Deasphalting of heavy crude oil by hexamethyldisiloxane: The effect of a solvent/oil ratio on the structure, composition, and properties of precipitated asphaltenes

Abstract

The use of hexamethyldisiloxane (HMDSO) for deasphalting heavy crude oil allows precipitating several times more compounds from crude oil compared to aliphatic hydrocarbons such as pentane or heptane. The asphaltenic products were obtained by varying the HMDSO/crude oil ratio in the range from 1/1 to 40/1. The structure, composition, and properties of the resulting products were then analyzed by elemental analysis, infrared spectroscopy, matrix-assisted laser desorption/ionization, gradient chromatography, X-ray diffraction, differential scanning calorimetry, and rotational rheometry. It turned out that HMDSO precipitates not only heptane-insoluble asphaltenes from heavy crude oil but also other polyaromatic compounds as well as resins and paraffin waxes. The product yield rises with increasing the HMDSO/crude oil ratio and reaches a maximum at the ratio of 15/1 (the yield is 5 times higher than when using heptane); then, the yield decreases due to an increase in the content of saturated compounds in the precipitate and the transition of a part of the polyaromatic compounds and asphaltenes into the solution of maltenes. The molecular weight, heteroatom content, resin concentration, and other characteristics of the asphaltenic product also pass through a maximum when the HMDSO/crude oil ratio changes. Despite the variability of structure and composition, all the asphaltenic products when heated above 60 °C pass from the glass state to the liquid state characterized by viscoelasticity and non-Newtonian behavior. Temperature variation allows changing the viscosity of the asphaltenic product over a wide range, which makes it possible for its processing by different methods.