Optimization of Waste Cooking Oil Biodiesel Production Using Eggshell-Derived CaO Heterogeneous Catalyst

Abstract

Waste cooking oil (WCO) is an inexpensive lipid feedstock for biodiesel production, but its variable free-fatty-acid (FFA), water and impurity contents complicate conventional alkaline transesterification. This methodology paper proposes an optimization framework for converting pretreated WCO into fatty acid methyl esters (FAME) using a reusable heterogeneous calcium oxide (CaO) catalyst prepared from waste eggshells. The procedure integrates feedstock conditioning, catalyst calcination and characterization, transesterification, Response Surface Methodology (RSM), product purification, fuel-property testing and catalyst reusability. Methanol-to-oil molar ratio, catalyst loading, reaction temperature and reaction time are selected as the principal independent variables. A Box-Behnken design is recommended to quantify linear, quadratic and interaction effects while minimizing experimental runs. FAME yield is the primary response, supported by acid value, density, kinematic viscosity and GC-based ester-content measurements. Literature up to 2021 shows that eggshell-derived CaO, modified CaO, bifunctional solids and supported heterogeneous catalysts can achieve high conversion while simplifying catalyst separation and reducing aqueous waste [1–7]. A literature-aligned illustrative response surface predicts an optimum near a 12:1 methanol-to-oil ratio, 3 wt% catalyst, 60 °C and 120 min, giving approximately 95% FAME. The proposed workflow offers a reproducible route for laboratory optimization and a basis for scale-up studies in circular-economy biodiesel production.