Page 113 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 89

Multiple-chamber incinerators, rotary kilns, and multiple-hearth furnaces are
most widely used in industrial waste disposal.
Incineration has in the past provided certain advantages, particularly
where land disposal sites are not available or are too remote for economic
hauling. A properly designed and carefully operated incinerator can be located
adjacent to a process plant and can be adjusted to handle a variety and quantity
of wastes. Not only can heat recovery through steam generation reduce operat-
ing costs, but it can also save on pollution-control equipment. Additionally, the
residue is a small fraction of the original weight and volume of the waste and
may be acceptable for landfill.

PYROLYSIS. The most acceptable route to recycling wastes in the future may be
through pyrolytic techniques in which wastes are heated in an air-free chamber
at temperatures as high as 3000°F (1650°C). Pyrolysis seems to provide several
advantages over incineration. These systems encounter far fewer air-pollution
problems, handle larger throughputs resulting in lower capital costs, provide
their own fuel, degrade marginally burnable materials, and have the added
potential for recovering chemicals or synthesis gas.

LANDFILL. Sanitary landfill is basically a simple technique that involves spread-
ing and compacting solid wastes into cells that are covered each day with soil.
Care needs to be exercised that wastes disposed of in this fashion are either
inert to begin with or are capable of being degraded by microbial attack to
harmless compounds. The principal problems encountered in landfill operation
are the production of leachates that may contaminate the surrounding ground-
water and the potential hazards associated with the accumulation of flammable
gases produced during the degradation of the waste material.

Thermal Pollution Control

Temperature affects nearly every physical property of concern in water quality
management including density, viscosity, vapor pressure, surface tension, gas
solubility, and gas diffusion. The solubility of oxygen is probably the most
important of these parameters, inasmuch as dissolved oxygen in water is
necessary to maintain many forms of aquatic life. This potential damage to the
aquatic environment by changes in temperature, the reduction in the assimila-
tive capacity of organic wastes due to increased temperature, and the federal
enactment of more stringent water-temperature standards has led design engi-
neers to investigate various offstream cooling systems to handle thermal dis-
charges from processes and plants. Cooling towers are most often considered
for this service, followed by cooling ponds and spray ponds in that order.
Cooling towers may be classified on the basis of the fluid used for heat
transfer and on the basis of the power supplied to the unit. In wet cooling
towers, the condenser cooling water and ambient air are intimately mixed.
Cooling results from the evaporation of a portion of the water and to a lesser