Page 90 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 79

Composition of Stripping Pinch: L = all components lighter than light key, not includ-
ing the light key
Components See Last Col. Above Revised Mol Fraction Cr, Cs = empirical constants
l=H 0.0794 0.0794/0.9714 = 0.0817
2=h 0.252 0.2585 Scheibel-Montross Empirical: Adjacent Key Systems:
3=1 0.64 o.659 Constant or Variable Volatility [61]
0.9714 0.9992

Although this method has not found as much wide
cr for each lighter than light acceptance when referenced to use by designers or con-
There is only One lighter than light key in this troversial discussion in the literature, it nevertheless allows
example, #4 a direct approximate solution of the average multicompo-
nent system with accuracy of 14% average. If the key com-
ponents are less than 10% of the feed, the accuracy is
probably considerably less than indicated. If a split key sys-
tem is considered, Scheibel reports fair accuracy when the
split components going overhead are estimated and com-
Reading curve, C, = 1.0 = C, (see Figure 848) bined with the light key, the balance considered with the
Evaluate Cs for No. 1: heavy key in the L/D relation.
[ (: - 1) a.] = [ (5 (0.25)] = (2 - 1) (0.25) = 0.25
-
1)

(8-148)
Reading curve, Cs = 0.875 (see Figure 848) XlF R' + (XhF +I: Xm)
Now, substitute into Colburn correlation, for check, aH

where xw = mol fraction of light key in feed
(1-
Z xn = sum of all mol fractions lighter than light key in
feed
R = pseudo minimum reflux
(0.572) (0.2.385) '(1 - ((1) (0.102))) Z XFH = sum of all mol fractions heavier than heavy key
(0.328) (0.659) in feed
XhF = mol fraction heavy key in feed
0.684 = (1 - 0.102) (1 - 0.0318) = (0.898) (0.9682) a1 = relative volatility of light key to heavy key at feed
0.684 0.868 tray temperature
aH = relative volatility of components heavier than
heavy key at feed tray temperature
aL = relative volatility of components lighter than
Because left side of equation is smaller than right side,
light key at feed tray temperatures
assumed was too large. Try a smaller value around 0.95. xit = mol fraction liquid at intersection of operating
Right side of equation is not so sensitive to change. lines at minimum reflux. (Calculated or from
graph.)
where L E all components lighter than light key xio = mol fraction light key in overhead expressed as
I; = sum of all components lighter than light key, fraction of total kqs in overhead.
L
does not include light key
Z = sum of all components heavier than heavy key, Pseudo minimum reflux:
H
does not include heavy key
p = pinch
r = rectifylng
s = stripping
1 = light key When the overhead contains only a very small amount
h = heavy key of heavy key, the second term in the equation may be
H = all components heavier than heavy key, not neglected.
including the heavy key Intersection of operating lines at Equilibrium Curve: