Azeotropic Distillation Methods
Dr. Stathis Skouras, Gas Processing and LNG
RDI Centre Trondheim, Statoil, Norway
Schedule
Tuesday 1/12/2015:
• 09.45 – 12.30: Lecture - Natural Gas Processing
• 14.00 – 17.00: Available for questions/discussion at TTPL
Wednesday 2/2/2015
• 14.00 – 17.00: Available for questions/discussion at TTPL
Thursday 3/12/2015
• 11.45 – 14.30: Lecture - Azeotropic distillation methods
• 15.00 – 17.00: Available for questions/discussion at TTPL
Friday 4/12/2015
• 13:30 – 15.00: Available for questions/discussion at TTPL
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Outline
• Introduction
− Importance and industrial relevance of azeotropic distillation
• Main part
− Theory: residue curve maps and distillation curve maps
− How to make residue curve maps at Aspen Plus
− Feasibility analysis of azeotropic distillation
− Azeotropic distillation methods
− Examples from the Oil & Gas Industry
• Summary
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Importance and industrial relevance of
azeotropic distillation
• Need for efficient recovery and recycle of organic solvents in chemical industry
• Distillation is the most common unit operation in recovery processes because of
its ability to produce high purity products
• Most liquid mixtures of organic solvents form azeotropes that complicate the
design of recovery processes
• 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
Knowledge of the limitations and possibilities in azeotropic
distillation is a topic of great practical and industrial interest
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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 (most common)
• Maximum boiling azeotrope: Azeotrope with higher boiling point than its
constituent components (less common)
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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-UnderwoodGilliland 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:
dxi
dW
 ( yi  xi )
dt
Wdt
and by considering the dimensionless time variable ξ
(dξ=dV/W)
A (TA)
dxi
 xi  yi
d
TA< TB < TC
• Integrating the above equation from any initial
composition (xw0) will generate a residue curve
The residue curve describes the change of
the still pot composition with time (trajectory)
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xW0
Still pot
composition
trajectory
C (TC)
B (TB)
Distillation curves
Total reflux
(V = L = R)
Condenser
• Consider the process of continuous distillation at
total reflux (45° line at McCabe-Thiele diagram)
V, yD
• Starting with a liquid composition at stage n (xi,n)
and by doing repeated phase equilibrium
calculations (E-mapping) upwards we get:
Yi,n-1
L, xD
Stage n-1
En
xi , n  yi , n
yi , n  xi , n  1
n1
E
xi , n  1 
 yi , n  1
yi , n  1  xi , n  2
yi,n
xi,n-1
Stage n
xi,n
...
• By doing this from any
initial composition (x0) the
distillation curve can be constructed
The distillation curve describes the change of the
component composition along the column (trajectory)
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Reboiler
xB
Singular points in RCM and DCM
• Pure component vertices and azeotropes are singular points in the RCM and DCM
dxi
 xi  yi  0
d
• 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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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
«Distillation synthesis» in Aspen Plus
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‘‘Find azeotropes’’
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‘‘Continue to Aspen Plus Residue Curves’’
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‘‘Residue curves’’
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Feasibility analysis based on RCM and DCM
D, xD
For a feasible separation the material
balances should be fulfilled:
F=D+B
F, zF
F zF = D xD + B xB
Feasibility rules
B, xB
xB
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
Products xD and xB must lie on the same distillation region
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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
Direct split: The most volatile is taken
at the first column
Indirect split: The less volatile is taken
at the first column
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F
F
Feasibility analysis based on RCM and DCM
Azeotropic mixtures
A
• One boundary exists (AzACAzAB)
• Two distillation regions (I and II)
• Different products for feed in regions I
and II
F1
AzAC
AzAB
• Feed F1 in Area I
F2
o AzAC as top product
o Component A as bottom product
• Feed F2 in Area II
• AzAC as top product
• Component B as bottom product
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C
B
Azeotropic distillation methods
1) Pressure swing distillation
No entrainer required
2) Homogeneous azeotropic (homoazeotropic) distillation
3) Heterogeneous azeotropic (heteroazeotropic) distillation
4) Extractive distillation
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Entrainer
enhanced
methods
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*
* Stichlmair and Fair, Distillation: Principles and practice, Wiley-VCH, (1998)
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2) Homogeneous azeotropic distillation
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• 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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F+E
2
1
1
E
Feasibility for homogeneous azeotropic distillation
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
D1
F´
A
•
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
AzAB=F
B
D1
F
F´
2
1
A
•
•
•
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Applicability of homoazeotropic distillation is limited
Restrictive feasibility rules (intermediate entrainers are rare)
Other distillation methods are preferably applied
B
E
3) Heterogeneous azeotropic distillation
• 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 columndecanter 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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Classic example: Ethanol/water + benzene (E)
3) Entrainer recovery column
1) Preconcentrator
•
•
•
Aqueous feed dilute in EtOH (F1)
EtOH-Water azeotrope at top (D1)
Pure water at bottom (B1)
•
•
•
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)
Benzene
A12E
H2O
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EtOH
4) Extractive distillation
• 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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Feasibility and synthesis for 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
Rectifying
section
Rectifying
section
Binary feed
(1 & 2)
F
Bottom
section
Bottom section
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Examples from Oil & Gas – CRAIER plant at Kårstø
• A common azeotrope in oil and gas industry is the ethane / CO2 azeotrope
• CO2 has a volatility between methane and ethane
• CO2 distributes between natural gas and NGL in a gas processing plant
CO2
C1
C2
C3
* Anette Kornberg, “Equation of State for the System CO2 and Ethane, Project report, NTNU, Dec. 2007
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CO2 Removal and Increased Ethane Recovery (CRAIER)
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CRAIER column
Az C2/CO2
~ 50 mol% CO2
C2 + CO2 (+ C1)
~ 20 mol% CO2
C2 product (pure)
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Examples from Oil & Gas – Ryan Holmes process
• Invented by Jim Ryan and Art Holmes*
• Cryogenic distillation process for the removal of CO2 from natural gas
• Suitable for removing high CO2 contents from natural gas
• Uses extractive distillation to ‘‘break’’ the CO2 / C2 azeotrope
• Uses Natural Gas Liquid (NGL) as entrainer, which is an internal product
of the process
• Various configurations with 2, 3 and 4 columns
* A. S. Holmes, J. M. Ryan, Cryogenic distillation separation of acid gases from methane, US patent, 1982
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Added
entrainer
De-C1
column
Extractive
column
CO2/C2+
Entrainer
recovery column
C2+
C4+
Entrainer (C4+) recycle
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MTBE Production and Separation Unit
Azeo
C4-MeOH
+ water (E)
Feed
C3 – C5+
MTBE
methanol
MeOHwater
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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
• 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
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Summary
• 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
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Presenters name: Dr. Stathis Skouras
Presenters title: Principal Researcher
E-mail: efss@statoil.com
Tel: +47-97695962
www.statoil.com
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