CBE 417“Chemical Engineering Equilibrium

Separations”

1

Lecture: 9

28 Sep 2012

Overview

2

• McCabe-Thiele graphical technique (binary systems)• Condensers• Reboilers

• Binary Shortcut Methods• AspenPlus:o Shortcut methods: DSTWUo Rigorous method: RADFRAC

• Efficiencies• Introduction to multicomponent distillation

Problem Solving Exercise

3

Problem Solving Exercise

4

McCabe Thiele: Total Condenser

5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Xa

Ya

Y=X line

1R

xD

D

y1

x1

xD

y2

xD

y1 V1

LO

y3

x3

x2

y4

1

3

2

McCabe Thiele: Partial Condenser

6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Xa

Ya

Y=X line

1R

xD

D

xOy1

x1

x0

y2

y1 V1

LO

y3

x3

x2

y4

1

3

2

yD

McCabe Thiele: Partial Condenser

7

D

xOy1

x1

x0

y2

y1 V1

LO

y3

x3

x2

y4

1

3

2

yD

Why consider using a partial condenser?•Desire vapor distillate?

•Need to control temperature in the column?

Concept: o control distillation column temperatures

by controlling the operating pressure.o Condenser design / column opn. Po Minimum desirable condenser

temperature is ~ 49 oC (120 oF).o Construction of column more expensive

at higher pressures (i.e. > 14.8 bar [200 psig])

o Bottoms T too high (decomposition or Tc approached)

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

McCabe Thiele: Partial Condenser

8

Seader & Henley, 2006

9

Seader & Henley, 2006

McCabe Thiele: Reboilers

Effect of Different Reflux Ratios

10

• Before we saw that..• increasing reflux resulted in an enrichment of vapor stream

in the “lighter” component

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.80

5

10

15

20

25

30

35

R/Rmin

N [

nu

mb

er

eq

uil

. sta

ge

s]

Effect of Different Reflux Ratios

11

• Increase R --- cost effect??

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8$0

$20,000

$40,000

$60,000

$80,000

$100,000

$120,000

$140,000

$160,000

$180,000

$200,000

$0

$5,000

$10,000

$15,000

$20,000

$25,000

$30,000

$ steam

$ water

R/Rmin

Ste

am C

osts

[$/

yr;

1991

]

Coo

ling

Wat

er C

osts

[$/

yr;

1991

]

Effect of Different Reflux Ratios

12

Seader & Henley, 2006

R = factor * Rmin

RRR optmin

50.105.1min

R

Ropt

Tie Together

13

• Found using McCabe-Thiele (knowing xD , xB , & feed condition):o Nmin (at total reflux)o Rmin (at infinite number of stages)

• If given actual R (or choose by R = factor * Rmin ) can find:o No NF (optimum feed stage number)

Suggests there is a relation between N, Nmin, and Rmin.

Gilliland Correlation

14

61 Data points over ranges:

1. No. components: 2 to 112. q : 0.28 to 1.423. P : vacuum to 42.4 bar4. : 1.11 to 4.055. Rmin : 0.53 to 9.096. Nmin : 3.4 to 60.3Molokanov Eqn:

5.0

1

2.11711

4.541exp1

X

X

X

XY

Seader & Henley, 2006

Binary Shortcut Methods

15

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

ln

11

ln

min

B

B

D

D

xx

xx

N

- binary; total condenser; ~ constant

31

BotFeedDistBotDist or

Binary Shortcut Methods

16

Minimum reflux ratio: Underwood Equation

1

11

min

F

D

F

D

zFxD

FzDx

F

L

- binary; assumes rectifying line intersects eq. line at feed line; ~ constant

D

LR min

min

R = factor * Rmin

Approximate number of equilibrium stages (N): Gilliland correlation

Binary Shortcut Methods

17

Optimum feed stage location (NF): Kirkbride Equation

206.02

1

1

D

B

x

x

z

z

N

N

D

B

F

F

S

R

- binary; approximate result

Binary Shortcut Methods

18

Summary Binary Shortcut Methods (FUG)

19

Fenske Equation

ln

11

ln

min

B

B

D

D

xx

xx

N

31

BotFeedDistBotDist or

Underwood Equation

1

11

min

F

D

F

D

zFxD

FzDx

F

L

D

LR min

min

Gilliland correlation (Nequil)

Kirkbride Equation (NF(opt))

206.02

1

1

D

B

x

x

z

z

N

N

D

B

F

F

S

R

R = factor * Rmin

Overview

20

• Questions on homework??• McCabe-Thiele graphical technique (binary systems)• Condensers• Reboilers

• Binary Shortcut Methods• AspenPlus:o Shortcut methods: DSTWUo Rigorous method: RADFRAC

• Efficiencies• Introduction to multicomponent distillation

AspenPlus: DSTWU

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