Page 104 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 81

or contribute to a violation of the Ambient Air Quality Standard (AAQS) or
maximum allowable increase over the baseline concentration of sulfur dioxide
and particulates in any area. (Three different clean air areas have been
designated, with class I the most pristine encompassing national parks and
forests.) Only when these steps indicate that the ambient air will not be
significantly impacted by the source may a construction permit be issued.
Air pollution control equipment can essentially be classified into two
major categories, those suitable for removing particulates and those associated
with removing gaseous pollutants,. Particulates are generally removed by me-
chanical forces, while gaseous pollutants are removed by chemical and physical
means.

PARTICULATE REMOVAL. The separating forces in a cyclone are the centrifugal
and impact forces imparted on the particulate matter. Similar forces account for
the particulate capture in mechanical collectors such as impingement and
dynamic separators. In settling chambers, the separation is primarily the result
of gravitational forces on the particulates. The mechanism in a wet collector
involves contact between a water spray and the gaseous pollutant stream.
Separation results primarily from a collision between the particulates and the
water droplets. Separation also occurs because of gravitational forces on the
large particles, or electrostatic and thermal forces on the small particles. The
main separating forces in a bag filter are similar to those described in the wet
collector, i.e., collision or attraction between the particle and the filter of the
bag. Finally, the principal components in an electrostatic precipitator are a
discharge plate and a collecting surface. The separation is effected by charging
the particles with a high voltage and allowing the charged particles to be
attracted to the oppositely charged collection plates.
To obtain the greatest efficiency in particulate removal, additional atten-
tion must be given to particle diameter and the air velocity. The particle size
determines the separating force required, while the effectiveness of the control
equipment is related to the stream velocity. Generally, the greater the relative
velocity between the air stream and the collision obstacle for the particulates,
the more effective the separating mechanism. The electrostatic precipitator is an
exception to this generality, since here the particle diameter influences the
migratory velocity and the power required to maintain the electrical field
influences the equipment performance. Figure 3-2 illustrates the characteristics
of various pollution particulates and the range of application for several control
devices as related to particle size.
A review of Table 8 and Fig. 3-2 indicates that large-diameter particles
can be removed with low-energy devices such as settling chambers, cyclones,
and spray chambers. Submicron particles must be removed with high-energy
units such as bag filters, electrostatic precipitators, and venturi scrubbers.
Intermediate particles can be removed with impingement separators or low-
energy wet collectors. Obviously, other equipment performance characteristics
as noted in Table 8 will also have their influence on the final equipment