Effect of operating pressure on particle temperature distribution in a fluidized bed with heat production

The effect of elevated pressure on gas-solid heat-transfer behavior in an olefin polymerization fluidized bed was numerically analyzed by using an in-house developed 3-D computational fluid dynamics model coupled with a discrete element model (CFD-DEM). To mimic the heat generation associated with the polymerization reaction, a constant volumetric heat production was incorporated in the particle thermal energy equation. Snapshots of the heat transfer driving force (the difference between single particle temperature and average gas temperature) are presented to provide insight into the particle temperature distribution in the fluidized bed. Furthermore, it was found from the Probability Distribution Function (PDF) of the particle temperature that with increasing operating pressure the average particle temperature drops and the bed becomes more isothermal. Moreover, the average particle-gas heat transfer coefficient, was found to increase with increasing operating pressure. However, it is independent from the superficial gas velocity when the bed was operated above the minimum fluidization condition at the same elevated pressure. Predictive based on a continuous stirred tank reactor (CSTR) approximation of the bed reveals that the average particle temperature at steady state is determined by the inlet gas temperature, the temperature difference of the two phases and the adiabatic temperature rise. In polymerization reactors the adiabatic temperature rise is relatively high, therefore the effect of the pressure on the average temperature of the particles can be entirely attributed to the increased gas density, whereas changes in the bed hydrodynamics have an effect on the spread of the particle temperature distribution. (C) 2016 Elsevier Ltd. All rights reserved.