What Is The Principle of Spray Dryer?
We can study the principle of spray dryer from the above picture :
In a word,spray drying is the process of dispersing the material into fog droplets and then dry them by hot air.The material can be solution, emulsion, suspension, melt or paste,but the finished product is powder or small particles.
The whole process is divided into three stages:
1.The material liquid is absorbed into the pipe by a pump and then turned to into fog droplets by the spray nozzle.
The purpose is to disperse the material liquid into droplets, the average diameter of which is generally 20-60 μ m. Therefore, it has a large surface area, and when it comes into contact with hot air, the moisture in the mist droplets rapidly evaporates and dries into powder or granular products. The size and uniformity of droplets have a great impact on product quality and technical and economic indicators, especially for the drying of heat sensitive materials. If the spray droplet size is very uneven, large particles have not reached the drying requirements, but small particles have been dried too much and deteriorated. Therefore, the material liquid atomizer(spray nozzle) is the key component of the spray dryer.
2.The drying of fog droplets
This state is carried in the spray dryer. It is the contact of hot air and droplets. The contact mode, mixing and flow state of the mist droplets and the hot air depend on the structural form of the hot air distributor, the installation position of the atomizer and the exhaust gas discharge mode. In the spray drying room, there are three ways of contact between droplets and hot air: co-flow type, counter flow type and mixed flow type. The different contact modes of fog droplets and hot air have a great impact on the temperature distribution in the spray drying chamber, the movement trajectory of fog droplets (or particles), the residence time of materials in the drying chamber and the product quality.
(1). Co-flow type
For co-flow type, the hottest hot air contacts the mist droplets with the largest moisture content, so the moisture evaporates rapidly, and the surface temperature of the mist droplets is close to the wet bulb temperature of the hot air at the inlet; At the same time, the hot air temperature is also significantly reduced; Therefore, the temperature of the material is not high in the whole process from the fog drop to the drying of the finished product, which is particularly advantageous for the drying of heat sensitive materials. Due to the rapid evaporation of moisture, the droplets expand and even break; Therefore, the dried products obtained by Co-flow are usually non spherical porous particles with low bulk density.
(2).Counter current type
For the counter current type, the mist droplets ejected from the top of the spray dryer are in contact with the wetter hot air from the bottom of the spray dryer; Therefore, the evaporation rate of the wet fraction is slower than that of the co flow type. The hottest hot air with the lowest humidity at the bottom of the dryer contacts the driest particles; Therefore, for non heat sensitive materials that can withstand high temperature, require low moisture content and high bulk density, counter current type is most suitable. In addition, in the counter current operation process, the average temperature difference and partial pressure difference of the whole process are large, and the material residence time is long, which is conducive to the heat and mass transfer of the process, and the utilization rate of heat energy is also high.
(3).Mixed flow type
For the operation of mixed flow, it is actually the combination of co flow and counter flow, and its characteristics are also between the two. For materials with high temperature resistance, this operation mode is most suitable. In the spray drying room, the drying process of materials is exactly the same as that in the conventional drying equipment, and also goes through two stages of constant speed drying and slow speed drying. When the droplet is in contact with the hot air, the heat is transferred to the droplet by the hot air through the saturated vapor film on the surface of the droplet, so that the moisture in the droplet evaporates. As long as the moisture inside the droplet diffuses to the surface enough to supplement the moisture loss on the surface, the evaporation proceeds at a constant speed; At this time, the surface temperature of the droplet is equivalent to the wet bulb temperature of the hot air, which is the constant speed drying stage. When the diffusion of moisture from the inside of the droplet to the surface is insufficient to maintain the wettability of the surface, the surface of the droplet gradually forms a dry shell. The dry shell thickens with the increase of time, and the diffusion speed of moisture from the inside of the droplet to the outside through the dry shell also decreases, that is, the evaporation rate gradually decreases. At this time, the surface temperature of the material is higher than the wet bulb temperature of the hot air, which is the deceleration drying stage.
3.Separation of spray dried products from waste gas.
Usually there are two ways of the Separation of drying products and waste gas(usually called gas-solid separation). One is that dry powder or granular products fall on the conical wall of the drying chamber and slide to the conical bottom, and are discharged through discharge equipment such as star discharge valve. A small amount of fine powder is collected by the gas-solid separation equipment along with the waste gas. The other is that all the dried products enter the gas-solid separation equipment together with the gas flow for separation and collection. The exhaust gas discharged must meet the emission standards for environmental protection to prevent environmental pollution. The gas-solid separation methods commonly used in spray drying system are as follows: only cyclone separator is used; Use only bag filters; Use only electrostatic precipitators; Combination of cyclone separator and bag filter; Combination of cyclone separator and wet dust collector, etc. In specific practice, which method is adopted mainly depends on process requirements and environmental protection requirements.