1. Sieder et. al. Chapter 9 and 13
Separation Trains
Sieder et. al. Chapter 9 and 13
Terry A Ring
Chemical Engineering
University of Utah

2. Use of Separation Units

3. Simple Separation Unit Operations
Flash
Quench
Liquid-liquid decantation
Liquid-liquid Flash
Crystallization
Sublimation
Filtration

4. MSA = Mass Separating Agent ESA = Energy Separating Agent

5. Criteria for the Selection of a Separation Method
Energy Separation Agent (ESA)
Phase condition of feed
Separation Factor
Cost of Energy
Mass Separation Agent (MSA)
Phase condition of feed
Choice of MSA Additive
Separation Factor
Regeneration of MSA
Cost of MSA
Phases I and II,
Components 1 and 2 (light key and heavy key)

6. Separation Reaction Hydrodealkylation of Toluene T+H2B+CH4 2B Biphenyl+H2
Reactor Effluent
T=1,350F
P = 500 psia

7. Reactor Effluent
Reaction Conditions
T=1,350F
P = 500 psia

8. After Flash to 500 psia
Recycled Reactants

9. Further Separation What separation units should be used?
Liquid Separation
Toluene, BP=110.6ºC
Benzene, BP=80.1ºC
What happens to the Methane (BP= ºC) and Biphenyl (BP=255.9ºC) impurities?
Gas Separation
Hydrogen
Methane
What happens to the Toluene and Benzene impurities?

10. After Flash to 500 psia
Recycled Reactants

11. Direct Distillation Sequence
B/TD, T/D
Direct Distillation Sequence
Biphenyl

12. Indirect Sequence BT/D, B/T Benzene Toluene Biphenyl
Which is the Cheapest Direct or Indirect?

13. Column Sequences No. of Columns No. of Possible Column Sequences
Nc=P-1
P= No. of Products
No. of Possible Column Sequences
Ns=[2(P-1)]!/[P!(P-1)!]
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
P=10, N.c=9, Ns=4,862
No. of Possible Column Sequences Blows up!

14. P = 4, Nc=3, Ns=5 Example A/BCD, B/CD, C/D A/BCD, BC/D, B/C
AB/CD, A/B , C/D
ABC/D, A/BC, B/C
ABC/D, AB/C, A/B

15. How do I evaluate which is best sequence?

16. Marginal Vapor Rate Marginal Annualized Cos t~ Marginal Vapor Rate
Marginal 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

17. 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

18. After Flash to 500 psia
Recycled Reactants

19. Simplified Marginal Vapor Flow Analysis
R assumed to be similar for all columns and R>1,
You should not make this assumption!
Because separations have different degrees of difficulty.

20. Separation Train Heuristics
1. Remove thermally unstable, corrosive, or chemically reactive components early in the sequence.
2. Remove final products one by one as distillates (the direct sequence).
3. Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed.
4. Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of the other components.
5. Sequence separation points to leave to last those separations that give the highest-purity products.
6. Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column.

21. Distillation Sequences for Separation
Direct
Biphenyl
Biphenyl Byproduct
Benzene
Toluene &
Biphenyl

22. Vapor Pressure vs Temp Benzene BP=80.1ºC 80.1°C P(48.8ºC)=101.4 kPa
=25 kPa ~0.25 atm
80.1°C

23. A Word About Column Pressure
Cooling Water Available at 90ºF (32.2ºC)
Distillate Can be cooled to 120ºF (48.8ºC) min.
Calculate the Bubble Pt. Pressure of Distillate Composition at 120ºF (48.8ºC)
equals Distillate Pressure
Bottoms Pressure = Distillate Pressure +10 psi delta P
Distillate P > Atm, Pressure generated by system.
For Vacuum, how is it that generated?

24. Steam Ejector Generates the Vacuum.
High Pressure
High Velocity
Steam
Velocity > Mach 1
Bernoulli’s Equation
Vacuum

25. Steam Ejectors

26. Pressure Vessel with Vacuum
Withstand atmospheric pressure exerted on it.
They always leak a little!
What is the leak rate?
1 Bar =
E is elastic Modulus of Steel, t is the thickness, R is the cylinder radius.

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