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CBE 417 “Chemical Engineering Equilibrium Separations”

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Presentation Transcript

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

McCabe Thiele: Partial Condenser

Why consider using a partial condenser?

yD

D

- Desire vapor distillate?

y1 V1

- 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)

LO

xO

y1

x0

1

x2

y3

x1

y2

2

x3

y4

3

- Running a partial condenser lowers the column operating pressure (i.e. don’t have to fully condense the distillate product

McCabe Thiele: Partial Condenser

Seader & Henley, 2006

Seader & Henley, 2006

Effect of Different Reflux Ratios

- Before we saw that..
- increasing reflux resulted in an enrichment of vapor stream in the “lighter” component

Effect of Different Reflux Ratios

- Increase R --- cost effect??

- 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.

61 Data points over ranges:

No. components: 2 to 11

q : 0.28 to 1.42

P : vacuum to 42.4 bar

: 1.11 to 4.05

Rmin: 0.53 to 9.09

Nmin: 3.4 to 60.3

MolokanovEqn:

Seader & Henley, 2006

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

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

Optimum feed stage location (NF): Kirkbride Equation

- binary; approximate result

Summary Binary Shortcut Methods (FUG)

Fenske Equation

Underwood Equation

R=factor* Rmin

Gilliland correlation (Nequil)

Kirkbride Equation (NF(opt))

- 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

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