Encyclopedia of Environmental Science and Engineering, Volume I and II

AIR POLLUTION SOURCES 105

Part 2. Major Air Pollution Sources (continued)

Chemicals manufacturing industry Nature of activity Type of air pollution problems

CHARCOAL Charcoal is generally manufactured by means

of pyrolysis, or destructive distillation of

wood waste from members of the deciduous

hardwood species. Four tons of hardwood

are required to produce one ton of charcoal.

In the pyrolysis of wood, all the gases, tars,

oils, acids, and water are driven off leaving

virtually pure carbon.

During pyrolysis of wood, carbon monoxide, hydrocarbons,

particulate crude methanol, and acetic acid are emitted

into the atmosphere. Some of these gases can be

recovered by utilizing a chemical recovery plant.

CHLOR-ALKALI Chlorine and caustic are produced concurrently

by the electrolysis of brine in either the

diaphragm or mercury cell.

Emissions from diaphragm and mercury cell chlorine

plants include chlorine gas, carbon dioxide, carbon

monoxide, and hydrogen. Other emissions include

mercury vapor, chlorine, wet scrubbers (alkaline) can

be utilized for emission reduction.

EXPLOSIVES An explosive is a material which, under the

influence of thermal or mechanical shock,

decomposes rapidly and spontaneously with

the evolution of large amounts of heat and

gas.

Emissions: Sulfur oxides and nitrogen oxides emissions

from processes which produce some of the raw

materials for explosives production can be

considerable.

HYDROCHLORIC ACID Hydrochloric acid is manufactured by a

number of different chemical processes.

Approximately 80% of the hydrochloric

acid is produced by the by-product hydrogen

chloride process. By-product hydrogen

chloride is produced when chloride is added

to an organic compound such as benzene,

toulene, and vinyl chloride.

The recovery of the hydrogen chloride from the

chlorination of an organic compound is the major

source of hydrogen chloride emissions. The exit gas

from the absorption or scrubbing system is the actual

source of the hydrogen chloride emitted.

HYDROFLUORIC ACID All hydrofluoric acid in the United States is

currently produced by reacting acid grade

fluorspar with sulfuric acid for 30–60

minutes in externally fired rotary kilns at a

temperature of 400–500°F.

The exist gases from the final absorber contain small

amounts of HF, silicon tetrafluoride (SiF 4 ), CO 2 , and SO 4

and may be scrubbed with a caustic solution to further

reduce emissions. Dust emissions may also result from

raw fluorspar grinding and drying operations.

NITRIC ACID The ammonia oxidation process (AOP) is the

principal method of producing commercial

nitric acid. It involves high temperature

oxidation of ammonia with air over a

platinum catalyst from nitric oxide. The nitric

oxide air mixture is cooled, and additional air

water added to produced nitric acid.

The main source of atmosphere emissions from the

manufacture of nitric acid is the tail gas from the

absorption tower, which contains unabsorbed nitrogen

oxides. These oxides are largely in the form of nitric

oxide and nitrogen dioxide.

PAINT AND VARNISH The manufacture of paint involves the

dispersion of a colored oil or pigment in a

vehicle, usually an oil or resin, followed by

the addition of an organic solvent for

viscosity adjustment.

Particulate emissions amount to 0.5 to 1% of the pigment

handled; 1 to 2% of the solvent are lost. Hydrocarbons

are the pollutant of primary concern.

PHOSPHORIC ACID Phosphoric acid is produced by two principal

methods, the wet process and the thermal

process. In the wet process finely-ground

phosphate rock is fed into a reactor with sulfuric

acid to form phosphoric acid and gypsum. In the

thermal process phosphate rock, siliceous flux,

and coke are vaporized and placed in contact

with water to produce phosphoric acid.

Emissions from the wet process are primarily gas

fluorides, consisting mostly of silicon tetrafluoride and

hydrogen fluoride. The principal emissions from the

thermal process acid are P 2 O 2 acid and acid mist.

Particulates are also emitted in fairly large quantities.

PHTHALIC ANHYDRIDE Phthalic anhydride is produced primarily by

oxidizing naphthaline vapors with excess

air over a catalyst, usually V 2 O 5. The

phthalic anhydride is then purified by a

chemical soak in the sulfuric acid.

The major source of emissions is the excess air from the

production system which contains some uncondensed

phthalic anhydride, maleic anhydride, quinines, and

other organics.

PLASTICS The manufacture of most resins or plastics

begins with the polymerization or linking of

the basis compound (monomer) usually a

gas or liquid, into high molecular weight

noncrystalline solids.

The air contamination from plastics manufacturing are the

emissions of raw material or monomer, emissions of

solvents or other volatile liquids during the reaction,

emissions of sublimed solids such as phthalic

anhydride in alkyd production, and emissions of

solvents during storage and handling of thinned resins.

(continued)

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