Gas Emission Control - Mathematics and Statistics for the Environment - page 450

Rain shadow effect —The phenomenon that occurs as a result of the movement of air masses
over a mountain range. As an air mass rises to clear a mountain, the air cools and precipita-
tion forms. Often, both the precipitation and the pollutant load carried by the air mass will
be dropped on the windward side of the mountain. The air mass is then devoid of most of
its moisture; consequently, the lee side of the mountain receives little or no precipitation
and is said to lie in the rain shadow of the mountain range.
Raleigh scattering —The preferential scattering of light by air molecules and particles that
accounts for the blueness of the sky. The scattering is proportional to 1/λ 4 .
Relative humidity —The concentration of water vapor in the air. It is expressed as the percent-
age that its moisture content represents of the maximum amount that the air could contain
at the same temperature and pressure. The higher the temperature the more water vapor
the air can hold.
Second law of thermodynamics —Natural law that dictates that with each change in form
some energy is degraded to a less useful form and given off to the surroundings, usually
as low-quality heat.
Secondary pollutants —Pollutants formed from the interaction of primary pollutants with
other primary pollutants or with atmospheric compounds such as water vapor.
Solute —The substance dissolved in a solution.
Solution —A liquid containing a dissolved substance.
Specific gravity —The ratio of the density of a substance to a standard density. For gases, the
density is compared with the density of air (= 1).
Stratosphere —Atmospheric layer extending from 6 to 30 miles above the Earth's surface.
Stratospheric ozone depletion —The thinning of the ozone layer in the stratosphere; occurs
when certain chemicals (such as chlorofluorocarbons) capable of destroying ozone accu-
mulate in the upper atmosphere.
Thermosphere —An atmospheric layer that extends from 56 miles to outer space.
Troposphere —The atmospheric layer that extends from the Earth's surface to 6 to 7 miles
above the surface.
Weather —The day-to-day pattern of precipitation, temperature, wind, barometric pressure,
a nd hu m id it y.
Wind —Horizontal air motion.
16.3 ABSORPTION
Absorption (or scrubbing) is a major chemical engineering unit operation that involves bringing
contaminated effluent gas into contact with a liquid absorbent so that one or more constituents of
the effluent gas are selectively dissolved into a relatively nonvolatile liquid. Key terms used when
discussing the absorption process include the following:
Absorbent —The liquid, usually water mixed with neutralizing agents, into which the con-
taminant is absorbed
Solute —The gaseous contaminant being absorbed (e.g., SO 2 , H 2 S)
Carrier gas —The inert portion of the gas stream, usually flue gas, from which the contami-
nant is to be removed
Interface —The area where the gas phase and the absorbent contact each other
Solubility —The capability of a gas to be dissolved in a liquid
Absorption units are designed to transfer the pollutant from a gas phase to a liquid phase. The
absorption unit accomplishes this by providing intimate contact between the gas and the liquid,
providing optimum diffusion of the gas into the solution. The actual mechanism of removal of a pol-
lutant from the gas stream takes place in three steps: (1) diffusion of the pollutant gas to the surface