Distillation
in Design
Terry A. Ring
ChE
University of Utah
www.che.utah.edu/~ring
Use of Separation Units
Criteria for the Selection of a
Separation Method
• Energy Separation
Agent (ESA)
– Phase condition of feed
– Separation Factor
– Cost
I
1
C
SF 
C
I
2
C
II
2
II
1
C
• Mass Separation
Agent (MSA)
– Phase condition of feed
– Choice of MSA
Additive
– Separation Factor
– Regeneration of MSA
– Cost
Phases I and II,
Components 1 and 2 (light key and heavy key)
Distillation
Distillation
Plate Types
• Bubble Cap Tray
• Sieve Tray
Packed Towers
• Random Packing
• Structured Packing
Note: Importance of
Distributor plate
Distillation
α=KL/KH
• Relative Volatility
• Equilibrium Line
Distillation
• Rectifying Section
– R= reflux ratio
– V=vapor flow rate
• Stripping Section
– VB= Boil-up ratio
• Feed Line
Minimum Reflux Ratio
McCabe-Thiele
Step Off Equilibrium Trays
What are you going to learn next year?
• Column sizing
– Diameter of Column
– Size of trays
– Height of packing
• Column Costing
• Optimization of column with respect to cost to run
(capital cost and operating cost)
• How to develop a distillation train.
• How to set up side streams in multi-component
distillation.
Marginal Vapor Rate
• Annualized Cost~ Marginal Vapor Rate
• Annualized Cost proportional to
–
–
–
–
–
Reboiler Duty (Operating Cost)
Condenser Duty (Operating Cost)
Reboiler Area (Capital Cost)
Condenser Area (Capital Cost)
Column Diameter (Capital Cost)
• Vapor Rate is proportional to all of the above
Direct Distillation Sequence
Column Sequences
• No. of Columns
– Nc=P-1
• P= No. of Products
• No. of Possible Column Sequences
– Ns=[2(P-1)]!/[P!(P-1)!]
• P= No. of Products
–
–
–
–
–
P=3, Nc=2, Ns=2
P=4, Nc=3, Ns=5
P=5, Nc=4, Ns=14
P=6, Nc=5, Ns=42
P=7, Nc=6, Ns=132
No. of Possible
Column Sequences
Blows up!
How do I evaluate which is best
sequence?
Marginal Vapor Rate
• Annualized Cost~ Marginal Vapor Rate
• Annualized Cost proportional to
–
–
–
–
–
Reboiler Duty (Operating Cost)
Reboiler Area (Capital Cost)
Condenser Duty (Operating Cost)
Condenser Area (Capital Cost)
Diameter of Column (Capital Cost)
• Vapor Rate is proportional to all of the above
Selecting Multiple Column
Separation Trains
• Minimum Cost for Separation Train will
occur when you have a
– Minimum of Total Vapor Flow Rate for all
columns
– R= 1.2 Rmin
– V=D (R+1)
• V= Vapor Flow Rate
• D= Distillate Flow Rate
• R=Recycle Ratio
Problem
Reactor
Flash
Distillation Train
After Flash to 100F @ 500 psia
Effluent
Vapor
Liquid
Component kmole/hr kmole/hr kmole/hr
Hydrogen
1292
1290
2
Methane
1167
1149
18
Benzene
280
16
264
Toluene
117
2
115
Biphenyl
3
0
3
Total
2859
2457
402
Recycled Reactants
Simplified Marginal Vapor Flow
Analysis
Liquid
kmole/hr
Hydrogen
2
Methane
18
Benzene
264
Toluene
115
Biphenyl
3
Total
402
Sequence Total
Direct Sequence
Distillate Flow Distillate Flow
Column 1
Column 2
Indirect Sequence
Distillate Flow
Distillate Flow
Column 1
Column 2
x
x
x
x
284
399
115
R assumed to be similar for all columns
V~D
x
x
x
x
x
x
x
399
683
284
Column Design
• Minimum Cost for Distillation Column will
occur when you have a
– Minimum of Total Vapor Flow Rate for column
– Occurs at
• R ~ 1.2 Rmin @ N/Nmin=2
– V=D (R+1)
• V= Vapor Flow Rate
• D= Distillate Flow Rate (=Production Rate)
• R=Reflux Ratio
How To Determine the Column
Pressure given coolant
• Cooling Water Available at 90ºF
• Distillate Can be cooled to 120ºF min.
