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Scrubber design query

Dear Freeman,

I have an customer who would like to have a acid scrubber on top of the fume hood. What kind of scrubber design in suitable ? These are mild acid H2SO4 fumes.

Sathya Murthy
laboratory infrastructure - Bangalore, Karnataka, India
2004

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2004

Hi Sathya !

Ah, I don't recommend a scrubber on the top of the fume hood. I've seen a picture of one, a mass transfer design, but it's ugly, large, inaccessible and a pain in the proverbial neck. But I have done this ... not mass transfer but a side access CT-120/2 eliminator design ... and I still don't recommend it on top of the scrubber. Consider, too, that most labs have a max. ceiling height of 9 feet so you have real space problems, too, during installation & possible servicing.

This was for an Assay lab in Alaska. This is what they WANTED and Mine was Not to Reason Why ...etc.etc.etc. Now I've designed Assay systems in Indonesia & Africa let alone in the Americas & I sure don't recommend this approach.

This was for 6 foot PER.LAB (mixed acids + Perchloric) hoods. This meant that to fit this eliminator in place, the vertical hood outlet had to be at the far end. This is bad design. Then it went into a 90 degree bend discharging out at the back, all within 6 feet. Yucks. A GOOD design would have meant that the unit had to be around at least 10 feet long (from 90 to 90) which allows, too, for good inlet transition lengths. With a poor design like this, you do not get the right efficiencies, also your SP rises.

You say it's for mild sulfuric. Oh yeah? In the assay labs they boil commercial full strength sulfuric at around 320 °C ... mind you, there's a fair amount of air dilution so by the time the fumes hit a 'scrubber' they'd be mild enough! Don't always believe what a customer tells you !

We have an expression for situations like this. It's called the K.I.S.S. factor and is much beloved by competent engineers.You may know it. This stands for Keep It Simple Stupid. Please apply it.

May I suggest that you provide your 'scrubber' away from the hood and tell your customer he might be up the creek if he insists of having it on the hood. Also, bearing in mind what you said about mild sulfuric (?), I'd just use a conventional blade type horizontal eliminator.

I hope that this helps you! By the way, the Imperial War Museum has come out with a book on the l942-45 war in Burma and it sure praises the Indian soldiers who garnered lots of V.C.s.

Let us know what your customer says!

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

What about design of a scrubber for chlorine from zirconium chloride production.

Kranti Kiran Pulla
- Hyderbad, Andhra Pradesh, India
2004

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February 19, 2008

Dear Freeman,

I would appreciate your guidance on how to make an Aqua regia fume scrubber for a gold refining plant of 5 kg per batch capacity. The HCl quantity will be about 14 liters and HNO3 about 3.5 liters. The acids would be heated to remain below 100 °C in the 50 L reactor, during the process. Presently I am using a condenser followed by a twin column caustic scrubber of 3 feet height each.

I do not know what the flow rate or volume of the fumes generated, but they are sucked through the vacuum pump which has a 24 Hg suction rating.

My idea was to make a 7-8 feet x 6-8 inch dia pp column with 2 sections. The bottom would have
packed polypropylene raschig rings through which the fumes would bubble, and the top section would have a spray of water onto a perforated sheet with 1/8 inch holes. The water would be reintroduced by a small pump. The fumes would be introduced to top section by a pipe. The fume outlet at the top of the scrubber would be connected to the vacuum pump. The aim is to have a caustic absorption a the bottom section and a water scrubbing at the top section.

Please let me know if this would make for an efficient scrubber. Any technical inputs
or suggestions would be most helpful.

Best regards,

Reggie Varghese
Product designer - Bangalore, Karnataka, India

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Hi,

sulfuric acid as with Perchloric acid is generated as a fine mist and no packed bed scrubber will remove these mists with great efficiency. Traditional packings (pall, saddle, raschig rings etc tend to work on a wetted surface area. These type packings require lower linear velocities thru the column. Mechanical scrubbing with new generation packing such as Lanpac XL is the answer. Typically this type packing (approx. 1500 mm high) and a good demister such as a Kimre or Airol (Coastal technologies) composite mesh pad will remove 99% of the acid droplets. NaOH may be used to neutralize the acid and extend the life of the scrubbing liquor.

Dave Kalk
- Johannesburg South Africa
March 16, 2009