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Unformatted text preview: onsistently good predictions. For two of three chemical systems (Korchinsky, Ehsani,
and Plaka, 1994), the Chan and Fair method gives the best predictions.
Chen and Chuang, in presenting their method (Ref. 25), criticize methods based on
absorption and desorption data because they may involve different interfacial areas. Their
method is based on separating the interfacial area from the mass-transfer coefficients. The
former is determined from a force balance suggested in a book by Levich (1962), and depends on
liquid viscosity, % hole area, F factor, liquid density, and surface tension. The mass-transfer
coefficients are both based on Higbe's penetration theory. Thus, they depend on diffusivities and
contact times. The Chen and Chuang method compares very well with FRI data on three
hydrocarbon systems over a very wide pressure range of 13.3 to 2758 kPa. No comparisons are
given for non-ideal organic or aqueous systems.
Finally, Koziol and Mackowiak [Chem. Eng. Technol., 15, 103-113 (1992)] criticize
methods for predicting mass-transfer coefficients in trayed columns with downcomers because
they ignore effects of entrainment and weeping. An exception, as discussed by Kooijman and
Taylor (Ref. 31) is the Chan and Fair method, which accounts for both weeping and entrainment
by using an empirical quadratic function of fraction of flooding to correlate gas-phase mass
transfer. Kooijman and Taylor also point out that although mass transfer in distillation is largely
controlled by the gas phase, the liquid-phase resistance may contribute up to 30% of the total
mass-transfer resistance. Exercise 12.8
Subject: Correlations for estimating binary-pair mass-transfer coefficients for packed columns.
Find: Advantages and disadvantages of the correlations for random and structured packings
In Section 12.3, the following correlations are listed:
1. Onda, Takeuchi, and Okumoto, Ref. 26, (1968).
2. Bravo and Fair, Ref. 27 (1982).
The correlations of Onda, Takeuchi, and Okumoto were developed from data on
absorption and desorption in columns packed with Raschig rings and Berl saddles. Separate
correlations are presented for the wetted area, the gas-phase mass-transfer coefficient, and the
liquid-phase mass-transfer coefficient. The wetted area depends on the packing surface area, the
surface tension, critical surface tension of the packing, and Reynolds, Froude, and Weber
numbers for the liquid. The gas-phase mass-transfer coefficient depends on the packing surface
area density, the gas diffusivity, the nominal packing size, and the Reynolds and Schmidt
numbers for the gas phase. The liquid-phase mass-transfer coefficient, obtained from aqueous
and organic solvent systems, depends on the liquid density and viscosity, packing surface area
density, nominal packing size, and Reynolds and Schmidt numbers for the liquid. The liquid
phase is correlated much better than the gas phase. This is unfortunate because the overall
resistance in distillation is often dominated by the gas phase.
Bravo and Fair use the mass-transfer coefficient equations of Onda, Takeuchi, and
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land