CONICET | Buscador de Institutos y Recursos Humanos

The elimination of free fatty acids and water from biodiesel is usually performed in the industrial practice using different units for neutralization with caustic, washing of the latter and drying of the fuel. Adjustment of the acidity can however be performed in only one operation by using bleaching tanks and commercial silicas as adsorbent. In this sense the current article explored the use of several TriSyl commercial silicas (types 3000, 450 and 300B), diatomaceous earth and impregnated activated carbon, and varying operation conditions (temperature, vacuum level, residence time) for the removal of free fatty acids from commercial biodiesel fuel. It was found that silica TriSyl 3000 (Grace Co.) was the best performing adsorbent, with a capacity for removal of free fatty acids of about 1 g g-1 at high values of biodiesel acidity Generally speaking the two factors mainly influencing the capacity for adsorbing free fatty acids are the temperature and the residual water content of the silica. The latter depends both on the temperature and the vacuum level of the pretreatment step. The variation of both the temperature and the vacuum level indicates that water sorption is mainly determined by the vacuum level while the temperature has a smaller effect. The effect of temperature was that expected from a thermodynamic point of view, i.e. higher temperatures decreased the adsorption capacity. It was also found that the adsorption curves were linear in the range of interest (0-2% acidity) and hence Henrys law could be used: Q=H CFFA. Values of H of 30.0-47.6 (dimensionless) were fitted in the case of TriSyl 3000, the best performing silica. H was decreased at higher temperatures and according to vant Hoffs rule the heat of adsorption was about -5.7 kCal mol-1. Water removal was found to be a strong function of the vacuum pressure. An efficient surface silica dehydration was necessary in order to prevent the occlusion of surface sites for free fatty acid adsorption. FFA uptake over TriSyl silicas in vacuum was 3-4 times larger than that obtained at atmospheric pressure. From a kinetic point of view FFA adsorption is rather slow despite the small diameter size of the particles used. The system was found to be highly constrained by the intraparticle mass transfer resistance. This resistance was attributed to a working mechanism of surface diffusion with a diffusivity value of about 10-15 m2 s-1. The system could be modeled by a linear driving force model with an overall coefficient K=0.013-0.035 min-1. The highest adsorption rate was obtained at 70 °C and 160 mmHg and corresponded to K=0.035 min-1. The bleaching procedure with silicas adjusts the biodiesel acidity level without adversely modifying the water content. The very slow kinetics of adsorption hinted the use of a countercurrent liquid-solid mode of operation. However, simulation of 2 and 3 serial bleachers working in countercurrent mode revealed that in the case of the linear adsorption isotherm no advantage can be got from this operation mode. Only for systems with a square irreversible or Langmuir-Freundlich isotherms this operation mode is convenient. FFA adsorption over silica, as depicted by Henrys law for adsorption is then better implemented in common tank bleachers working alone or in parallel.