Research progress in analytical theory of drying system

Drying is the spontaneous heat energy transfer and conversion between two independent systems of heterogeneous wet materials and finite media in the thermodynamics field. In addition to general thermodynamics, there are mostly complex interactions among a numbers of stochastic factors along the system boundary. Thus, the energy utilization cannot be real-time and quantitatively evaluated in actual using thermodynamic entropy parameters. Specifically, the diffusion model cannot obtain the mathematical solution of the transfer coefficient with strict meaning, thus causing a deviation significantly away from reality. The drying model is also limited by the undetermined physical quantity (pre-exponential factors, activation energy, and mechanism functions) in the drying kinetics model under reaction engineering. A major theoretical challenge can still remain to systematically analyze the actual process for the analytical solution in the fundamental scientific research field of thermodynamic application since ancient times. The thermodynamic basis has been focused on non-homogeneous systems and the drying exergy analysis over the decade. Taking the free energy consumption of exergy transfer and conversion during drying as the unified scale, water activity was considered as the common attribute for the drying systems. The inherent functions were obtained for the theoretical solutions during drying. The fundamental theory was enriched in the thermodynamic application. Accordingly, the present review aims to analyze the development history and research status of analytical theory during drying. The analytical theories were summarized to reveal the theoretical process of material drying. The energy efficiency of the drying process and equipment was evaluated to propose the technical paths for the application and development of analytical theories. Some scientific references were provided to reveal the transfer mechanism. The adaptive control of drying can be realized to develop the evaluation standards for energy efficiency of drying.