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Unformatted text preview: int, the pressure = 165 psia = 23.9
psia. Assume a saturated liquid feed. The bubble-point temperature is estimated to be 215o F.
Take an average temperature = 205oF, at which the vapor pressures for benzene and toluene,
respectively, are 23 psia and 9 psia respectively. From Eq. (2-44), the corresponding K-values at
the average section pressure of 21.5 psia are 1.07 and 0.42. The relative volatility, from Eq. (221) = 1.07/0.42 = 2.55. From the Fenske equation (9-12), applied between the distillate and feed
N min =
= 7.1 stages
log α B,T
(b) For the section between the feed and the vapor sidestream, again take the LK as benzene and
the HK as toluene. The pressure at the vapor sidestream = 180 kPa = 26.1 psia. A dew-point on
the vapor sidestream gives a temperature of 265oF. Assume an average temperature for the
section of 240oF, at which the vapor pressures for benzene and toluene, respectively, are 37 psia
and 16 psia respectively. From Eq. (2-44), the corresponding K-values at the average section
pressure of 25 psia are 1.48 and 0.64. The relative volatility, from Eq. (2-21) = 1.48/0.64 = 2.31. Exercise 9.5 (continued) Analysis: (b) (continued) From the Fenske equation (9-12), applied between the feed and vapor sidestream compositions:
N min = fB
vsB log α B,T log
= 260 79.4
log 2.31 = 5.3 stages (c)
In the section between the vapor sidestream and bottoms, take toluene as the LK and
biphenyl as the HK. At the bottom of the column, the pressure = 200 kPa = 29 psia. Biphenyl
boils at 255oC = 491oF at 14.7 psia, which is much higher than toluene. From Perry's Handbook,
the following vapor pressure data are obtained for biphenyl:
Vapor pressure, psia 243
7.74 A bubble point on the bottoms pressure gives 465oF. At this temperature, the relative volatility,
αT,BP = vapor pressure of T/vapor pressure of BP = 220/10.6 = 20.8.
At the temperature of the vapor sidestream, 265oF, αT,BP = vapor pressure of T/vapor pressure of
BP = 27.9/0.37 = 75.4. Use a geometric mean for this wide range. Thus, αT,BP = [20.8(75.4)]1/2 =
From the Fenske equation (9-12), applied between the vapor sidestream and bottoms
N min =
= 2.3 stages
log α B,T
log 40 Exercise 9.6
Comparison of minimum number of stages by Fenske equation and McCabeThiele method for a non-ideal system of acetone (A) and water (W).
Given: A feed of 25 mol% A and 75 mol% W. Distillation at 130 kPa (975 torr) to obtain a
distillate of 95 mol% A and a bottoms of 2 mol% A. Infinite-dilution liquid-phase activity
coefficients of 8.12 for acetone and 4.13 for water.
Find: Minimum stages by the Fenske equation and by the McCabe-Thiele method.
Analysis: Because the pressure is close to 1 atm, estimate vapor-liquid equilibria from the
modified Raoult's law, Eq. (4) in Table 2.3. Obtain vapor pressures fr...
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- Spring '11
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