Gas Emission Control - Mathematics and Statistics for the Environment - page 469

• Physical adsorption —The forces active in physical adsorption are electrostatic in nature.
These forces are present in all states of matter: gas, liquid, and solid. They are the same
forces of attraction that cause gases to condense and cause gases to deviate from ideal
behavior under extreme conditions. Physical adsorption is also sometimes referred to as
van der Waals' adsorption . The electrostatic effect, which produces the Van der Waal's
forces, depends on the polarity of both the gas and solid molecules. Molecules in any
state are either polar or nonpolar depending on their chemical structure. Polar substances
exhibit a separation of positive and negative charges within the compound. This separa-
tion of positive and negative charges is referred to as a permanent dipole . Water is a prime
example of a polar substance. Nonpolar substances have both their positive and negative
charges in one center so they have no permanent dipole. Because of their symmetry, most
organic compounds are nonpolar.
• Chemical adsorption —Chemical adsorption or chemisorption results from the chemical
interaction between the gas and the solid. The gas is held to the surface of the adsorbent
by the formation of a chemical bond. Adsorbents used in chemisorption can be either pure
substances or chemicals deposited on an inert carrier material. One example is using pure
iron oxide chips to adsorb H 2 S gases. Another example is the use of activated carbon that
has been impregnated with sulfur to remove mercury vapors.
All adsorption processes are exothermic whether adsorption occurs from chemical or physical
forces. In adsorption, molecules are transferred from the gas to the surface of a solid. The fast-
moving gas molecules lose their kinetic energy of motion to the adsorbent in the form of heat.
16.4.3 a dsorption e quilibrium r elationships
Most available data on adsorption systems are determined at equilibrium conditions. Adsorption
equilibrium is the set of conditions at which the number of molecules arriving on the surface of the
adsorbent equals the number of molecules leaving. The adsorbent bed is said to be “saturated with
vapors” and can remove no more vapors from the exhaust stream. The equilibrium capacity deter-
mines the maximum amount of vapor that can be adsorbed at a given set of operating conditions.
Although a number of variables affect adsorption, gas temperature and pressure are the two most
important in determining adsorption capacity for a given system. Three types of equilibrium graphs
are used to describe adsorption capacity:
• Isotherm at constant temperature
• Isostere at constant amount of vapors adsorbed
• Isobar at constant pressure
16.4.3.1 Isotherm
The most common and useful adsorption equilibrium data is the adsorption isotherm. The isotherm
is a plot of the adsorbent capacity versus the partial pressure of the adsorbate at a constant tem-
perature. Adsorbent capacity is usually given in weight percent expressed as grams of adsorbate
per 100 grams of adsorbent. Figure 16.13 is a typical example of an adsorption isotherm for carbon
tetrachloride on activated carbon. Graphs of this type are used to estimate the size of adsorption
systems as illustrated in Example 16.7.
■ EXAMPLE 16.7
Problem: A dry-cleaning process exhausts a 15,000-cfm air stream containing 680-ppm carbon
tetrachloride. Given Figure 16.13 and assuming that the exhaust stream is at approximately 140°F
and 14.7 psia, determine the saturation capacity of the carbon (USEPA, 1981, p. 5-8).