Coextraction of water

When coextraction of water is an important consideration, the diluent can heve a large effect on the amonm of water coextracted and also can improve the separation of water in a subrequeni distillation step. It can be worthwhile to choose a diluanl with a lower equilibrium distribution coefficient for the solute, if the ratio of solute to water extracted is increased. 

In the recoveiy of ethanol and many other otganic solutes from aqueous solution, coextraction of water has a large effect on the process economics. Solvents may be compared hy plotting the selectivity (k = separation factor between ethanol and water) versus the solvent capacity for ethanol, expressed as Kff. Figure 15.2-6 la such a plot for extraction of ethanol from relatively dilute aqueous solution by many different solvents.3 This figure includes data fiom Roddy 35 Souissi and Thyrion,37 and Munson and King. 

The surfactant used was therefore the potassium salt of DOLPA. The resulting organic phase was a water-in-oil microemulsion since the potassium transfer was accompanied by the coextraction of water molecules. Following liquid-liquid separation of the bulk phases, the organic phase was used for material synthesis. 

In addition to high capacity at low solute concentration, the other major potential advantage of separation processes based on chemical complexation is selectivity. Since the complexation reaction can be selective for solutes with particular fbnctional groups, these processes have the potential to separate only certain solutes from a complex mixture. They also can give less coextraction of water than separsxions with more conventional solvents. 

Coextraction of water can be an important economic disadvantage. The complexing agent should be selected to minimize coextraction of water and/or facilitate subsequent removal of that water. 

The water concentration term in Eq. (2.129) is usually omitted considering that the change in water activity is small. For extraction systems with the same amine, the reported in the literamre stoichiometry and a number of complexes are not always in agreement. This results primarily from an interpretation of similar, but not exactly the same experimental conditions. The factors such as the nature and initial concentration of diluent, pH, temperature, concentration range and coextraction of water change the form of extraction isotherms and such changes are attributed to differences in the stoichiometry of formed complexes. Similar procedures based on the chemical model (formation of complexes and dimers), were also applied to more complicated simations when citric acid is separated from small amounts of carboxylic acids which are always present in fermentation broths [232, 235, 241, 249, 253, 255]. 

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