Fractional distillation is a method for separating a mixture of liquids based on their different boiling points. Unlike simple distillation, which works well when liquids boil at temperatures more than 25 °C apart, fractional distillation can separate liquids with very similar boiling points by forcing the vapor through a tall column where it condenses and re-evaporates many times over. It’s the process behind everything from refining crude oil into gasoline and jet fuel to pulling pure oxygen and nitrogen out of thin air.
How the Process Works
Every liquid has a boiling point: the temperature at which its vapor pressure equals the surrounding air pressure. In a mixture of two liquids, the one with the lower boiling point produces more vapor at any given temperature. When you heat the mixture, both liquids contribute vapor, but the lighter (lower-boiling) liquid is overrepresented in that vapor compared to the liquid below.
Fractional distillation exploits this by running the vapor up through a tall column packed with material that gives it plenty of surface area to condense on. As vapor rises and hits a cooler surface, it condenses back into liquid. That liquid gets reheated by the hot vapor still rising from below, and it evaporates again. Each time this condensation-and-re-evaporation cycle happens, the vapor becomes richer in the lower-boiling component. Chemists call each of these cycles a “theoretical plate,” and each one is equivalent to performing a separate simple distillation. A column with 10 theoretical plates effectively distills the mixture 10 times in a single pass.
The result is that the lightest component exits from the top of the column as a nearly pure vapor, while heavier components condense and flow back down. In a column with many theoretical plates, you can tap off different products at different heights, each one representing a different boiling range.
What Makes It Different From Simple Distillation
Simple distillation heats a liquid mixture, captures the vapor, and condenses it once. That single vaporization step gives you a distillate that’s enriched in the lighter component but far from pure. For mixtures where the boiling points differ by less than about 25 °C, simple distillation can’t achieve meaningful separation.
Fractional distillation solves this with the fractionating column, which forces dozens or even hundreds of vaporization-condensation cycles. The more theoretical plates the column provides, the better it resolves close-boiling liquids. In practice, this means you can separate compounds that boil just a few degrees apart, something simple distillation cannot do.
The Fractionating Column
The column is the heart of the process, and its job is to maximize the number of theoretical plates in a given height. Several designs accomplish this in different ways.
- Vigreux columns have pointed glass indentations along the inner wall. They offer the least surface area of common designs and work best when the boiling point gap is moderate.
- Packed columns are filled with glass beads, ceramic rings, or steel wool. Glass beads provide high surface area and are a strong choice for separating close-boiling components. Steel wool falls in the middle, and its performance depends on how tightly it’s packed.
The general principle is straightforward: more surface area means more places for vapor to condense and re-evaporate, which means more theoretical plates and sharper separation. Engineers measure column efficiency using something called HETP, or “height equivalent to a theoretical plate.” A lower HETP means the column packs more separation power into less height.
Crude Oil Refining
The largest-scale application of fractional distillation is in petroleum refineries, where crude oil is heated to roughly 315 to 400 °C (600 to 750 °F) and fed into massive distillation towers. Crude oil is a complex mixture of hydrocarbons with a wide range of boiling points, and the tower sorts them into groups called fractions.
The lightest fractions rise to the top. Naphtha, the feedstock for gasoline, has a boiling range of roughly 50 to 200 °C (122 to 400 °F). Kerosene and jet fuel overlap in the middle, with final boiling points around 300 °C (572 °F). Diesel fuel is heavier still, with a distillation temperature around 340 °C (640 °F) at the 90-percent recovery point. The heaviest residues, used for asphalt and heavy fuel oil, never make it off the bottom of the tower.
Each fraction is drawn off at a different level of the column, where the temperature corresponds to its boiling range. A single refinery tower can produce half a dozen or more products from one stream of crude.
Separating Air Into Its Components
Fractional distillation also works on gases, as long as you cool them into liquid form first. Industrial air separation plants chill air to around negative 200 °C, turning it into a liquid mixture of nitrogen, oxygen, and argon. These three gases have boiling points within just a few degrees of each other, so separating them requires tall, highly efficient columns.
Nitrogen, with the lowest boiling point, rises to the top and can be collected at purities above 99.99%. Oxygen-enriched liquid collects at the bottom at around negative 180 °C, reaching purities of 99.8% or higher. Argon, which boils between the two, is drawn from a side column and can be purified to 99.99%. These gases supply hospitals, welding shops, semiconductor factories, and food packaging operations worldwide.
Where Fractional Distillation Hits a Limit
Some liquid mixtures form what’s called an azeotrope: a specific ratio at which the vapor has the exact same composition as the liquid. At that point, no amount of re-evaporation and condensation will change the ratio, and fractional distillation stalls. The most familiar example is ethanol and water. You can distill an ethanol-water mixture to roughly 95% ethanol, but beyond that the two liquids behave as a single substance. Reaching higher purities requires a different technique entirely, such as adding a third chemical to break the azeotrope or using molecular sieves to absorb the remaining water.
Safety Considerations
In a laboratory setting, the main risks are pressure buildup and flammable vapors. If a column becomes clogged or a vent is blocked, pressure can spike and crack glassware. Water trapped in a hot system is particularly dangerous: if it contacts a liquid well above 100 °C, it can flash to steam and cause an explosive release of vapor.
At the industrial scale, refineries manage these risks with pressure relief valves, flare systems that safely burn off excess vapor, and constant monitoring of temperature and pressure inside each tower. Leaks in heaters or exchangers are a persistent fire hazard because the vapors involved are often highly flammable, and the equipment itself can serve as an ignition source.