Spray dryer is the use of heat to vaporize water in materials, and the formation of steam into the external environment of the process. Drying of wet materials is affected by external environmental conditions and material structure.
In the drying process, on the one hand is the surface vaporization process, heat transfer from the surrounding environment to the material surface, so that the surface moisture evaporation.:On the other hand, heat transfer and moisture diffusion and migration occur simultaneously in the interior of the material. This process of surface vaporization and internal diffusion occurs simultaneously. The purpose of drying is not only to vaporize the surface water, but also to keep the internal moisture diffusing outwards. When the moisture diffused from the interior to the surface is greater than that evaporated from the surface, the surface of the material remains wet and produces intense moisture distribution, and the heat transferred to the material is almost all consumed by the evaporation of moisture. The surface temperature of the material is approximately equal to the temperature of the dried medium. At this time, the drying rate is constant and the drying rate is constant. Fast drying stage. In the constant speed drying stage, the drying rate depends on the evaporation rate of moisture on the surface of the material, and is controlled by external conditions.
As the drying process proceeds, the moisture content of the material decreases gradually. Critical moisture pointCWhen the evaporation pressure decreases, the internal moisture diffusion rate is less than the surface vaporization rate. Because the moisture can not diffuse outward in time, the surface of the material dries first, and the evaporation surface transfers to the interior. Some of the heat energy is used for the heat and mass transfer necessary for drying, while the other part makes the material warm up and the drying rate. Began to fall into the drying stage. The drying rate in the drying stage is mainly affected by the moisture diffusion rate inside the material.
Obviously, critical moisture pointCoIt is a key point. It is the dividing point between the constant speed and the slow down drying stage. When the critical point is reached, the rate of moisture migration to the surface within the material can not catch up with the rate of moisture vaporization on the surface of the material. Due to the different structure, shape and size of the material, the material has different moisture transfer rate, and the operating conditions of the drying medium will make the material have different surface vaporization rate. Therefore, the critical moisture content is related to the characteristics of the material and the drying medium. If the drying rate is too fast in the constant drying stage, or the material layer is thicker, it is easy to make the moisture transfer rate from the inside to the surface less than the surface moisture vaporization rate when the moisture content of the material is higher, so the critical moisture content of the material will be higher.
Therefore, from the point of view of drying technology, it is very important to reduce the critical moisture content of materials as much as possible, so that the drying process is at the stage of constant drying speed to shorten the required drying time, avoid excessive temperature of materials, cause material deterioration or cause excessive energy consumption, so as to obtain a lower temperature. Dry high quality products.
The small spray dryer can widen the moisture range of the constant speed drying stage. In a gas-solid suspension in an airflow drying tube, the bonded material is dispersed due to the action of a high-speed airflow, which is equivalent toLIn the process of drying, the critical moisture content decreases, which makes it more close to the equilibrium moisture content of the material, and keeps the uniform moisture discharge on the surface of the particles to the maximum extent, so that the drying process is in the stage of constant speed drying as far as possible. Air dryer uses high temperature and high speed air as drying medium to strengthen the drying process. The contact time between gas and solid is very short. So it can only be used in the constant speed drying stage of surface evaporation. The moisture content in the material should be mainly wetting water, pore water or capillary water with thicker pipe diameter. Under this condition, the wet material can be dried in the air dryer, and the final moisture content can be divided into two parts.0.3%~ 1%Dry materials. For adsorptive or cytoplasmic materials, it is difficult to dry it to water content.2%~3%Below, however, those wet materials whose moisture is mainly diffused in the material are not suitable for air drying.
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