How it works
In a fluidized bed, an upward gas stream suspends the particles so the bed behaves like a boiling liquid — intense particle motion gives excellent gas–solid contact and very uniform temperature. Each particle is wetted with liquid; the hot gas supplies heat that evaporates that liquid from the particle surface into the gas. While the surface stays wet, drying runs at a constant rate set by the heat and mass transfer between gas and particle.
Once the surface moisture falls below a critical value, drying slows: internal moisture transport now limits the rate, and the rate tapers toward an equilibrium moisture the particle can’t dry below at the given gas humidity. Because fine particles can be carried out of the bed by the gas (elutriation), the dryer also separates: small particles leave with the exhaust while the bed retains and discharges the product over a weir. The whole process is dynamic — bed mass, moisture, and temperature evolve in time toward steady operation.
The model
Dyssol Pro models the fluidized-bed dryer dynamically with two ideally-mixed phases — a solid phase carrying the particles and their moisture, resolved per particle size class, and a gas phase carrying the evaporated vapor — coupling their mass and heat balances at every time point of the flowsheet, resolving how moisture and temperature develop across the particle size distribution.
Evaporation follows a two-stage drying curve: a constant rate while the surface stays wet, set by convective heat and mass transfer between gas and particles, then a falling rate below the critical moisture as internal transport limits it, tapering toward the equilibrium moisture the particle cannot dry past at the given gas humidity. Because the bed is well mixed, moisture and temperature are resolved size class by size class as the bed approaches steady operation.
Fine particles carried out of the bed by the gas leave through an elutriation separation, while product is discharged over a weir, so the dryer also classifies as it dries. On this basis Dyssol Pro covers fluidized-bed drying from continuous well-mixed and plug-flow beds to batch and spouted-bed units, and even material-specific drying behavior beyond the standard description can be brought into the flowsheet.
Equipment this model can represent
Any fluidized-bed drying duty for free-flowing granular solids.
Continuous well-mixed FB dryers
A back-mixed bed with continuous feed and weir overflow discharge.
Plug-flow / vibrated FB dryers
Long beds giving a narrower residence-time distribution.
Batch fluidized-bed dryers
Charge-and-dry units common in pharma.
Spouted beds
For coarse, sticky, or irregular particles that don’t fluidize smoothly.
Typical engineering studies
What teams investigate with the fluidized-bed dryer model.
Product moisture & temperature
Predict product moisture and temperature dynamically for given inlet gas conditions.
Drying-rate studies
Study inlet air temperature, gas flow, and bed height against drying rate and final moisture.
Elutriation & fines recovery
Quantify elutriation losses and couple a cyclone/gas filter downstream to recover fines.
Transient operation
Investigate startup, feed disturbances, and the approach to steady state.
Calibration & scale-up
Calibrate the drying-curve parameters to lab data, then scale up.
Existing application example
Continuous vibrated fluidized-bed drying
A dynamic model for continuous vibrated fluidized-bed dryers was implemented in Dyssol Pro and validated across materials, geometries, and vibration settings.
Technical FAQ
How to avoid overdrying in a fluidized bed dryer?
Overdrying wastes energy and can damage product, so you target the equilibrium-moisture plateau and stop there. Dyssol Pro tracks the bed’s moisture dynamically against the critical and equilibrium moisture, so you can tune inlet temperature, gas flow, and residence to hit the target moisture without driving past it.
Why are particles sticking together in my fluidized bed dryer?
Stickiness arises when surface moisture or temperature crosses a material’s sticky point. Dyssol Pro tracks surface moisture and temperature per size class, so you can study operating points that keep them out of the sticky regime.
How can I reduce energy consumption in fluidized bed drying?
Energy falls if you avoid overdrying and recover exhaust heat, using the minimum air and temperature that meet the moisture target. Dyssol Pro’s heat and mass balances let you study inlet temperature and gas flow against final moisture and exhaust humidity to find the lean operating point.
What causes uneven drying in a fluidized bed?
Uneven drying comes from poor fluidization, channeling, or a wide residence-time spread. Dyssol Pro models a well-mixed bed with size-resolved moisture, so it shows how size classes dry at different rates and how residence time sets the spread.
How do inlet air temperature and air velocity affect drying rate?
Higher inlet temperature raises the driving force and air velocity raises heat/mass transfer — both speed drying, up to entrainment limits. Dyssol Pro takes both as inputs to the transfer correlations, so you can map drying rate and final moisture against them directly.
