In order to remove water content from waste fryer oils, I have been
considering a combined heat/vacuum system.
I have been wondering if a standard electric water heater vessel can
handle a vacuum of ~24" of mercury vacuum...anyone tried this yet?
The boiling point of water is around 140degF at ~24" of mercury...
In this way...:
1. Fill water heater with filtered oil
2. Heat to 140DegF
3. Meanwhile draw 24" vacuum
4. Allow water to boil off, until moisture indicator (as used by HVAC
contractor) shows no water vapor content in vacuum vapor output.
5. Release vacuum
6. Allow oil to cool a few degrees (below methanol boiling point)
7. Start biodiesel reaction (or transfer for use as WVO fuel)
I am aware that drawing the vacuum uses energy, it just seems that
heating oil produces quite a bit of wasted energy, as the heat
eventually is just dissipated (hence wasted), so using a vacuum to
reduce the amount of heating MAY be a more efficient way to go...?
thoughts?
I experimented with vacuum drying a few years ago. While it should work
more efficiently than simply heating the oil above 212 degrees F., some
devilish details get in the way of implementing the elegant theory.
Water in vegetable oil can exist as free water, which will eventually
settle to the bottom of a vessel; as suspended droplets, which may
settle if the oil is heated, or the droplets are coalesced; and as water
in solution with other impurities in the oil. Free water is the easiest
to remove. The droplets are removed most efficiently by coalescing and
draining. Suspended droplets that cannot be coalesced and water in
solution are more problematic.
Boiling off the water is more difficult than it appears on the surface.
Colligative properties of solutions (and some mixtures) can make make
removal of the last traces of water almost impossible. Water mixed with
oil will not boil at the same temperature and pressure as pure water.
As water is removed, more heat or lower pressure will be required to
remove more water. If the oil contains salts or semi-soluble fatty
acids, distillation is even more difficult.
As to equipment used in vacuum distillation, here are a few pointers:
1. No air leaks can be tolerated. None.
2. Vacuum pumps that do not use oil are best.
3. Insulate the vessel well.
4. Heat input should closely match condensation rate.
Good luck.
Dale Scroggins
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