Three stages of spray drying

- Feb 10, 2020-

First stage

The purpose of material liquid atomization is to disperse the material liquid into fine droplets. 

The average diameter of the droplets is generally 20-60um, so it has a large surface area. 

When it comes into contact with hot air, the water in the droplets quickly evaporates. 

And dried into powder or granular products.

The size and uniformity of the droplets have a great impact on product quality and technical and economic indicators, especially the drying of heat-sensitive materials is particularly important. 

If the size of the spray droplets is very uneven, large particles will not meet the drying requirements, but small particles will be over-dried and deteriorated.

 Therefore, the material liquid atomizer is a key component of the spray dryer.


Second stage

The second stage of spray drying-the contact and drying of mist droplets and hot air, the contact, mixing and flow of mist droplets and hot air are the heat and mass transfer processes (that is, the drying process) performed simultaneously in the spray dryer. 

The contact mode, mixing and flow state of the droplets and hot air depends on the structure of the hot air distributor, the installation position of the atomizer, and the exhaust gas exhaust method.

In the spray drying chamber, there are three ways for the droplets to contact the hot air: co-current, counter-current and mixed-flow.

The contact methods of mist droplets and hot air are different, which have a great influence on the temperature distribution in the spray drying chamber, the movement trajectory of the droplets (or particles), the residence time of the materials in the drying chamber, and the product quality. 

For the countercurrent type, the hottest hot air is in contact with the droplets with the highest moisture content, so the moisture quickly evaporates, and the surface temperature of the droplets is close to the wet bulb temperature of the hot air at the inlet. 

At the same time, the temperature of the hot air is also significantly reduced. 

During the entire process of drying the finished product, the temperature of the material is not high, which is particularly advantageous for the drying of heat-sensitive materials. 

Due to the rapid evaporation of moisture, the droplets swell or even rupture; therefore, the dry products obtained by co-flow are often non-spherical porous particles and have a low bulk density. 

For the counter-current type, the droplets sprayed from the top of the spray dryer are in contact with the moist hot air from the top of the spray dryer; therefore, the evaporation rate of the moisture content is slower than that of the parallel-flow type. 

The hottest air with the lowest humidity at the bottom of the dryer is in contact with the driest particles; therefore, for non-heat-sensitive materials that can withstand high temperatures, require low moisture content, and high bulk density, the counterflow method is most suitable. 

In addition, during the countercurrent process, the average temperature difference and partial pressure difference of the whole process are large, and the material residence time is longer, which is beneficial to the heat and mass transfer of the process, and the heat energy utilization rate is also high.


The third stage

The third stage of spray drying-separation of the dried product from the exhaust gas, and separation of the spray dried product from the exhaust gas (commonly referred to as gas-solid separation). 

One is that the dried powder or granular product falls on the main wall of the drying chamber and slides to the bottom of the cone, and is discharged through a discharge device such as a star discharge valve. 

A small amount of fine powder is collected with the exhaust gas into the gas-solid separation device. 

The other is that all the dried products enter the gas-solid separation equipment to be collected with the airflow. 

The exhaust gas must meet the environmental protection emission standards to prevent environmental pollution. 

There are several ways of gas-solid separation commonly used in spray drying systems: cyclone separators; bag filters; electrostatic precipitators; combination of cyclones and bag filters; combinations of cyclones and wet dust collectors, etc. 

In practice, which method is used depends mainly on process requirements and environmental requirements.