Page 54 - 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

Aerosols 277

The use of devices to disperse quantities of pesticides or pulverized coal injection to provide an inlet stream for
for agricultural or public health applications has been efficient combusion. Diesel engines, turbines, and certain
widespread over the world. Their application ranges from kinds of rocket engines use fuel spray injection. The pro-
individual household and domestic activity to very large- cess of combustion concerns the steps of transport of fuel
scale, systematic treatment as an integral part of agricul- and oxidizer to the reaction or flame zone. Because of its
tural practice. technological importance, considerable research has been
In general, the control of pests or disease involves the done on the burning of finely divided particles.
distributionofasmallamountofpesticideoveraverylarge The main design objectives for combustion devices us-
surface area. This may include surfaces of buildings, vege- ing finely divided fuels are the following:
tation, or soil. The dispersal of pesticides is accomplished
by suspending material in a liquid, usually water, and then 1. A high combustion intensity
spraying or by dusting with a finely divided powder. A 2. A high combustion efficiency so that as little
variety of sprayers are available for dispersing pesticides; unreacted fuel leaves the combustion chamber as
techniques have been developed for different applications, possible
involving ground-based or aircraft operations. 3. A stable flame
Some optimum droplet size range is recognized as the 4. The minimum deposition of soot or solid on the
most effective for each pesticide and for each formulation combustion chamber walls
used for a specific control problem. Maximum effective 5. The maximum rate of heat transfer from the flame to a
control of a target organism with minimum use of toxic heat sink or exchanger
materials and minimum adverse impact on the surround-
ing ecosystem is the objective. This simple statement cov- The most common method of firing liquid fuels is to
ers a highly complex physical and biological phenomenon atomize the liquid before combustion. The fuel is intro-
that occurs during and after an area application of pesti- duced into the combustion chamber in the form of a spray
cides. Research toward this objective has been conducted of droplets, which has a controlled size and a velocity dis-
for many years. The earliest work with Paris green and tribution. The main purpose of atomization is to increase
toxic botanicals progressed through petrochemical prod- the surface area of the liquid in order to intensify vaporiza-
ucts, culminating in the extensive use of the synthetic pes- tion, to obtain good distribution of the fuel in the chamber,
ticides, organochlorines, as well as organophosphorus and and to ensure easier access of the oxidant to the vapor. In-
carbamate materials. jection of powdered fuels follows similar principles.
In some applications of aerosol technology, the capabil- After atomization, combustion takes place through a
ity of generating aerosols with large volumes of suspended series of processes, the most important of which include
material has been developed. Some situations dictate un- the following:
controlled smoke dispersal, as in military applications.
However, others can involve at least a degree of control 1. Mixing of the fuel particles with air and hot
of particle diameter and the integrity of chemical com- combustion products, a process usually occurring
position of the aerial suspension. Another application of under turbulent conditions
control requirements for sprays and mists is in the area of 2. Transfer of heat to the particles by convection from
medical research and therapy. For example, aerosols have the preheated oxidant and recycled combustion gases
been used for therapeutic treatment of respiratory disease. and by radiation from the flame and the chamber walls
Here, the medication must be dispersed with a controlled 3. Evaporation of particles; often accompanied by
particle size and volume for a long period of time under cracking of vapor
conditions when any chemical change in the suspended 4. Mixing of the vapor with air and combustion gases to
material is negligible. Requirements for the study of the form an inflammable mixture
influence of air contaminants on respiratory disease led to 5. Ignition of the gaseous mixture (depending on the
the engineering of large exposure chambers with carefully mixing conditions, an ignition may occur at the
controlled air properties. The controlled environment and oxygen-rich boundaries of eddies containing many
clinical conditions can be applied equally well to clean vaporizing particles or may occur as a microscale
room environments. The latter is an important adjunct process surrounding an individual particle)
to particle control technology in modern industry, where 6. Formation of soot, with residual fuels
manufacturing requires very sterile conditions. 7. Combustion of soot, a relatively slow process
The dispersal of material by spraying or by dusting of
solid particles plays an important role in combustion tech- In practice, these processes often occur simultaneously,
nology. Large industrial boilers or furnaces employ oil fuel resulting in an extremely complex aerosol system. Owing