CHE425: Problem set #5
1. (Seader 4.23) An equimolar mixture of ethane, propane, n-butane, and n-pentane is
subjected to flash vaporization at 610oR and 205 psia. What are the expected amounts and
compositions of the products? Determine the temperature and pressure of a single-stage flash
to recover 70% of the ethane in the vapor without losing more than 5% of the nC4 to the
vapor.
Equilibrium K values for light hydrocarbon systems
=============================================================
ln K = A/T2 + B  C ln(P) + D/P + E/P2, where P is in psia, T is in oR
Compound
A
B
C
D
E
=============================================================
Ethane
687248.2
7.90694
.866
0
49.02654
Propane
970688.6
7.15059
.76984
6.90224
0
n-Butane
1280557
7.94986
.96455
0
0
n-Pentane
1524891
7.33129
.89143
0
0
=============================================================
Solution
P(psia) = 205
Drum temperature T(R) = 610
Flash temperature, T(R) = 610.00, V/F = 0.4537
Species: Ethane Propane n-Butane n-Pentane
Feed z = 0.2500 0.2500 0.2500 0.2500
x=
0.1007
0.1956
0.3169
0.3868
y=
0.4298
0.3155
0.1695
0.0853
Ki =
4.2692
1.6124
0.5348
0.2205
Percentage recover of ethane in vapor = 78.0008
Percentage lost of n-butane in vapor = 30.7537
>> y11s5p1
P(psia) = 14.6
Drum temperature T(R) = 420
Flash temperature, T(R) = 420.00, V/F = 0.2570
Species: Ethane Propane n-Butane n-Pentane
Feed z = 0.2500 0.2500 0.2500 0.2500
x=
0.1002
0.2463
0.3198
0.3336
y=
0.6831
0.2607
0.0480
0.0082
Ki =
6.8156
1.0583
0.1502
0.0246
Percentage recover of ethane in vapor = 70.2132
Percentage lost of n-butane in vapor = 4.93896
2) 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.
3) Run program MCCABE. 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 MCCABE 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.
4) Run program VLE. 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 VLE and press “ENTER” to run the program. Copy and report the performance
number from the program. Type “e” or “exit” and press “ENTER” to exit the DOSBox
program.
5) Run program BASIS. 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 BASIS and press “ENTER” to run the program. Copy and report the
performance number from the program. Type “e” or “exit” and press “ENTER” to exit the
DOSBox program.
6) A 1000 lb-mole per hour mixture of 5% methane, 20% ethane, 25% propane, 30% nbutane, and 20% n-pentane at 47 psia and 90F is to be separated by a flash drum. Write a
Matlab program using the data in Table 2 to determine the fraction of the feed vaporized and
the compositions of the vapor and liquid streams leaving the flash drum. Compare the results
with those from Provision.
Table 2 Equilibrium K values for light hydrocarbon systems
=============================================================
(1) ln K = A/T2 + B  C ln(P) + D/P2
(2) ln K = A/T2 + B  C ln(P) + D/P
Compound
A
B
C
D
Form
=============================================================
Methane
292860
8.2445
.8951
59.8465
(1)
Ethane
687248.2
7.90694
.866
49.02654
(1)
Propane
970688.6
7.15059
.76984
6.90224
(2)
n-Butane
1280557
7.94986
.96455
0
(1)
n-Pentane
1524891
7.33129
.89143
0
(1)
========================================================
Ans:
V/F = 0.97709
Species
Methane
Ethane
Propane
n-Butane
n-Pentane
Feed mole fraction
x
0.05
.00108
0.20
.02001
0.25
.08247
0.3
.29914
0.2
.59730
Solution
y
.05115
.20422
.25393
.30002
.19068
Flash temperature, T(R) = 550.00, V/F = 0.9771
Species: Methane
Ethane Propane n-Butane
n-Pentane
Feed z = 0.0500 0.2000 0.2500 0.3000 0.2000
x=
0.0011
0.0200
0.0825
0.2992
0.5973
y=
0.0511
0.2042
0.2539
0.3000
0.1907
Ki =
47.3323 10.2064
3.0789
1.0029
0.3192
7) The equilibrium data for the acetone (1)-water (2) system are given at 25oC as follows:
x1
0.0194
0.0289
0.0449
0.0556
0.0939
y1
0.5234
0.6212
0.7168
0.7591
0.8351
50.1
61.8
81.3
91.9
126.1
P(mmHg)
At 25oC the vapor pressures are: P1sat = 230.05 mmHg and P2sat = 23.76 mmHg. Use these
data to calculate the following for the air-water-acetone system.
Ans:
(a) The vapor mol fraction of acetone in equilibrium with a liquid containing 0.0939 mol
fraction acetone at 25oC and a pressure of 2 atm.
(b) The vapor mol fraction of water in equilibrium with the liquid described in part (a).
a) 0.0693
b) 0.0137
8. 1A distillation column is fed 100 kmol/h of a mixture of 50 mol % benzene and 50 mol %
toluene at 101.32 kPa abs pressure. The feed is liquid at the boiling point. The distillate is to
contain 90 mol % benzene and the bottoms 10 mol % benzene. The reflux ratio is 4.52:1.
Calculate the kmol/h distillate, kmol/h bottoms, and the number of theoretical trays needed
using the McCabe-Thiele method. Equilibrium data for benzene-toluene system at 101.32
kPa are given below, where x and y are mole fraction of benzene.
x
y
0
0
0.130
0.261
0.258
0.456
0.411
0.632
0.581
0.777
0.780
0.900
1.000
1.000
Solution
From the graph, we have about 6 theoretical stages or 5 theoretical trays plus a reboiler.
1
Geankoplis, C.J., Transport Processes and Separation Process Principles, 4th edition, Prentice Hall, 2003, p.
753