Froth regime

Is believed to be a froth regime (liquid in continuous phase above the tray and gas present as bubbles in the liquid) phenomenon rather than a spray regime (gas in continuous phase above the tray and liquid present... 

In the froth regime, which is between the spray and emulsion ones, flooding may be by either mechanism, depending on the tray spacing and the particular combination of vapour and liquid loads. 

FIG. 14-17 Schematic of a tray operating in the froth regime. (Based on H. Z. Kister, Distillation Design, copyright 1992 by McGraw-Hill reprinted by... 

For decades, the Fair correlation [Pet/Chem. Eng., 33(10), 45 (September 1961)] has been used for entrainment prediction. In the spray regime the Kister and Haas correlation was shown to be more accurate [Koziol and Mackowiak, Chem. Eng. Process., 27, p. 145 (1990)]. In the froth regime, the Kister and Haas correlation does not apply, and Fair s correlation remains the standard of the industry. Fair s correlation (Fig. 14—34) predicts entrainment in terms of the flow parameter [Eq. (14-89)] and the ratio of gas velocity to entrainment flooding gas velocity. The ordinate values XF are fractions of gross liquid downflow, defined as follows ... 

Effect of liquid rate. At low liquid rates, entrainment diminishes with higher liquid loads, while at high liquid rates entrainment increases with liquid loads (22,24,26,33,40,53-55 Fig. 6,15), When most of the dispersion is in the form of a spray, entrainment diminishes with higher liquid loads (22,24,27). The point at which the trend reverses, and entrainment begins to increase with liquid rate, has been interpreted either as the point where the dispersion changes from partially developed spray to froth (40,53), or where the dispersion changes from the spray to froth regime (22-24,45,55). 

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... 

The froth regime is the most common operating regime in distillation practice, and ite hydraulics is reasonably well approximated by the classical hydraulic model (Sec. 6.2.1 Fig. 6.5). 

Figuf 7 J>ay action in the froth, spray, and emuisian regimes. Horitontal bare Indicate height above tray floor in inches, (a) Froth regime (6) Spray regime. (Ati porta... 

Effect of pressure. In vacuum columns, vapor velocities are generally high and liquid flow rates are low, which coincides with an operating point in the spray regime. If the column operates at high liquid loads, it may operate in the froth regime. The emulsion regime is unlikely to occur in vacuum columns. 

In atmospheric and low-pressure (<100-p i) distillation, the column is likely to operate in the froth regime, but depending on the liquid and vapor rates, it may also operate in either the spray or emulsion regime. 

An early study (91) reports that a correlation derived by Barber and Wijn for sieve tray froth-to-spray transition is also applicable to valve trays. A more recent study by Dhulesia (112) disagrees, and reports that valve trays have a stronger tendency to operate in the froth regime than sieve trays. Dhulesia proposed an alternative froth-spray transition correlation for valve trays, but this correlation is based on air-water data from a single type of valve tray, and its extension to other situations has not been tested. 

The difference in the behavior of the aerated mass between the emulsion and froth regimes is far less apparent than the difference between the froth and spray regime. 

Mass transfer. Tray efficiency increases with pressure in the froth regime (118,119), but decreases with increased pressure in the emulsion regime (44,104,105). The efficiency decrease in the emulsion regime is caused by the greater vapor recycle (44,104,105). In general, there is otherwise little difference between mass transfer in the froth and emulsion regimes. 

For low-viscosity absorption systems, one set of data (186) shows that in the froth regime tray efficiency increases as surface tension is reduced, while for high-viscosity absorption systems, surface tension had little effect on mass transfer (166). 

Pressure. Tray efficiency slightly increases with pressure in the froth regime (17,105,119). The apparent pressure effect could be a reflection of the rise in efficiency with a reduction in liquid viscosity and in relative volatility. (Note As distillation pressure rises, so does the equilibrium temperature this in turn leads to a decrease in liquid viscosity.)... 

Fr The Froude number, defined by Eq. (6.66) for froth regime clear... 

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