Optimization and kinetic study of esterification reaction of oleic acid using [HMIM]HSO4 as catalyst
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Biodiesel Biodiesel is an alternative fuel to petrodiesel. It can be produced from a wide range of raw materials such as vegetable oils and animal fats. Yet, the use of sources that do not compete with the food market, such as waste cooking oils - which usually feature high levels of free fatty acids (FFA’s), can lead to problems in the process of biodiesel production through alkaline transesterification. Therefore, new methodologies to successfully apply acidic oils need to be developed.
Biodiesel is a promising fuel, since its similarity to petrodiesel allows the application in regular diesel engines without major changes, while still promoting a reduction in environmental impacts caused by fossil diesel. Meanwhile, the employment of high quality feedstock, for instance the usual edible oils, has disadvantages, such as competition with the food market. The replacement of those oils by waste cooking oils (WCO) brings major advantages, amongst them, the reduction of the costs associated to the feedstock and the value added to a material usually considered as waste.
The current work, proposes an optimization study using a Response Surface Methodology (RSM), focusing on two responses: (i) the conversion of oleic acid (OA), used as a model compound to simulate the FFAs present in the WCO, estimated on the acid value drop between initial OA and the biodiesel samples produced, determined according to EN 14104 and (ii) the FAME content, measured according to EN 14103. The chosen design was Box-Behnken Design (BBD), with 4 variables (time, temperature, molar ratio methanol/oleic acid, and catalyst dosage) and 3 levels, leading to 27 experimental runs. The optimization was followed by a kinetics study, where time, molar ratio and catalyst dosage remained constant and set to optimal conditions, and the temperature was varied from 70 to 110°C. The acid value of the samples collected throughout the reaction was measured, and the conversion was calculated, allowing the establishment of the reaction evolution with time, which was used to predict the respective activation energy and the pre-exponential factor, according to Arrhenius theory.
