Froth flow regime

Monsanto Enviro-Chem under the Dyna-Wave trademark case histories can be downloaded from a web site (http //www.enviro-chem. com/airpol/common/rjstop.html). Nonatomizing froth scrubbing is described in those patents as occuring within defining boundaries on a new dimensionless velocity versus dimensionless liquid/gas ratio two-phase flow regime map, shown here as Fig. 17-51. 

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... 

Three flow regimes are commonly encountered in industrial columns spray, froth, and emulsion. 

FIG. 14-20 Distillation flow regimes schematics and photos, (a) Froth, (b) Emulsion, (c) Spray. [Schematics from H. Z. Kister, Distillation Design, copyright 1992 by McGraw-Hill, Inc. reprinted by permission. Photographs courtesy of Fractionation Research Inc. (FRI).]... 

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... 

Flow regime. Since the trays are unlikely to operate in the spray regime (Sec. 6.4.2), it is best to first examine the froth-emulsion transition. This check requires using the Hofhuis correlation for clear liquid height (Sec. 6.3.5). 

It is worth emphasizing that Eqs. (13-61) to (13-68) hold regardless of the models used to calculate the interphase transport rates and EJ. With a mechanistic model of sufficient complexity it is possible, at least in principle, to account for mass transfer from bubbles in the froth on a tray as well as to entrained droplets in a spray, as well as transport between the phases flowing over and through the elements of packing in a packed column. However, a completely comprehensive model for estimating mass-transfer rates in all the possible flow regimes does not exist at present, and simpler approaches are used. 

Spray regime operation is desirable (321) for negative-surface-tension systems (i.e., where the mixture s surface tension decreases from the top tray toward the bottom tray). Froth regime operation is desirable for positive-surface-tension systems (surface tension increases from the top tray down), and when liquid entrainment needs to be minimized (321). In most commercial applications, vapor and liquid loading requirements override these desirability considerations and dictate the tray flow regime. For optimum tray performance, the tray layout must therefore accommodate the expected flow regime. 

Two additional considerations need to be taken into account in the spray regime. First, at reduced vapor rates, the flow regime may change to froth. Second, and most important, experience teaches that downcomer sealing is particularly difficult at low liquid loads (144, 179, 211), and there is a strong incentive to make downcomer clearance as low as possible. 

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