Mike Pelly's biodiesel method

To determine the correct amount of lye required, a titration must be performed on the oil being transesterified. This is the most difficult step in the process, and the most critical -- make your titration as accurate as possible.

IMPORTANT: The lye must be dry -- keep it away from water, store it in an airtight container.

Make up a solution of one gram of lye to one liter of distilled water. Make sure it dissolves completely. This sample is then used as a reference tester for the titration process. It's important not to let the sample get contaminated, it can be used for many titrations.

Mix 10 milliliters of isopropyl alcohol in a small container with a 1 milliliter sample of WVO -- make sure it's exactly 1 milliliter. Take the WVO titration sample from the reaction vessel (Figure 5 #1) after it's been warmed up and stirred.

Add to this solution 2 drops of phenolphthalein, an acid-base indicator that's colorless in acid and red in base.

IMPORTANT: Phenolphthalein has a shelf life of about a year, it is very sensitive to degradation by light so after a while it will start giving erroneous readings.

Using a graduated eye dropper (with increments marked in tenths of milliliters) or some other calibrated instrument (from medical supply outlets), while carefully keeping track of the amounts, drop measured amounts of the lye/water solution a couple of tenths of milliliters at a time into the WVO/isopropyl/phenolphthalein solution.

Follow each drop with vigorous stirring of the solution. In cold weather the WVO might congeal and not work so you might need to do the titration in a heated room. If conditions are right eventually the solution turns pink (magenta), and stays pink for 10 seconds. This is the indicator color for a pH range of 8-9 (see the photograph in the left column of this page, "Color of titrated liquid sample when at the correct pH"). It's important to find the exact amount, to just reach this pH without dropping in too much!

It's a good idea to do this entire process more than once to ensure that your number is correct. I've found that depending on the type of WVO, how hot it got in the fryer, what was cooked in it and how long it was used, the amount of lye/water solution needed to titrate it is usually 1.5 to 3 milliliters. You can also use litmus paper or a digital pH tester instead of the phenolphthalein. Try it with fresh cooking oil from your kitchen too, it should need much less lye to reach pH 8-9.

The calculation

The next step is to determine the amount of lye needed for the reaction. Take the number of milliliters derived from the titration and multiply by the number of liters of WVO to be transesterified.

There is one more thing to be included in the calculation. Every liter of neat vegetable oil (fresh -- never been cooked) needs 3.5 grams of lye for the reaction. So for every liter of WVO to be transesterified add an additional 3.5 grams of lye.

Example: The titration determined that it took 2.4 milliliters to reach pH 8-9 and you'll be transesterifying 150 liters of oil.

2.4 grams times 150 liters equals 360 grams lye

Plus 3.5 grams times 150 liters equals 525 grams lye

360 + 525 = 885 grams lye

If the titration result was 1.8 milliliters to reach pH 8-9, the final amount of lye needed for the reaction would be 795 grams.

I've found over time that the number of grams of lye needed per liter of WVO has generally been between 6 and 7.

Test batches

The first few times you do this process or if you're planning on transesterifying a lot of WVO it is a good practice to first try out your lye amounts on a 1 liter batch in a kitchen blender. This works really well and you don't need to heat up the WVO too much, just enough so it will spin well in the blender. Blenders are very thorough at mixing the ingredients so heating is not as critical.

Start by mixing up the lye and methanol in a blender (one that will never be used for food again). First make sure the blender and all utensils used are dry. Forming the exothermal sodium methoxide polar molecule will heat up the blender container a bit. Keep mixing until all the lye has been dissolved.

Once the sodium methoxide is prepared, add to the blender 1 liter of WVO. Make certain all your weights and volumes are precise. If you're unsure of the titration result numbers then use 6-6.25 grams of lye per liter of used WVO, or 3.5 grams for fresh vegetable oil. Blender batches need only be run for about 15-20 minutes for separation to be completed before switching off. The settling takes some time to complete. The solution can be poured from the blender into another container right after switching off the blender.

It is good to do a few batches with varying amounts of lye recorded so later when checking results one can choose the lye quantity that did the best job.

When too much lye is used the result can be a troublesome gel that is tough to do anything with. (See Glop soap.) When not enough lye is used the reaction does not go far enough so some unreacted WVO will be mixed with the biodiesel and glycerine. This will form three levels with biodiesel on top above unreacted WVO with glycerine on the bottom. If there is too much water in the WVO it will form soaps and settle right above the glycerine forming a fourth level in the container. This layer is not too easy to separate from the unreacted WVO and glycerine layers.

