5 replies to this topic

[Vineeth Dasaraju]

Dear all,

I had been recently asked to design a stripping column for the removal of ammonia from an effluent stream coming from the plant. For this, air would be used to remove the ammonia present in the column. I have designed a column based on a method given in a book by the name of "Waste Water Engineering - Treatment and Reuse" by George Tchobanoglous, Franklin L. Burton, Metcalf & Eddy, H. David Stensel.

By following that method, I got an unexpected result. Diameter was 3 times the height of the column. I am attaching the excel sheet containing the calculations and the procedure used in for calculating the height and diameter.

I am not able to figure out what I did wrong. Your advises would be helpful.

Vineeth

Attached Files

•  Scrubber Design.xls   106KB   223 downloads

[breizh]

Hi ,
Let you try this resource to support your query :

[breizh]

Here the attachement .
breizh

[Vineeth Dasaraju]

Hi,

Thanks for ur link. It has helped me get a better understanding of the design. I have been referring to an example in a book. I have attached the scanned copy of that example. Please tell me the mistake in this approach because the answer that is obtained is absurd. There is one mistake : L/G is taken as 2.5 but not G/L.
Apart from that, what are the other mistakes that the author has committed.

Regards,
Vineeth

Attached Files

•  untitled1 copy.jpg   1.21MB   81 downloads
•  untitled2 copy.jpg   1.13MB   67 downloads

[cnaren]

For packed tower design, try the HETP approach

Assuming pure water, the partial pressure of 4% Ammonia at 20*C is about 3.5 KPa. Henry's law seems reasonably valid below this range.

For feed concentration of 100ppm or 0.01%, the partial pressure with Henry's law is 8.6e-5 atm. This means 8.6e-5 moles ammonia per mole of air is the equilibrium with 100ppm ammonia liquid. Converting to weight fractions, this is 5e-5 kg NH3 per kg of air.

Your waste water flow is 8,000 kg/h, containing 0.8 kg/h ammonia. The minimum air required (infinitely long column) for stripping ammonia is 0.8 / 5e-5 = 16,000 kg/h = Gmin

Since we cannot have an infinitely long column, take G=1.5xGmin = 24,000 kg/h. The operating curves are plotted in enclosed attachment. Four (4) theoretical plates are required.

The HETP of column packings is available in various literature (Ludwig, etc.). You can take the Norton Generalized formula ln HETP = n-0.187 ln(Surf.tension) + 0.213 ln (viscosity). n varies with packing size.


Column height for 2" PP Pall rings is approx. 1,000mm x 4 = 4,000mm column height.

The pressure drop correlation is the one you have used. For a 40mm/m pressure drop (Total 160mm drop) and 60% flooding, I got a column diameter about 1.6m.

Hope this is useful. Ammonia stripping is difficult when there is mild acidity, even dissolved CO2 in the waste water. You have to then consider lime dosing to neutralize the solution.

Attached Files

•  HETP.pdf   52.07KB   160 downloads

Edited by cnaren, 22 June 2011 - 11:55 PM.

[siretb]

Hi
I would like to add a few lines of comments
1) I am not sure that air stripping is a very good solution in your case. The air to liquid ratio is high, and a question immediately araises: What do you do with the ammonia in the stripping air?

2) you are talking of an effluent waste. IT may contain other components that may interfere with the ammonia recovery. If any acid is present, it will combine with the ammonia and the ammonia will stay in the water stream.

3) I would assume that the stream you have does not contain only ammonia, so it is likely that the liquid stream out of your stripper (bottoms) will need to go to a waste water treatment plant, anyway.
One option could be to add a bit of caustic soda so as to ensure tha pH will remain well above the pKa everywhere. Unless you have plenty of buffering components,the amount would be small. If you do that check scaling issues.

4) With air stripping, the higher the temperature the better. For this reason steam stripping is considerd, but in your case, that would mean a large amount of steam. But may be you could manage to feed the waste stream at a higher temperature, using some waste heat source?

5) I do not trust the HETP approach for this king of design. Use HTU / NTU. I would not expect that you need more than 2 meters of packing for your case, but of course calculations are needed.