Fluidized beds bubble properties

Fig. 16. Computer-generated solidity distribution showing the single bubble in a 2D gas fluidized bed. Physical properties of density, 2660 kg/m. Bed dimensions width, 0.58 m height, 1.0 m.

The bed height and particle density can therefore be eliminated as contributing factors in the windbox pressure increase, since they are the same for both ores. We therefore turn our attention to the bubble fraction and particulate phase voidage in the fluidized bed. These properties are a combined function of the gas flow rate through the bed and the particle shape, particle size and particle size distribution. 

To provide the pr equisite knowledge for designing the three-phase fluidized-bed reactors with new modes, the hydrodynamics such as phase holdup, mixing and bubble properties and heat and mass transfer characteristics in the reactors have to be determined. Thus, in this study, the hydrodynamics and heat and mass transfer characteristics in the inverse and circulating three-phase fluidized-bed reactors for wastewater treatment in the present and previous studies have been summarized. Correlations for the hydrod3aiamics as well as mass and heat transfer coefficients are proposed. The areas wherein future research should be undertaken to improve... 

Litka, T., and Glicksman, L. R., The Influence of Particle Mechanical Properties on Bubble Characteristics and Solid Mixing in Fluidized Beds, Powder Technol., 42 231 (1985)... 

In contrast to the strong effect of gas properties, it has been found that the thermal properties of the solid particles have relatively small effect on the heat transfer coefficient in bubbling fluidized beds. This appears to be counter-intuitive since much of the thermal transport process at the submerged heat transfer surface is presumed to be associated with contact between solid particles and the heat transfer surface. Nevertheless, experimental measurements such as those of Ziegler et al. (1964) indicate that the heat transfer coefficient was essentially independent of particle thermal conductivity and varied only mildly with particle heat capacity. These investigators measured heat transfer coefficients in bubbling fluidized beds of different metallic particles which had essentially the same solid density but varied in thermal conductivity by a factor of nine and in heat capacity by a factor of two. 

Temperature of the fluidized bed is another parameter that could influence the heat transfer coefficient. Increasing bed temperature affects not only the physical properties of the gas and solid phases, but also increases radiative heat transfer. Yoshida et al. (1974) obtained measurements up to 1100°C for bubbling beds of aluminum oxide particles with 180 pm diameter. Their results, shown in Fig. 6, indicate an increase of over 100% in the heat transfer coefficient as the bed temperature increased from 500 to 1000°C. Very similar results were reported by Ozkaynak et al. (1983) who obtained measurements for bubbling beds of sand particles (dp = 1030 pm) at temperatures up to 800°C. 

Gas logging, the adherence of small bubbles to particles, causing them to rise to the surface in the reactor and form an inefficient packed bed with poor mass transfer properties, can be a problem in various fermentations and in wastewater treatment. A double entry fluidized bed reactor has been developed with simultaneous top (inverse) and bottom (conventional) inlets to overcome this problem (Gilson and Thomas, 1993). 

For the discrete bubble model described in Section V.C, future work will be focused on implementation of closure equations in the force balance, like empirical relations for bubble-rise velocities and the interaction between bubbles. Clearly, a more refined model for the bubble-bubble interaction, including coalescence and breakup, is required along with a more realistic description of the rheology of fluidized suspensions. Finally, the adapted model should be augmented with a thermal energy balance, and associated closures for the thermophysical properties, to study heat transport in large-scale fluidized beds, such as FCC-regenerators and PE and PP gas-phase polymerization reactors. 

Gas bubbles in liquid metals and in fluidized beds have been the subject of special studies because of their practical importance and because of the experimental difficulties associated with studying bubble properties in opaque media. Much of the work has been carried out in so-called two-dimensional columns, where a sheet of liquid or fluidized particles, typically 1 cm thick, is confined between two parallel transparent walls. Bubbles span the gap between the front and rear faces and can be observed with backlighting. 

The distribution of gas flow in the fluidized bed is important for the analysis of the fundamental characteristics of transport properties in the bed. One common method to estimate the gas flow division is based on the two-phase theory of fluidization, which divides the superficial gas flow in the bed into two subflows, i.e., bubble phase flow and emulsion phase flow, as shown in Fig. 9.14. According to the theory, the flow velocity can be generally expressed as... 

For the bed-to-surface heat transfer in a dense-phase fluidized bed, the particle circulation induced by bubble motion plays an important role. This can be seen in a study of heat transfer properties around a single bubble rising in a gas-solid suspension conducted... 

Bayle, J., Mege, P., and Gauthier, T., Dispersion of bubble flow properties in a turbulent FCC fluidized bed, in Proceedings of 10th Engineering Foundation Conference, Fluidization X" (M. Kwauk, J. Li, and W.-C. Yang Eds.), pp. 125-132. Beijing, China (2001). 

The ultimate cause of bubble formation is the universal tendency of gas-solid flows to segregate. Many studies on the theory of stability [3, 4] have shown that disturbances induced in an initially homogeneous gas-solid suspension do not decay but always lead to the formation of voids. The bubbles formed in this way exhibit a characteristic flow pattern whose basic properties can be calculated with the model of Davidson and Harrison [30], Figure 5 shows the streamlines of the gas flow relative to a bubble rising in a fluidized bed at minimum fluidization conditions (e = rmf). The characteristic parameter is the ratio a of the bubble s upward velocity u, to the interstitial velocity of the gas in the suspension surrounding the bubble ... 

Fluidization quality in terms of material properties, particle characteristics, and particle group behavior thus needs to be assessed on three scales gross scale of the fluidized bed (macro scale), aggregate scale of gas bubbles, and particle clusters (meso scale), and scale of the discrete, individual particles (micro scale), as described in Chapter 4. 

Large-diameter solid particles in a three-phase fluidized-bed system cause bubbles to be small, whereas, in a fine particle slurry, the bubbles can become large. Henriksen and Ostergaard40 showed that the large bubbles in the latter case can break as a result of Taylor instability at the root of the bubble. The wake properties of bubbles in a three-phase fluidized-bed system have been studied by Rigby and Capes.115 They showed that bubble wakes in a three-phase system consist not only of a stable portion carried with the bubbles but also of vortices shed by the bubbles. 

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