Unformatted text preview: relative volatility of nheptane with respect to toluene is about 1.7.
At low concentrations of nheptane and 50 mol% phenol in the liquid, the relative volatility of nheptane with respect to toluene is even higher at about 2.8. Thus, phenol makes the separation of
nheptane and toluene quite favorable.
Extractive Distillation Column:
The following Chemcad results, using the Tower (insideout) model, give a reasonable nheptane
distillate product. Convergence was achieved in 15 iterations. Other combinations of stages,
reflux, and solvent and feed stage locations could also give satisfactory results.
Number of stages = 21 (includes total condenser and partial reboiler)
Solvent enters stage 6 from the top
Feed enters stage 13 from the top
Reflux ratio = 5
Bottoms flow rate = 1400 lbmol/h (based on toluene + phenol)
Initial guesses: Distillate rate = 200 lbmol/h
Reflux rate = 1000 lbmol/h
o
o
Temperatures: Stage 1, 200 F Stage 21, 300 F
Stage 2, 220oF
Convergence was achieved in 34 iterations with the following material balance:
Solvent Feed Distillate Bottoms
Temperature, oF
220
200
229.2
323.4
Flow rates, lbmol/h:
nHeptane
0
200
198.2
1.8
Toluene
0
200
1.2
198.8
Phenol
1200
0
0.6
1199.4 Exercise 11.12 (continued)
Analysis: Extractive Distillation Column (continued)
The liquid phase composition profile is shown below. The condenser and reboiler duties are
15.99 million Btu/h and 24.30 milllion Btu/h, respectively. The extractive distillation column
was sized with Chemcad for sieve trays, with:
24inch tray spacing
12% downcomer area
10% hole area
1/4inch holes
80% of flooding
The resulting column inside diameter was determined to be 8 feet
For a final design, the computations should be repeated taking into account tray pressure drop
and tray efficiency. Exercise 11.12 (continued)
Analysis: (continued)
Ordinary Distillation Column to separate toluene from phenol:
A starting point for this separation is obtained by using the ShortCut Column (FUG)
model of Chemcad with an operating reflux ratio of 1.3 times the minimum ratio, with an
operating pressure of 20 psia. The feed is assumed to be at the bubble point. To achieve
reasonable product purities, the following material balance is assumed:
Component
nHeptane
Toluene
Phenol
Total: lbmol/h:
Feed Distillate
1.8
1.8
198.8
196.8
1199.4
1.2
1400.0
199.8 Bottoms
0.00
2.0
1198.2
1200.2 The specifications for the ShortCut Column model are 196.8/198.8 = 0.99 for the recovery of
toluene to the distillate, and 1.2/1199.4 = 0.001 for the recovery to the distillate of phenol.
The results are:
Minimum number of equilibrium stages = 7.7
Minimum reflux ratio = 1.50
Reflux ratio = 1.3(1.50) = 1.95
Number of equilibrium stages = 15.5
Feed stage location (Kirkbride equation) = 11.4 from the top
The Tower model of the Chemcad program is then used to make a rigorous calculation based on
the following input data, using the FUG results:
Pressure constant at 20 psia.
Number of stages = 16 (includes the total condenser and...
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 Spring '11
 Levicky
 The Land

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