Page 108 - Plant design and economics for chemical engineers

84 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

Gas-liquid absorption processes are normally carried out in vertical, coun-
tercurrent flow through packed, plate, or spray towers. For absorption of
gaseous streams, good liquid-gas contact is essential and is partly a fu tion of
proper equipment selection. Optimization of absorbers or scrubbers (as a plied
ncb
to noxious gas removal) is also important. The power consumption of a modem,
high-energy scrubber at its peak can be considerable because of the high
pressure drop involved. The latter difficulty has been alleviated in the spray
scrubber with its low pressure drop even when handling large volumes of flue
gases. In addition, sealing and plugging which can be a problem in certain
scrubbing processes (e.g., using a limestone-slurry removal of sulfur dioxide
from a flue gas) do not present difficulties when a spray is used in a chemically
balanced system.
The use of dry adsorbents like activated carbon and molecular sieves has
received considerable attention in removing final traces of objectional gaseous
pollutants. Adsorption is generally carried out in large, horizontal fixed beds
often equipped with blowers, condensers, separators, and controls. A typical
installation usually consists of two beds; one is onstream while the other is being
regenerated.
For those processes producing contaminated gas streams that have no
recovery value, incineration may be the most acceptable route when the gas
streams are combustible. There are presently two methods in common use:
direct flame and catalytic oxidation. The former usually has lower capital-
cost requirements, but higher operating costs, particularly if an auxiliary fuel is
required. Either method provides a clean, odorless effluent if the exit-gas
temperature is sufficiently high.
Each technique for removing pollutants from process gas streams is
economically feasible under certain conditions. Each specific instance must be
carefully analyzed before a commitment is made to any type of approach.

Water Pollution Abatement
Better removal of pollutants from wastewater effluents was originally mandated
by the federal government in the Water Pollution Control Act of 1970 (P.L.
92-500). Since then the performance requirements for the various treatment
technologies have been raised to new and higher standards with additional
legislation aimed at regulating the amounts of toxic and hazardous substances
discharged as effluents. Increased legal and enforcement efforts by various
governmental agencies to define toxic and hazardous substances give evidence
of the demands that will be placed on pollution technology in the future. The
trend in effluent standards is definitely away from the broad, nonspecific
parameters (such as chemical oxygen demand or biochemical oxygen demand)
and towards limits on specific chemical compounds.
The problems of handling a liquid waste effluent are considerably more
complex than those of handling a waste gas effluent. The waste liquid may
contain dissolved gases or dissolved solids, or it may be a slurry in either