Particle Collection in Wet Scrubbers - Diffusion

Diffusion

Very small particles (less than 0.1 µm in diameter) experience random movement in an exhaust stream. These particles are so tiny that they are bumped by gas molecules as they move in the exhaust stream. This bumping, or bombardment, causes them to first move one way and then another in a random manner, or to diffuse, through the gas. This irregular motion can cause the particles to collide with a droplet and be collected (Figure 2). Because of this, diffusion is the primary collection mechanism in wet scrubbers for particles smaller than 0.1 µm.

The rate of diffusion depends on the following:

1. The relative velocity between the particle and droplet
2. The particle diameter
3. The liquid-droplet diameter.

For both impaction and diffusion, collection efficiency increases with an increase in relative velocity (liquid- or gas-pressure input) and a decrease in liquid-droplet size.

However, collection by diffusion increases as particle size decreases. This mechanism enables certain scrubbers to effectively remove the very tiny particles (less than 0.1 µm).

In the particle size range of approximately 0.1 to 1.0 µm, neither of these two collection mechanisms (impaction or diffusion) dominates. This relationship is illustrated in Figure 3.