(331b) Geldart Classification and Limitations of the Kinetic Theory of Granular Flow in Solid-Gas Modelling

Numerical simulation of the hydrodynamic features of a bubbling bed over a broad range of operating conditions is presented and validated against experimental measurements. The two-fluid model (e.g. Ocone et al. AIChE J., 39, 1261-1271, 1993) is solved using MFIX and employing the constitutive relations derived by the kinetic theory of granular flow (KTGF). To validate the theory experiments were conducted in a cold conventional gas-solid bubbling column of 13.8 cm internal diameter. Different particle groups, in the size range of 125 – 800 μm (belonging to Geldart group A/B, B and D), were fluidised over a broad range of gas velocity. Electrical Capacitance Tomography (ECT) was used to record the cross-sectional particle-gas distribution over 70 sec operations. The measurements were then used to estimate the bubble characteristics.

It is shown that the model predictions significantly improve as the particle size increases. The accuracy of the quantified bubble characteristics (e.g. velocity, frequency) is very sensitive to the particle size; improvement is noticed at increased gas velocity. A systematic analysis is then undertaken by studying the model response to changing the gas drag law, the particle restitution coefficient and the numerical grid size. Additionally, the particulate phase stress tensor is investigated by modifying the inter-particle forces by including cohesion and liquid-bridge forces in slightly wet particulate flow. The conclusions from the work are in support of the recent growing concern about the validity of the classic kinetic constitutive relations in capturing the mechanism of particle-particle interactions in bubbling fluidised beds.