Drying theory and application of porous media

The drying of wet materials, particularly through thermal means, is fundamentally a coupled process of heat and mass transfer. Heat is absorbed from the drying medium, moving from the surface into the material, while moisture inside the material migrates outward, gradually reducing its moisture content to meet desired specifications. This dynamic interaction between heat and mass transfer within the fluid boundary layer surrounding the solid material is a central focus in the study of drying processes. According to Luikov's classification, dried porous solids can be categorized into three types: typical colloids, typical capillary porous media, and colloidal-capillary porous media. These materials are widely present across various industries, including rocks, ceramics, building materials, insulation products, catalysts, plant tissues, and agricultural goods. As such, porous media represent a key subject in industrial drying technologies. It is important to note that the theory and evolution of moisture migration during drying, along with the models describing moisture transport in solids, have become essential components in advancing drying technologies. Traditional models, like the continuous medium hypothesis, have provided foundational insights. However, they often fail to capture the complexities of mesostructures. To address this, the volume average theory was developed for porous media, offering a more accurate representation of internal behavior. Over the past decade, research trends in drying theory have increasingly integrated knowledge from neighboring disciplines. This has led to a deeper understanding of continuum hypothesis models, revealing their strengths and limitations. Among these, pore network models, multi-scale methods, and fractal theory have emerged as promising approaches. These concepts not only enhance our comprehension of moisture movement in porous media but also open new avenues for technological development. Compared to traditional continuous medium assumptions, the pore network model represents a significant shift in theoretical thinking. It focuses on the mesoscale structure of porous materials, aiming to link microstructural features with macroscopic transport phenomena. As research progresses, this approach is expected to play an even greater role in improving drying efficiency. Multi-scale methods have proven effective in tackling complex systems, especially where other techniques fall short. By analyzing the morphology and internal heat-mass transfer characteristics of porous media, it is reasonable to apply multi-scale approaches to drying. A successful multi-scale study should involve scale classification, representative methods at each level, and ultimately, the integration of multi-scale information. Pore network and fractal theories are particularly useful in this context. Fractal geometry, with its growing applications in various fields, also shows great potential in the study of moisture movement in both dry and wet porous media. Its ability to describe complex, irregular structures makes it a powerful tool for future research in drying technology. Together, these advancements are shaping the next generation of drying solutions.

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