Distillation explained, Dr Stone S1E1

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Right after producing his wine, Senku moves to the next step: “Time to learn wine distillation, how to make brandy.” He assembles a makeshift alembic (a jar, a narrow neck, a cooled tube) and announces, with his usual enthusiasm, that the Mesopotamians were already distilling 3,000 years before our era “in clay pots.”

The stakes go beyond brandy. Distillation is the pivotal technical lever of the whole season: it’s what will let Senku concentrate ethanol to make nital, then extract essences, acids, and perfumes.

The subject in depth

A gap in volatility

Everything rests on a simple observation. At normal atmospheric pressure (1,013 hPa), water boils at 100 °C but ethanol boils at 78.4 °C. If you heat a mixture of the two to an intermediate temperature, say 80 °C, the ethanol evaporates considerably faster than the water. The resulting vapor is therefore enriched in ethanol compared to the liquid left behind.

But that vapor isn’t pure ethanol either. A physical law governs what evaporates: Raoult’s law (1882) states that the vapor pressure of a component in an ideal mixture is proportional to its mole fraction in the liquid ^[Livre] . The practical consequence: the vapor contains both components, just in different proportions than the liquid. The further you move from either pure boiling point, the greater the volatility gap, and the more effective the separation.

From gas back to liquid: condensation

The second half of the process is purely mechanical: the enriched vapor is passed through a cooled tube (a coil surrounded by cold water in a modern alembic, a damp clay tube in the primitive version). The vapor loses kinetic energy on contact with the cold wall, falls back as droplets, and is collected in a second vessel. The resulting distillate is more concentrated in ethanol than the starting liquid.

A simple distillation starting from a 12% wine typically gives a distillate at 35 to 50% ethanol. To go higher, you have to redistill: a second pass toward 70%, a third toward 85–90%. This is how modern eaux-de-vie are made.

The unbreakable limit: the water-ethanol azeotrope

Keep distilling, and you hit a physical wall. At 95.6% ethanol by mass (at atmospheric pressure), the composition of the vapor becomes strictly identical to that of the liquid ^[Wikipedia] . Simple distillation no longer separates anything. This point is called an azeotrope .

The consequence: no number of successive distillations can produce pure (100%) ethanol from a water-ethanol mixture. To reach the “absolute alcohol” used in analytical chemistry or as a fuel additive, an extra step is needed, typically azeotropic distillation (adding benzene or cyclohexane, which forms a third, wetter azeotrope) or molecular sieves, which selectively adsorb water molecules.

This is precisely the limit Dr Stone never mentions. Over the course of the manga, several processes require highly concentrated ethanol: Senku works around the problem, but the story never spells out the difficulty.

Simple, fractional, vacuum: the distillation family

What Senku does is a simple distillation: one vaporization-condensation cycle per batch. It works well as long as the two liquids have very different boiling points (at least 30 to 40 °C apart). For closer mixtures, fractional distillation is used: the vapor travels up a column packed with trays where dozens, even hundreds, of successive vaporization-condensation cycles take place (the “theoretical plates” theory of McCabe and Thiele). This is what’s used to split crude oil into gasoline, kerosene, diesel, and fuel oil.

When the compounds being separated break down at high temperature (perfumes, fragile medicines), distillation is done under partial vacuum: at reduced pressure, boiling points drop. Products that would destroy themselves at 200 °C can then be distilled at 40 to 60 °C.

The fact-checker’s eye

Going further

• Discipline: Chemistry: all chemistry entries on the site
• Coming soon: Wine: the source of ethanol distilled in the previous step
• Coming soon: Brandy: the finished product of this distillation
• Coming soon: Nitric acid: combined with this ethanol to produce nital

Glossary

Azeotrope ↗

A mixture of two liquids whose vapor has exactly the same composition as the liquid. Simple distillation can no longer separate them.

Raoult's law ↗

A physical-chemistry law stating that the vapor pressure of a component in an ideal mixture is proportional to its mole fraction in the liquid.

Fractional distillation ↗

A variant of distillation using a tray column where dozens of vaporization-condensation cycles occur in sequence, to separate liquids with close boiling points.

Sources

1. Distillation, French Wikipedia ^[Wikipedia]
2. Azeotrope, French Wikipedia ^[Wikipedia]
3. Robert J. Forbes, Short History of the Art of Distillation, E. J. Brill, 1948 ^[Livre]
4. Peter Atkins & Julio de Paula, Physical Chemistry, De Boeck, 2013 ^[Livre]