Jet Flood Percent Criteria 1

[NickVil]

Hello,
Do you know what is the recommended jet flood for the following column?
Naphtha Splitter: the gasoline from crude distillation column would go to an splitter, (TBP cut : 95ªC).
We have an existing column, so, I think there is a minimum and a maximun jet flood to consider for reusing columns.
Also we think to install a new column, so, there is a minimum and a maximum recommended value for new columns.
The old or new column would have valve plates or packing. Maybe the percent flooding jet recommended would be different.

Please advice.

Regards,

 

[sshep]

Jet flood correlations such as Fair (Perry, 5th, fig 18-10) are based on a maximum superficial velocity possible at a given tray spacing, liquid to gas ratio, densities, and surface tension. I find this correlation to be right on for atmospheric towers. Typically 80% of jet flood is used for design. Since the velocity is calculated from diameter, this correlation provides a good way to evaluate existing towers shells based on height and diameter, or design new. Most computer simulations allow for some tray sizing or rating based on jet flood- i.e. tray diameter and spacing.

Weep point depends on the actual tray hole layout where as jet flood (being based on superficial velocity) does not. Typically my experience is that vendors generally show conventional sieve trays to be good down to 50% of design vapor rate (or 40% of jet flood)- valve trays can go lower, and bubble caps have almost complete turndown. This 50% is just an observation of what seems typical since weep point correlations exist. For example fig 18-11 (Perry, 5th) has a correlation based on tray liquid height and head loss across the tray.

best wishes,
sshep

 

[NickVil]

Thanks for the comments,
I think there is some guidelines that are normally employed by the designers. For example 75% max. of jet flood is usual. But that value will depend of the kind of column that we have.
In our case, if we design a new column naphtha splitter, the maximum jet flood would be 70 a 75%. But, if we use and old column for a new service (splitter), the max jet flood would be 80% (maybe 85% with 10% of overdesign).

Best Regards

 

[25362]

Foam height on the trays, vapor velocity, fouling, and even carbon steel metallurgy (tendency to stick), all seem to affect the % of jet flood.

Lieberman and Lieberman in their book A working guide to process equipment (McGraw-Hill) suggest, for valve and sieve trays, the formula:

k = (28)([Δ]P)[÷][(NT)(TS)(SG)]​

where
[Δ]P = the pressure drop across a tray section, psi
NT = number of trays
TS = tray spacing, inches
SG = specific gravity of clear (not foaming) liquid at working temperature

Their observation:

k = 0, liquid level on tray is zero
k = 0.10-0.12, low efficiency due to leakage
k = 0.18-0.25, closest to best efficiency point
k = 0.35-0.40, heavy entrainment, increased reflux reduces efficiency
k = 0.50+, fully developed flooding