Geometric confinement growth of wax crystals by star acrylate polymer for enhanced cold flowability of waxy crudes

Wax precipitation causes numerous challenges in the transportation and refining of crude oil. Wax deposition can be decreased and the flowability of waxy crude oil in cold temperatures can be increased by addition of polymer additives in the crude oil. Sequentially well-structured polymers synthesized by reversibly additive fragment chain transfer (RAFT) polymerization enhance the capability to inhibit the growing of wax crystals. In this paper, star chain-transfer agents (star-CTA) was synthesized by esterifying polyol with a tri-thiocarbonate RAFT reagent. Then, poly octadecyl acrylate-maleic anhydride (POM) copolymers with linear and star structure (3, 4, 6 arms) were synthesized via RAFT polymerization. The chemical structures were then characterized using FT-IR, 1 H NMR and GPC approaches. The effects of star copolymers on wax crystallization and cold flowability were analyzed by DSC, XRD, polarized light microscopy and rheology. The transformation from lamellar to fibrous crystals and a decrease in fractal dimensions of crystal growth upon adding all polymers were observed through kinetics of isothermal crystallization and non-isothermal crystallization (NIC) using DSC. Upon further calculations, the crystallization activation energy of NIC increased from 34.40 kJ/mol to 63.22 kJ/mol with the increase of arms. Revealed by XRD, both of the relative crystallinity index and the grain size of crystals are reduced by polymers. Further, the growth along the  x  and z axes is suppressed, resulting in fibrous growth. The viscosity initially decreases and then increases as the number of arms of polymer increases, with 4-arms POM (4X-POM) reducing the viscosity by 64.63 %. The present work reveals the mechanism of inhibiting wax crystal growth by star polymers, providing valuable theoretical guidance for the practical application.