14-11-2012, 01:16 PM
LIQUID-LIQUID EXTRACTION
LIQUID-LIQUID EXTRACTION.pdf (Size: 141.58 KB / Downloads: 91)
Object
The object of the experiment is to demonstrate how a mass balance performed on the extraction
column, and to measure the mass transfer coefficient and its variation with flow rate with the
aqueous phase as the continuous medium.
Theory
Many processes in chemical engineering require the separation of one or more of the components
of a liquid mixture by treating the mixture with an immiscible solvent in which these
components are preferentially soluble. In some cases purification of a liquid may be the function
of the process, in others the extraction of a dissolved component for subsequent processes may
be the important aspect. An example of the former is the preparation of the pure organic liquids
from products of the oil industry. Liquid-liquid extractions may also be used as energy saving
processes by, for example, eliminating distillation stages. It is possible, of course that the
substance of interest may be heat-sensitive anyway and that distillation is accordingly an
unacceptable process.
When separation by distillation is ineffective or very difficult, liquid extraction is one of the main
alternatives to consider. Close-boiling mixtures or substances that cannot withstand the
temperature of the distillation, even under a vacuum, may often be separated from impurities by
extraction, which utilizes chemical differences instead of vapor pressure differences. For
example, penicillin is recovered from fermentation broth by extraction with a solvent such as
butyl acetate. Another example for liquid extraction is recovering acetic acid from dilute aqueous
solutions; distillation would be possible in this case, but the extraction step considerably reduces
the amount of water to be distilled.
When either distillation or extraction may be used, the choice is usually distillation, in spite of
the fact that heating and cooling are needed. In extraction the solvent must be recovered for reuse
(usually by distillation), and the combined operation is more complicated and often more
expensive than ordinary distillation without extraction. However, extraction does offer more
flexibility in choice of operating conditions, since the type and the amount of solvent can be
varied as well as the operating temperature. In many problems, the choice between the methods
should be based on a comparative study of both extraction and distillation.
In liquid-liquid extraction, as in gas absorption and distillation, two phases must be brought into
contact to permit transfer of material and then be separated. Extraction equipment may be
operated batchwise or continuous. The extract is the layer of solvent plus extracted solute and the
raffinate is the layer from which solute has been removed. The extract may be lighter or heavier
than the raffinate, and so the extract may be shown coming from top of the equipment in some
cases and from the bottom in others. The operation may of course be repeated if more than one
contact is required, but when the quantities involved are large and several contacts are needed,
continuous flow becomes economical.
The rate at which a soluble component is transferred from one solvent to another will be
dependent, among other things, on the area of the interface between the two immiscible liquids.
Therefore it is very advantageous for this interface to be formed by droplets and films, the
situation being analogous to that existing in packed distillation columns.