Electrochemical Conversion of Fatty Acids to Produce Green Diesel

Electrochemical conversion of fatty acids to produce green diesel (aka renewable diesel)

We are developing an electrochemical process to convert fatty acids to green diesel. Under appropriate conditions of temperature, current density, solvent mixtures etc., free fatty acid salts can be decarboxylated in a specialized electrolysis cell. The resulting alkyl fragments form hydrocarbons with a carbon chain shortened by one atom, producing C₁₅ and C₁₇ hydrocarbons from typical, lipid-based fatty acid feedstocks.

The chemistry is relatively straightforward. An unsaturated fatty acid, e.g. oleic acid, subjected to this process is transformed into unsaturated C17-hydrocarbons:

CH₃(CH₂)₇CH=CH(CH₂)₇COO^- → CH₃(CH₂)₇CH=CH(CH₂)₅CH= CH₂ + CO₂ + H⁺ + 2e^-

Traditional refining and upgrading processes, such as “hydrotreating”, are commonly being run at 300-500 Celsius as well as thousands of psi of pressure. The process under development permits the stereospecificity (i.e. cis-double bonds) in oleic and other unsaturated acids to be retained. This contributes to the excellent low-temperature performance of the hydrocarbons so produced. Thus, this fuel has properties that easily meets or even exceeds those of fossil-derived diesel fuel.

As shown in the equation above, the process also produces molecular hydrogen as well as carbon dioxide as byproducts. Hydrogen is a valuable byproduct, useful in other areas of a biorefinery, or directly as a fuel or salable commodity. The carbon dioxide can be easily separated and used as is, or fed back to an algae colony or other CO₂-consumer.

The talk presents results from experiments with a prototypical reactor, that has processed pure oleic acid, palmitic acid and commercially available mixtures to produce high quality hydrocarbon fuel products. The effects of pH, temperature, and current density are evaluated.