Phase Behaviour of Mixtures of Heavy Oil and n-Butane

Abstract

Solvent-based methods are a potential alternative to thermal methods for the recovery of heavy oils because they are less water and energy intensive. n-Butane is a solvent of interest for these processes because it has a saturation pressure that is close to the operating (reservoir) pressure of many heavy oil reservoirs. Operating at a pressure near the saturation pressure maximizes the solubility of the solvent in the oil while avoiding the formation of a second liquid phase. To design these processes, it is first necessary to predict the phase behaviour but there are few data available in the literature upon which to base a prediction. The phase behaviour of heavy oil and n-butane mixtures was examined at temperatures up to 230°C and pressures up to 10 MPa. Both vapour-liquid and liquid-liquid regions were observed. The amount of the heavy pitch phase was measured at temperatures from 20 to 180°C and pressures up to 10 MPa. The liquid phase composition (in terms of C5-asphaltene, maltenes, and n-butane) were measured at 130°C and 10 MPa. A ternary diagram was constructed for the liquid-liquid region and equilibrium ratios (K) were determined and used to confirm the consistency of the data. The phase behaviour data for mixtures of n-butane and bitumen were consistent with the trends in the phase boundaries and masses versus carbon number observed with propane (Mancilla-Polanco et al., 2018) and n-pentane diluted bitumen (Johnston et al., 2017b). The Peng-Robinson Equation of State was used to match the data by adjusting the binary interaction parameters with two approaches: 1) temperature-dependent parameters (TDvdW model); 2) composition-dependent parameters (CDvdW model). The TDvdW model matched both the vapour-liquid and liquid-liquid boundaries to within the uncertainty of the measurement but significantly under-predicted the heavy phase masses. The CDvdW model matched not only the phase boundaries but also the phase masses and compositions generally to within the experimental error. The model deviations increased above the critical temperature of n-butane.