Effect of Fe3O4 Nanoparticle Size on the Performance of Magnetic Nanocomposite Pour Point Depressants

Waxy crude oils exhibit poor low-temperature flowability due to wax crystallization and the formation of a three-dimensional gel network, leading to operational challenges in production and transportation. In this work, a series of Fe3O4@ethylene–vinyl acetate (EVA) nanocomposite pour point depressants (F-NPPDs) with different Fe3O4 particle sizes were synthesized via hydrothermal preparation, surface modification, and solution blending. The effects of Fe3O4 particle size on the physicochemical properties, wax crystallization behavior, and low-temperature rheology of a 20 wt% wax model oil were systematically investigated. Results show that all F-NPPDs significantly improve flowability compared to neat EVA. Among the samples prepared, F2-NPPD exhibits the best overall performance, reducing the pour point to 20 °C and markedly decreasing apparent viscosity and gelation temperature. Rheological and DSC analyses confirm that F-NPPDs effectively lower wax crystallization temperature, weaken gel strength, and reduce both storage and loss moduli. Polarized light microscopy reveals that the addition of Fe3O4@EVA transforms wax crystals from dense needle-like structures into dispersed spherical or flower-like morphologies, thereby disrupting the interconnected wax network. The enhanced performance is primarily attributed to Fe3O4-induced heterogeneous nucleation, which increases nucleation density and refines wax crystal size. Particle size is identified as the dominant factor governing performance, while magnetic interactions play a secondary role. Overall, this study demonstrates that tailoring Fe3O4 particle size is an effective strategy to optimize EVA-based nanocomposite pour point depressants for the improved low-temperature flow assurance of waxy crude oils.