Pneumatic conveying involves the transportation of solid particles using a moving gas, typically air, as the carrier. The drag force created by the slip between the gas and the particles, or groups of particles like a packed bed, surpasses frictional losses and gravitational pull, providing the essential energy to move the particles from one location to another. When the gas velocity surpasses a particle’s terminal velocity, that particle gets carried along with the gas flow. However, for bulk solids, this velocity tends to be slightly higher due to particle clustering.
Depending on the gas velocity and material properties, various flow patterns emerge. At a constant feed rate of solids, as gas velocity increases, the packed bed transitions into a moving slug flow, which then transforms into dunes or unstable dunes. Eventually, the particles become fully entrained in the gas stream, resulting in homogeneous flow. In homogeneous flow, particles move swiftly but at a lower solids concentration. Conversely, increasing solids concentration at a constant gas velocity reverses this sequence, leading to dune flow, slug flow, and ultimately a packed bed.
This diverse range of flow dynamics offers a broad spectrum of applications. Homogeneous flow is relatively straightforward to manage, while dune and slug flows are more challenging but essential for specific applications. Conveying with lower gas velocity and higher solids concentration is termed dense phase conveying, whereas higher gas velocity and lower solids concentration is known as dilute phase conveying. Medium dense or strand phase (occasionally called mixed phase) conveying falls between these two and necessitates precise control of gas velocity at the material pickup point to prevent unstable flow conditions.
To distinguish between dilute and dense phase conveying, examining the solids flow rate (carrying capacity) or pressure gradient against gas flow rate in a horizontal pipe is helpful. The carrying capacity peaks at a given system pressure drop as gas flow rate increases. The higher the system’s pressure drop, the higher this maximum carrying capacity. Generally, this maximum marks the transition between dense and dilute phase conveying, though it is not an ideal operating point due to unstable flow dynamics corresponding to the saltation flow. Dilute phase systems operate slightly above the saltation velocity, while dense phase systems operate below it.
The pressure gradient against gas flow rate curve for single-phase gas flow (zero solids rate) follows a monotonic, velocity-squared trend. However, introducing solids into the horizontal conveying line drastically alters the pressure gradient due to additional pressure drop factors such as particle acceleration, particle shear stresses, and particle-wall friction, which are more significant than their gaseous counterparts.
For optimal operation in terms of pressure gradient versus gas flow rate, it is advisable to operate near but not too close to the saltation flow line, where the pressure gradient is lowest, resulting in reduced capital and power consumption. In dense phase flow, which occurs to the left of the saltation flow line, excessively low gas flow can lead to a packed bed plugging the line. Dense phase mode exhibits higher pressure gradients and fluctuations, manifesting as vibrations in the conveying line. Conversely, in dilute phase flow, which occurs at high gas velocities to the right of the saltation flow line, pressure fluctuations are minimal, reducing concerns about mechanical stress from vibration.
Dilute versus Dense Phase Conveying
Dilute phase conveying takes place when the gas flow rate (velocity) is sufficiently high to fully suspend and entrain the transported solids. Typically, superficial gas velocities exceed 4,000 ft/min, with solids loading less than 15% of the gas mass (i.e., 15 lbm of solids per lbm of gas). In this scenario, particles are fully suspended without stationary layers, moving dunes, strands, or plugs at the bottom of a horizontal conveying line section or recirculation patterns in a vertical section. As mentioned, slightly above the saltation velocity should be considered the minimum gas velocity for operating a dilute phase conveying line.