Membrane Diffusion Measurements Do Not Detect Exchange between Asphaltene Aggregates and Solution Phase

Abstract

Although the aggregation of petroleum asphaltenes has been measured by a variety of methods, little is known about the rate of exchange between aggregated and dissolved components. This work studied the diffusion of asphaltenes from several heavy oils and bitumens in dilute toluene solutions using a stirred diffusion cell equipped with ultrafiltration membranes (Ultracel YM and Anopore). The pore sizes were varied between 3 and 20 nm to retain aggregates while allowing free molecules to diffuse. The permeate was continuously monitored using in situ UV−vis spectroscopy. These experiments demonstrated that the sizes of the asphaltene aggregates at a concentration of 1 g/L in toluene at 25 °C were between 5 and 9 nm and that rates of exchange of material between the aggregates and free solution were extremely low. An increase in the temperature results in an increase in asphaltene mobility but does not reduce the size of the asphaltene structures below 5 nm. Likewise, a decrease in the concentration to 0.1 g/L did not result in a decrease in size. The origin of the asphaltenes (Athabasca, Safaniya, and Venezuela) did not have a significant impact on the observed behavior; therefore, the above observations are widely applicable to C7 asphaltenes. The diffusion of vanadyl porphyrins (vanadyl meso-tetraphenylporphyrin, vanadyl octaethylporphyrin, and native petroporphyrins) in the presence of asphaltenes showed that the native petroporphyrins were larger than the model vanadyl porphyrins based on the appearance of hindered diffusion within smaller pores. An increase in the temperature resulted in an increase in petroporphyrin mobility as per the Stokes−Einstein relationship, although decreasing the asphaltene concentration did not. The mobility of the vanadyl petroporphyrins varied with the origin of the sample (Safaniya, Venezuela, and Athabasca) and is therefore not universal.