principles of gas liquefaction and cylinderization, critical temperature, critical pressure and critical volume and Inversion Temperature
Question: What are Critical Temperature, Critical Pressure, Critical Volume, and Inversion Temperature?
Critical Temperature:
For every gas, there is a specific temperature above which it cannot be liquefied, no matter how much pressure is applied. However, at or below this temperature, the gas can be easily liquefied by applying pressure. This specific temperature is called the critical temperature of the substance.
For example: The critical temperature of CO2 is 31.1°C. This means above 31.1°C, it cannot be liquefied regardless of the pressure applied. But at or below 31.1°C, CO2 gas can be transformed into a liquid by applying the necessary pressure.
Critical Pressure:
The minimum pressure required to liquefy 1 mole of a gas exactly at its critical temperature is called its critical pressure.
For example: To liquefy 1 mole of CO2 gas at 31.1°C, a pressure of 72.9 atm must be applied. Therefore, the critical pressure of CO2 is 72.9 atm.
Critical Volume:
The volume occupied by 1 mole of a gas at its critical temperature and critical pressure is called its critical volume.
For example: The critical volume of CO2 gas is 95.65 mL.
Inversion Temperature:
There is a specific temperature for every gas below which the Joule-Thomson effect is applicable (where the gas cools upon expansion). This temperature is called the inversion temperature of the gas.
For example: The inversion temperatures for hydrogen and helium are -80.15°C and -240°C, respectively.
Question: Explain the basic principles of gas liquefaction and cylinderization.
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Applying Boyle’s and Charles’s Laws:
Various gases can be compressed and liquefied by applying Boyle’s and Charles’s laws. According to the postulates of the kinetic theory, as temperature decreases, the kinetic energy of gas molecules decreases. At a sufficiently low temperature (at or below the critical temperature), gas molecules come closer to each other, and the intermolecular force of attraction increases, causing the gas to turn into a liquid. The core principle is that, according to Boyle’s law (V ∝ 1/P), applying very high pressure decreases the volume of the gas. This brings the molecules closer, and the increased intermolecular forces liquefy the gas. This mechanism is applied for gas cylinderization.
However, all gases might not be liquefied simply by applying pressure alone. In reality, along with applying pressure, the temperature of the gas must also be decreased. According to the kinetic theory of gases, as the temperature of a gas falls below its critical temperature, the kinetic energy of the molecules decreases, and simultaneously the volume of the gas decreases (V ∝ T); this is a special form of Charles’s law. The logic here is that at sufficiently low temperatures, the slow-moving gas molecules coming closer cannot overcome the intermolecular force of attraction. Under the influence of this force, the gas turns into a liquid according to Charles’s law.
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Using Freezing Mixtures or Faraday’s Method:
Gases can be liquefied by reducing their temperature using freezing mixtures. However, the temperature cannot be lowered very much using this method. Therefore, this method is only applicable to a limited number of gases like CO2, NH3, Cl2, etc. In this method, a temperature of -20°C can be achieved using a mixture of NaCl and ice, -54°C using molten CaCl2 and ice, and up to -110°C using a mixture of solid CO2 (dry ice) and ether. Scientist Faraday used this method to liquefy gases.
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Linde’s Method:
Scientist Linde liquefied gases by applying the Joule-Thomson effect. The Joule-Thomson effect is the phenomenon where the temperature of a gas decreases when it is suddenly expanded after being highly compressed. The reason is that in the highly compressed state, strong intermolecular forces of attraction exist between the gas molecules. During sudden expansion, as the molecules move away from each other, work has to be done against this intermolecular attraction. The energy required for this work is absorbed from the internal energy of the gas itself. That is why the temperature of the gas drops due to the Joule-Thomson effect. By applying this method, scientist Linde invented a process for liquefying gases on a commercial scale. In this method, when a gas is repeatedly compressed and expanded, its temperature gradually decreases according to the Joule-Thomson effect. Once it reaches the critical temperature, the gas turns into a liquid.
