Heroic drying introduces you to the basic stage of spray dryer.

Spray drying process can be divided into three basic stages: material atomization is droplet, droplet contact with hot air, mixing and flow, namely droplet drying, drying product and exhaust gas separation.

The first stage of spray drying: atomization of slurry

The size and uniformity of droplets have great influence on product quality and technical and economic indexes, especially the drying of heat sensitive materials. If the size of the spray droplets is very uneven, it will appear that the large particles have not reached the drying requirements, but the small particles have been over dried and deteriorated. Therefore, the slurry atomizer is the key component of the spray dryer.

Second basic stages of spray drying: contact and drying of droplets with hot air

The contact, mixing and flow of droplets and hot air are simultaneous heat and mass transfer processes (i.e. drying process) in a spray dryer. The contact mode, mixing and flow state of droplets and hot air depend on the structure of hot air distributor, the installation position of atomizer and exhaust gas exhaust mode. In the spray dryer room, there are three ways of droplet contact with hot air: parallel flow, counterflow and mixed flow.

The different contact modes between droplets and hot air have great influence on the temperature distribution, droplet (or particle) trajectories, the residence time of materials in the drying chamber and the quality of the products. For the parallel flow type, the hot air contacts with the droplet with large moisture content, so the moisture content evaporates rapidly, and the surface temperature of the droplet is close to the wet bulb temperature of the inlet hot air; at the same time, the temperature of the hot air is significantly reduced; therefore, the temperature of the material is not high during the whole process from the droplet to the drying of the finished product, which is particularly beneficial to the drying of heat sensitive materials. Because of the rapid evaporation of moisture, the droplets expand or even break; therefore, the drying products obtained by parallel flow are usually non spherical multi empty particles with low bulk density. For counterflow, the droplets sprayed from the top of the spray dryer contact with the wetter hot air at the bottom of the spray drying. Therefore, the evaporation rate of wet evaporation is slower than that of the flow pattern. The hot air at the bottom of the dryer is hot and humid, and the hot air with low moisture content is in contact with dry particles; therefore, for non heat sensitive materials that can withstand high temperature, require low moisture content and high bulk density, it is appropriate to adopt the counter flow type. In addition, in the countercurrent operation process, the average temperature difference and partial pressure difference of the whole process are larger, and the logistics residence time is longer, which is conducive to the heat and mass transfer of the process, and the utilization rate of heat energy is also high. For mixed flow operation, it is actually a combination of parallel flow and counter flow, and its characteristics are also between the two. For materials with high temperature resistance, this operation mode is suitable.

In the spray dryer room, the drying process of material is the same as that of conventional drying equipment. It also goes through two stages: constant speed drying and slow drying. When the droplet contacts with the hot air, the heat is transferred to the droplet by the hot air through the saturated vapor film on the droplet surface, so that the moisture content in the droplet evaporates. As long as the amount of moisture inside the droplet diffuses to the surface enough to compensate for the moisture loss on the surface, the evaporation is carried out 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 in the droplet to the surface is not enough to keep the surface wet. The dry shell gradually forms on the surface of the droplet, and the dry shell thickens with the increase of time, and the diffusion speed of moisture from the inside of the droplet through the dry shell decreases, that is to say, the evaporation rate decreases gradually. At this time, the surface temperature of the material is higher than the wet bulb temperature of the hot air, which is the stage of speed reduction drying.

Third basic stages of spray drying: separation of dry products from exhaust gases

There are two ways to separate the spray drying products from the exhaust gas (usually called gas-solid separation). One is that the dry powder or particle product falls on the cone wall of the drying chamber and slides to the bottom of the cone, and is discharged through discharging equipment such as star discharge valve. A small amount of fine powder enters the gas-solid separation equipment and is collected with the waste gas. The other is that all the dried products are collected by the gas-solid separation equipment along with the air flow. The exhaust gas must meet the emission standards of environmental protection to prevent environmental pollution. Heroic drying is recommended for you to recommend gas solid separation for spray drying system in the following ways: only cyclone separator; bag filter only; electrostatic precipitator only; cyclone separator combined with green gas; cyclone separator and wet dust collector combination. In practice, which method is adopted mainly depends on the process requirements and environmental protection requirements.

Above is the basic principle of spray dryer. Changzhou Hao Mei drying Engineering Co., Ltd. is a professional manufacturer of spray dryer. According to the production process requirement of every customer, we design detailed optimization plan. Welcome to inquire!