azeotropic distillation

Azeotropic distillation methods Dr. Stathis Skouras

Gas Treating Technologies, Statoil RDI Centre Trondheim, Norway

2014-02-07 Classification: Internal

Schedule

Tuesday 11.02.2014: 09:45 – 11:30

• Lecture: Natural Gas Processing (Part 1-2)

Thursday 13.02.2014: 11:45 – 14:30

• Lecture: Distillation of azeotropic mixtures

Tuesday 18.02.2014: 09:45 – 11:30

• PC-lab

o Lecture: Natural Gas Processing (Part 3)

o HYSYS exercise: Dew Point Control Unit (DPCU)

o Aspen exercise: Extractive Distillation (Acetone/methanol + water)

2

Outline

• Introduction

− Lecture objectives

− Importance and industrial relevance of azeotropic distillation

• Main part

− Overview of azeotropic distillation methods

− Azeotropic phase equilibrium diagrams - residue curve maps and distillation

curve maps

− Tools for feasibility analysis and conceptual design of azeotropic distillation

• Summary

3

Objectives

Introduce you to the topic of azeotropic distillation (enhanced distillation)

Present the theory behind azeotropic phase equilibrium diagrams

Show you how such diagrams can be used for analysis of the behaviour of

azeotropic mixtures in distillation columns

Present methods for feasibility analysis and conceptual design of distillation

processes in a systematic and comprehensive manner

4

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

synthesis and conceptual 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

• Thus, knowledge of the limitations and possibilities in azeotropic distillation

is a topic of great practical and industrial interest

5

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

6

Overview: Azeotropic distillation methods

i) Pressure swing distillation

ii) Hybrid methods (membrane + distillation)

iii) Homogeneous azeotropic (homoazeotropic) distillation

iv) Heterogeneous azeotropic (heteroazeotropic) distillation

v) Extractive distillation

7

No entrainer required

Entrainer

enhanced

methods

i) Pressure swing distillation

• Principle: Overcome the azeotropic composition by changing the pressure

• Key factors: Azeotropic composition sensitivity in pressure changes, recycle

ratio which increases costs

• Application: Tetrahydrofuran/water*

8

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

ii) Hybrid methods (distillation + membrane)

• Principle: Membrane used as the mass separating agent by absorbing and diffusing

one of the azeotrope-forming components (Pervaporation-distillation hybrid)

• Key factors: Membrane efficiency (high mass transfer flux, selectivity, life-time)

• Applications: dehydration of alcohols, removal of Volatile Organic Components

(VOC) from wastewater*

9

* Hilmen, Separation of azeotropic mixtures: tools for analysis and studies on batch distillation operation, PhD thesis, NTNU, Norway, 2000

iii) Homogeneous azeotropic (homoazeotropic) distillation

• Definition:

o The distillation is carried out in a conventional

single-feed column

o Entrainer completely miscible with the original

components. It may form homoazeotropes with

the original azeotropic components

• Principle: The addition of the entrainer results

in a ternary phase equlibrium diagram

promising for separation

• Applicability: Limited because the entrainers

resulting in promising phase diagrams are rare

• Examples: hydrochloric acid/water + sulphuric

acid *, nitric acid/water + sulphuric water **

10

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

** Perry et al., Perry’s chemical engineers handbook, 7th ed., 1997

iv) Heterogeneous azeotropic (heterozeotropic) distillation

• Definition:

o The distillation is carried out in a combined

column-decanter column

o Entrainer forms heteroazeotrope with at least

one of the original azeotropic components

• Principle: Liquid-liquid immiscibilities provide a

powerful way to overcome azeotropic

compositions

• Applicability: Widely used in the industry

• Examples: Dehydration of ethanol by using

benzene, diethylether or pentane, separate

acetic acid/water by using ethyl acetate *

11

* Seader et al., Separation Process Principles, 3rd ed., 2011

v) Extractive distillation

• Definition:

o The distillation is carried out in a two-feed

column with a heavy entrainer added

continously in the top stages

o Entrainer has a boiling point substantially

higher than the original azeotropic components

and interacts selectively with one of them

o The entrainer is recovered as a bottom product

in the second column

• Principle: The entrainer alters the relative

volatility of the original components

• Applicability: The most widely used method

in the industry

• Examples: Acetone/methanol by using water*

12

* Seader et al., Separation Process Principles, 3rd ed., 2011

Azeotropic phase equilibrium diagrams –general issues

• 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, operating lines

and composition trajectories are used to

identify feasible distillation schemes and

screen infeasible specifications

13

Residue curve maps (RCM)

