1. Unit Operations Lecture 19
7 Nov 2012

2. Overview Review rigorous methods / RADFRAC Multicomponent systems:
Residue curves
DSTWU / RADFRAC
Rules of thumb
Complex (Enhanced) distillation
Column internals
Batch distillation

3. Distillation Heuristics (Rules of Thumb)
Remove corrosive, dangerous, and reactive components first
Don’t use distillation if LK-HK < ~1.05 to 1.10
Do easy separations first ( large).
Next split / separation / remove components in excess, then most volatile components (trying to minimize size of downstream columns).
Try to do most difficult separations as binary and last (taller columns will have smaller diameter.
Remove products and recycle streams as distillates (rust & heavy contaminates can accumulate in bottoms).
Whatever you add you normally have to remove at some point.

4. Distillation Heuristics (Turton)

5. Distillation Heuristics (Turton)

6. Distillation Heuristics (Turton)

7. Distillation Heuristics (Turton)

8. Variety of Phase Diagrams
Stichlmair & Fair (1998)

9. Complex or Enhanced Distillation
Ethanol – Water Binary (1.013 bar):

10. Complex Distillation
Ethanol – Water Binary (1.013 bar):

11. Extractive Distillation
EtOH
H2O
Ethylene Glycol

12. Extractive Distillation
Solvent:
Lower volatility than feed mixture
Add above the feed stage
Add few trays below the top stage
No azeotrope formed with other components
Should interact differently with other components
Typically add 1:1 with feed (molar basis)

13. Unit Operations Lecture 20
09 Nov 2012

14. Overview Review rigorous methods / RADFRAC Multicomponent systems:
Residue curves
DSTWU / RADFRAC
Rules of thumb
Complex (Enhanced) distillation
Column internals
Batch distillation

15. Azeotropic Distillation
Perry’s 8th ed.

16. Azeotropic Distillation
Water with (binary system at atmospheric P):
methanol - no azeotrope
ethanol - azeotrope
propanol - azeotrope
In-class exercise:
using Aspen Plus, explore if we can use distillation to separate the binary mixture of water and n-butanol.

17. Heterogeneous Azeotropic Distillation
VLLE (NRTL)

18. Heterogeneous Azeotropic Distillation
Perry’s 8th ed.

19. Residue Curve Maps

20. Complex Multicomponent Systems

21. Pressure Swing Distillation

22. Pressure Swing Distillation

23. Complex or Enhanced Distillation
Extractive Distillation
Higher boiling solvent
Enters near top of column
Interacts with other components to affect volatility or activity coefficients
Homogeneous Azeotropic Distillation
Add entrainer that forms min/max boiling point AZ w/ 1 or more of feed compds
Added near top or bottom, depending upon if AZ is min or max BP
Heterogeneous Azeotropic Distillation
Add entrainer to form min BP heterogeneous azeotrope (EtOH-H2O + benzene)
Pressure – Swing Distillation
For pressure sensitive azeotropes and distillation boundaries
Salt Distillation
Alter relative volatilities of feed compds by dissolving a soluble ionic salt into the reflux
Steam Distillation
Steam is added to reduce temperature of distilling organic mixture
Reactive Distillation
add reactant &/or catalyst to cause a reversible/selective reaction with one of the feed components
Reaction and distillation occur in same vessel

24. Column Internals

25. Column Internals

26. Column Internals

27. Column Internals

28. Column Internals
Jaeger Product Bull

29. Column Internals

30. Random and Structured Packing

31. Packed Columns (Distillation)
Usually for small diameter columns
Usually more economical for columns < ~75 cm (2.5 ft)
Lower pressure drop than trayed columns
Good for vacuum operations
Wide choice of packing materials (random or structured)

32. Column Internals

33. Packed Columns (Distillation)
Column diamter
Dcol/Dpacking ~ 8 – 12 (rule of thumb)
If Dcol/Dpacking > ~40, watch for channeling
Sized based usually on approach to flooding or acceptable pressure drop
Packing height

34. Trayed Columns (Diameter)
Chap 10 (p 314, Wankat) “Fair’s Procedure”
Considers entrainment flooding (most freq.)
Downcomer flooding (sometimes) – need different procedure
Downcomer flooding rare if (1- f) ≥ 10%
Used in AspenPlus
James R. Fair ( )

35. Trayed Columns (Diameter)
Plate spacing (selected for maintenance, performance). Typ:
12 – 16” for Dia < 5’
24” larger columns
Calc Dia & round up to nearest ½ foot (USA)
2.5’ minimum dia.
If < 2.5’ consider packed tower

36. Overview Questions from last week?? Review rigorous methods / RADFRAC
Multicomponent systems:
Residue curves
DSTWU / RADFRAC
Rules of thumb
Complex (Enhanced) distillation
Column internals
Batch distillation

37. Batch (Rayleigh) Distillation
Usually for small capacity systems
1 column handle multi-”campaigns”
Produce sample new products
Batch upstream processes
Feed contains solids/foulants
Material Balance:
leads to Rayleigh Equation
Seader & Henley (2006)
where:

38. Batch (Rayleigh) Distillation
a) P = constant; K = f(T) only
b) Binary with  = constant
c) y = K x ; but K = f(T,x)
Solve graphically or numerically

39. Multistage Batch Distillation
Seader & Henley (2006)
Modes of operation:
Constant reflux rate or ratio
xD varies with time
easily implemented (flow sensors)
Relatively simple and cost effective
Constant distillate composition
R or D varies with time
Requires fast response composition sensors
Sensors might not be available or only justified for larger batch systems
Optimal control mode
xD and R varied with time
Designed to:
Minimize operation time
Maximize amount of distillate
Maximize profit
More complex control scheme

40. Multistage Batch Distillation
Removing volatile impurities.
Flexible, multi-purpose system
Seader & Henley (2006)

41. Questions?

44. Go Over Homework: