Ethylene–Vinyl Acetate Copolymer and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 2. Effect of Asphaltene Content

In part 1 (10.1021/acs.energyfuels.7b03657), the synergistic effect of ethylene–vinyl acetate copolymer (EVA) pour point depressant (PPD) and rensin-stabilized asphaltenes on improving the flowability of synthetic waxy oil has been verified. This paper is a continuous work studying the effect of the asphaltene content (0.01–3 wt %) on the synergistic effect between EVA PPD and resin-stabilized asphaltenes. The results showed that, in the absence of EVA and with the increase of the asphaltene content, the precipitated wax crystals of the waxy oil tend to grow gradually from initial big needle-like to smaller and more regular (spherical-like) particles with a larger amount; therefore, adding aphaltenes can only decrease the apparent viscosity of waxy oil at the temperature range slightly lower than the wax precipitation temperature (WPT) (the precipitated wax crystal amount is low), and the temperature range is broadened by increasing the asphaltene content. When the temperature is decreased far below the WPT of the oil, however, the apparent viscosity of oil rises up with increasing the aphaltene content as a result of the large amount of wax crystals and asphaltenes. In addition, only a part of the asphaltenes participates in the wax precipitation process, and the rest of the asphaltenes disperses in the oil phase as asphaltene aggregates, which could adhere or adsorb on the existing wax crystal flocs, strengthening the interactions between wax flocs. After asphaltenes are added together with EVA, EVA molecules can adsorb onto the asphaltene aggregates to generate the formation of the EVA/asphaltene composite particles, and the synergistic effect of the EVA/asphaltene composite particles on the flowability of waxy oil improves first with the increase of the asphaltene content and then somewhat deteriorates at a higher asphaltene content (3 wt %). When the asphaltene content is low, the wax crystal modification by the composite particles is insufficient and the formed large wax flocs have a very loose structure, which favor the wax crystal structure building. When the asphaltene content is too high (3 wt %), EVA/asphaltene composite particles disperse the precipitated wax flocs into relatively small spherical-like wax flocs with a larger amount. Although the structure of the wax flocs is compact, the large amounts of wax flocs and asphaltene aggregates in the oil phase lead to somewhat deterioration of the synergistic performance of EVA and asphaltenes. At the middle contents of asphaltenes (0.75–1.5 wt %), EVA/asphaltene composite particles cause the formation of relatively large spherical-like wax flocs with a compact structure and the asphaltene content is moderate, both of which greatly promote the flow behavior improvement of the oil.