DOWNCOMER CHOKE FLOODING

[Tremendo]

I am evaluating the performance of a distillation column for a revamp.
I don't know what's the maximum allowable velocity of clear liquid at the downcomer entrance. Some people tell me that above 0.13 m/s (200 gal/ft2) the downcomer will choke, but the literature presents values of 0.17 - 0.18 m/s (250 - 270 gal/ft2).
I have the feeling that de value of 0.13 m/s is for a new design, not for a revamp...

Can anybody help me?

 

[davefitz]

I think the max choke velocity is a function of the level of liquid above the inlet, and also the use of a bellmouth with anti-vortex plates.

 

[katmar]

I agree with davefitz - there is more to it than just the superficial velocity. Check the backup in the downcomer as well. I like to have it roughly 50% of the tray spacing. A simplistic superficial velocity calc will not show you if the pressure drop under the downcomer is excessive and causing backup.

Also check the residence time. You need 3 to 4 seconds for the bubbles to collapse before the liquid enters the lower tray. I like to be conservative here and if possible I would go for >5 seconds.

regards
Katmar

 

[StoneCold]

Tremendo
A great way to check out your column performance against any model numbers you might generate is to increase the reflux ratio until the column begins to flood.

 

[25362]

The total height of a liquid in the downcomer depends on a series of factors:

- The departing velocity onto the tray below as affected by
the clearance under the DC.
- The crest of the liquid over the oulet weir.
- The weir height.
- The ratio of top to botttom area for a sloped DC.
- The drop in pressure of the vapor flowing through the
tray dumping liquid into the downcomer, which frequently
represents about 50% of the liquid height.
- Bubbling area and residence time, froth or foam, as
mentioned by Katmar, may represent an aeration factor of
50%, doubling, in fact the liquid height.
- Small downcomer top area: for clean services a cross-
sectional area of 150 GPM/ft² of top area
(~0.1 m/s) has been mentioned as a reasonable ROT for
most services.
- Operating pressures. These may influence the V/L
separation due to the proximity of the V/L densities.
- The thermophysical and chemical properties of the fluids
that may affect surface tension, viscosity, etc.,
directly affecting V/L separation.

Have a look at Chapters 6 and 14 in DISTILLATION-OPERATION- by Henry Z. Kister (McGraw-Hill) for layouts of DC and trays and tips to avoid flooding.