• 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

14

( )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

Residue curve maps (RCM)

Properties

• A residue curve map is a representation of

the VLE

• Residue curves move along the direction of

increasing temperature

• Residue curves cannot intersect each other

• Residue curve split the composition space

into distillation regions and boundaries exist

• Residue curves do not say anything about

the ease of separation

15

Residue curves represent the liquid

composition profile in a packed

distillation column at infinite reflux

Distillation curve maps (DCM)

Distillation curves (or distillation lines)

• 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 map (DCM) can be constructed

16

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

Properties

• DCM is also a representation of the VLE

• Distillation curves move along the direction of

decreasing temperature (xy), opposite that of

residue curves

• BUT this is a matter of definition. The curve will be

the same on the opposite direction (yx)

• Distillation curves are discrete vectors (tie-lines) but

smoothed curves are usually constructed

• Distillation curves cannot intersect each other

• Distillation curves split the composition space into

distillation regions and boundaries exist

• Distillation curves (discrete) say something about

the ease of separation (long line – easy separation)

17

Distillation curve maps (DCM)

Distillation curves represent

the liquid composition

profile in a staged distillation

column at total reflux

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

• The behaviour of the residue curves at the vicinity of singular points depends on the

two eigenvalues, of the above equation

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 – Cannot be recovered as

pure product in distillation. Residue curves move towards and then away from

these points – One positive and one negative eigenvalue

18

Singular points in RCM and DCM

0i

i idx

x yd

Relationship between RCM and DCM

• Both are pure representations of the VLE

• Provide the same information and can be equally

used for synthesis of azeotropic distillation

• Have the same topological structure and singular

points

• Sometimes are sketched in different directions

(opposite singular points) but this is a matter of

definition and does not change the synthesis work

• They DO NOT coincide to each other

19

Rule of azeotropy – ternary systems

• The singular points (SN, S, UN) should

always satisfy the rule:

2N3 + N2 + N1 = 2S3 + S2 + 2

where:

o N3 (S3) is the number of ternary nodes (saddles)

o N2 (S2) is the number of binary nodes (saddles)

o N1 (S1) is the number of pure component nodes

(saddles)

• This rule dictates feasible phase diagrams

for azeotropic mixtures (Serafimov’s

topological classes)

• Experimental VLE data indicates the

natural occurence of 16 out of 26 classes

20

Feasible phase diagrams for azeotropic mixtures

Distillation regions and boundaries

a) Zeotropic mixture (Class 0.0-1)

• No distillation boundary/one distillation region

• Same distillation products at any feed (F)

b) Azeotropic mixture (Class 1.0-2)

• One distillation boundary/two distillation regions

• Different distillation products at feeds (F1 and F2)

c) Azeotropic mixture (Class 3.1-2)

• Three distillation boundaries/three distillation regions

• Different distillation products at feeds (F1, F2 and F3)

21

F1 F2

F1

F2 F3

F

Feasibility analysis based on RCM and DCM

Simple zeotropic mixture (Class 0.0-1)

• Only one distillation region exists

• No matter where the feed (F) is located

the products will be the same

• Direct split: The most volatile is taken at

the first column

• Indirect split: The less volatile is taken at

the first column

22

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

23

Simple zeotropic mixture (Class 0.0-1)

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 curve (distillation curve)

Feasibility analysis based on RCM and DCM Azeotropic mixture (Class 2.0-2b)

• One boundary exists

• Two distillation regions

• Different porducts for feeds F1 and F2 at

different regions

Question: What will the products be?

a) At region I – Feed F1?

