Separating Azeotropic Mixtures

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 Octobe
r 3, 2005 Contains Material Developed by Dr.
Daniel R. Lewin, Technion, Israel
2
Process Design/Retrofit Steps

• Detailed Process Synthesis -Algorithmic Methods

• SECTION B

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
Homogeneous Azeotropes 14

• At equilibrium

10
Homogeneous Azeotropes 24

• Example Phase diagrams for benzene-toluene
  mixture at 90 oC

11
Homogeneous Azeotropes 34
For non-ideal mixtures, the activity coefficients
are different from unity
12
Homogeneous Azeotropes 44
13
Heterogeneous Azeotropes

• Homogeneous Azeotrope

• Heterogeneous Azeotrope

14
Residue Curves 13
Simple Distillation

• Mass balance on species j

15
Residue Curves 23

• Residue Curves ? Liquid Compositions at Total
  Reflux

16
Residue Curves 33

• 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
Product Compositions Regions

• For zeotropic systems
• L Lowest boiling component, I Intermediate
  boiling component, H Highest boiling compoent,
  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
• SSL 7.14b-d, 7.15
• Due Monday October 17
• Midterm Exam
• October 17 during lecture
• Open book or closed book?
• Next Lecture
• Part 1 Sequencing Azeotropic Distillation
  Columns (SSL 271-299)
• Part 2 Review for Midterm Exam
• Miscellaneous
• Updates made to lecture plan, remember to
  download new copy