Page 169 - Applied Process Design For Chemical And Petrochemical Plants Volume II

158 Applied Process Design for Chemical and Petrochemical Plants

Blowing checked relative to the effect on maximum cap vapor
capacity and entrainment, and on liquid gradient and
A bubble tray blows when the vapor rate is extremely buildup in the downcomer.
high, regardless of the liquid rate, causing large vapor
streams or continuous bubbles to be blown through the Total Tray Pressure Drop
liquid. The efficiency and contact is low and entrainment
is usually high. Here also low slot seals contribute to the This is normally taken as the wet bubble cap pressure
sensitivity of the tray to such action. drop plus the "mean dynamic slot seal" in inches of clear
or unaerated liquid on the tray.
coning Guide values for normal operations, drop per tray.

A bubble tray cones when the liquid seal over the slot is Pressure Vacuum (500 mm Hg and below)
low and the vapor rate is so high as to force the liquid com- 2-4 in. water 2-4 mm Hg
pletely away from the cap, thus bypassing the liquid entire-
ly. Obviously, efficiency is unsatisfactory. The dynamic slot Liquid Height Over Outlet Weir
seals recommended in Table 8-18 normally will prevent
such action. For a straight (non-circular) weir, the head of liquid
over its flat top is given by the modified Francis Weir rela-
Entrainment tion (Figure 8-104; also see Figure 8-63):

A bubble tray has high entrainment when mist and liq- how = 0.092 F, (Lg/lw)2/3 (8-222)
uid particles carry up in the vapor from the liquid on one
tray through the riser and cap on to the tray above. Bub- The modlfying factor Fw developed by Bolles [5] for
ble caps tend to entrain by jetting liquid-vapor mixtures restriction at the shell due to segmental downcomer appli-
high above the tray. Sufficient tray spacing must be avail- cation is determined from Figure 8-105.
able to prevent the quantity of material from significantly When how values exceed 1% to 2 in., consider special
affecting the efficiency of the system. The quantitative pre- downcomers or down pipes to conserve cap area for high
sentation of entrainment in later paragraphs is designed vapor loads.
to work to this end. Notched outlet weirs (usually 60" V-notch) are only used
for low liquid flow rates, and the head over this type of
Overdesign weir with notches running full [13].

Overdesign is often necessary in designing a tray, L, = 14.3 (lw/n) [h0w5/2 - (h, - hd5I2] (8 - 223)
although caution must be exercised to prevent a piling-up
or accumulation of safety factors resulting in numbers For notches not running full
which are totally unrealistic for performance. In other L, = 13.3 (l,/n) (how,)5/2 (8 - 22311)
words, the magnitude, effect, and significance of overca-
pacity figures must be continuously monitored as each fac- where how = height of liquid crest over flat weir, in.
tor is calculated. A factor of 10-15% on liquid and vapor 1, = length of weir (straight), feet
rates is usually acceptable. However, each should be Lg = liquid flow rate, gallons per minute, tray or tray
section
n = depth of notches in weir, in.
Table 8-18 how' = height of liquid above bottom of notch in weir, in.
Suggested Slot Seals h, = depth of notches in weir, in.

Taver Operating Static Slot Seal Dynamic Slot Seal For circular weirs (pipes) h,, = L&O d,,, (8 - 224)
Pressure c1.51, In. PI, In.
Vacuum, 30-200 mm where d, = diameter of circular weir, in.
Hg. abs 0-0.25 0.5-1.5
Atmospheric 0.5 1.0-2.0 Slot Openine;
50-100 psig 1 .o 1.5-3.0
300 psig 1.5 2.04.0 The slot opening is the vertical opening available for
500 psig 1.5 2.04.0 vapor flow during operation of the cap under a given set
Used by permission, Bolles, W. L., Petroleum Aocessing, Feb. thru May of conditions. It has been found to be essentially indepen-
(1956), and Davies, J. A., Pet. RefinerV. 29, No. 93 (1950), Gulf Pub. Co. dent of surface tension, viscosity and depth of liquid over