Page 212 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 201

= 8.083 ft maximum allowable liquid rate (at flooding) to the mini-
Weir crest @ 304 gpm, L, (see Figure 8-104): mum allowable operating throughput.
Downcomer liquid handling:
how = 0.092 (Lg/1,)*13 = Based on clear liquid, downcomer velocity:
= 0.092 (504/8.08)*13
= 1.45 in. liquid Vd = wm = 504
(7.48) (60) (Ad) (7.48) (60) (10.3)
In such a large column, the weir constriction factor
(Figure 8-105) is not significant and is not applied to the = 0.106 ft/sec
above hoLv.
Aeration: Referring to Table 8-20 for low foaming hydrocarbons
From Figure 8-126: on 24in. tray spacing, this velocity of 0.106 fps is quite
F,,, = v, (P.;)O.~ for active area “safe” compared to a suggested range of 0.55-0.60 fps.
F,, = (347/65.59) (0.168)O.’ = 2.168 Based on tray spacing of 24 in., assume 50% downcom-
Read figure; aeration factor, fi = 0.38 er full, then:
Then, the wet-tray pressure drop is:
height of liquid = 12 in. = 1 ft-0 in. then,
1. Operating liquid seal loss, clear liquid on tray residence time = 1 ft/O.l06 fps = 9.43 sec
hl = p (h, + hotv) = 0.58 (2 in. + 1.45 in.) This is compared to about 3 sec reported by Bolles
= 2.00 in. liquid [ 1901. This should be checked, and the tray spacing may

2. Total tray pressure drop: have to be increased, depending on the recalculation for
the entire tray.
ht = hh + p (hw + how)
h, = 1.98 + 2.00 = 3.98 in. liquid Liquid Gradient

Weep Point Referring to equations for aerated liquid pressure drop,

Surface Tension Head:
hf =-- 28 h1 - 1 - 2.001/[2(0.58) - 1]= 12.3 in.
0.0403
0.0405 (21)
_-
h, = -
--
pi dh 43.3 (0.1875)
= 0.1034 in. liquid Velocity of froth:
lf= 504/(7.48 gal/ft3) (60 sec/min) = 1.12 cfs
Then: Ah/& = 8.82/65.59 = 0.134
Referring to Figure 8132:

hl, = h, + h,,v, in. liquid, height of clear liquid at overflow weir Note Tower diameter = 10.5 ft

hl,, = 2 in. + 1.45 = 3.45 in. 8.083
Weir length = -
18.58 ft
hh + h, = 1.98 + 0.103 = 2.08 in. liquid
Average length for lfi%, 18.58/2 = 9.29 ft
=
Reading the intersection of 3.45 vs. 2.08 shows that for
either weep point curve, the weep point is well below the (cross section)
values for operation, so this design not near the weep point. Hydraulic radius of aerated mass, RH = (wetted perimeter) , ft
hf Df (12.5) (9.29)
Turndown Ratio R* = = = 0.859 ft
2hf + 12Df (2) (12.63) + (12) (9.29)
By trial and error the tray can be examined to deter- Reynold’s Modulus:
mine the rates that will coincide with the weep point.
Thus, the entrainment can establish the upper limit of
operation, and the liquid weeping through the perfora-
tions represents the lower limit of stable operations; that
is, turndown is generally used to represent the ratio of the = 1.260 x lo5