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Azeotropic Distillation

Post-IFAC Conference

I-Lung Chien

Department of Chemical Engineering

National Taiwan University of Science and Technology

Taipei 106, TAIWAN

July 14, 2008

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Typical methods for separating mixtures with azeotrope

˙Not adding third component- Pressure-swing distillation (THF-H2O)

- Mixtures with binary heterogeneous azeotrope (n-butanol-H2O)

- Hybrid distillation with pervaporation (membrane)- Hybrid distillation with adsorbent (molecular sieve)

˙Adding third component- Homogeneous azeotropic distillation (IPA-H2O+DMSO)

- Heterogeneous azeotropic distillation (Two systems: IPA-H2O+CyH and HAc-H2O+IBA)

- Salt distillation (saline extractive distillation)

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Thermodynamic Model

˙Extremely important for any design study˙Check your application and select the proper class of

property method˙Use Aspen Plus built-in model parameters or the

parameters from literature to predict VLE (Txy, xy), LLE, and azeotropic compositions and azeotropictemperatures

˙Verify from data in DECHEMA, Azeotropic Data – III (Horsely, 1973), Azeotropic Data (Gmehling, 2004), and also from literatures

˙You may need to re-fit model parameters using parameter estimation capability in Aspen Plus

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Pressure-Swing Azeotropic Column System(Example from: Luyben’s book “Plantwide Dynamic Simulators in Chemical Processing and Control”)

˙Can be used in systems where there is significant change in the azeotropic composition with pressure.

˙Azeotrope: @20 psia→ 80.6 mol% THF, 164ºF@115 psia→ 65.1 mol% THF, 280ºF

˙Minimum-boiling homogeneous azeotrope varies with pressure.

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Design Flowsheet

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Remarks about the Design Flowsheet

˙Two pressures are design variables to be optimized, as well as the number of trays in each column and feed-tray locations.

˙The larger the difference in the two pressures, further apart the azeotropic compositions, less recycle is required and the lower of the energy consumption.

˙However, the lower the pressure in the low-pressure column, the larger the diameter and the coolant required in the condenser. The higher the pressure in the high-pressure column, the higher the pressure of the steam that must be used in the reboiler and other problems with high temperature at reboiler.

˙Possible heat integration of the condenser (HPCOL) and the reboiler (LPCOL).

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Isobaric Phase Diagrams for Binary Azeotropic Mixtures

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Separation of a Binary Heterogeneous Azeotropic Mixture

(Example from Doherty and Malone, “Conceptual Design of Distillation Systems)

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Design when Feed Composition is in miscible Region (e.g. 20% water)

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Examples of Binary Mixture Systems

• No need to add entrainer as in pressure-swing azeotropic distillation system.

• Examples include: purifying water-hydrocarbon mixtures (e.g., water with any one of the following components: C4-C10, benzene, toluene, xylene, etc.).

• Water-alcohol mixtures (e.g. butanol, pentanol, etc.) as another example.

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Homogeneous Azeotropic Distillation(Minimum-boiling azeotrope with intermediate-boiling entrainer example)

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Extractive Distillation

• Separating minimum-boiling binary azeotrope by using heavy entrainer.

• Two-column sequence with first extractive column separating out light product and second entrainerrecovery column separating out entrainer and another product.

• Most widely used form of homogeneous azeotropicdistillation in industries

• Examples include: n-butane-butadiene using furfural; dehydration of ethanol using ethylene glycol; acetone-methanol using water; pyridine-water using bisphenol.

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Case Study of an Homogeneous Azeotropic Distillation System

• Isopropyl Alcohol (IPA) is widely used in semi-conductor industry as a cleaning agent, thus the recovery of IPA from waste solvent stream is an important issue worthy of study.

• Dehydration of IPA using Dimethyl Sulfoxide (DMSO) as entrainer.

• Minimum-boiling azeotrope with heavy entrainer, thus an extractive distillation system.

• Two-column system with an extractive distillation column and an entrainer recovery column.

• Optimum design and control of the overall system.

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RCM and Conceptual Design Flowsheet

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Equivolatility Plots(adding DMSO keeps water toward the bottom of the column)

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Optimum Design of the Flowsheet

• The design variables include: total stages for extractive column and recovery column; feed location for extractive column, recycled entrainer location, and feed location for recovery column; ratio of recycled entrainer and fresh feed; and recycled entrainer feed temperature.

• Equal molar fresh feed composition of IPA and water.• IPA product spec. at 99.9999 mol% for semi-conductor

industry usage, bottom spec. of extractive column set at xIPA/(xIPA+xH2O)=0.001, and Water spec. at 99.9 mol%.

• Do optimization for the extractive distillation column first and then for the overall flowsheet.

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Resulting Optimal Flowsheet

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Material Balance Lines for the System

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Liquid Composition Profiles for the two Columns

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Conclusions for Extractive Distillation Study

• Extractive agent (DMSO) was added to alter the relative volatility between IPA and H2O.

• IPA goes toward top of the extractive column and water goes toward bottom of this column.

• Two-column design to obtain pure IPA and H2O.• A pre-concentrator column is needed for diluted

fresh feed.

• Simple control strategy is developed with only one tray temperature control loop in each column to handle feed variations.

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Same IPA Dehydration Example Using Heterogeneous Azeotropic Distillation

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Alternative Design with a Pre-concentrator Column to Reduce Internal Flows

NF2

Decanter

C-3C-2

IPA Water

IPA-Water

Cyclohexane makeup

Organic reflux

Aqueous outlet flow

Water

C-1

NF1

D1 D3NF3

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Proposed Design with Combined Pre-concentrator/Recovery Column

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Conclusions for Heterogeneous AzeotropicDistillation Study (IPA Dehydration)

• Heterogeneous Azeotropic Distillation able to cross distillation boundary and obtain products at different distillation regions.

