Freely bubbling beds

Equation 8 has not been rigorously tested on freely bubbling beds because of the difficulty of making precise independent measurements of Dg or Ug under such conditions. It has been verified for slugging beds, however, as is shown in Figure 3. In the case of slugging, the surface level of the bed oscillates considerably, and in this case, the maximum bed height is used for H in Equation 8. 

Pass an excess of gas upward through a bed of fine particles. With a large enough bed diameter we get a freely bubbling bed of fast bubbles. As simplifications, assume the following ... 

The first term in each case arises from bulk flow of gas into the floor of an isolated bubble and out the roof, as required by the hydrodynamic model of Davidson and Harrison (27). The weight of experimental evidence, from studies of cloud size (28,29), from chemical reaction studies (e.g. 30), and from interphase transfer studies (e.g. 31,32), is that this term is better described by the theory proposed by Murray (33). The latter leads to a reduction in the first term by a factor of 3. Some enhancement of the bulk flow component occurs for interacting bubbles (34,35), but this enhancement for a freely bubbling bed is only of the order of 20-30% (35), not the 300% that would be required for the bulk flow term Equations (1) and (2) to be valid. 

In reactor modeling we need to deal with the behavior of the bubbling bed as a whole rather than single rising bubbles. In extending the simple two-phase theory, Davidson and Harrison [29] proposed that the average velocity of bubbles in a freely bubbling bed can be approximated by... 

McKeen T, Pugsley T Simulation and experimental validation of a freely bubbling bed of... 

Geldart (1975) also developed an equation for bed expansion in freely bubbling beds with sandlike powders as follows ... 

Bubble Size and Velocity in Freely Bubbling Beds... 

Studies concerning bubbles characteristics in three-phase fluidized beds can be grouped into three categories. The first category includes the analysis of the relationship between bubble size and the velocity of single bubbles. The second group considers the characteristics of bubbles in freely bubbling beds. 

In some industrial, and nearly all pilot-plant, fluidized bed reactors the aspect ratio is such that slugging behaviour can be expected, as Davidson and his co-workers have suggested. Scale-up therefore frequently involves a jump from slugglng-bed behaviour to freely bubbling bed behaviour. 

Although the studies completed and proposed have direct application only to slugging fluidized bed reactors there will be insights given into the behaviour of freely bubbling beds. 

Integration variable in PBE source on flux form Average rise velocity of bubbles in a freely bubbling bed (m/s)... 

To examine this hypothesis, the two-phase, two-dimensional particle bed model has been used to simulate the two matched systems (Chen et al., 2001). The geometric arrangement for the simulations is shown in Figure 16.6. The gas flux was 217m/in both cases. Instantaneous pressure and void fraction measurements were recorded for data analysis at the points A, B, C and D. The initial conditions corresponded to beds at the point of minimum fluidization gas rates were then set to 2C/m/, resulting in the development of freely bubbling beds. The results reported below are representative of all the measured data, and confirm the equivalence of fluidization quality in the two matched beds. 

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