Making Bio-diesel

Adapted from procedure provided by Dr. Victoria Collins

Warren Wilson College

Asheville, North Carolina

Background

(Note: The project has been adapted from the American Chemical Society’s publication, Introduction to Green Chemistry) Vegetable oil, like any natural fat, contains three large hydrocarbon rich parts called fatty acids. Typically fatty acids contain 12 to 20 carbons. These three fatty acids are joined to a small glycerol molecule, making a triglyceride. These triglycerides are huge molecules. A typical triglyceride formula would be C₅₇H₁₀₀O₆, with a molecular weight of 880. Such large molecules are viscous and not very volatile: they burn but are difficult to use as liquid fuels.

The process of making biodiesel converts the triglyceride into three smaller parts, called methyl esters of the fatty acids, and a small waste product, glycerol. The smaller methyl esters are easier to pump and burn than the original triglyceride. Three moles of methanol are required for each mole of triglyceride. Essentially, methanol replaces glycerol on each fatty acid. This replacement process, called transesterification, is very slow in the absence of a catalyst (recall the definition of a catalyst). The simplest catalyst is NaOH, also called lye. Water is highly detrimental to the transesterification process, so we use a very concentrated solution of NaOH as the catalyst. Please note that concentrated NaOH is a seriously dangerous chemical, burning skin and capable of causing blindness. We will remove the catalyst in the last step, and the assistants will dispose of it responsibly.

Procedure

SAFETY NOTES:

WEAR SAFETY GLASSES AT ALL TIMES!

SODIUM HYDROXIDE USED IN THIS EXPERIMENT CAN CAUSE BLINDNESS.

Place 25mL of vegetable oil into a 50mL conical vial or centrifuge tube. Carefully add 4.5mL of methanol to the vegetable oil. You should see a small layer on the top. Use a small plastic dropper to slowly add 0.25mL of 9M NaOH to the methanol/vegetable oil mixture. Add a few drops, mix, wait a few minutes, add a little more, etc. Stir or swirl the mixture for ten minutes (this is important: make sure you mix for ten minutes. Mix gently: you don’t need to shake vigorously.) Allow mixture sit and separate (this may take up to an hour or, even, overnight).

When the mixture has cleared, you should find two layers: a big layer on top, containing the biodiesel, and a small layer on bottom of waste glycerol and NaOH. Record the volume of each layer by reading the graduations on the side of the vial. Carefully remove top layer with a large plastic pipette, into another centrifuge tube, placing the the waste layer in the proper container in the fume hood.

Wash the biodiesel product with 2.5mL of deionized water, and mix. Allow the mixture to sit and separate. The bottom water layer contains most of the NaOH catalyst. Carefully remove top layer containing clean biodiesel, using a plastic pipette, into the third centrifuge tube. Measure the amount of biodiesel that you have collected, and compare to the amount of vegetable oil you started with.