Separating Gases from Liquid Air

Introduction

Every breath you take is a mixture: roughly 78% nitrogen, 21% oxygen and about 0.9% argon, with small amounts of carbon dioxide, water vapour and other gases. Hospitals need pure oxygen, food packers need pure nitrogen and welders need pure argon. Industry obtains all three from the same free raw material — the air around us. The trick is to turn air into a liquid and then use fractional distillation, exactly the method used to separate miscible liquids.

Core explanation

Why not separate air as a gas? The gases in air are fully mixed and do not settle into layers. At room temperature they are all far above their boiling points, so there is no difference in behaviour that a simple method can use. Distillation needs a liquid mixture whose parts boil at different temperatures, so the first job is to liquefy the air.

Boiling points of the main gases:

Gas Boiling point (°C) Share of dry air --- --- --- Nitrogen, N₂ −196 about 78% Argon, Ar −186 about 0.9% Oxygen, O₂ −183 about 21%

These values are all far below room temperature, and they are close together — only 13 °C separates nitrogen from oxygen. That is why a tall fractionating column is needed rather than simple distillation.

Stage 1: cleaning the air. Air is drawn in and filtered to remove dust. Water vapour and carbon dioxide are then removed. This matters because water freezes at 0 °C and carbon dioxide turns solid at about −78 °C; long before the air liquefies, these would form ice and "dry ice" that block the pipes.

Stage 2: cooling and liquefying. The clean air is compressed, which warms it, and the heat is removed. It is then allowed to expand, which cools it sharply. Repeating this compression–cooling–expansion cycle lowers the temperature to about −200 °C. At this temperature nitrogen, argon and oxygen are all liquids.

Stage 3: fractional distillation. The liquid air enters a fractionating column that is warmer at the bottom and colder at the top. As it warms slightly, nitrogen , with the lowest boiling point, evaporates first and rises to the top of the column, where it is drawn off as a gas. Oxygen , with the highest boiling point, stays liquid and collects at the bottom. Argon , with a boiling point in between, is concentrated in the middle of the column and removed from there, often for further purification.

The logic is the same as for any fractional distillation: the lower the boiling point, the higher up the column a substance travels before it condenses.

Step-by-step reasoning

To predict where each gas is collected:

1. List the boiling points: N₂ −196 °C, Ar −186 °C, O₂ −183 °C. 2. The lowest boiling point (nitrogen) evaporates most easily, so it leaves from the top. 3. The highest boiling point (oxygen) stays liquid longest, so it collects at the bottom. 4. The substance in between (argon) is taken from the middle.

Visual explanation

Picture a tall insulated tower with a thermometer at each level. The bottom reads about −183 °C and holds a pale blue pool of liquid oxygen. The top reads about −196 °C, and nitrogen gas streams out of a pipe there. Halfway up, a side pipe draws off an argon-rich stream.

Real-world analogy

Imagine a queue of people leaving a cinema as the lights come up. Those sitting nearest the exit leave first, and those in the back row leave last. In the column, nitrogen is "nearest the exit": it needs the least warming to boil, so it escapes first.

Real-world example

Oxygen from air separation plants is used in hospitals for patients with breathing difficulties and in huge quantities in steelmaking to burn impurities out of molten iron. Nitrogen fills crisp packets so the food does not go stale, and liquid nitrogen is used to freeze food and store biological samples. Argon shields hot metal from oxygen during welding and fills some light bulbs.

Why?

Why must water vapour and carbon dioxide be removed first? Both turn solid at temperatures much higher than −200 °C. As the air cools, they would freeze onto the inside of the pipes and heat exchangers, blocking the flow and stopping the plant from working.

Common misconception

"Oxygen boils off first because it is the most important gas." The order depends only on boiling point, not on usefulness or amount. Nitrogen has the lowest boiling point, so it evaporates first; oxygen, with the highest, remains as a liquid at the bottom.

Worked example

Question: Neon has a boiling point of −246 °C. If a small amount of neon were present in the liquid air, where in the column would it end up compared with nitrogen?

Reasoning: Neon's boiling point is even lower than nitrogen's (−196 °C). A lower boiling point means it evaporates more easily and travels further up the column.

Answer: Neon would be found at or above the nitrogen, at the very top of the column. In practice it is collected from the coldest part of the plant.

Quick check

1. Which of the three main gases in air has the highest boiling point? Answer: Oxygen, at about −183 °C.

Exam focus

Learn the three boiling points and the order in which the gases separate. Be ready to explain why air is cooled to about −200 °C, why carbon dioxide and water are removed first, and to link each gas to a use. Remember that this is fractional distillation because the boiling points are close together.

Advanced insight

Liquid gases are hazardous in ways that ordinary liquids are not. Contact with them causes cold burns similar to heat burns. Large releases of nitrogen or argon can displace oxygen in a closed room without any smell or warning, and liquid oxygen makes many materials burn far more fiercely. Air separation plants therefore rely on insulation, oxygen monitors, ventilation and strict handling rules.

Summary

Air is a mixture of mainly nitrogen, oxygen and argon. To separate them, air is cleaned, then compressed and cooled to about −200 °C until it becomes liquid. Fractional distillation then separates the gases by boiling point: nitrogen (−196 °C) leaves from the top, oxygen (−183 °C) collects at the bottom and argon (−186 °C) is taken from the middle.

Practice questions

1. Why must air be liquefied before its gases can be separated by fractional distillation? Answer: Distillation separates liquids by boiling point; at room temperature all the gases are far above their boiling points, so they must be cooled until they become liquids. 2. State why carbon dioxide is removed from the air before it is cooled. Answer: Carbon dioxide turns solid at about −78 °C, so it would freeze and block the pipes as the air is cooled. 3. Put nitrogen, argon and oxygen in order of how high up the column they are collected, highest first. Answer: Nitrogen (top), argon (middle), oxygen (bottom), matching their boiling points from lowest to highest. 4. Give one use each of oxygen and argon obtained from air. Answer: Oxygen: medical breathing support or steelmaking. Argon: an inert shielding gas in welding or filling light bulbs.