Freely bubbling fluidized

In a freely bubbling fluidized bed, small bubbles often tend to coalesce into larger... 

A freely bubbling fluidized bed has a higher flow rate of the gas than the minimum fluidization limit. It will reach the case of three phases solid, gas in contact with solid, and gas in bubbles inside the tube. 

Wiman J, Almstedt AE. Influence of pressure, fluidization velocity and particle size on the hydrod5mamics of a freely bubbling fluidized bed. Chem Eng Sci 53 2167-2176, 1998. 

Glicksman LR, Yule T. Prediction of the particle flow conditions in the freeboard of a freely bubbling fluidized bed. AIChE J, 1991c. 

There are advantages and disadvantages of fluidized beds for catalytic reactions. Advantages of fluidized beds (freely bubbling)... 

Their study has resulted in the following two modified forms of Eqs. (74) and (75) for a freely bubbling, two- or three-dimensional fluidized bed. For two-dimensional beds, the modified equation has the form... 

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. 

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. 

The direct contact model has some difiiculties, however. In fluidized beds, gas bubbles of very low solid content are usually considered to exist in the dense phase (H14, K13, T19). Also, the cloud layer is negligibly thin, due to small (/ r for the usual fluid catalyst beds, according to equa-ticMis of Davidson and Harrison (D3) and Murray (M47). The streamlines of gas phase through a bubble have been observed to pass through the cloud, but not through the bubble wake (R17). Thus there seems little possibility of believing that the bubble gas is in direct contact with a substantial amount of catalyst in the bubble phase (see also Secticxi VI,A). Furthermore, the direct contact model is applied to the data by Gilliland and Knudsen, and v in Eq. (7-9) is calculated to fit the data. Calculation (M26) shows that the volume of catalyst, with an apparent density the same as for the emulsion, which contacts the bubble gas freely exceeds the volume of bubble gas itself (v/ib = 3.3, 2.0, and 1.5, respectively, for Uc. = 10, 20, and 30 cm/sec). This seems to be unsound physically. 

In this case, the fluidizing fluid moves upward relative to the bubble motion. This case is usual for beds of large particles and small bubbles. Since R is less than zero in this case, the majority of fluidizing fluid enters the void at the base and leaves from the roof. The fluidizing fluid, in essence, uses the bubble void as the shortcut. The fluidizing fluid penetrates the particulate phase freely from the bubble except for a small fraction of fluid in the shaded area, as shown in Fig. 12 in a circle of radius a, expressed in the following equation. 

For the estimation of the average bubble velocity, Wb, the minimum fluidization velocity Wfn( is required. If the bed resides in a condition of minimum fluidization, the particles float freely. The gravitation force is thus compensated for by the pressure drop in the bed. Let us consider a bed cross-section (A), where there are n pieces of particles. The gravitation force, AF, is then given by... 

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