CHE425: Problem set #5
1) Run program CASCADES. You need to copy the folder CHE425 into the H: drive or your
flash drive. Open the folder CHE425 and double click on DOSBox.exe. When the prompt
“C:\>” appears, type CASCADES and press “ENTER” to run the program. Copy and report
the score and performance number from the program. Type “e” or “exit” and press “ENTER”
to exit the DOSBox program.
2) 1A liquid mixture containing 10 mol % n-heptane and 90 mol % n-octane is fed at its
boiling point to the top of a stripping tower at 101.32 kPa. Figure 2 depicts a stripping tower
where the feed stream is the saturated liquid and the distillate stream is the saturated vapor.
There is no reboiler or condenser in a stripping tower. The bottoms are to contain 98 mol %
n-octane. For every 3 mol of feed, 2 mol of vapor is withdrawn as product. Calculate the
composition of the vapor and the number of theoretical plates required. The equilibrium data
are given below
x
y
0
0
0.012
0.025
0.039
0.078
0.067
0.131
0.097
0.184
0.284
0.459
V=D
F
B
Figure 2 Stripping tower with feed F and product D = V.
1
Geankoplis, C.J., Transport Processes and Separation Process Principles, 4th ed., Prentice Hall, 2003, p. 754
3) 2A liquid feed at the boiling point contains 3.3 mol % ethanol and 96.7 mol % water and
enters the top tray of a stripping tower shown in Figure 3.
V, y D
F
xF
L
x
V
y
S (Steam)
B, x B
Figure 3 Stripping tower and direct steam injection.
Saturated steam is injected directly into liquid in the bottom of the tower. The overhead
vapor which is withdrawn contains 99% of the alcohol in the feed. Assume equimolar
overflow for this problem. Equilibrium data for mole fraction of alcohol are as follows at
101.32 kPa abs pressure.
x
y
0
0
0.0080
0.0750
0.020
0.175
0.0296
0.250
0.033
0.270
(a) For an infinite number of theoretical steps, calculate the minimum moles of steam
needed per mole of feed. (Be sure and plot the q-line)
(b) Using twice the minimum moles of steam, calculate the number of theoretical steps
needed, the composition of the overhead vapor, and the bottoms composition.
4. 3(a) For the cascade shown in Figure 4, calculate the composition of streams V4 and L1.
Assume constant molar overflow, atmospheric pressure, saturated liquid and vapor feeds, and
the vapor-liquid equilibrium data given below.
x
y
2
3
0
0
0.10
0.20
0.30
0.50
0.50
0.68
0.70
0.82
0.90
0.94
1.000
1.000
Geankoplis, C.J., Transport Processes and Separation Process Principles, 4th ed., Prentice Hall, 2003, p. 754
J. D. Seader and E. J. Henley, Separation Process Principles, Wiley, 1998
Total
condenser
100 moles
70% alcohol
30% water
V4
V4
LR
D
50 moles
4
4
3
3
2
2
1
1
100 moles
30% alcohol
70% water
L1
(a)
100 moles
30% alcohol
70% water
L1
(b)
Figure 4 Vapor-liquid equilibrium cascade.
(b) Given the feed compositions in cascade (a), how many stages would be required to
produce a V4 containing 85% alcohol?
(c) For the configuration in cascade (b), with D = 50 moles what are the composition of D
and L1?
(d) For the configuration in cascade (b), how many stages are required to produce a D of
50% alcohol?
5. Determine the overall efficiency of a distillation column used to separate methanol from
water. The average temperature and pressure of the column are 80oC and 1 atm, respectively.
The feed is saturated liquid with mole fraction of methanol equal to 0.40 corresponding to
equilibrium vapor mole fraction of 0.718.
6. Determine the plate efficiency of a distillation column used to separate methanol from
water. The plate is at 80oC and 1 atm. The cross-sectional area of the column is 0.50 m2. The
vapor volumetric flow rate is 0.95 m3/s. The following data are provided
hw (m)
Ah (m2)
L (kg/m3)
v (kg/m3)
L(kg/ms)
v(kg/ms)
806
0.826
0.336×10-3
10.6×10-6
50×10-3
0.038
Atomic volume of
m3/kmol
C
0.0148
H
0.0037
O
0.0074
7. A liquid mixture of benzene-toluene is to be distilled in a fractionating tower at 760
mmHg. The feed is saturated liquid with a flow rate of 100 kmol/h, containing 50 mol %
benzene and 50 mol % toluene, and enters at 327.6 K. We want to obtain a distillate
containing 95 mol % benzene and 5 mol % toluene and a bottoms containing 2 mol %
benzene and 98 mol % toluene. The reflux ratio is 2.0. The average heat capacity of the feed
is 159 kJ/kmol and the average latent heat 32,099 kJ/kmol. Use enthalpy balances to
determine the temperatures, flow rates, and composition of the streams entering and leaving
the first two stages of the column (after the condenser).
Physical property data are given in the following Table 11.
Component
Tb(oC)
Cp(kJ/kmolK)
Cpy(kJ/kmolK)
Benzene (A)
80.1
138.2
96.3
Toluene (B)
110.6
167.5
138.2
(kJ/kmol)
30,820
33,330
Vapor pressure: Psat (mmHg), T(oC)
log10 PAsat = 6.90565  1211.033/(T + 220.79)
log10 PBsat = 6.95334  1343.943/(T + 219.377)
You need to provide the following information
Stage 1: T1(oC) = ________
Stage 2: T2(oC) = ________
L0(kmol/h)
L1(kmol/h)
L2(kmol/h)
x0
x1
x2
V1(kmol/h)
V2(kmol/h)
V3(kmol/h)
y1
y2
y3
Note: x and y are the mole fraction of benzene.
1
Geankoplis, C.J., Transport Processes and Separation Process Principles, 4th edition, Prentice Hall, 2003, p.
736