Separating Azeotropic Mixtures

CHEN 4460 – Process Synthesis, Simulation and Optimization

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 6 – Review of Non-ideal Thermodynamics

September 27, 2011

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

PART II

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 H2O

0.65 DTBP

0.2 TBA

0.2 DTBP

Basics: Ternary Phase Diagrams

0.6 H2O

0.2 DTBP

0.2 TBA

Homogeneous Azeotropes 1:4

At equilibrium:

V L

j jff_ _

V L

j j j j jy P x f

s

1 1 1yP xP

s

2 2 2yP x P

s s s s

1 1 2 2 1 1 1 2P xP x P xP 1 x P( )

s s s

2 1 2 1P P P x( )

At fixed temperature

Homogeneous Azeotropes 2:4

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

Homogeneous Azeotropes 3:4

For non-ideal mixtures, the activity coefficients are different from unity:

S

1 1 1 1yP x P

S

2 2 2 2yP x P

s s

1 1 1 1 2 2P x P 1 x P( )

If the mixture has a minimumminimum-boiling azeotrope

i1

Example – Phase diagrams for Isopropyl ether-Isopropyl alcohol

Homogeneous Azeotropes 4:4

For non-ideal mixtures, the activity coefficients are different from unity:

S

1 1 1 1yP x P

S

2 2 2 2yP x P

s s

1 1 1 1 2 2P x P 1 x P( )

If the mixture has a maximummaximum-boiling azeotrope

i1

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.

i1

Homogeneous Azeotrope Heterogeneous Azeotrope

Residue Curves 1:3

j j j j j jLx ydL Lx Ldx xdL dLdx j 1 C 1, , ,

Mass balance on species j:

j j j jLx L y L L x x j 1 C 1( ) ( )( ), , ,

As L 0:

Simple Distillation

j

j j j j

dxx y x 1 K T P x y

dL L( { , , , })

/

Rearranging:

j

j j

dxx y

dt

Residue Curves 2:3

Residue curves for Azeotropic

system

j

j j

dxx y

dt

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:

n

n n 1

dxx x

dh

n 1 n 1 n nDL x Dx Vy

Approximation for liquid phase:

Substituting:

n n

n n D

n 1 n 1

dx V Dx y x

dh L L

At total reflux, D = 0 and Vn = Ln-1

n

n n

dxx y

dh

Rectifying section of distillation

column

Sketching Residue Curves

j

j j

dxx y

dt

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

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– 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.