the visbreaking process, the following research question was raised: would process conditions that, in principle, lead to the same conversion also lead to the same asphaltenes content in the final product? The answer to this question was evaluated in this work, where vacuum residue deasphalted oil was submitted to visbreaking. Isoconversion conditions were obtained by the ERT concept. As expected, an increase in n-pentane-insoluble material was obtained at all process conditions studied. The kinetics of asphaltenes formation was different to the kinetics of vacuum residue conversion because the amount of asphaltenes formed were different at the same conversion and vice versa. Subsequent analysis of the new n-pentane-insoluble material revealed that asphaltenes were participating in hydrogen transfer reactions. The amount of hydrogen transferred during conversion at 438 degrees C were around 20 mg of H/g of asphaltenes in the product. Free radicals were formed as a consequence of hydrogen transfer reactions, and their recombination resulted in the formation of heavier material, which was reflected in an increase in n-pentane-insoluble material during visbreaking. Further investigation revealed that the free radicals remained reactive upon storage, which resulted in a 9 wt % increase in asphaltenes after 210 days of storage under nitrogen. Results also showed that isoconversion was not achieved as predicted by the ERT definition and the differences between predicted and experimental values are discussed on the basis of the assumptions to calculate ERT.