Considering all the above-mentioned statements, making biofuels through bio­feedstocks refining can be an appealing alternative. However, the co-processing of triglyceride-based biomass in a refinery is necessary to enforce several previous studies. The stability of refining streams in the units and conditions of a refinery are well known, as well as the compatibility with the materials of the different systems. Nevertheless, this behaviour is unknown with pure vegetable oils or animal fats streams and their mixtures with petrol feedstocks. Stability problems during their storage might occur as a consequence of their low thermal and oxidative stability, and corrosion might arise from the free fatty acids that contain vegetable and residual oils and animal fats. Storage conditions can lead to density, viscosity or acidity changes, which might affect the processing of renewable materials (vegetable oils and animal fats) in the FCC unit (Geller et al., 2007). For example, in the FCC unit, viscosity of the sample is very important to control its vaporization. Likewise, potential polymers formed under storage conditions of vegetable oils and animal fats could lead to the deposition of gums in the tubes of the heat exchangers and the transfer lines prior to the FCC unit. Moreover, although acidity of vegetable oils or animal fats has a different origin compared to the acidity of oil products (the first one is referred to free fatty acids and the second one to naphtenic acids), acid limitation for refining streams (1.5 mg KOH/g approximately) (Humphries and Sorell, 1976; Piehl, 1988) might be a problem if free fatty acids cause corrosion. Corrosion problems associated with the mentioned free fatty acids of oils and fats are not important in the reaction section of a refinery unit, as the acids react rapidly because of the high temperatures reached. However, it cannot be said the same with the parts of the unit upstream the reactor as storage system where free fatty acids are intact. However, these issues have been poorly addressed in the literature.

We have recently studied the storage stability and corrosivity of a petrol feedstock and renewable materials mixtures under high temperature similar to that found in feed lines and heat exchangers prior to the FCC reactor (Melero et al., 2010a). Precisely, a low-saturated vegetable oil (soybean oil), a highly saturated vegetable oil (PO), animal fat unfit for human consumption and waste cooking oil were selected, whereas vacuum gasoil, hydrotreated vacuum gasoil and atmospheric residue were taken as petrol feedstocks. Storage stability studies were performed by means of an accelerated oxidation process in the presence of oxygen at 140°C according to the UOP 174-84 method (UOP, 1984). Physical properties as well as distillation curve of the samples studied were statistically unchanged after oxidation treatment. Likewise, water and/or sediment content in the samples were not evidenced after thermal treatment. Hence, according to the UOP 174-84 method, the different mixtures can be considered stable in storage at 77°C for periods of at least 180 days. Corrosion studies were also carried out following the UOP 174-84 method slightly modified by the presence of a carbon metal probe ASTM A 293 Gr C. The leaching of metallic species was monitored after thermal treatment. The results showed a negligible leaching of metallic species for pure petrol samples as well as for their mixtures with renewable materials. Hence, this preliminary study opens up good perspectives for the co-processing of triglyceride biomass feedstocks in the existing infrastructure of petroleum refineries, although further studies must be performed in the future.