Numerical Study of Heavy Crude Oil Pyrolysis in Supercritical Water

Pyrolysis of heavy oil in supercritical water (SCW) occurs under elevated pressures and temperatures, when two phases often co-exist in the reacting mixture. Based on published experimental data in the literature, we propose a fractional-order lumped reaction kinetic model considering two phases (an oil-rich phase and a water-rich phase). Using our kinetic model, the pyrolysis of a heavy oil droplet (D=0.5mm) in a bulk of SCW is simulated as the coupling of kinetics, thermodynamics and transport phenomena. Results show that the phase behavior plays an important role in the pyrolysis reactions of heavy oil: (1) asphaltenes are heavily concentrated in the center of the original oil droplet, which speeds up the reaction compared to the process without water; and (2) maltenes quickly diffuse into the water-rich phase and become diluted, which slows down the coking from maltenes and thus suppress the overall coke formation. Modeling of this study can help design the heavy oil pyrolysis reactor using SCW, and indicates that phase behavior may also be important in other reactors using supercritical fluids.