Fluid Bed Dryers
Fluid bed dryers are found throughout all industries, from heavy mining through food, fine chemicals and pharmaceuticals. They provide an effective method of drying relatively free-flowing particles with a reasonably narrow particle size distribution. The feed may take the form of powders, granules, crystals, pre-forms and nonfriable agglomerates. Technology for processing of liquids in fluid bed systems using host media does exist, but it will not be discussed.
Fluid bed dryers can process a wide variation of feed rates, from a few pounds to several hundred tons per hour. Three principle types of fluid bed dryers exist. The first type is referred to as a static fluid bed because the dryer remains stationary during operation. Static fluid bed dryers can be continuous or batch operation and may be round or rectangular. The second type of fluid bed dryer is a vibrating fluid bed dryer, where the body of the dryer vibrates or oscillates, assisting the movement of material through the unit. Vibrating fluid bed dryers are almost exclusively rectangular in shape. The third type of fluid bed dryer fluidizes the material from the top by means of tubes that deflect on a solid pan lifting the material on the deflected airflow. This technology will not be discussed in this column. Fluid bed dryers may use a direct, indirect or combination heat source to provide the energy required to achieve drying.
Principle of OperationIn general, fluid bed dryers operate on a through-the-bed flow pattern with the gas passing through the product perpendicular to the direction of travel. In batch operation, a dose of feed is metered into the dryer bowl and processed until the desired final moisture has been attained. For continuous systems, wet feed is introduced by being metered into the drying section (zone) by means of a suitable feeder. The dry product is discharged from the same section.
In direct systems, hot gas is introduced into the dryer's wind box or plenum. The plenum is a "pressurized box" that distributes the gas evenly across the distributor or fluidizing plate that separates the product from the wind box. The fluidizing plate is a proprietarily designed perforated plate that has either nondirectional or directional holes (nozzles), with or without caps, through which the gas passes.
The process gas passes from the wind box into the drying chamber and interacts with the feed, lifting and maintaining the feed in a fluidized state termed a bed. The bed attains fluid-like properties, flowing and mixing in a similar fashion to a liquid. The fluidization provides intimate contact between each material particle and the gas stream, creating an efficient transfer device.
Once the gas has transferred its energy to the material, it enters the expansion chamber, where the velocity of the gas is reduced to allow entrained particles to fall out of suspension back into the bed. From there, the gas is induced into a dust collection system such as a cyclone and baghouse, scrubber or electrostatic precipitator (ESP). Fluid bed systems require dust control due to the nature of the gas/product interaction.
Fluid bed dryers commonly are provided with both forced and induced draft fans. Depending on the product bulk density, the static pressure required for fluidization can be high, requiring large motors on the fans -- particularly the forced draft or fluidizing fan. The systems are designed to provide the zero or null pressure point in the expansion chamber above the bed.
Residence time within the dryer most typically is controlled by means of a weir on the product discharge. This weir controls the bed depth and can be automated by responding to various inputs. Alternatively, the feed rate can be modulated (by using a variable speed drive on a vibrating or screw feeder) with a constant weir setting by means of the same system inputs. In these systems, the inlet air temperature, bed temperature, exhaust air temperature, wind box pressure, bed differential pressure and product temperature are monitored frequently.