When distributor between beds needed and when not?

[RicoB]

Hello Everybody

I was wondering when a distributor between each bed is necessary and when not. Is there something like a rule of thumb by bed height or bed diameter or packing?
I have a De-Ethanizer with two bed but with out distributor between.

Thanks
Rico

 

[EmmanuelTop]

That is quite interesting setup. My experience with liquid distributors and redistributors in packed columns is that they are normally installed above the top of each bed. Some experiments have shown that up to certain bed depth, liquid distribution is not considered critical. According to R. Strigle:

"A liquid re-redistributor is required at the top of each packed bed. The liquid flow from a typical support plate is not sufficiently uniform to properly irrigate the next lower packed bed. The multi-beam type of packing support plate, which is widely used, tends to segregate the liquid downflow from a packed bed into a pair of parallel rows of liquid streams about 2-in. apart with a 10-in. space between adjacent pairs. Gas distributing sup port plates, likewise, do not give a sufficiently uniform liquid irrigation pattern, because gas riser locations take precedence in the design of such a plate."

"A liquid distributor is required at all locations in the tower where an external liquid stream is introduced"

"Packed bed heights typically vary from 20 ft to 30 ft [3]. Many times the location of manholes to provide access to the redistributor will determine the packed depth. Whenever more than 15 theoretical stages are required in one packed bed, good liquid distribution is critical."

(R.Strigle, "Packed Towers and Applications", Gulf publishing, 1994).

Dejan IVANOVIC
Process Engineer, MSChE

 

[RicoB]

Hello EmmanuelTop

Thanks for your response. I do have a distributor on top of the upper bed but not between both beds. The only different between both beds is the type/size of packing.
However, I will have a look in the book you refer to.

Thanks

 

[don1980]

Good distribution is critically important for good performance from packed beds. Poor distribution can hugely affect the performance, and bad distribution can render the bed virtually ineffective. If you were talking about a small, non-important column then maybe I could possibly imagine not have a distributor between the beds, but in this case you're taking about a de-ethanizer which is obviously a very critical part of the plant. I think you're overlooking something. This is not a detail that a packing supplier would miss when they put together their design proposal. Check your drawings from the internals of this tower, and if you still don't see anything, contact the vendor to ask them how the gas & liquid are re-distributed between the beds.

 

[sshep]

Dear RicoB,

I subscribe to the belief that redistribution should occur between each bed. The basic theory supports this requirement- i.e. as the liquid spreads out in a cone below each drip point, it will build up against the wall. Redistribution at a minimum is needed to put liquid loading evenly onto the surface of bed below. In some cases more elaborate collection and redistribution is used to mix the liquid into a homogeneous composition before redistribution.

Although the theory would make optimum bed depth a function of packing type and tower diameter, 20-30ft beds are common as EmmanuelTop has written.

I also believe you probably have some redistribution and are missing it.

best wishes,
SShep

 

[AME137]

I have experience in packed bed design and redistribution should be done every 20ft or so. It depends on the size of the packing, gas factor, liquid load, tower diameter mainly. Let me know if you have particular questions.

 

[georgeverghese]

There is some discussion on this topic on page 14-50 of Perry Chem Engg Handbook 7th edn, and subject to meeting some other criteria, the following abstract appended here may be useful:
------------------
Liquid migration to the wall appears to be favored by small-column
diameter-packing diameter ratios (for random packings) and can be
corrected by the use of side wipers or redistributors. Inhibition of lateral
dispersion can be caused by the geometry of certain types of
structured packing, according to Huber and Hiltbrunner (op. cit.).

With careful attention given to the five causes of maldistribution, it is
possible to design commercial packed columns for heights of 8 to 9 m
between redistributors

-----------------------

It also says that packed bed mass transfer eff is significantly affected by the distributor design.

 

[Jobst]

If the column is working as intended at design point is there really a problem with not having a redistributor?

I would investigate, document, and file it away for when it is actually causing a problem.