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CBE 417 “Chemical Engineering Equilibrium Separations”. Lecture: 9. 28 Sep 2012. Overview. McCabe-Thiele graphical technique (binary systems) Condensers Reboilers Binary Shortcut Methods AspenPlus : Shortcut methods: DSTWU Rigorous method: RADFRAC Efficiencies

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CBE 417

“Chemical Engineering Equilibrium Separations”

Lecture: 9

28 Sep 2012


Overview

  • McCabe-Thiele graphical technique (binary systems)

    • Condensers

    • Reboilers

  • Binary Shortcut Methods

  • AspenPlus:

    • Shortcut methods: DSTWU

    • Rigorous method: RADFRAC

  • Efficiencies

  • Introduction to multicomponent distillation


Problem Solving Exercise


Problem Solving Exercise


McCabe Thiele: Total Condenser

y1 V1

LO

D

y1

xD

xD

1

x2

y3

x1

y2

2

x3

y4

3


McCabe Thiele: Partial Condenser

yD

D

y1 V1

LO

xO

y1

x0

1

x2

y3

x1

y2

2

x3

y4

3


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


McCabe Thiele: Reboilers

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??


Effect of Different Reflux Ratios

R = factor* Rmin

Seader & Henley, 2006


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.


Gilliland Correlation

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


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


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


Binary Shortcut Methods

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))


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


AspenPlus: DSTWU


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