|MadSci Network: Engineering|
The ethanol water mixture separation isn't a problem until the azeotrope reaches an ethanol purity of @96%. At this point, the ethanol water vapor is also @96% purity and the separation stops. The polar hydroxyl group holds on to the water molecule and requires additional steps to free those water molecules, dehydration. Dehydration can be achieved with an azeotrope "buster" like benzene, or an absorbant like zeolite. The miscibility of ethanol with gasoline decreases with the increase in water content. The dehydration step is necessary. Ethanol for use in fuel needs to achieve a 98-99% purity. The butane separation you suggested would also suffer from a lack of miscibility. The ethanol content of the butane would be low and the cost considerable.
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