Environmental Engineering Reference
In-Depth Information
the removal is called the solvent. During absorption, a low relative volatility, K ,value as
defined in Chapter 3 is beneficial. Stripping works best at conditions that yield a high K
value.
Absorption can be either physical or chemical. Physical absorption takes advantage
of solubility differences of the gaseous component to be removed between the gas and
the liquid phases. Chemical absorption (which is not covered here) requires the gas to
be removed to react with a compound in the liquid solvent and to remain in solution.
Chemical absorption involves irreversible and reversible reactions. Reversible reactions
make it possible to recover and recycle the liquid solvent, while irreversible processes
allow for only a one-time solvent use.
The most common method of treating wastewaters to reduce the level of organic con-
taminants is steam stripping, particularly when the contaminant's boiling point is lower
than the boiling point of water (e.g., methylene chloride, acetone, methanol, benzene, and
toluene) [1].
One environmental example of absorption is the removal of ammonia gas from an
air stream with water as the mass-separating agent. With the proper choice of a liq-
uid solvent MSA, acid gases and other contaminants can also be removed from air
streams using absorption processes. Air-stripping towers used to remove volatile or-
ganic compounds (VOCs) from water are a common application of stripping. Again,
with the right gas phase MSA, many volatile contaminants can be removed from liquid
streams.
The primary advantage of absorption and stripping processes is that they usually do
not require reboilers or condensers. One exception is that heat input to a stripping col-
umn can often make a non-volatile contaminant adequately volatile for removal by the
solvent. Another advantage is that the absorption solvent can often be recycled; usually
through the use of a stripping column to recover the solvent. A weakness of these technolo-
gies is that chemical absorption typically has fairly low Murphree efficiencies for staged
columns.
To optimally remove a solute from a feed stream with a solvent in either absorption
or stripping, it is best to have the highest possible contact between the two phases for an
extended period of time. This can be achieved in one of two column designs: equilibrium
stage or packed. The gas flows from the bottom of the column to the top and the liquid
falls countercurrent from the top down in both designs. Spray columns, which are a
somewhat simplified form of packed towers, are also occasionally used. The mathematical
characterization of the process is different for staged and packed column design; staged
columns are analyzed with equilibrium as the controlling mass transfer mechanism while
packed columns are analyzed in terms of resistance to mass transfer between the phases.
Both analyses will be presented in this chapter.
In general, continuously packed columns are basically identical whether they are
used for absorption or stripping applications. Kohl provides a helpful guide for
choosing the best type of equipment for various stripping and absorption systems
(Table 6.1) [2].
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