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Prewashing is commonly used by those making biodiesel at home. It is used before regular water wash methods to decrease the chances of an emulsion, to decrease the number of washes needed, and to increase the yield of biodiesel.
A prewash halts transesterification and causes some of the methanol, lye, and glycerin suspended in the biodiesel to join the glycerin layer which decreases the soaps, thus decreasing chances of emulsion during the regular wash.
• A 5% pre wash increases the fuel yield by about 3%.
• The glycerin can be drained after only an hour or so, instead of settling it overnight.
• It can decrease the number of washes needed, and to allow for faster, more aggressive methods of washing.
• It makes it harder to remove methanol from the glycerin. After the prewash, you may need a fractionating column (which uses more energy) instead of just a pot still to
remove the methanol.
• It introduces water into the processing tank.
How to Do a 5% Prewash:
Figure out how much water you will need (which is just 5% of the volume of your batch.)
Process your biodiesel batch as normal. Then, when the batch is complete (as in you've allowed enough time for transesterification to take place), instead of turning off the mixer, add the water for the pre-wash and let it continue mixing for 15 to 20 minutes. (Yes, the water is added before the glycerin is drained.)
Allow the mixture to settle for an hour or more, and drain the glycerin.
Finish washing using one of these
water wash methods.
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