How can I prevent product attrition in a fluidized bed dryer?
Attrition is mechanical breakage from particle collisions, worsened by vigorous fluidization at high gas velocity, so it is limited by drying at the lowest gas velocity and residence that still meet the duty. Dyssol Pro computes the gas velocity and residence an operating point needs, so you can steer toward the gentler settings; where attrition must be quantified, a dedicated breakage unit represents it in the same flowsheet.
How do I determine the minimum fluidization velocity?
Minimum fluidization velocity follows from a force balance on the bed — the classic Ergun-type correlation using particle size and density and the gas density and viscosity. Dyssol Pro tracks the particle size distribution, solids density, and gas properties in the stream, so you have the inputs that correlation needs to set the operating gas velocity above minimum fluidization.
Why is my fluidized bed dryer producing too much dust?
Dust is fines elutriated out of the bed with the exhaust. Dyssol Pro models elutriation with a size-dependent grade efficiency, so you can study how gas velocity and the feed’s fine fraction drive the loss and size a downstream cyclone or filter to recover it.
How can I control final product moisture content?
Final moisture is set by the balance of heat input, residence, and exhaust humidity against the equilibrium moisture. Dyssol Pro predicts it dynamically, so you can study which combination of inlet temperature, gas flow, and bed (weir) height holds the product on target.
How do I model heat and mass transfer in a fluidized bed dryer?
This is the core of the unit: convective heat and mass transfer between gas and particles, coupled to a two-stage drying curve. In Dyssol Pro you set the geometry and material moisture parameters and it solves the coupled balances dynamically.
How can I avoid product overheating in a fluidized bed dryer?
While the surface is wet, evaporative cooling holds particle temperature down; overheating risk rises once the product dries out. Dyssol Pro tracks particle temperature and moisture per size class, so you can find the inlet temperature and residence that finish drying without overheating the product.
Why does my fluidized bed collapse during drying?
Bed collapse (defluidization) is a hydrodynamic failure, often from stickiness or too-low gas velocity. Dyssol Pro tracks the surface moisture and temperature that precede sticky defluidization, so you can steer the drying conditions that keep the bed safely fluidized.
How do I prevent channeling in a fluidized bed dryer?
Channeling is uneven gas distribution, where gas finds preferential paths instead of fluidizing the whole bed. It is prevented by a well-designed distributor plate with adequate pressure drop, keeping the gas velocity comfortably above minimum fluidization, and avoiding overwet, cohesive material that resists fluidization.
How does particle size distribution affect fluidization quality?
A wide PSD fluidizes differently — fines elutriate while coarse particles need more gas. Dyssol Pro resolves drying and elutriation per size class, so you see how the PSD affects fines loss and size-dependent drying.
What causes agglomeration during fluidized bed drying?
Agglomeration occurs when wet particle surfaces bridge and bind together. Dyssol Pro tracks the surface moisture that drives that tendency, and where agglomeration must be quantified a dedicated agglomeration or granulation unit represents it in the same flowsheet.
How can I improve drying uniformity across the bed?
Uniformity comes from good mixing and a narrow residence-time spread. Dyssol Pro assumes a well-mixed bed and shows size-resolved drying within it, so you can study how bed height and throughput set residence and even out the drying.
How do I optimize exhaust air humidity in drying?
Exhaust humidity reflects how fully you’ve used the air’s drying capacity — higher humidity means leaner air use but slower final drying. Dyssol Pro computes exhaust humidity and temperature, so you can optimize gas flow and inlet temperature to balance energy use against drying rate.
How can I reduce elutriation of fine particles?
Elutriation falls with lower gas velocity and fewer fines in the feed. Dyssol Pro’s elutriation model quantifies the loss against gas velocity and feed PSD, so you can find a velocity that still fluidizes the bed while cutting fines carryover.
How do I scale up a fluidized bed dryer?
Scale-up transfers the material drying parameters and the heat/mass transfer basis to a larger bed and gas flow. Dyssol Pro lets you fit those at lab scale and run the model at production geometry and throughput to check moisture and energy before building.
How can I use PAT sensors to control fluidized bed drying?
PAT (e.g. NIR moisture, bed temperature) is an instrumentation-and-control layer. Dyssol Pro complements it by predicting the moisture and temperature trajectories the sensors should see, providing a model basis for control strategies and soft sensors.