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Calculated values of HETP were:
Methanol: 1.1 to 1.8 m
Ethanol: Very wide range
Water: 0.8 to 1.25 m
A pressure drop of 0.06 atm was estimated. Estimated column diameters were 1 m above and
1.28 m below the feed entry. Exercise 12.20
Subject: Absorption of benzene from a near-air stream by n-tridecane in a sieve-tray tower
using the rate-based method.
Given: A gas feed at 1 atm, 300 K, and 0.01487 kmol/s, with 75.05 mol% nitrogen, 19.95 mol%
oxygen, and 5.0 mol% benzene. Absorption oil at 1 atm, 300K, and 0.005 kmol/s of 99.5 mol%
n-tridecane and 0.5 mol% benzene. Tray geometry with 0.5 m tray spacing, 0.05 m weir height,
0.003 m hole diameter, and 0.002 m tray thickness. 80% of flooding.
Find: Number of actual trays required, column diameter, and Murphree vapor tray efficiency of
benzene for all possible combinations of vapor and liquid flow patterns.
Analysis: The Chemsep program was used with non-equilibrium, absorber options and the SRK
EOS was applied for K-values and enthalpies. Default properties and the Hayduk-Minhas
method for liquid diffusivities were specified. The Chan-Fair method was used to estimate masstransfer coefficients and interfacial area. For each combination of flow patterns, the number of
actual trays was varied to achieve close to 96% absorption of benzene. In all cases the material
balance for the other components was essentially the same as follows:
Flow rate, kmol/h
Total: Feed gas
18.00 Lean gas
50.99 Rich oil
20.53 The number of actual trays and the Murphree vapor tray efficiency for the combinations of flow
patterns were as follows. For all three cases, the column diameter was 0.63 to 0.65 m. Vapor flow Liquid flow
Plug Number of trays
7.5 Benzene tray efficiency, %
49 to 55
64 to 67
73 to 75 The number of equilibrium stages was determined to be about 6. In this exercise, with only one
component (benzene) undergoing any significant mass transfer, an equilibrium-stage method is
adequate, but only if a good estimate of the tray efficiency is available. For the relative small
column diameter, the mixed-mixed flow-pattern case is probably best. Exercise 13.1
Subject: Evaporation of a small amount of n-heptane (C7) from a drum of toluene (T). Given: A drum of toluene containing 2 mol% C7. Simple batch differential (Rayleigh)
distillation at 1 atm. Vapor-liquid equilibrium data for the C7-toluene system at 1 atm.
Assumptions: Perfect mixing in the still. Exiting vapor in equilibrium with the liquid.
Find: (a) Composition of the residue in the pot as a function of wt% distilled. Composition of
residue and the cumulative distillate at 50 wt% distilled.
(b) Composition of the cumulative distillate and the residue...
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