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CHAPTER
18
LIQUEFACTION OF GASES

If the temperature and pressure of a gas can be brought into the region between the saturated liquid and
saturated vapour lines then the gas will become ‘wet’ and this ‘wetness’ will condense giving a liquid.
Most gases existing in the atmosphere are extremely superheated, but are at pressures well below their
critical pressures. Critical point data for common gases and some hydrocarbons are given in
Table 18.1.
Figure 18.1 depicts qualitatively the state point of oxygen at ambient conditions and shows that it is
a superheated gas at this pressure and temperature, existing at well above the critical temperature, but
below the critical pressure. If it is desired to liquefy the gas, it is necessary to take its state point into the
saturated liquid – saturated vapour region. This can be achieved in a number of ways. First, experience
indicates that ‘heat’ has to be taken out of the gas. This can be done by two means:
1. cooling the gas by heat transfer to a cold reservoir, i.e. refrigeration;
2. expanding the gas in a reversible manner, so that it does work.

18.1 LIQUEFACTION BY COOLING – METHOD (I)
This method is satisfactory if the liquefaction process does not require very low temperatures.
A number of common gases can be obtained in liquid form by cooling. Examples of these are the
hydrocarbons butane and propane, which can both exist as liquids at room temperature if they are
contained at elevated pressures. Mixtures of hydrocarbons can also be obtained as liquids and these
include liqueﬁed petroleum gas (LPG) and liqueﬁed natural gas (LNG).
A simple refrigerator, and the refrigeration cycle, are shown in Fig. 18.2(a) and (b) respectively.
Consider the throttling process in Fig. 18.2, which is between 4 and 1. The working ﬂuid enters the
throttle at a high pressure in a liquid state, and leaves it at a lower pressure and temperature as a wet
vapour. If the mass of working ﬂuid entering the throttle is 1 kg then the mass of liquid leaving the
throttle is (1 x 1 ) kg. If this liquid were then withdrawn to a vessel, and the mass of ﬂuid in the system
were made up by adding (1 x 1 ) kg of gas at state 3 then it would be possible to liquefy that gas. The
liquefaction has effectively taken place because, in passing through the throttle the quality (dryness
fraction) of the ﬂuid has been increased, and the energy to form the vapour phase has been obtained
from the latent heat of the liquid thus formed: the throttling process is an isenthalpic one, and hence
energy has been conserved. If the working ﬂuid for this cycle was a gas which it was desired to liquefy
then the liquid could be withdrawn at state 1’ on Fig 18.2.
The simple refrigeration system shown in Fig. 18.2 might be used to liquefy substances which boil
at close to the ambient temperature, but it is more common to use refrigeration plant in series, referred
to as a cascade, to achieve reasonable levels of cooling. The cascade plant is also more cost-effective
because it splits the temperature drop between two working ﬂuids. Consider Fig. 18.2, the difference

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