Separation Trains Azeotropes

1. Separation TrainsAzeotropes 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 HelpDiagnosing 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. x Binary DistillationIPA/IPE IPA-IPE x Mininum-boiling Azeotropes x

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

14. 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 Homogeneous Azeotropes (Cont’d)

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. Homogeneous Azeotrope Heterogeneous Azeotrope Two Types of Min. Boiling Azeotropes 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 0.65 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 0.2 DTBP 0.6 H2O TBA = Tertiary-butyl alcohol DTBP = Di-tertiary-butyl peroxide

21. Basics: The Lever Rule

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

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

30. Nodes

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

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

36. D1 M2 M1 S1 S2 Ethanol/Water Distillation with Toluene to Break Azeotrope Distillation Line Tie Line

37. Ethanol/Water Distillation with BenzeneTo 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. Example: Dehydration of Tetrahydrofuran (THF) T-x-y diagrams for THF and water Pressure-swing Distillation (Cont’d)

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