JohnBee
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Mission impossible, filtration issues...
I've been tearing my hair out trying to filter a solution that just won't seem to quit.
My setup consists of a 2000ml Filter Device w/Buchner core fitting. I'm using .42um PTFE Membrane Filters and a oil free diaphragm vacuum pump. All
joints are sealed and confirmed, system is pulling very good suction.
The problem: No matter how many times I run the solution, it comes out full of floaters and specs!
That said, there must be a logical explanation for this, but I just can't figure it out. Any ideas?
[Edited on 14-6-2015 by JohnBee]
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Sulaiman
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I can imagine some possibilities
. large holes in filter
... unlikely
. the vacuum stretches parts of the filter (e.g. where the Buchner holes are) so much that the pore size increases significantly
... try reduced vacuum or support the filter on a finer support ... e.g. stainless steel mesh
. the initial stock chemicals had not completely reacted and the reaction continues after filtration
... try using a different component of the reaction as the limiting reagent
... give more time and agitation to ensure the reaction is complete before filtering
. the Buchner filter or something on the receiving side of the filter was not clean
... try pulling individual liquid components of the reaction through the filter individually to check for contamination.
..... if the filter is clean then you will not loose any valuable reagents
. particles are sneaking around the edge of the filter then under it
... check that the filter covers all of the Buchner holes with some margin around the edge and that the filter is not so large that it is bent up at
the edges by the wall of the Buchner
... examine the filter after filtration to see if there is any evidence of channels around the underside edge of the filter
[Edited on 14-6-2015 by Sulaiman]
[Edited on 14-6-2015 by Sulaiman]
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macckone
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Another possibility is that the filtrate has really fine particles that are
coagulating after filtration. This is classic with substances with low
solubility. One of the best examples is calcium carbonate which causes
all kinds of filtration issues such as clogging pores and particles coagulating
after filtration.
It might help to know what the reactants, products and solutes are.
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JohnBee
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Thanks alot guys.
I decided to take a break to tear-down my system and sanitize it before trying again.
I went through too many filters already and it just didn't look like I was making progress.
Needless to say, filtration is proving to be far more challenging that I had anticipated.
As for the possibilities, I'm leaning toward the apparatus itself.
My guess is that there must be some contaminants in the system itself(in the funnel portion) that are contributing to the contaminants. Or should I
say, I hope this is the issue.
As the remainder may prove to a challenge to rectify. ie, the filter apparatus is glass and the pore plate is of the white crystalline type. Which is
not easily cleaned.
That said, what about the decompression phase?
When I'm done pulling the solution through the filtration system, I have to purge the flask before decanting the solution. And so I was wonder if this
could be the cause? And if so, what options I'd have to avoid that from happening. ie, I tried turning off the pump, but the system help a vacuum for
over 2 hours with only a very minimal drop in pressure. :/
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macckone
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It sounds like you have multiple problems.
If it is maintaining vacuum after the pump is shut off
then you are clogging your filter.
It sounds like you are using filter paper on top of a
pore plate. Without knowing what your are filtering
it is difficult to give recommendations.
The best cleaning method depends on the substance
that is being filtered. Pore plates are best cleaned
with either a solvent for the possible contaminant
or an acid. If you don't know what might have contaminated
the filter then you need to do a series of cleanings.
And you need very pure distilled solvents and acids.
For solvents the obvious first choice is distilled and deionized
water. Deionized only may contain non-volatile organics
although it is unlikely. I prefer distilled because I have
the equipment to produce it.
After distilled water you probably want to use isopropyl alcohol.
You need to make sure that it does not leave a residue on
evaporation. Acetone is also popular with the same caveat.
Finally you want to use a non-polar solvent with a low boiling
point. DCM is a good choice although not truly non-polar.
If that doesn't clean things up then resort to acids.
Nitric is probably the best as most nitrates are soluble.
Note that you don't want to use this if your contaminant is
primarily an organic that is easily nitrated. You need to use
something else. Next up is hydrochloric, but if your contaminant
falls in the class of metal compounds that have insoluble chlorides
you don't want to use this. Finally, pirhana solution. This stuff
is nasty so it is a last resort. It also forms insoluble sulfates
with both organics and inorganics. So you really need to know
what you are dealing with before resorting to this. One weak acid
that is worth mentioning is acetic acid. Most salts are soluble
and good distilled acetic acid will evaporate without reside.
You have to distill it not the stuff in a grocery store labelled
distilled vinegar.
Things that may damage your pore plate are very strong bases
and phosphate cleaners. Mild phosphates are ok and weak bases
are ok but if you use strong ones, then expect your pore size to
change.
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