Liquefaction of Gases
Table of contents
- Introduction to Liquefaction of Gases
- Methods for Liquefaction
- Andrews Experiments on Critical Phenomena
- Continuity of State
- Difference between Vapour and Gas
- Importance of Critical Temperature
- Critical constants in terms of van der Waals constants
The liquefaction of a gas is a phenomena which takes place when the intermolecular forces of attraction increases to such an extent that they combine the gas molecules together forming a liquid state. Liquefaction of gas can be increased by increasing the intermolecular forces of attraction, which in turn can be increased either by increasing the pressure, which will reduce the distance between the molecules or decreasing the kinetic energy by cooling the gas making them slower. Hence, a gas can be liquefied by cooling or by application of pressure or the combined effect of both.
Gases like ammonia, Sulphur dioxide, hydrogen chloride, carbon dioxide etc. could be liquefied by any one of the modes mentioned above that is, either by application of pressure (at room temperature) or by cooling (at atmospheric pressure). However, the effect of temperature on the liquefaction of gases is found to be very important as higher the temperature of the gas, more difficult it is to liquefy it and higher is the pressure required. For Example, pressures required to liquefy carbon dioxide at different temperatures are given below:
Simply by application of pressure, some gases like hydrogen, helium, oxygen, nitrogen etc. could not be liquefied at room temperature no matter whatever the pressure is. Hence, at one time, these gases were called ‘permanent gases’. Now, it is well known that each of these gases could also be liquefied provided first it is cooled down to or below a particular temperature. In other words, for each gas, there is a particular temperature above which it cannot be liquefied, howsoever high pressure may be applied on the gas. This temperature is known as Critical Temperature. Thus,
“Critical temperature of a gas may be defined as that temperature above which it cannot be liquefied howsoever high pressure may be applied on the gas”
Critical pressure is the amount of pressure required to liquefy the gas at its critical temperature. The volume occupied by one mole of the gas at the critical temperature and the critical pressure is called the Critical Volume. All the three collectively are called critical constants of the gas and are represented by TC, PC and VC. For Example, critical constants of CO2 are:
Tc= 30.98°C (304.10K), PC= 73.9 atm, VC= 95.6 cm3 mol-1
Hydrogen liquefies at a temperature of -240°C.
There are two types of Methods for Liquefaction:
- Linde’s Method for Liquefaction
- Claude’s Method of Liquefaction
Linde’s Method of liquefaction works on the principle of Joule Thomson effect, using which we can liquefy air and many other gases. First step in this process is compression of gas upto 200 atm and then it enters the inner tube as shown in fig. no. 2. The part E is opened from the tip of J allowing the gas to expand in the region below:
This expansion results in cooling of gas, thereby reducing the pressure of the gas upto 50 atm. The gas is now allowed to pass through tube coming out of D. Here gas which is coming in is initially cooled by gas which is going out. The cooled gas is recompressed and recirculated inside the apparatus. Then we repeat the process of cooling and compression followed by expansion. After the completion of process the gas is liquefied and the liquid air drops out taken out from the G.
Claude’s method of liquefaction of air is done by mechanical work of expansion at Y as shown in figure no. 3. Because of this external work kinetic energy of the gas gets reduced and the temperature reduces. This process also uses Joule-Thomson effect. Compression of air takes place upto 200 atm from where the gas is passed through the pipe T as shown in fig. no. 3. At point X, air divides into two from which one stream moves towards jet nozzle J passing via spiral C while another stream of the air moves towards piston in Y cylinder. Cooling is created by the movement of air in the piston. The incoming compressed air is passed through liquefying chamber from where the precooled incoming compressed air experiences Joule-Thomson expansion when passed through nozzle J and is cooled further. The process is repeated until gas is liquefied completely.
