Page 223 - Applied Process Design For Chemical And Petrochemical Plants Volume II

21 2 Applied Process Design for Chemical and Petrochemical Plants

H
L
L‘
0
c-(
0
m
U
3c
.-
4d
V
m
LI
ru
0
Flooding
0.1 ~
0
0 50 100 150 200 250 300 350 400 450 500

Pressure, psia
*Vload/Tower Area x (24/Tray Spacingslos
Figure 8-151. Graphical correlation of sieve tray ultimate capacity for hydrocarbons. Used by permission, Capps, R. W., The American Insti-
tute of Chemical Engineers, Chem. Eng. Prog. V. 89, No. 3 (1993), p. 35, all rights reserved.

Capps analyzes that from Figure 8-151 [ 1881, which was 1. Jet flooding occurs due to liquid entrainment induced
derived from data of Fractionation Research, Inc. in com- by vapor jets passing through the liquid flowing on the
mercial scale tests, a 450-psig deethanizer operating at a tray. The entrained droplet may carry into the tray
capacity factor of 0.18 in the rectifying section may not be area above and reduce tray efficiency and capacity.
worth retraying to debottleneck a process, while a 30-psig 2. System limit jlooding is similar to jet flooding, due to
crude column at a capacity factor of 0.25 may provide a low surface tension and low density difference
good economic rate of return for retraying operation/or between liquid and vapor. Terminal velocity of some
revamp. These generalized decisions are established by entrainment droplets is less than the upward vapor
spotting the capacity factors on the chart and noting the velocity, and hence they are carried up into the tray
potential improvement possible to reach the appropriate above, thus reducing tray efficiency and capacity.
L/V curve. 3. Downcomer backup flooding results from pressure drop
Note that “jet flooding” capacity is fairly insensitive to at bottom outlet of downcomer, causes liquid to back-
system physical properties, but that the “system limit” up in the downcomer and flood the tray above. Gen-
capacity is strongly dependent on physical properties. erally the cause is due to excessive tray pressure drop.
Generalized mechanical performance of high pressure 4. Downcomer twqhase jlooding results from vapor failing
and vacuum tray hydrocarbon distillation are shown in to disengage from downcomer liquid, and causing twc-
Figures 8-152A and 8-152B. The representations are for phase flow to pass through the downcomer bottom
concepts only and do not represent any published data outlet, causing backup in the downcomer to the tray
per se. The charts illustrate the effects of physical proper- above. Generally, this occurs in high pressure systems
ties and pressure on flooding situations. Because flooding with low surface tension and low density differences.
is an important condition that limits the performance and
capacity of a column, it deserves attention and under- where AB = bubbling area of tray, f;‘
standing. The four mechanisms of flooding are [ 1881 : ADCT = downcomer top area, ft’