− Distillate: AzAB

− Bottom: B

b) At region II – Feed F2

− Distillate: AzAB

− Bottom: C

24

A

B C

AzAB

AzBC

Special case: Crossing distillation boundaries

In special cases of quite curved boundaries,

boundary crossing tecnhiques can be employed

• Case 1: Feed F1 can be separated into products

D1 and B1, which lie on distillation curve (a)

The feed and the distillation products lie on the

same distillation region (Region 1)

• Case 2: Feed F1 can also be separated into D2

and B2 (or B3), which lie on a distillation curve (b)

The feed and the distillation products lie on

different distillation regions

25

• Boundary crossing is theoritically possible but difficult in practice

• Require an advanced column control system to maintain product purities

and feed compositions at the right place at various disturbances

Feasibility and synthesis for homogeneous azeotropic distillation

• Definition:

o Entrainer completely miscible with the original components. It may

form homoazeotropes with the original azeotropic components

o The distillation is carried out in a conventional single-feed column

• Here we consider ordinary distillation of ternary mixtures

o At least one binary homoazeotrope formed by the original

components

o The entrainer may or may not form new azeotropes in the system

o The only criteria is that the resulting ternary system should form a

VLE diagram that is promising for separation

• Two possibilities exists:

o Original components lie within a single distillation region (no

boundary crossing)

o Original components lie in different regions but some type of

boundary crossing technique can be empoyed

• The applicability of the process is limited

o Difficult to find entrainers that do not introduce additional boundaries

o Practically difficult to implement boundary crossing techniques

26


• Heavy and light entrainers are infeasible

o Class 1.0-1a: Original components are saddles

o Class 1.0-2: Boundary exists (unless boundary

crossing is possible)

• Intermediate entrainers are feasible

o Class 1.0-1b: Component 1 is a stable (unstable)

node and can be taken as bottom (top) product,

Component 2 is a saddle and can be taken as a

side-product under certain operating conditions

o In practice intermediate entrainers are difficult

Rare to find entrainer with such a boiling point

(class 1-0-1b is rare)

Separation sensitive to operating conditions

27

Case 1: Entrainer does not form azeotropes with the original components


• Class 2.0-1: Original components are

saddles

• Class 2.0-2c: Infeasible due to boundary

(unless boundary crossing is possible)

• Class 2.0-2b: Infeasible due to boundary


• Class 3.0-2: Infeasible due to boundaries


• Class 2.0-2a: Feasible (same as class 1.0-

1b in previous slide)

• Class 3.0-1a: Feasible (same as class 2.0-

2a above)

28

Homoazeotropic distillation offers in

general limiting possibilities and

therefore other azeotropic distillation

methods are preferably applied

Case 2: Entrainer forms azeotropes with the original components

Feasibility and synthesis for heterogeneous azeotropic distillation

• Definition:

o Entrainer forms heteroazeotrope with at least one of the original azeotropic components

o The distillation is carried out in a combined column-decanter system

• Principle: Liquid-liquid immiscibilities provide a powerful way to overcome

azeotropic compositions and cross distillation boundaries

29

* Example: Acetone- water – 1-butanol (Class 1.0.2)

Feasibility and synthesis for heterogeneous azeotropic distillation

Most classic example - Class 3.1-2 (26%)

• Original components form a homogeneous azeotrope (saddle A12)

• Entrainer forms two binary homogeneous azeotropes with each one of the orginal components (saddles A2E

and A1E)

• Entrainer forms a ternary heterogeneous azeotrope with the orginal components (unstable node A12E)

• The ternary heteroazeotrope will be distilled on the top as an unstable node

• The original azeotropic components 1 and 2 are stable nodes of two distillation regions. The feed composition

is located in one region and one end of the liquid-liquid tie-line is located in the other

30

* Example: Ethanol dehydration by benzene or cyclohexane (Class 3.1-2)

