We know that relative volatility provides a useful index for selection of suitable separating agent. According to the definition of relative volatility, obviously a]2 larger than unity is desired. Thus, the following conclusions can be drawn from Table 3:
(1) Adding a little of salt to DMF can improve the relative volatility of ethanol to water.
(2) The separation ability of ethylene glycol is greater than that of single DMF, and approximate to the mixture of DMF and NaSCN (0.20 g NaSCD / ml DMF).
(3) The separation ability of ion-exchange resins is the least among all the separating agents. It isn't good that ion-exchange resins are used for separating agent for the separation of ethanol and water. The reason may be that the molecular weight of ion-exchange resins is so high that the amount of function group Na+ is very little and can't effectively influence the relative volatility of ethanol to water.
Therefore, by combining the results of Fig. 3 and Table 3, it is evident that the best separating agent is the mixture of ethylene glycol and CaC^, and the corresponding separation process is just extractive distillation.
So the separation technique using active packing materials for separating ethanol and water may seem very attractive, but can't replace common extractive distillation process by now because molecular sieves and ion-exchange resins as solid separating agent are much more difficult to be regenerated than liquid separating agent, and the separation effect is also less than that of the separating agents currently used in the extractive distillation process.
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