Pneumatic conveying uses air to either push (positive pressure) or pull (negative pressure) solid material through a conveying line. The characteristics of the solid materials being conveyed play a significant role in how the pneumatic conveying system is designed and operated. Factors such as particle size, shape, bulk density and abrasiveness can impact the flow mode, conveying system efficiency and wear rates. This one-page reference outlines material characteristics that influence solids behavior in pneumatic conveying systems.
Dense-phase versus dilute-phase
FIGURE 1. In dilute-phase conveying, the air-to-solids ratio is higher than for densephase conveying [3]
Pneumatic conveying systems are generally categorized as either dilute-phase, where the air-to-product ratio is higher, and dense-phase, which injects less air and more product into the conveying line (Figure 1). Most systems use dilute-phase conveying, but dense-phase systems are used as applications dictate. Dilute-phase systems employ higher velocity rates than dense-phase systems. [1]
Material characteristics
The following properties influence the behavior of flowing powders in pneumatic conveying systems.
Particle size and distribution. In general, smaller particles are more easily suspended in flowing air, which allows higher conveying rates with the same pressure drop. Since larger particles require higher air velocities to be conveyed effectively, pressure drops will generally be higher. Maximum particle size, as well as average particle size, are important pieces of information because finer particles have higher surface-area-to-volume ratios, leading to more cohesion among particles and adhesion to system walls.
Bulk density. Bulk density helps determine what volume of air (ft3 per min, or CFM) is necessary to move a material through the conveying line. A widely cited rule of thumb says materials with bulk densities between 25 and 55 lb/ft3 will convey readily, while heavier materials require more power and larger vacuum receivers. Fine powders with low bulk density, such as fumed silica, present another set of challenges and may require more filtration, and therefore larger vacuum receivers, than medium-density powders [1]. The compressibility of a bulk solid (bulk density over a range of pressures) is also a factor in conveying. For example, granulated sugar is not compressible, whereas sawdust is highly compressible.
Gas interaction. How a solid or powder material interacts with air or other gases is a key area. Gas permeability refers to the ability of a packed bed of solids to resist gas flow through it [2]. Fluidization is the ability of a material to take on fluid-like characteristics as gas is incorporated. Dense-phase pneumatic conveying systems often rely on fluidization to achieve high solids-to-air ratios. Deaeration is the time required for a material to settle after it has been fluidized.
Cohesion (stickiness). Cohesion refers to the tendency for particles to stick to other particles and other surfaces. Particle size, shape, moisture content and interparticle interactions can all affect stickiness. Cohesion can hinder particle motion and conveying system performance. Friction between bulk solids and a conveyor’s surface is influences operational energy requirements of the conveying system.
Abrasiveness. Abrasiveness is a function of a solid’s pressure, velocity and the relative difference in hardness between the particles and a surface. Abrasive bulk solids (such as sand, fly ash or gravel) can wear equipment surfaces during pneumatic transport. For highly abrasive solids, alternative conveyor types may be more suitable.
Particle breakage. Mechanical forces in the conveying line can lead to particle attrition (wearing away of solids through contact). A bulk solid’s ability to resist breakage can be quantified through specialized testing under scenarios of impact, shear, rubbing and compression [2]. High levels of particle breakage in pneumatic conveying systems can lead to dust formation and its associated issues. Particle attrition affects particle-size distribution, which influences product flow properties.
Segregation. Segregation refers to the separation of groups of particles by size, shape or density. Several mechanisms of segregation exist industrially, including sifting, fluidization and dusting segregation. Pneumatic conveying can exacerbate segregation due to the flow of air and particle motion in conveying systems.
Health and environmental hazards. Many bulk solids have properties that pose hazards for personnel and facilities, and the hazards can often depend on the particle size. Fine powders have high surface-area-to-volume ratios, which means they can exhibit high chemical reactivity and biological exposures, as well as a higher risk of flash fires and dust explosions in conveying applications [2]. Materials considered benign as bulk solids can pose toxicological, ecological and explosion risks when suspended in air as fine particles. A material safety data sheet (MSDS) provides the basic risks associated with a material and should be a starting point for a hazard assessment in conveyor selection.
References
1. Pendleton, D., Pneumatic Conveying Basics, Chem. Eng., April 2023, pp. 44–45.
2. Maynard, E., Dilute- or Dense-Phase Pneumatic Conveying? Chemical Engineering Progress, November 2022.
3. Wilms, H. and Dhodapkar, S., Pneumatic Conveying: Optimal System Design, Operation and Control, Chem. Eng., October 2014, pp. 59–67.
4. Jenike & Johanson, How to Optimize Pneumatic Conveying Design, J&J Blog post, November 5, 2024, www.jenike.com.