Why Can You Not Distil Alcohol Past 96 Percent? The Mixture Fights Back

Separating two liquids by boiling relies on the vapour being richer in the more volatile component than the liquid is. Condense that vapour and you have a slightly better mixture, repeat the operation and it improves again, and a column with many stages performs the repetition continuously. The whole method depends on that difference in composition between liquid and vapour existing at every stage. Where it disappears, the method has nothing left to work with, and no amount of additional equipment or patience changes the result.

At one particular composition the two become identical:

• •The vapour has exactly the same composition as the liquid
• •So condensing it produces no improvement whatsoever
• •The mixture boils at a single fixed temperature like a pure substance
• •That temperature can be lower than either pure component
• •Or in other mixtures higher than either
• •The composition is fixed by the chemistry, not by the apparatus

Ethanol and water form one at about ninety six percent alcohol by volume, boiling at a temperature slightly below that of pure ethanol. That is why spirits produced by distillation alone stop at that strength and why laboratory alcohol is sold at it. Producing anhydrous alcohol requires something other than ordinary distillation, and the usual industrial routes add a third substance that changes the behaviour of the mixture, or pass the vapour through a material whose pores admit water molecules and exclude the larger alcohol ones. Both work because they stop treating it as a two component problem.

Mixtures behave this way for two opposite reasons and the difference shows up in the boiling point. Where the two components dislike each other, each escapes more readily than it would alone, so the mixture boils below either pure substance and the sticking point is a minimum. Alcohol and water work this way. Where the components attract each other strongly, both are held back, the mixture boils above either pure substance and the sticking point is a maximum, which is what nitric and hydrochloric acid do with water. Distilling a maximum type concentrates the mixture in the pot rather than in the distillate.

The phenomenon constrains a great deal of chemical manufacturing and is a routine design problem rather than a curiosity. Nitric acid cannot be concentrated past about sixty eight percent by boiling, and hydrochloric acid stops around twenty percent, which is why those strengths are the commercial standards. Recovering solvents for reuse frequently runs into the same barrier, so a mixed solvent stream may be impossible to separate economically. Refrigerant blends are sometimes chosen to behave this way deliberately, because a blend that boils at a single temperature behaves predictably in equipment and does not change composition if it leaks.

Distillation improves a mixture only while the vapour differs in composition from the liquid, and at one particular composition the two become identical, so repetition achieves nothing. Ethanol and water reach that point at about ninety six percent, which fixes the strength of distilled spirit. Getting past it requires adding a third substance or filtering by molecular size.