5 replies to this topic

I am currently on internship

My objective is to rate a column, where an incoming slurry is counter-currently stripped off ammonia using steam. The problem lies in the large size of the column ( 8.5 ft); also the design allows for several bubble-cap trays, followed by a a packed section.

My calculations based on exisitng flow-rates for the packed section result in almost negligible pressure-drop (on the basis of Generalized Pressure Correlation), indicating that the current flow-rates are way too low.

With such a large size, and a 'hybrid' design, what factors should I be looking at for improving the performance?

The system may primarily be considered as ammonia-water
I know the top and bottom pressure-temperatures...(nearly atmospheric, 90-120 C)

the ammonia needs to be almost completely removed

[Art Montemayor]

Bubble:

Never - but never - use a bubble cap tray on a slurry fluid. It is the wrong application and the system will plug up in no time at all.

Depending on the amount of solids you have in the slurry, you can employ sieve trays or just plain solid plates.

Bubble:

Never - but never - use a bubble cap tray on a slurry fluid. It is the wrong application and the system will plug up in no time at all.

Depending on the amount of solids you have in the slurry, you can employ sieve trays or just plain solid plates.

Well- thanks for your reply

but that is again another confusing aspect- these aren't conventional bubble caps but large mushroom-shaped gas pasettes- almost like nozzles with caps
only one per plate... and 4-5 ft in diameter

i would be grateful if u could shed some light on tray hydraulics on this 'ancient' design

[Art Montemayor]

Bubble:

You're not considering that you are receiving advice based on 48 years of experience. If you persist on applying a bubble cap design in a slurry application ("slurry" = mixture of solids + liquid), you will have a fiasco on your hands. It simply won't work for long.

What you propose doesn't make for logical engineering sense: if you are only going to employ ONE plate, then install several flat plates (sloped) instead.

Bubble plate hydraulics are simple and straight forward. You can find bountiful information in text books on the subject. I recommend Ludwig's "Applied Process Design for Chemical & Petrochemical Plants".

[Zauberberg]

This column design is really ancient, and I think - based on your description - it's the same as first commercial distillation tower (kerosene stripper built in USA, if I remember well - with a single bubble cap per tray).

I've never seen structured or random packing being used in slurry applications - it's definitely wrong and poor practice. Disc-and-donut trays or shed decks are the type of internals which are most commonly used in such services, due to extreme fouling potential of tower feed.

Since you are working on column rating (and not design), it's relatively easy to determine the number of theoretical plates and tray/packing efficiency, once when feed and product qualities are known. From this point you can start to think about possible improvements but, as Mr Montemayor already has said, heavy fouling nature of slurry will hardly allow for replacement of existing tower internals with high-efficiency devices. It will simply not work.

Best regards,