Environmental Engineering Reference
In-Depth Information
ln 200
8
1
6
L
10 4
=
.
15
×
hr
·
mg
1
M
mg TOC
kg GAC
10 5
10 3
×
.
11
×
/
7
.
5
×
L/hr
hr · mg 200 mg TOC
720 m 3 1000 L
m 3
6
L
10 4
.
15
×
L
.
10 3
7
.
5
×
L/hr
10 3
Therefore, M
=
1
.
65
×
kg, close to the value calculated in Example 7.1.
7.11
Remember
Adsorption is a UNIT OPERATION. Regardless of what chemicals are being separated,
the basic design principles for adsorption are always similar.
Adsorption requires an interface between phases (solvent and adsorbent), and a driving
force must exist for the adsorbate to accumulate on the adsorbent. Electrostatic forces
or chemical bonding reactions are examples of the necessary driving force which allows
adsorption to occur.
Adsorption can be reversible or irreversible; it is sometimes possible to regenerate the
adsorbent (usually some losses in the amount of adsorbent and its capacity to adsorb
are encountered).
Adsorption has been used in wastewater treatment primarily for taste and odor control,
but it is growing more popular for removal of contaminants such as synthetic organic
chemicals, color-forming organics, disinfection chemicals and their by-products (the
most notorious being the trihalomethanes), and heavy metals.
Adsorption may be chosen over distillation when: (1) undesirable reactions occur during
distillation; (2) an azeotrope is encountered or when the boiling range of one set of
components overlaps the range of another set; (3) throughputs are less than a few
tons per day; or (4) corrosion, precipitation, or explosive conditions make distillation
impossible.
An adsorption isotherm relates the amount of the substance adsorbed at thermodynamic
equilibrium to the amount present in the liquid or gas stream (concentration or partial
pressure) at a constant temperature. Langmuir and Freundlich isotherms are the most
common adsorption isotherms.
An adsorption isotherm is useful for scaling up small-scale batch processes usually
carried out in a laboratory. Once the laboratory data are fitted to an isotherm, one can
predict the amount of adsorbent required to reach a specific effluent solute concentration
(in terms of a batch reactor) or the breakthrough time (for a plug-flow column).
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