CHE425: Problem set #6
1) Run program ABSORP. 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 ABSORP 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) 1We have 10 kmol/h of a saturated liquid feed that is 40 mol % benzene and 60 mol %
toluene. We desire a distillate composition that is 0.992 mole fraction benzene and a bottoms
that is 0.986 mole fraction toluene. CMO is valid. Assume a constant relative volatility with
BT = 2.4. Reflux is returned as a saturated liquid. The column has a partial reboiler and a
total condenser.
(a) Use the Fenske equation to determine Nmin.
(b) Use the Underwood equation to find (L/D)min
(c) For L/D = 1.1(L/D)min, use the previous results and the Gilliland correlation to estimate
the total number of stages and the optimum feed stage location.
Ans:
Nm = 10.37, Rmin = 1.7488, N = 24.6
3) 2We have an existing column that acts as 30 equilibrium contacts. We are separating a
multicomponent mixture where LK = 1.1 and LK = 1.0. A recovery fraction of 98% is
required for the light key in the distillate. Find the recovery fraction of the heavy key in the
bottoms. Feed is a saturated liquid that contains zLNK = 0.1, zLK = 0.4, zHK = 0.3, zHNK = 0.2.
LNK = 1.25 and HNK = 0.75. Operation is at total reflux.
Ans:
FR of HK = 0.263
4) The following feed of 100 mol/h at the boiling point and 60 psia pressure is fed to a
fractionating tower: n-butane (xa = 0.40), n-pentane (xb = 0.25), n-hexane (xc = 0.20), and nheptane (xd = 0.15). This feed is distilled so that 95% of the n-pentane is recovered in the
distillate and 95% of the n-hexane in the bottoms. Calculate the following.
(a) Moles per hour and composition of distillate and bottoms.
(b) Top and bottom temperature of tower.
(c) Minimum stages for total reflux and distribution of other components (trace components)
in the distillate and bottoms, i.e, moles and mole fractions.
(d) (L/D)min using the Underwood equations.
(e) For L/D = 1.3(L/D)min, use the previous results and the Gilliland correlation to estimate
the total number of stages and the optimum feed stage location.
Table 4 Equilibrium K values for light hydrocarbon systems
=============================================================
(1) ln K = A/T2 + B  C ln(P) , where P is in psia, T is in oR
1
2
J. D. Seader and E. J. Henley, Separation Process Principles, Wiley, 1998
J. D. Seader and E. J. Henley, Separation Process Principles, Wiley, 1998
1
Compound
A
B
C
Form
=============================================================
n-Butane
1280557
7.94986
.96455
(1)
n-Pentane
1524891
7.33129
.89143
(1)
n-Hexane
1778901
6.96783
.84634
(1)
n-Heptane
2013803
6.52914
.79543
(1)
Ans:
(a) Moles per hour and composition of distillate and bottoms.
D = 64.75 mol/h, B = 35.25 mol/h
(b) Top and bottom temperature of tower.
Dew point temperature, T(R) = 607.22, T(C) = 64.19
Bubble point temperature, T(R) = 733.02, T(C) = 134.08
(c) Minimum stages for total reflux and distribution of other components (trace components)
in the distillate and bottoms, i.e, moles and mole fractions.
Nmin = 7.74
Table C Product distribution
Distillate
xd
Bottoms
xb
-3
nC4
40
0.6178
2.38×10
6.75×10-5
nC5, light key
23.75
0.3668
1.25
0.0355
nC6, heavy key
1
0.0154
19
0.5390
nC7
2.0788×10-3
3.21×10-5
15
0.4255
64.75
1.0000
35.25
1.0000
(d) (L/D)min using the Underwood equations.
Rmin = 0.5658
(e) For L/D = 1.3(L/D)min, use the previous results and the Gilliland correlation to estimate
the total number of stages and the optimum feed stage location.
Ne + Ns = 18.7, Ne = 10.2, the feed tray is 10 trays from the top.
5. Run the ChemSep program and produce Table 5 in the example: Distillation
Calculation using ChemSep Software. Copy the ChemSepL folder from the
Distribution folder to your H: drive. Open the ChemSepL folder and double click on cs.bat
to start the program. Note: This program only works properly in your H: drive. If you want
this program on your flash drive you need to download ChemSep-LITE from the website:
http://www.chemsep.org/program/index.html and install it on your flash drive.
Example: Distillation Calculation using ChemSep Software
“ChemSep is a software system for simulation of distillation, absorption, and extraction
operations. ChemSep integrates flash calculations, the classic equilibrium stage column
model and a nonequilibrium or rate-based column model in one easy to use program”.1
ChemSep-LITE is available for FREE. It only includes the equilibrium column model and is
limited to 10 components and 150 stages.
