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186 6 Separation of Particles from a Gas

The buildup of particles inside and/or on a filter surface results in more or less
structural changes in the filter and consequently alters the filtration efficiency and its
resistance to flow. These particles deposited on the filter surface can themselves act
as filter media. These factors should be taken into consideration in engineering
design and practices. And it is discussed in Chap. 10, Sect. 10.2.2.

6.5.5 Granular Filtration

When the filter media is made of granules rather than fibers, the corresponding
filters are called granular filters. It can be considered as an internal filter with respect
to a pack of granules or surface filter with respect to each single layer. The earliest
application of granular filtration was for water treatment. Layers or deep beds of
solid granules (e.g., sands) have been used for a long time for precleaning drinking
water. Recently, it has been tested for air emission control, especially for high-
temperature applications. Low-cost granules such as sand, silicate, or alumina
gravel can function very well at temperatures of as high as 450–500 °C. However,
at higher temperatures, sintering of the granules and fine particles on granules may
take place, leading to extreme filter clogging.
Granular filtration model was mainly based on the numerical analysis by
Rajagopalan and Tien [30] predicting the trajectories of the particles moving around
the filter granules under various conditions. By taking advantage of regression
analysis, they developed the classic Rajagopalan–Tien model, referred to as
RT-model hereby. Similar to the single fiber analysis, a single granule also captures
aerosol particles by diffusion, interception, sedimentation efficiency, and electro-
static precipitation (ESP).
Starting from the single granule analysis, they derived the total filtration effi-
ciency of a granular column filled with uniform granules. The correlation between
the single granule efficiency and the overall packed bed efficiency is

h

g d p ¼ 1 exp 1:5f ag sG ð6:107Þ
d G
where f is an empirical fitting factor, representing the fraction of contacts between
particles and collector granules; g sG is the single granule efficiency; h is the height
of the packed bed filled with granules; a is the filter solidity that depends on d G , the
diameter of the granules.
The internal pores of the individual granules are disregarded in the equations.
The average bed solidity can be determined using the equations proposed by
Pushnov [27].