Roots Blowers for Fluidized-Bed Processes in Chemical Plants

Fluidized-bed processes in chemical plants use controlled gas flow to suspend or agitate solids for drying, coating, reaction or other process steps. When gas flows upward through a suitable distributor at sufficient velocity, the particles become suspended and the bed begins to behave like a fluid.

This behaviour provides effective mixing, relatively uniform temperature distribution and high rates of heat and mass transfer. Fluidized beds are therefore used for drying, cooling, granulation, coating, catalytic reaction, calcination and other gas-solid operations.

A roots blower can supply the fluidizing gas where the process requires a stable gas volume at low or moderate differential pressure. Its positive-displacement operating principle allows it to maintain the required airflow as the resistance of the distributor, particle bed and downstream filtration system changes within the permitted operating range.

Roots blower selection is not automatic. Centrifugal fans may be more efficient for very-high-volume, low-pressure systems, while pressurized reactors can require compressors. The appropriate technology must be determined from the complete pressure and flow duty.

What Is Fluidization?

A fixed bed consists of particles resting on a support or gas distributor. At low upward gas velocity, the gas passes through the voids between the particles without causing significant particle movement.

As gas velocity increases, the pressure drop across the bed also increases. When the upward drag force becomes approximately equal to the effective weight of the particles, the bed reaches minimum fluidization.

The corresponding gas velocity is known as the minimum fluidization velocity, U_mf.

Above this point, the bed expands and the particles become mobile. Depending on gas velocity, particle properties and equipment geometry, the bed may operate in bubbling, turbulent or circulating fluidization.

Fluidization Regimes

Fixed-Bed Operation

Below minimum fluidization velocity, the particles remain substantially stationary. Gas follows channels through the void spaces, and pressure drop increases with gas velocity.

A roots blower operating below the required flow may provide airflow without establishing fluidization. The process may then experience poor mixing, temperature gradients and localized channeling.

Minimum Fluidization

At minimum fluidization, the pressure drop across the bed approximately equals the effective weight of the particles per unit cross-sectional area.

The approximate bed pressure drop is:

This relationship represents an idealized force balance. Actual pressure drop can be affected by wall friction, non-uniform gas distribution, particle-size variation and cohesive forces.

Bubbling Fluidization

For many materials, gas supplied above minimum fluidization forms bubbles that rise through the particle bed. Bubble movement promotes mixing but can also cause particle attrition and entrainment.

The bubble size and behaviour depend on:

• Gas velocity
• Distributor design
• Particle size and density
• Bed depth
• Vessel diameter
• Internal equipment
• Particle cohesion

Excessive airflow can produce large bubbles, slugging or unstable bed behaviour.

Turbulent and Circulating Fluidization

At higher velocity, the distinction between bubbles and the dense particle phase becomes less clear. Particle entrainment increases, and solids-separation equipment becomes increasingly important.

In a circulating fluidized bed, a significant quantity of solids leaves the reactor with the gas and is returned through a cyclone or other separator.

The gas flow required for these systems can be very high. roots blower suitability should be evaluated against centrifugal and compressor technologies based on pressure, efficiency, turndown and process scale.

Chemical Fluidized-Bed Applications

Fluidized-bed equipment may be used for:

• Drying chemical powders and granules
• Cooling hot particles
• Granulation
• Particle coating
• Catalyst activation
• Catalyst regeneration
• Gas-solid chemical reactions
• Adsorption and desorption
• Calcination
• Thermal treatment
• Polymer processing
• Product conditioning
• Solvent removal
• Powder agglomeration

The required gas may be atmospheric air, heated air, nitrogen, recycled process gas or another process-specific medium.

When combustible solvents, reactive products or oxygen-sensitive materials are present, the fluidizing gas and blower package must be selected from a formal process-safety assessment.

Roots Blower Operating Role

In an open fluidized-bed system, atmospheric gas enters the roots blower, is delivered through conditioning equipment and then passes through the gas distributor into the particle bed.

A typical gas path may include:

• Inlet filter
• Roots Blower
• Silencer
• Gas heater or cooler
• Flow-control equipment
• Fluidized-bed gas distributor
• Particle bed
• Cyclone or product separator
• Exhaust filter or gas-treatment system

Installing the gas heater downstream of the blower allows the machine to handle gas at a lower inlet temperature. This increases gas density, limits blower discharge temperature and protects seals and lubricants from unnecessary thermal exposure.

In a closed-loop system, the gas leaving the bed may be filtered, cooled, dehumidified or condensed before it returns to the Roots process blower.

Superficial Gas Velocity

Fluidized-bed behaviour is generally evaluated using superficial gas velocity. This is the velocity calculated as though the vessel contained no particles:

The actual interstitial velocity between particles is higher than the superficial velocity because gas flows only through the open void space.

Estimating Minimum Fluidization Velocity

For an initial estimate involving approximately spherical particles, the Archimedes number can be calculated as:

An empirical estimate of the Reynolds number at minimum fluidization is:

This correlation is suitable for preliminary evaluation. Irregular, cohesive, porous or broadly distributed particles may behave differently. Experimental testing is preferable for critical chemical processes.

Particle Classification and Behaviour

Particle size, density and cohesion strongly affect fluidization.

Fine cohesive powders may form channels, cracks or agglomerates instead of fluidizing uniformly. Medium-sized particles often fluidize more predictably. Large or dense particles require higher gas velocity and can impose a significant pressure requirement.

Chemical powders are frequently described using Geldart-type behaviour:

Group A materials tend to expand before bubbling and can fluidize smoothly.

Group B particles usually begin bubbling near minimum fluidization.

Group C powders are cohesive and difficult to fluidize.

Group D particles are relatively large or dense and require high gas velocity.

This classification is useful for preliminary understanding but does not replace material testing.

Moisture, electrostatic forces, particle shape and temperature can shift the actual behaviour away from that expected from particle size and density alone.

Glossary

Bed pressure drop: Pressure difference across a particle bed during gas flow.

Bed-expansion ratio: Fluidized-bed height divided by the settled-bed height under stated conditions.

Distributor grid: Plate or nozzle system that distributes gas across a solids bed.

Distributor pressure drop: Pressure loss across the grid or plate that distributes gas into the bed.

Minimum fluidization velocity: Gas velocity at which drag balances the effective weight of bed particles.

Particle entrainment: Carrying of particles out of the bed by the gas stream.

Reaction-gas composition: Make-up of gases supplied to or produced by the chemical process.

Superficial gas velocity: Gas flow divided by the empty cross-sectional area of the bed.