Isopropanol drying entrainers

These developments will have a wide impact. Reaction enhancement will be a major beneficiary, but a look at the simpler field of solvent dehydration shows that the innovation process is very application dependant. Pervaporation (with vapor permeation) is progressively displacing other techniques in solvent dehydration. Replacing entrainer distillation for drying ethanol and isopropanol, pervaporation at initial stages is always now preferred to techniques, where a third component must be added to shift equlibria. The handling of entrainers and/or calcium chloride or caustic with the attendant environmental risks and costs is no longer a viable option. 

In a few cases, this does not present a problem. DMF forms low-boiling azeotropes with heptane and xylenes. The addition of water in the fractionation system as an entrainer allows the formation of azeotropes with these hydrocarbons, but not with DMF. The hydrocarbon/water azeotrope at the column top splits into two liquid phases, so that the hydrocarbon can be removed and the water recycled. Similarly, in drying ethanol and isopropanol the unwanted water forms a separate phase which can be removed while the entrainer is recycled to pick up more water. 

Entrainers such as benzene, chloroform and carbon tetrachloride were used in the 1930s. Such materials, despite the fact that their other properties may be attractive, would be considered too toxic to be introduced into use today. A TLV of less than 10 ppm would be disqualified unless the solvent to be dried required handling precautions of the same level. Ethanol with a TLV of 1000 ppm or isopropanol (IPA) of 400 would not justify a highly toxic entrainer. 

While it is therefore more like ethanol and isopropanol, which require the addition of an entrainer to dry them by distillation, sec-butanol is not very hydrophilic and LLE can be linked to distillation in water removal processes. 

Table 16.13 shows the effectiveness of entrainers similar to those used for drying ethanol and isopropanol. In all three cases the MEK content of the water phase separating at the column top is low enough to consider sending it to effluent treatment rather than recycling it to try to improve the yield of the process. 

---