Unformatted text preview: to, but present a new correlation for the mass-transfer area, which accounts for masstransfer area other than the packing wetted area. The use of the packing wetted area as the area
for mass transfer is criticized by Bravo and Fair because it ignores the area provided by
suspended and falling droplets, gas bubbles within liquid, ripples on the liquid film surface, and
wall films. Their area depends on the packing area, a capillary number, a Reynolds number for
the vapor, the surface tension, and the height of the packing. Because the liquid-phase Reynolds
number is a function of the area density for mass transfer, Bravo and Fair compute values of the
liquid-phase mass transfer coefficient different from Onda, Takeuchi, and Okumoto. The
correlation of Bravo and Fair is based on data for Pall rings as well as Raschig rings and Berl
saddles. It is also recommended for Intalox saddles. As discussed by Shariat and Kunesh, 1995,
the efficiency of the reflux distributor can have a large effect on mass transfer. As discussed by
Brierley, 1994, the efficiency of random packings for high pressure operation may be lower than
predicted. Data and correlations are needed for the newer high void-fraction packings. Exercise 12.8 (continued)
In Section 12.3, the following correlations are listed:
1. Bravo, Rocha, and Fair, Ref. 28 (1985).
2. Bravo, Rocha, and Fair, Ref. 29 (1992).
3. Billet and Schultes, Ref. 30 (1992).
Reference 28 presents a method for estimating HETP for distillation in columns with
structured packing of the gauze type, such as Sulzer BX and Gempak 4BG. The correlations
consist of: (a) gas-phase mass-transfer coefficient in terms of an equivalent flow channel
diameter, gas diffusivity, vapor density and viscosity, vapor and liquid velocities, and vapor
Schmidt number; (b) liquid-phase mass-transfer coefficient in terms of liquid diffusivity, liquid
channel velocity, and channel side dimension; and (c) area for mass transfer equal to the packing
surface area. The equivalent flow channel diameter is based on the hydraulic radius concept.
The correlation was tested on four chemical systems distilled in columns ranging in diameter
from 0.07 to 1.0 m, and at pressures from 10 to 760 torr. Good agreement was obtained with
measured HETP, computed from equations in Table 6.7 and 7.6.
Reference 29, and a more recent article by the same authors, Rocha, Bravo, and Fair, Ind.
Eng. Chem. Res., 35, 1660-1667 (1996), extend the study of Ref. 28 to more structured packings
(Fexipac, Intalox 2T, Maxpak, and Mellapak) and to other binary systems, covering a wide
pressure range of 0.021 to 20.4 bar, and column diameters of 0.07 to 1.2 m. The gas-phase masstransfer coefficient uses a modified hydraulic diameter and is a function of a Reynolds number
that involves both liquid and gas velocities, the Schmidt number, and the gas diffusivity. The
liquid-phase mass-transfer coefficient uses a modified exposure time to take into account slower
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