Page 67 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 49
understanding of the sources of contaminants to which workers are exposed is
important for the recognition, evaluation, and control of occupational health
hazards. For example, mechanical abrasions of solid materials by cutting,
grinding, or drilling can produce small particles which can form an airborne
dust cloud or solid aerosol. Liquid aerosols, on the other hand, may be
produced by any process that supplies sufficient energy to overcome the surface
tension of the liquid. This process occurs intentionally in spray coating and
unintentionally when oil mist is generated from lubricants or coolants used on
high-speed machinery. Liquid aerosols can also be produced by condensation.
Contaminant vapors are normally formed by allowing the liquid to evapo-
rate into the air. A significant source of mercury poisoning is from worker
exposure in laboratories where mercury has been spilled, trapped in cracks, and
then evaporates at room temperature to exceed the TLV of 0.05 mg/m3.
Gases are usually stored or processed in closed systems. Contamination of
air with such gas occurs from fugitive emissions (leaks) or from venting.
Essentially all closed systems leak to some degree. [The Environmental Protec-
tion Agency (EPA) through various studies has determined that emissions from
just the synthetic organic chemical manufacturing industry in the United States
are greater than 80,000 Mg/yr before emission controls are applied.] Obviously,
the tightness of a system is directly related to the engineering and leak
monitoring effort expended. This, in turn, depends on the consequences result-
ing from these emissions. High-value and very toxic materials are usually very
tightly controlled. Contaminants that are neither valuable nor toxic but that
create an undesirable atmosphere in neighboring communities are also con-
trolled to maintain good public relations. Flammable materials likewise are
carefully controlled because a leak may lead to a fire and a possible major loss
in life and facility. Table 1 lists potential sources of air contamination in the
chemical process industry, noting whether these are intermittent or continuous
sources, whether workers are directly involved in the emission operation, the
relative importance of the emission source, and the most probable control of the
emission.
In typical well-maintained plants, pumps and valves are probably the
major source of fugitive emissions. Monitoring and maintenance efforts are
therefore generally focused on these sources. Taken as a whole, fugitive emis-
sions, even without major seal failure, are the origin of the continuous back-
ground exposure of workers. This source of exposure may not, by itself, result in
overexposure; but its presence reduces the margin within which other emissions
may vary while still remaining under the acceptable TLV.
The continuous movement of materials through a process unit generally
does not involve any situations for emission release and consequent exposure.
However, some material-handling steps are difficult to accomplish with total
containment. For example, whenever quantities of materials are allowed to
accumulate in storage and then are removed for further processing, the possibil-
ity of release needs to be considered; for example, liquids entering fixed tankage
generally displace air that must be vented to”avoid overpressuring the tank.
Control of such liquid-transfer operations can be achieved by using variable