Extractive distillation usually introduces a heavy boiling entrainer below the column top to change the separation factor between the components to be separated. The separation factor or relative volatility alpha between 2 components 1 and 2 can be expressed as

An entrainer influences relative volatility by changing the ratio of the activity coefficients.

Typically, potential entrainers are evaluated by the ratio of the activity coefficients at infinite dilution of the components in the entrainer.

The following plot shows the concentration regions and relative volatilities relevant for the process. In the yellow field, the entrainer is separated from the top product. The blue and red regions are the most important for the separation in the extractive column. Note that below a liquid feed, the entrainer concentration is lower than above.

The grey area is relevant for the separation of the bottom product from the entrainer.

At Rareytec, we employ a relatively simple technique for entrainer evaluation. A binary mixture of the components to be separated is weighted into headspace vials with different concentrations of entrainer.

This yields 3 data points in the vicinity of the extractive distillation operating region. All vials are brought to thermal and phase equilibrium, and the vapor phase is analyzed via gas chromatography. Only the peak areas of the volatile components are required.

The effect of the entrainer is represented by the competitive factor D, which is ratio of the volatile component vapor mole fractions relative to the value without the entrainer.

The following plot shows example results from our laboratory.

Compared to infinite dilution activity coefficients, the results are more significant for the entrainer effect in the actual process. Only 3 GC-samples need to be analyzed for each entrainer or entrainer blend.

A similar strategy is applied for the determination of binary interaction parameters for the ternary mixture.