1
http://www.chemsep.org/program/index.html
2
Double click on the cs.bat and the following screen will appear
You can click on the menu: Components, Operation, Properties, Feeds, and
Specifications on the left column to specify your operation and conditions. When all the
necessary specifications are entered into the program, the menu will have a green check next
to it. Let consider the following example:
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A butane-pentane splitter is to operate at 8.3 bar with the following feed composition
Saturated liquid feed (kmol/s)
Propane,
C3
.05
Isobutane,
iC4
.15
Normal butane,
nC4, light key
.25
Isopentane,
iC5, heavy key
.20
Normal pentane
nC5
.35
For a specification of a reflux ratio of 2.5 and split between two components in the bottom
iso-pentane/n-pentane = 0.45, determine the compositions of the product streams.
You can enter the data given in the example and the following information into the program:
Operation
Equilibrium column
Simple distillation
Total condenser
Partial reboiler
Number of stages: 14
Feed stage: 6
Specifications
Constant pressure
Column heat loss: 0
Default stage efficiency: 1
Thermodynamics Properties
K-value: EOS
Equation of state: Soave-RK
Enthalpy: Soave-RK
Leave blank for
Thermodynamic Model
Parameters
Column Specifications
Split between two components
iso-pentane/n-pentane = 0.45
Feeds
State: Pressure & Vapor
fraction
You will need to explore this program to be familiar with the menu and sub-menu. You
should read the ChemSep Help and ChemSep Book to know more about the program and
its functions. After you enter all the necessary information into the program you will arrive at
the following screen:
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Choose Quick Solve in the menu and you will get the following results:
Table 5 Result
Stream
Stage
Pressure (N/m2)
Vapour fraction (-)
Temperature (K)
Enthalpy (J/kmol)
Entropy (J/kmol/K)
Feed1
6
Top
1
830000
0.000000
355.242
-1.428E+07
-41313.7
Bottom
14
830000
830000
0.000000 0.000000
338.339 384.767
-1.548E+07 -1.032E+07
-48735.5 -33575.1
Mole flows (kmol/s)
Propane
0.0500000 0.0499994 5.5389E-07
Isobutane
0.150000 0.149712 2.8757E-04
N-butane
0.250000 0.246904 0.00309628
Isopentane
0.200000 0.0630532 0.136947
N-pentane
0.350000 0.0456737 0.304326
Total molar flow
Mole fractions (-)
Propane
Isobutane
1.00000
0.555342
0.444658
0.0500000 0.0900335 1.2456E-06
0.150000 0.269586 6.4674E-04
5
N-butane
Isopentane
N-pentane
0.250000 0.444597 0.00696328
0.200000 0.113539 0.307983
0.350000 0.0822443 0.684406
6) Use ChemSep to simulate a distillation column with 100 mol/h of feed at the boiling point
and 410 psia pressure and the following composition: n-butane (xa = 0.40), n-pentane (xb =
0.25), n-hexane (xc = 0.20), and n-heptane (xd = 0.15). You can use the following
specification:
Operation
Equilibrium column
Simple distillation
Total condenser
Partial reboiler
Number of stages: 19
Feed stage: 10
Specifications
Constant pressure
Column heat loss: 0
Default stage efficiency: 1
Thermodynamics Properties Feeds
K-value: DECHEMA
State: Pressure & Vapor
Activity Coefficient: Regular fraction
solution
Vapor pressure: Antoine
Enthalpy: Soave-RK
Column Specifications
Reflux ratio: 0.75
Split between two components
N-pentane/N-hexane = 0.004
Report the following.
(a) Moles per hour and composition of distillate and bottoms.
(b) Top and bottom temperature of tower.
Click on the FUG Result and report the following
(c) Minimum stages for total reflux
(d) Minimum reflux ratio.
(e) The total number of stages and the optimum feed stage location.
Stream
Stage
Pressure (N/m2)
Vapour fraction (-)
Temperature (K)
Enthalpy (J/kmol)
Entropy (J/kmol/K)
Mole flows (kmol/s)
N-butane
N-pentane
N-hexane
N-heptane
Feed1
Top
10
1
410000
0.000000
341.454
-6.363E+06
-21979.0
Bottom
19
410000
410000
0.000000 0.000000
327.571 406.294
-7.984E+06 5.9245E+06
-34525.7 10996.5
40.0000 40.0000 1.0500E-05
25.0000 24.9237 0.0763508
20.0000 0.912304 19.0877
15.0000 1.8929E-04 14.9998
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Total molar flow
Mole fractions (-)
N-butane
N-pentane
N-hexane
N-heptane
100.000
65.8361
34.1639
0.400000 0.607569 3.0736E-07
0.250000 0.378571 0.00223484
0.200000 0.0138572 0.558710
0.150000 2.8751E-06 0.439055
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