Thermodynamic analysis of the drying kinetics of coffee grounds under different heat transfer mechanisms

Coffee grounds are an agro-industrial waste product generated in large quantities, the reuse of which can contribute to the circular bioeconomy through the recovery of high value-added bioactive compounds. This study aimed to investigate the drying kinetics of coffee grounds, estimate the effective moisture diffusivity, and determine the thermodynamic parameters associated with the process under different operating conditions. Drying was performed by conduction and convection, with and without the application of infrared radiation, at temperatures of 313, 333, and 353 K. The experimental drying curves were fitted to the Page and Fick diffusion models, with the Page model being the most suitable for describing the kinetic behavior under all conditions evaluated. The effective diffusivity ranged from 7.23 × 10⁻¹² to 3.26 × 10⁻¹¹ m²/s, increasing with rising temperature and with the use of infrared radiation, especially in convective drying. Thermodynamic analysis indicated activation enthalpy values between 7.9 and 19.2 kJ/mol, with a progressive reduction as the temperature increased, reflecting lower energy demand for moisture diffusion. The entropy values were negative (−360 to −361 J/mol·K), suggesting greater organization of the system during the drying process. Gibbs free energy showed positive values between 1.21 × 10⁵ and 1.46 × 10⁵ J/mol, increasing with temperature, characterizing a non-spontaneous process dependent on external energy supply. The dried samples were used for the extraction of phenolic compounds and flavonoids, showing that drying conditions simultaneously influence mass transfer and the potential for sustainable recovery of coffee grounds.