Liquefaction of Gases: Introduction and Methods

You must be familiar with the arrangement of particles in gases. A gas is composed of many molecules that lie far apart, and it is this arrangement that makes it difficult to transport gas from one place to another. In order to do so, the gases are converted into liquids. In this article, we will discuss how it is done and the various methods of doing the Liquefaction of gases.

Liquefaction of Gases

Liquefaction is a process in which the gaseous substance transforms into a liquid state.

For example, Oxygen generally occurs as a gaseous substance and can be converted to a liquid by applying sufficient pressure and reducing the temperature.

In order to liquefy a gas, it is necessary to bring the molecules closer. This can be achieved by lowering the temperature and increasing the pressure. When the pressure on a gas is increased, the molecules come closer and closer, and at a certain pressure, they merge to form liquids.

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On the other hand, when the temperature of a gas is lowered, the molecules lose kinetic energy, which results in a considerable decrease in their velocity. The slow-moving molecules cannot resist the force of attraction and come closer and closer, and finally, aggregate to form the liquid.

Thus, a lowering in temperature and an increase in pressure leads to the liquefaction of gases.

Conditions Necessary for Liquefaction of Gases

There are two conditions necessary for the liquefaction of gases which are as follows:

1. Low temperature
2. High pressure

Critical Temperature and its Importance

T. Andrews, in \(1869\) studied the phenomenon of liquefaction of gases and succeeded in liquefying several gases. He found that for each gas, there is a certain temperature above which it cannot be liquefied, no matter how high is the pressure applied on it. This temperature is called the critical temperature of the gas. It may be defined as follows:

The temperature above which it is not possible to liquefy a gas by applying any amount of pressure is called the critical temperature of that gas.

For example, the critical temperature of carbon dioxide is \(30.98{\,^{\rm{o}}}{\rm{C}}.\) This implies that carbon dioxide cannot be liquefied above \(30.98{\,^{\rm{o}}}{\rm{C}},\) howsoever high may be the pressure applied on it.

The critical temperature is represented by \({{\rm{T}}_{\rm{c}}}{\rm{.}}\) It is given by-

\({{\rm{T}}_{\rm{c}}}{\rm{ = }}\frac{{{\rm{8a}}}}{{{\rm{27bR}}}}\)

Critical pressure and critical volume:

The minimum pressure required to liquefy a gas at the critical temperature is called the critical pressure of the gas. It is represented by \({{\rm{P}}_{\rm{c}}}\) and is given by-

\({{\rm{P}}_{\rm{c}}}{\rm{ = }}\frac{{\rm{a}}}{{{\rm{27}}{{\rm{b}}^{\rm{2}}}}}\)

Where \({\rm{a}}\) and \({\rm{b}}\) are Van der Waal’s constant.

The volume occupied by one mole of a gas at critical conditions is known as the critical volume of the gas. It is represented by \({{\rm{V}}_{\rm{c}}}\) and is given by-

\({{\rm{V}}_{\rm{c}}}{\rm{ = 3b}}\)

where \({\rm{b}}\) represents the effective volume of the molecules per mole of the gas.
\({{\rm{T}}{\rm{c}}},\,{{\rm{P}}{\rm{c}}},\) and \({{\rm{V}}_{\rm{c}}}\) are collectively referred to as critical constants of a gas.

Isotherms of Carbon dioxide

T. Andrews carried out a series of experiments on carbon dioxide, studied the pressure-volume relationship for the gas at different temperatures, and plotted the points. These curves are referred to as \({\rm{P – V}}\) isotherms of carbon dioxide. The isotherm was obtained at \(0{\,^{\rm{o}}}{\rm{C}},21{\,^{\rm{o}}}{\rm{C}},31.1{\,^{\rm{o}}}{\rm{C}}\) and \(50{\,^{\rm{o}}}{\rm{C}}.\)

At point \({\rm{A,}}\) the lowest temperature employed, i.e. at \(13.1{\,^{\rm{o}}}{\rm{C}},\) at low pressure, carbon dioxide exists as a gas.

It can be observed that as the pressure is increased, the volume of the gas decreases along the curve.

On attaining \(21.5{\,^{\rm{o}}}{\rm{C}},\) carbon dioxide behaves like a gas until point \({\rm{B}}{\rm{.}}\)

At point \({\rm{B}}{\rm{,}}\) the gas exists in a dual form that is both liquid and gas.

At point \({\rm{C}}{\rm{,}}\) all the carbon dioxide condenses and further compression results in the rise in pressure.

When the liquefaction of the gas starts, volume decreases rapidly because the liquid has much less volume than the gas.

Once the liquefaction is complete, the increase in pressure has very less effect upon volume because liquids are very little compressible. Hence, a steep curve is obtained. The steep line represents the isotherm of liquid. 

Below \(30.98{\,^{\rm{o}}}{\rm{C}},\) the behaviour of the gas on compression is quite different  and each curve shows the similar trend. Only length of the horizontal line increases at lower temperature, and at critical point horizontal portion of the isotherm merges into one point.

