Biodiesel Transesterification

Biodiesel is an alternative diesel fuel. It is made from renewable resources such as vegetable oils and animal fats. Being made from renewable esources, t has a carbon neutral footprint. Vegetable oil is too thick to flow through modern diesel engines without causing damage, so we can lower it’s viscosity through a process called transesterification.

Transesterification is the chemical process which replaces one type of alcohol for another in an ester. An ester is made by combining an alcohol with an acid.

Vegetable oil is an ester of glycerol with long chain fatty acids. The formula for vegetable oil is C3H5(RCOOH)3, with the fatty acids represented by RCOOH attached to a glycerol (C3H5(OH)3) molecule. Examples of fatty acids are Stearic acid, Palmitic acid, Linoelic acid, and Oleic acid. There are many different fatty acids found in vegetable oils and animal fatty acids containing from 12 to 20 carbons.

Methanol (CH3OH) is used to replace glycerol (C3H5(OH)3). Glycerol has three sites upon which fatty acids can be attached, while methanol has only one. Therefore, three moles of methanol are needed for every mole of vegetable oil.

A strong alkali is used as a catalyst to break apart the fatty acids from the glycerol. We typically see Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH) used as the catalyst in homebrewing. In commercial production we typically see Sodium Methylate (CH3NaO) dissolved in methanol used as the catalyst.

The chemical formula for biodiesel transesterification is: C3H5(RCOOH)3 + 3CH3OH <-> 3RCOCH3O + C3H5(OH)3

The biodiesel transesterification process is slightly reversible making it difficult to get 100% conversion. To push the reaction to it most complete status we use LeChâtelier's Principle and offset the reactants to drive the reaction in a more favorable direction. The formula above calls for 3 moles of methanol for every mole of vegetable oil, but we double that to six moles of methanol to take advantage of LeChâtelier's Principle. After we convert from moles to volume we end up with 1 part methanol to 5 parts vegetable oil.

We will use NaOH in our recipe as the catalyst. Recommendations range from 0.3% to 1% depending on who is making the recommendation. Commercial production tends to be higher than homebrew. We will use 0.6% or 5 grams of NaOH per liter of oil. If the oil contains free fatty acids, then we add enough extra NaOH to neutralize the acids as determined by a titration test.

The simplest biodiesel recipe is:
Mix 5 grams of NaOH with 200ml of methanol.
Stir until the NaOH is completely dissolved.
Heat 1 liter of new vegetable oil to 55C.
Mix the methanol with the vegetable oil.
Continue to stir for 15 minutes while the transesterification reaction takes place.
Allow the glycerin to settle out for 2 hours.
The stuff on top is raw biodiesel. It needs to be cleaned before it can be used.

For a more in depth discussion of how to make biodiesel at home, please visit www.make-biodiesel.org