Page 204 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 193

0.2
CSB, flood = 0.340 (g) = 0.328
Calculate column diameter using Uflood reduced by
15-25%, or increase the calculated column area by about
nus, percent flood = 0.278 x 100 = 85%
0.328 25% and convert to a working diameter.

From Figure &138, I$ = 0.055 Maximum Hole Velocity Flooding

0’055
Finally, entrainment = - (200,000) = 11,60Olb/hr The maximum hole velocity will give a liquid build up in
0.945 the downcomer of 50% of the tray spacing.
= 11,600 lb/hr To determine the maximum velocity:
If the dry efficiency at this point in the column is 9096,
the wet efficiency is calculated by means of Equation 291: 1. Assume a hole velocity.
2. Calculate liquid height in downcomer, Hd by Equa-
tion 8-269.
0.90
EW = = 0.855 = 85.3% 3. If Hd = ?4 S,, the assume hole velocity is satisfactory; if
1 + 0.90 (-) 0.055 not, repeat until a close balance is obtained.
0.945
Design Hole Velocity
Experimental flooding and entrainment data for sieve
trays are not plentiful, and measurements are not precise. The design velocity for selection of the holes also sets
Accordingly, it has been necessary to relate correlations of the minimum tower diameter. To take advantage of as
flooding and entrainment to those of the well-known much flexibility in operation as possible throughout the
device, the bubble-cap tray. It appears that the two devices expected operating range, the following points should be
have about the same flooding limits, so long as usual considered in setting this velocity.
design practice is followed. However, the sieve tray shows A. Select a design velocity near the weep point if:
entrainment advantages, especially when used in vacuum
and atmospheric service. 1. The design vapor rate is, or is very close to, the mini-
The flooding capacity for sieve trays has been set into mum rate.
mathematical equation by Ward [187] using Fair’s equa- 2.All change in capacity is to be as an increase over
tion [183] and Figure 8-137. This is turn allows for the design rate.
determination of the column diameter, assuming that an 3. Reduction in efficiency can be tolerated if vapor rate
allowance is made in the flooding velocity so as not to falls to weep point minimum or below.
design for flooding, but perhaps 25% below. I have not 4. Low tray pressure drop is required, as for vacuum sys-
personally verified the equation of Ward [187], but Ward tems. Design with extra caution under vacuum, since
does show comparison curves, i.e., his with Fair’s. Ward’s data correlations have not been checked in this region.
equation for sieve tray flooding capacity factor:
B. Select a design velocity near the maximum velocity if:
0.26S, - 0.029 S‘,
CF = , ft / sec (8 - 296) 1. The design vapor rate is the maximum expected. All
c1+6%2 s, 0.7498 f.3 change will be to lower rates.
2. High efficiency is required.
FP = F1, = Flow Parameter = (L‘/V’) (~~/pl)O.~ (8-29’7) 3. High pressure drops are acceptable.

where S, = tray spacing, ft Tray Stability
L’ = liquid mass flow, lb/sec
V’ = vapor mass flow, lb/sec Figure 8-141A of Huang and Hodson [30] and Figure
pv = vapor density, lb/ft3 at flowing conditions 8-141B can be prepared from an evaluation of limits of
p1 = liquid density, lb/ft tray performance using the relations set forth herein, or as
presented in the original reference using slightly different
Ward [ 1871 reports the best fits for the curves at tray (or analysis.
plate) spacing in the range of 0.5 to 3.0 feet, and at the Unstable liquid oscillations on a tray have received only
ends of the curves. limited examination when compared to perhaps tray
By analogy to Fair’s [ 1831 work, weeping, flooding and froth build-up. Biddulph [87] pro-