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Unformatted text preview: tion. To confirm this, two runs were made with the SCDS model of Chemcad
for the conditions below. The runs differed only in the bottoms mole flow specification, with
Case 1 being the sum of the toluene solvent and the acetone in the feed, in an attempt to obtain a
distillate of methanol. For Case 2 the bottoms mole flow specification was the sum of the
toluene and the methanol in the feed, in an attempt tot obtain a distillate of acetone. The results,
given on the next page, confirm the effect of the distillation boundary. In Case 1, the distillate is
not methanol, but is close to the distillation boundary near the M-T azeotrope. In Case 2, the
distillate is not acetone, but is close to the distillation boundary and near the A-M azeotrope.
Solvent (toluene) flow rate = 60 mol/s 60oC
Pressure constant at 1 atm
Number of stages = 46 (includes the total condenser and the partial reboiler)
Solvent stage = 10 from the top
Feed stage = 30 from the top
Reflux ratio = 5
Bottoms mole flow rate: Case 1, 90 mol/s
Case 2, 70 mol/s
Estimated distillate rate: Case 1, 10 mol/s
Case 2, 30 mol/s
Estimated reflux rate:
Case 1, 50 mol/s
Case 2, 150mol/s (reflux ratio of 5)
Estimated temperatures: Stage 1, 55oC Stage 46, 75oC Stage 2, 60oC
Stage 45, 70oC Exercise 11.11 (continued)
Case 1 material balance:
Case 2 material balance:
70.00 For both cases, most of the methanol moves to the distillate because, in the presence of a
substantial fraction of toluene, methanol is more volatile than acetone. Exercise 11.11 (continued)
Analysis: (continued) Exercise 11.12
Subject: Separation of n-heptane from toluene by extractive distillation with phenol.
Given: 400 lbmol/h of a feed of an equimolar mixture at 200oF and 20 psia of n-heptane and
toluene. 1200 lbmol/h of phenol solvent at 20 psia and 220oF.
Assumptions: . Column operations at 20 psia UNIFAC for K-values. Negligible tray pressure
drop. 100% tray efficiency.
Find: Suitable column designs to obtain reasonable product purities, with only a small loss of
Using the vapor pressure data in Chemcad, the boiling points at 20 psia are found to be: 229.1oF
for n-heptane, 251.4oF for toluene, and 379.0oF for phenol. Thus, n-heptane appears to be the
most volatile component. Although n-heptane and toluene do not form an azeotrope at 20 psia,
their relative volatility at high concentrations of n-heptane is almost 1.0. Therefore, ordinary
distillation of the mixture is not practical. Phenol does not form an azeotrope with either
n-heptane or toluene, and with its higher boiling point is a possible solvent. Its selection is
further enhanced because, as might be expected on molecular structure considerations, phenol at
say a mole fraction in the liquid of 0.5 causes a larger liquid-phase activity coefficient for nheptane than for toluene. For example, for 50 mol% phenol in the liquid and at low
concentrations of toluene, the...
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