Waste Eggshell-Derived Heterogeneous CaO Catalyst for Sustainable Valorization and Response Surface Optimization of Castor Oil Biodiesel Production

An eco-friendly and cost-effective route for castor oil biodiesel production was developed using a waste-eggshell-derived heterogeneous CaO catalyst. Response Surface Methodology optimized four parameters: methanol-to-oil ratio, catalyst loading, reaction time, and temperature, yielding optimal conditions of 10.8:1, 2.9 wt.%, 57 min, and 61℃, with a predicted yield of 90.2%. The produced biodiesel exhibited favorable fuel properties, including a higher calorific value of 39.73 MJ/kg, flash and fire points of 192℃ and 203℃, density 877.04 kg/m3, specific gravity 0.877, acid value 0.45 mg KOH/gm, dynamic and kinematic viscosities of 7.8 cP and 9.13 mm²/s, pour point −7℃, and cloud point 3℃, satisfying biodiesel quality requirements. The solid catalyst simplified separation and reduced environmental burden, demonstrating the viability of biowaste valorization and circular economy integration. Further investigations will focus on catalyst life-cycle assessment, engine performance, pollution mitigation, and pilot-scale continuous reactor validation.