Elucidating the Structure-Property Relationship of Pour Point Depressants: The Case of Polyoctadecyl Methacrylate

Polymeric additives are used as active agents to reduce or prevent the formation and deposition of paraffin waxes during the production and transportation of crude oil, a common flow assurance problem in the petroleum industry. This study aims to elucidate the relationship between the additive structure and its ability to depress the pour point of waxy crude oil. To that end, we have synthesized octadecyl methacrylate polymers with well-defined hierarchical structures and evaluated their performance as pour point depressants (PPDs) for model crude oils (MCOs) differing in wax concentration and carbon number distribution of n-alkanes. The results showed that brush-shaped polymers significantly depressed the pour point of MCO comprising lower-carbon-number n-alkanes. In contrast, the comb-shaped polymers were not effective PPDs for all of the MCOs tested. The effectiveness of the PPDs correlated well with their effect on the reduction of the wax appearance temperature (WAT) of the MCOs. Furthermore, the brush polymer significantly reduced the viscosity of the waxy gel phase formed on cooling the MCOs, thus improving their cold-flow behavior. Our investigation on the effect of carbon number distribution of n-alkanes on PPD performance revealed that if the alkyl chain length of the polymer additive is closer to the mean carbon number of n-alkanes in the MCO, better is the cocrystallization ability of the additive and hence its pour point depressing efficacy. By analyzing the WAT, wax crystal morphologies, and interaction between the polymer and wax during crystal formation, we propose a mechanism to explain the effect of polymer structure on the PPD performance. Accordingly, the brush-shaped polymer cocrystallized with the wax molecules through a template-induced nucleation mechanism and thereafter inhibited growth of the wax crystals. Concurrently, the polymer hindered aggregation of the wax crystals formed at low temperatures and thus lowered the pour point of MCO. In contrast, the alkyl pendants in the comb polymer provided steric hindrance to the assembly of the paraffin molecules, thereby preventing it from cocrystallizing with the wax.