1. Membrane of liquid- liquid extraction:
Extraction of a solute from water to an organic liquid, or vice versa,
can be carried out using membranes to separate the phases & provide a
high surface area for mass transfer. Hollow fiber or flat sheet membranes
can be used, and the mass transfer area is then fixed by the design and
does not depend on variables such as flow rate, viscosity, and surface
tension, which affect the area of liquid-liquid dispersions. The membrane
extractor can be arranged to have counter flow of the two phases with no
flooding limitations, unlike the situation in packed or spray columns. A
further advantage is that there is no need for a settling tank or deemulsifier,
because the phases are kept separate by the membrane.however, the
membrane does introduce an additional resistance to mass transfer, and
this must be minimized to make the process attractive.
If a dense polymer film were used in an extractor, the membrane
resistance would be quite large because of the very low diffusivity in solid
polymers. Using an asymmetric membrane would reduce the membrane
resistance, because diffusion is more rapid in the open substructure than in
the dense skin.however, the minimum resistance is obtained with a porous
membrane, one that has pores extending completely through the
membrane. Phase separation is maintained by choosing a membrane that
is not wetted by one of the phases. E.g., membranes made of Teflon or
polypropylenes are hydrophobic, and water will not enter the pores except
at high pressure. The critical entry pressure depends on the contact angle
and the size and shape of the pores and is as high as 50 lbf/in2.for some
commercial membranes.
An extract0r with hollow fibers of polypropylene could operate with
the aqueous phase inside the fibers of at a pressure slightly greater than
the pressure of the organic phase on the outside. The pores of the
membrane would fill with the organic solvent, and the liquid-liquid interface
would be at the pore mouths. The concentration gradients are sketched in
fig. e.g. where the equilibrium solute concentration is much higher in the
organic phase. The overall resistance for this case is

2. The coefficients for the water phase, kw, the organic, ko, and the
membrane, de.o/z, are generally about the same magnitude, but if the
distribution coefficient “m” is large, most of the resistance is in the water
phase. Here m is the ratio of the solute concentration in the organic phase
to that in the water phase.
Fig. (Liquid-Liquid extraction with a porous hydrophobic membrane)
If a hydrophilic membrane is used, the pores fill with the water phase, and
the organic phase must be kept at a high pressure to prevent water from
passing through the pores and forming drops in the organic phase. For the
system shown in fig. Use of a hydrophilic membrane would mean two water
phase resistances and a lower overall coefficient, as shown by the equation

3. If the distribution coefficient for the solute strongly favors the water phase
(m<<1), the organic phase has the controlling resistance, and a hydrophilic
membrane might be selected to make the membrane resistance smaller.
Selective extraction of a solute from an aqueous phase into another
aqueous the phase can be accomplished with a hydrophobic membrane,
the pores of which are filled with a polymeric liquid that has a high partition
coefficient for the solute.
The overall mass transfer coefficient kw for such a process is found
from the equation.
Where km is the mass transfer coefficient for the membrane and kp is the
partition coefficient for the solute distributed b/w water and the polymeric
liquid. The recovery and concentration of phenol from a dilute salt solution
using this process have been demonstrated using commercial radial flow
modules.
Other hollow fiber extractors have been tested in the lab. And the
mass- transfer rates are generally consistent with theory, although
satisfactory correlations for the external coefficient may not be available.
These devices should find practical application for systems where it is
difficult to get a good dispersion or where emulsification makes the final
phase separation difficult.