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The molecules in a gas must be pushed closer together to liquefy it. This can be performed by lowering the temperature and increasing the pressure. When you increase the pressure on a gas, the molecules come closer and closer until they unite to form liquids at a certain pressure. When a gas’s temperature is reduced, the molecules lose kinetic energy, resulting in a considerable decrease in velocity. Slow-moving molecules cannot resist the force of attraction, so they clump together to form a liquid. A reduction in temperature and an increase in pressure cause the liquefaction of gases.
History on the Liquefaction of Gases :
Thomas Andrew was the first to investigate the transition of carbon dioxide from a gas to a liquid state. Most genuine gases, it was later discovered, behave like Carbon Dioxide (CO2) and shift from gases to liquids when the right physical adjustments in temperature and pressure are made.
Andrews discovered that the gases cannot liquefy at high temperatures, despite great pressure, in his CO2 experiment. The gases also deviate significantly from their optimum behaviour as the temperature rises. At 30.98° C, the gas began to change into a liquid in the instance of carbon dioxide.
Necessary Conditions :
Critical Temperature : Gas cannot be liquefied above a certain temperature, regardless of how much pressure is applied to it. This temperature is referred to as the gas’s critical temperature. It is possible to define it as follows:
The critical temperature of a gas is the temperature above which it is impossible to liquefy it with any amount of pressure.
The critical temperature is denoted by the symbol TC
TC=8a27bR
Where , a and b are the Van der Waal constant and R denotes gas constant
Critical Pressure : The critical pressure of a gas is the minimum pressure required to liquefy it at the critical temperature. It is symbolised by PC
PC=a27b2
Where a and b are Van der Waal constants
Critical Volume : The critical volume of a gas is the volume occupied by one mole of the gas at critical conditions and it is denoted by VC
VC=3b
All these three constants are collectively known as Critical Constants of Gas
Carbon Dioxide Isotherms :
T. Andrews carried out a series of carbon dioxide tests, analysing and graphing the pressure-volume relationship for the gas at various temperatures. These curves are referred to as carbon dioxide P–V isotherms. The isotherm was obtained at 0°C, 21°C, 31.1°C, and 50°C.
At point A, the lowest temperature used, i.e. 13.1°C at low pressure, carbon dioxide exists as a gas.
As can be seen, the volume of the gas decreases as the pressure increases along the curve.
When carbon dioxide hits 21.5°C, it behaves like a gas until point B.
At point B, the gas is in a dual state, i.e. it is both liquid and gas.
All of the carbon dioxide condenses at point C, causing an increase in pressure.
The volume of the gas rapidly reduces as liquefaction occurs because the liquid has a considerably lower volume than the gas.
Due to the low compressibility of liquids, after liquefaction is complete, the increase in pressure has very little effect on volume. As a result, the curve becomes steep. The liquid isotherm is represented by the steep line.
Below 30.98°C, the behaviour of the gas under compression is quite different, and each curve follows a similar pattern. At lower temperatures, only the length of the horizontal line increases, and the horizontal portion of the isotherm merges into one point at the critical point.
Despite the fact that enormous pressure can be applied, the gas cannot liquefy over 30.98oC. As a result, carbon dioxide’s critical temperature is 30.98oC.
It was revealed that all gases react identically to carbon dioxide due to isothermal compression.
Methods of Liquefaction :
Linde’s Process : The Linde method liquefies air by compressing, cooling, and expanding it repeatedly, each expansion resulting in a significant fall in temperature. Because the molecules travel more slowly and take up less space at lower temperatures, the air changes phase to liquid.
Claude’s Process : Claude’s technique, in which the gas is allowed to expand isentropically twice in two chambers, can likewise liquefy air. As it passes through an expansion turbine, the gas has to work while expanding. Because the turbine would be destroyed if the gas became liquid, it is not yet liquid. By expanding the air to supercritical pressures, commercial air liquefaction plants avoid this problem. Isenthalpic expansion in a thermal expansion valve is used to complete the liquefaction process.
Conclusion :
We looked at the liquefaction of gases and an example in this topic. We also looked into the circumstances that are required for gas liquefaction and the ways that can be used. We know that critical temperature is important in gas liquefaction, and that due to isothermal compression, all gases behave identically to carbon dioxide.
