1. “Chemical Engineering Equilibrium Separations”
CBE 417
“Chemical Engineering Equilibrium Separations”
Lecture: 9
28 Sep 2012

2. Overview McCabe-Thiele graphical technique (binary systems) Condensers
Reboilers
Binary Shortcut Methods
AspenPlus:
Shortcut methods: DSTWU
Rigorous method: RADFRAC
Efficiencies
Introduction to multicomponent distillation

3. Problem Solving Exercise

4. Problem Solving Exercise

5. McCabe Thiele: Total Condensery1 x1 xD y2
y1 V1 LO y3 x3 x2 y4 D 1 2 3

6. McCabe Thiele: Partial CondenseryD D y1 x1 x0 y2
y1 V1 LO y3 x3 x2 y4 1 3 2 xO

7. McCabe Thiele: Partial Condenser
Why consider using a partial condenser? yD D
Desire vapor distillate? y1 x1 x0 y2
y1 V1 LO y3 x3 x2 y4 1 3 2
Need to control temperature in the column?
Concept:
control distillation column temperatures by controlling the operating pressure.
Condenser design / column opn. P
Minimum desirable condenser temperature is ~ 49 oC (120 oF).
Construction of column more expensive at higher pressures (i.e. > 14.8 bar [200 psig])
Bottoms T too high (decomposition or Tc approached) xO
Running a partial condenser lowers the column operating pressure (i.e. don’t have to fully condense the distillate product

8. McCabe Thiele: Partial Condenser
Seader & Henley, 2006

9. McCabe Thiele: Reboilers
Seader & Henley, 2006

10. Effect of Different Reflux Ratios
Before we saw that..
increasing reflux resulted in an enrichment of vapor stream in the “lighter” component

11. Effect of Different Reflux Ratios
Increase R cost effect??

12. Effect of Different Reflux Ratios
R = factor * Rmin
Seader & Henley, 2006

13. Suggests there is a relation between N, Nmin, and Rmin.
Tie Together
Found using McCabe-Thiele (knowing xD , xB , & feed condition):
Nmin (at total reflux)
Rmin (at infinite number of stages)
If given actual R (or choose by R = factor * Rmin ) can find: N
NF (optimum feed stage number)
Suggests there is a relation between N, Nmin, and Rmin.

14. Gilliland Correlation
61 Data points over ranges:
No. components: 2 to 11
q : to 1.42
P : vacuum to 42.4 bar
 : to 4.05
Rmin : to 9.09
Nmin : 3.4 to 60.3
Molokanov Eqn:
Seader & Henley, 2006

15. Binary Shortcut Methods
McCabe-Thiele is good at showing procedure and helping explain concepts for binary distillation. It can be extended to multiple feeds and side draws (Wankat).
When consider multicomponent mixtures, McCabe-Thiele is not quite as useful. However, having some shortcut methods to aid in final column design is useful.
Minimum number of stages: Fenske Equation
- binary; total condenser;  ~ constant

16. Binary Shortcut Methods
Minimum reflux ratio: Underwood Equation
- binary; assumes rectifying line intersects eq. line at feed line;  ~ constant
R = factor * Rmin
Approximate number of equilibrium stages (N): Gilliland correlation

17. Binary Shortcut Methods
Optimum feed stage location (NF): Kirkbride Equation
- binary; approximate result

18. Binary Shortcut Methods

19. Summary Binary Shortcut Methods (FUG)
Fenske Equation
Underwood Equation
R = factor * Rmin
Gilliland correlation (Nequil)
Kirkbride Equation (NF(opt))

20. Overview Questions on homework??
McCabe-Thiele graphical technique (binary systems)
Condensers
Reboilers
Binary Shortcut Methods
AspenPlus:
Shortcut methods: DSTWU
Rigorous method: RADFRAC
Efficiencies
Introduction to multicomponent distillation

21. AspenPlus: DSTWU