Weeping and Entrainment

At high vapor rates, entrainment becomes significant and decreases tray performance. To take this into account, Bennett et al. (1997) defined the fractional entrainment E as 

Example 4.8 Weeping and Entrainment in a Sieve-Tray Ethanol Absorber 

Estimate the entrainment flow rate for the ethanol absorber of Examples 4.4,4.6, and 4.7. Determine whether significant weeping occurs. 

check whether weeping is significant. From equation (4-43), using information from the previous examples, we have 

It can be concluded from this result that weeping is not a problem under these circumstances. Next, estimate the entrainment flow rate. From equation (4-47), 

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DESIGNER also contains different hydrodynamic models (e.g., completely mixed liquid-completely mixed vapor, completely mixed liquid-vapor plug flow, mixed pool model, eddy diffusion model) and a model library of hydrodynamic correlations for the mass transfer coefficients, interfacial area, pressure drop, holdup, weeping, and entrainment that cover a number of different column internals and flow conditions. 

The Chan and Fair correlation does not include a term that per se accounts for weeping and entrainment. In principle, however, the FF... 

Because of their simplicity and low cost, sieve (perforated) trays are now the most important of tray devices. In the design of sieve trays, the diameter of the tower must be chosen to accommodate the flow rates, the details of the tray layout must be selected, estimates must be made of the gas-pressure drop and approach to flooding, and assurance against excessive weeping and entrainment must be established. 

Uncorrected for effects of weeping and entrainment (used to subscript Murphree tray efficient ). 

FIG. 14-43 Efficiency reduction when fractional hole area is increased, also showing little effect of vapor and liquid loads on efficiency in the normal operating range (between excessive weeping and excessive entrainment). Also shown is the small increase in efficiency with pressure. FRI data, total reflux, Dt = 1.2 m, S = 610 mm, hw = 50.8 mm, d% = 12.7 mm. (Reprinted with permission from T. Yanagi and M. Sakata, Ind. Eng. Chem. Proc. Des. Dev. 21, 712 copyright 1982, American Chemical Society.)... 

A liquid level is maintained with an overflow weir while the vapor comes up through the perforated floor at sufficient velocity to keep most of the liquid from weeping through. Hole sizes may range from 1/8 to 1 in., but are mostly l/4-l/2in. Hole area as a percentage of the active cross section is 5-15%, commonly 10%. The precise choice of these measurements is based on considerations of pressure drop, entrainment, weeping, and mass transfer efficiency. The range of conditions over which tray operation is... 

From these results, we conclude that the plate design is appropriate from the pont of view of pressure drop, entrainment, weeping, and plate efficiency. The next step in the design is to determine the number of real stages required. Since the plate efficiency does not change much from one end of the tower to the other, we will assume a constant value taken as the arithmetic average of the efficiency values at the tower ends Emce = (0.77 + 0.73)/2 = 0.75. 

Fig. 2-4. Examples of typical scan profiles obtained for various problems met on distillation columns, respectively from up and left to bottom and right normal column, collapsed tray, flooding, entrainment, weeping and foaming (IAEA, 2002).

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