Scrubbers Using A Combination Of Designs

Besides the preceding basic scrubber types, a number of scrubber designs use two or more of the collection mechanisms in Table 5.18.1. With a process known as flux force/condensation scrubbing (Calvert and Jhaveri 1974), collection phenomena such as diffusiophoresis or ther-mophoresis combine with condensation to improve particle removal. This process uses steam to increase the deposition of scrubbing liquid on the dust particles by condensation, making the resulting dust-liquid droplets easier to remove. Temperature gradients or concentration fluxes enhance particle collection. This process is particularly attractive if the incoming dusty gas stream is hot and humid. Otherwise, the process is more costly than more conventional scrubber designs.

Another approach enhances scrubber collection efficiency using electrostatics. The electrostatics can take the form of wet ESPs, charged-dust/grounded-liquid scrubbers, charged-drop scrubbers, or charged-dust/charged-liquid scrubbers (Calvert 1977).

the collecting liquid (Overcamp and Bowen 1983). These high velocities enhance the impaction efficiency. Because fine particles rapidly attain the velocity of their surroundings, continually mixing the dirty air stream with the collecting liquid is necessary for good impaction, and short distances between the particles and the collecting surface are required for efficient diffusive collection (Nonhebel 1964).

Considering all collection mechanisms in an ideal scrubber, Figure 5.18.13 shows the shape of a typical efficiency versus particle diameter curve for impaction plus diffusion. In the figure a minimum collection efficiency is apparent that is typical of the low efficiency in the transition from impaction collection processes to diffusion-related processes. Above this minimum, efficiency varies roughly as a function of the dust particle diameter squared, and below it roughly as (1/d2) (Reist 1993). Efficiency is approximately proportional to particle density. As might be expected, because of the variety of scrubbing configurations available, no one collection mechanism is dominant, and no single approach determines all scrubber performance (Crawford 1976).

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