Page 88 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 77

Component No. 3: x, = mol fraction of a component in the liquid part of
the feed where the feed is part vapor
Dx~2 0.428
Bxgg = - - 0.0214 rf = 0.450/0.225 = 2.0
=
=
(Sr)i 20 XU, = (0.5) (0.10) = 0.05
Component No. 4: This component will not be in the bot- “light /heavy key = ,.5 = 2.0
toms because it is lighter than the light key
Overhead: 2.0 2
=
xn (approx)= (1 + 2.0) (1 + 0.05) = - 0.635
3.15
+
DXD~ DXD~ + DXD~ + DXD~ = D
In terms of heavy key:
0 + 0.01072 + 0.428 + 0.223 = D
al/h = 2.0
D = 0.66372 mols overhead product/mol feed
al/l= 0.25
Composition of Overhead
al/h = (al/h) (al/l) = (2.0) (0.25) = 0.5
Component DxDi Mol%
1
1 0 0 approx (k) = - (E)
2 0.01072 1.6 min (2.0-1) 0.635
3 0.428 64.6
4 0.223 33.9 xD3 0.646
0.66372 100.1%
Mmin
Composition of Bottoms: = 1.017approx.

Component BxBi Mol %
1 0.10 29.9 Now: Use Colburn’s more detailed method:
2 0.214 63.8 (:)
3 0.0214 6.3 Assume = 1.017
4 0 0 min
0.3354 100.0%
1 1
To have some idea of what value to use in Colburn’s ($)mi*=- 1+(+) = 1+- 1
“exact” method for minimum reflux, use Colburn’s min 1.017
“approximate” method to establish the order-of-magni-
tude of the minimum reflux: = 0.506
($) min = 1.017+1
1.017
V=L+D

V D
where XD and x, = top and pinch composition of a given light -=I+-
component L L
xhD and Xhn = top and pinch composition of the heavy _=- 1
L
key component
D
a = relative volatility of the given component 1+-
with reference to the heavy key L
L, = (0.506) (\’,)
Estimating Pinch Composition:
And L, = (1.017) (D,)
x, (approx)= 4
(l+q)(l+Zm, 1 Then: (0.506) Vr= (1.017) (D,) = (1.017) (0.66372)
where rf = ratio of liquid composition of light to heavy key v, = (1’017) (0.6637) = 1.332 mols/molfeed
component on feed plate 0.506
x, = mol fraction of a component in the liquid in the
rectifying column pinch L, = (1.017) (0.6637) = 0.674 mols/mol feed.