Asphaltenes are the heaviest and most polar components of crude oils and they can precipitate during the production and processing of crude oil, increasing the risk of deposition and fouling. For example, during refining when native and reacted streams are blended and the blend can be unstable versus asphaltene precipitation. Simple and reliable methods are needed to characterize these streams and to predict the conditions at which precipitation occurs.
One approach for predicting the stability of these blends is the Modified Regular Solution Model (RSM) which has been successfully applied to model asphaltene precipitation in upstream fluids and their blends. Input parameters to the regular solution model are the mole fractions, molar volume, and solubility parameters of the pseudo-components making up the crude oil and the n-alkane used in the mixture. Few data are available for some of these pseudo-components such as saturates, aromatics, and resins (SAR). Also, the effect of feedstock processing such as thermal cracking and hydrocracking on the component properties is not known.
The main objective of this work is to characterize the non-distillable (SAR) fractions of native and reacted fluids and develop property correlations as inputs to the RSM. Molecular weights, density and refractive index were measured for saturates, aromatics and resins whereas the solubility parameter of saturates and aromatics were back-calculated from fitting asphaltene solubility measurement in these solvents with the Modified Regular Solution Model. In addition, property correlations between the model inputs and the refractive index were sought because it is a reliable indicator property, easy to measure, and proven to relate to valuable properties.
The SAR fractions were found to form non-ideal solutions in the solvents used in this study, toluene and heptane. Excess volume mixing rules for the density and refractive index of SAR fractions and solvents were experimentally identified. A correlation was developed for the binary interaction parameter that quantifies the excess properties. This correlation was used to determine the density and refractive index of samples for which a direct measurement was not possible (solid, high viscosity, limited sample volume) and only mixture data were available. The refractive index was found to correlate to density of these fractions even at elevated temperatures. Although cracking changed the properties of the SAR fractions, the correlations applied to both native and reacted materials.
Asphaltene solubility was measured in solutions containing toluene, heptane, saturates or aromatics. New methods were developed to measure asphaltene precipitation yields in low volume solutions containing a SAR fraction. Solubility parameters of the SAR fractions were calculated from RSM modeling of asphaltene yield data from these solutions. The back-calculated solubility parameters were correlated to both refractive index and density for the unreacted and thermally cracked oil fractions.
Overall, a set of methods were developed to determine the molecular weight, density, refractive index, and solubility parameter of limited amounts of SAR fractions from native and reacted streams. A dataset was collected for a variety of samples and a set of correlations was developed to determine these properties from a limited set of measurements. The data and correlations are a key part of the characterization of refinery blends for regular solution modeling.