The small spray dryer can be divided into three basic stages.:The atomization of the material, the contact between droplet and drying medium and drying.;The drying product is separated from the exhaust gas.
(1)The atomized material droplets are dispersed into small droplets with large specific surface area. After zeroing, enter the drying tower and heat it into hot air. The moisture in the zero drops evaporates rapidly. Dried into powder or granular products in a very short time. The size and uniformity of droplets have great influence on the quality of products, especially for the drying of heat sensitive materials. If the size of the spray droplets is not uniform, there will be some large particles have not yet reached the drying requirements, and small particles are deteriorated due to excessive drying. Therefore, the zeroizer used in the material liquid zero is the key part of zero spray drying. Commonly used atomizers are airflow, pressure and rotary.
（TwoThe droplets contact with the drying medium and the droplets are sprayed into the hot air. The droplets have a certain speed and contact with the air. This process is also a process of heat and mass transfer. The contact mode between droplet and air is parallel flow, counterflow and mixed flow. The direction of motion of droplets is called parallel flow, and the opposite direction of motion is called counter-current first and then the same, that is, first counter-current and then parallel flow is called mixed flow. The contact mode between droplets and air is related to the installation position of atomizer in the tower and the position of hot air inlet. Different contact modes between droplets and air have great influence on temperature distribution, movement track of droplets and particles, particle size and moisture content of products in drying tower.
Droplet drying process also experienced constant speed and slow down stage. In the constant velocity stage, water evaporation occurs on the surface of the droplet, and the evaporation rate is controlled by the diffusion rate of the vapor through the surrounding gas film. Its driving force is the temperature difference between hot air and liquid droplets. The greater the temperature difference, the greater the drying rate. When the water diffusion rate decreases and the water content is insufficient to maintain the evaporation of the surface, the drying enters the stage of slowing down.
（ThreeDrying products are separated from waste gas and spray dried products are powders. Most of them use cyclone separator and tower bottom discharge, and some fine powders are discharged with exhaust gas. In order to improve the product yield and reduce production costs, it is necessary to increase the separation equipment to collect fine powder. At the same time, in order to protect the environment, the emissions must meet the environmental protection standards. Separation equipment is generally bag filter, cyclone separator, Venturi scrubber, electrostatic precipitation equipment. All kinds of methods can be combined, multi-stage separation operation can be carried out, the product can be graded, improve separation efficiency.
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