Joule–Thomson Effect
Table of Contents
The Joule–Thomson effect is the temperature change that occurs when a real gas expands from high to low pressure through a porous plug, a narrow opening, or a throttle valve under insulated conditions. This type of expansion is also called throttling or Joule–Thomson expansion. [1–4]
The effect is named after James Prescott Joule and William Thomson, who investigated this behavior of gases in 1852.
What Happens During Joule–Thomson Expansion
During the process, no heat is exchanged with the surroundings, and no external (shaft) work is obtained from the gas. As a result, the enthalpy of the gas remains constant. The gas may cool, warm, or show little to no temperature change depending on its nature, starting temperature, and pressure. [1–4]
Real gases show the Joule–Thomson effect because their molecules experience intermolecular attractions and repulsions. As the gas expands, the average distance between molecules changes, redistributing energy between molecular motion and intermolecular forces. When attractive forces are dominant, a portion of the gas’s kinetic energy is used to overcome these forces, leading to a decrease in temperature. When repulsive effects dominate, the gas may warm instead.
On the other hand, an ideal gas does not show the Joule–Thomson effect because it is assumed to have no intermolecular forces. The enthalpy depends only on temperature, so a constant–enthalpy expansion produces no temperature change.
Joule–Thomson Coefficient
The Joule–Thomson coefficient indicates whether a gas will cool or warm during Joule–Thomson expansion. It is represented by μJT and is defined as: [1–5]
μJT = (∂T/∂P)H
Where:
T: Temperature
P: Pressure
H: Enthalpy
The subscript “H” means that the change is measured at constant enthalpy, i.e., ΔH = 0. In simple terms, the Joule–Thomson coefficient shows how the temperature of a gas changes with pressure during an isenthalpic expansion. No heat is exchanged with the surroundings, and no shaft work is produced.
The sign of μJT is important:
| Sign of μJT | Effect during expansion |
|---|---|
| Positive | The gas cools as pressure decreases, meaning temperature also decreases. |
| Negative | The gas warms as pressure decreases, meaning temperature increases. |
| Zero | The gas shows no temperature change. |
The inversion temperature is the temperature at a given pressure where the Joule–Thomson coefficient changes sign. At this temperature, the gas does not change temperature during Joule–Thomson expansion. This point marks a boundary between cooling and warming behaviors in gases.
When below the inversion threshold, a gas cools as it expands; above it, the gas may warm. Each gas has its own inversion curve, and its inversion temperature changes with pressure.
Under ordinary room-temperature conditions, most gases cool during Joule–Thomson expansion. On the other hand, hydrogen, helium, and neon may warm unless they are precooled below their inversion temperatures.
Applications
The cooling that occurs during Joule–Thomson expansion makes the effect useful in refrigeration and low-temperature applications. It is also central to the Linde process, where gases are repeatedly compressed, cooled, and expanded to produce liquids such as liquid oxygen, nitrogen, and argon. [5]
