Separation TrainsAzeotropes

Chapt. 8

What is an Azeotrope?

Introduction Separation sequences are complicated by the presence of

azeotropes, often involving mixtures of oxygenated organic compounds:AlcoholsKetonesEthersAcidsWater

In these cases, distillation boundariesdistillation boundaries limit the product compositions of a column to lie within a bounded regionPrevents the removal of certain species in high

concentrations

Binary Distillation

IPA/IPE

x

x

x

IPA-IPE

Mininum-boiling orMaximum-boilingAzeotropes

Can multi-component Distillations have Azeotropes?

•Yes!

Azeotrope Conditions

• Conditions on the Activity Coefficient

• Minimum Boiling, γjL> 1

• Maximum Boiling, γjL< 1

• xj=yj, j=,1,2,…C

Law sRaoult' from Deviations Negative,...2,1,1

Law sRaoult' from Deviations Positive,...2,1,1

)1( 221111

Cj

Cj

PxPxP

Lj

Lj

sLsLT

Raoult’s Law

satii PxP

Homogeneous Azeotropes (Cont’d) 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 (Cont’d) 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

Importance of Physical Property Data Set

• In all cases– Need sophisticated liquid phase model to

accurately predict the activity coefficient for the liquid.

• For High Pressure Cases Only– Also need sophisticated (non-ideal) gas phase

fugacity model

Two Types of Min. Boiling Azeotropes

• Homogeneous Azeotrope • Heterogeneous Azeotrope

Overlay withLiquid/Liquid Separation which is sometimes best separation method

Instructional Objectives

Be able to sketch the residue curves on a tertiary phase diagram

Be able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagram

Be able to define the PFD for a heterogeneous azeotropic distillation system

Be able to define the PFD for a pressure swing distillation system

When you have finished studying this unit, you should:

Concepts Needed

• Phase Diagram for 3 phases

• Lever Rule on Phase Diagram

• Residue Curves

0.15 H2O

0.65 DTBP

0.2 TBA

0.2 DTBP

Basics: 3-Phase Diagrams

TBA = Tertiary-butyl alcoholDTBP = Di-tertiary-butyl peroxide

0.6 H2O

0.2 DTBP

0.2 TBA

Basics: 3-Phase Diagrams (Cont’d)

Basics: The Lever Rule

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

Residue Curves

Mass balance on species j:

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

As L 0:

Distillation still

j

j j j j

dxx y x 1 K T P x y

dL L( { , , , })

/

Rearranging:

j

j j

dxx y

dt

Multi-component Azeotropes

• Residue Curve Map– dxj /dť = dxj /d ln(L) = xj – yj

• Integrate from various starting points

Arrows from low to High TempPath of the residue composition

Sketching Residue Curves (Exercise)j

j j

dxx y

dt

Distillation

• XB, XF and YD form a line for a Distillation Column

• Line can not cross Feasible Region line

For Partial Condenser

For TotalCondenser

Distillation Boundaries

• Equilibrium Trays in Total Reflux

• Distillation Lines

– xn and yn lie on equilibrium tie lines– Tangent to Residue Curve

n1-n111

VL 0,Dfor ,

nnDn

nn

nn

n

yxyL

Dy

L

Vx

dh

xd

0,1,...n allfor ,1 nn yx

To Create Residue Maps

• AspenPlus– After putting in the components and selecting

the physical property method– Choose

• Tools/Analysis/Property/Residue

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

Stripping section of distillation column

Nodes

Residue curves for Azeotropic

system

Residue Curves (Cont’d)j

j j

dxx y

dt

Residue curves for zeotropic

system

Defining Conditions for Multi-component Azeotrope

t goes from 0 to 1, ideal to non-ideal to find Azeotrope

Product Composition Regions for Zeotropic Systems

Product Composition Regions for Azeotropic Systems

Example: Dehydration of Ethanol

Heterogeneous Azeotropic Distillation

Try toluene as an entrainer

What are the zones of exclusion?

Ethanol/Water Distillation with Toluene to Break

Azeotrope

Distillation LineTie LineS1

S2

M1M2

D1

Ethanol/Water Distillation with

BenzeneTo Break Azeotrope

How To Break Azeotropes with Entrainer

• Separation Train Synthesis– Identify Azeotropes– Identify alternative separators

• Some distillations are not Azeotropic and can be accomplished relatively easily

– Select Mass Separating Agent or Entrainer– Identify feasible distillate and bottoms product

compositions• Residue Curve Analysis

Pressure Swing to Break Azeotrope

Temp. of Azeotropevs. Pressure

Mole Fraction of Azeotrope

T-x-y diagrams for THF and water

Example: Dehydration of Tetrahydrofuran (THF)

Pressure-swing Distillation (Cont’d)

Other Multi-component Distillation Problems

• Multiple Steady States – Run same distillation column with same set

points but different computational starting point

• Get Two or More Different Results– Top or bottom compositions– This is real in that the column will have two

different operating conditions!– Happens most often with multi component

distillation