Roots Blowers for Pneumatic Conveying of Chemical Powders

Pneumatic conveying of chemical powders depends on the material, containment requirements and resistance of the complete route. Pneumatic conveying provides an enclosed method of transferring these materials through pipelines using air, nitrogen or another compatible gas.

Roots blowers are widely used as conveying-gas sources in low- and moderate-pressure pneumatic conveying systems. As positive-displacement machines, they deliver an approximately constant gas volume per revolution and can maintain the required conveying flow when pipeline resistance varies within the allowable operating range.

The chemical product does not pass through the roots blower. The blower handles clean conveying gas, while the solid material is introduced downstream in a positive-pressure system or separated upstream in a vacuum system. Protecting the blower from chemical dust is essential for reliability, product purity and process safety.

Chemical Materials Handled by Pneumatic Conveying

Pneumatic systems can transport a wide range of chemical bulk materials, including:

• Polymer pellets and powders
• Polyethylene and polypropylene granules
• PVC powders
• Soda ash
• Sodium bicarbonate
• Lime and hydrated lime
• Limestone
• Fertilizer products
• Urea granules and prills
• Ammonium salts
• Detergent ingredients
• Pigments
• Carbon black
• Activated carbon
• Catalyst powders and pellets
• Resins
• Silica
• Alumina
• Mineral fillers
• Plastic additives
• Fine inorganic chemicals
• Intermediate chemical products

These materials have substantially different conveying characteristics. A system that successfully transports polymer pellets may be unsuitable for cohesive pigment, abrasive alumina or moisture-sensitive soda ash.

Pneumatic conveying equipment should therefore be selected from representative material properties rather than from the chemical name alone.

Why Pneumatic Conveying Is Used in Chemical Plants

An enclosed pneumatic conveying system can provide several advantages:

• Reduced product exposure to the environment
• Controlled dust emissions
• Flexible pipeline routing
• Transfer between several process levels
• Fewer moving mechanical parts along the conveying route
• Reduced risk of external contamination
• Operation under pressure or vacuum
• Capability to use a selected conveying gas
• Automated material routing
• Easier integration with silos, reactors and mixers

The enclosed nature of the pipeline does not automatically make the installation safe. Toxicity, chemical reactivity, electrostatic charging, combustible dust and overpressure must still be evaluated.

Positive-Pressure Pneumatic Conveying

In a positive-pressure system, the roots blower is installed upstream of the material feeding point. Clean conveying gas passes through the blower and the chemical powder is introduced into the pressurized pipeline.

Common feeding devices include:

• Rotary airlock valves
• Blow-through rotary valves
• Screw feeders
• Pressure vessels
• Double-valve arrangements
• Venturi eductors for suitable low-capacity duties
• Specialized dosing equipment

Positive-pressure conveying is commonly considered when:

• Material must be transported over a relatively long distance
• The route includes significant vertical elevation
• One source supplies one or several destinations
• The receiving vessel can accommodate the conveying gas
• Continuous or batch transfer is required
• Moderate pipeline pressure is acceptable

The feeding device must introduce the material while limiting reverse gas leakage. Leakage through a rotary valve can increase the required blower capacity and disturb material flow in the feed hopper.

The preliminary airflow balance is:

All flow quantities must be converted to the same pressure and temperature conditions before being added.

Vacuum Pneumatic Conveying

In a vacuum conveying system, the roots blower operates downstream of the receiving vessel and filtration equipment. The pressure inside the conveying pipeline remains below atmospheric pressure, drawing material toward the receiver.

Vacuum conveying may be preferred when:

• Material must be collected from several pickup points
• Outward leakage of toxic dust must be minimized
• Bags, drums or containers are unloaded manually
• The pickup point cannot be pressurized
• A centralized destination receives material from multiple sources
• Product escape into the workplace is unacceptable

Because pipeline pressure is below atmospheric pressure, leakage is generally inward. This helps contain dust but increases the gas flow handled by the vacuum blower.

The total blower capacity should include system leakage:

The material must be separated before the gas reaches the roots blower. A receiver filter and, where necessary, a secondary protection filter should prevent chemical particles from entering the rotor chamber.

Combined Vacuum and Positive-Pressure Systems

Some chemical plants use vacuum conveying to collect material from several sources and positive pressure to transfer it to a remote process or storage destination.

In this arrangement:

• Material is collected under vacuum.
• A receiver separates the solids from the conveying gas.
• The material enters an intermediate hopper or pressure vessel.
• A positive-pressure system transfers it to the final destination.

The vacuum and pressure sections may require separate roots blowers because they operate at different flow and pressure conditions. Combining them into one machine should only be considered after evaluating operating sequence, contamination risk and required pressure range.

Dilute-Phase Conveying

Dilute-phase conveying uses a relatively high gas velocity to suspend and transport particles through the pipeline. It is widely applied because it can handle many powders and granules using comparatively simple feeding equipment.

Potential advantages include:

• Continuous transfer
• Straightforward control
• Broad material applicability
• Relatively simple system layout
• Compatibility with rotary-valve feeding

Possible disadvantages include:

• Particle degradation
• Pipeline and bend wear
• High energy consumption
• Increased fines generation
• Greater receiver-filter loading
• Electrostatic charging
• High impact velocity at the destination

Dilute phase is often suitable for robust, non-abrasive powders. Friable pellets, abrasive minerals and sensitive granules may require lower-velocity conveying.

Dense-Phase Conveying

Dense-phase conveying transports material at a higher solids loading and lower gas velocity. Depending on the material and system design, the solids may move as plugs, dunes, waves or a partially fluidized bed.

Potential benefits include:

• Lower pipeline velocity
• Reduced product degradation
• Lower bend wear
• Reduced fines generation
• Lower gas consumption

Dense-phase conveying is not created simply by increasing pressure or reducing airflow. The material must possess suitable permeability, air-retention and flow characteristics.

Some chemical powders deaerate so rapidly that stable plug formation is difficult. Other cohesive powders may compact and block the pipeline. Material testing is therefore especially important.

A roots blower may serve certain low- or moderate-pressure dense-phase duties, but high-pressure dense-phase systems commonly require a compressor and pressure-vessel feeder.

Glossary

Dense-phase conveying: Pneumatic transport with relatively high solids concentration.

Dilute-phase conveying: Pneumatic transport with relatively low solids concentration.

Dust-tight feeder: Feeder designed to limit powder leakage where solids enter a conveying line.

Filter receiver: Vessel and filter that separate solids from transport gas.

Inert conveying gas: Non-reactive gas used to transport powder while limiting oxidation or reaction.

Powder bridging: Stable arch of powder across an outlet that interrupts gravity discharge.

Powder containment: Enclosure and filtration that limit release of chemical dust.

Rotary valve: Rotating pocket feeder that meters solids into a conveying line.