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RE: Recovery of painting solvents from air stream



Just an additional comment on the suggestion to use nitrogen as the
regenerant for the saturated activated carbon. Regeneration of the carbon is
caused by one or both of two phenomena: Heat, which pushes the adsorption
equilibrium back toward the vapor phase, and displacement of the adsorbate.
While either steam or nitrogen could heat up the carbon, and promote
desorption, water is more attractive to the carbon as an adsorbate replacer.
Consequently, you would expect the adsorbate to come off in the steam at a
higher concentration than it would with nitrogen. Secondly, you can't
(unless cryogenically) condense nitrogen. The adsorbate will be at a low
concentration (and partial pressure), so condensing it from the nitrogen
will be inefficient. Steam, on the other hand, is easily condensed. By
condensing the steam, you get the everything. The efficiency of recovery of
the solvents from the condensate depends on what they are and how
concentrated.

-----Original Message-----
From: Alfonso Manrique [mailto:almanriq@mafalda.univalle.edu.co]
Sent: Tuesday, May 30, 2000 1:48 PM
To: Robert Michalowicz
Cc: 'P2TECH'
Subject: Re: Recovery of painting solvents from air stream


On Tue, 30 May 2000, Robert Michalowicz wrote:

> Greetings All:
> 
> Your assistance in the following problem would be appreciated!
> 
> A manufacturer paints parts and the solvent used in the paint
> formulation,  n-butyl acetate, gets into the exhaust air (13,000 CFM) in
> trace amounts (500 ppm).  The air is passed over activated carbon to
> remove the n-butyl acetate.  The activated carbon is then re-activated
> using low pressure steam.  The steam is condensed and most of the
> n-butyl acetate is decanted as a separate phase from the water and
> re-used.  However, the n-butyl acetate is slightly soluble in the water
> and some remains in the water along with decomposition products - acetic
> acid and n-butanol.  The BOD of this wastewater is 10,000 mg/L and it
> needs to be reduced before discharge.
> 
> I have several questions:
> 
> 1)  Does the acetic acid contribute to the BOD?
Yes, is one of the path of organic compounds biological degradation

> 2)  Is there a better way to approach solvent recovery from dilute air
> streams?

No, I think is the better way if you are really interested in recovery,
otherwise use Biofiltration or Incineration

> 3)  What would be the advantages/disadvantages of using nitrogen to
> re-activate the activated carbon?

May be the advantage is just to condense the organic compounds alone,
without water but I supposse the cost of the Nitrogen is higher than steam
and the condensacion temperature lower in order to have low amount of
organic compound in the gas waste, And may be require some treatment for
it at the pipe end ( Biofiltration )

> 4)  What is the best treatment technology for this wastewater containing
> n-butyl acetate, acetic acid, n-butanol?

May be an anaerobic system

> 5)  Steam stripping has been suggested, but doesn't the steam have to be
> re-condensed, getting the problem of dilute organics in a wastewater
> stream once again?  Can the organics be recovered from a steam stripper?

Or air stripping, in that case the waste gas probably must be treated in
the same facilities as biofiltration.
> 
> 6)  How about oxidation with peroxide and UV?  What is the removal
> efficiency?

I dont know
> 
> Thanks,
> Rob Michalowicz, P.Eng.
> Chem Process & Environment Inc.
> 
>