Significant increasing a packed distillation column capacity just with new bed supports?

[guerky]

I will open a small distillation column (3.000 x 300 mm) for maintenance in a few days. This column is packed with 1" Intalox Saddles (void fraction: 77%) and have a PTFE support plate with 15mm holes spaced by 4mm (free area: 41%) (there are the support beams right bellow, that block about 50% of the free area, but those i intend to left untouched).

I designed a support plate in SS316 (no corrosion issues with this material, thermal expansion issue covered, as well as the mechanical resistance), with less spacing between the 15mm holes (2mm), increasing the free area to 59%).

If Lieberman is to be trusted, i can significantly increase the capacity(both for gas and liquid) of my packed column just with this new support plate, since the bottleneck in a internals interface would be the product of the free areas, not just the smaller free area.
This collumn capacity limitation would cease to be 77% x 41% x 50% = 15,8% to be 77% x 59% x 50% = 22,5%, almost a 50% increase with a simple drilled plate.

This looks too good to be true.

Please, correct me if i'm wrong or if i misunderstood some concept.




In God we trust. All the others must bring data.

 

[sshep]

Hey guerky,

The bed support you describe sounds poorly thought out in temrs of capacity limitations. Maybe it wasn't an issue inthe original design and done cheap and simple.

I know Lieberman's check. I am not sure void fraction can be used as open area in the calculation of effective open area. I also calculated more open area on the support tray. On the otherhand, I do think you might be on to something if the tower is hydraulically limited.

Best wishes,
Sshep

 

[georgeverghese]

There are some better options for the selection and design of these support plates in packed columns suggested in pages 14-46/47 and figures 14-55, 56 and 57 in the 7th edition of Perry Chem Engg Handbook that would avoid the disadvantages with countercurrent flow support plate designs, as with the perforated plate you've got now.

 

[shvet2008]

You can find more usefull practicies in a great Kitzer's "Distillation Operation". See chapter 8 "Internal Unique to Packed Towers".

 

[guerky]

sshep,
You're correct.The beams were originally designed to support 1 1/2" packing directly over then, without support plate. The packing was changed to reach better separation, probably without taking these internal details in consideration.

Currently, my major problem is due to poor liquid distribution (reason for maintenance). Its a distillation coupled in a batch synthesis reactor). I operate with a reflux of 250 L/h, when the flooding normally start at 270 L/h. I dont need to operate at higher reflux rates than 300 L/h, so even a slight improvement in the capacity would be enough. The question was more for a better theoretical understandig.

But why the void fraction can't be used as open area? Yes, its a volumetric number, but seems like a good aproximation to the real sectional open area.









In God we trust. All the others must bring data.

 

[25362]

Shvet2008, probably you mean Henry Z. Kister's DISTILLATION -OPERATION- Mc Graw Hill.

 

[georgeverghese]

The recommendation in Perry is for the net free area of the support plate to exceed that of the packing. Net free area of the support plate is given by the gross free area of the plate minus ( gross free area occupied by (1- free area) of the packing).

 

[shvet2008]

to 25362

Yes Kister Distillation Operation.