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SAMPLE PROBLEM #18:SAMPLE PROBLEM #18:
A distillation column with a total condenser and a A distillation column with a total condenser and a partial reboiler is separating acetone and ethanol. partial reboiler is separating acetone and ethanol.
We desire a bottoms composition of 5 mole % We desire a bottoms composition of 5 mole % acetone and a distillate composition of 90 mole % acetone and a distillate composition of 90 mole % acetone. Reflux is a saturated liquid. Pressure is 1 acetone. Reflux is a saturated liquid. Pressure is 1 atm.atm.
A distillation column with a total condenser and a A distillation column with a total condenser and a partial reboiler is separating acetone and ethanol.partial reboiler is separating acetone and ethanol.
The feed is 70 mole % acetone, flows at 1000 The feed is 70 mole % acetone, flows at 1000 moles/hr, and is a superheated vapor for which 1 moles/hr, and is a superheated vapor for which 1 mole of liquid must vaporize on the feed plate to cool mole of liquid must vaporize on the feed plate to cool 4 moles of feed.4 moles of feed.
1000 mols/h
Feed
xF= 0.7
Superheated
Vapor
(v/F= ¼)
Distilling Column
1atm
TC
PR
L0
sat liqD
B
xD = 0.9
xB = 0.05(a) Nmin?(b) Rm?(c) nF,opt & N, if Lo/D = 2(Lo/D)min?
SOLUTION
Minimum number of equilibrium stagesMinimum number of equilibrium stages Acetone-Ethanol System
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
mol frac acetone in liq, x
mo
l fra
c a
cet
on
e in
va
p, y
xB xF xD
Nmin = 6.8
Minimum External Reflux Ratio
F
L V
vv/F = 1/4
L V
L = L – v = L – (1/4)F
L
Fq
LL
F
F
F
F14
=L -
=
--
==
14 1
4-
-
1slope
= == =qq 51-
14
-
14
- - 1
14
-
4- 5
Acetone-Ethanol System
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
mol frac acetone in liq, x
mo
l fra
c ac
eto
ne
in v
ap, y
xBxF xD
y-int = 0.39
xDy-int = 0.39 ,Rm = 1.33R + 1
0.9==
R + 1nF, opt & N, if Lo/D = 2(Lo/D)min
R = 2Rm = 2(1.33) = 2.67
Rectifying Line
y-int = xD
R + 1
0.9=
2.67 + 1= 0.25
Feed Line
if x = 0.26, y = 0.44
Acetone-Ethanol System
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
mol frac acetone in liq, x
mo
l fra
c a
cet
on
e in
va
p, y
xB xF xD
Rectifying Operating Line
Stripping Line
N = 9.4
nF, opt = 5
Nmin = 6.8 Rm = 1.33 nF,opt = 5 & N = 9.4
