Azeotropic Distillation Methods

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Dec 3, 2015 - Azeotropes make separation impossible by normal distillation but can be also ... Homogeneous azeotrope: Azeotrope where the forming ...
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) 5

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 2

• 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



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 25

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: [email protected] Tel: +47-97695962

www.statoil.com

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