Cattle fat valorisation through biofuel production by hydrogenation in supercritical carbon dioxide

The use of residues or wastes in biofuel production is a potential solution to the questioned increase of the food market price. The hydrogenation (more specifically hydrodeoxygenation) of residues or wastes rich in free fatty acids seems to be an interesting option for the valorisation of this type of feedstock. The major obstacle of this process is the high temperature and high H₂ pressure process requirement. The way to reduce it is the use of a co-solvent e.g. supercritical fluids. At supercritical conditions, CO₂ is completely miscible with H₂ and at the same time is also a great solvent for non-polar liquids allowing enhanced efficiency of the hydrogenation leading to the ability to perform reactions at milder conditions. This work deals with the heterogeneous hydrogenation of cattle fat in the presence of CO₂. Total reaction pressures up to 140 bar at 340 °C and the residence time from 1 to 6 hours as well as the reuse of catalyst were examined. Depending on the chosen reaction conditions a diverse end products can be obtained. The CO₂ presence influences positively the concentration of hydrocarbons minimising the presence of glycerides' content in the final product mixture. At elevated CO₂ pressure, a high concentration of hydrocarbons simultaneously with a low presence of free fatty acids and glycerides' content in the mixture was obtained. A similar effect was observed for reaction carried out for 6 hours, when above 90 wt% of all hydrocarbons present in the mixture were the saturated ones. Additionally, the reuse of catalyst increases the saturated hydrocarbons' content to values similar to those obtained for reactions with prolonged residence time. In the CO₂-assisted reactions, the diesel-like hydrocarbons were formed with at least two fold higher content than in reactions without CO₂.