John J. Walsh | American Drying Consultants
Many questions can come up when choosing a fluid-bed dryer. In this column, we’ll review a few basic questions that can help when determining the best fluid-bed dryer for your application.
Fluid-bed dryers come in a myriad of configurations. How do you decide which features will best suit your application? Should you select a stationary fluid-bed dryer or a vibrating unit? Should you use a circular or rectangular drying chamber? Should you use indirect heating elements in the material bed, and if so, what type? Should your dryer be zoned? What type of gas distributor screen and material feed should you choose?
Let’s start with the basics. A fluid-bed dryer is a vessel that allows a bed of moving particles to have intimate contact with a noncocurrent (cross-flow) drying gas stream. A horizontal gas distributor screen separates the dryer’s heated inlet air plenum (called a windbox) from the material bed above the screen. Wet feed material enters the dryer from the top. As the drying gas flows from the heated inlet air plenum up through the screen into the material bed, two distinct areas form — a fluidized bed and a material-disengaging area (that is, the freeboard area above the bed and below the drying chamber’s roof). The drying gas transfers heat to the particles, and the particles’ moisture is evaporated and carried away by the exiting gas stream. Dried material discharges via an overflow weir.
Stationary vs. vibrating
Fluid-bed dryers can fluidize a material bed in one of two ways. A stationary fluid-bed dryer depends on the drying gas’s fluidization velocity, where the gas drag forces through the bed overcome the material’s weight. The drying chamber in a vibrating fluid-bed dryer has a mechanical oscillating motion that promotes fluidization, so the dryer requires less gas velocity.
Realize that gas velocity isn’t the same as drying gas flowrate. Rather, gas velocity is determined by the screen area for a given drying gas flowrate. The drying gas flowrate is set by heat balance and exiting gas approach to saturation.
If your material has large particles and a high specific gravity, you’ll need a high fluidization velocity for the stationary fluid-bed dryer. If your material requires a high fluidization velocity and a long residence time so that you’d need a drying gas flowrate much higher than that set by your application’s heat balance and saturation approach conditions, you should consider using a vibrating fluid-bed dryer.
If your material has a wide particle size distribution, the gas velocity needed to fluidize the coarse fraction may elutriate a significant portion of the fines with the exiting drying gas. Thus, the smaller particles will be flash-dried with a very short residence time that may not be long enough to reach the final product moisture content. But if you dry the same material in a vibrating unit, you can use a lower gas velocity, and the mechanical vibration will fluidize the coarse fraction, allowing all the material to be dried together within the fluid-bed chamber.
Circular vs. rectangular
A circular (or cylindrical) fluid-bed dryer resembles a giant, upright soup can. A rectangular fluid-bed dryer resembles a giant shoebox. Inside the two, the basics are the same — a gas distributor screen separates the windbox from the material — but the rectangular bed may also have baffles to direct the material flow and divide the drying chamber into zones.
The type of drying you do will affect your choice of dryer shape. For batch drying, or if your material’s particles are difficult to fluidize, you’ll want a dryer that backmixes (mixes all particles together turbulently). The more turbulent the fluidization and the deeper the material bed, the greater the backmixing. A circular dryer is ideal for backmixing because it doesn’t have any corners where particles can stagnate.
By their very nature, fluid-bed dryers are good mixers. However, if you do continuous drying and want first-in first-out flow with all particles having the same heat exposure and residence time, you’ll want a dryer that provides less backmixing and, instead, provides material flow approaching plug flow.
In plug flow, all the particles constantly move forward with minimal mixing between first-in and last-in particles. For a fluid-bed dryer to approach plug flow, there must be a long, narrow material flow path, a low material bed height, and a lower fluidzation velocity for a less violent degree of fluidization. These characteristics are most easily achieved in a rectangular fluid-bed dryer, either stationary or vibrating. In a stationary fluid-bed dryer, you can use zigzag baffles to increase the flow path length and decrease the flow path width. In addition, intermittent overflow weirs along the material flow path will decrease backmixing effects.
For further reading
John J. Walsh, PE, (651-263-3697) has a BE in chemical engineering from CCNY and has worked in the field of solids thermal processing for more than 40 years. He holds several process and equipment patents and has written numerous technical articles. He has been an instructor on industrial drying at the Center for Professional Advancement, New Brunswick, NJ, and Amsterdam, the Netherlands.
American Drying Consultants • St. Paul, MN
651-263-3697 • www.ameridrycon.com
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