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Why Pre-treat Oil?
When dealing with WVO from restaurants, the quality of oil as it is added to the processor is a major factor in your ability to make biodiesel. If the oil has excessive water or excessive Free Fatty Acids (FFA) it will make lots of soap and not much biodiesel.

If you have water in the oil, the lye and the water combine with the oil to make soap. The oil that’s left does not have enough lye left in it to convert to biodiesel. How much water is too much? The consensus seems to be about 5% to 10% water is enough to cause serious problems with the base only process.

The Base only process does not convert FFA into usable biodiesel. It instead reacts with the lye to form soap. The problem is that this side reaction also creates water that makes more soap. If you have too much FFA then you end up starving the reaction for lye and not getting conversion.

What methods are in use by Homebrewers to pre-treat the oil?

  • Selection – Take the best and leave the rest.
  • Time – Let it settle and only use the good oil that floats to the top.
  • Filter – Remove the crud and you remove the water.
  • Heat – Drive the water out quickly.
  • Vacuum – Dry it using vacuum.
  • Acid Esterfication – Lowers the FFA content.
  • All or some of the above.

Test the oil before collecting. Three tests are used to determine what is the best:
1. Visually inspect the oil for water, crud, color and dead animals.
2. Titrate before collecting and take only oil that titrates less than 2 or 3 g/l.
3. Use a portable frying pan to heat a sample of oil to check for water.

Let the oil sit for weeks or months in an open topped container in warm dry environments. As the oil on top clears, use it to make biodiesel.

Particulate matter like old french fries and batter absorb water like a sponge and help keep large amounts of water suspended in the oil. Examples of filters are screen wire mesh, cloth (old blue jeans and shirts), bag filters, coffee filters. Filter too fine and the filter clogs quickly, filter too coarse and leave stuff in the oil. If the temperature is low the oil may turn to solid and be difficult to filter.  more on filtering WVO…

Heat the oil and boil the water off. This is sometimes done in the reactor or using a “heat plate.” Heating also tends to break the emulsion causing the water to drop to the bottom of the oil quickly. Mixing the oil and water are needed to prevent violent boiling. Using heat alone usually requires high temperatures and so much energy that it is not often recommended.

Vacuum can be used to remove the water, but vacuum drying systems are not usually used by homebrewers because of the complexity and cost. They can be a good fit for Co-ops and Fleet Scale producers.

Acid Esterfication
If the oil is really high in FFA, adding an acid esterfication phase to the processing will convert some of the FFA in to biodiesel. This subject is really too complex to cover here, but it does deserve mention.

Heat/Filter – Sometimes light heating is needed in order to get the oil to flow through a filter.
Heat/Time – Light heating can break the emulsion of oil and water allowing settling to work. Filter/Time – If the oil is liquid, settling before or after filtering is sometimes used. Selection/Heat – Pick oil low in FFA and boil off the water.

Whatever method you use, pre-treating the oil will result in more predictable reactions and improved consistency.