Page 182 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 171

Vapor Distribution Tray Spacing, 24 in.; Type outlet weir: End
Note: Tray spacing was set when the Gft, 0-in. diameter
The vapor distribution approximaiion was determined.
No. downcomers/tray 1; Located: End
Rv = A/hc
Cap Data:
is an indication of the uniformity of vapor flow through the
caps on the inlet side of the tray to those on the outlet side (1) Cap I.D., ID, 3% in., Spacing: 5% in. A 60" centers
and the tendency of inlet caps to stop bubbling. Davies [ 14, (2) Total height, 3% in.
151 recommends values for the ratio of 0.4, not to exceed (3) No./tray, Nc, 129
0.6; Bolles recommends 0.5. Only at values around 0.1 is (4) Slots: No., N,, 50
essentially uniform vapor flow maintained through all the (5) Height, H,, 1% in.
caps. As the RV ratio increases, a smaller percentage of the (6) Width, w,, % in.
vapor flows through the inlet tray caps and a larger per- (7) Skirt Height, s, 54 in.
centage is shifted to the outlet caps. This cross-flow of (8) Shroud Ring height, hSr, % in.
vapor increases the effect of A on the tray and accentuates (9) Height of inside surface of cap above tray, 3.94 in.
the problem. Unfortunately it is often overlooked in many (10) Riser I.D., 2.68 in.
tray examinations. When the ratio reaches the value equal (1 1) Riser height above tray floor, 3 in.
to the cap drop at full slot capacity (usually 1.75 to 2.5 in.)
liquid will flow or dump back down the risers at the inlet Areas:
row of caps. This is definitely improper tray operation, pro-
viding markedly reduced tray efficiencies. (12) Riser inside cross-sectional, a,, 5.43 in.2 per riser
For stepped caps (tray level, risers of different heights) (13) Total riser inside cross-sect. area/tray, A,, 4.95 ft2
01- cascade tray (tray consisting of two or more levels) care (14) Riser outside cross-sectional area aro, 5.94 in2 per
must be taken to analyze all the conditions associated with riser. Riser is 2%-in O.D.; z2.7?i2/4 = 5.94
level changes on the tray. Reference 5 discusses this in (15) Cap inside cross-sectional area a,, 11.79 in2 per
some detail. cap. Cap is 3%in. I.D.; x(3?6)*/4 = 11.79
(16) Total cap inside cross-sectional area, 4, 10.53 ft'
Bubble Cap Tray Design and Evaluation (17) Annular area per cap, &, in2, (1 1.79 - 3.94) = 5.85
(18) Total annular area per tray, &, 5.24 ft2
Example 8-36: Bubble Cap Tray Design (19) Reversal area per cap, ar', in.2 = n(2.69) (3.94 - 3.0)
= 7.95. d = (2.75 + 2.63)/2 = 2.69 in.
Check the trays for a finishing tower operating under (20) Total reversal area, per tray, Afr, ft2 (129/144)
vacuum of 75 mm Hg at the top. The estimated pressure (7.95) = 7.12
drop for the twenty trays should not exceed 50-60 mm Hg. (21) Slot area per cap, a,, (50) (X) (1.5) = 9.39 in.2
Fifteen trays are in the rectifying section, five in the strip- (22) Total slot area per tray, &, 8.40 €t2
ping section.
Following the suggested form, and starting with the Tray Detaik
standard tray design existing in the unit, the calculations
will be made to check this tray, making modifications if (23) Length of outlet overflow weir, l,, 4.0 ft
necessary. The 6-ft 0-in. diameter tower was designed orig- (24) Height of weir (weir setting) above tray floor, h,,
inally for a significantly different load, but is to be consid- 2.5 in.
ered for the new service. The tray features are outlined in (25) Inlet weir (downcomer side) length (if used), 4.0 ft
Table 8-1 6. It becomes obvious that the tray is too large for (26) Inlet weir height above tray floor, 3 in.
the requirements, but should perform reasonably well. (27) Height of top of cap slots above tray floor, 2 in.
The weakest point in performance is the low slot velocity. (28) Static slot submergence or static slot seal (2.5-2.0),
hss, 0.5 in.
(29) Height of bottom of downcomer above tray floor,
Tray Design ++
2% in.
Tower application or service: Product Finishing (30) Downcomer flow areas: (a) Between downcomer
Tower Inside Diameter: 6 ft, 0 in. and tower shell, 0.886 ft2
Tray Type: Cross Flow (31) (b) Between bottom downcomer and tray floor,
0.710 ft2
*Adapted by permission from Ref. 15, modified to suit recommendations (32) (c) Between downcomer and inlet weir, 0.740 ft2
offered in this prescntadon. (33) Riser slot seal, (3.0 - 2), 1.0 in.