Spray dryer is a drying equipment for processing solutions, suspensions or slurry materials. It uses the method of spraying to make the material become mist and disperse in the hot air. The material and the hot air contact each other in the way of co-current, counter-current or mixed flow, so that the water evaporates quickly to achieve the purpose of drying. With this drying method, unit operations such as concentration, filtration, and crushing can be omitted, and granular products of 30 to 500 μm can be obtained. And the drying time is very short, the general drying time is 5~30s. It is suitable for drying highly heat-sensitive materials and easily dispersed materials in the process of liquid concentration. The product has good fluidity and instant solubility.
The gas-solid two-phase contact surface area in the spray dryer is large, but the gas-solid two-phase flows in a dilute phase, so the volumetric heat transfer coefficient is small, generally 20~100kcal/m3, and the hot air inlet temperature is 260~500 in parallel operation ℃, 200~300℃ during countercurrent operation. Industrial-scale spray dryers generally have a thermal efficiency of 30-50%. The thermal efficiency of spray drying with waste heat recovery in foreign countries can reach 70%, but this kind of equipment is only economically meaningful when the production capacity is greater than 100kg (water)/h.
The basic process of spray drying is as follows. The material liquid passes through the atomizer, sprays into droplets and disperses in the hot air flow. The air is sent to the air heater to be heated by the blower, and then enters the spray dryer, where it is dried in contact with the spray droplets. Part of the product falls into the bottom of the tower, and part of it is sucked into the first-stage cyclone separator by the first-stage induced draft fan. After separation, the tail gas is vented. The product at the bottom of the tower and the product collected by the cyclone separator are extracted by the secondary exhaust fan, separated by the secondary cyclone separator and then packaged.
The spray-dried products are fine particles. In order to meet the requirements of environmental protection, it is not enough for the spray-drying system to use a cyclone separator to separate the products and purify the exhaust gas. Generally, a bag filter is also used for purification. Make the dust content in the tail gas lower than 50mg/Nm3 gas, or use a wet scrubber to reduce the dust content of the tail gas to 15~35mg/Nm3 gas.
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