I have been working along the same exact line of thinking. Tom is
probably right about stiring 5 gal of oil with a bar magnet. However
where a magnetic stirrer really shines is in mixing methoxide. Stirring
bars work great so long as they are confined to the area of the rotating
magnetic field. The easiest way to do this is to have a small motor
beneath the pot with a permanent magnet coupling to the stir bar through
the stainless. I decided to get fancy and made a motorless stirrer
using two home made electromagnets and some circuitry to create a
rotating magnetic field. Yes there is a reason those motorless stirrers
are so expensive to buy. You can make one for a lot less money if you
want to do all this work, BUT now consider that when the thing is off
there is nothing to hold the stir bar in place in the bottom of the big
mixing vessel where it needs to be to work properly. In a small beaker
the bar can not escape the field. A motor with a permanent magnet will
hold the bar in position all the time, but with the motorless type the
bar can end up anywhere in the pot when you pour in the ingredients.
The little 1/2 inch bars are great for doing titrations. In fact the
motorless stirrer I made is really only useful for this job. As an
added benefit the home made electromagnets get warm after a little while
and this is just great for warming the oil and isopropyl alcohol for the
titration. Knowing what I know now though I wouldn't have spent all the
time building the motorless stirrer. It works just as well to put the
little titration beaker into a small jar of warm water to heat it and
stir with a bar and a motor driven permanent magnet underneath.
Bottom line....don't be like me, remember the KISS principle of
engineering ( Keep It Simple Stupid) yeah, I find it necessary to teach
myself that one from time to time....
Joe
Tom Irwin wrote:
Dear Skapegoat,
You're probably going to have difficulty scaling up to 5 gallons using magnetic
stir bars. The largest bars I have used are only 6.3 cm(2.5 inches). They can
stir a liter or 2 reasonable well but 10 times that volume I have my doubts if
it will mix well enough. You would have to extend your stirring times at the
very least. You could also baffle your reactor for more turbulent mixing.
Bigger bars and more powerful magnets may help but I have not found a source
for these as yet. In thick fluids mag stirrers have a tendency to disengage and
sort of whirl around the outside of the beakers I've used. Pumping may be the
best alternative.
Tom Irwin
Has anyone created a home made magnetic stir plate? I am scaling up
from lab scale 100 mL batches to a 5 gallon reactor, and have some ideas
for a homemade magnetic stir plate in lieu of the more typical agitation
methods, and I wanted to see if others have gone down this path with any
success (or failure).
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