Unit Operations Lecture 19

Unit OperationsLecture 19

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7 Nov 2012

Overview

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• Review rigorous methods / RADFRAC• Multicomponent systems:o Residue curveso DSTWU / RADFRAC o Rules of thumb

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

Distillation Heuristics (Rules of Thumb)

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• 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

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

have 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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Distillation Heuristics (Turton)

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Distillation Heuristics (Turton)

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Distillation Heuristics (Turton)

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Distillation Heuristics (Turton)

Variety of Phase Diagrams

8Stichlmair & Fair (1998)

Complex or Enhanced Distillation

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Ethanol – Water Binary (1.013 bar):

Complex Distillation

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Ethanol – Water Binary (1.013 bar):

Extractive Distillation

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H2O

EtOH

Ethylene Glycol

Extractive Distillation

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

components• Should interact differently with

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

basis)

Unit OperationsLecture 20

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09 Nov 2012

Overview

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• Review rigorous methods / RADFRAC• Multicomponent systems:o Residue curveso DSTWU / RADFRAC o Rules of thumb

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

Azeotropic Distillation

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Perry’s 8th ed.

Azeotropic Distillation

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Water with (binary system at atmospheric P):• methanol - no azeotrope• ethanol - azeotrope• propanol - azeotrope

In-class exercise:• using Aspen Plus, explore if we can use distillation to separate the binary

mixture of water and n-butanol.

Heterogeneous Azeotropic Distillation

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VLLE (NRTL)

Heterogeneous Azeotropic Distillation

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Perry’s 8th ed.

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Residue Curve Maps

Complex Multicomponent Systems

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Pressure Swing Distillation

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Pressure Swing Distillation

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Complex or Enhanced Distillation

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

• 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 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

o 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

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Jaeger Product Bull. 400-09

Column Internals

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Random and Structured Packing

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Packed Columns (Distillation)

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• Usually for small diameter columnso Usually more

economical for columns < ~75 cm (2.5 ft)

o Lower pressure drop than trayed columns

o Good for vacuum operations

• Wide choice of packing materials (random or structured)

Column Internals

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Packed Columns (Distillation)

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• Column diamtero Dcol/Dpacking ~ 8 – 12 (rule of thumb)o If Dcol/Dpacking > ~40, watch for channelingo Sized based usually on approach to flooding or acceptable pressure drop

• Packing height

stagesltheoreticanumberheightpacking

NH

HETPTH

packing

Trayed Columns (Diameter)

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• Chap 10 (p 314, Wankat) “Fair’s Procedure”o Considers entrainment

flooding (most freq.)o Downcomer flooding

(sometimes) – need different procedure

o Downcomer flooding rare if (1- f) ≥ 10%

• Used in AspenPlus

floodufracVfDia

*1,1,2

1

velocityfloodingtoapproachfractionalfrac

sftvelocityvaporfloodingu

flowvaporforavailablefractionAtray

flood

cs

James R. Fair (1920 -2010)

Trayed Columns (Diameter)

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• Plate spacing (selected for maintenance, performance). Typ:o 12 – 16” for Dia < 5’o 24” larger columns

• Calc Dia & round up to nearest ½ foot (USA)o 2.5’ minimum dia.o If < 2.5’ consider packed

tower

V

VLfsbflood Cu

2.0

, 20

factorcapacityC

cmdynetensionsurface

fsb

,

Overview

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• Questions from last week??• Review rigorous methods / RADFRAC• Multicomponent systems:o Residue curveso DSTWU / RADFRAC o Rules of thumb

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

Batch (Rayleigh) Distillation

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Seader & Henley (2006)

x

xo oxy

dxWWln

• Usually for small capacity systems

• 1 column handle multi-”campaigns”

• Produce sample new products• Batch upstream processes• Feed contains solids/foulants

Material Balance:leads to Rayleigh Equation

where:][moleschargeinitialFW

chargeinitialoffractionmolexx

o

Fo

Batch (Rayleigh) Distillation

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x

xo oxy

dxWWln

a) P = constant; K = f(T) only

oo xx

KWW ln

11ln

b) Binary with = constant

o

oo

xx

xx

WW

11lnln

11ln

c) y = K x ; but K = f(T,x) Solve graphically or numerically

Multistage Batch Distillation

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Modes of operation:• Constant reflux rate or ratio• xD varies with time• easily implemented (flow sensors)• Relatively simple and cost effective

• Constant distillate composition• R or D varies with time• Requires fast response composition sensors• Sensors might not be available or only

justified for larger batch systems• Optimal control mode• xD and R varied with time• Designed to:

Minimize operation time Maximize amount of distillate Maximize profit

• More complex control scheme

Seader & Henley (2006)

Multistage Batch Distillation

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Removing volatile impurities.

Seader & Henley (2006)Flexible, multi-purpose system

Questions?

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Go Over Homework:

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