1. Separation Trains Azeotropes
Chapt. 8

2. What is an Azeotrope?

3. Introduction
Separation sequences are complicated by the presence of azeotropes, often involving mixtures of oxygenated organic compounds:
Alcohols
Ketones
Ethers
Acids
Water
In these cases, distillation boundaries limit the product compositions of a column to lie within a bounded region
Prevents the removal of certain species in high concentrations

4. Binary Distillation IPA/IPEx
Binary Distillation IPA/IPE
IPA-IPE x
Mininum-boiling or
Maximum-boiling
Azeotropes x

5. Can multi-component Distillations have Azeotropes?
Yes!

6. Azeotrope Conditions Conditions on the Activity Coefficient
Minimum Boiling, γjL> 1
Maximum Boiling, γjL< 1
xj=yj, j=,1,2,…C

7. Raoult’s Law

8. Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the activity coefficients are different from unity:
If the mixture has a minimum-boiling azeotrope
Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol

9. Homogeneous Azeotropes (Cont’d)
For non-ideal mixtures, the activity coefficients are different from unity:
If the mixture has a maximum-boiling azeotrope
Example – Phase diagrams for Acetone-Chloroform

10. Importance of Physical Property Data Set
In all cases
Need sophisticated liquid phase model to accurately predict the activity coefficient for the liquid.
For High Pressure Cases Only
Also need sophisticated (non-ideal) gas phase fugacity model

11. Two Types of Min. Boiling Azeotropes
Homogeneous Azeotrope
Heterogeneous Azeotrope
Overlay with
Liquid/Liquid Separation which is sometimes best separation method

12. Instructional Objectives
When you have finished studying this unit, you should:
Be able to sketch the residue curves on a tertiary phase diagram
Be able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagram
Be able to define the PFD for a heterogeneous azeotropic distillation system
Be able to define the PFD for a pressure swing distillation system

13. Concepts Needed Phase Diagram for 3 phases Lever Rule on Phase Diagram
Residue Curves

14. Basics: 3-Phase Diagrams
0.2 TBA
DTBP
TBA = Tertiary-butyl alcohol
DTBP = Di-tertiary-butyl peroxide
TBHP =Tertiary-butyl hydroperoxide
Rxn
TBA + TBHP ->DTBP
0.2 DTBP
0.15 H2O
TBA = Tertiary-butyl alcohol
DTBP = Di-tertiary-butyl peroxide

15. Basics: 3-Phase Diagrams (Cont’d)
0.2 TBA
TBA = Tertiary-butyl alcohol
DTBP = Di-tertiary-butyl peroxide
0.2 DTBP
0.6 H2O

16. Basics: The Lever Rule

17. Residue Curves Mass balance on species j: Rearranging:
Distillation still
Mass balance on species j:
Rearranging:

18. Multi-component Azeotropes
Residue Curve Map
dxj /dť = dxj /d ln(L) = xj – yj
Integrate from various starting points
Arrows from low to High Temp
Path of the residue composition

19. Sketching Residue Curves (Exercise)

20. Distillation XB, XF and YD form a line for a Distillation Column
Line can not cross Feasible Region line
For Partial Condenser
For Total
Condenser

21. Distillation Boundaries
Equilibrium Trays in Total Reflux
Distillation Lines
xn and yn lie on equilibrium tie lines
Tangent to Residue Curve

23. To Create Residue Maps AspenPlus
After putting in the components and selecting the physical property method
Choose
Tools/Analysis/Property/Residue

24. Residue Curves  Liquid Compositions at Total Reflux
Species balance on top n-1 trays:
Stripping section of distillation column
Approximation for liquid phase:
Substituting:
At total reflux, D = 0 and Vn = Ln-1

25. Nodes

26. Residue Curves (Cont’d)
Residue curves for zeotropic system
Residue curves for Azeotropic system

27. Defining Conditions for Multi-component Azeotrope
t goes from 0 to 1, ideal to non-ideal to find Azeotrope

28. Product Composition Regions for Zeotropic Systems

29. Product Composition Regions for Azeotropic Systems

30. Heterogeneous Azeotropic Distillation
Example: Dehydration of Ethanol
Try toluene as an entrainer
What are the zones of exclusion?

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

32. Ethanol/Water Distillation with Benzene To Break Azeotrope

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

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

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

36. 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