Natural fatty alcohols are one of the major oleochemicals and can be produced by catalytic hydrogenation of fatty acid methyl esters. In the commercial multiphase process, the hydrogen availability to the solid catalyst limits the reaction rate. By adding propane to the reaction mixture, we can utilize the unique properties of supercritical fluids, properties between those of gases and liquids. Using propane, a substantially homogeneous supercritical phase is created, whereby hydrogen has complete access to the solid catalyst. At high substrate concentrations, a rapid fall of the reaction rate was observed, and the benefits of the propane addition were completely lost. This fall depends on a split of the supercritical reaction mixture into two phases (a substrate-rich and a hydrogen-rich phase). If this phase split occurred using small catalyst particles (e32 µm), the pressure drop over the catalyst bed increased sharply because the formed liquid droplets blocked the void space in the porous catalyst bed. These two phenomena were used to deduce the product and substrate solubility in the reaction mixture. The product showed the most unfavorable solubility which increased with higher pressure. Under our process conditions (150 bar, 280 °C, and 11 mol % hydrogen), a single phase was observed up to 2 mol % (i.e., 15% by mass) product. Besides the minimum pressure in the catalyst bed, substrate transport limitation could be shown to be an important factor in process optimization. Therefore, egg-shell catalysts or fine catalyst particles (100-300 µm) should preferably be used in the continuous supercritical reactors.

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Cis and trans isomers of fatty acid methyl esters, fatty alcohols, and triacylglycerols were analyzed with a silverion high-performance liquid chromatography system. Gradients of n-heptane and acetonitrile were used to elute molecules with up to nine cis double bonds. The analyses were as fast and reliable and had a resolution similar to that of the best published analyses. However, published analyses were performed with chlorinated solvents, and these solvents are carcinogenic and mutagenic. The solvents we used, heptane and acetonitrile, are less dangerous to the analyst.

Extremely rapid hydrogenation of fatty acid methyl esters (FAME) to fatty alcohols (FOH) occurs when the reaction is conducted in a substantially homogeneous supercritical phase, using propane as a solvent, over a solid catalyst. At these conditions, the limitations of hydrogen transport are eliminated. At temperatures above 240°C, complete conversion of the starting material was reached at residence times of 2 to 3 s, which is several orders of magnitude shorter than reported in the literature. Furthermore, formation of by-products, i.e., hydrocarbons, could be prevented by choosing the right process settings. Hydrogen concentration turned out to be the key parameter for achieving the above two goals. As a result of the supercritical conditions, we could control the hydrogen concentration at the catalyst surface independently of the other process parameters. When methylated rapeseed oil was used as a substrate, the hydrogenation catalyst was deactivated rapidly. However, by using methylated sunflower oil, a catalyst life similar to that obtained in industrial processes was achieved. Our results showed that the hydrogenation of FAME to FOH at supercritical conditions is a much more efficient method than any other published process.