Page 217 - Applied Process Design For Chemical And Petrochemical Plants Volume II

206 Applied Process Design for Chemical and Petrochemical Plants

8. Set tray spacing at twice the selected value of hd. Because it is known that the entrainment from perfo-
9. Check entrainment at maximum vapor rate. rated trays is considerably less than for bubble caps, the 2-
10. Physical arrangement: refer to Figure 8-146. ft, 8-in. diameter would be very conservative and perhaps
excessively large.
For new towers, the designs will usually develop to utilize Tower diameters in the 1-ft, 6in. to 2-ft range are not
the entire tower cross-section. However, for existing towers usually economical as tray installations. A packed tower
with perforated trays being installed to replace bubble caps might prove the best economically. Trays can be installed
or packing, the optimum active tray area may not utilize on a central rod and spacer arrangement, with seals
the entire cross-section. If the number of holes required is between trays and tower shell. Such an arrangement usu-
small compared to available area, it is better to group the ally brings the cost of the installation up to that of a 2-ft, 6-
holes on 2.5 do to 3.5 do than to exceed these limits. Holes in. tower. This is the smallest practical size that a man can
separated by more than 3 in. are not considered effective crawl through.
in tray action so necessary for good efficiency. Blanking For the purpose of this design, assume that a cost study
strips may be used to cover some holes when more than has verified the above remarks, and a 2-ft, Gin. tower will
required have been perforated in the tray. be used. This means that entrainment will be very low on
If trays are punched, the sharp hole edge side should a 15-in. tray spacing. Therefore, a smaller spacing should
face the entering vapor. be considered. From usual fabrication costs, 12-inch spac-
ing is about the closest spacing to consider.
Example 8-40: Design of Perforated Trays Without
Downcomers The allowable velocity by Hunt for this spacing, S‘ = 8.25, vc = 4
(8.25/11.25) = 2.94 ft/sec
A tower separates a weak ammonia solution. Design
trays using perforated plates without downcomers for the Tower area = z (2.3)2/4 = 4.9 ft2
following conditions as determined from the column per-
formance calculations. Actual tower velocity = 5.22/4.9 = 1.06 ft/sec

Top Trav Bottom Tray Therefore 12-in. spacing should be O.K. entrainment-
Liquid, gpm 40.8 17.8 wise, check aeration later.
Lb/ft3 38.8 54.2
Dynes/cm <13 59 Plate Activation Velocities (Minimum)
Vapor, ft3/sec 5.22 4.3
Lb/ft3 0.593 0.408 Top:
Liquid rate, L = (40.8 gpm/7.48) (38.8) (60)
Estimated Tower Diameter = 12,200 Ib/hr (ft2)

e, = 0.22 (73/a) (v,/~’)3.2 From Figure 8-147, read Fh2 = 1.0 @ %win. holes, 23%
open area.
Allowable velocity: assume S’ = 15 in. - 2.3 (1.5 in.) = 11.25 in.
From Figure 8121 for e, = 0.05 and assumed 15-in. tray spacing (v&1/2)2 = 1.0 = V2hpv
at top, tower velocity v, = 4 ft/sec
v, = (l/pJ = (1/0.593) = 1.298 ft/sec
at bottom, v, = 6.4 ft/sec
Tower area at 4 ft/sec limiting: = 3.22/4 = 1.30 ft2 Bottom:
Diameter = [(4/x) 1.30]1/2 = 1.29 ft. Say 1 ft 6 in.
L = 17.8 (54.2) (60)/7.48 = 7,730 lb/hr (ft2)
Comparison:
Souders-Brown, Figure 8-83 at top tray conditions, which From Figure 8147 read Fh2 = 1.0 @ %in. holes, 23% open area
are limiting. Vh = (1/0.408)1/2 = 1.56 ft/SeC

W = 2000 Ibs/hr (ft2) Max. allowable hapor velocity Note that Figure 8-147 indicates the operating liquid
Top vapor rate = 5.22 (0.593) (3600) = 11,130 lb/vapor/hr minimum range is quite stable in the region of design for
these trays. The vapor rate must never fall below the above
Required area = 11,130/2000 = 5.3 ft2
values or instability will immediately set in and dumping
Diameter = 2.64 ft, Say 2 ft, 8 in. will result.