Characterization of asphaltenes precipitated with three light alkanes under different experimental conditions

Asphaltene precipitation plays an important role in both oil production and refining processes. In this paper, asphaltenes are precipitated from a heavy oil sample under different experimental conditions by using three different light alkanes, i.e., propane, n-pentane, and n-heptane. A variety of analytical techniques are applied to characterize the precipitated asphaltenes and deasphalted heavy oil (i.e., maltenes), such as the density and viscosity measurements, vapour-pressure osmometry, freezing-point osmometry, scanning electron microscope (SEM) imaging, nuclear magnetic resonance (NMR) measurement, and simulated distillation for compositional analysis. It is found that the yields and properties of the precipitated asphaltenes and remaining maltenes strongly depend on the specific precipitant tested and the liquid precipitant-to-oil volume ratio used. The asphaltene yield decreases as the carbon number of an alkane increases, while it increases monotonically and finally reaches a plateau if the liquid precipitant-to-oil volume ratio increases up to 20–40 for n-pentane and n-heptane, respectively. As a result, n-heptane-precipitated asphaltenes (C₇-asphaltenes) have the highest molecular weight and aromaticity among the three kinds of precipitated asphaltenes. C₇-asphaltenes are bright and black particles, whereas n-pentane-precipitated asphaltenes (C₅-asphaltenes) are dull and brown powders. Propane-precipitated asphaltenes (C₃-asphaltenes) together with some amount of co-precipitated resins are found to be highly viscous and semi-solid like immediately after the flashed-off process but become more and more liquid-like afterward. Compositional analysis results of the original heavy crude oil and three different maltenes indicate that the carbon numbers of most precipitated asphaltenes are higher than C₅₀.