4. Preparing the sodium methoxide

Generally the amount of methanol needed is 20% of the WVO by mass. The densities of these two liquids are fairly close so measuring 20% of methanol by volume should be about right. To be completely sure, measure out a half-liter of both fluids, weigh, and calculate exactly what 20% by mass is. Different WVOs can have different densities depending on what type of oil it originally was and how long it was used in the deep fryer.

Example: When transesterifying 100 liters of WVO, use 20 liters of methanol.

The methanol is mixed into a solution with the sodium hydroxide (lye), creating sodium methoxide in an exothermic reaction (ie it gets warm from bonds forming). Keep all utensils the lye comes in contact with as dry as possible.

CAUTION:
Treat sodium methoxide with extreme caution! Do not inhale any vapors! If any sodium methoxide gets splashed on your skin, it will burn you without your feeling it (killing the nerves) -- wash immediately with lots of water. Always have a hose running when working with sodium methoxide.

Sodium methoxide is also very corrosive to paints. Lye reacts with aluminum, tin and zinc. Use glass, enamel or stainless steel containers -- stainless steel is best. Used restaurant equipment supply stores and scrap metal recycling yards are two good places to look for this type of equipment. Braze on plumbing fittings for drains, etc. where needed.

5. Heating and mixing

Pre-heat waste vegetable oil reclaimed from a restaurant's waste grease barrel to 120-130 deg F (48-54 deg C).

A propeller or paint stirrer coupled to a 1/2-inch electric drill held securely in a jig works fine as a mixer.

Too much agitation causes splashing and bubbles through vortexing and reduces mix efficiency. There should be a vortex just appearing on the surface. Adjust the speed, or the pitch or size of the stirrer to get the right effect.

If you want a quieter processor, an electric pump plumbed to form a mixing loop for stirring the WVO would do a nice job. Mount the pump above the level that glycerine will gel at to prevent clogging up the pump (see below).

Add the sodium methoxide to the WVO while stirring; stir the mixture for 50 minutes to an hour. The reaction is often complete in 30 minutes, but longer is better.

The transesterification process separates the methyl esters from the glycerine. The CH3O of the methanol then caps off the ester chains and OH from the NaOH (lye) stabilizes the glycerine.

6. Settling and separation

Allow the solution to sit and cool for at least eight hours, preferably longer. The methyl esters -- biodiesel -- will be floating on top while the denser glycerine will have congealed on the bottom of the container forming a hard gelatinous mass (the mixing pump must be mounted above this level).

An alternative method is to allow the reactants to sit for at least an hour after mixing while keeping the brew above 100 deg F (38 deg C), which keeps the glycerine semi-liquid (it solidifies below 100 deg F). Then carefully decant the biodiesel.

This can be done by draining the reactants out of the bottom of the container through a transparent hose. The semi-liquid glycerine has a dark brown color; the biodiesel is honey-colored. Keep a watch on what flows through the sight tube: when the lighter-colored biodiesel appears divert it to a separate container. If any biodiesel stays with the glycerine it is easy to retrieve it later once the glycerine has solidified.

If you left the mixture in the tank until the glycerine gelled, reheat the tank just enough to liquify the glycerine again. Don't stir it! Then decant it out as above.

Figure 4 shows one example of a system for separating two liquids with different densities. This could make a good pre-filter or be used to separate oils, soap and glycerine/esters goop. Marine/boat fuel cleaning systems do a good job of filtering.

Figure 4	Figure 5

Figure 5 shows the reaction vessel (#1) where the WVO is mixed (also could be done with a pump) and heated. Heating is accomplished by (#2) a hotplate, propane-burner, or a diesel stove would work well.

Figure 6

A proposed alternative using very little electricity is illustrated in Figure 6. This system would use a furnace-type burner run on reclaimed esters to heat its reaction vessel. The vessel's stirring action is created by thermo inversion currents generated by the vessel's external cooling tubes and a baffled exhaust vent that runs up through its center.

Figure 5 also shows a blender (#3) used to mix up the sodium methoxide. When making 16 liter (5 gal) batches, I use a yard-sale glass blender for the sodium methoxide solution (and for nothing else!), but I can't fit it all in at once, so I measure out three separate portions.

Glycerine