Structure of Asphaltenes during Precipitation Investigated by Ultra-Small-Angle X-ray Scattering

Abstract

Time-resolved size and structure measurements of asphaltenes while in the process of precipitating were monitored for the first time using ultra-small-angle X-ray scattering. The results revealed that asphaltenes precipitating from a heptane-toluene mixture demonstrate a hierarchical structure of an asphaltene-rich phase (e.g., droplet) that further agglomerates into fractal flocs. The fractal flocs that form by the agglomeration of the asphaltene-rich phase are what is commonly detected by optical microscopy above the precipitation detection point. The surface of the asphaltene-rich phase is initially rough and transitions to a smooth interface, as would be expected for a highly viscous liquid. Simultaneous small-angle X-ray scattering measurements were also performed to investigate the structure of soluble asphaltenes, providing comprehensive structural characterization from the nanometer- to micrometer-length scales as a function of time. Further, the results demonstrate that the size and concentration of asphaltenes remaining in solution (e.g., soluble asphaltenes) do not change during precipitation, whereas the structure of insoluble asphaltenes varies. The ability to measure the properties of asphaltenes as they undergo precipitation opens new opportunities for understanding the fundamental mechanisms of asphaltene deposition and aggregation and the impact of chemical inhibitors to alter these processes. The universality of these conclusions and how specific properties vary as a function of asphaltene source and solution properties can provide valuable insight into asphaltene behavior.

Distributed Properties of Asphaltene Nanoaggregates in Crude Oils: A Review

“…This process is often described as precipitation or flocculation because the second phase that forms is a powdered solid, and the radius and morphology of the solid particles can be modeled through flocculation kinetics. − Once asphaltene separation begins, the formation of particles of dimensions from 200 nm to 2 μm is almost instantaneous. These primary particles then flocculate to give porous particles with mean diameters up to 400 μm depending on the concentration and shear forces. − Detailed studies of asphaltene precipitation show that the kinetics of the primary particle formation are extremely fast, with a rapid increase in dimension from aggregated molecules ca. 5–20 nm to primary particles of diameters up to 400 μm in a fraction of one second .…”

Fractionation and Characterization of Petroleum Asphaltene: Focus on Metalopetroleomics

“…This technique covers the gap in the characterization capabilities of asphaltenes over the length scales between 50 and 500 nm, which has potentially severe consequences [124]. Yang et al [125] monitored the process of asphaltenes precipitating with USAXS for the first time, and results revealed that asphaltenes precipitating in a heptane−toluene mixture show a hierarchical structure of an asphaltene-rich phase and further agglomerate into fractal flocs. In addition, the size and concentration of soluble asphaltenes do not change during precipitation, whereas the structure of insoluble asphaltenes varies.…”

Nanoscale Investigation of Asphaltene Deposition under Capillary Flow Conditions

“…The addition of heptane to toluene solvent destabilized asphaltenes and triggered their fast aggregation in a shorter period of time. 41 SEM images of deposits from 100 ppm of asphaltene in heptol solutions revealed a large accumulation of particles in the vicinity of the onset of precipitation (i.e., 50:50 H/T), with relatively small amounts at low and high heptane dilution (Figure 8). Analysis of the segmented images is presented in Figures 9 and 10, where a peak in the average size and number and mass densities was obtained with an increasing heptane percent.…”