Slurry inLaboratory spray dryerThe atomizer forms droplets. Once the droplets come into contact with dry hot air, the heat transfer process from hot air to the inside of the droplets and the mass transfer process of water diffusion from the inside of the droplets to the surface of the droplets are rapidly established. Different products show different evaporation characteristics. In the evaporation process, the size distribution of droplets will change. If the evaporation process is not well controlled, the droplets will easily expand, collapse, break up, or split again, resulting in hollow spheres or mushroom-shaped, apple-shaped, irregular shapes, or form a bond, agglomeration. Particles. The spray drying process is shown in the diagram.
Spray drying process
The rate of droplet drying in a laboratory spray dryer is determined by the rate of heat and mass transfer. Heat and mass transfer rates are functions of temperature, humidity, air transfer characteristics around each droplet, droplet diameter, and relative velocity between the droplet and air. Too fast drying causes particles to break, hollow particles and hollow spheres. Too slow drying causes sticky or reunited.
lab spray dryerThe droplet drying process can be divided into two stages: constant speed drying stage and descending speed drying stage. In the constant speed drying stage, the internal diffusion rate of moisture is higher than the evaporation rate, and the surface moisture is sufficient. The evaporation rate is mainly controlled by external conditions, and the evaporation rate is almost unchanged in this stage. Most of the moisture in the droplet evaporates at this stage.
With the continuous evaporation of water, the water content in the droplets was gradually reduced. The surface water was insufficient to maintain the surface evaporation, and the internal diffusion rate was lower than the water evaporation rate. At this point the surface is no longer wet, the temperature begins to rise, drying rate continues to decline, this is the slow drying stage.
If the attractive air temperature is very high, so that droplets of water evaporation quickly, it is easy to form a shell on the surface of the liquid, to play a role in the preservation of internal moisture. If the attractive air temperature is low, the initial drying rate is low, and the droplet surface will maintain the wet bulb temperature for a long time.
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