Continuous biodiesel production in a packed-bed reactor using waste-derived catalyst: Kinetics, thermodynamics, and cost analysis

Development of low-cost heterogeneous catalysts is essential for advancing biodiesel production toward industrial implementation. Renewable solid base catalysts derived from agricultural and biogenic wastes: calcined Guanacaste pod husk ash (CGPHA), calcined banana peel ash (CBPA), and calcined chicken eggshell powder (CCESP), were synthesized and applied for biodiesel production from waste sunflower oil (WSO). Batch transesterification screening identified CGPHA as an effective catalyst, achieving ∼78 wt% biodiesel yield under the same reaction conditions. CGPHA was subsequently immobilized on pumice to enhance mechanical stability and enable continuous packed-bed reactor (PBR) operation. The supported catalyst exhibited a surface area of 1.28 m 2 g −1 and a pore volume of 0.0071 cm 3 g −1 , providing accessible mesoporosity for triglyceride diffusion and catalytic contact under flow conditions. Continuous transesterification experiments demonstrated increasing WSO conversion with temperature and residence time, reaching ∼94% at 60 °C, 90 min, and 1.6 mL min −1 . Steady-state kinetic analysis confirmed pseudo-first-order behavior with flow-rate-independent apparent rate constants, indicating intrinsic kinetic control, and yielded an apparent activation energy of 25.1 kJ mol −1 . Thermodynamic evaluation yielded apparent negative Gibbs free energy values (ΔG° = −3.50 to −7.65 kJ mol −1 ) together with a positive enthalpy change (ΔH° = 61.1 kJ mol −1 ) and positive entropy change (ΔS° = 205.2 J mol −1 ·K −1 ), confirming a thermodynamically favourable and endothermic reaction. Cost analysis estimated biodiesel production at $1.934 kg −1 and catalyst preparation at $3.385 kg −1 , indicating economic feasibility. Chromatographic analysis indicated ∼95% ester content, and the produced biodiesel met ASTM D6751 and EN 14214 specifications. • Waste-derived base catalysts were screened; CGPHA gave ∼78 wt% biodiesel yield. • Pumice-immobilized CGPHA achieved ∼94% conversion in a continuous PBR. • Apparent activation energy of 25.1 kJ mol −1 confirmed intrinsic control. • Biodiesel cost estimated at $1.934 kg −1 with catalyst cost of $3.385 kg −1 . • WSO biodiesel (∼95% FAME) met ASTM D6751 and EN 14214 standards.