Page 46 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 35

)If- 6 = 1.35 (.15) - .01736 = 0.186
Xf- 6= 0.092 Substituting, slope = - - -+3
-1.5
-1.5
-
=
yf- 7= 1.35 (.092) - .01736 = 0.107 1- 1.5 - 0.5
xf- 7= 0.05 (The desired bottoms composition)
Total theoretical trays: Referring to calculations of Example 8-10, for an
equimolal mixture of benzene and toluene in feed:
rectifylng section = 4
feed tray =1 overhead product, D = 50 mols/100 mols feed
stripping section = 7
total = 12 Trays calculate ($) =-, XD - Yc
Example: 81 1 Thermal Condition of Feed min YC -XC

Using the same operating reflux (same fraction times where XD = 0.95
the minimum) as was used in Example 8-10, calculate the yc = 0.774”
theoretical plates required for feed of the following ther- xC = 0.59*
mal conditions: Use Figure 8-27. - 0.95 - 0.774
-
(a) q = 1.5 0.774- 0.59
(b) q = 0 0.176
(c) q = -1.5 =- 0.184

A. Fmq = 1.5 (L/D)min = (LR/D)~~~ 0.956 min. reflux ratio,
=
reflux/produc t
Slope of “q” line = -q/l - q
*Read from graph at intersection of =q” line for 1 .?i and minimum reflux
operating line.
Equilibrium Curve,
Slope of Operating Line at Min. Rejlux:

L/D 0.956 o.49
(3 =(?)min =L/D=.956+1=

(Graph reads 0.59 but this depends much on accuracy
of plot.)

calculating

(+) XD -Yc
min =- XD -x~
- - .95 - .774 = 0.49
.95 - 39

Actual Operating Line:

Operating reflux ratio = (1.5) (L/D) = 1.5 (0.956)
= 1.432 reflux/product

Slope of actual operating line:

L
Figure 8-27. Equilibrium curve; benzene-toluene for Example 8-1 1 _=-- L/D --= 1.432 o.5g
(curve data only). Used by permission, Robinson, C. S. R. and V L/D+l 1,432+1
Gilliland, E. R., Elements of Fractional Distillation, 4th Ed. McGraw-
Hill Book Co. (1950), all rights reserved.