The authoritative public forum
  for Metal Finishing since 1989
-----

Scrubber Chemicals

I have a scrubber on my aluminum bright dip tank that is currently working at about 50% effienceny rate, but we need to up this. Do you know of any chemicals that we can either add to the caustic we are currently using, or that we can use instead of the caustic that will give us better NOx removal efficiencies. We currently scrub at about 55,000CFM.

Amy Kruse
- Newnan, GA
2001

---

Have you discussed this with the manufacturer of your scrubber? NOx is hard to remove, but 50 % is a terrible removal rate.

Typically, you need a longer packed bed and possibly a better packing. Have you checked it for salt buildup? You may have a reduced effective area which would increase the air velocity and reduce the residence time for contact with the water.

Are you maintaining the pH with a doser that is calibrated regularly and is working properly. Higher water flow might help a tiny bit. Higher pH might help, but this has problems also. Slowing down the air volume, thus velocity and still staying in design parameters might help-IF you still have enough ventilation of the tank.

Speak to your vendor first and if that fails, find a consultant. (a competitor is very cheap for information). You probably are going to have to do some equipment modification and or replacement.

James Watts
- Navarre, Florida
2001

---

2001

Amy,

What James Watt says about the efficiency rate is l00% correct, your emissions are bloody awful.

You didn't say if they have always been like that (i.e., a failure of the scrubber design) or if this is recent,(i.e., a failure of another sort, i.e., blockage or an increased airflow). Most mass transfer scrubbers operate at max.max. 500 fpm and for bright dip, I'd guess @ 400 fpm. One must have all the facts.

Then, as a doubting Thomas, I ask myself the perennial question, You say that You have a 50% carry-over of NOX but I say to myself, oh yeah? Really? Are you sure it's NOX and not sulfuric, too? In other words, the proper diagnosis is needed before you spend more $$$.

Please go in the archives and have a look-see at # 8270.

James mentioned the possibility of build-up. Then there's the question of just WHOSE packing you are using ... maybe the packing is partially plugged up, maybe it has deteriorated. Facts are missing.

True NOX is awful to remove. Maybe your process uses excessive amounts of Nitric (I thought that bright dip used between 4% and 6 percent which is very small). Chilling or cooling the 'reagent' certainly helps. Maybe your process is not for aluminum but for brass in which case, oh Dear oh Dear, as the nitric strength increases so does the problem.

Then there's retention time or, as James W said, slowing down the airflow. In the true NOX scenarios (digestion of gold etc in pure nitric retorts), retention time is extremely important ... but surely that shouldn't apply to your scenario.

Lastly, 'NOX' emissions don't always consist of varying compounds of nitric acid. Sometimes or partially ... they can consist of ultra fine invisible droplets (brown or otherwise) and that, Mizz Amy, is where a first class mist eliminator can prove to be pretty efficient and pretty inexpensive ($ & SP-wise) because many scrubbers either lack or have awful mist eliminators or they have mesh eliminators which can be extraordinarily good but are terribly prone to plugging up. Be careful of packing suppliers who will try to sell another packed section to act as a mist eliminator. Dubious efficiency. Largish droplet emissions (over 40 microns, for sure). I have added numerous eliminators to packed tower scrubbers ... and saved their bacon.

I hope that this will be of some help to you.

Cheers!

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).

---

Hi,
I am interested in knowing the Nox scrubber design. Please let me know if anybody knows about calculation , sizing etc.

Himanshu
air pollution control - Houston, Texas
2006

---

our NOX scrubber is giving a problem of becoming saturated(ph=0) within 1-4 hrs.what can work best between caustic flakes vs caustic solution

Nchadi Manonyane
- South Africa
September 30, 2009