• Combined pre-concentrator/recovery column design not only reduce TAC and operating cost but also save equipments and instrumentations.

• Simple control strategy is developed with only one tray temperature control loop in each column.

• Fresh feed goes into the combined column first, thus the fresh feed disturbances were dampened by the control at this simple column.

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Case Study (II): Acetic Acid Dehydration

• No azeotrope exists for the acetic acid dehydration system.

• VLE exhibits tangent pinch near pure water end.

• Needing many trays if using simple distillation.

• Adding entrainer via heterogeneous azeotropicdistillation to help the separation.

• Study the entrainer selection, design, and control of this system.

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Conceptual design of the separation system

Water0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Liquid-Liquid Equilibrium CurveFeed CompositionOrganic Phase Recycle CompositionAqueous Phase Recycle CompositionCombined Feed CompositionBottom CompositionDistillation Top Vapor Composition

IBA

Acetic acid

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Conceptual design of the separation system

Additional Aqueous Reflux

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RCM and LLE for i-Butyl Acetate

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Comparison of TAC for the alternative systems

1.86×10504.37×1041.42×1053750No

entrainer

1.73×1056.08×1042.78×1048.44×1041131NBA

1.03×1051.70×1041.80×1046.81×104930IBA

1.64×1055.40×1044.20×1046.84×104216EA

TAC($)

EntrainerCost ($)

Utility Cost ($)

Capital Cost ($)

Optimum Feed Stage

Optimum Total

Stages

Entrainer

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Column composition profile for the HAc+H2O+IBA system

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Conclusions for Heterogeneous AzeotropicDistillation Study (HAc Dehydration)

• Using acetate as entrainer can help in the separation of HAc and H2O via heterogeneous azeotropic distillation.

• Optimum design of three candidate entrainers are compared using TAC as objective function.

• TAC with i-butyl acetate as entrainer is only about 55% of the TAC for no entrainer system.

• Simple control strategy is developed with only one tray temperature control loop.

• This control strategy is able to hold both bottom HAcproduct and top aqueous product at high-purity despite feed composition or feed flow rate disturbances.

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Another related industrial study with small feed impurity of PX or MX

(working with Prof. Hsiao-Ping Huang from NTU)

• This small feed impurity from upstream process can not be removed from either the top or the bottom product streams, thus accumulation of this feed impurity inside the column will occur.

• Additional design of the proper sidedraw location and sidedraw flow rate.

• Design and control of this heterogeneous azeotropicdistillation system with sidedraw.

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Process flowsheet of an industrial unit

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Acknowledgement

• Professor Hsiao-Ping Huang, NTU – HAc with feed impurity

• Dr. Hao-Yeh Lee, NTU – HAc with feed impurity

• Former master students: K. L. Zeng, H. Y. Chao, and Saiful Arifin (NTUST) – IPA dehydration

• Former master students: J. H. Liu and C. L. Kuo(NTUST) – HAc dehydration

• Former master students: T. K. Gau and C. H. Wang (NTU) – HAc with feed impurity

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Paper References1. Chien, I-Lung; Zeng, K. L.; Chao, H. Y. Design and Control of a Complete

Heterogeneous Azeotropic Distillation Column System. Ind. Eng. Chem. Res.2004, 43 (9), 2160-2174.

2. Arifin, Saiful; Chien, I-Lung Combined Preconcentrator/Recovery Column Design for Isopropyl Alcohol Dehydration Process. Ind. Eng. Chem. Res. 2007, 46 (8), 2535-2543.

3. Arifin, Saiful; Chien, I-Lung Design and Control of an Isopropyl Alcohol Dehydration Process via Extractive Distillation Using Dimethyl Sulfoxide as an Entrainer. Ind. Eng. Chem. Res. 2008, 47 (3), 790-803.

4. Chien, I-Lung; Zeng, K. L.; Chao, H. Y.; Liu, J. H. Design and Control of Acetic Acid Dehydration System via Heterogeneous Azeotropic Distillation Column. Chem. Eng. Sci. 2004, 59 (21), 4547-4567.

5. Chien, I-Lung and Kuo, Chien-Lin Investigating the Need of a Pre-Concentrator Column for Acetic Acid Dehydration System via Heterogeneous AzeotropicDistillation. Chem. Eng. Sci. 2006, 61 (2), 569-585.

6. Chien, I-Lung; Huang, Hsiao-Ping; Gau, Tang-Kai; Wang, Chun-Hui. Influence of Feed Impurity on the Design and Operation of an Industrial Acetic Acid Dehydration Column. Ind. Eng. Chem. Res. 2005, 44 (10), 3510-3521.

7. Huang, Hsiao-Ping; Lee, Hao-Yeh; Gau, Tang-Kai; Chien, I-Lung Design and Control of Acetic Acid Dehydration Column with p-Xylene or m-Xylene Feed Impurity. 1. Importance of Feed Tray Location on the Process Design. Ind. Eng. Chem. Res. 2007, 46 (2), 505-517.

8. Huang, Hsiao-Ping; Lee, Hao-Yeh; Chien, I-Lung Design and Control of Acetic Acid Dehydration Column with p-Xylene or m-Xylene Feed Impurity. 2. Bifurcation Analysis and Control. Ind. Eng. Chem. Res. 2008, 47 (9), 3046-3059.