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average width of flow path, and the vapor Schmidt number, which however was not varied. The
effect of pressure was determined from data on the acetone-benzene system over a range from 20
to 92 psia. This system was largely gas-phase controlling because the slope, m, in Eq. (7-50) was
kept small. After correcting for a small mass-transfer resistance in the liquid phase and mixing,
the results for the acetone-benzene system were in reasonable agreement with the correlation
developed from the ammonia-air-water system at 1 atm.
The transfer-unit correlation for the liquid phase is a function only of the liquid
diffusivity, the contact time of the liquid on the tray, and the F-factor. The correlation was
developed from data on the desorption of small amounts of oxygen from water into air. Because
of the small solubility of oxygen in air, the mass-transfer resistance is almost wholly in the liquid
phase. To check the difference between an aqueous system and an organic system, the normal
pentane-paraxylene system was used at very low concentrations of normal pentane such that λ in
Eq. (7-48) was very large and, thus, the system was controlled by mass transfer in the liquid
phase. Corrections for gas-phase resistance and mixing were more difficult, but the results
compared well with the correlation based on the oxygen-air-water system. Exercise 12.7 (continued)
For both AIChE correlations, the effect of physical properties was not explored over any
significant range. Based on previous published work, the Schmidt number exponent was
assumed to be -0.5 in the NV correlation and the liquid diffusivity exponent was assumed to be
0.5 in the NL correlation. Because it was not found necessary to incorporate any bubble-cap
geometry into the correlation, it was assumed by others that the AIChE method could be applied
to sieve and valve trays as well. This was fortunate because soon after the AIChE program,
bubble-cap trays fell into disfavor for new installations because of the good efficiency and
capacity, together with lower pressure drop, for the less expensive sieve and valve trays.
An advantage in the application of the AIChE method is that it is independent of tray
design features such as bubble-cap size and spacing or hole diameter and hole area for sieve
trays. However, it has received the following criticisms:
1. It is based on a very narrow range of Schmidt number in the gas phase.
2. It is based on data for absorption and desorption, and not for distillation (Chen and Chuang,
3. It overpredicts point efficiencies for liquid-phase controlled systems operating at low liquid
flow rates (Dribika and Biddulph, 1992).
4. It is based on systems where the mass-transfer resistance is confined entirely to either the gas
phase or the liquid phase (Chen and Chuang, 1995).
5. It greatly overpredicts NL for distillation systems (Chen and Chuang, 1995).
6. It underpredicts point efficiencies for sieve trays with small holes (Dribika and Biddulph,
7. It significantly underpredicts point efficiencies of distillation systems (Korchinsky, Ehsani,
and Plaka, 1994).
The method of Harris (Ref. 21) h...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land