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RE: Distillation column cleaning

In a post to P2TECH, David Foulkes (Dave.Foulkes@epa.state.oh.us) asked (in
part) --

> One of our inspectors is interested in finding a alternative 
> method or chemical for cleaning distillation columns. 

You did not mention what, exactly, was being cleaned from the column.  This
is important information, since there are a number of places where grunge
accumulates in distillation columns (and not the good kind of grunge, like
my son's rock band -- "The Weapons of Mass Distraction" -- plays, but the
nasty, baked-on chemical sort of grunge.  Though speaking of nasty
chemicals, I do have my concerns about their bass player.....) 

In any event, a couple of mechanisms leading to grunge accumulation in
columns includes (1) deposition of thermal decomposition or polymerization
byproducts on heated surfaces in or associated with the column itself (e.g.,
in the reboiler) and/or  (2) deposition of settled solids from upstream
processes (catalyst fines, byproducts and impurities, etc) in portions of
the column where fluid velocities might not be high enough to entrain the
materials (dead spots on column internals, and again, in the column bottom
or reboiler).  

Since you would ideally want to prevent the material from accumulating in
the first place (hence pre-empting the need to clean), it is important to
discern the mechanism by which the material is showing up in the first
place.  Lacking any other information, I'd bet on the former mechanism
(degradation in the column), since it sounds like the material requires some
effort to clean.  

If the material needing to be removed is of the former type (thermal
degradation/polymerization residue being generated in the column), any of
the following strategies MAY contribute to reductions.  Note that almost all
of these methods are addressing the same root cause, which is high contact
temperature at the heated surface(s):

- Lower column pressure (reduces the temperature profile in the column).
- Reduce reboiler temperature (e.g., using lower pressure steam,
de-superheated steam, installing thermocompressor, using intermediate
transfer fluid).
- Modify reboiler design (falling film or pumped recirculation reboilers,
high-flux tubes) -- this can be particularly effective for viscous materials
- Retray or repack column -- this can increase .

- Preheat column feed (e.g., by cross exchange with another stream) - this
can reduce heat duties in the reboiler, again reducing need for high tube
- Add insulation to the column 

If the material appears to be coming into the process from upstream
processes, and you cannot eliminate it at the source, you might also try
creating a settling tank for this material upstream of the column.  I am
aware of at least one case study in an Adiponitrile manufacturing operations
where the decanting system was upgraded, permitting higher recovery rates of
catalyst and higher yield in the process. Solid wastes were reduced by
approximately 1,000,000 pounds per year, and the investment in new equipment
paid for itself in approximately 18 months.

You can find out more about these and related strategies for P2 in columns
by visiting the ChemAlliance "Virtual Plant Tour" at
http://www.chemalliance.org/Handbook/plant/index.htm and clicking on the
distillation column; view the GMP's and Case Studies to view the P2

If none of these purely preventative approaches appear to be applicable, you
might also look into some of the high pressure nozzles that are available
for vessel cleaning; these can help increase the cleaning effectiveness of a
solvent, allowing less solvent to be used, or a less
aggressive/objectionable solvent to be used.   

Hope this helps,

Scott Butner (scott.butner@pnl.gov)
Director, ChemAlliance 
Senior Research Scientist, Data and Knowledge Engineering Group
Pacific Northwest National Laboratory
MS K6-04
PO Box 999, Richland, WA  99352
(509)-372-4946 voice/(509)-372-4995 fax