This study focuses on the design, optimization, and evaluation of a biodiesel production process involving the transesterification of waste cooking oil (WCO) using a heterogeneous calcium oxide (CaO) catalyst derived from waste eggshells. The work is divided into two main parts. The first focuses on the laboratory preparation, characterization, and performance of the CaO catalyst, while the second translates the experimentally optimized conditions into a process-scale model using Aspen HYSYS to assess industrial feasibility. Waste eggshells were cleaned, dried, ground, and calcined at high temperature to produce the CaO heterogenous catalyst. The catalyst was characterized by Simultaneous Thermogravimetric-Differential Scanning Calorimetry (TG-DSC) and Fourier Transform Infrared Spectroscopy (FTIR). Transesterification experiments were conducted in a batch round-bottom flask reactor where CaO was added to sunflower oil and methanol, and multiple operating parameters were varied to determine the optimal conditions. The catalyst exhibited its best performance after calcination at 900 °C for 2 h. A maximum biodiesel yield of 95 wt.% was obtained at a methanol-to-oil molar ratio (MOMR) of 9:1, reaction time of 2 h, stirring speed of 700 rpm, temperature of 60 °C, and catalyst amount of 3 wt.%. In addition, the eggshell-derived CaO catalyst maintained a biodiesel yield close to 95% over three consecutive reuse cycles, demonstrating good reusability and catalytic stability. The produced biodiesel complied with ASTM standards. Based on these results, the process was then scaled up by simulating a continuous industrial biodiesel production plant using Aspen HYSYS. The model proved practical, achieving a biodiesel purity of 99.85%. Further process optimization, including methanol recovery and heat integration, reduced fresh methanol consumption by 60% and overall energy requirement by 25%. The combined experimental and simulation results demonstrate that energy efficiency and waste valorization enable a biodiesel production pathway that is both environmentally and economically sustainable and aligned with circular economy principles and sustainable development goals.