Azeotropic Distillation

Azeotropic Distillation Methods Dr. Stathis Skouras, Gas Processing and LNG

RDI Centre Trondheim, Statoil, Norway

Schedule

Tuesday 09.12.2014: 09:45 – 12:30

• Lecture: Natural Gas Processing

Thursday 11.12.2014: 11:45 – 14:30

• Lecture: Distillation of azeotropic mixtures

Tuesday 16.12.2014: 09:45 – 11:30

• PC-lab / HYSYS exercises

o Dew Point Control Unit (DPCU)

o Extractive Distillation (Acetone-methanol with water as entrainer)

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Outline

• Introduction

− Phase diagrams of azeotropic mixtures (Prof. E. Voutsas)

− Importance and industrial relevance of azeotropic distillation

• Main part

− Theory: residue curve maps and distillation curve maps

− Feasibility analysis of azeotropic distillation

− Examples of azeotropic distillation methods

• Summary

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Importance and industrial relevance of azeotropic distillation

• Need for efficient recovery and recycle of organic solvents in chemical industry

• Most liquid mixtures of organic solvents form azeotropes that complicate the

design of recovery processes

• Distillation is the most common unit operation in recovery processes because of

its ability to produce high purity products

• Azeotropes make separation impossible by normal distillation but can be also

utilised to separate mixtures not ordinarily separable by normal distillation

• Azeotropic mixtures may often be effectively separated by distillation by adding a

third component, called entrainer

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Knowledge of the limitations and possibilities in azeotropic

distillation is a topic of great practical and industrial interest

Terminology

• The methods and tools presented in this lecture also appply for:

− Azeotropic mixtures, close boiling systems, low relative volatility systems

• Original components A and B: The components that form the azeotrope and need

to be separated

• Entrainer: A third component (E or C) added to enhance separation

• Binary azeotrope: Azeotrope formed by two components

• Ternary azeotrope: Azeotrope formed by three components

• Homogeneous azeotrope: Azeotrope where the forming components are miscible

• Heterogeneous azeotrope: Azeotrope where the forming components are

immiscible

• Minimum boiling azeotrope: Azeotrope with lower boiling point than its constituent

components

• Minimum boiling azeotrope: Azeotrope with lower boiling point than its constituent

components

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Overview: Azeotropic distillation methods

1) Pressure swing distillation

2) Homogeneous azeotropic (homoazeotropic) distillation

3) Heterogeneous azeotropic (heteroazeotropic) distillation

4) Extractive distillation

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No entrainer required

Entrainer

enhanced

methods

Theory: Residue curve maps (RCM) and distillation curve maps (DCM)

• For ordinary multicomponent distillation

determination of feasible schemes and column

design is straightforward

• McCabe-Thiele method and Fenske-Underwood-

Gilliland equations are powerful tools

• Azeotropic phase equilibrium diagrams such as

residue curve maps (RCM) or distillation curve

maps (DCM) are sometimes nicknamed the

McCabe-Thiele of azeotropic distillation and

provide insight and understanding

• RCM or DCM sketched together with material

balance lines and operating lines are used to

identify feasible distillation schemes and products

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Residue curves

• Consider the process of differential (open)

distillation (Rayleigh distillation)

• The component mass balance is written:

and by considering the dimensionless time variable ξ

(dξ=dV/W)

• Integrating the above equation from any initial

composition (xw0) will generate a residue curve

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( )i

i idx dW

y xdt Wdt

ii i

dxx y

d

A (TA)

B (TB) C (TC)

TA< TB < TC

xW0

Still pot

composition

trajectory The residue curve describes the change of

the still pot composition with time (trajectory)

Distillation curves • Consider the process of continuous distillation at

total reflux (45° line at McCabe-Thiele diagram)

• Starting with a liquid composition at stage n (xi,n)

and by doing repeated phase equilibrium

calculations (E-mapping) upwards we get:

• By doing this from any initial composition (x0) the

distillation curve can be constructed

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1

, ,

, , 1

, 1 , 1

, 1 , 2

...

nE

nE

i n i n

i n i n

i n i n

i n i n

x y

y x

x y

y x

Stage n

Stage n-1

Yi,n-1

yi,n

xi,n

Condenser

Reboiler

xB

Total reflux (V = L = R)

V, yD L, xD

xi,n-1

The distillation curve describes the change of the

component composition along the column (trajectory)

• Pure component vertices and azeotropes are singular points in the RCM and DCM

• The behaviour at the vicinity of singular points depends on the two eigenvalues

a) Stable node ( ): Point with the highest boiling point – Bottom product in

distillation. All residue curves end at this point - Both eigenvalues negative

b) Unstable node ( ): Point with the lowest boiling point – Top product in distillation.

