Transcript Lecture 18-19

Unit Operations
Lecture 19
7 Nov 2012
1
Overview
•Review rigorous methods / RADFRAC
• Multicomponent systems:
o Residue curves
o DSTWU / RADFRAC
o Rules of thumb
• Complex (Enhanced) distillation
• Column internals
• Batch distillation
2
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
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.
3
Distillation
Heuristics
(Turton)
4
Distillation
Heuristics
(Turton)
5
Distillation
Heuristics
(Turton)
6
Distillation
Heuristics
(Turton)
7
Variety of Phase Diagrams
Stichlmair & Fair (1998)
8
Complex or Enhanced Distillation
Ethanol – Water Binary (1.013 bar):
9
Complex Distillation
Ethanol – Water Binary (1.013 bar):
10
Extractive Distillation
EtOH
H2O
Ethylene Glycol
11
Extractive Distillation
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)
12
Unit Operations
Lecture 20
09 Nov 2012
13
Overview
•Review rigorous methods / RADFRAC
• Multicomponent systems:
o Residue curves
o DSTWU / RADFRAC
o Rules of thumb
• Complex (Enhanced) distillation
• Column internals
• Batch distillation
14
Azeotropic Distillation
Perry’s 8th ed.
15
Azeotropic Distillation
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.
16
Heterogeneous Azeotropic Distillation
VLLE (NRTL)
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Heterogeneous Azeotropic Distillation
Perry’s 8th ed.
18
Residue Curve
Maps
19
Complex Multicomponent Systems
20
Pressure Swing
Distillation
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Pressure Swing Distillation
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Complex or Enhanced Distillation
• Extractive Distillation
o Higher boiling solvent
o Enters near top of column
o Interacts with other components to affect volatility or activity coefficients
• Homogeneous Azeotropic Distillation
o Add entrainer that forms min/max boiling point AZ w/ 1 or more of feed compds
o Added near top or bottom, depending upon if AZ is min or max BP
• Heterogeneous Azeotropic Distillation
o Add entrainer to form min BP heterogeneous azeotrope (EtOH-H2O + benzene)
• Pressure – Swing Distillation
o For pressure sensitive azeotropes and distillation boundaries
• Salt Distillation
o 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
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Column Internals
24
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
columns
o 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)
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Column Internals
32
Packed Columns (Distillation)
• Column diamter
o Dcol/Dpacking ~ 8 – 12 (rule of thumb)
o If Dcol/Dpacking > ~40, watch for channeling
o Sized based usually on approach to flooding or acceptable pressure drop
• Packing height
HETP 
H
packing
N TH

packing height
number theoretica l stages
33
Trayed Columns (Diameter)
• 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

Dia  f V

1
2
,
1

,
1
frac * u flood
James R. Fair (1920 -2010)



  tray Acs fraction available
u flood  flooding vapor velocity
for vapor flow
 
ft
s
frac  fractional approach to flooding velocity
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Trayed Columns (Diameter)
• 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
u flood  C sb , f
 


20


0 .2
 L  V
V
  surface tension  
C sb , f  capacity
dyne
cm
factor
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Overview
• Questions from last week??
• Review rigorous methods / RADFRAC
• Multicomponent systems:
o Residue curves
o DSTWU / RADFRAC
o Rules of thumb
• Complex (Enhanced) distillation
• Column internals
• Batch distillation
36
Batch (Rayleigh) Distillation
• Usually for small capacity
systems
• 1 column handle multi”campaigns”
• Produce sample new products
• Batch upstream processes
• Feed contains solids/foulants
Seader & Henley (2006)
W
ln 
 Wo




x

xo
dx
yx
where:
Material Balance:
leads to Rayleigh Equation
x o  x F  mole fraction of initial charge
W o  F  initial charge [moles ]
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Batch (Rayleigh) Distillation
W
ln 
 Wo




x

xo
dx
yx
a) P = constant; K = f(T) only
W
ln 
 Wo

 x 
1



 K  1 ln  x 

 o 
b) Binary with  = constant
 1 x
1   xo 
 Wo 

ln 
ln


ln




1 x
 W   1   x 
o

c) y = K x ; but K = f(T,x)
Solve graphically or numerically




38
Multistage Batch Distillation
Seader & Henley (2006)
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
39
Multistage Batch Distillation
Removing volatile impurities.
Flexible, multi-purpose system
Seader & Henley (2006)
40
Questions?
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Go Over Homework:
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