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Separating Azeotropic Mixtures

This lecture covers non-ideal thermodynamics in the separation of azeotropic mixtures, involving oxygenated organic compounds like alcohols, ketones, and acids. The discussion includes residue curves, distillation boundaries, phase diagrams, and the Lever Rule application. Various azeotrope examples are analyzed along with heterogeneous and homogeneous azeotropes. Additionally, the lecture explores techniques for sketching residue curves on ternary phase diagrams and determining feasible product compositions through distillation processes.

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Separating Azeotropic Mixtures

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  1. Separating Azeotropic Mixtures CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University Lecture No. 6 – Review of Non-ideal Thermodynamics September 25, 2012 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

  2. Assess Primitive Problem • Plant-wide Controllability Assessment • Development of Base-case • Detailed Design, Equipment sizing, Cap. Cost Estimation, Profitability Analysis, Optimization Process Design/Retrofit Steps • Detailed Process Synthesis -Algorithmic Methods • PART II

  3. Algorithmic Methods

  4. Lecture 6 – 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. • This prevents the removal of certain species in high concentrations.

  5. Lecture 6 – Objectives • Be able to sketch the residue curves on a ternary phase diagram • Be able to define the range of possible product compositions using distillation, given the feed composition and the ternary phase diagram

  6. Basics: The Lever Rule

  7. Basics: Ternary Phase Diagrams 0.2 TBA 0.65 DTBP 0.2 DTBP 0.15 H2O

  8. Basics: Ternary Phase Diagrams 0.2 TBA 0.2 DTBP 0.6 H2O

  9. At fixed temperature Homogeneous Azeotropes 1:4 • At equilibrium:

  10. Homogeneous Azeotropes 2:4 • Example – Phase diagrams for benzene-toluene mixture at 90 oC

  11. If the mixture has a minimum-boiling azeotrope • Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol Homogeneous Azeotropes 3:4 For non-ideal mixtures, the activity coefficients are different from unity:

  12. 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 4:4

  13. For a minimum-boiling azeotrope with large deviation from Raoult’s law ( ), phase splitting may occur and a minimum-boiling heterogeneous azeotrope forms, having a vapor phase in equilibrium with two liquid phases. Heterogeneous Azeotropes • Homogeneous Azeotrope • Heterogeneous Azeotrope

  14. Rearranging: Residue Curves 1:3 Simple Distillation • Mass balance on species j:

  15. Residue curves for zeotropic system • Residue curves for Azeotropic system Residue Curves 2:3 • Residue Curves  Liquid Compositions at Total Reflux

  16. Rectifying section of distillation column Residue Curves 3:3 • 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

  17. Sketching Residue Curves • Plot pure components on vertices along with Tb • Plot all azeotropes on diagram along with their Tb • Plot residue curves connecting all azeotropes, azeotropes & vertices, and finally vertices & vertices with arrow heads pointing towards increasing boiling point temperatures • Plot additional residue curves that “arch” towards intermediate temperatures on the way to the end point

  18. Pure L distillate Pure H bottoms Product Compositions Regions • For zeotropic systems • L: Lowest boiling component, I: Intermediate boiling component, H: Highest boiling component, F: Feed composition

  19. Product Compositions Regions • For azeotropic systems • Shaded regions: Feasible distillate and bottoms product compositions • Three binary azeotropes and one ternary azeotrope • Two binary azeotropes

  20. Summary – Non-ideal Thermo On completion of this part, you should: • Be able to sketch the residue curves on a ternary phase diagram • Be able to define the range of possible product compositions using distillation, given the feed composition and the ternary phase diagram

  21. Other Business • Homework • SSLW: 8.14b-d, 8.15 • Due Tuesday October 2 • Next Lecture (October 2) • Part 1: Sequencing Azeotropic Distillation Columns (SSLW 230-251) • Part 2: Review for Midterm Exam • Midterm Exam • October 9 during lecture • Open book or closed book?

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