Equivolatility curves

One final comment about the iso- and equivolatility curves is that we should not solely rely on the location where the isovolatility curve intercepts the IPA-entrainer edge of the triangle to determine which one is the more effective entrainer. We will show a... 

In the first part of this chapter, the important factors of an effective entrainer for separation of an azeotrope using extractive distillation have been demonstrated using IPA dehydration as an example. The isovolatility curve can be used to determine the feasible distillate produet in the extractive distillation column. The isovolatility and equivolatility curves can be used to determine which candidate entrainer is more effeetive in enhancing the relative volatility of the azeotropic mixture. The VLB information of the binary pairs between the candidate entrainer and either of the two original eomponents should also be plotted to make sure there is no problem for the separation in the leetilying section of the extractive distillation column and in the entrainer reeovery eolumn. 

The RCMs and the equivolatility curves of this chemical system ean be seen in Figure 13.1, where the numbers in the equivolatility emwes denote the relative volatility of acetone versus methanol in the presence of water. The RCM indicates that any mixture of acetone and methanol, even premixed with water, will produce the acetone-methanol azeotrope at the top of the column. However, by continuously adding water (a heavy entrai-ner) into the column, it can be seen from the equivolatility curves that the acetone is becoming more and more volatile than the methanol in the extractive section. Acetone and methanol can then be separated in the extractive section if the number of trays in this section is sufficient. Acetone will go toward the top of the column while methanol will be carried with the water toward the column bottom. In the rectifying section, owing to the lack of methanol in this section, only the separation of acetone and water is performed. Pure acetone will preferably go to the top of the batch extractive distillation column. After the draw-off of the acetone product and a slop-cut period, where the acetone in the column is completely depleted, the methanol product can be collected at the top of the column. The heavy entrainer (water) can be collected at the column bottom. 

Figure 13.9 RCM and equivolatility curves of an IPA-water-DMSO system.

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