Hydrodynamics, heat transfer and kinetics reaction of CFD modeling of a batch stirred reactor under hydrothermal carbonization conditions

Computational Fluid Dynamics simulation was used to study hydrothermal carbonization of avocado stone (AS) in a batch stirred reactor, using an open-loop controller system. The corresponding simulations were carried out in COMSOL Multiphysics 5.2. The different biomass-to-water ratio was investigated. The hydrodynamic study shows that the ideal stirring speed to obtain a homogeneous mixture inside the reactor is 550 rpm, due to the high density of AS particles (1547.64 ± 27.33 kg/m3). However, a stagnant zone was observed just below the impeller. To validate the CFD simulations temperature profiles with experimental data, the heat transfer coefficient of the insulator was determined (11.66 W/m2.K), this value was used to set the heat loss in the CFD simulation. According to the model, the difference between the thermal properties of biomass and water under hydrothermal carbonization conditions is negligible. However, experimentally, an increase in temperature was observed with increasing biomass to-water-ratio; this is due to the global hydrothermal carbonization of AS is exothermic reaction. The heat released during 8 h, including heat-up time, was −7.25 ± 0.32 MJ/kg of feedstock. Finally, a kinetic model was proposed taking into account the influence of temperature, heat-up time, reactor volume, and biomass concentration.