Environmental Engineering Reference
In-Depth Information
other. Leaching, or solid extraction, is a similar process in which solute(s) are removed
from a solid by a liquid mass-separating agent for which they have an affinity. Extraction
is typically analyzed as an equilibrium-limited separation.
An environmental application of liquid extraction is the removal of trace organics from
water. Examples are the separation of acetic acid-water mixtures and removal of solvents,
insecticides, pesticides, etc., from water. It can also be applied to the separation of liquids
with close boiling points or those that form azeotropes, such that distillation is not useful.
In addition, zero- or low-volatility compounds, such as metals and organometallic deriva-
tives, can be separated by liquid extraction as can mixtures of water-hydrogen bonded
compounds, such as formaldehyde. Solid extraction (leaching) can be used to remove
organics or heavy metals from contaminated soils, sludges or contaminated equipment.
One advantage of extraction separations is that they can be performed at ambient tem-
peratures. Thus, extraction is relatively energy efficient and can be applied to separations
involving thermally unstable molecules. In addition, extraction processes can accommo-
date changes in flowrates and the solvent (mass-separating agent) can be recovered and
recycled for reuse. The primary disadvantage of extraction is the complexity arising from
the addition of a mass-separating agent to the system. While in distillation the simplest
system is a binary, in extraction it is a ternary. The solvent must be stored, recovered, and
recycled. Additionally, some of the equipment required in extraction systems is compli-
cated and expensive. Because it is based upon solubility differences between phases, it
is not possible to remove all the solute from the feed phase to the solvent phase, so pure
products are not possible. Finally, models to predict efficiency and capacity in extraction
are more complex than those for distillation, as is scale-up.
In general, when either distillation or extraction is feasible to achieve a separation,
distillation is the method of choice. In distillation there is no mass-separating agent to be
recovered. In extraction, on the other hand, the solvent is recovered continuously for reuse,
usually by distillation. The addition of a new species to any system requires a separation
process for its recovery. Thus, extraction separations must include two separation steps,
while distillation separations require only one.
Because it is key to the success of any extraction process, the optimal solvent must
meet several criteria. The distribution (partition) coefficient must be high under operating
conditions. The distribution coefficient is a ratio describing the solubility of the solute in
the solvent to that in the original feed stream. A high number indicates the solute has a
higher affinity for the solvent than for the feed material. The solvent should have a low
solubility in the feed material, such that the solvent does not replace the solute as the
contaminating species. Likewise, the other feed material should not be highly soluble in
the solvent and the affinity of the solvent for the solute should not be so high that recovery is
prohibitive. The solvent should be compatible with both the feed stream (non-reactive) and
the physical system (non-corrosive). The solvent should also have a low vapor pressure,
viscosity and freezing point for easy handling and storage. In addition, the solvent should
be non-toxic and non-flammable.
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