Entrainment Correlation For Sieve Plate

Weir height

V2-3 in

Entrainment Correlation For Sieve Plates

Figure 6.1$ Fair's entrainment correlation. (J. Ft. Fair, Petrol Chem Engr. 33 (JO), p. 45,1961, reprinted courtesy of Petroleum. Engineer International, Dallas, Texas.)

Figure 6.1$ Fair's entrainment correlation. (J. Ft. Fair, Petrol Chem Engr. 33 (JO), p. 45,1961, reprinted courtesy of Petroleum. Engineer International, Dallas, Texas.)

dependently by the extensive data analysis of Koziol and Mackowiak (55c). In the froth regime, the Kister and Haas correlation does not apply, and Fair's correlation has little competition. Fair's correlation (Fig. 6.16) predicts entrainment in terms of the flow parameter [Eq. (6.7)], and the ratio of gas velocity to flooding gas velocity. Fractional entrainment is defined as

The restrictions applying to the use of Fair's flooding correlation (Sec. 6.2.6) also apply to the use of Fair's entrainment correlation (5).

Several other entrainment correlations have been reported in tbe literature (17,27,29,39,40,54). Some of their limitations were described elsewhere (12,36,40). In the spray regime, Koziol and Mackowiak

(55a) described the agreement between two of these correlations (17,29) and experimental data as "rather poor."

6.2.12 Sieve tray weeping

Weeping is liquid descending through the tray perforations. Under weeping conditions, part of the liquid flows over the outlet weir while the rest descends through the perforations. The liquid descending through the perforations short-circuits the primary contacting zone, causing a reduction in tray efficiency.

At the tray floor, the static liquid head tends to force liquid down through the perforations. The vapor pressure drop counteracts the downward force and acts to keep liquid on the tray. Weeping takes place when the liquid head on the tray exceeds the pressure drop that is holding the liquid on the tray.

For most systems and tray designs, weeping takes place to some extent under practically all conditions. Sloshing and oscillation of liquid on the tray cause the liquid depth to vary instantaneously at different locations on the tray. With the pressure essentially constant in the vapor space below, weeping occurs at those locations where the liquid head is temporarily high, even when the average head on the plate does not exceed the tray vapor pressure drop.

Weeping may not be uniform across the tray. Tests by Banik and Lockett (56,57) in a 4 ft x 2 ft rectangular simulator showed fairly uniform weeping at low weir heights. With higher weirs (2 in and more), most of the weep issued from the inlet half of the tray at low liquid rates (< 3 gpm/in of outlet weir), while at high liquid rates (> 6 gpm/in of outlet weir) most of the weep issued from the exit half of the tray. A tray with high fractional hole area was observed to weep mostly from the inlet region even at high liquid flow rates. When weeping was nonuniform, some areas appeared to be totally bubbling while others totally weeping.

Some weep can be tolerated without appreciably affecting tray efficiency (4,58,59). Some mass transfer to and from the weeping liquid occurs, and this reduces the impact of bypassing on efficiency (60). Weep from the exit half of the tray is far less detrimental to tray efficiency than weep from the inlet half of the tray (57), and can be tolerated to a much greater degree.

The weep point is defined as the vapor rate when weeping first becomes noticeable. At that point, little efficiency is lost. As vapor rate is reduced below the weep point, the fraction of tray liquid falling through the holes increases, and the reduction in efficiency becomes more noticeable. When this fraction is sufficiently large to effect a sig nificant reduction in tray efficiency, the actual lower tray operating limit is reached.

The mechanism of weeping is not well understood. Lockett and Banik (56) observed the weeping mechanism to vary with hole diameter and with the weep rate (Fig. 6.17). In one mechanism, weeping liquid bridged into a layer that covered the underside of the tray, sometimes extending over holes (Va-in holes), at other times not (Vb-in holes, slight weep). Liquid disengaged as streams from this layer. In another mechanism, liquid jets disengaged directly from the holes. The jets sometimes filled the hole (Vi-in holes; heavy weep from V2-in holes), at other times did not (Vh-in holes, moderate weep).

Y11W FROM MM Of TW TMT

□ INMCATIS LArCR

Of LIQUID

□ INMCATIS LArCR

Of LIQUID

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