All residue curves start at this point - Both eigenvalues positive

c) Saddles ( ): Point with an intermediate boiling point – Residue curves move

towards and then away from these points – One positive and one negative

eigenvalue

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Singular points in RCM and DCM

0i

i idx

x yd

Relationship between residue curves and distillation curves

• Both are pure representations of the VLE and no

other information needed to construct them

• Have the same topological structure and singular

points

• Distillation boundaries exist and split the

composition space into distillation regions

• DO NOT completely coincide to each other

• BUT provide the same information and can be

equally used for feasibility analysis

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Residue curve

------- Distillation curve

Feasibility analysis based on RCM and DCM

For a feasible separation the material

balances should be fulfilled:

F = D + B

F zF = D xD + B xB

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Feasibility rules

a) The top (xD) and bottom (xB) compositions

must lie in a straight line through feed (zF)

b) The top (xD) and bottom (xB) compositions

must lie on the same residue (distillation) curve

xD

xB

Products xD and xB must lie on the same distillation region

Feasibility analysis based on RCM and DCM

Zeotropic mixture

• No distillation boundaries

• Only one distillation region exists

• No limitations regarding possible

products independently of feed location

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Direct split: The most volatile is taken

at the first column

Indirect split: The less volatile is taken

at the first column

F F

Azeotropic mixtures

• Case 1

o One boundary exists

o Two distillation regions (I and II)

o Different porducts for feeds F1 and F2

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C

Feasibility analysis based on RCM and DCM

Case 1

A

B

AzAB

AzBC

Case 2 • Case 2 - Possible products?

o Feed F1 - Distillate: AzAB

- Bottom: B

o Feed F2 - Distillate: AzAB

- Bottom: C

1) Pressure swing distillation

• Principle: Overcome the azeotropic composition by changing the

system pressure

• Key factors: Azeotrope sensitive to pressure changes, recycle ratio

which increases costs

• Application: Tetrahydrofuran/water*

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* Stichlmair and Fair, Distillation: Principles and practice, Wiley-VCH, (1998)

• Definition:

o Entrainer completely miscible with the original

components

o Entrainer may (or not) form additional azeotropes

with the original components

o The distillation is carried out in a sequence of

columns

• Principle:

o The addition of the entrainer results in a residue

curve map promising for separation

o Both original components must belong to the

same distillation region

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2) Homogeneous azeotropic distillation

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Example – Use of intermediate entrainer

• Original components A and B form a min. AzAB

• Components A and B belong to the same distillation region

• Original feed (F) is close to the azeotrope AzAB

• Total feed (F´) is a mix of fresh feed (F) and entrainer (E)

• Component A is taken as bottom product in Column 1

• Component B is taken as top product in Column 2

• Entrainer (E) is recovered as bottom product in Column 2

• Entrainer (E) is recycled to Column 1

Applicability of homoazeotropic distillation is limited

• Quite restrictive feasibility rules

• Other distillation methods are preferably applied

A B AzAB=F

Feasibility for homogeneous azeotropic distillation

F´

D1

A

1 2

B

E

F F´

D1

• Definition:

o Entrainer is immiscible and forms azeotrope with at

least one of the original azeotropic components

o The distillation is carried out in a combined column-

decanter column

o Entrainer is recovered and recycled to the first column

• Principle:

o Liquid-liquid immiscibilities are used to overcome

azeotropic compositions

o Distillation boundaries can be crossed by immiscibility

• Applicability:

o Widely used in the industry

o One of the oldest methods of azeotropic distillation

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3) Heterogeneous azeotropic distillation

Classic example – ternary heterogeneous azeotrope

• Entrainer forms a ternary heterogeneous azeotrope with orginal components

(unstable node Az12E)

• Ternary heteroazeotrope will be the top product and will split in two liquid phases

