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 23, 2008 Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

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

Algorithmic Methods

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.

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

Basics: The Lever Rule

Basics: Ternary Phase Diagrams 0.15 H 2 O 0.65 DTBP 0.2 TBA 0.2 DTBP

Basics: Ternary Phase Diagrams 0.6 H 2 O 0.2 DTBP 0.2 TBA

Homogeneous Azeotropes 1:4 At equilibrium: At fixed temperature

Homogeneous Azeotropes 2:4 Example – Phase diagrams for benzene- toluene mixture at 90 o C

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

Homogeneous Azeotropes 4:4 For non-ideal mixtures, the activity coefficients are different from unity: maximum If the mixture has a maximum-boiling azeotrope Example – Phase diagrams for Acetone-Chloroform

Heterogeneous Azeotropes 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. Homogeneous Azeotrope Heterogeneous Azeotrope

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

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

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 V n = L n-1 Rectifying section of distillation column

Sketching Residue Curves  Plot pure components on vertices along with T b  Plot all azeotropes on diagram along with their T b  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

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

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

Summary – Non-ideal Thermo  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 On completion of this part, you should:

Other Business Homework –SSL: 7.14b-d, 7.15 –Due Tuesday October 7 No Lecture Next Week –I will be in Canada for a meeting – if you need help or have questions Next Lecture (October 7) –Part 1: Sequencing Azeotropic Distillation Columns (SSL ) –Part 2: Review for Midterm Exam Midterm Exam –October 14 during lecture –Open book or closed book?