1. ChEN 5253 Design II Chapter 13 Terry A. Ring University of Utah
Distillation
ChEN 5253 Design II
Chapter 13
Terry A. Ring
University of Utah

2. Distillation
Add extra Tanks and control system for liquid level

3. Plate Types
Bubble Cap Tray
Sieve Tray

4. Packed Towers Random Packing Structured Packing Note: Importance of
Distributor plate

5. Distillation
α=KL/KH
Relative Volatility
Equilibrium Line

6. Distillation Rectifying Section Stripping Section Feed Line
R= reflux ratio
V=vapor flow rate
Stripping Section
VB= Boil-up ratio
Feed Line

7. Minimum Reflux Ratio

8. McCabe-Thiele

9. Step Off Equilibrium Trays

10. 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

11. 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 N/Nmin=2,
Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK]
Rmin~(F/D)/(α-1)
V=D (R+1)
V= Vapor Flow Rate
D= Distillate Flow Rate (=Production Rate)
R=Reflux Ratio

12. 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

13. Design Issues Packing vs Trays
Column Diameter from flooding consideration
Trays, DT=[(4G)/((f Uflood π(1-Adown/AT)ρG)]1/2 eq
Uflood= f(dimensionless density difference), f =
Packed, DT =[(4G)/((f Uflood πρG)]1/2 eq
Uflood= f(flow ratio), f =
Column Height
Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] eq. 13.1
N=Nmin/ε (or 2 Nmin/ ε)
Column Height = N*Htray
Tray Height = typically 1 ft (or larger)
Packed Height = Neq*HETP (or 2 Neq*HETP)
HETP(height equivalent of theoretical plate)
HETPrandom = 1.5 ft/in*Dp Rule of thumb p. 392
Tray Efficiency, ε = f(viscosityliquid * αLK,HK) Fig 13.3
Pressure Drop
Tray, ΔP=ρLg hL-wier N
Packed, ΔP=Packed bed (weeping)

14. Tray Efficiency
μL * αLK,HK

15. Costing

16. 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

17. CPI

18. Problem Methanol-Water Distillation Feed Desired to get 10 gal/min
50/50 (mole) mixture
Desired to get
High Purity MeOH in D
Pure Water in B

19. Simulator Methods - Aspen
Start with simple distillation method
DSDTW or Distil
Then go to more complicated one for sizing purposes
RadFrac
Sizing in RadFrac
Rate Based Calculation – Efficiency is calculated for you
Costing

20. 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)
Or R~1.2*(F/D)/(α-1)
May need pump around loop estimate.
Determine αLK,HK* viscosity
(use Plots Tab to determine extra trays) determine Nmin and feed tray
Use Fig 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
R/Rmin=1.2

21. Figure 13.1

22. Tray Efficiency
μL * αLK,HK

23. Distillation Problems
Multi-component Distillation
Selection of Column Sequences
Azeotropy
Overcoming it to get pure products
Heat Integration
Decreasing the cost of separations