The first to study the critical phenomena experimentally using CO2 gas was Andrews in the year 1861. At different constant temperatures, he studied the effect of pressure on the volume of CO2. The plots obtained are called Isotherms. Some of the isotherms thus obtained are shown in figure below:
At the lowest temperature employed that is, 13.1°C, at low pressure, CO2 exists as a gas, as shown at the point A. The volume of the gas decreases along the curve, AB as the pressure is increased. Liquefaction of the gas starts at the point B, which results in the decrease of volume rapidly along BC because liquid has much less volume than the gas. Liquefaction process gets completed at the point C. As the liquids are almost incompressible, the increase in pressure has very little effect upon volume. Hence, a steep curve CD is obtained. As the temperature is increased, horizontal portion becomes smaller and smaller and at 30.98°C, it is reduced to a point, P. This means that above 30.98°C, the gas cannot be liquefied at all, however high pressure may be applied. Thus, 30.98°C is the critical temperature. The corresponding pressure to liquefy the gas at the critical temperature is its critical pressure, PC (that is, 73.9 atm). The volume occupied by 1 mole of the gas under these conditions is its critical volume, VC (that is, 95.6 mL).
Joining the end points of the horizontal portion of the different curves, we get a dome-shaped curve a shown in figure no. 4. We observe that a point like A in the figure represents gaseous state. A point like D represents a liquid state and a point within the dome shaped area represents existence of liquid and gaseous CO2 in equilibrium. All gases on isothermal compression show the same behavior as shown by CO2.
Gases can be converted to liquid state and liquid can be converted to gaseous state in such a manner that there is always a single phase present. For Example, in figure no. 4 by increasing the temperature, we can move from A to F vertically, and by compressing the gas at constant temperature along the 31.1°C isotherm, we can reach the point G. As the pressure is increasing, by lowering the temperature we can move vertically down towards pC. When the point H is crossed on the critical isotherm, liquid is obtained. Thus, at no stage during the process, we pass through two-phase region. This is called continuity of state between the gaseous and the liquid state.
It is important to know that below the critical point (that is,critical temperature and critical pressure), the surface of separation between the liquid and its vapour is clearly visible. As we approach towards the critical point, the density of the liquid decreases while that of the vapour increases due to compression. At the critical point, the densities of the liquid and that of the vapour become equal and the surface of separation disappears that is, the liquid and the gaseous state become indistinguishable. In other words, the meniscus is no longer visible. The fluid which is now a homogeneous mixture is called Supercritical Fluid. Thus any fluid having temperature higher than its critical temperature and pressure is called a Supercritical Fluid.
These supercritical fluids dissolve many organic substances. They are used for quick separation of a mixture into its components. For Example, CO2 above 31.1°C and above 600 bar pressure has a density about 1 cm3. It is used to dissolve out caffeine from coffee beans as it is better substitute for chlorofluorocarbons which are harmful for the environment.
In terms of critical temperature, by applying pressure, a gas can be liquefied below the critical temperature. Therefore, above the critical temperature, it is a gas but below the critical temperature, it is vapour. For Example, when temperature of CO2 is below its critical temperature it is called CO2 Vapour.
The measure of the strength of the intermolecular forces of attraction of that gas is called its critical temperature. If the intermolecular forces are weak, it becomes difficult to liquefy the gas and hence critical temperature of that gas would also be less. For example, helium and hydrogen have weak intermolecular forces and it is very difficult to liquefy as they have low critical temperatures. But, CO2 and NH3 can be liquefied easily as they have strong intermolecular forces of attraction and their critical temperatures are high which are above room temperature. van der Waals constant ‘a’ is also a measure of the intermolecular forces of attraction. Hence, it is found that the values of the constant ‘a’ increase in the same order as the critical temperature. A comparision of van der Waals constant ‘a’ and
the critical temperature, Tc for a few gases is given below:
a (atm L2 mol-2)
Thus, the ease of liquefaction of some of the gases in the order:
SO2 > Cl2 > NH3 > HCl > CH4 > CO > N2 > H2 > He
The critical constants are related to van der Waals constant by the expressions
(a and b are van der Waals constant, while R is gas constant)
Further, they are related to each other as:
And if we know the value of critical constants, van der Waals constants can be calculated, using the above expressions.