Introduction of spray drying chamber for counter flow and mixing flow of spray dryer
Countercurrent spray drying chamber
The atomizer and the hot air distributor of the counter flow spray drying chamber are respectively located at the top and bottom of the drying chamber. This structure is mainly used in spray drying tower with small diameter and high nozzle atomizer. The hot air distributor makes the hot air entering the drying chamber rotate differently. However, because of the higher drying chamber, the rotation movement can not be maintained in the upper part of the drying chamber.
Mixed flow spray drying chamber
In the mixed flow spray drying chamber, when the gas drying medium and product pass through the drying chamber, there are both parallel flow and countercurrent, that is, the material flows in one direction while the drying medium flows in two directions. The hot air intake pipe, exhaust gas pipe and atomizer are all located at the top part of the drying chamber, and the gas drying medium flows in one direction. While the material moves in two directions, the atomizer sprays upward at the bottom of the drying chamber, and the fog enters the drying medium from the upper part, and the drying product is discharged from the bottom of the drying chamber.
The intake speed is large, resulting in strong swirling flow, resulting in high evaporation rate. The residence time of the material in the drying chamber is very short (5 ~ 10s). The strong rotating motion of the air stream cleans the inner wall of the drying chamber and separates the material from the air stream. Dry materials are discharged from the bottom of the drying chamber. Due to the high ratio of powder to hot air in the drying chamber, a considerable amount of powder is retained in the air stream and taken out from the top of the drying chamber.
It is a new structure with built-in fluidized bed. The feed liquid is transported to the atomizer (rotary or nozzles) at the top of the spray drying chamber by the feed pump, and is then sprayed downwards into the built-in fluidized bed on the bottom of the drying cone, and the hot air distributor is also located at the top of the drying chamber. The second hot air comes into the built fluidized bed from the bottom of the cone of the drying chamber. All the exhaust gas is discharged from the top of the drying chamber. The desiccator can be used for continuous treatment of products with high moisture content. Its outlet temperature is relatively low and its thermal efficiency is relatively high. Therefore, this type of dryer is especially suitable for the drying of hygroscopic products, producing agglomerate or granular powder products to meet the industrial requirements of dust-free powder.
The feed is sprayed from the bottom up to the swirling air flow from the hot air distributor at the top of the drying chamber. As the fog moves upward, it slows down to zero, then moves downward completely under the control of the air flow, and finally falls back to the bottom of the drying chamber. This structure is often referred to as "fountain type spray dryer". With this structure, the product can be carried away by air flow, and can also be discharged separately from the conical bottom of the drying chamber and the air flow.
The wall material of spray drying chamber can be stainless steel, carbon steel, aluminum, special alloy (such as haste alloy), or fiber or wood. At present, the drying chamber wall materials are mostly stainless steel, mainly used in dairy products, food, fine chemicals and pesticides drying process; carbon steel is mainly used in bulk chemical products, minerals and fuel gas desulfurization. The selection of drying chamber materials should meet the requirements of non-polluting drying products, convenient cleaning (if necessary), corrosion resistance and so on. Austenitic stainless steels (such as AISI304, 316, 321, etc.) usually meet these requirements. They have the advantages of high strength, easy manufacture, beautiful appearance and good corrosion resistance. Corrosion damage may occur if the selected drying chamber material is not completely corrosion resistant to certain products and the necessary preventive measures are not taken. For example, for the drying process of dairy products, food and pesticides, the potential corrosion mainly includes: (1) cleaning solution - acid and alkali; (2) water; (3) bactericide - sodium hypochlorite; (4) various forms of chloride ion.
There are many forms of corrosion damage of stainless steel drying chamber, such as intergranular corrosion, crevice corrosion, pitting corrosion, stress corrosion and electric corrosion.
Intergranular corrosion is caused by the precipitation of chromium carbide at the grain boundary of austenite under certain heating (e.g. welding). A series of cracks are formed between the grain boundaries under the action of corrosive medium.
(2) Crevice corrosion: caused by insufficient chromium oxide protective film formed on the surface of stainless steel as an anticorrosion agent.
_Electrical erosion: corrosion caused by the presence of different metals in corrosive electrolyte solutions (such as saline water).
Generally speaking, high content of molybdenum, nitrogen and chromium have good pitting resistance. Copper is very acid resistant. High content of nickel and chromium and molybdenum have strong stress corrosion resistance.
Corrosion damage of most stainless steel drying chambers is directly or indirectly related to the environment containing chloride ions. Therefore, the chloride content in the water in the feed or in the washing water in the device must be detected regularly and, if necessary, reduced. In dairy industry, the condensate obtained by evaporator can be considered as a chloride free water source.