The dominant means of PM capture in most industrial wet scrubbers is inertial impaction of the
PM onto liquid droplets. Brownian diffusion also leads to particle collection, but its effects are only
significant for particles approximately 0.1 micrometer (:m) in diameter or less. Direct interception is
another scrubber collection mechanism. Less important scrubber collection mechanisms utilize
gravitation, electrostatics, and condensation.
Inertial impaction in wet scrubbers occurs as a result of a change in velocity between PM
suspended in a gas, and the gas itself. As the gas approaches an obstacle, such as a liquid droplet, the
gas changes direction and flows around the droplet. The particles in the gas will also accelerate and
attempt to change direction to pass around the droplet. Inertial forces will attempt to maintain the
forward motion of the particle towards the object, but the fluid force will attempt to drag the particle
around the droplet with the gas. The resultant particle motion is a combination of these forces of fluid
drag and inertia. This results in impaction for the particles where inertia dominates, and by-pass for
those particles overwhelmed by fluid drag. Large particles, particles i.e. greater than 10 μm are more
easily collected by inertial impaction because these particles have more inertial momentum to resist
changes in the flow of the gas and, therefore, impact the droplet. Small particles (i.e. particles <1 μm)
are more difficult to collect by inertial impaction because they remain in the flow lines of the gas due to the predominance of the fluid drag force.
Collection by diffusion occurs as a result of both fluid motion and the Brownian (random)
motion of particles. This particle motion in the scrubber chamber results in direct particle-liquid
contact. Since this contact is irreversible, collection of the PM by the liquid occurs. Diffusional collection effects are most significant for particles less than 0.1 μm in diameter. Direct interception
occurs when the path of a particle comes within one radius of the collection medium, which in a
scrubber is a liquid droplet. The path can be the result of inertia, diffusion, or fluid motion.
Gravitational collection as a result of falling droplets colliding with particles is closely related to
impaction and interception, and is a minor mechanism in some scrubbers. Gravitational settling of
particles is usually not a factor because of high gas velocities and short residence times. Generally,
electrostatic attraction is not an important mechanism except in cases where the particles, liquid, or
both, are being deliberately charged, or where the scrubber follows an electrostatic precipitator.
Some scrubbers are designed to enhance particle capture through condensation. In such cases, the
dust-laden stream is supersaturated with liquid (usually water). The particles then act as condensation
nuclei, growing in size as more liquid condenses around them and becoming easier to collect by inertial impaction.
The collection mechanisms of wet scrubbers are highly dependent on particle size. Inertial
impaction is the major collection mechanism for particles greater than approximately 0.1 :m in
diameter. The effectiveness of inertial impaction increases with increasing particle size. Diffusion is
generally effective only for particles less than 0.1 :m in diameter, with collection efficiency increasing with decreasing particle size. The combination of these two major scrubber collection mechanisms contributes to a minimum collection efficiency for PM approximately 0.1 :m in diameter.
The exact minimum efficiency for a specific scrubber will depend on the type of scrubber, operating conditions, and the particle size distribution in the gas stream.
PM onto liquid droplets. Brownian diffusion also leads to particle collection, but its effects are only
significant for particles approximately 0.1 micrometer (:m) in diameter or less. Direct interception is
another scrubber collection mechanism. Less important scrubber collection mechanisms utilize
gravitation, electrostatics, and condensation.
Inertial impaction in wet scrubbers occurs as a result of a change in velocity between PM
suspended in a gas, and the gas itself. As the gas approaches an obstacle, such as a liquid droplet, the
gas changes direction and flows around the droplet. The particles in the gas will also accelerate and
attempt to change direction to pass around the droplet. Inertial forces will attempt to maintain the
forward motion of the particle towards the object, but the fluid force will attempt to drag the particle
around the droplet with the gas. The resultant particle motion is a combination of these forces of fluid
drag and inertia. This results in impaction for the particles where inertia dominates, and by-pass for
those particles overwhelmed by fluid drag. Large particles, particles i.e. greater than 10 μm are more
easily collected by inertial impaction because these particles have more inertial momentum to resist
changes in the flow of the gas and, therefore, impact the droplet. Small particles (i.e. particles <1 μm)
are more difficult to collect by inertial impaction because they remain in the flow lines of the gas due to the predominance of the fluid drag force.
Collection by diffusion occurs as a result of both fluid motion and the Brownian (random)
motion of particles. This particle motion in the scrubber chamber results in direct particle-liquid
contact. Since this contact is irreversible, collection of the PM by the liquid occurs. Diffusional collection effects are most significant for particles less than 0.1 μm in diameter. Direct interception
occurs when the path of a particle comes within one radius of the collection medium, which in a
scrubber is a liquid droplet. The path can be the result of inertia, diffusion, or fluid motion.
Gravitational collection as a result of falling droplets colliding with particles is closely related to
impaction and interception, and is a minor mechanism in some scrubbers. Gravitational settling of
particles is usually not a factor because of high gas velocities and short residence times. Generally,
electrostatic attraction is not an important mechanism except in cases where the particles, liquid, or
both, are being deliberately charged, or where the scrubber follows an electrostatic precipitator.
Some scrubbers are designed to enhance particle capture through condensation. In such cases, the
dust-laden stream is supersaturated with liquid (usually water). The particles then act as condensation
nuclei, growing in size as more liquid condenses around them and becoming easier to collect by inertial impaction.
The collection mechanisms of wet scrubbers are highly dependent on particle size. Inertial
impaction is the major collection mechanism for particles greater than approximately 0.1 :m in
diameter. The effectiveness of inertial impaction increases with increasing particle size. Diffusion is
generally effective only for particles less than 0.1 :m in diameter, with collection efficiency increasing with decreasing particle size. The combination of these two major scrubber collection mechanisms contributes to a minimum collection efficiency for PM approximately 0.1 :m in diameter.
The exact minimum efficiency for a specific scrubber will depend on the type of scrubber, operating conditions, and the particle size distribution in the gas stream.
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