General Description

In the scrubbing process, gas containing dust particles contacts a scrubbing liquid (often water). Here, some of the dust particles are captured by the scrubbing liquid. The scrubbing liquid can also condense on the dust particles, forming liquid droplets which are more easily removed than the dry dust particles. Finally, the mist droplets, which may or may not contain dust particles, are removed in an entrainment separator. Figure 5.18.1 schematically shows the process. The primary collection mechanisms for particle removal are inertial impaction, interception, diffusion or diffusiophoresis, and electrical attraction. At times, other mechanisms, such as those listed in Table 5.18.1, are involved in collection. However, in all cases, the dominant mechanism is inertial impaction followed by diffusion.

Scrubbers can be divided into three categories. In the first category, the scrubbing liquid is a spray, and collection occurs when particles are embedded by impaction in the scrubbing liquid surface (see part a in Figure 5.18.2). In the second category, collection occurs by impingement

Clean Gas Out

Removal of Wetted Particles in the Collecting Surface

Cleaned Gas and Mist

Particle-droplet Growth Liquid In

Cleaned Gas and Mist

Wetting of the Particles by the Scrubbing Liquid

Dirty Gas In

Clean Gas Out

Removal of Wetted Particles in the Collecting Surface

Cleaned Gas and Mist

Cleaned Gas and Mist

Wetting of the Particles by the Scrubbing Liquid

Dirty Gas In

' Collected Mist

Liquid Out Particles Out

FIG. 5.18.1 Schematic diagram of scrubbing process.

Removal of Particles from the Device

' Collected Mist

Humidification and / Droplet Growth

Saturation

Liquid

Liquid Out Particles Out

FIG. 5.18.1 Schematic diagram of scrubbing process.

on a wetted surface (see part b of Figure 5.18.2). Finally, in the third category, particle-laden air is bubbled through the scrubbing liquid, and particles are removed by impingement (see part c of Figure 5.18.2).

The main factor influencing a scrubber's efficiency is the size of the dust particles being removed. Scrubber collection efficiency also varies with scrubber design and operating conditions. Figure 5.18.3 shows examples of typical grade efficiency curves. Because of the efficiency variation due to particle size, collection efficiency is often expressed as a function of one particle size, the 50% cut diameter, that is, the particle diameter for which collection efficiency is 50% (Calvert et al. 1972; Calvert 1974, 1977). The rationale for this definition is that the grade efficiency curve for all scrubbing is steepest at the 50% point, and for estimating purposes this curve is considered to be a step function—all particles greater than this size are collected, all smaller are not (see dotted line on Figure

Removal of Particles from the Device

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