Page 75 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: FWD Revised Pages
Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

298 Aerosols

These are outlined in the form of emission limits or air- gas through a large plenum chamber, sometimes equipped
borne particle concentration limits. with horizontal plates to facilitate deposition. Removal ef-
When the emission limitations for a planned operation ficiency depends on the number of shelves, the size of the
are estimated to be in conformance with air quality regula- chamber, and the air flow rate.
tions,aseriesofprocessdesigninteractionsmaytakeplace
to ensure compliance. To reduce emissions of existing and
2. Centrifugal Separation
new facilities, considerable innovation may be required in
process and system engineering. In general, gas cleaning Centrifugal separation or cyclones rely on inertial forces
and atmospheric emissions can be minimized by reducing in curvilinear flow to induce deposition on the walls of
to the greatest degree possible the amount of contaminants the collector. The collection efficiency of such devices
entering exhaust gas streams. Process, operational, and depends on the air flow at the inlet, the spinning rate, the
system control will concentrate contaminants in the small- collector dimensions, and the particle size. Efficiencies of
est possible air volumes. As an engineering principle, this 90% or greater can be achieved for particles of 10-µm
is important since the cost of control equipment is based diameter and larger. Cyclones have been designed with
mainly on the volume of gas that has to be handled and various configurations to achieve high gas rotation rates
not on the amount of material to be removed. Also, most for given inlet velocities. Usually rotation is induced in
cleaningequipmentismoreefficientathighconcentration, a helical path such that layers of collected particles can
all else being equal. Emission control with modern, highly slide by gravity down the walls to an accumulator.
efficient industrial cleaning devices can cost over $2 per
cubic meter per second of installed capacity and entails
3. Wet Scrubbing
high, continuous operational and maintenance expense.
Thus, innovation in the process or system control steps Inertial collection can also be achieved effectively by tak-
represents major opportunities for minimizing air cleaning ing advantage of liquid gas contactors through droplet–
control costs. Such practice can also be vital to improving particle collision and contact with liquid sheets during
employee safety inside the plant. Emission reductions or gas passage through a liquid spray. Sprays are added to
their elimination can be achieved in a variety of ways, in- cyclones, for example, to improve the collection efficiency
cluding substituting products, changing processes, revis- of the latter for small particles. Wet scrubbing also uses
ing plant layout, or revising internal ventilation systems. interception and diffusion processes as a supplement to
inertial effects. Wet scrubber collection efficiencies de-
pend on the spray droplet diameter, the particle diameter,
A. Particle Removal Technology
and the flow rates of the aerosol and the liquid spray. Ef-
Methods for the removal of particles from industrial ficiencies of 99% or greater are achieved in scrubbers for
streams rely on the same generic collection techniques particles of 1-µm diameter or larger.
used for sampling and collecting particles for examination A variety of designs have been developed to maximize
and characterization. These methods include: (1) gravita- the relative motion between particles and spray droplets
tional settling, (2) centrifugal separation, (3) inertial cap- or the contact between liquid and particles. These include
ture by wet scrubbing, (4) filtration, and (5) electrostatic jet impingement devices, packed or sieved plate towers,
precipitation (see Fig. 1). preformed and gas atomized spray towers, and venturi
In all cases, the efficiency of removal using a control sprayers.
device is given by the collection efficiency, equal to the
ratio amount collected in the device to the amount in the
4. Filtration
inlet gas stream. Another common measure of efficiency is
the penetration, which is 1 minus the collection efficiency. Industrial filtration has become increasingly attractive for
The decontamination factor is the reciprocal value of the gas cleaning with new high-volume flow configurations;
penetration. Modern gascleaning machinery aims for very it is possible to achieve very high collection efficiencies
high collection efficiencies exceeding 99%, at least over for finely divided particles less than 1 µm in diameter.
a given particle size range. Industrial filters have efficiencies of well over 99% for
particles over a full range of size. The primary design
factors are pressure drop (air pumping expense) and ability
1. Gravitational Settling
to clean filters.
Removal of particles by gravitational fallout is the least Filter units are broadly classified into two types: (1)
efficient of available techniques. It is primarily used for fabric or cloth bag systems and (2) deep columns packed
very large particles when the settling rate is rapid. Grav- with rock or other contactors such as fiber plates or glass
ity removal is inexpensive and generally involves flowing beads or rings. Commerical fiber materials are selected for