Keynote Talk: Investigation of the Bubble Dynamics in a Semi-Cylindrical Gas-Solid Fluidized Bed

Fluidized beds are used in various processes such as mixing, catalytic and non-catalytic reactions, drying, and adsorption since they provide uniform temperature distributions, high mass/heat transfer rates, and low pressure drops. Almost every aspect of fluidized beds operating in the bubbling regime, including gas interchange rate between phases, gas and particle residence times, reaction conversion, particle entrainment and elutriation, and mass and heat transfer are significantly controlled by the bubble characteristics of size, shape, and velocity. Accordingly, a great deal of effort in measuring and characterization of bubbles has been spent in the published fluidization literature. More researches have been dedicated to the development of various correlations for predicting bubble size and velocity. However, it is not easy to visualize motion of bubbles in three-dimensional fluidized beds. Although two-dimensional beds can be used for direct visual observation, their results are qualitative and have a limitation in applicability to three-dimensional fluidized beds. It has been shown that semi-cylindrical bed is a useful tool which allows direct observation of in-bed phenomena with minor wall interactions. Therefore, the semi-cylindrical fluidized bed is appropriate for direct visual observation of bubbles due to its flat wall. Moreover, the measurements would be more accurate and applicable to three-dimensional beds compared with two-dimensional beds.

In the present work, bubble dynamics was studied (i.e., bubble frequency, bubble size, bubble rise velocity, and gas hold up) in a gas-solid semi-cylindrical fluidized bed. A semi-cylindrical fluidized bed with a transparent flat wall, in order to observe bubbles visually, and a cylindrical fluidized bed were employed to investigate the effect of cross-section of the column on the bubble behavior. Both beds were made of Plexiglas with diameter of 14 cm filled with glass beads as the bed material. Air was entered into the column through a perforated plate distributor and its flow rate was controlled by a mass flow controller. All experiments were carried out in atmospheric pressure cold fluidized beds while the superficial gas velocity was varried in the range of 0.2-0.8 m/s. In order to investigate the effect of particle size on the bubble dynamics in both beds, experiments were conducted at three glass bead sizes 120, 290 and 450 μm (Geldard B) at the same static bed height of 21 cm (L/D=1.5). In the semi-cylindrical bed, the bubble size and bubble rise velocity were directly measured at the bed height of 14 cm (L/D=1) by digital imaging technique, when using a camera in front of the flat wall. Bubble size and bubble rise velocity were measured by a reflection-type fiber optic probe in the cylindrical fluidized bed, located at 14 cm above the distributor. Gas hold up in both beds was measured by the same fiber optic probe.

It was shown that the semi-cylindrical fluidized bed is proper for direct visual observation of bubbles through its flat wall. The experimental results were compared with the existing correlations obtained for three-dimensional gas-solid fluidized beds. Results showed that increasing the particle size results in increase in the bubble chord length and bubble rise velocity in both beds. Besides, no effect of particle size on bubble chord length and bubble rise velocity was observed for different particle sizes when compared at the same excess gas velocity (U_G- U_mf). Correlation of Mori and Wen for bubble growth was found to be appropriate for both fluidized beds. Also, the expressions for bubble rise velocity under the present conditions developed for three-dimensional beds by Davidson and Harison, was found to be applicable to both beds. This finding indicates that in one hand the bubble size and bubble rise velocity in semi-cylindrical fluidized bed is in compliance with the cylindrical fluidized bed. On the other hand, measuring these parameters in the semi-cylindrical bed is much easier than in the cylindrical bed. This is ensuring that the semi-cylindrical fluidized bed is a very useful tool for being employed increasingly for cold model works in laboratories.