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RE: Distilling halogenated solvents



Bob
The answer will vary with the solvent used and the stabilizer package 
present.  As a perspective, PPG, in a 1986 manual on vapor degreasing 
using their solvents, indicated critical pH (measured on 1 part solvent & 3 
parts neutral water) requiring action were:
pH=5.0 for perc
pH=6.0 for TCE
while critical acid acceptance numbers are:
0.01 for perc compared to the recommended level of 0.1
0.03 for TCE (0.18 recommended)
Acid acceptance numbers are the better indicator of whether the solvent is 
in good condition - pH can change quickly if acid acceptance is low.

Adding lime to a stand alone still seems like treating symptoms since it 
does nothing about acid contamination on degreaser surfaces - which is 
important since acid catalyzes further hydrolysis.  There is an ASTM method 
for handling acid degreasers - D 4579-86


On Sat, 03 Jul 1999 18:48:15 -0700,
rpojasek@sprynet.com wrote...
>Does anyone have any information on the buildup of hydrochloric acid in 
the
>still bottoms when halogenated solvents (yes there are still companies
>using them) are distilled.  I am trying to find out what a typical
>concentration of HCl might be in the moisture laden still bottom.l  I am
>also trying to find more information about the hydrolysis reaction that
>forms the acid.  I understand that many storage tanks corroded from the
>indisde due to the buidup of acid both in the still bottoms and in the
>solvent as acid must have been carried over into the distillate.  Many
>people now add lime to the distillation pot to neutralize the acid,
>although not everyone knows about this practice.  Any information would be
>useful.  Thank you.
>
>
>Bob
>
>Dr. Robert B. Pojasek
>Adjunct Faculty Lecturer
>Harvard School of Public Health
>P.O. Box 1333
>E. Arlington, MA 02474-0071
>(781) 641-2422
>(617) 788-0288  fax
>
>rpojasek@hsph.harvard.edu
>
>http://www.hsph.harvard.edu/facres/pojasek.html
>
>