Process optimisation and techno-economic analysis of vacuum ultraviolet photolysis for ethylene removal in apple storage

Abstract

The nutritional values of apples (Malus domestica) is renowned globally. Apples are known to contribute antioxidants, fibre, vitamins, and essential nutrients to the human diet. During storage and transportation, apples remain metabolically active, producing ethylene, a gas responsible for accelerating ripening and senescence, thus contributing to food waste and economic losses. Therefore, the removal of ethylene in overhead atmosphere during transportation and storage is of utmost importance to preserve their shelf-life and the reduction of food waste. Various techniques have been implemented to ensure the mitigation of ethylene effects during the post-harvest management of fruits and vegetables; however, due to limitations such as rapid saturation, high operational cost, toxicity, additional waste management requirements, and the frequent need for replacement render these conventional methods unsuitable for extended storage. This study explored vacuum ultraviolet photolysis (VUV) as a promising technology for the removal of ethylene in storage atmosphere. To more accurately represent the process, two complementary modelling approaches were utilized. The first model focused on the fundamental chemical and physical mechanisms that govern ethylene degradation, while the second model examined how different process parameters in the storage system influence performance. Combined, these approaches offered both mechanistic insight and practical relevance. The first model was formulated using a coupled mass and energy balance with ethylene degradation described as a first-order reaction. To assess model sensitivity, key kinetic parameters, including the pre-exponential factor (k₀, 0.10 and 1.00 s⁻¹) and activation energy (Ea,15,000 to 25,000 J·mol⁻¹) were varied. This model achieved high predictive accuracy (R²=0.988, RMSE=0.0572), closely reproducing experimental concentration decay and temperature rise. However, the first model did not incorporate relative humidity (RH) effects. To address this, a second percentage ethylene removal (PER) model was constructed from a Box–Behnken experimental design, incorporating relative humidity, initial ethylene concentration, and lamp wattage as independent variables. Relative humidity improved removal efficiency at low ethylene concentrations; however, an unexpected result was observed at 50 ppm and 3W. Under these operating conditions, increased relative humidity slightly reduced the percentage of ethylene removal. This was hypothesized to be due to photon competition limiting oxidant generation. Optimisation combined outputs from both models to minimise total operating cost, defined as the sum of energy use and spoilage-related losses. Cost was more sensitive to incomplete. degradation than to power consumption, meaning that faster conversion directly reduced total cost by lowering spoilage penalties. Sensitivity analysis identified lamp power and activation energy as the most influential parameters, with k₀ and the heat-transfer coefficient exerting smaller effects. The optimal configuration,12 W lamp power, Ea=19,600 J·mol⁻¹, k₀=0.47 s⁻¹, and he=15–30 W·m⁻²·K⁻¹, achieved near-complete degradation within 780 s, with a moderate temperature rise (~2.9× initial) and a cost of 0.148 R/kg apple. Industrial-scale projections showed a ~90.5 % reduction in ethylene concentration (from 113.27 ppm to 10.81 ppm), spoilage reduction from 31.72 % to 3.03 %, and ~10.5× shelf-life extension. For Golden Delicious apples, this equated to ~R5,785 savings per pallet and ~R 1.82 million per day for 15 truckloads at optimal operation. These findings confirm that VUV photolysis, operated within the identified parameters, offers a scalable, efficient, and cost effective ethylene control method. By combining mechanistic modelling with environmental sensitivity, the dual model offers a robust foundation for optimising ethylene management in diverse storage systems.