Glycerol removal through dry purification of biodiesel produced from waste cooking oils using biomass-based adsorbents

Mestrado de dupla diplomação com a UTFPR, Universidade Tecnológica Federal do Paraná

The great demand for renewable energy sources has increased interest in biofuels in particular, with biodiesel being a viable alternative for replacing conventional diesel. Biodiesel is usually produced using vegetable oils, including waste cooking oils (WCOs), through a transesterification process. However, this process generates by-products such as glycerol, which needs to be removed to guarantee the quality of the biodiesel. Purification is mostly done by wet washing, which generates large quantities of effluent. A possible alternative is the dry purification process, namely through adsorption which can be carried out using natural adsorbents derived from agro-industrial waste. This study therefore evaluated the removal of glycerol from biodiesel using adsorbents produced from several sources of agro-industrial wastes such as almond shells, rice husks, olive pits and walnut shells. For this purpose, WCO was collected, filtered and characterised to analyse acidity, free fatty acid profile and density. Biodiesel was then produced via transesterification by the ethanolic route, using a ratio of 1:7.5 oil/alcohol and 0.5 wt% of sodium hydroxide as a catalyst, at 30 °C. The content of 0.16 wt% of free glycerol in crude biodiesel was determined, and purification was necessary to meet the established standards specification, which require values below 0.02 wt%, according to EN 14105/14016 and ASTM D6584 norms. The adsorbents were used in natural form and with physical activation at 800 °C for 60 minutes, totalling eight materials for testing. Analyses were carried out to determine the point of zero charge, Fourier transform infrared spectroscopy, textural properties, thermogravimetric analysis and scanning electron microscopy. A preliminary test was carried out with 3% adsorbent at 25 °C for 360 min, and the natural materials showed the best removal performance. Hence, ricehusk, with 54% removal, and walnut husk, with 51.03%, were chosen to carry out the adsorption studies. The kinetic studies were done at three temperatures (25 °C, 35 °C and 45 °C), reaching equilibrium in approximately 480 min. The results indicated that the pseudo-second order model best described the process, suggesting that adsorption was controlled by chemical interactions between glycerol and the active sites of the adsorbent. The final concentration of free glycerol was approximately 0.05 wt% a temperature of 45°C. The adsorption isotherms were carried out by varying the adsorbent concentration between 3% and 10% at 45 °C for 480 min and showed that the data best fitted the Freundlich model, indicating a heterogeneous surface and the formation of multiple adsorbate layers. Finally, even though the materials did not allow the achievement of the value established by the standards, the studied adsorbents revealed great potential for in glycerol removal from biodiesel. Further studies are needed to: optimise the adsorption conditions, namely regarding temperature, add multiple steps in the adsorption process, or combine commercial and natural adsorbents.