Transesterification of Rubber Seed Oil to Biodiesel over a Calcined Waste Rubber Seed Shell Catalyst: Modeling and Optimization of Process Variables

In the present study, waste rubber seed shell (RSS) obtained from our previous study was investigated as a plausible solid base catalyst for the transesterification of esterified rubber seed oil (RSO) to biodiesel. TGA, XRF, XRD, SEM, and N-2 adsorption/desorption analysis (BET) were used to characterize the catalyst. Central composite design (CCD) was employed to design the experiments conducted to study the influence of the process variables (reaction time, methanol/oil ratio, and catalyst loading) on biodiesel yield. Response surface methodology (RSM) technique, was used to optimize the process, and the quadratic model developed was statistically significant with F-value of 12.38 and p-value (<0.05). The optimum conditions obtained from RSM are as follows: reaction time (60 min), methanol/oil ratio (0.20 vol/vol), and catalyst loading (2.2 g) with a maximum biodiesel yield of 83.11% which was validated experimentally as 83.06 +/- 0.013%. Reusability test of the catalyst at optimum conditions shows that the biodiesel yield was over 80% after fourth cycle of usage and the leached Ca2+ ion content of biodiesel was 3.26 mg/kg (ppm). The ester content determined by a precalibrated gas chromatography and the oxidation stability of the biodiesel are 96.7% and 7.8 h, respectively. The characterized biodiesel complied with ASTM D 6751 and EN 14214 biodiesel standards.