Storage stability and corrosion studies of renewable raw materials and petrol mixtures: A key issue for their co-processing in refinery units

Mixtures of renewable raw materials such as vegetable oils (crude palm and soybean oil and waste cooking oil) and animal fat unfit for human consumption with petrol feedstock were assessed for considering their co-processing in refinery units. Vacuum gasoil, hydrotreated vacuum gasoil and atmospheric residue were taken as petrol feedstocks. Ageing of these mixtures under storage conditions has been deeply studied. Significant changes in physical properties such as density, viscosity and acidity may promote some problems in the processability in the FCC unit. Moreover, potential polymers formed under storage conditions could lead to the deposit of gums in the tubes of the heat exchangers and the transfer lines previous to the FCC unit. Storage stability studies were performed by means of an accelerated oxidation process according to the UOP 174-84 method. Physical properties including density, viscosity and acidity were measured before and after oxidation treatment. In addition, distillation curve of the mixtures before and after treatment was determined by means of simulated gas chromatography (ASTM D-2887). Physical properties as well as distillation curve of samples were statistically unchanged after oxidation treatment. Likewise, presence of water in the raw feedstock and sediment content in the samples after thermal treatment were not evidenced. Hence, according to the UOP 174-84 method, the different mixtures under study can be considered stable in storage at 77 °C for periods of at least 180 days. Corrosion studies were also carried out to elucidate if the presence of free fatty acids in vegetable oils and animal fats might cause problems in storage systems of refinery units. The assays were performed 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 feedstock samples as well as for their mixtures with renewable materials.