Exploring the Conversion Limits of Bitumen Visbreaking through a Molecular Reaction Model

In this work, a visbreaking reaction model was extended to represent a regime where coke formation becomes significant (>1 wt %) in order to better understand the conversion limits when the process is applied to partial upgrading of bitumen. The proposed model reconstructs the molecular composition of the feedstock and then simulates thermal cracking reactions at the molecular level using Monte Carlo algorithms. The model's reactivity parameters were calibrated against bitumen visbreaking data from a pilot plant. Structural and theoretical solubility criteria were implemented to identify hydrocarbon molecules in the feed and products as saturates, aromatics, resins, asphaltenes, and coke. Model simulations extending over the entire range of conversion (11-36%) obtained in the pilot plant enabled tracking changes in product composition and properties fairly accurately and were able to predict behaviors that characterize the visbreaking conversion limit, such as the formation of new asphaltenes above 20% conversion and the onset of coking at 26% conversion. In the second part of this work, the model was applied to study the effect of asphaltene removal from the feed without making changes to the original reaction model parameters. The model was proven to give reasonable predictions for the cracking of the deasphalted feedstock in terms of product distribution and new asphaltene formation in the conversion range tested experimentally (13-56%). However, the predictions for coke yields were not validated due to the absence of experimental data, and for this reason, the conversion limit of the deasphalted feedstock was not characterized in detail. The issues identified in the predictions for the deasphalted feedstock were largely attributed to insufficient analytical information on the feed to obtain a more precise representation of its composition and reaction chemistry.