Environmental Engineering Reference
In-Depth Information
The solvent is recovered in a second step, typically by distillation. So, the solute-solvent
pair should not form an azeotrope and the more volatile component should be the minor
component in the mixture.
5.3
Environmental applications
Liquid/liquid extraction (LLE) is used for the removal of low levels of organic compounds
or heavy metals from aqueous streams. It is normally evaluated relative to steam stripping
or distillation. There are several factors to consider:
1Physical properties of solute (contaminant) and water. If the boiling point of the sol-
ute is significantly below that of water, the separation is usually performed by steam
stripping. One important exception is the formation of non-ideal liquid-phase solutions
and minimum temperature azeotropes. This allows the solute to be removed by steam
stripping even though the pure solute has a higher boiling point. So, high-boiling-point
organics are good candidates for LLE. They may form azeotropic mixtures with water
but, if the boiling point of the azeotrope is close to that of water, there is not an advantage
for steam stripping.
An additional mechanism for non-ideal liquid-phase solutions is hydrogen bonding.
An example is formaldehyde that has a boiling point (
19 C) much lower than that
of water. But, it hydrogen bonds very strongly with water so it is difficult to remove by
stripping. LLE is an effective approach for this separation.
2Low concentrations of carboxylic acids. Acetic acid and formic acids have boiling
points near (formic) or above (acetic) water. Acetic acid does not form an azeotrope
and the boiling point of the formic acid-water azeotrope is 107 C. So, LLE is a viable
option to extract these acids from water.
3 Metals and organometallic derivatives. These compounds are non-volatile and thus
cannot be removed by stripping. LLE is typically used to remove these compounds. A
chemical complexing agent (typically an ion-exchanger) is added to the solvent phase
to extract the metal. An acid is contacted with the solvent phase to strip the metal and
regenerate the complexing agent.
5.4
Definition of extraction terms
Solute ( A ): the species to be removed from the liquid diluent stream.
Diluent ( D ) : the component containing the solute which is to be removed.
Solvent ( S ) : the second liquid stream which will remove the solute from the diluent.
Raffinate ( R ) : the exiting phase which has a high concentration of diluent (and
less A).
Extract ( E ) : the exiting phase which has a high concentration of solvent (and more A).
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