Continuing my discussion on fluid bed dryers begun in my last column, in this issue I will look at fluid bed dryer configuration, heat sources and zoning.
Most fluid bed dryers are single-pass systems in that the process gas is exhausted to atmosphere after passing through the bed once. Some systems are designed for recirculation or recuperation, depending on the economic feasibility of the operation.
Products that have large particle-size variations or high bulk densities can benefit from a vibrating bed design. Any larger particles that fall out of suspension or the bed are introduced back into the bed or propelled along the length of the dryer by the vibrating action. Vibrating systems also will reduce the operating cost associated with the power requirements for fluidization.
Indirect fluid bed processing is employed when the process dictates or when a low cost form of energy (such as steam) is available. The principles are similar; the most fundamental difference is that with indirect units, a tube bundle or series of plates are incorporated into the drying chamber, allowing the feed to intimately contact the heated surface, thereby transferring energy primarily by means of conduction.
Fluid bed dryers can use almost any heat source. For gaseous or liquid fuels, the wind box doubles as a wind box/combustion chamber combination that is suitably refractory lined. Obviously, the higher the allowable temperature of the process gas, the smaller the volume of air required and the smaller the unit. This reduces the initial capital investment for the equipment. With the correct design, fluid bed dryers can operate at extremely high temperatures, providing the potential for unit reactions such as calcining. These designs would incorporate refractory lining the wind box, drying chamber and expansion chamber.
Fluid bed dryers also can be designed with multiple zones with different inlet conditions for each zone. This is especially useful in applications where altering the inlet temperature can benefit the process or for sensitive products. Additionally, fluid bed dryers can be designed with an integral cooling section in the dryer, eliminating the need for a separate cooler in those applications that require one.
Fluid bed dryers are a gentle method of handling many products. They require relatively small real estate and are low maintenance due to the few moving components in the system. Their principle limitations include:
- Inadequate bed formation due to poor fluidizing plate designs.
- Relatively high operating costs associated with the power requirements for the fans.
- Potential reduction of product size due to attrition and impact.
- The possibility of product buildup in the wind box on loss of power.
Also, depending on product characteristics, they can tend to agglomerate, which may be an advantage or disadvantage, depending on operational requirements.