Mechanistic insights of free radical evolution and mechanism of oil from -180°C to 200°C using in-situ electron spin resonance spectroscopy

Abstract

Unconventional oils are difficult to process due to their high viscosity, complex chemistry, and tendency to form coke during upgrading. In this study, in-situ Electron Spin Resonance (ESR) spectroscopy was used to monitor the free radical evolution of a medium-heavy crude oil over a wide temperature range (i.e. −180°C to 200°C). ESR revealed three key regimes: a gradual radical increase of about 147% from 3.0 × 10¹⁶ to 7.5 × 10¹⁶ spins/g between −180°C and −80°C due to trapped radical release; a sharp 323 % rise to 3.2 × 10¹⁷ spins/g between −80°C and 30°C linked to enhanced radical release due to elevated molecular mobility; and a slower increase up to 4.3 × 10¹⁷ spins/g from 30°C to 190°C, nearly 35 %. A sharp decline occurred at 200°C, remaining stable over 50 mins, indicating a balance between radical generation and termination. These findings reveal the critical temperature zones where radical activity transitions, offering insight into free radical reactions as well as coke initiation and pathways to optimize low-severity thermal upgrading of heavy oils. For refinery practice, the identified transition zones can inform temperature selection and process monitoring during low-severity upgrading. Changes in the ESR-derived radical population may provide an early indication of feed instability before substantial coke becomes evident. Translation to process equipment will require validation under continuous-flow and pressurized conditions.