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 lb f/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

1Kw

=1kw

+1m ( 1ko+ z

De .0 )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

1Kw

= 1kw

+ zDe ,w

+ 1mk o

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.

1Kw

= 1kw 1

+ 1kmK p

+ 1kw 2

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.