• Calculate the Bubble Pt. Pressure of Distillate Composition at 120ºF
– equals Distillate Pressure
– Bottoms Pressure = Distillate Pressure +10 psia delta P
• Compute the Bubble Pt. Temp for an estimate of the Bottoms
Composition at Distillate Pressure
•
•
– Gives Bottoms Temperature
P > Atm, Pressure generated by system.
For Vacuum, how is it that generated?
• Not Near Critical Point for mixture
Steam Ejector Generates the
Vacuum.
High Pressure
High Velocity
Steam
Velocity > Mach 1
Vacuum
Bernoulli’s Equation
Design Issues
• Packing vs Trays
• Column Diameter from flooding consideration
– Trays, DT=[(4G)/((f Uflood π(1-Adown/AT)ρG)]1/2
eq. 14.11
– Packed, DT =[(4G)/((f Uflood πρG)]1/2
eq. 14.14
• Uflood= f(dimensionless density difference), f = 0.75-0.85 eq. 14.12
• Uflood= f(flow ratio), f = 0.75-0.85
eq. 14.15
• Column Height
– Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK]
– N=Nmin/ε
eq. 14.1
• Tray Height = N*Htray
• Packed Height = Neq*HETP
– HETP(height equivalent of theoretical plate)
– HETPrandom = 1.5 ft/in*Dp
• Tray Efficiency, ε = f(viscosityliquid * αLK,HK)
• Pressure Drop
• Tray, ΔP=ρLg hL-wier N
• Packed, ΔP=Packed bed
eq. 14.9
Fig 14.3
Tray Efficiency
μL * αLK,HK
Costing
Column Costs
• Column – Material of Construction gives ρmetal
– Pressure Vessel Cp= FMCv(W)+CPlatform
• Reboiler CB α AreaHX
• Condenser CB α AreaHX
• Pumping Costs – feed, reflux, reboiler
– Work = Q*ΔP
• Tanks
– Surge tank before column, reboiler accumulator
(sometimes longer (empty) tower), condensate
accumulator
Problem
• Methanol-Water Distillation
• Feed
– 10 gal/min
– 50/50 (mole) mixture
• Desired to get
– High Purity MeOH in D
– Pure Water in B
Simulator Methods - Aspen
• Start with simple distillation method
– DSTWU – Winn-Underwood-Gilliland Method
• Min # stages, Rmin – Fenske-Underwood
• Min # stages vs R - Gilliland
– Distil – short cut Edmister Method
• Then go to more complicated one for sizing
purposes
– RadFrac – rigorous method
– Sizing in RadFrac
Eric Carlson’s Recommendations
Figure 1
Polar
Non-electrolyte
E?
Electrolyte NRTL
Or Pizer
Electrolyte
Real
All
Non-polar
Peng-Robinson,
Redlich-Kwong-Soave
Lee-Kesler-Plocker
R?
Polarity
R?
Real or
pseudocomponents
P?
Pressure
E?
Electrolytes
See Figure 2
Pseudo & Real
P?
Vacuum
Chao-Seader,
Grayson-Streed or
Braun K-10
Braun K-10 or ideal
Yes
Figure 2
Yes
LL?
P < 10 bar
ij?
(See also
Figure 3)
P?
NRTL, UNIQUAC
and their variances
No
Yes
No
No
Yes
P > 10 bar
P?
Pressure
ij?
Interaction Parameters
Available
UNIFAC LLE
LL?
Polar
Non-electrolytes
LL? Liquid/Liquid
WILSON, NRTL,
UNIQUAC and
their variances
ij?
No
UNIFAC and its
extensions
Schwartentruber-Renon
PR or SRK with WS
PR or SRK with MHV2
PSRK
PR or SRK with MHV2
Hexamers
Figure 3
Yes
DP?
Dimers
Wilson
NRTL
UNIQUAC
UNIFAC
VAP?
DP?
Wilson, NRTL, UNIQUAC,
or UNIFAC with special EOS
for Hexamers
VAP?
No
Wilson, NRTL, UNIQUAC,
UNIFAC with Hayden O’Connell
or Northnagel EOS
Wilson, NRTL,
UNIQUAC, or UNIFAC*
with ideal Gas or RK EOS
Vapor Phase Association
Degrees of Polymerizatiom
UNIFAC* and its Extensions
Distillation Problems
• Multi-component Distillation
– Selection of Column Sequences
– Selection of tray for side stream
• Azeotropy
– Overcoming it to get pure products
• Heat Integration
– Decreasing the cost of separations