Separations
ChEN 4253 Design I
Chapter 19
Terry A. Ring
University of Utah
Simple Separation Units
• Flash
– Quench
• Liquid-liquid decantation
– Liquid-liquid Flash
• Sublimation
– Solid/Vapor Flash
• Crystallization
• Filtration
Use of Separation Units
Separation
Reaction
Hydrodealkylation of
Toluene
T+H2B+CH4
side reaction
2B Biphenyl+H2
Reactor Effluent
T=1,350F
P = 500 psia
Reactor Effluent
Reaction Conditions
T=1,350F
P = 500 psia
Component
Hydrogen
Methane
Benzene
Toluene
Biphenyl
Total
kmole/hr
1292
1167
280
117
3
2859
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
Separation
• Vapor Separation
– CH4 from H2
• Liquid Separation
Further Separation
What separation units should be used?
• Liquid Separation
– Toluene, BP=110.6ºC
– Benzene, BP=80.1ºC
• What happens to the Methane (BP= -161.5ºC) and Biphenyl
(BP=255.9ºC) impurities?
• Gas Separation
– Hydrogen
– Methane
• what happens to the Toluene and Benzene impurities?
Direct Distillation Sequence
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
Marginal Vapor Rate
• Marginal Annualized Cost~ Marginal Vapor Rate
• Marginal 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
Short cut to Selecting a 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 or see Fig 19.1
– V=D (R+1)
• V= Vapor Flow Rate
• D= Distillate Flow Rate (=Production Rate)
• R=Reflux Ratio
Figure 19.1
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
– Give Bottoms Temperature
• Not Near Critical Point for mixture
Design Issues
• Packing vs Trays
• Column Diameter from flooding consideration
– Trays, DT=[(4G)/((f Uflood π(1-Adown/AT)ρG)]1/2
eq. 19.11
– Packed, DT =[(4G)/((f Uflood πρG)]1/2
eq. 19.14
• Uflood= f(dimensionless density difference), f = 0.75-0.85 eq. 19.12
• Uflood= f(flow ratio), f = 0.75-0.85
eq. 19.15
• Column Height
– Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK]
– N=Nmin/ε (or 2 Nmin/ ε)
Fenske eq.19.1
• Column Height = N*Htray
• Tray Height = typically 1 ft (or larger), 2 inch weir height
• Packed Height = Neq*HETP (or 2 Neq*HETP)
– HETP(height equivalent of theoretical plate)
– HETPrandom = 1.5 ft/in*Dp Rule of thumb
• Tray Efficiency, ε = f(viscosityliquid * αLK,HK)
• Pressure Drop
• Tray, ΔP=ρLg hL-wier N
• Packed, ΔP=Packed bed (weeping)
eq. 19.9
Fig 19.3
Tray Efficiency
19.3
μL * αLK,HK
Costing
Column Costs
• Column – Material of Construction gives ρmetal
–
–
–
–
Pressure Vessel Cp= FMCv(W)+CPlatform
Height may include the reboiler accumulator tank
Tray Cost = N*Ctray(DT)
Packing Cost = VpackingCpacking + Cdistributors
• Reboiler CB α AreaHX
• Condenser CB α AreaHX
• Pumping Costs – feed, reflux, reboiler
– Work = Q*ΔP
• Tanks
– Surge tank before column, reboiler accumulator, condensate accumulator
– Pressure Vessel Cp= FMCv(W)+CPlatform
CPI
Distillation Problems
• Multi-component Distillation
– Selection of Column Sequences
• Azeotropy
– Overcoming it to get pure products
• Heat Integration
– Decreasing the cost of separations
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
– DSDTW or Distil
• Then go to more complicated one for sizing
purposes
– RadFrac
– Sizing in RadFrac
• Costing
Simulation Methods- ProMax
•
•
•
•
•
•
•
•
•
•
•
Start with 10 trays (you may need up to 100 for some difficult separations)
set ΔP on column, reboiler, condenser and separator
set ΔT on condenser
Create a component recovery for HK in bottom with large ±
Set Reflux ratio = 0.1 (increase to get simulation to run w/o errors).
May need pump around loop estimate.
Determine αLK,HK, viscosity
(use Plots Tab to determine extra trays) determine Nmin and feed tray
Use Fig. 19.1 to determine Rmin from R, N from Nmin
Redo calc with tray efficiency defined see Figure 19.3 correlation.
Recommendations for final design
– Use N/Nmin=2 (above and below feed tray)
– R/Rmin=1.2
Figure 19.1
Tray Efficiency
μL * αLK,HK