Can a Cubic Equation of State Model Bitumen–Solvent Phase Behavior?

Abstract

Cubic equations of state (CEoS), such as the advanced Peng−Robinson (APR) EoS, are convenient for use in commercial simulators and have successfully fit saturation pressures and asphaltene onset points for bitumen−solvent systems using simple quadratic mixing rules. However, this approach does not accurately predict asphaltene precipitation yields. In this study, the APR EoS with several sets of asymmetric mixing rules is evaluated against saturation pressure and asphaltene yield data for n-pentane diluted bitumen. The asymmetric van der Waals, Sandoval et al., and two forms of Huron−Vidal mixing rules with an NRTL (non-random liquid theory) activity coefficient model are considered. The use of asymmetric mixing rules significantly improves the match to asphaltene yield data; however, the yields are still underpredicted at high solvent contents, and the tuning parameters that give the best match for asphaltene yield data are not predictive or easily correlated for other solvents. The APR EoS with symmetric van der Waals mixing rules is also evaluated with compositionally dependent binary interaction parameters. The use of compositionally dependent solvent/asphaltene binary interaction parameters allows the model to fit asphaltene yield data over the entire composition range. A set of interaction parameters is recommended that fits both asphaltene yield and saturation pressure data. The merits of this methodology as a practical option for modeling heavy oil−solvent behavior are discussed.

The Phase Behavior of Heavy Oil and Propane Mixtures

“…The P-X diagrams are qualitatively similar to those obtained for n-pentane diluted bitumen (Johnston et al, 2017b). The saturation pressures are higher for the relatively more volatile propane.…”

“…Symmetric mixing rules are unable to adequately represent the phase behavior of mixtures involving species with significantly different polarities (Castellanos-Diaz et al, 2011;Agrawal et al, 2012). For these mixtures, asymmetric van der Waals mixing rules, can be employed to better match their highly non-ideal phase behavior of mixtures (Johnston et al, 2017b). The simplest asymmetric mixing rules have two distinct…”

“…This deficiency could not be overcome with the binary interaction parameters used in this study. Johnston et al, (2017b) evaluated this CEoS approach with several sets of asymmetric mixing rules to model n-pentane diluted bitumen systems. The match to the asphaltene yield data was significantly improved; however, the yields at high dilution were under-predicted.…”

“…The non-associating species (approximated as maltenes) and the associating species (approximated as C5-asphaltenes) were characterized separately. The maltenes fraction was characterized following the methodology recommended by Agrawal et al (2012), and the C5asphaltene fraction characterization was lumped into a single pseudo-component as recommended by Johnston et al (2017b). The bitumen characterization was performed on the WC-B-B3 bitumen.…”

“…If the model was tuned to match the yield, the predicted propane content at the onset would be much lower than the measured value. The same problem occurred when using a symmetric CEoS to model asphaltene precipitation for n-pentane diluted bitumen (Agrawal et al, 2012;Johnston et al, 2017b). Johnston et al, (2017b) developed composition dependent binary interaction parameters to address this issue.…”