Biodiesel production using supercritical methanol

The catalyst-free transesterification of oil (triglycerides) in supercritical methanol at a temperature and pressure above 239⁰C and 8.1MPa, respectively, provides a new way of producing biodiesel. High reaction temperature and pressure help to accelerate the transesterification because the supercritical methanol becomes non-polar and has enhanced contact with oil. The supercritical transesterification of soybean oil was carried out in a 170 ml volume high-pressure batch reactor. The pressure within the batch reactor was self-generated from heating the contents since no pumps were used. Transesterification of soybean oil with supercritical methanol readily produced biodiesel. Since the supercritical transesterification is carried out without a traditional acid/base catalyst and does not produce any saponified byproducts, the recovery of pure biodiesel product becomes much simpler. After the reaction, the products readily separated into two phases of biodiesel (fatty acid methyl esters) and glycerol. Process variables such as pressure, volume, and temperature were studied. Also, the effect of supercritical CO₂ as a potential co-solvent was investigated. Several 7 ml volume mini-reactors were fabricated and used to evaluate the supercritical transesterification reaction and develop an analytical method for lipid composition determination. Reaction temperature, reaction time, and the methanol-to-oil molar ratio were studied. The effect of water, free fatty acids, hexane, co-solvents, and reaction with different alcohols (n-propanol, isopropanol, allyl alcohol) were also studied. A sequential analytical process was developed by extracting lipids directly from oil seeds using hexanes and converting them to fatty acid esters, using supercritical alcohol --Abstract, page iv