(66e) Ultra Small-Angle X-Ray Scattering (USAXS) Assessment of Asphaltene Precipitation and Inhibitor Effectiveness

The mechanisms of asphaltene precipitation and inhibition remain unsolved despite nearly a century of focused investigation. Previous characterization approaches either study the flocculation and dynamics of asphaltenes >500 nm in size with optical microscopy or on length scales <50 nm using X-ray and neutron scattering approaches. Ultra-Small-Angle X-ray Scattering (USAXS) is a technique that has not been previously applied to precipitating asphaltene systems and provides valuable insight into the structure of soft systems on length scales from 1 nm to approx. 5 microns. The experimental results to be presented demonstrate the unique capabilities of USAXS to measure time-resolved values of size, molecular weight, and internal structure (i.e., fractal dimension) of precipitating asphaltenes. The results demonstrate that USAXS is a valuable new tool to assess asphaltene phase behavior. The experimental results have numerous applications in aggregation model validation, fundamental mechanistic insight, and inhibitor screening. Asphaltene precipitation with and without chemical inhibitors (nonylphenol and nonylphenol formaldehyde resin) were monitored by adding n-heptane to a toluene-diluted vacuum residue sample. Both of the inhibitors screened demonstrated an ability to slow the precipitation growth rate of the asphaltene clusters. The results demonstrate that one of the inhibitors (nonylphenol) more effectively reduces the rate of asphaltene precipitation. The experimental results provided by USAXS go beyond assessing the rate of growth and can also provide valuable insight into the mechanisms of inhibition. The internal structure of precipitating asphaltene clusters or aggregates can also be assessed with USAXS to monitor morphological changes as a result of solvent or inhibitor addition, providing valuable design and mechanistic insight.