• Liquid-liquid tie-lines are located in different distillation regions

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Feasibility for heterogeneous azeotropic distillation

A1E A2E A12E

A12E

Example: Ethanol/water + benzene (E)

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1) Preconcentrator • Aqueous feed dilute in EtOH (F1)

• EtOH-Water azeotrope at top (D1)

• Pure water at bottom (B1)

A12E

EtOH H2O

Benzene

3) Entrainer recovery column • Aqueous phase from decanter is column feed

• Pure water is taken at the bottom (B3)

• Top product (D3) is close to the EtOH-water

azeotrope + some benzene left

2) Azeotropic column • Ternary heterogeneous azeotrope (A12E) at top

• Splits in two liquid phases in a decanter

• Benzene-rich phase is recycled at the top

• Pure EtOH is taken at bottom (B2)

• Definition:

o Heavy entrainer is used with high boiling point

o Distillation is carried out in a two-feed column with

a heavy entrainer added continously at the top

o Entrainer is recovered in a second column

• Principle:

o The entrainer alters the relative volatility of the

original components

o The entrainer has a substantial higher affinity to

one of the original components and ‘‘extracts’’ it

downwards the azeotropic column

• Applicability:

o Most widely used method in the industry

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4) Extractive distillation

• Pure component 1 (D1) is taken as top product from extractive column

• Entrainer “extracts” component 2 at the bottom (B1)

• Entrainer recovery column separates entrainer from component 2

• Pure entrainer (E) is recovered at bottom (B2) and used as reflux in extractive column

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Feasibility and synthesis for extractive distillation

Rectifying

section

Bottom section

Rectifying

section

Bottom

section

Binary feed

(1 & 2)

F

• Invented by Jim Ryan and Art Holmes*

• Cryogenic distillation process for the removal of CO2 from natural gas

• Uses extractive distillation to ‘‘break’’ the CO2/ C2 azeotrope

• Uses Natural Gas Liquid (NGL) as entrainer, which is extracted from the

feed stream itself

• Various configurations with 2, 3 and 4 columns

Examples from Oil & Gas: Ryan-Holmes process

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* A. S. Holmes, J. M. Ryan, Cryogenic distillation separation of acid gases from methane, US patent, 1982

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CO2/C2+

C2+

Added

entrainer

Entrainer

recovery column

Entrainer (C4+) recycle

Azeo

CO2/C2

C4+

Extractive

column

De-C1

column

MTBE Production and Separation Unit

Feed

C3 – C5+

MTBE

methanol

MeOH-

water

+ water (E)

Azeo

C4-MeOH

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Process description • Feed to 1st separation column

− C3

− C4 (with excess isobutylene)

− C5+

− Water

− Methanol

• First Column (Distillation Column)

− Bottom: MTBE

− Top: C4 – methanol azeotrope

• Second Column (Extraction Column)

− Addition of water countercurrent to flow

− Methanol has more affinity for water pass to aqueous phase

− Top: Raffinate (C3,C4,C5+)

− Bottom: Methanol/Water

• Third Column (Distillation Column)

− Top: methanol

− Bottom: Water

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Summary

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• Separation of azeotropic mixtures is a topic of great practical and

industrial interest

• Azeotropic mixtures are impossible to separate by ordinary distillation,

but may be effectively be separated by adding a third component, called

entrainer

• Residue curve maps (RCM) and distillation curve maps (DCM) are

representations of the thermodynamic behavior (VLE and VLLE) of

azeotropic mixtures

• RCM and DCM are used to identify feasible distillation schemes

Summary

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• Homogeneous azeotropic distillation

o Only few RCM and DCM lead to feasible schemes

o Limiting use in the industry

• Heteroazeotropic distillation

o Ordinary distillation combined with a decanter is used

o Liquid-liquid immiscibilities are used to overcome azeotropic compositions

o Method widely used in the industry

• Extractive distillation

o Heavy entrainer used that ‘‘extracts’’ one of the original components and

enhances separation

o Broad range of feasible entrainers (no liquid-liquid immiscibility required)

o The most widely used method in the industry

Presentation title: Azeotropic distillation methods

Presenters name: Stathis Skouras

Presenters title: Principal researcher

E-mail address [email protected]

Tel: +47-97695962 www.statoil.com

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