1. Terry A. Ring Chemical Engineering University of Utah
Distillation
Terry A. Ring
Chemical Engineering
University of Utah

2. Marginal Vapor Rate Marginal Annualized Cost~ Marginal Vapor Rate
Marginal Annualized Cost proportional to
Reboiler Duty (Operating Cost)
Reboiler Area (Capital Cost)
Condenser Duty (Operating Cost)
Condenser Area (Capital Cost)
Diameter of Column (Capital Cost)
Vapor Rate is proportional to all of the above

3. Revisit Marginal Vapor Flow Rate – HW3
Case B
Marginal Vapor Flow Rate (MVFR)
Sum of Vapor Flows for all columns in the Sequence
V=ΣD
Case A =
Direct Sequence
A/BC, B/C
Is Case B- MVFR
Direct Squence
Indirect Sequence
AB/C, A/B
Distillate Products
Species
Boiling Pt (C)
Feed Flow
Direct Secuence
Indirect Sequence
(kmole/hr) C1 C2
Benzene
80.1
100
Toluene
110.6
Ethylbenzene
136
p-xylene
138
m-xylene
139
Total D
200
300
Case A

4. HW 3 Case A – Direct Sequence
Using ΣDi only, Case B – Best via Marginal Vapor Flow Rate
Direct Sequence
These are both the same! Your boss is an idiot!!

5. Revisit Marginal Vapor Flow Rate – HW3
Case B
Marginal Vapor Flow Rate
Sum of Vapor Flows for all columns in the Sequence
V=Σ[D*(1.2*Rmin+1)]
Distillate Products
Species
Boiling Pt (C)
Feed Flow
Direct Secuence
Indirect Sequence
(kmole/hr) C1 C2
Benzene
80.1
100
Toluene
110.6
Ethylbenzene
136
p-xylene
138
m-xylene
139
Total D
200
300
Case A

6. Aspen Simulation with DSTWU Columns

7. DSTWU Direct Sequence Results

8. Direct Sequence – Case A Results
Case A Case B
Species
Boiling Pt (C)
Feed Flow
Direct Secuence = A/BC, B/C
Indirect Sequence = AB/C, A/B
(kmole/hr)
R.min
D for CI
D for C2
D for C1 C2
Benzene
80.1
100
1.26
1.32
Toluene
110.6
3.98
1.37
Ethylbenzene
136
p-xylene
138
m-xylene
139
Total V= Σ[D*(1.2*Rmin+1)] =
828.8
787.2
Note: Rmin is in the row of the light key component
Kmole/Hr

9. DSTWU Indirect Sequence Results

10. Marginal Vapor Flow Rates
Case A Case B (MVFR Case)
Species
Boiling Pt (C)
Feed Flow
Direct Secuence = A/BC, B/C
Indirect Sequence = AB/C, A/B
(kmole/hr)
R.min
D for CI
D for C2
D for C1 C2
Benzene
80.1
100
1.26
1.32
Toluene
110.6
3.98
1.37
Ethylbenzene
136
p-xylene
138
m-xylene
139
Total V= Σ[D*(1.2*Rmin+1)] =
828.8
787.2
Note: Rmin is in the row of the light key component

11. Azeotropic Distillation
Multi-component Distillation with Azeotropes
Breaking a Binary Distilation Azeotrope

12. Heterogeneous Azeotropic Distillation
Example: Dehydration of Ethanol
Liquid-Liquid Equilibrium Line
Try toluene as an entrainer
What are the zones of exclusion?

13. Ethanol/Water Distillation with Toluene to Break AzeotropeM1 M2 D1
Distillation Line
Tie Line

14. Ethanol/Water Distillation with Benzene To Break Azeotrope (black line)
Liquid-Liquid Equilib Line

15. How To Break Azeotropes with Entrainer
Separation Train Synthesis
Identify Azeotropes
Some distillations are not Azeotropic and can be accomplished relatively easily
Identify alternative separators
Select Mass Separating Agent or Entrainer
Identify feasible distillate and bottoms product compositions
Residue Curve Analysis

16. Pressure Swing to Break Azeotrope
Temp. of
Azeotrope
vs. Pressure
Mole Fraction
of Azeotrope

17. Pressure-swing Distillation (Cont’d)
Example: Dehydration of Tetrahydrofuran (THF)
T-x-y diagrams for THF and water

18. Other Multi-component Distillation Problems
Multiple Steady States
Run same distillation column with same set points but different computational starting point
Get Two or More Different Results
Top or bottom compositions
This is real in that the column will have two different operating conditions!
Happens most often with multi component distillation
Multiple
Solutions
Starting Point yD