Page 95 - Applied Process Design For Chemical And Petrochemical Plants Volume II

84 Applied Process Design for Chemical and Petrochemical Plants

Eduljee [lo71 evaluated published data and corrected 1
relationships for determining the number of actual trays ($)min = 1+ (D/L),in = 1+1/1.017 =0.506
versus actual reflux with reasonably good agreement:
First Trial.
0.506 V, = 1.017 (D,) = 1.017 (0.664)
when 1.1 < R/Rm 7 2.0 V, = 1.332 mols per mol of feed
L, = 1.017 (0.664) = 0.674 mols/mol feed
q = 1.0
L, = (0.674) + (1) (1) = 1.674 mols/mol feed
when R/Rm > 2.0 V, = 1.332 - (1) (1 - 1) = 1.332 mols/mol feed

If the number of actual trays, S, calculates to be 27 or
greater, then revert to the following for better accuracy: Operating values:
Second Trial:
Operating (L/D), = (1.5) (1.017) - 1.525
when 1.1 < R&, 7 2.0

=0.603
S = 2.71 (Rm/R) (S,) + 0.38 (8-163) ($),= 1+1/1.525
when R/R, > 2.0
Vr (1’525) (o’664) = 1.68 mols/molfeed
S = [0.67 (Rm/R) + 1.021 (S,) + 0.38 (8-164) 0.603

where n = number of theoretical trays in the rectifying section L, = (1.323) (0.664) = 1.013 mols/mol feed
R = reflux ratio (O/D) q = 1.0
S = number theoretical trays in the column, including L, = 1.013 + (1) (1) = 2.013
reboiler V, = 1.68 - (1) (1 - 1) = 1.68
(L/V), = 2.013/1.68 = 1.198
Subscript
For graph:
m = minimum
[($)s (3, =1.198(&) -l=O.985
-11,
The feed plate location, for either rectlfylng or strip
ping sections:
1
-
For R/%, from 1.2 to 3.6: [ ( $), (E), 11 = (1.255) (-) 0.506 - 1 = 1.48
min
(n/nm) (R/Rm) = 1.1 + 0.9 (R/Rm) (8-165)
Read curve for “greater than 8” minimum equilibrium
Hengstebeck [224] presents a technique for locating steps:
the feed tray by plotting.
at 0.985/1.48 = 0.666
Example 847: Operating Reflux Ratio
Curve reads: So/S~ = 1.64
The minimum reflux ratio (L/D)min has been deter-
mined to be 1.017. Using the Brown and Martin graph Theoretical stages at reflux (L/D) = 1.525
[9], evaluate the theoretical number of trays at an operat- so = s, (1.64) = 22.1 (1.64)
ing reflux of 1.5 times the minimum. The minimum num-
ber of stages was determined to be 22.1 including the So = 36.2 stages
reboiler. See Figure 8-49.
The column will have a total condenser. Product rate D is Theoretical trays at the operating reflux (L/D) = 1.525
0.664 mols/mol feed, and the feed is a boiling point liquid.
Minimum values: No = 36.2 - 1 (for reboiler) = 35.2 trays in column