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where N is the total number of mole of solvent in the dilution cell at time t, and Ac, and A are respectively the original peak areas of the solute and the peak area at intervals of the time t.

For extractive distillation, we usually take on the selectivity or relative volatility at infinite dilution as the standard of evaluating the solvent. The selectivity S°J and relative volatility a°J at infinite dilution are respectively given by the following equations:

It is evident that the selectivity S" and relative volatility or™ are consistent in evaluating the separation ability of the solvent. For the sake of simplicity, it is better to select a™ . Comparing with , the data of a™ is easy to be obtained accurately because it needn't measure N and t which perhaps bring extra errors from the experiment. Thus if the peak area A of the solute at different time is known, a™ is able to be calculated according to

Eq. (23) in which only the peak area A is required. It has been verified by Lei et al. [64] that the results obtained by this method are reliable, and very suitable for the evaluation of different solvents.

The comparison of gas-liquid chromatography method (method 1) and inert gas stripping and gas chromatography method (method 2) for separating C4 mixture with ACN and DMF is shown in Table 9 [56, 64], where we use the subscript (1-5) for butane, 1-butene, 2-trans-butene, 2-cis-butene and 1,3-butadiene respectively. It can be seen that the difference between these two methods is below 15%.

Table 9

The comparison of gas-liquid chromatography method (method 1) and inert gas stripping and gas chromatography method (method 2)

Method 1 Method 2 Deviation Method 1 Method 2 Deviation _%_%

It is concluded by comparison of these four methods that inert gas stripping and gas chromatography method should be firstly recommended, secondly the HSGC technique in the direct method and finally gas-liquid chromatography method. However, it is hard to find the references about ebulliometric method in the recent years, which may be a cool point.

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