Fluidization bubbles

Bubbles and Fluidized Beds. Bubbles, or gas voids, exist in most fluidized beds and their role can be important because of the impact on the rate of exchange of mass or energy between the gas and soflds in the bed. Bubbles are formed in fluidized beds from the inherent instabiUty of two-phase systems. They are formed for Group A powders when the gas velocity is sufficient to start breaking iaterparticle forces at For Group B powders, where iaterparticle forces are usually negligible, and bubbles form immediately upon fluidization. Bubbles, which are inherently... 

Cocurrent three-phase fluidization is commonly referred to as gas-liquid fluidization. Bubble flow, whether coeurrent or countereurrent, is eonveniently subdivided into two modes mainly liquid-supported solids, in which the liquid exeeeds the minimum liquid-fluidization veloeity, and bubble-supported solids, in whieh the liquid is below its minimum fluidization velocity or even stationary and serves mainly to transmit to the solids the momentum and potential energy of the gas bubbles, thus suspending the solids. 

Group B powders They give only bubbling fluidization. Bubbles are formed as soon as the gas velocity exceeds the minimum fluidization velocity ( bm = u(n). Most particles... 

Incipient fluidization Bubbling regime FIGURE 10 Schematic of the bubbling bed. 

In bubbling fluidization, bubble motion becomes increasingly vigorous as the gas velocity increases. This behavior can be reflected in the increase of the amplitude of the pressure fluctuations in the bed. With further increase in the gas velocity, the fluctuation will reach a maximum, decrease, and then gradually level off, as shown in Fig. 9.16. This fluctuation variation marks the transition from the bubbling to the turbulent regime. 

D 1,000 coarse e.g., wheat can be spouted mix poorly when fluidized appreciable particle attrition rapid elutriation of fines relatively sticky materials can be fluidized bubbles cloudless slow bubbles bubble velocity less than interstitial gas velocity... 

As a phase-reversed counterpart of bubbling fluidization, bubbles are replaced by clusters, which are subject to rapid dissolution and reformation, thus leading to improved G/S contacting. 

In the last several decades, many gasification processes have been developed using all three regimes of fluidization bubbling fluidized bed, circulating fluidized bed, and transport reactor. Each gasification process exploits the particular characteristics of a fluidization regime. 

In this section the solution method employed by Lindborg et al [119] simulating fluidized bubbling bed reactors is outlined. 

The fluidization is characterized as bubbling , B (aggregate fluidization). Bubbling consists of two phases ... 

Roy, N.K., Guha, D.K., and Rao, M.N. (1963), Fractional gas holdup in two-phase and three-phase batch-fluidized bubble-bed and foam-systems, Indian Chemical Engineer, 1963 27-31. 

Collins, R. A model for the effects of the voidage distribution around a fluidization bubble. Chem. Engng Sci. 44, 1,481-1,487 (1989). 

Figure 9. Variation of burning rate with the diameter of char particies in a turbulent fluidized bed [2]. 1) Kinetic limit [2] 2) Diffusion limit 3) Model of Haider et al. for burning rate of char particles in turbulent fluidized beds [2] o Haider et al. for char particles burning in a turbulent fluidized bed with air as fluidizing medium [2] c La Nauze for petroleum coke particles burning in air fluidized bubbling bed [17] a Chakraborty and Howard for char particles burning in air fluidized bubbling bed [38].

Bubbling fluidization bubbles form at the bottom of the fluidized bed and grow by coalescence while ascending, until they burst at the surface of the bed. These bubbles are visually identifiable they do not contain any particles. The bed becomes inhomogeneous in terms of porosity. The formation of bubbles produces pressure fluctuations within the bed. 

Johnsen K, Grace JR, Elnashaie SSEH, Kolbeinsen L, Eriksen D (2006) Modeling sorption-enhanced steam reforming in a dual fluidized bubbling bed reactor. Ind Eng Chem Res 45 4133 144... 

Dense phase fluidization Gas fluidized beds are considered dense phase fluidized beds as long as there is a clearly defined upper limit or surface to the dense bed. The dense-phase fluidization regimes include the smooth fluidization, bubbling fluidization, slugging fluidization, and turbulent fluidization regimes. In a dense-phase fluidized bed the particle entrainment rate is low but increases with increasing gas velocity. 

The characteristics of B powders are relatively simple. Directly beyond the point of minimum fluidization bubbles are formed and the average dense phase porosity doesn t change. In large scale units bubbles grow rapidly to large sizes by coalescense, dense phase mixing is moderate and the apparent "viscosity" is high. 

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