Liquefaction of Gases

Gases are very difficult to transport because of their bulk and weight. The process of converting gas into a liquid accomplishes the same result as turning a liquid into a gas. Based on this study, we may conclude that liquefaction significantly alters the properties and structure of the gas. Furthermore, it provides a vast lot of information on the general structure of matter. 

Liquefaction of Gases

Whatever has to be transformed from a gaseous state to a liquid state must be liquefied first. Temperature, pressure, and volume are only a few of the physical factors contributing to this transformation. Thomas Andrew was the one who discovered that Carbon Dioxide could change the state from a gas to a liquid state. Since then, it has been shown that most true gases behave similarly to carbon dioxide, turning from a gas to a liquid at the proper temperature and pressure.

Because of his experiments with CO2, Andrews concluded that gases could not be liquefied even at very high temperatures and pressure levels. Additionally, the gases diverge dramatically from their optimal behavior. Carbon dioxide started to convert into a liquid at 30.98𝆩 Celsius.

Limits for Pressure, Volume, and Temperature 

The results of Andrews’ experiment revealed that, regardless of how high the pressure was applied, the gas sample could not be liquefied above a specific temperature. Gases can only become liquids when their critical temperature is lowered below a certain point. The pressure required to liquefy a gas increases in direct proportion to the temperature of the gas. Temperatures far higher than this one were the only ones that resulted in gas turning to a liquid state. It’s referred to as the critical temperature point or Tc.

The crucial constants are the variables that determine the state of matter. The most critical constants are pressure, temperature, and volume. If one mole of a gas volume liquefies at the critical temperature, it is called Vc or Pc. When the required volume (Vc) and necessary pressure (Pc) are reached, the mole and pressure are referred to as Vc and Pc.

The Method of Liquefaction of Carbon Dioxide

It is vital to monitor changes in both volume and pressure. The methods of liquefaction of gases of carbon dioxide in the isotherm.

Andrews who performed a series of experiments on the pressure-volume relationship of carbon dioxide under varied conditions of temperature. Names for these curves include P–V isotherms of CO2. The isotherm was found at 0°C, 21.1°C, 31.1°C, and 50°C.

• Point A, the lowest temperature selected, i.e. 13.1°C at low pressure, is where carbon dioxide appears as a gas.
• As can be seen, the volume of the gas decreases as the pressure rises along the curve.
• When carbon dioxide hits 21.5°C, it acts as a gas until point B.
• When it comes to point B, the gas has a dual state: it is both liquid and gas.
• To put it another way, at point C all of the carbon dioxide condenses, leading to a rise in pressure.
• When liquefaction occurs, the volume of the gas quickly reduces due to the liquid’s lower volume than the gas’s.
• Once the liquefaction process is complete, the increase in pressure has very little effect on the volume of the liquid. In the end, this produces a sharp curve. Slope demonstrates that the liquid isotherm is present.
• Below 30.98°C, the gas behaves quite differently under compression, and each curve exhibits a distinct pattern. Just the horizontal line length grows at lower temperatures, and at the critical point, the horizontal component of the isotherm merges into one point.
• As long as pressure is maintained above 30.98°C, the gas will not condense. As a consequence, 30.98°Cis determined to be the threshold temperature for CO2.
• It was observed that all gases behave similarly to carbon dioxide because of the isothermal compression.
• The critical temperature is vital in the liquefaction of gases. Liquefaction is possible only if the gas temperature falls below a certain threshold. NH3, CO2, SO2, and other gases with a high critical temperature may be liquefied by increasing the pressure.

We observe that, as a result of the constant temperature or isothermal compression, all gases behave like CO₂. Isothermal compression is a word used to describe the identical behavior of gases when compressed at a constant temperature, regardless of the temperature. It is impossible to liquefy H2, He, and other low-critical-temperature gases by applying pressure alone. To liquefy, they must be refrigerated to a temperature below their critical point and then subjected to the right amount of pressure. 

Liquidification of Gases Requires Specific Environmental Conditions

Gas cooling may be done using the following principles:

• By reducing the gas’s temperature below the critical point.During an adiabatic transition from a high-pressure zone to a low-pressure zone, the temperature of the gas drops below its inversion temperature. This phenomenon is known as the Joule–Thomson effect. The Joule-Thomson effect is a well-known phenomena used in the liquefaction of gases.
• A gas loses part of its kinetic energy as it expands adiabatically via mechanical effort, and the temperature decreases.

Conditions

• The critical temperature of a gas is when liquefaction starts to occur in a certain amount of time.
• The strength of intermolecular forces of attraction increases as the critical temperature rises, making gas liquefaction more straightforward at higher temperatures.

Methods of Air Liquefaction

• Linde’s method of liquefaction of gases

Linde’s method of liquefaction of gases which repeatedly compresses, cools, and expands air to reduce its temperature, is used as a Method of Air Liquefaction. Soluble air may be formed as the air temperature drops since the molecules move more slowly and take up less space.

• Claude’s method

When air is heated and allowed to expand isentropically twice in two chambers, it may also be liquefied. As it passes through an expansion turbine, the gas is forced to exert energy throughout its expansion. The turbine would be destroyed if the gas became liquid. By expanding the air to supercritical pressures, commercial air liquefaction facilities avoid this issue. Thermal expansion valves are used to complete the final liquefaction process.

Methods of liquefaction of gases Require Cooling and Compression

Does a gas liquefies need both cooling and compression to achieve this liquefaction? To change the state of gases that have a positive deviation from the compressibility factor (Z), it is essential to cool and compress them simultaneously.

As for Explanation:

With a Freezing combination, it is not possible to drop the temperature of a gas very much. Ammonia, for example, has a critical temperature of 406K (271.4°F [133°C].). Because this temperature is just over the freezing point of water, converting ammonia gas to a liquid is a simple matter of applying sufficient pressure. This means that for most gases, such NH3 and CO2, the process temperature may be reduced to as low as -20°C using a salt and ice combination. For the other gases, however, the process temperature can be reduced to as low as -54°C by mixing calcium chloride with ice and Ether. Even though the colder the gas is to begin with, the less pressure is necessary to compress it, the pressure at its critical temperature is 112.5 atmospheres.

Changes in Temperature as a Result of Compression and Heating

As a consequence of compression, the pressure of gas molecules rises as the molecules squeeze together. This procedure has the effect of bringing molecules closer together. A decrease in random mobility occurs as soon as the molecules come into touch with one another, caused by the reduced temperature in their surroundings. This is due to the combined impact of compression and cooling, which causes intermolecular connections to be activated. Because of this intermolecular contact, the molecules move closer to one another, causing changes in their physical states.

an example of a problem on liquefaction of gases that were solved:

Q: While N2 has a critical temperature of 126 K, It has just 5.3 K. Why is this? Which of the following gases has the greatest likelihood of liquefaction?

When mixed with water, N2 (nitrogen) forms a liquid that dissolves swiftly. There are significant intermolecular forces in gases with high critical temperatures, which causes a fast state change to occur in these gases. For example, raising the pressure and lowering the temperature of oxygen in its gaseous state may transform oxygen into a liquid state.

Conclusion 

Liquidizing a gas necessitates the gathering of its constituent components in one place. This may be accomplished by lowering the temperature while increasing the pressure in the system. When a gas is subjected to a specific amount of pressure, the molecules must grow closer together for the gas to become a liquid.

Despite this, when the temperature of a gas is reduced, the molecules lose kinetic energy, resulting in a significant fall in the velocity of the gas. A liquid is formed due to the attraction between molecules that are too slow to resist the force of the attraction.

Gas liquefaction is caused by a combination of low temperature and high pressure.