Drying

AVA supplies vertical and horizontal drying mixers that can be operated either in batch mode or continuously.

Contact Drying

The most common application for AVA drying mixers is in contact drying. The energy flow takes place via heated surfaces that are in contact with the product. Heating takes place mostly using thermal oil, hot water, or steam applied through double-walled heat chambers. The AVA agitator moves the product against the heated surfaces within the drying mixer where it is heated to the boiling point of the liquid component(s). The liquids evaporate, and the product can now be dried to the desired residual moisture content. Due to the permanent homogenizing of the product, uniform product characteristics are always obtained throughout the entire processing space. For high temperature applications, electrical resistance heating units featuring a high power density are positioned directly on the drying mixer shell. Using this AVA solution, heating temperatures of up to 900°C can be achieved.

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Convective Drying

AVA drying mixers are used for a variety of applications that require convective drying. Here, the product to be dried is brought into intensive contact with a gas stream, causing the liquid components to evaporate and leave the system via the gas stream. Filter systems or cyclones ensure separation of entrained particles. Energy is normally applied by the cooling drying gas in the product space, or additionally via the heated drying mixer shell (superimposed convective and contract drying). Product characteristics or quality grades can be determined at the AVA test center. Air, flue gases, or superheated steam are mainly used for convective drying. For fibrous products, materials with a porous structure, or even temperature-sensitive products, convective drying is usually superior to contact drying.

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Stripping

The stripping process is a special convective drying process, in which a liquid component within a product is targeted for evaporating. This is done by producing the largest possible partial pressure gradient of the substance to be evaporated between the product surface and gas phase. The process is particularly efficient in the vertical conical apparatus as the entire material to be dried is selectively streamed from bottom to top. Pressure and temperature can be precisely adjusted to achieve the desired product characteristics, for example, to bring about condensation effects. Commonly used stripping media are nitrogen or superheated steam.

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Evaporating

The AVA HVW series (vertical/conical dryer) is an ideal tool for evaporating liquids. Advantages include its favorable surface-to-volume ratio, which ensures high evaporating rates, in particular when using heated mixers. Heat transfer is implemented using the Mitrava helix, which is conveyed alongside and at a short distance from the side wall. By increasing the helix speed, the entire heated vessel surface can be used for heat transfer, even at lower fill levels. The intense fluid movement along the side wall as well as the short distance from the wall prevent caking on the heated surfaces. Evaporating generally takes place under vacuum conditions. This results in a higher driving temperature differential between the heated surfaces and the product and at the same time allows for lower product temperatures when processing delicate products. Through the application of high-torque drives and robust agitators, even difficult product consistencies can be processed. To save energy, it is possible to implement a multistage evaporating process by combining the AVA conical evaporator with AVA pre-evaporating stages.

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Crystallizing

The advantages of AVA crystallizers lie in their wide range of applications, their handling of harder consistencies, and their ability to perform all process steps (evaporation, crystallization, drying) in one device. Crystallization is generally achieved in batches by feeding a saline solution into the AVA crystallizer. Solvent evaporation transforms the solution into a supersaturated state. Subsequently, the crystallization process begins by further supersaturation through evaporating or by changing the temperature or pH level. Crystal growth can be influenced by means of the mixing intensity. Once the crystals are sufficiently formed, they can be further treated in a solid–liquid separation process, or evaporation can be continued to obtain solid matter. When processing waste materials, it is possible to empty the suspended crystals into containers to form a solid residue through cooling and crystal water retention.

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