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Introduction
Liquid-liquid extraction is a process for separating
chemicals in solution depending on the solubility of each chemical. This difference in solubility allows one
chemical to be removed from a mixture and be attracted to another
chemical.
A liquid-liquid extraction column has two inlet streams,
one at the top and one at the bottom.
The bottom stream is fed into the column as a mixture containing the
component desired to be separated. This
mixture also contains a component that is not capable of being mixed in the top
stream. The upward flow of the bottom stream is a result of this immiscibility
and the density difference between the two inlet streams.
The desired component is extracted by a solvent flowing
down the column. (A solvent is a fluid
that dissolves another substance.) This
results in two outlet streams, where the top outlet stream is more pure, and
the bottom stream is the new mixture.
For example, the liquid-liquid extraction experiment in the
Unit Operations Lab uses a mixture of ethanol and heptane as the bottom feed
and water as the solvent. These feed streams are run through the extraction
column where ethanol contacts water so that it can be drawn away from
heptane. The result is two products,
one of which was mostly heptane and the other a mixture of ethanol and water.
Extraction is used most often with petroleum and
petrochemicals, pharmaceuticals, foods, and metals. This process is used when other methods are impractical or too
expensive.
Using the extraction column,
one may wish to determine the
water flow rate required for a very pure product with and without the agitation
of the plates. In addition, one might
want to calculate the number of theoretical plates required for the
separation. By accomplishing
these objectives, one can gain more insightful knowledge concerning this
device.
Last modified:
November 7, 2001
Created by Team
#2