Page 91 - Applied Process Design For Chemical And Petrochemical Plants Volume II

80 Applied Process Design for Chemical and Petrochemical Plants

Intersection of Operating Lines:

I 0.2 1
(2 - 1$ (1 - 2.33) 17) - 2 - (-2.33) 2
02+02

(2 - 1) (1 - 2.33)

/ 0.2
+ 4 (- 2.33) (2 - 1) (1 - 2.33)
2m (a1 - 1)
L
2(- 2.33) (2 - 1)
The proper value for xit is positive and between zero
and one. Actually this is fairly straightforward and looks xit = 0.610, or -0.459 (not acceptable)
more difficult to handle than is actually the case.
Pseudo ratio of liquid to vapor in feed Pseudo minimum reflux ratio:

(8- 151) 19.3 = 0.975
x10 = 19.5 + 0.5

where XL = mol fraction of feed as liquid R’ = 0.975 - (1.0 - 0.975) (2)
x, = mol fraction of feed as vapor (2 - 1) (0.610) (1 - 0.610) (2 - 1)
FL = mols of liquid feed
FV = mols of vapor feed R‘= 1.472
ZFH = total mols of components heavier than heavy key
in feed Minimum reflux ratio:
ZFL = total mol of components lighter than light key in
feed.

Example 8-25: Scheibel-Montross Minimum Reflux [61]

A tower has the following all liquid feed composition: +””( 4 l+:)

Feed Overhead Bottoms
Component Mols/hr Mols/hr Mols/hr
A 30 30.0 - (L/D)min = 0.912
B (light key) 20 19.5 0.5
C (heavy key) 20 0.5 19.5 Minimum Number of Trays: Total Rdkx-Constant
D 30 - 30.0 volatility
100 50.0 50.0
The minimum theoretical trays at total reflux can be
Relative volatilities referenced to the heavy key, C: determined by the Fenske relation as previously given

aA = 4.0
ag = 2.0 = a1 (8- 152)
aC =l.O=ah
a~ = 0.5
..
iuote mat ~,i* is me number or trays in me column and
~
XL - Z X 1.0 - 0.30
Calculate : m = - - = - 2.33 does not include the reboiler. When a varies considerably
xV - X X ~ 0 - 0.30 through the column, the results will not be accurate using