Process Technology: An Introduction - Haan A.B. 2015

11 Particle removal from gases
11.3 Gravitational separators

The simplest way to separate particles from gases is by gravity. The gravity settling chamber is one of the oldest forms of gas-particle separation. It may be nothing more than a large room where the well-distributed gas enters at one end and leaves at the other (Fig. 11.5). In the settling chamber the gas stream is slowed down sufficiently to allow particles to settle under their own weight. In theory a very large settling chamber would give sufficient time for even very small particles to be collected. Practical size limitations, however, restrict the applicability of these chambers to the collection of coarse particles. In the horizontal type chamber, an average gas velocity can be assumed to represent piston flow through the chamber. This can be simply derived from the gas-flow rate Q and the height H and width W of the chamber:

Image

(11.4)

From the length L of the chamber the residence time t of the gas can be calculated as

Image

(11.5)

During this residence time a particle of size dp will settle a distance h equal to the product of the terminal settling velocity vt of the particle and the residence time t:

Image

(11.6)

where the steady-state settling velocity of spherical particles in the Stokes regime is given by

Image

(11.7)

In general nonspherical particles settle more slowly than spherical ones of the same size. If a particle settles a distance h, then h/H represents the fraction of particles of this size that will be collected. If h is equal to or greater than H, all particles of that size or larger will be collected in the settling chamber. A curve of h/H ratios for the different sizes of a material provides the fractional or grade efficiency curve for the settling chamber:

Image

(11.8)

Gravity settlers have largely disappeared because of bulky size and low collection efficiency. They are generally impractical for particles smaller than 50 µm. Improved settling chamber efficiency can be achieved by decreasing the height a particle has to fall before being collected. This has been applied in the Howard settling chamber (Fig. 11.6), where a number of collecting trays have been inserted in the chamber. Because gravity settling is technically feasible only for coarse particles, these settlers have negligible importance in gas-particle separations.

Image

Fig. 11.6: Multitray settling chamber.