Spray dryer is one of the fastest growing and most widely used forms in the drying field. It is suitable for liquid raw materials such as solutions, emulsions and pumpable suspensions to generate powder, granular or blocky solid products. The different drying characteristics of the material being dried, such as thermal sensitivity, viscosity, and flowability, as well as the quality requirements of the product's particle size, particle size distribution, residual moisture content, bulk density, particle shape, etc., determine the use of different atomizers, airflow movement methods, and drying chamber structural forms. Spray drying is a method of systematic technology applied to material drying. After atomization of the dilute material in the drying room, the moisture rapidly vaporizes upon contact with hot air, resulting in the dried product. This method can directly dry solutions and emulsions into powder or granular products, eliminating processes such as evaporation and crushing.

The spray dryer is a thermal process that enables liquid materials to be atomized into fine atomized droplets through nozzles, and then contact with hot media in the drying tower to be dried into powder. The feed can be a solution, suspension, or paste, and atomization can be achieved through rotary atomizers, pressure atomizers, and air flow atomizers. The operating conditions and design of the drying equipment can be selected according to the required drying characteristics of the product and the specifications of the powder particles.

There are also many classification methods for spray dryers, such as parallel flow type (forward flow type), counter flow type and mixed flow type according to the flow direction of gas and liquid; According to the installation method of the atomizer, there are two types: upper spray and lower spray, and lower spray and upper spray; According to the system, there are open, partially cyclic, and closed types, etc. As we all know, there are many kinds of spray dryers, but according to its atomization principle, atomizers can be divided into three types: centrifugal, pressure and airflow. Traditionally, people classify spray dryers according to the way of atomization, that is, according to the structure of atomizers. Spray drying is divided into three types: rotary (centrifugal), pressure (mechanical) and airflow. A large number of spray dryers have been used for nearly 20 years, and the air flow spray dryer should be the first to be industrialized in China. However, with the successful development of centrifugal and pressure spray dryers, the disadvantage of large energy consumption of air flow spray dryers has emerged (the energy consumption of atomizers is 4~8 times that of the other two). In recent years, this model has gradually been replaced by two other models in large-scale industrial production, but it is still in use due to the special needs of the pharmaceutical industry. The high-speed atomizer of the centrifugal spray dryer is the key equipment, and the amplification problem has high technical requirements. At present, China has the ability to produce the centrifugal atomizer of airflow, machinery and electromechanical integration, and can handle 45t water per hour. There are professional atomizer manufacturers in Hangzhou, Xining, Wuxi, Jingjiang and other places. At present, the centrifugal spray dryer has formed a series of models from several kilograms per hour to dozens of tons. The production and manufacturing technology is basically mature. The products obtained from the pressure spray dryer are particles, which are widely used in the synthesis of detergents, dyes, water treatment agents, etc. At present, the pressure spray dryer designed and manufactured by our country can reach a diameter of 8m, a total of more than 50 meters, and the evaporation capacity can reach several tons of water per hour.

The spray dryer is a drying equipment for handling solution, suspension or slurry materials. It uses spray method to make materials become droplets and disperse in hot air. Materials and hot air contact each other in parallel, countercurrent or mixed flow, so that water can evaporate quickly to achieve the purpose of drying. By using this drying method, unit operations such as concentration, filtration, and crushing can be eliminated, and granular products of 30-500 μ m can be obtained. And the drying time is extremely short, usually between 5-30 seconds. It is suitable for drying high heat sensitive materials and materials that are easily dispersed during the concentration process of the material liquid. The product has good fluidity and solubility.

In the spray dryer, the gas-solid two phase contact surface area is large, but the gas-solid two phase flow is dilute, so the volume heat transfer coefficient is small, generally 20~100kcal/m3 • h • ℃, the inlet temperature of hot air is 250~500 ℃ in parallel flow operation, and 200~300 ℃ in countercurrent operation. The thermal efficiency of industrial scale spray dryer is generally 30-50%. The thermal efficiency of spray drying with waste heat recovery abroad can reach 70%, but this kind of equipment has economic significance only when the production capacity is greater than 100kg (water)/h.

The basic process of spray drying is as follows: the feed liquid is sprayed into droplets and dispersed in the hot air flow through the atomizer. The air is sent to the air heater through the blower for heating, and then enters the spray dryer to dry with the droplets. Part of the product falls into the bottom of the tower, while another part is drawn into the first stage cyclone separator by the first stage induced draft fan. After separation, the exhaust gas is released. The products at the bottom of the tower and the products collected by the cyclone separator are extracted by the secondary exhaust fan and separated by the secondary cyclone separator before being packaged.

The products of spray drying are fine particles. In order to meet the requirements of environmental protection, it is not enough for spray drying system to use only cyclone separator to separate products and purify tail gas. Generally, bag filter is also used for purification. By reducing the dust content in the exhaust gas to below 50mg/Nm3 gas or using a wet scrubber, the dust content in the exhaust gas can be lowered to 15-35mg/Nm3 gas.