The combination of material and moisture in drying equipment

According to the degree of difficulty in removing water contained in materials, it can be divided into the following two types:

(1) And unbound water:

The non binding water includes the moisture absorbed by the material when the material is in direct contact with the moisture, such as the wetting water and pore water existing on the material surface. It is easy to remove because of its low binding strength with materials.

(2) Combined water content:

It includes the moisture contained in the material cell or fiber tube wall and capillary. This kind of water can be subdivided into chemically bound water, physicochemical bound water and mechanical bound water. Among them, the chemical bound water mainly includes crystal water, which is difficult to remove because of its high binding strength. The process of removing crystal water is not a drying process; Physicochemical bound water includes water of adsorption, permeability and structure. The combination of adsorbed water and material is the strongest. Water can be adsorbed on both the external surface and the internal surface of the material. The heat is released when the water is adsorbed, but it needs to be absorbed when it is removed. The combination of permeable component and material is due to the difference of the concentration of dissolved matter inside and outside the material tissue wall The structure moisture exists in the material tissue, which is combined with water during the formation of colloid. The dissociation of this kind of water can be caused by evaporation, external pressure or destruction of tissue

The capillary water exists in the wet materials with fibers or small particles, and its binding strength with the materials is weak. Materials containing combined moisture are called absorbent materials, such as wood, grain, leather, fiber and its fabrics, paper, synthetic resin particles, etc. Materials containing only unbound moisture are called non absorbent materials, such as molding sand for casting and various crystalline particles. In terms of the difficulty of drying, non absorbent materials are much easier to dry than water absorbing materials. The crystal water of the material is chemically bound water, which can not be removed in the drying process. The binding energy of water with different structures is about 100 ~ 3000j / mol. The different combination forms of materials and moisture make the energy consumption of water removal different, which means that the heat energy required for drying is also different.

According to the material in a certain drying conditions, its moisture can be divided into equilibrium moisture and free moisture. In our daily life, we often encounter the phenomenon that some materials return to moisture in the air with high humidity, and these materials will return to their "dry" state in dry air. No matter whether the process of "returning to moisture" or "drying", after reaching a certain limit, the moisture content in the material will tend to a certain value, which is called the equilibrium moisture in this air state. The part of the material containing more than the equilibrium moisture can be removed from the wet material during drying, which is called free moisture.