Fractional Distillation of Liquid Air

Introduction

Hospitals need pure oxygen, food factories need pure nitrogen and welders need argon. All three come from the same free and endless raw material: the air. Because air is a mixture, its gases can be separated by a physical process. The method used industrially is fractional distillation of liquid air . It relies on the fact that each gas in air has its own boiling point. This page explains how air is prepared, liquefied and then separated.

Core explanation

The key data. The main gases in air have different, very low boiling points:

Substance Boiling point --- --- Nitrogen −196 °C Argon −186 °C Oxygen −183 °C

Because these values differ, the gases can be separated when air is turned into a liquid and then gently warmed.

Stage 1: cleaning the air. Air is drawn in and filtered to remove dust. Water vapour and carbon dioxide are removed next, because they would freeze into solid ice and "dry ice" at the very low temperatures used, blocking the pipes. They are taken out by passing the air through beds of adsorbent material, or by cooling so they solidify and are separated.

Stage 2: liquefying the air. The clean air is compressed to many times atmospheric pressure, which warms it. The heat is removed by cooling. The compressed air is then allowed to expand rapidly. Expansion makes a gas cool down, and repeating the compression–cooling–expansion cycle, with the cold outgoing gas used to pre-cool the incoming air, eventually brings the temperature to about −200 °C . At this temperature most of the air has become a pale blue liquid.

Stage 3: fractional distillation. The liquid air is fed into a tall fractionating column . The column is slightly warmer at the bottom and colder at the top.

- Nitrogen has the lowest boiling point (−196 °C), so it boils most easily. It rises as a gas and is collected at the top of the column. - Oxygen has the highest boiling point (−183 °C), so it stays liquid and collects at the bottom . - Argon , with a boiling point in between, collects part-way up the column and is drawn off from the side. Because its boiling point is so close to that of oxygen, extra columns are needed to purify it.

Neon, krypton and xenon are also separated in small amounts from special parts of the process.

Why "fractional"? Inside the column, rising vapour meets falling liquid again and again. Each time, the vapour becomes richer in the lower-boiling substance and the liquid becomes richer in the higher-boiling one. After many such stages, the gases emerge highly pure. Each separated product is called a fraction .

It is a physical process. No chemical reactions happen. The molecules of nitrogen, oxygen and argon are the same in the products as in the air; they have simply been sorted by boiling point. This is strong evidence that air is a mixture.

Step-by-step reasoning

To predict where each gas leaves the column:

1. List the boiling points of the gases. 2. The lowest boiling point boils most readily, rises highest and leaves at the top. 3. The highest boiling point stays liquid longest and leaves at the bottom. 4. Gases with intermediate boiling points are taken off at intermediate heights.

Visual explanation

Picture a tall column with a thermometer scale down its side: about −196 °C at the top and about −183 °C near the bottom. Liquid air enters in the middle. Arrows of blue nitrogen gas rise and leave from the top; a pool of pale blue liquid oxygen gathers at the base; a side pipe near the lower middle draws off argon.

Real-world analogy

It is like several runners who start together but each drop out at a different point on a staircase depending on their stamina. Nitrogen has the most "energy to escape" at these temperatures and reaches the top, oxygen drops out first near the bottom, and argon stops part-way.

Real-world example

Liquid nitrogen from air separation plants is used to freeze food very quickly, which keeps its texture, and to store biological samples such as blood and embryos at −196 °C. Hospitals keep large insulated tanks of liquid oxygen, which is far more compact than gas; it is warmed and turned back into gas to be piped to wards.

Why?

Why must the carbon dioxide be removed before cooling? Carbon dioxide turns directly into a solid at −78 °C, and water freezes at 0 °C. As the air is cooled towards −200 °C, both would form solid deposits that block the narrow pipes and valves, so they must be taken out while the air is still warm.

Common misconception

"Oxygen comes off the top because it is the most useful." The order has nothing to do with usefulness or with how much of each gas there is. It depends only on boiling point: nitrogen boils at the lowest temperature, so it leaves at the top; oxygen boils at the highest, so it collects at the bottom.

Worked example

Question: Neon boils at −246 °C and krypton at −153 °C. Where would each tend to collect in a fractionating column compared with nitrogen and oxygen?

Reasoning: Neon's boiling point is lower than nitrogen's (−196 °C), so neon is even more volatile and would go to the very top. Krypton's boiling point is higher than oxygen's (−183 °C), so it would stay in the liquid at the very bottom with the oxygen.

Answer: Neon at the top (above nitrogen); krypton at the bottom (with the oxygen).

Quick check

1. Which gas leaves the top of the column, and why? Answer: Nitrogen, because it has the lowest boiling point (−196 °C).

Exam focus

Learn the three boiling points and the three stages: remove dust, water and carbon dioxide; compress, cool and expand to liquefy; fractionally distil. Explain the order of separation by boiling point, and state that the process is a physical separation, not a chemical reaction.

Advanced insight

Liquid oxygen and liquid nitrogen are hazardous. They can cause severe cold burns, and as they boil they produce very large volumes of gas — liquid nitrogen expands about 700 times — which can burst sealed containers or displace oxygen in enclosed rooms. Liquid oxygen also makes nearby materials burn far more fiercely, so plants are carefully designed and ventilated.

Summary

Air is separated industrially by fractional distillation of liquid air. Dust, water vapour and carbon dioxide are removed; the air is compressed, cooled and expanded to about −200 °C to liquefy it. In the fractionating column, nitrogen (−196 °C) leaves at the top, argon (−186 °C) part-way down and oxygen (−183 °C) at the bottom. The separation is physical and depends only on boiling points.

Practice questions

1. Why must water vapour and carbon dioxide be removed from air before it is liquefied? Answer: They would freeze into solids at the low temperatures used and block the pipes. 2. Put nitrogen, oxygen and argon in order of boiling point, lowest first. Answer: Nitrogen (−196 °C), argon (−186 °C), oxygen (−183 °C). 3. Explain why oxygen collects at the bottom of the fractionating column. Answer: It has the highest boiling point of the three main gases, so it remains liquid while nitrogen and argon boil off. 4. Why is fractional distillation of liquid air described as a physical process? Answer: No new substances are formed; the gases are separated unchanged, simply by their different boiling points. 5. Why is argon difficult to separate completely from oxygen? Answer: Their boiling points (−186 °C and −183 °C) are very close, so many more separation stages are needed.