1. Separation Trains Azeotropes
S,S&L Chapter 9.5
Terry A. Ring
Chemical Engineering
University of Utah

2. Distillation Recycle Loops

3. Closing Recycle Loops Matrix Mathematics Without Recycle Loop
[P] x = y
Straight forward martix multiplication gives y
With Recycle Loop
[P] x + r [P] y = (1-r) y
Non-linear model
Non-linear matrix manipulation
Unit Op y x
recycle
Unit Op x y
[P] (x+ry)

4. Tear Streams in Aspen Hidden Tear Stream is added to Recycle Loop
Tear = β*Recycle
By Component
Teari = β*Recyclei
β is increased from δ to 1
Issues
how fast do you step β?
Convergence
(is mass & energy & P in balance at mixing point )
Tear
Recycle
Unit Op y x

5. Aspen Help Diagnosing Tear Stream Convergence
Most of the time the problem is the inside loop U1 U2

6. What is an Azeotrope?

7. 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

8. Binary Distillation IPA/IPEx
Binary Distillation IPA/IPE
IPA-IPE x
Mininum-boiling Azeotropes x

9. Binary Distillation IPA/IPE
Acetone/Chloroform
Maximum-boiling
Azeotropes x

10. Can multi-component Distillations have Azeotropes?
Yes! Possibly Multiple!

11. Raoult’s Law
γLi

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

13. 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

14. 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

15. 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 (> 10 bar) Cases Only
Also need sophisticated (non-ideal) gas phase fugacity model

16. Two Types of Min. Boiling Azeotropes
Homogeneous Azeotrope
Heterogeneous Azeotrope
A B
A B
Overlay with Liquid/Liquid Separation
which is sometimes best separation method (costs much less)

17. 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

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

19. 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, TBHP =Tertiary-butyl hydroperoxide
DTBP = Di-tertiary-butyl peroxide

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

21. Basics: The Lever Rule

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

23. 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

24. Sketching Residue Curves (Exercise)

25. 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

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

28. To Create Residue Maps AspenPlus
After putting in the components and selecting the physical property method
Choose
In Properties Choose Residue Curves
In Simulation Choose Distillation Search

29. 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

30. Nodes

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

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

33. Product Composition Regions for Zeotropic Systems

34. Product Composition Regions for Azeotropic Systems

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

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

37. Ethanol/Water Distillation with Benzene To Break Azeotrope

38. 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

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

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

41. 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