Page 218 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 207

Design Hole Vdocity Total Tower Tray Pressure Drop
Set at 1.5 times the activation velocity, or 2.34 ft/sec at For 15 trays, the maximum expected drop using the
the top and 1.95 ft/sec at the bottom. expected tray drop at the top:

Hole Arrangement = 13 (1.12) = 16.8 in. liquid

%in. dia. on 60”A pitch, spaced on %in. centers
This gives 22.6% open area (Figure 8-143), close
enough to the 23% selected. A more precise approach requires evaluation of the wet
tray drop for the bottom condition also. However, since it
No. holes/in2 = 8.3
will give a lower value (by inspection), the higher result is
Area required for holes: preferred as long as a vacuum tower is not being designed.
Here, the careful approach is justified.
TOP: 5.22/2.34 = 2.23 f3 It must also be remembered that the data used in estab
No. holes = 2.23 (144) (8.3) = 2670 lishing the design criteria are not accurate to better than
Bottom: 4.3/1.95 = 2.2 & 10-20%.
No. holes = 2.2 (144) (8.3) - 2640
Height of Aerated Liquid on Tray
Since these are so close the same number of holes can
be perforated in all plates. These should be drilled hd = 1.25 F, + .0005L + (0.54/8) - 2.45
(punched) in a 1-ft, &in. x 1 ft, 6-in. square area as per Fig-
ure 8-145. This will reduce costs of trays slightly as com- At top:
pared to custom perforating to different number of holes.
The tower area is 4.9 ft2; therefore the entire tray will = 1.25 [(1.06) (0.593)”2] + .0005 (12,200) + 0.34/0.226 - 2.43
not be perforated for the conditions of design.
If there is the possibility of lapor and liquid rates being = 1.002 + 6.1 + 2.39 - 2.45
reduced to 50% of the indicated values, this would place hd = 7.06 in. (evaluated as air-water)
the trays as selected above at the dumping point, or acti-
vation point, which is not a good operating condition. In Trajl Spacing
this situation the number of holes should be reduced in
order to maintain a velocity of vapor through the holes From this value of ha] it is essential that the trays be
greater by at least 15% than the activation velocity. spaced no closer than 2 (7.06), say 15 in. If lab tests indi-
cate these mixtures foam more than water-air, the value of
Mkt Tray Pressure Drop 15 in. should be increased in relative proportion.

(a) Dry Tray Pressure Drop Proprietary Valve Trays Design and Selection

hdt = 0.003 (v, p,,) (PPL -EEL. )(l-B2)/C: For proper design of proprietary valve trays (see previ-
ous illustrations), the design operating data, basic ther-
modynamics, and physical properties of the fluids must be
Top appears to be region of greatest pressure drop hdt = submitted to the appropriate manufacturer’s technical
0.003 [(2.34)2 (0.593)] (62.3/38.8) (1 -
(0.226)2) (0.82) department for evaluation and final design recommenda-
tion. Often the designer for the operating company has
C, = 0.82 for %in. hole in %in. tray (Figure 8-128) established sufficient computer programs to essentially
hd, = 0.0222 in. liquid make the equivalent studies as the tray manufacturer, for
example see Figure 8-54. To examine the variations in
(b) Effective Head expected tray(s) performance, several select studies must
Assume clear liquid height on tray = 1 in. (note, 1 be made, and results plotted, to analyze the variables and
in. may be a slightly better value than the 1.5 in. their effects on performance. Billet [208] has reviewed
assumed when determining ew) . the progress in the design and performance of valve trays.
From Figure 8-130, effective head, he = 1.1 in. liq-
uid at F, = 2.34 (0.593)1/2 = 1.81 Klein [201] has evaluated from the literature including
(c) Total wet tray pressure drop manufacturer’s, Le., Glitsch [202], Koch [203], and Nutter
[204], design procedures for their respective valve type tray
h, = 0.022 + 1.1 = 1.12 in. liquid