Thermogravimetric heat and mass transfer: Modeling of bitumen pyrolysis

Mobile reservoirs heat bitumen to temperature exceeding 240 °C thereby reducing the viscosity to pump and make it easier to spread to seal roof tops. Aerosols, vapors and gases evolve at this temperature but more fumes are generated at the surface of the burner where it exceeds 300 °C; these fumes represent a safety hazard. The Cleveland open cup flash point was 320 °C and the Pensky-Martens closed cup flash point was 310 °C for the bitumen tested. Based on thermo-gravimetric analysis, bitumen pyrolysis occurs in three stages. A first fraction (35%) evolves at 250 °C, a second fraction (65%) releases at 350 °C and finally a third fraction (5%) at 400 °C, with activation energies of 52 kJ mol−1, 132 kJ mol−1 and 228 kJ mol−1, respectively. Increasing the heating rate shifts the thermo-gravimetric curve towards higher temperature and raises the maximum decomposition rate. We attribute the shift in the curve to heat and mass transfer resistance between the bitumen surface and the inert gas flow. The heat transfer coefficient between the gas phase and the liquid in the crucible varies from 10 W m−2 K−1 to 20 W m−2 K−1, which was estimated by evaporating a solution of glycerine and isobutanol. These ideal compounds were chosen to derive the heat and mass transfer rates, which we applied to the bitumen experimental data.