Unit OperationsLecture 8

2 June 2010

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Overview• Questions from last week??Questions from last week??• Review rigorous methods / RADFRAC• Multicomponent systems:

R ido Residue curveso DSTWU / RADFRAC o Rules of thumb

• Complex (Enhanced) distillation• Column internals• Batch distillationBatch distillation

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Multicomponent Rigorous MethodsRigorous Methods

Rigorous Methods: solution of stage by stage material

balances energy balances andbalances, energy balances, and thermodynamic equilibrium

equations.

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AspenPlus: RADFRACRigorous two- or three-phase fractionation for single columns. Models absorbers, strippers, reactive distillation, and so forth.

• RadFrac is a rigorous model for simulating all types of multistage vapor liquid fractionation operations. In addition to ordinary distillation, it can simulate:

o absorption reboiled absorption stripping reboiled stripping extractive ando absorption, reboiled absorption, stripping, reboiled stripping, extractive and azeotropic distillation

• RadFrac is suitable for:o three phase systems, narrow boiling and wide boiling systems, systems

exhibiting strong liquid phase nonideality• RadFrac can detect and handle a free water phase or other second liquid phase

anywhere in the column. You can decant free water from the condenser. • RadFrac can handle solids on every stageRadFrac can handle solids on every stage.• RadFrac can model columns where chemical reactions are occurring. Reactions can

have fixed conversions, or they can be:o equilibrium, rate controlled, electrolyticd h d diff h i l• RadFrac can model columns where two liquid phases exist and different chemical

reactions occur in the two liquid phases. RadFrac can also model salt precipitation.• RadFrac can operate in rating mode or design mode. • With Aspen Rate-Based Distillation feature, RadFrac can perform rate-based

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W t spe ate ased st at o eatu e, ad ac ca pe o ate baseddistillation calculations (packed tower configurations).

Multicomponent MethodsAspenPlus (methanol, ethanol, 1-propanol):

ComponentP*I

(64.5 oC) KeysMeOH 1.013 bar AspenPlus TxyEtOH 0.5709

1‐Propanol 0.2534

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“Residue” From Batch Distillation

Seader & Henley (2006)

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Residue Curve Map

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Residue Curve Map

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DSTWUZi = 1/3 MeOH recovery (dist.) = 99.9%

R = 1.2 * Rmin EtOH recovery (dist.) = 0.1%

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RADFRACZi = 1/3 F = 100 kmol/hR = 5.2N = 63NF = 31NF 31D = 33 kmol/h

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Residue Curve Map

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Plot Wizard

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NF = 21

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Distillation Heuristics (Rules of Thumb)• Remove corrosive, dangerous, and reactive components first• Don’t use distillation if LK-HK < ~1.05 to 1.10• Do easy separations first ( large).• Next split / separation remove components in excess then most volatile• Next split / separation remove components in excess, then most volatile

components (trying to minimize size of downstream columns).• Try to do most difficult separations as binary and last (taller columns will

have smaller diameterhave smaller diameter.• Remove products and recycle streams as distillates (rust & heavy

contaminates can accumulate in bottoms).• Whatever you add you normally have to remove at some point.

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Variety of Phase Diagrams

15Stichlmair & Fair (1998)

Complex or Enhanced DistillationEthanol – Water Binary (1.013 bar):

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Complex DistillationEthanol – Water Binary (1.013 bar):

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Extractive DistillationEtOH

H2O

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H2OEthylene Glycol

Extractive DistillationSolvent:• Lower volatility than feed mixture• Add above the feed stage• Add few trays below the top stage• No azeotrope formed with otherNo azeotrope formed with other

components• Should interact differently with

other components• Typically add 1:1 with feed (molar

basis)

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Azeotropic DistillationPerry’s 8th ed.

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Heterogeneous Azeotropic DistillationVLLE (NRTL)( )

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Heterogeneous Azeotropic Distillation

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Perry’s 8th ed.

Residue Curve MapsMaps

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Complex Multicomponent Systems

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Pressure Swing DistillationDistillation

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Pressure Swing Distillation

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Complex or Enhanced Distillation• Extractive Distillation

o Higher boiling solvento Enters near top of columno Interacts with other components to affect volatility or activity coefficients

• Homogeneous Azeotropic Distillation• Homogeneous Azeotropic Distillationo Add entrainer that forms min/max boiling point AZ w/ 1 or more of feed compdso Added near top or bottom, depending upon if AZ is min or max BP

• Heterogeneous Azeotropic Distillationo Add entrainer to form min BP heterogeneous azeotrope (EtOH-H2O + benzene)

• Pressure – Swing Distillationo For pressure sensitive azeotropes and distillation boundaries

• Salt Distillation• Salt Distillationo Alter relative volatilities of feed compds by dissolving a soluble ionic salt into

the reflux• Steam Distillation

o Steam is added to reduce temperature of distilling organic mixture• Reactive Distillation

o add reactant &/or catalyst to cause a reversible/selective reaction with one of the feed components

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feed componentso Reaction and distillation occur in same vessel

Column Internals

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Column Internals

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Column Internals

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Column Internals

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Column Internals

Jaeger Product Bull. 400-09

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Column Internals

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