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)
17
Heterogeneous Azeotropic Distillation
Perry’s 8th ed.
18
Residue Curve
Maps
19
Complex Multicomponent Systems
20
Pressure Swing
Distillation
21
Pressure Swing Distillation
22
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
23
Column Internals
24
Column Internals
25
Column Internals
26
Column Internals
27
Column Internals
Jaeger Product Bull. 400-09
28
Column
Internals
29
Random and Structured Packing
30
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)
31
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
34
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
35
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 ]
37
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?
41
42
43
Go Over Homework:
44