At present, the spray dryer is one of the most widely used dryers in a large number of drying apparatuses and is a drying apparatus for treating a solution, a suspension, or a slurry-like material. The ability to dry directly from a liquid into a powder is the biggest advantage of a spray dryer; its disadvantages are low thermal efficiency, large volume, low production capacity, and high investment. Spray drying has the special advantages of powdered products which can be directly obtained from the solution or suspension system. It is widely used in chemical, light industry, food and other industries. The dye industry is the most widely used in the chemical industry.
The working principle of the spray dryer is: spray the material into droplets dispersed in the hot air, the material and the hot air in parallel flow, countercurrent or mixed flow contact each other, so that the rapid evaporation of water to achieve the purpose of drying.
With this drying method, may be omitted concentration, filtration, pulverization unit operations can be obtained is 30 ~ 500 μ m granulated product. And drying time is short, the general drying time is 5 ~ 30s . Spray drying is suitable for the drying of easily dispersible materials in the process of concentration of highly heat-sensitive materials and liquids. The product has good flowability and fast dissolvability.
The gas-solid two-phase contact surface area in the spray dryer is large, but due to the dilute phase flow of gas-solid two phases, the volumetric heat transfer coefficient is small, generally 20~100kcal/m3 · h ·°C, and the hot air inlet temperature is concurrent operation. The time is 250~500 °C , and the countercurrent operation is 200~300 °C . Industrial-scale spray dryers typically have a thermal efficiency of 30% to 50% . In foreign countries, spray drying with waste heat recovery can achieve a thermal efficiency of 70% . However, this equipment is only economically significant when its production capacity is greater than 100kg (water) /h .
The spray dryer process is: feed liquid through the atomizer, spray droplets dispersed in the hot gas flow. Air is blown into the air heater through a blower and heated, and then enters a spray dryer where it contacts and dries. Part of the product falls into the bottom of the tower, and part of it is sucked by a first-level induced draft fan into a cyclone separator. After separation, the exhaust gas is vented. The products at the bottom of the tower and the products collected by the cyclone separator are extracted by a secondary air extractor and separated by a secondary separator.
The product of the spray dryer is fine particles. In order to adapt to the requirements of environmental protection, the spray drying system only uses the cyclone separator to separate the products, and the exhaust gas is still not enough to be purified. Generally, the bag dryer is also used to purify the exhaust gas to make the dust content in the exhaust gas. Below 50 mg/m3 , or using a wet scrubber, the dust content of the tail gas can be reduced to 15 to 35 mg/m3 .
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