AND ITS CONTROL

W. L.AWRENCE FAITH Consulting Chemical Engineer San Marino, Calif.

According to the 1958 Census of Manufactures, there are nearly 300,000 manufacturing establishments in the United States. No two of them are exactly alike in their emissions of air pollutants. Those with the greatest pollutant potential may be classified into five groups: the dusty industries, the smoky industries, those which emit odors, those emitting irritating or toxic substances, and for lack of a better term, those which emit photochemical pollutants. Obviously, some industrial units fall into more than one class.

Dusty industries.-The major dusty industries and the specific operations producing dust are shown in table 1. One can readily see that wherever a solid is broken down into fine particles (by blasting, crushing, grinding, pulverizing, or sanding) or wherever the fine particles are subsequently handled (conveying, classifying, mixing, drying, or calcining), dust problems may arise.

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The various dusts emitted differ one from another in chemical composition, density, particle size range, and obviously in the amounts emitted. Generally dusts vary in size from less than 0.1 micron 1 (carbon black) to 1,000 microns (sawdust). (See table 2.) The smaller particles, particularly those from 0.3 to 1 micron, tend to remain in the atmosphere and restrict visibility. Particles greater than 40 microns tend to fall rapidly to the ground fairly close to the source; those between 10 and 40 microns tend to fall somewhat farther away. The intermediate sizes are variable in their actions. But, generally, primary problems resulting from dust emissions are the dustfall nuisance from the larger particles and the visibility restriction from the smaller ones.

One may also consider metallurgical fumes as dusts, although they are extremely fine particles formed by the condensation of metallic vapors. These fine particles act like fine dusts and, unless they are toxic, their chief effect is visibility reduction.

Smoke. The combustion of fuels is the common denominator of all industry. Manufacturing operations require heat or power, and usually both, and both (except in the case of water power) arise from the burning of fuels. Smoke and its control will be discussed by speakers to follow me, so it is sufficient here just to say, again in generalities, that most smoke arises from the inefficient burning of bituminous coal and waste wood. Another all too common source of industrial smoke is trash burning, either for waste disposal or for salvage operations. The chief effect of black smoke is the soiling or blackening of surfaces. Emission of dense smoke is unnecessary and inexcusable in this day and age, but others will tell you more of this.

Odors. Probably the most annoying and difficult of all air pollution problems is the odor problem, and industry emits its share. A list of odorproducing industrial operations is shown in table 3. Most of the odors arise from the evaporation and subsequent loss of simple organic liquids from chemical operations, e.g., hydrocarbons, alcohols, aldehydes, amines, and mercaptans; from handling and use of solvents; or from the putrefaction of animal or vegetable matter. Fermentation operations (e.g., bread baking) also produce noticeable odors. Highly odorous hydrogen sulfide may be emitted wherever sulfur compounds are handled.

The big difficulty with odors is the fact that one can measure them only by the nose, and noses are notoriously variable and undependable. Further,

one can become accustomed to some odors and never notice them. Then there are the differences

among people as to likes and dislikes. One person will drive blocks out of his way to sniff the aroma from a neighborhood bakery, but someone who lives next door to the bakery may complain that the intense odor interferes with his sleep.

More often than not, odors are the spark to neighborhood complaints of air pollution. I am convinced that the real problem of petroleum refineries, for instance, is not hydrocarbons or smoke, but the typical odor noticeable in and near the refinery.

The chief effect of industrial odors is the esthetic offense against the sense of smell, and to many this is a distinct nuisance. Occasionally a strong odor can produce nausea, but this effect is very unusual.

Irritating and toxic substances.-The most common irritating or potentially toxic gas emitted industrially is sulfur dioxide, but here again chiefly in flue gases from the burning of coal and oil.

A few industries use sulfur dioxide in their processing operations and lose some in the process. The major industries of this type are nonferrous smelting, the manufacture of wood pulp, some chemical operations, particularly sulfuric acid manufacture, and the wet milling of corn. In high concentrations, sulfur dioxide affects those with respiratory diseases; at lower concentrations it is irritating to the nose and throat. However, the more common air pollution effects of sulfur dioxide, in the concentration range most likely to be encountered, are marking of vegetation (high concentrations kill some plants) and corrosion of metal and stone structures.

Another potentially toxic industrial pollutant is a group of fluorides, both in gaseous (hydrofluoric acid) and particulate form (sodium and calcium fluorides). Chief emitters of fluorides, in those cases where fluoride-containing dusts are handled, are the steel industry (sintering and open-hearth operations), aluminum manufacture (electrolytic cell), superphosphate manufacture, and various ceramic industries, particularly brick and tile manufacture.

Fluoride dusts settling on forage may be ingested by cattle with subsequent damage to the animals. Certain flowers, e.g., gladioli, are sensitive to fluorides, and some citrus growers suspect fluorides affect yield of fruit.

Occasionally acid and alkali mists are discharged from general industrial operations with consequent

damage to painted and enamelled surfaces. The toxic gas, carbon monoxide, is emitted from some industrial operations but in nowhere near the amounts emitted in vehicular exhaust.

Photochemical pollutants.-Photochemical smog, of the type commonly encountered in Los Angeles and on the west coast, and less frequently elsewhere, results from a photosensitized atmospheric reaction between reactive hydrocarbons (chiefly olefins), perhaps certain organic solvents, and oxides of nitrogen. The chief source of olefinic hydrocarbons and nitrogen oxides is motor vehicle exhaust, but some of each are also emitted industrially. All burning operations produce nitric oxide. Hydrocarbons are emitted in the manufacture, transportation, and marketing of gasoline. Solvent losses occur in a variety of operations, principally in painting and application of other protective coatings.

There has been a tendency in some quarters, in my opinion, to overestimate the role of solvents, the more stable hydrocarbons (particularly paraffins and aromatics), and flue gases containing low concentrations of nitrogen oxides, in the production of photochemical smog. Certainly this type of smog is a genuine nuisance, but it comes almost entirely from motor vehicle exhaust.

Control methods.-A variety of control methods are utilized by industry to reduce emissions to the point where air quality is acceptable. Occasionally an air pollution problem can be cured by changing fuels (e.g., natural gas to replace coal) or by shifting raw materials (use of granular material instead of a fine dust), but such changes are seldom economically feasible. In a few areas, plants are so located with respect to people, plants, and animals that no diminution of emissions is necessary, but this situation is rare. Occasionally, the value of material recovered will pay for collection equipment.

In the vast majority of cases, control equipment must be installed solely to prevent or greatly reduce emissions to the atmosphere. Control equipment requires a capital investment and involves extra operating costs over and above an uncontrolled process. Thus an economic consideration comes into the picture. For an equitable solution the cost of control must be weighed against the frequency, severity, and nature of the effect caused by the pol lutant. Most of the "hassles" between government and industry, or between the public and both government and industry, are incurred by differences of opinion in weighing the equities involved.