Fluidization is achieved when the solid materials display fluid-like behaviors, such as the ability to flow. As the temperature and pressure rises, evaporation occurs and moisture inside the materials becomes a gas and diffuses with the air. Dehydration can also occur if the materials are dried long enough.
Fluid bed dryers are used for small particles or materials that are sticky, temperature sensitive or fragile because fluidization can be reached at temperatures lower than the boiling point and is relatively gentle. The material should be relatively uniform in size, shape and moisture content to ensure that all particles are properly and completely dried.
Utilized in the pharmaceutical, agricultural, food and chemical industries for powders and agglomerates, fluid bed dryers are used with pesticides, detergents, pigments, milk powder, yeast, powdered flavorings and more. Depending on the application, the permeable plate can vibrate to achieve fluidization for certain materials while a circulating bed causes the particles to become suspended.
Fluid bed dryers are generally used for industrial-sized batches, and standard capacities contain loads from 30 to 25,000 pounds and range from 24 to 72 inches in diameter.
Fluid bed dryers are made from stainless steel and can be horizontally or vertically oriented depending on the space available. A large rounded tank or rectangular box with multiple intake and output valves contains the permeable surface and holds the materials. The supply hose for the gas is connected at the base of the tank because gas will rise once heated.
Pipes are fitted to transport the product into and out of the tank, and there may be multiple internal chambers in the tank. Once the material has entered the tank, the heated air is released. Heating panels or tubes immersed in the material can also contribute to the rising temperatures. The velocity of the air increases to a point where it exceeds the velocity of gravity of the particles and fluidization occurs.
The particles display fluid-like behavior and every piece of the material shakes and moves, allowing direct contact between the air and the complete outer surface of the material. This prevents clusters from forming and allows each particle to reach the same moisture content and level of dryness. Once fluidization is reached, the product is able to flow through the bed and exit the tank.
Continuous fluid bed dryers allow a constant stream of material to enter the tank as the dried product is removed. This process is much faster than drying material in batches, and some models of fluid bed dryers reach up 99% efficiency.