Oif

Liquid Out

FIG. 5.18.6 Centrifugal scrubbers.

Cleaned Gas Out t

Cleaned Gas Out t

FIG. 5.18.7 Plate scrubbers.

Cleaned Gas Out t

Cleaned Gas Out t

FIG. 5.18.8 Massive packing and fibrous packing scrubbers.

exceed 30 m/sec to prevent reentrainment of the droplet-particle mixture.

IMPINGEMENT ON A WETTED SURFACE—TYPE II

With impingement on a wetted surface, collection occurs by impingement. These scrubbers include plate scrubbers and scrubbers with massive packing or fibrous packing.

Plate Scrubbers

Plate scrubbers have plates or trays mounted within a vertical tower at right angles to the axis of the tower (see Figure 5.18.7). Gas flows from the bottom of the tower up through slots, holes, or other perforations in the plates where mixing with the scrubbing liquid occurs. Collection efficiency increases as the cut diameter decreases. A cut diameter of about 1 /m (aerodynamic diameter) is typical for Ak-in holes in a sieve plate (Calvert et al. 1972). Calvert (1977) points out that increasing the number of plates does not necessarily increase collection efficiency. Once particles around the size of the cut diameter are removed, adding more plates does little to increase collection efficiency.

Massive Packing

Packed-bed towers packed with crushed rock or various ring- or saddle-shaped packings are often used as gas scrubbers (see a in Figure 5.18.8). The gas-liquid contact can be crossflow, concurrent, or countercurrent. Collec-tion efficiency rises as packing size falls. According to Calvert (1977), a cut diameter of about 1.5 /m (aerodynamic) can be achieved with columns packed with 1-in Berl saddles or Raschig rings. Smaller packings give higher efficiencies; however, the packing shape appears to have little importance.

Fibrous Packing

Beds of fibers (see part b in Figure 5.18.8) are also efficient at removing particles in gas scrubbers. Fibers can be made from materials such as steel, plastic, or even spun glass. With small diameter fibers, efficient operation is achieved. Efficiency increases as the fiber diameter decreases and also as gas velocity increases. The collection is primarily by impaction and interception. Diffusion is important for small particles, although to increase diffusional collection, lower gas velocities are necessary. Cut diameters can be as low as 1.0 to 2.0 /m (aerodynamic) or in some cases as low as 0.5 /m (aerodynamic) (Calvert et al. 1972). A major difficulty with fibrous beds is that they are prone to plugging; even so, they are widely used.

BUBBLING THROUGH SCRUBBING LIQUID—TYPE III

In scrubbers that bubble gas through a scrubbing liquid, the particulate-containing gas is subjected to turbulent mixing with the scrubbing liquid. Droplets form by shear forces, or the dust particles are impacted directly onto the scrubbing liquid. Droplets are also formed by motor-driven impellers (that are either submerged or in the free air of the scrubber). These impellers serve not only to form small droplets but also to enhance impaction of the dust particles.

Baffle and Secondary-Flow Scrubbers

These units remove particulates from an air stream by continually changing the flow direction and velocity as the gas flows through the unit (see Figure 5.18.9). This motion results in intimate particle mixing in the gas and the spray droplets of scrubbing liquid. Zig-zag baffles or louvers are examples of how flow direction and velocity are altered internally. With these units, cut diameters as low as 5 to 10 /m can be achieved with low pressure drops. Plugging can be a problem with some heavy particle loadings.

Impingement and Entrainment Scrubbers

With this type of scrubber design, the particle-gas mixture is bubbled through or skimmed over the scrubbing liquid surface which atomizes some droplets and mixes the particles with the scrubbing liquid (see Figure 5.18.10). Both particle impaction on the liquid surface and on the atomized drops and some diffusion contribute to particle collection, which is effective with high-velocity entrainment for cut diameters down to 0.5 / m (aero-

Cleaned Gas Out

Liquid In

Gas Plus Dust In

Cleaned Gas Out

Liquid In

Gas Plus Dust In

Liquid In

FIG. 5.18.9 Baffle-type scrubber.

Liquid In dynamic diameter). The pressure drop for this class of device is high.

Mechanically Aided Scrubbers

With mechanically aided scrubbers, a motor-driven device in the scrubber produces spray droplets and mixes the incoming gas and scrubbing liquid more intimately (see Figure 5.18.11). The motor-driven device can be a fan with the scrubbing liquid introduced into the fan rotor, or it can be a paddle arrangement (disintegrator) in the scrubbing liquid that produces droplets. These units have cut diameters down to 2.0 /m (aerodynamic), or even 1.0 /m for some disintegrator designs (Calvert et al. 1972).

Fluidizied- (Moving) Bed Scrubbers

Fluidized-bed scrubbers are similar to packed-bed units except that the packing material is light enough to float in the gas stream (see Figure 5.18.12). The packing material expands to about twice its original depth. This expansion intimately mixes the particulates and scrubbing liquid and permits effective collection down to cut diameters of 1 /m (aerodynamic) (Calvert 1977).

Gas Plus Dust In

Liquid oo

Cleaned Gas Out ->

FIG. 5.18.10 Impingment scrubber.

Liquid Out

Liquid

Liquid

FIG. 5.18.9 Baffle-type scrubber.

FIG. 5.18.11 Mechanically aided scrubber.

Cleaned Gas Out

Liquid In

Gas Plus Dust In

Liquid Out

FIG. 5.18.12 Fluidized- or mobile-bed scrubber.

Liquid Out

FIG. 5.18.12 Fluidized- or mobile-bed scrubber.

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