Above \({30.98^{\rm{o}}}{\rm{C,}}\) the gas cannot be liquefied at all however high pressure may be applied. Thus, \({30.98^{\rm{o}}}{\rm{C}}\) is the critical temperature of carbon dioxide.

We get a dome-shaped curve.

It was observed that all gases behave similarly to carbon dioxide because of the isothermal compression.

Importance of Critical Temperature

Critical temperature plays an important role in the liquefaction of gases. Gas can only be liquefied when it is below its critical temperature.

The gas whose critical temperature is high like that of \({\rm{N}}{{\rm{H}}_3},{\rm{C}}{{\rm{O}}_2},{\rm{S}}{{\rm{O}}_2},\) etc., can be liquefied by applying suitable pressure only.The gases which possess low or very low critical temperature like \({{\rm{H}}_{\rm{2}}}{\rm{,He,}}\) etc., cannot be liquefied just by applying pressure on them. They can be liquefied only when they are cooled below their respective critical temperature and then subjected to adequate pressure.

Liquefaction of Gases can be Achieved By:

The cooling of gas can usually be carried out by making use of the following principles:

1. By compressing the gas below its critical temperature.
   
2. Joule-Thomson effect: When a highly compressed gas, at a temperature below its inversion temperature, is passed through a throttle (a porous plug or jet) from a region of high pressure to low pressure under adiabatic conditions, it suffers a fall in temperature. This phenomenon is called the Joule-Thomson effect and is widely used for the liquefaction of gases.
3. Adiabatic expansion involving mechanical work: When a gas is subjected to adiabatic expansion involving mechanical work,it loses some of its kinetic energy, and there is a fall in the temperature.

Methods of Liquefaction of Gas

There are several methods used for the liquefaction of gases. Among these, the two important ones used for the liquefaction of air are described below:

1. Linde’s Method

This method works on the Joule-Thomson effect. Pure and dry air is introduced into a compressor, where it is compressed to about \(200\) atmospheres. It is then passed through a pipe cooled by a refrigerating liquid such as liquid ammonia, which removes the heat of compression. The compressed air is then passed through a spiral pipe having a jet at its end and fitted in an insulated chamber. As the compressed air passed through the jet, it suffers expansion resulting in a considerable decrease in its temperature. The expanded air moves up the chamber and cools the fresh air coming through the spiral tube. It is then collected through a pipe and again sent to the compressor. The process is repeated over and over again when the air gets sufficiently cooled and gets liquefied.

2. Claude’s Method

This method uses both the Joule-Thomson and adiabatic expansion effect of the gas involving mechanical work.

Pure and dry air is admitted into the compressor, where it is compressed to about \(200\) atmospheres. A refrigerating liquid then cools it to remove the heat of compression. The compressed gas is taken to an insulated chamber through a tube. Here, it is bifurcated into two parts. One part is passed through a spiral tube having a jet at the end, where it suffers Joule-Thomson expansion and records a fall in temperature. The other part is taken to the cylinder of an engine, where it does mechanical work by pushing back the piston and gets cooled. It then enters the insulated chamber and mixes with the air coming out of the jet. It then cools the pipe carrying the incoming air. The cooled air is collected and taken to the compressor again. The entire process is repeated over and over again when the air gets sufficiently cooled and gets liquefied.

Summary

In this article, we studied in detail the liquefaction of gases and their example. We also studied the conditions necessary for the liquefaction of gases and the methods that can be employed for this process. We know that critical temperature plays a major role in the liquefaction of gas, and all gases behave similarly to carbon dioxide because of the isothermal compression.

Frequently Asked Questions (FAQs) on Liquefaction of Gases

Q.1. What is the process of liquefaction?
Ans: Liquefaction is a process in which the gaseous substance transforms into a liquid state.
For example, Oxygen generally occurs as a gaseous substance and can be converted to a liquid by applying sufficient pressure and reducing the temperature.

Q.2. What is the significance of critical temperature in the liquefaction of gases?
Ans: Critical temperature plays an important role in the liquefaction of gases. Gas can only be liquefied when it is below its critical temperature.

Q.3. What are the uses of the liquefaction of gases?
Ans: The most important use of gas liquefaction is that they can be stored and transported in a much more compact form than done in the gaseous state. For example, the two most important gases are LPG and Liquid Oxygen.

Q.4. How does pressure help in the liquefaction of gases?
Ans: On applying enough pressure to the gas, the space between the particles of gas reduces, and the gas starts to compress, and the gases start liquefying.

Q.5. What is the importance of the liquefaction of gases?
Ans: The importance of liquefaction of gas is that they can be stored and transported in a much more compact form than done in the gaseous state. For example, the two most important gases are LPG and Liquid Oxygen.

Q.6. What is the principle of the liquefaction of gases?
Ans: The principle is that the liquefaction of gases requires high pressure and low temperature. Increasing the pressure will bring the molecules of the gas close to each other, and lowering the temperature causes to increase in the attractive forces.

Q.7. Which are the two major steps involved in the process of liquefaction of gas?
Ans: The two major steps necessary for the liquefaction of gases are as follows-
1. First, the pressure is increased, which brings the molecules of the gas close to each other.
2. Secondly, the temperature is lowered that causes to increase in the attractive forces.
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