Easy and Efficient Drying of Molecular Sieve and Silica Gel
Everyone probably has had the experience of working in a lab and
being told the importance of that dry solvent and yet the bottle
with the magical molecular sieve at the bottom just gets topped up
and no one can seem to remember it ever being regenerated, right?
Maybe it's time to regenerate those pellets!
3A zeolite ceramic molecular sieve pellets used for solvent
drying in the EM lab can only absorb so much water and then need
to be heated to 250C to regenerate by driving off the water.
Most lab ovens are rated 1 to 2 kW and barely get to 250C. This
is both marginal for drying and wastes a lot of energy to bake
out a small volume of sieves. The baking is very easily
accomplished using a heating mantle driven by a Variac (Variable
AC Autotransformer) or some other power control device. Every
lab/department seems to have both of these items lurking in back
corners. For the 380 Watt Heating Mantle shown, the Variac set
to "60" gives the right temperature; measure once with a
thermocouple placed in the molecular sieve to find the proper
setting and it should remain the same for that heating mantle..
- Air dry the molecular sieve from empty solvent bottles in
a fume hood draft until they appear dry before heating at
elevated temperatures. While the sieves bind much water, they
also hold some solvent and you don't want to heat the pellets
very hot with a lot of solvent present. (Fire hazard.)
- Operate the unit in a fume hood and make sure
that no ignition sources are present and no solvent
containers are anywhere near the heating mantle.
- The apparatus can be used for drying silica gel or
molecular sieves. Indicating silica gel gives an easy
indication of dryness when it turns very dark blue.
- The aluminum cups shown fit the heating mantle nicely;
they came from an old camping mess kit. A single or double
layer of heavy aluminum foil would do as well. Aluminum melts
at 660C so there is little possibility of melting it.
- Heat the drying agent at 250C for about 2 hours in a
shallow layer (not over a few cm deep) with a cover that lets
water escape (but retains much heat).
- Use tongs to transfer the cup with hot sieves to an glass
dessicator with a porcelain support to allow cooling without
much atmospheric water uptake.
- When cool, pour into a bottle that will then receive the
solvent. Glass bottles with "Poly-Seal" lined closures are
very good and dry sieve can be stored in them and solvent
added later. If not bottled, dried sieve can be stored
in tight "paint cans" like the cans used to ship OsO4.
- It is good to gently agitate the solvent/sieve (a
lab rocker table is ideal) for a while after combining, but
then let it stand and settle overnight before using and do not
further disturb the sieve layer so as not to resuspend any
fine ceramic particles that may be present - they could stick
to samples and cause damage to sectioning knife edges. Gently
decant, or else pipette solvent from near the top with a clean
Heating Mantle with cover
Heating Mantle showing aluminum cup with silica gel
A "Variac", or Variable AC autotransformer to control input power to
the heating mantle. The heating mantle shown above required a
setting of "60" (about half of line voltage) to reach 260C.
Dale A. Callaham