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44 / September 2020 powderbulk.com PBE DRYING DESK Fluid-bed dryer basics John J. Walsh, American Drying Consultants gas transfers heat to the particles, and the particles' moisture is evap- orated 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 mate- rial'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 dry- ing gas flowrate. The drying gas flowrate is set by heat balance and exiting gas approach to saturation. If your material has large par- ticles and a high specific gravity, you'll need a high fluidization velocity for the stationary fluid-bed dryer. If your material requires a M any 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? Fluid-bed dryers 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 noncocur- rent (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-disen- gaging area (that is, the freeboard area above the bed and below the drying chamber's roof). The drying high fluidization velocity and a long residence time so that you'd need a drying gas flowrate much higher than that set by your appli- cation'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. "If your material has large particles and a high specific gravity, you'll need a high fluidization velocity for the stationary fluid-bed dryer."