During the production of myriad goods, the manufacturing process requires that a material be dried, heated, cooled or reacted to meet proper production and quality requirements for that process. This can be accomplished with an array of apparatuses, including direct or indirect drying equipment.
Often, the equipment selected is a heated or cooled, mechanically agitated and efficient device for adding or removing energy from a process mass: a vibrating fluid-bed dryer. In addition to drying, cooling, heating and reacting, the processes involved can include sterilizing, torrefying, melting, pasteurizing, stripping solvents, calcining, roasting, cooking or crystallizing. Such thermal processes are performed on materials such as chemicals, pharmaceuticals, biosolids, food, waste, minerals, polymers and metals.
In many cases, the equipment is used primarily for drying, a process whereby liquid is transferred from a solid state into a vapor phase. In most cases, the drying process is accompanied by the application of heat, which facilitates the phase change and expedites the drying operation.
Direct and Indirect Drying or Cooling
With direct drying or cooling, the material to be processed is intimately mixed with treated gas to add or remove energy from the material. The treated gas transfers energy and liquid to and from the material. In direct drying or cooling processes, the process mass comes in direct contact with the heated or cooled gas. Direct drying or cooling is a viable alternative to indirect drying or cooling.
Indirect drying or cooling is a process of energy transfer, where the process mass is indirectly heated via saturated steam or thermal fluid, or cooled via a water/glycol mixture. The process mass and heating or cooling mediums are kept separated by a barrier. In most cases, a stainless steel wall is used to provide isolation between the mass and heating medium due to its durability and corrosion resistance. Alternative materials of construction also can be used.
The heating medium’s energy is conducted through the metal wall of the equipment to the material on the process side of the thermal processor during a drying process, and vice versa in a cooling process. In the case of drying, the solvent in the process mass is brought to its boiling point. As the material dries, the liquid changes phases and enters the vapor space above the process mass. The vapor then leaves the indirect dryer through the off-gas outlet. One example of this style of equipment is a paddle dryer/cooler.
A typical paddle dryer/cooler has dual counter-rotating agitators with intermeshing, hollow, wedge-shaped paddles. This design results in uniform heating (or cooling) and optimized heat transfer. The use of hollow heated or cooled paddles and a jacketed trough provides for a large amount of heat transfer area in a compact machine. Localized mixing around each self-cleaning paddle creates a homogeneous mixture.
Some applications where paddle dryer/coolers have been used effectively include processing chemicals, organic compounds, inorganic compounds, solvent stripping, thermal reactions and calcining.
In other applications, restrictions limit indirect-drying capabilities. For instance, thermal decomposition may occur at the solvent’s boiling temperature. Other restrictions can be related to residence time, the use of temperature-sensitive materials with low residual moisture requirements, or the feed material size. These types of applications may be better suited for direct drying/cooling equipment such as a vibrating fluid-bed dryer.
The vibrating fluid-bed dryer provides a way to process many pastes, cakes, powders and granules using direct heat transfer in a controllable manner. The vibrating fluid-bed dryer causes the material to flow like a fluid via a combination of vibration and air that passes upward through it, allowing for uniform heat and mass transfer.
In conclusion, a vibrating fluid-bed dryer can be used in many heating, drying and cooling applications. The vibrating fluid-bed dryer can handle material with a minimum particle size of approximately 100 microns.
This type of unit also offers a fully enclosed design that prevents outside contamination from infiltrating the process material. The material to be processed enters the dryer via the feed inlet and is conveyed along the vibrating drying zone in a continuous movement. Treated gas is passed through a perforated plate for direct heat transfer and removes moisture from the material. As a result, a vibrating fluid-bed dryer can achieve uniform product moisture content.
The treated gas is discharged from the off-gas outlet at the top of the unit while the dry material is discharged at the product outlet. The off-gas and ultra-fines entrained in the off-gas are sent through the off-gas outlet to a cyclone and baghouse. The cyclone allows the coarse particles to drop out while the off-gas is sent downstream to a baghouse for final separation before ejecting into the atmosphere.
One way to determine if a vibrating fluid-bed dryer is a good fit for your application is to perform pilot testing. Some dryer manufacturers’ facilities are equipped to run bench- and pilot-scale tests for liquid/solids separation and thermal processing applications. The technical facility also may offer rental equipment for field testing or small production runs. Testing allows a prospective user of a vibrating fluid-bed dryer to obtain process design information for equipment scaleup and to develop optimized processes.