Page 199 - Applied Process Design For Chemical And Petrochemical Plants Volume II

188 Applied Process Design for Chemical and Petrochemical Plants

diameter. Kister [94] discusses the effects of hardware Pressure: 1.5-500 psia (Note 1)
relationships on spray and froth entrainment. Gas velocity: 1.5-13 ft/s
For sieve trays Kister’s [94, 1841 final correlation is for Liquid load: 0.5-12 gpm/in of outlet weir (Notes 2,3,5)
the Souders-Brown flooding coefficient and is essentially Gas density: 0.03-10 lb/ft3 (Note 1)
independent of pressure. The Kister and Haas correla- Liquid density: 20-75 lb/ft3
tion: [94] [184] Surface tension: 5-80 dyne/cm
Liquid viscosity: 0.05-2.0 cp
Tray spacing: 14-36 in. (Notes 4,5)
Hole diameter: M1 in.
Fractional hole area: O.OWl.20 (Note 5)
Correcting for the froth-to-spray regime transition [94]: Weir height: e3 in.
NOTES:
1.At pressures above 150 psia, downcomer flood is
often the capacity limitation. This limitation is not
predicted by the correlation. Caution is required.
where [94] 2. At high liquid loads (above 7-10 gpm/in.), down-
comer flood is often the capacity limitation. This lim-
0.29 Af-0.791 dH0.833
(hct )water = (8 - 289) itation is not predicted by the correlation. Caution is
(1 + 0.0036 Q-0.59 Af-l.”) required.
3. Equation 8-289 does not apply for liquid loads lower
C~B Gfactor at flood, ft/sec than 0.5 gpm/in. of weir (35)*. For this reason, this
=
Vflood = superficial vapor velocity at flood, ft/sec, based on correlation must not be extended to lower liquid rates.
net area, AN, ft2
& = fractional hole area, AH/AB 4. At lower tray spacing, entrainment flooding may be
Ah = hole area, ft2 (net) related to lifting of the froth envelope and to froth
AB = bubbling area, column cross-section area less total rather than spray height. This correlation must not
of downcomer areas, downcomer seal areas and be extended to lower tray spacing.
areas of any other non-perforated region, ft2 5. The correlation does not apply when the following
AN = net area (column cross-section area less downcom- three conditions occur simultaneously. (a) Ratio of
er top area) ft2 flow path length to tray spacing is high (> 3); (b) liq-
hct = clear liquid height at transition from froth to spray uid rate is high (> 6 gpm/in of weir); and (c) fi-action-
regime, in. liquid. al hole area is high (> 11%). Under these conditions,
dH = hole diameter, in. entrainment flooding is related to vapor channeling
S = tray spacing, in. and vapor cross flow rather than spray height.
n = a parameter in the spray regime, in.
u = surface tension, dynes/cm
p~ = liquid density, lb/ft3 Fair’s [183] design procedure to establish an entrain-
p~ or p~r = vapor density, lb/ft3 ment flooding condition or “point” is as follows: Design
QJ = liquid load, gpm/in. of outlet weir length Procedure (From Fair, Reference 183, by permission)
The design method presented in this article is best sum-
Kister and Haas [184] recommend using 25 dynes/cm marized by a stepwise procedure:
in Equation 8-286 when the actual surface tension is z 25
dynes/cm. This correlation is reported [94, 1841 to give 1. Establish liquid and vapor flow rates and densities.
better effects of physical properties, and predicts most Obtain or estimate liquid surface tension. If condi-
sieve and valve tray entrainment flood data to d5 to 20%, tions vary significantly across the tower, apply this
respectively. method to each section of interest wherein condi-
Kister and Haas [184] analyzed sieve and tray data as tions can be considered constant.
earlier described [94] and then related their results to the 2. Calculate the flow parameter, FP = L/G
application for sieve and valve trays. 3. Estimate the flood point from Figure 8-137, which
accounts for liquid flow effects and is a ratio of liq-
Recommended Range of Application. The Kister and Haas uid/vapor kinetic effects [79]. Flooding velocity is
[I 841 Flood Correlation (used by pmission, Kister; H. Z., obtained from
Distillation Design, McGrawHilZ, Inc., 1992)
Flooding mechanism: Entrainment (jet) flood only
Tray types: Sieve or valve trays only *References in ( ) are from the original source.