1. Distribution Coefficient
This is the ratio (at equilibrium) of the concentration of solute in the extract and raffinate phases. It gives a measure of the affinity of the solute for the two phases.
A distribution coefficient other than unity implies that the solute must have different affinity in the two phases. If only one solute is involved (such as in the recovery of an impurity from an effluent stream), only the distribution coefficient need be considered, and it is desirable for this to be as large as possible.
2. Selectivity (Separation Factor)
If there are more than one solutes (say two solutes A and B), then consideration should be given to the selectivity of the solvent for solute A as against B. The selectivity between the 2 solutes A and B is defined as the ratio of the distribution coefficient of A to the distribution coefficient of B. For all useful extraction operation the selectivity must exceed unity. If the selectivity is unity, no separation is possible.
3. Insolubility of Solvent
The solvent should have low solubility in the feed solution, otherwise the separation is not "clean". For example, if there is significant solubility of solvent in the raffinate stream, an additional separation step is required to recover the solvent.
4. Recoverability
It is always necessary to recover the solvent for re-use, and this must ordinarily be done by other means, eg. distillation. If distillation is to be used, the solvent should form no azeotrope with the extracted solute and mixtures should show high relative volatility. The solvent should also be thermally-stable under the distillation temperature.
5. Density
A large difference in density between extract and raffinate phases permits high capacities in equipment. This is especially important for extraction devices utilizing gravity for phase separation.
6. Interfacial Tension
The larger the interfacial tension, the more readily coalescence of emulsions will occur but the more difficult the dispersion of one liquid in the other will be. The more readily coalesces the emulsions the easier phase separation will be. Low interfacial tension aids dispersion and thus improves contacting mass-transfer efficiency. Coalescence is usually of greater importance, and interfacial tension should therefore be high.
7. Chemical Reactivity
The solvent should be stable chemically and inert toward the other components of the system and toward the common materials of construction.
8. Viscosity, Vapour Pressure, Freezing Point
These should be low for ease in handling and storage, for example, a high viscosity leads to difficulties with pumping , dispersion and mass-transfer rate.
9. Availability and Cost
An excellent solvent may not be commercially available. Or it may represent a large initial cost for charging the system, and a heavy continuing expense for replacing inevitable operating losses.
10. Other Criteria
Toxicity and flammability of the solvent are important occupational health and safety considerations. Stability of the solvent (i.e. resistance to breakdown), particularly in the recovery steps, is significant, especially if the breakdown products might contaminate the products of the main separation. Corrosivity of the solvent leads to the usual problems with materials of construction. Finally, compatibility of the solvent with the mixture to be separated can have many manifestations, particularly when easily contaminated materials such as food or pharmaceuticals are being handled.
