The Stability of Visbroken Heavy Oil Against Asphaltene Precipitation

Abstract

Visbreaking is used in refinery and oilfield upgrading to reduce the viscosity of heavy oil (including bitumen) and residue feedstocks. However, if carried too far, this process can cause asphaltene precipitation and coke formation. A model that is able to predict the conditions at which the asphaltenes start to precipitate is required to design and optimize these thermal cracking processes. The Modified Regular Solution (MRS) approach has been widely used to model asphaltene precipitation from native and live oils (Tharanivasan, 2012, Akbarzadeh et al., 2005, and Akbarzadeh et al., 2005) and preliminary property correlations have been developed for solubility fractions from reacted oils (Yarranton et al., 2018, Powers et al., 2016). The goal of this thesis is to complete these correlations and apply the MRS model to asphaltene precipitation from visbroken heavy oils. An Athabasca bitumen was thermally cracked in a visbreaker pilot plant at five different temperatures and residence times. The thermal conversion for each visbroken product was calculated based on SimDist data. The feed and each product were characterized into 5 pseudo-components (distillates, saturates, aromatics, resins, and asphaltenes) using distillation and SARA assays. Properties required for the model input (molecular weight, density, and solubility parameters) were measured or estimated for each pseudo-component. New correlations were proposed for the distillate fractions as a function of conversion and the existing correlations for the SARA fractions were updated. The model inputs are then the feedstock composition, the feedstock pseudo-component properties, and conversion. The RSM model was able to match the measured asphaltene precipitation yields from visbroken heavy residues and visbroken whole oils with average absolute deviations of 1.2 wt% and 0.4 wt%, respectively. At this point, the model is valid for Western Canadian heavy oils derived from the same source oil as the sample used for this thesis.