Page 316 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

318 Chapter 11 Distillation

Estimating minimum number of ideal stages for total condenser using Fenske equation (Eq. 11.13)

log x LK;D =x HK;D x HK;B =x LK;B
S m ¼ðN min þ 1Þ¼ avg
log a LK HK
¼ 2:34
log½ð0:95=05Þ ð0:1209=3263ފ
¼
logð2:303Þ
Estimating minimum reflux ratio considering saturated liquid feed (q ¼ 1) using Underwood
equation (Eq. 11.14a)
" avg #
1 1 x LK;D
x LK;D
LK HK
a
R min ¼ avg
a LK HK 1 x LK;F 1 x LK;F

1 :95 2:303 ð1 :95Þ
¼ 1:947
:35
¼
ð2:303 1Þ ð1 :35Þ
Estimating number of ideal stages at a reflux ratio R (R > R min ) using Gilliland correlation (Eq. 11.
16)
0:5124
N N min R R min S S min
; this can also be written as
N þ 1 R þ 1 S
¼ 0:7591 0:7532
0:5124
R R min
¼ 0:7591 0:7532
R þ 1
Say, for R/R min ¼ 1.2, R ¼ 1:2 1:947 ¼ 2:336,
S 2:34 2:336 1:947 0:5124
S S m
¼ 0:5087
¼
S S 2:336 þ 1
¼ 0:7591 0:7532
i.e., S ¼ 2:34=ð1 0:5087Þ¼ 4:76, the total number of ideal stages in the column.
Locating feed tray using Kirkbride correlation (Eq. 11.20)
Ratio of number of rectification trays to number of stripping trays is given by
#
" 2 0:206 " 2 0:206
#
S r N r þ 1 x HK;F x LK;B B 0:25 0:3263 1367
¼ ¼ ¼
S s N s þ 1 x LK;F x HK;D D 0:35 0:05 521:9
¼ 2:464
Actual number of trays to be provided assuming 70% tray efficiency,
Number of rectification trays in column ¼ 4:76 f2:464 =ð1 þ 2:464Þg=0:7 ¼ 4:84, say five
trays.
Number of stripping trays in column ¼ 4:76 f1 2:464 =ð1 þ 2:464Þg=0:7 ¼ 1:96, say two
trays.
One tray is additional for the feed entry section.
Condenser load, inlet to condenser is vapour at 98.6 C and exit is at 90 C.