(b) Decreasing boiling point

In general, the solvent has a much higher boiling point than the components to be separated in order to ensure the complete recovery of the solvent. It indicates that at the bottom of the solvent recovery column, a water stream with a higher temperature than the boiling point of the solvent is needed to heat the solvent to reach its boiling point at normal pressure. This means that much energy with high level has to be consumed. If the solvent recovery column is operated at below normal pressure, the degree of vacuum pressure must not be too high due to the restriction of the temperature of condensing water at the top. Therefore, the best strategy is to find the suitable additive to make into a mixture to decrease the boiling point. It is evident that the boiling point of the additive is relatively lower than the basic solvent, but higher than the components to be separated. Table 7 illustrates the examples of the mixture of the solvents, in which the function of the additive is to decrease the boiling point [61, 62],

Table 6

Selectivity Sn of the mixture of the solvents for separating n-pentane (1) and 1-pentene (2);

adapted from the reference [60]

No. Mixture of the solvents Concentration

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