Comparative thermokinetic study of vegetable oils and deoxygenated derivatives

Abstract

This study shows the usefulness of thermokinetic analysis in the characterization of vegetable oils and their deoxygenated derivatives (bioparaffin mixtures). The approach allows obtaining information about the complexity and nature of the process rate-controlling steps (inter- or intra-molecular), to establish quality criteria and to estimate the cohesive forces in liquids and fuels in general. The study also reveals the influence of the gaseous products, Ps, on the limit temperature, Tl, at which the liquid–vapor interface coexists or vanishes. The presence of multiple steps and stages, and the energy barrier variability between ∼ 120 and > 250 kJ mol−1 in the starting vegetable oils, demonstrates the importance of the origin and composition of triglyceride and the effect of unsaturation degree (C = C)n in the fatty acid degradation pathway. Besides, the isoconversional analysis of the deoxygenated products (bioparaffins) describes a kinetics controlled by a single-step, with almost constant activation energy barriers ∼ 70 kJ mol−1, related to the inter-molecular cohesive forces, usually E(α) < 100 kJ mol−1, equivalent to the vaporization enthalpy, ΔvH. The results are validated by a complementary thermokinetic analysis of pure solvents (n-decane and n-hexadecane). Finally, the study describes several experimental observations of great relevance and interest to understand and control the nature of thermally activated processes in vegetable oils and biofuels.