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Separating Azeotropic Mixtures

Separating Azeotropic Mixtures. CHEN 4460 – Process Synthesis, Simulation and Optimization Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 6 – Review of Non-ideal Thermodynamics September 27, 2011

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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 27, 2011 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 4 • Next Lecture (October 4) • Part 1: Sequencing Azeotropic Distillation Columns (SSLW 230-251) • Part 2: Review for Midterm Exam • Midterm Exam • October 11 during lecture • Open book or closed book? • Lab Session Tonight • I have to take our seminar speaker to dinner, so I am unable to be there. Vivi and Charlotte will be there as usual though.

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