Froth flow

Fig. 3. Air-sparged hydrocyclone, where A represents the tangential feed that estabHshes swid flow B, the area of small bubbles formed by high shear at the porous wall and C, the outlet for the (D) hydrophilic particles rejected by the swid flow. The (B) hydrophobic particles are in the axial froth flow.

C) The DNB in a low-quality froth flow is caused by a bubble burst under the bubbly liquid layer. 

Perhaps the simplest classification of flow regimes is on the basis of the superficial Reynolds number of each phase. Such a Reynolds number is expressed on the basis of the tube diameter (or an apparent hydraulic radius for noncircular channels), the gas or liquid superficial mass-velocity, and the gas or liquid viscosity. At least four types of flow are then possible, namely liquid in apparent viscous or turbulent flow combined with gas in apparent viscous or turbulent flow. The critical Reynolds number would seem to be a rather uncertain quantity with this definition. In usage, a value of 2000 has been suggested (L6) and widely adopted for this purpose. Other workers (N4, S5) have found that superficial liquid Reynolds numbers of 8000 are required to give turbulent behavior in horizontal or vertical bubble, plug, slug or froth flow. Therefore, although this classification based on superficial Reynolds number is widely used... 

A knowledge of the mechanics of vertical cocurrent bubble, slug, and froth flow is important for an understanding of the performance of gaslift pumps. These devices will not be discussed here, but reference may be made to a recent article by Nicklin (Nl). 

Downcomer operation is often described in terms of a non-flooded downcomer, where complications arise from the cascade of froth flowing over the weir onto a froth layer in the downcomer, causing entrainment into-, and gross circulation of the froth layer, in a manner analogous to a waterfall. 

Figure 6.5 illustrates the classical hydraulic model of a fractionation tray. Liquid enters the tray from the downcomer of the tray above. The liquid entering the tray is aerated with vapor rising from the tray below to form froth on the tray. The froth flows across the tray until it reaches the outlet weir. The froth then flows over the weir into the downcomer, where the vapor is disengaged from the liquid,... 

The Bennett et al. correlation. This correlation was shown (31) to predict experimental sieve tray pressure drop data more accurately than Fair s correlation. The correlation is based on froth regime considerations and is not applicable to the spray regime. The Bennett et al. calculation of dry pressure drop is identical to Fair s, using Eqs. (6.42) and (6.43) and the Liebson et al- correlation (Fig. 6.21a). To calculate the h, term in Eq. (6.41), Bennett et al. depart from the concept of clear liquid flow corrected for aeration effects [Eq. (6.47a)]. Instead, they use Eq, (6.476) and a model of froth flow across the weir. Their residual pressure drop, hn, is a surface tension head loss term, which is important for trays with very small holes ([Pg.317]

Sieve trays, troth regime. Most dear liquid height and froth density correlations (35,68,81-86) are based on the Francis weir formula. A correlation by Colwell (68), based on a model of froth flow over the weir, was demonstrated to agree with experimental data better than other published correlations. Colwell s correlation is recommended by the author and by Lockett (12), and was successfully used as a building block in weeping correlations (56,63,69) and in froth regime entrainment correlation (40). Colwell s correlation is... 

In froth flow the gas and the liquid are intimately mixed as a froth, with the liquid forming very thin films surrounding the gas bubbles. This flow is created by passing the gas through a glass frit. The froth flow is usually unstable but can be stabilized by surfactants. 

Cocurrent downflow with slug or Taylor flow has been most widely used. Other possible designs, e.g., cocurrent upflow and froth flow, have to our knowledge been tested only in laboratory and pilot plant reactors. Consequently, we will focus on downward slug flow, and the main areas of interest are scale-up, liquid distribution, space velocity, stacking of monoliths, gas-liquid separation, recirculation, and temperature control. 

Figure 13. The pattern map for an upward vapor-liquid flow of R21 refrigerant through the assemblage with plain fins (8FPI). Here 1, 2, 3, 4 are the areas of annular flow, cell flow, froth flow, plug and bubble flows respectively. Solid lines indicate transition between flow modes and dashed lines indicate constant mass flux condition.

The mechanics and applications of multiphase flow has been an area of continuing interest to chemical, environmental, and civil engineers (23,77). The multiphase flow patterns may be classified as bubble flow, plug flow, stratified flow, wave flow, slug flow, annular flow, spray flow, and froth flow. Typical sketches of these various flow patterns are shown in Fig. 3. They are self-explanatory. In the field of absorptive bubble separation processes, only multiphase bubble flow and froth flow are of interest to the process engineer. 

Type of flow pattern(s) involved in an adsorptive bubble separation system depends on the type of process used. For example, bubble fractionation involves two-phase (gas-phase and liquid-phase) bubble flow, while solvent sublation involves multiphase bubble flow in their vertical bubble cells. Foam fractionation involves a two-phase bubble flow in the bottom bubble cell, and a two-phase froth flow in the top foam cell. However, all froth flotation processes (i.e., precipitate flotation, ion flotation, molecular flotation, ore flotation, microflotation, adsorption flotation, macroflotation, and adsorbing colloid flotation) involve multiphase bubble flow and multiphase froth flow. 

Bubble or Froth Flow. Bubbles of gas are dispersed throughout the liquid and are characterized by bubbles of gas moving along the upper part of the pipe at approximately the same velocity as the liquid. It occurs for liquid superficial velocities of about 5-15 ft/sec (1.5 to 4.5 m/sec) and gas superficial velocities of about 1-10 ft/s (0.3 to 3 m/sec). 

From Figure 14.1, it is observed that liquid and froth flow down the downcomer from... 

The liquid and froth flowing over the weir partially fill the downcomer, creating a backup of height h f (Figure 14.1). If the overall relative froth density in the downcomer is (t),i, the equivalent clear liquid height is... 

Even though froth actually flows over the weir (unless calming zones are nsed), h is expressed on an eqnivalent clear liqnid basis, assuming that the Francis relationship also represents froth flow. Later, in connection with mass transfer in tray froths, we will discnss a relative froth density ... 

And where the froth flowed over from the pot And the hot drops spattered the ground beneath,... 

Bubble Flow. The gas flows in the form of bubbles along the upper surface of the liquid and the gas bubbles move at about the same velocity as that of the liquid. This pattern is found when there is a high liquid to gas ratio. If the bubbles are dispersed then it is called froth flow. 

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