Key Material Characteristics in Drying Mechanics

In the field of drying mechanics, the hard work of researchers studying the dynamics of drying is indispensable. Without their relentless experimentation, we would not have ready-to-use process parameters for drying or baking products using carbon fiber heater elements. Drying dynamics involve studying the dehydration of materials during the drying process and its relationship with various governing factors. These factors are divided into two main categories.

The first category is the characteristics of the materials themselves, including structural features, biological characteristics, pear flower characteristics, and thermophysical properties. These are intrinsic factors.

The second category includes the heating conditions, which comprise heating parameters and methods. Heating parameters include radiation heating temperature, heating power, dry and wet bulb temperatures, airflow direction and speed. Heating methods encompass constant and variable temperature supply, rapid or slow temperature rise, constant temperature duration, and cooling methods. These are external factors.

By integrating internal and external factors, the goal is to understand how moisture inside different materials diffuses to their surface and evaporates from there. This involves studying the resistance encountered by moisture as it migrates or diffuses inside materials, and how these resistances relate to the structure of the materials and their ability to absorb external energy. This is an exploration of the heat and mass transfer characteristics of wet materials. Drying dynamics, based on the search for material drying laws, establish optimized heating schemes and drying cycles. This provides a theoretical basis for designing new processes and retrofitting aging equipment, with the goal of both saving energy and ensuring drying quality.

During the drying process, the properties of the material being dried—such as structure, shape, size, thermal stability, and chemical stability—are crucial in determining the drying technique. Particularly, the new material characteristics produced by the combination of moisture with different types of materials are the most significant factors affecting the drying process. The way moisture combines with solid materials affects the ease of moisture removal from the materials. To master the dehydration rules, it is first necessary to study the binding modes of water with materials.

Let’s delve into how substances in nature can be classified based on their interaction with water into three categories:

1. Capillary Porous Bodies, also known as porous media. In these materials, the size changes little or not at all with water content changes. However, they become brittle as moisture decreases, and some can turn into powder, such as coke, charcoal, soil, sand, bricks, and some building materials. The capillary forces in these materials greatly exceed their circumferential forces, thus they alone determine the internal distribution of moisture. When neutral compared to capillary forces, these materials are referred to as porous bodies. In porous bodies, the binding of materials with water is primarily through capillary forces, which is a relatively simple binding method where water molecules exist in a free radical state.
2. Colloids. These materials change in size and volume as their water content varies. There are two types of colloids: those that expand infinitely upon water absorption, losing their original geometric dimensions and eventually dissolving, known as infinitely expandable bodies, like gum arabic; and those that only absorb a certain amount of water, expand to a certain extent, and retain their geometric shape, known as finitely expandable bodies, like gelatin. In these materials, the micro-capillaries are very small, comparable in size to the molecules of the materials, making dehydration challenging.
3. Capillary Porous Colloids. These materials possess the characteristics of both of the above categories, having a capillary porous structure with colloidal properties in their capillary walls or cell walls, which are elastic and can absorb water to expand and contract during dehydration, such as wood, skin, grains, and foods.

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