(68g) Experimental Study on Flow Regimes Transition and Bubble Characteristics in a Pressurised Fluidised Bed

Gas-solids fluidised beds have been widely used in process industries due to their excellent mixing and heat/mass transfer performance. Many industrial processes require high operating pressure to improve the overall throughput and reduce the size of the reactors. However, despite considerable studies on atmospheric fluidised beds have been undertaken, understanding the flow characteristics and regime transition at elevated pressure conditions is still insufficient.

In this research, two non-intrusive measurements, i.e. electrical capacitance tomography (ECT) technique and pressure fluctuation measurement, are introduced in a pilot-scale fluidised bed operated at four pressure conditions from 0.2 to 1.3 MPa. A wide range of fluidisation velocities, covering flow regimes from incipient fluidisation to fast fluidisation, is enabled in each operating pressure condition. The flow regime transition, bubble characteristics and effect of pressure, by means of ECT image reconstruction and statistical analyses based on the particle volume fraction (PVF) and pressure fluctuations, are presented and discussed.

The results show that for Geldart B particles, the flow regime transition velocities, namely, the minimum fluidisation velocity (U_mf), the onsets of turbulent flow (U_c) and fast fluidisation flow (U_k) show significant decrease as the pressure increases. However, the corresponding fluidisation dimensionless numbers (i.e. U_c/U_mf, and U_k/U_mf) are almost independent of the operating pressure.

Both the bubble diameter and bubble rise velocity, estimated by image reconstruction and cross-correlation analysis on the dual-plane ECT measurement, increase with the increase of the fluidisation velocity under the same pressure condition. Furthermore, the pixel-by-pixel correlation gives the bubble velocity distribution image across the monitoring cross-section, from which the uneven distribution feature between the centre and near-wall region is clearly observed.

This work demonstrates the high suitability of the ECT technique, applied to a gas-solids pressurised fluidized bed, for the real-time measurement without intruding the flow field. The experimental results, in terms of the pressure effect on gas-solids flow characteristics, bubble properties and regime transition are believed to provide useful information for the proper design and optimisation of pressurised fluidised beds.