Biodiesel Production Department Editor: Kate Torzewski Chemical Engineering©

Biodiesel can be produced from vegetable oils by three types of reactions: base catalyzed transesterification of the oil; direct acid-catalyzed transesterification of the oil; and conversion of the oil to its fatty acids, and then to biodiesel.

Biodiesel is typically produced by a base-catalyzed reaction (Figure 1). This method of production has several advantages, including the following: low temperature (150˚F) and pressure (20 psi) reaction that requires only standard materials of construction; direct conversion to biodiesel with no intermediate compounds; and high conversion (98%) with minimal side reactions and a low reaction time.

In the chemical reaction for basecatalyzed biodiesel production, vegetable oil is reacted with a short chain alcohol (signified by ROH) in the presence of a catalyst to produce glycerin and biodiesel. The fatty acid chains associated with the oil, which are mostly palmitic, stearic, oleic, and linoleic acids for naturally occurring oils, are represented by R', R'' and R''' (Figure 2).

Production steps

Mixing of alcohol and catalyst.
The catalyst is typically sodium hydroxide (caustic soda) or potassium hydroxide (potash). It is dissolved in the alcohol using a standard agitator or mixer. Methanol or ethanol is commonly used as the alcohol.

Reaction
The mixture of alcohol and catalyst is charged into a closed reaction vessel, and the oil is added.

The reaction mix is kept just above the boiling point of the alcohol, 160°F, to speed up the reaction, although it is sometimes recommend to run the reaction at room temperature. The reaction time can vary from 1–8 h.

Excess alcohol is used to ensure total conversion of the oil to its esters. The amount of water and free fatty acids in the incoming oil must be monitored, because if either level is too high, it can inhibit soap formation and the separation of glycerin downstream.

Separation
Glycerin and biodiesel are the two main products of reaction, with each containing an amount of unreacted alcohol. Since the glycerin phase is much more dense than biodiesel phase, the two phases can be separated by gravity in a settling vessel, with glycerin simply drawn off the bottom of the settling vessel. Alternatively, a centrifuge can be used to separate the two materials more quickly.

Glycerin neutralization
The separated glycerin contains unused catalyst and soaps, which are neutralized with an acid. Water and alcohol are removed to produce glycerin at 80–88% purity to sell as crude glycerin. Alternatively, glycerin can distilled to 99% purity or higher for selling to the cosmetic and pharmaceutical industries.

Methyl ester wash
After the biodiesel is separated from glycerin, residual catalyst or soaps can be removed with a gentle warm water wash.

Alcohol removal
Unreacted alcohol in both the glycerin and biodiesel phases is removed by flash evaporation or distillation. The recovered alcohol is then reused for mixing with the catalyst. Alcohol removal can occur after the wash and neutralization, as shown in Figure 2 to the right, but it can occur before these steps as well.

Product quality and registration
Prior to use as a commercial fuel in the U.S., the finished biodiesel must be analyzed to ensure it meets ASTM specifications. Additionally, all biodiesel produced must be registered with the U.S. Environmental Protection Agency (Washington, D.C) under 40 CFR Part 79.

References
1. Biodiesel Production & Quality Standards, July, 2008. National Biodiesel Board, www.biodiesel.org/resources/fuelfactsheets