(118f) Further Investigation into Regime Transition from Bubbling to Turbulent Fluidization

Over the last fifty years gas-solids fluidized beds have played a very important role in many areas in the process industry, especially in chemical and petroleum industries. The existence of different flow regimes at different operating conditions has been known for a long time and numerous studies have been carried out to define the flow regimes. It is well accepted that with increasing gas velocity, the bed goes through particulate fluidization, bubbling fluidization, turbulent fluidization, to fast fluidization and pneumatic transport. A common way to classify the flow regimes is to determine the transition velocities by certain measurement techniques, but the transition velocities are influenced by many factors such as particle properties, solids inventory, column diameter, distributor design, operating temperature and pressure, the measuring techniques and data analysis methods. In addition, despite the wide range of application of fluidized beds operated in the turbulent flow regime, its hydrodynamic properties are still not well understood and the regime transition from bubbling to turbulent fluidization are not completely clear.

This work has (1) further investigated the transition from bubbling to turbulent fluidization based on the measurements of optical fiber probes (local) and differential pressure (global) transducers; (2) compared the global and local regime transition phenomena and velocities; 3) studied the effects of static bed height and interval distance between pressure measurement ports on the regime transition velocities. Extensive experiments were performed to investigate the regime transition from bubbling to turbulent fluidization in the fluidized bed using differential pressure transducers and optical fiber probes.

It was found that the transition velocity, Uc, determined from differential pressure fluctuations, increased with decreasing axial locations, suggesting that regime transition from bubbling to turbulent fluidization occurs first at upper region of the fluidized bed and develops downward. With increasing static bed height H0 from 0.9 to 1.2 m, Uc increased from 0.6 to 0.7 m/s at H = 0.8 m and from 0.8 to 1.0 m/s at H = 0.4 m. The results also indicate that the relative distance from the investigated position to upper bed surface has greater influence on the corresponding Uc than the absolute distance above gas distributor. For the same H0, increasing the spacing between two pressure tubes leads to lower pressure fluctuations and the appearance of two transition velocities.

The local flow regime transition was studied using the standard deviation of local solids concentrations, measured at 11 different radial positions from three radial directions. Two transition points (U1 and U2) are clearly found and they shift to higher velocity with moving outward towards wall region. Furthermore, the range between these two point also increases with moving outwards, and in near wall region (r/R = 0.92 ~ 0.98), the second transition point is no longer found.

The experimental results suggest that demarcating the regime transition from bubbling to turbulent fluidization with just one transition velocity is not adequate. Our analysis reveals that the transition velocity strongly depends on both axial and radial positions, and it is important to investigate the flow system in details to properly characterize how the regime transition occurs. The standard deviation analysis of local solid concentration measured provides a good understanding of the local regime transition. Comparison of the transition velocity results determined by local solid concentration fluctuations (U1 and U2) and differential pressure fluctuations (Uc) reveals that Uc is always higher than U1 and lower than U2 at all radial positions, indicating that the differential pressure measurements represent averaged flow behaviour of the measuring section between two pressure tubes.