azeotropic destillation

054402 DESIGN AND ANALYSIS LECTURE SIX

Daniel R. Lewin, Technion1

6 - Azeotropic DistillationDESIGN AND ANALYSIS - (c) Daniel R. Lewin1

054402 Design and Analysis

LECTURE 6: SEQUENCING OF AZEOTROPIC DISTILLATION COLUMNS

Daniel R. LewinDepartment of Chemical Engineering

Technion, Haifa, Israel Ref: Seider, Seader and Lewin (2004), Section 7.5


Assess Primitive Problem

Steps in Process Design and Retrofit

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




Section B: Algorithmic Methods


IntroductionSeparation 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 region

Prevent the removal of certain species in high concentrations




Instructional Objectives

Be able to sketch the residue curves on a tertiary phase diagramBe able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagramBe able to define the PFD for homogeneous and heterogeneous azeotropic distillation systemsBe able to define the PFD for a pressure swing distillation system

When you have finished studying this unit, you should:


Basics: The Lever Rule




0.15 H2O

0.2 DTBP

Basics: 3-Phase Diagrams


Basics: 3-Phase Diagrams (Cont’d)




Homogeneous Azeotropes

At equilibrium:=

V L

j jf f_ _

φ = γV L

j j j j jy P x f

= s

1 1 1y P xP

= s

2 2 2y P x P

= + = + −s s s s

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

= + −s s s

2 1 2 1P P P x( )

At fixed temperature


Homogeneous Azeotropes (Cont’d)

Example – Phase diagrams for benzene-toluene

mixture at 90 oC




Homogeneous Azeotropes (Cont’d) For non-ideal mixtures, the activity coefficients are different from unity:

S

1 1 1 1y P x P= γ

S

2 2 2 2y P x P= γ

s s

1 1 1 1 2 2P x P 1 x P( )= γ + − γ

If the mixture has a minimumminimum-boiling azeotropei1γ >

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 1y P x P= γ

S

2 2 2 2y P x P= γ

s s

1 1 1 1 2 2P x P 1 x P( )= γ + − γ

If the mixture has a maximummaximum-boiling azeotropei1γ <

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.

i1γ

Homogeneous Azeotrope Heterogeneous Azeotrope


j j j j j jLx y dL Lx Ldx x dL 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

d t∧ = −




Residue curves for Azeotropic system

Residue Curves (Cont’d)

j

j j

dxx y

d t∧ = −

Residue curves for zeotropic system


Residue Curves ≡≡≡≡ Liquid Compositions at Total Reflux

Species balance on top n-1 trays:

n

n n 1

dx x xdh −

≈ −

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 xdh L L

− −

≈ − +

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

n

n n

dx x ydh

≈ −

Rectifying section of distillation column




Sketching Residue Curves (Exercise)

j

j j

dxx y

d t∧ = −


Product Composition Regions for Zeotropic Systems




Product Composition Regions for Azeotropic Systems


Example: Dehydration of Ethanol

Heterogeneous Azeotropic Distillation

Try toluene as an entrainer




S1

S2

M1

D2

M2D1B2B1

D2D1F M1 S1

S2

M2


Pressure-swing Distillation




T-x-y diagrams for THF and water

Example: Dehydration of Tetrahydrofuran (THF)

Pressure-swing Distillation (Cont’d)


Azeotropic Distillation - Summary On completing this unit, you should:

Next week:Next week: HEN SynthesisHEN Synthesis

Be able to sketch the residue curves on a tertiary phase diagramBe able to define the range of possible product compositions using distillation, given the feed composition and the tertiary phase diagramBe able to define the PFD for homogeneous and heterogeneous azeotropic distillation systemsBe able to define the PFD for a pressure swing distillation system

azeotropic destillation

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