Relative bubble velocity

In Section I, a qualitative schematic description of the main connection between increased agitation intensity and increased total mass-transfer rate was given. It can readily be seen from this description that further research in gas and liquid flow patterns and in the area of relative bubble velocities in dispersions will contribute to the basic knowledge necessary for understand ing the real mechanisms occurring in these systems. 

All batch adsorptive bubble separation processes involve no net movement of liquid, but steady bubbling of gas through the stagnant liquid. The relative bubble velocity in the bubble cell is the function of buoyancy component and the superficial gas velocity. 

In the bubble cell of any continuous adsorptive bubble separation process, the relative bubble velocity is the function of buoyancy component, the superficial gas velocity, and the superficial liquid velocity. The superficial velocity of gas and liquid are caused by the continuous entry of the gas and liquid phases into the bubble cell. Figures 4 (A) and 4 (B) show the relative bubble velocity in various two-phase bubble flow systems (43). 

Fig. 4. (A) Relative bubble velocity in a two-phase bubble flow—systems (a), (b), and (c).

Thulasidas et al. [6] compared their and all other available experimental data on relative bubble velocity, m, with the literature correlations. There was a considerable scattering of the experimental data at the lower values of Ca, particularly for Ca < 10, while Bretherton s correlation was found to underpredict all the data. The results showed that m reached a limiting value of 0.58 at large values of Ca. 

The model in its present form cannot be used for the design of gas-liquid contacting systems, for several reasons. The model requires a knowledge of the average bubble velocity relative to the fluid, U, a variable that is not available in most cases. This model only permits the calculation of the average rate per unit of area, and unless data are available from other sources on the total surface area available in the vessel, the model by itself does not permit the calculation of the overall absorption rate. 

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... 

Fig. 11. Mean length of bubble signals in fluid beds operated under relatively high velocity (M43) perf. = perforated.

So far, the influence of bubble wake on mean bubble velocity i b relative to the column wall has not been mentioned, since Eq. (5-3) has been formulated on the basis of uy,o, which already includes the effect of the wake (although it lacks a correction for wake fraction). In bubbling-bed models (FIO, F12, K24, L5, S18) an upward flow of solid carried by the bubbles and bubble wakes leads to a downflow of solid (that has been assumed uniform) in the remainder of the bed. Then the bubble velocity b relative to bed wall should be smaller than the slip velocity of the bubble Ms relative to emulsion, since the bubble phase is retarded by downflow of... 

The bubble wake released at the free surface, joined with the upflow, constitutes the downflow so that there is no downward return flow through the bubble layer. Under these circumstances, the mean bubble velocity b relative to the column wall is... 

Hagesffither et al [29, 30] did not make any firm conclusion on the relative importance of the various collision density contributions, as the turbulent bubble velocity closures used by them are at best inaccurate. 

---