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Liquid extraction   INTRODUCTION
SUMMARY
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Introduction General principles
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Nadine LE BOLAY Gilbert CASAMATTA
GENERAL SUMMARY
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INTRODUCTION
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INTRODUCTION
Introduction
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A process is often made up of a reaction step
followed by separations.
When analyzing the process in details, one can
see that it consists of a reactor
fed with reagents
and energy.
The products of the reactions (main reaction, but
also secondary reactions) flow out of the
reactor, as well as the reagents that have not
reacted.
The purpose is then to separate the different
products leaving the reactor. In that way,
separation equipment (generally several items)
are implemented.
In most cases, distillation is carried out.
However, in certain conditions, it is not
suitable. Then, one may choose other separation
techniques, such as absorption, adsorption or
liquid extraction, also called solvent
extraction.
It is particularly the case when

• Liquids have boiling points with are close to
  each other
• Their boiling points have a high value
• The mixture forms an azeotrope
• Distillation must be performed under high vacuum
  conditions
• The products are heat-sensitive (recovery of
  antibiotics or vitamins)
• The mixture to be processed is a very diluted
  aqueous solution (the latent heat of vaporization
  of water is very high)

We will study the case of liquid extraction which
is a physico-chemical separation of constituent
mixtures during a homogeneous phase
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GENERAL PRINCIPLES
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GENERAL PRINCIPLES
Généralités
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Let us consider a homogeneous liquid mixture
constituted by a diluent A and a solute B which
are miscible.
The objective is to recover the solute.
Let us add a liquid solvent phase, non miscible
with A.
Under the effect of agitation, the solvent phase
is dispersed as droplets in the diluent phase.
A part of the solute is transferred from the
diluent phase to the solvent phase.
When agitation is stopped, the two phases are
separated by decantation,
the lighter phase (it will be
supposed here that the solvent is lighter than
the diluent) being concentrated in the upper part
of the vessel.
, the
diluent phase is called the raffinate.
The solvent phase is called the extract
The two phases are at equilibrium.
This equilibrium is defined by an ideal stage.
Extract
A B
Raffinate
A stage is considered as ideal when the
contact between the phases is long enough in
order to reach the solute distribution
equilibrium between the extract and the raffinate
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In practice, the diluent and the solvent are
often partially miscible.

A small volume of
the diluent may thus be contained in the extract,
while a small volume of the solvent may be
present in the raffinate.

The extraction will thus be
followed by as many separations as necessary in
order to recover the different constituents.

At that stage, separations may eventually be
distillations if the limitation imposing
extraction use no longer exists (for example, the
solvent and the solute do not form an azeotrope
which existed between the diluent and the
solute).
The phases containing mainly the solvent can be
recycled in the extraction apparatus.
Numerous solvents are used in liquid extraction.
However all of them are not suitable. The feed
phase being imposed, the solvent choice must be
optimized. This choice is influenced by -its
physico-chemical properties (allowing an easy
recovery of the solute or of the solvent), -a
negligible solubility of the solvent in the
diluent (post-extraction processes have to be as
cheap as possible), -physical characteristics
offering acceptable dispersion and separation
times of the post-contact phases (viscosity,
interfacial tension, density difference compared
to the feed),
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-favorable properties (Mass transfer kinetics
equilibrium after contact less than a few minutes
- Economy cheap and available solvent -
Safety of use low toxicity, low flammability,
low volatility, low corrosion in comparison with
usual construction materials -),
but particularly a property called selectivity.
The selectivity b is defined as the ratio
The necessary and sufficient condition for an
extraction to be possible is that b is greater
than 1. The higher the value of b compared to 1,
the easier the extraction.
The selectivity can be written in a different
form
Consequently, a sufficient condition for a
solvent to be selective is m greater than 1.
It should be noted that the higher the value of
m, the higher the value of b and thus the more
selective the solvent.
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No solvent offers all possible favorable
conditions. Thus, a compromise has to be found
between all the constraints.
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Once the solvent has been chosen, the extraction
process has to be defined and designed. Four
extraction techniques will be studied in this
course
To design an extraction apparatus, it is
necessary to - determine the number of ideal
stages - determine the phases flowrates, as well
as the solute distribution between the
phases - choose the most adapted apparatus -
study the hydrodynamics of the apparatus -
determine the size and the configuration of the
apparatus
In the following chapters, we will learn how to
determine the number of ideal stages, the
transfer of the solute in the extract, and the
flowrates of the phases with respect to the feed
flow rate. The other concepts will be studied
in another course.
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The course will be divided into 6 chapters
- CHARACTERISTICS AND PROPERTIES OF SOLVENTS
- LIQUID EQUILIBRIUM
- SINGLE STAGE EXTRACTION
- CROSS-CURRENT EXTRACTION
- COUNTER-CURRENT EXTRACTION
- COUNTER-CURRENT EXTRACTION WITH REFLUX
END OF THE INTRODUCTION
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