Unit OperationsLecture 9

16 June 2010

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Overview• Last timeLast time…• Column Internals; sizing• Batch (Rayleigh) Distillation

E ti• Evaporation • Crystallization

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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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Random and Structured Packing

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Packed Columns (Distillation)• Usually for small diameter y

columnso Usually more

economical for columns < 75 cm (2 5columns < ~75 cm (2.5 ft)

o Lower pressure drop than trayed columns

o Good for vacuum operations

• Wide choice of packing materials (random ormaterials (random or structured)

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Column Internals

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Packed Columns (Distillation)• Column diamter

o 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• Packing height

stagesltheoreticanumberheightpacking

NH

HETPTH

packing

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Overview• Last timeLast time…• Column Internals; sizing• Batch (Rayleigh) Distillation

E ti• Evaporation • Crystallization

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Batch (Rayleigh) Distillation• Usually for small capacityUsually for small capacity

systems• 1 column handle multi-

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

Seader & Henley (2006) Material Balance:leads to Rayleigh Equation

x

xo oxy

dxWWln where:

][moleschargeinitialFWchargeinitialoffractionmolexx

o

Fo

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Batch (Rayleigh) Distillation

x dxWl

xo o

xyWln

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

xKW

W ln1

1ln

oo xKW 1

b) Binary with = constant

oo xxW 1lnln1ln b) Binary with = constant

oxxW 1

lnln1

ln

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

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Multistage Batch DistillationModes of operation:p• Constant reflux rate or ratio

• xD varies with time• easily implemented (flow sensors)• Relatively simple and cost effective• 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

• x and R varied with timeSeader & Henley (2006)

• xD and R varied with time• Designed to:

Minimize operation time Maximize amount of distillate Maximize profit

• More complex control scheme

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Multistage Batch Distillation

Removing volatile impurities.

Flexible multi-purpose system

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Seader & Henley (2006)Flexible, multi purpose system

Questions?

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