Entrainment flooding

These two types of flooding are usuaUy considered separately when a plate column is being rated for capacity. For identification purposes they are caUed entrainment flooding (or priming ) and downflow flooding. When counterflow action is destroyed by either type, transfer efficiency is lost and reasonable design hmits have been exceeded. 

Entrainment Flooding The early work of Souders and Brown [Ind. Eng. Chem., 26, 98 (1934)] based on a force balance on an average suspended droplet of hquid led to the definition of a capacity parameter C,i, ... 

An alternate method for predicting the flood point of sieve and valve plates has been reported by Kister and Haas [Chem. Eng. Progi , 86(9), 63 (1990)] and is said to reproduce a large data base of measured flood points to within 30 percent. It applies to entrainment flooding only (values of Flc less than about 0.5). The general predictive equation is... 

For distillations, it is often of more interest to ascertain the effect of entrainment on efficiency than to predic t the quantitative amount of liquid entrained. For this purpose, the correlation shown in Fig. 14-26 is useful. The parametric curves in the figure represent approach to the entrainment flood point as measured or as predicted by Fig. 14-25 or some other flood correlation. The abscissa values are those of the flow parameter discussed earher. The ordinate values y are fractions of gross hquid downflow, defined as follows ... 

Most studies have used the Souders-Brown [67] droplet settling velocity concept to relate entrainment flooding. In this mechanism, flooding develops due to a sufficiently high upward vapor velocity through the cross-section of the net area of the column to suspend droplets, and is expressed as the Souders-Brown flooding constant, Csb> [94, 183, 184]. 

Kister and Haas [184] recommend using 25 dynes/cm in Equation 8-286 when the actual surface tension is a 25 dynes/cm. This correlation is reported [94, 184] to give better effects of physical properties, and predicts most sieve and valve tray entrainment flood data to 15 to 20%, respectively. 

At lower tray spacing, entrainment flooding may be related to lifting of the froth envelope and to froth rather than spray height. This correlation must not be extended to lower tray spacing. 

Fair s [183] design procedure to establish an entrainment flooding condition or point is as follows Design Procedure (From Fair, Reference 183, by permission)... 

Figure 8-137 is used for estimating the entrainment-flood point. Liquid particle entrainment is generally considered as reducing tray efficiency (performance). 

The calculated entrainment values may be as good or better than measured values [183]. Figure 8-139 illustrates comparison of entrainment between bubble cap and sieve trays. Fair [183] concludes that for vacuum to moderate pressure applications, sieve trays are advantageous from an entrainment-flooding stand-point. 

Example 8-37 Sieve Tray Splitter Design for Entrainment Flooding Using Fair s Method (used by permission [183])... 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

The entrainment flooding prediction methods described here are based primarily on spray entrainment flooding. Considerations unique to froth entrainment flooding can be found elsewhere (Kister, Disfiliation Design, McGraw-Hill, New York, 1992). 

The Souders and Brown constant CSB is the C-factor [Eq. (14-77)] at the entrainment flood point. Most modern entrainment flooding correlations retain the Souders and Brown equation (14-80) as the basis, but depart from the notion that CSB is a constant. Instead, they express CSb as a weak function of several variables, which differ from one correlation to another. Depending on the correlation, CSB and us,flood are based on either the net area or on the bubbling area AB. 

A correlation for valve tray entrainment flooding that has gained respect and popularity throughout the industry is the Glitsch Equation 13 (Glitsch, Inc., Ballast Tray Design Manual, 6th ed., 1993 ... 

The Fair correlation [Pet/Chem Eng. 33(10), 45 (September 1961)] for decades has been the standard of the industry for entrainment flood prediction. It uses a plot (Fig. 14-31) of surface-tension-corrected Souders and Brown flood factor CSB against the dimensionless flow parameter shown in Fig. 14-31. The flow parameter represents a ratio of liquid to vapor kinetic energies ... 

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 ... 

Fair (paper presented at the AIChE Annual Meeting, Chicago, 111., November 1996) correlated data for efficiency reduction due to the rise of entrainment near entrainment flood, getting... 

Alternatively, at 80 and 90 percent of entrainment flood, the median value of f from Eq. (14-99) is... 

Vapor Channeling All the correlations in this section assume an evenly distributed tray vapor. When the vapor preferentially channels through a tray region, premature entrainment flood and excessive entrainment take place due to a high vapor velocity in that region. At the same time, other regions become vapor-deficient and tend to weep, which lowers tray efficiency. 

As the liquid holdup increases, the effective orifice diameter may become so small that the liquid surface becomes continuous across the cross section of the column. Column instability occurs concomitantly with a rising continuous-phase liquid body in the column. Pressure drop shoots up with only a slight change in gas rate (condition C or C ). The phenomenon is called flooding and is analogous to entrainment flooding in a tray column. 

Equations (3.89) and (3.90) equate the tray active area vapor loading VN to the maximum VM for determining the gas in liquid entrainment flooding of the tray. The early work of Souders and Brown [12], based on a force balance on an average suspended droplet of liquid, led to the definition of a capacity parameter VM- Both VN and Where refer to the active area of the tray. This active area is simply the net tower cross-section internal area less the downcomer areas. The downcomer areas include both the downcomer inlets and outlets. 

Equation (3.89) is the sieve tray liquid entrainment flood gas loading equation. Equation (3.89) sets the maximum gas rate VM- At a higher Vm, excess gas-liquid froth buildup would reach the tray above and recycle liquid to it. This liquid recycle would build up to a point at which the liquid would block any vapor passage, resulting in a flooded column and costly shutdown. 

Equation (3.91) is the jet flood equation. The chief difference between this equation and the entrainment flood equations, (3.88) through (3.90), is the area references. Equation (3.91) is based on the total sieve tray hole area for gas passage, and Eq. (3.88) through (3.90) are based on the tray active area. Again, the tray active area is simply the tower cross-sectional area less the total downcomer area. 

Entrainment flooding is predicted by an updated version of the Souders and Brown correlation. The most popular is Fair s (1961) correlation (Fig. 20), which is suitable for sieve, valve, and bubble-cap trays. Fair s correlation gives the maximum gas velocity as a function of the flow parameter (L/G)V(Pg/Pl), tray spacing, physical properties, and fractional hole area. 

Tray area is usually determined from an entrainment flooding correlation. Trays are normally designed to operate at 80 to 85% of flood at the maximum expected throughput. Downcomer area is usually determined from the downcomer choke criteria. The design is then checked to ensure that downcomer backup flood does not occur. 

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