Asphaltene fractionation based on adsorption onto calcium carbonate: Part 3. Effect of asphaltenes on wax crystallization

Abstract

Asphaltenes fractionation procedures were developed and tested in two preceding articles (doi: https://doi.org/10.1016/j.colsurfa.2016.02.011, doi: https://doi.org/10.1016/j.colsurfa.2016.11.035), and asphaltene fractions were characterized in terms of composition, adsorption and self-aggregation properties. In this article, the effect of asphaltene fraction on wax crystallization was studied. Asphaltene fractionation on calcium carbonate was developed further to a column separation procedure, which yielded asphaltene fractions with even larger differences according to FTIR analysis. Wax crystallization was studied using differential scanning calorimetry (DSC), cross-polarized microscopy (CPM), rheometry, and pour point tests (PPT). Asphaltenes were found to change wax crystal morphology and decrease gel yield strength with increasing concentration. The effect on wax appearance temperature (WAT) was generally less. The greatest effect on WAT and pour point was made by bulk asphaltenes, which is the lowest polarity fraction from separation on calcium carbonate. Irreversibly adsorbed asphaltenes, containing significant amounts of carbonyl, carboxylic or derivative groups, induced the largest changes in wax crystal morphology. Still, high pour point and gel yield strength indicated low performance for this fraction. Overall, wax inhibition performance tended to be best for asphaltene fractions with low solubility and mixed functional groups. In isothermal titration calorimetry (ITC), unfractionated whole asphaltenes interacted more strongly with crystallized wax than asphaltene fractions bulk and irreversibly adsorbed. The findings indicate that wax crystallization is influenced the most by an ensemble of asphaltenes with different functional groups, rather than groups with more distinct properties.