Heterogeneous Fluidization Bubbling

Behavior of bubbles for a typical fluidized Group B powder (Geldart s classification). 

In most solid-gas systems bubbling occurs when the gas velocity is increased well over the minimum fluidizing velocity. The simplest description of the expansion of a gas-fluidized bed comes from the two-phase theory of fluidization (Rhodes, 1998), originally attributed to Toomey and Johnstone (1952). This theory suggests that the bubbling fluidized bed is composed of two phases the bubbling phase and the particulate phase (also known as the emulsion phase), and that all the gas in excess of that required to fluidize the system will pass through the bed in the form of bubbles. 

The lower and upper limits of gas velocity at which a uniform particulate phase can exist are the minimum fluidizing velocity and the minimum bubbling velocity, respectively. It is quite difficult to define the latter precisely, since it will depend critically on the previous history of the bed, on the nature 

Powders of small particle size and low density are difficult to fluidize initially, but once they reach the fluidized state they can give origin to beds that can be expanded over a wide range before bubbling occurs. The formation of large bubbles in gas-fluidized systems is often prevented by introducing standard tower packings into the bed to produce what is known as a packed-fluidized bed. 

The bed expansion can be expressed in terms of the fraction occupied by bubbles Eg  

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It is clear from previous sections of this chapter that fluid-dynamic phenomena can account for a wide spectrum of behaviour patterns in unstable heterogeneous fluidized beds, and there is no reason to suppose that the same should not be so for stable homogeneous systems. The problem becomes that of identifying a relevant criterion for characterizing such differences. As no minimum-bubbling condition is predicted for fully homogeneous beds, fluidization quality criteria based on Smb and Aa are inapplicable in these cases. 

In showing how the minimum bubbling point can be determined from an examination of the structure of heterogeneous fluidization, as an alternative to the traditional approach based on analysis of the homogeneously fluidized state, we have both confirmed the internal consistency of the particle bed model formulation, and generalized the Wallis criterion to encompass finite perturbations. 

Bubbling Fluidized Bed Catalytic Reactors (Heterogeneous Two-Phase System)... 

As a first approximation a convective term in the film region has been negleted, u is the superficial gas velocity and u f denotes the gas velocity at minimum fluidization conditions. Tne specific mass transfer area a(h) is based on unit volume of the expanded fluidized bed and e OO is the bubble gas hold-up at a height h above the bottom plate. Mathematical expressions for these two latter quantities may be found in detail in (20). The concentrations of the reactants in the bubble phase and in film and bulk of the suspension phase are denoted by c, c and c, respectively. The rate constant for the first order heterogeneous catalytic reaction of the component i to component j is denoted... 

The attrition of solid particles is an unavoidable consequence of the intensive solids motion resulting from the presence of bubbles in the fluidized bed. The attrition problem is especially critical in processes where the bed material needs to remain unaltered for the longest possible time, as in fluidized-bed reactors for heterogeneous catalytic gas-phase reactions. Catalyst attrition is important in the economics of such processes and may even become the critical factor. 

E.U. Schlunder, On the mechanism of mass transfer in heterogeneous systems—in particular in fixed beds, fluidized beds and on bubble trays, Chem. Eng. ScL 32 845 (1977). 

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