Spin flash Dryer drying process

The heat exchange of the Dryer is mainly based on the air flow and the particles, and the two heat exchanges between the cylinder wall and the particles. The drying process is essentially the diffusion process of water, which depends on the process that the water in the material becomes steam and diffuses outward.

The Rotary Flash Dryer adopts high-speed hot air flow into the cylinder in the tangential direction. Due to the spiral motion of the airflow in the cylinder, on the one hand, the temperature of the medium around the particles is reduced, and the flow rate and temperature of the medium are increased at the same time, which greatly improves the speed of external diffusion. On the other hand, the high-temperature airflow impacts the particles or wet materials in the lower part of the cylinder at high speed, and at the same time, the stirring effect of the stirring blades in the cylinder makes the particles broken, the particle size is reduced, and the diffusion resistance of water is reduced. Most of these particles are repeatedly broken in the cycle of high temperature and high stirring intensity at the lower part of the cylinder, which eliminates the agglomeration of materials, and also promotes the diffusion of water in the powerful measures, which strengthens the evaporation of water.

The flow mode of particles and hot air flow: there are both convection and co-current in the lower part of the cylinder, and for coarse particles, it is convection and co-current repeated heat exchange. For fine-grained materials, it is paralleled with the hot air flow, so the drying process can be completed in an instant. For the drying of particles, hot air drying with high temperature and low humidity is actually used. These particles are mainly adsorbed by water molecules and filled between the voids of the particles. Under the conditions of high temperature and low humidity, the heat conduction of the whole particle is slow, resulting in local stress concentration and drying, cracking, breaking, dispersing, and accelerating the drying process.

After the material enters the dryer, the hot air first transmits heat to the surface of the material, causing the evaporation of the surface water, and the water inside the material particles continuously diffuses to the surface and diffuses to the outside, and finally achieves the drying of the entire particle. If the material does not undergo chemical reaction during the drying process, the changes in moisture content, temperature and drying time of the material can be divided into the following stages:

1. Speed-up drying stage: The particles are placed in a heat transfer medium with a high temperature and a relative humidity of less than 100%, and the surface is heated to the wet bulb temperature of the drying medium in a short period of time, and the water evaporation rate increases rapidly. After time, the heat absorbed by the particles is equal to the heat consumed by the evaporated water, and the equilibrium is reached. The time of this stage is very short, and after the discharge of water is not large, it enters the isokinetic stage.

2. Isokinetic drying stage: In this stage, the water evaporated from the surface of the particles is continuously replenished from the inside to the surface of the particles, and the surface is always kept wet. At this time, the drying rate remains unchanged, and the surface temperature of the particles remains unchanged. The evaporation rate is related to the difference in water vapor concentration and temperature between the particle surface and the surrounding medium. The greater the difference, the greater the drying rate. In addition, the drying rate is also related to the air velocity on the particle surface. Increasing the air flow speed on the particle surface can increase the drying rate. Drying continues for a certain period of time, the diffusion rate of moisture inside the particles begins to be lower than the surface evaporation rate, and the moisture in the particles can no longer fully wet the surface to maintain the evaporation of the surface, and then enter the next drying stage.

3. Deceleration drying stage: After the moisture reaches a certain level, the moisture inside the particles is not enough to fully wet the surface, the wet surface gradually decreases, and the drying rate gradually decreases. At this stage, the evaporation rate and heat consumption are greatly reduced, and the surface temperature of the particles is higher than that of the medium. The wet bulb temperature increases gradually, and the temperature difference with the heat carrier decreases until it is close to or the same.

4. Equilibrium stage: At this time, the moisture absorption and evaporation on the surface of the particles reach equilibrium, and the drying rate is zero.

Due to the small particles of the material after drying, the Rotary Flash Dryer stays in the dryer for a short time, usually 1 to 3 seconds. Therefore, the drying of the particles is in the constant speed drying stage, and the surface temperature is the wetness of the drying medium. ball temperature. Using a rotary flash dryer, the particle size of the material is uniform, which is conducive to ensuring product quality.