Feasibility and synthesis for extractive distillation

• Definition:

o Entrainer has a boiling point substantially higher than the original

azeotropic components and interacts selectively with one of them

(mainly in the liquid phase)

o The distillation is carried out in a two-feed column with a heavy

entrainer added continously in the top stages

o The entrainer is recovered as a bottom product in the second

column

• Principle: The entrainer interacts selectively with one of

the original components and alters their relative volatility

• Extractive distillation is the oldest and most widely used

enhanced distillation method

“Distillation in the presence of a substance which is relatively non-

volatile compared to the components to be separated and which,

therefore, is charged continuously near the top of the distilling

column so that an appreciable concentration is maintained on all

plates of the column.” (Benedict and Rubin, 1945)

31


32

• Original components form a homogeneous

azeotrope (unstable node A12)

• Entrainer (E) is heavy boiling and forms no

azeotropes with the original components

(stable node E)

• The entrainer can be recovered as a bottom

product in the second column

• The original components (1 and 2) become

saddles

• Based on the RCM the original azeotropic

components (saddles) cannot be recovered as

top or bottom products in a distillation column


33

Rectifying section 1+E

1+2+E

2+E

E

2


• A main characteristic of extractive distillation is that a saddle is obtained as a distillate product

which is “against” the residue curves

• In the rectifying section, component 1 is the unstable node of the binary edge between

component 1 and the entrainer E. Thus, component 1 is recovered in the distillate

• Component 2 is removed as bottom product with the entrainer E. Thus, the complete

extractive distillation scheme requires a sequence of two columns, one extractive distillation

column and one ordinary column (or stripper) to separate component 2 from the entrainer

34

Rectifying

section

Bottom section

Rectifying

section

Bottom

section


• A common restriction on the “extractive distillation” is that the entrainer should not

introduce new azeotropes in the system, that is, no distillation boundaries.

However, this restriction seems unnecessary

• High reflux may be harmful in extractive distillation because it weakens the

extractive effect (decreases the entrainer concentration in the extractive column

section)

• Previous studies on continuous extractive distillation have shown that there is a

maximum as well as a minimum reflux ratio to make the separation scheme

feasible

• Re-extractive distillation is the symmetrical process of separating a maximum-

boiling azeotrope by using a light entrainer added continuously below the original

mixture feed point, or together with it

35

• 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

Example of extractive distillation: Ryan-Holmes process

36

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

37

CO2/C2+ C2+

Added

entrainer

Entrainer

recovery

column

Entrainer recycle

CO2/C2+

extractive

column

Summary

38

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

and industrial interest

• Most liquid mixtures of organic components form nonideal systems that often

results in the formation of azeotropes. Binary homoazeotropes are impossible

to separate by ordinary distillation, but may be effectively separated by

membrane-distillation hybrids or distillation methods where a third component

(entrainer) is added to the system

• Analysis of phase equilibrium diagrams is an efficient tool to predict feasible

separations for entrainer-addition distillation processes

• Insights into to the thermodynamic behavior of azeotropic mixtures is

fundamental for the development of separation system involving azeotropic

mixtures

Summary

39

Three different entrainer-addition distillation methods depending on the

properties of the entrainer and the column configuration, exist:

• Homogeneous azeotropic distillation

Only a few VLE diagrams lead to feasible separations by ordinary distillation in

sequences of single-feed columns. The separation schemes are complex and

may be very energy intensive, or difficult to control and maintain

• Heteroazeotropic distillation

Several VLLE diagram structures that involves one or more heteroazeotropes

are possible to separate by ordinary distillation combined with decantation. The

separation schemes are fairly simple, but the range of feasible entrainers is

rather limited

• Extractive distillation

The most general distillation method for separating homoazeotropic mixtures.

The scheme is not sensitive to the type of original mixture as there is a broad

range of feasible entrainers

Presentation title: Azeotropic distillation methods

Presenters name: Stathis Skouras

Presenters title: Principal researcher

E-mail address [email protected]

Tel: +47-97695962 www.statoil.com

40

azeotropic distillation

Documents