The drying process requires the consumption of a large […]
The drying process requires the consumption of a large amount of heat energy. In order to save energy, some high moisture content materials, suspensions or solutions containing solid materials are generally first mechanically dehydrated or heated to vaporize, and then dried in a dryer to obtain dry solids [8 ].
The purpose of drying is for material use or further processing needs. For example, the drying of wood before the production of wooden molds and woodware can prevent the deformation of the product. The drying of the ceramic material before calcination can prevent the finished product from cracking. In addition, the dried material is also easy to transport and store, such as drying the harvested grain below a certain moisture content to prevent mildew. Since natural drying is far from meeting the needs of production development, various mechanized dryers are increasingly being used.
During the drying process, the heat and mass (moisture content) transfer must be completed at the same time to ensure that the moisture partial pressure (concentration) of the moisture on the surface of the material is higher than the moisture partial pressure of the moisture in the external space and the heat source temperature is higher than the material temperature.
The heat is transferred from the high temperature heat source to the wet material in various ways, vaporizes the surface of the material and escapes to the external space, so that the difference in moisture content occurs on the surface and inside of the material. The internal moisture spreads to the surface and vaporizes, so that the moisture content of the material is continuously reduced, and the overall drying of the material is gradually completed.
The drying rate of the material depends on the surface vaporization rate and the diffusion rate of internal moisture. Generally, the drying rate in the early drying stage is controlled by the surface vaporization rate; then, as long as the external conditions of drying are not changed, the drying rate and surface temperature of the material remain stable. This stage is called constant-rate drying stage; when the moisture content of the material is reduced to a certain level To a certain extent, when the diffusion rate of internal moisture to the surface decreases and is less than the surface vaporization rate, the drying rate is mainly determined by the internal diffusion rate, and decreases continuously with the decrease of the moisture content. This stage is called the deceleration drying stage.