Liquefaction of Gases
Liquefaction of Gases
First complete
data on pressure – volume – temperature relations of a substance in both
gaseous and liquid state was obtained by Thomas Andrews on carbon dioxide. He plotted
isotherms of carbon dioxide at various temperatures. Later on it was found that
real gases behave in the same manner as carbon dioxide. Andrews noticed that at
high temperature isotherms look like that of an ideal gas and the gas cannot be
liquefied even at very high pressure. As the temperature is lowered, shape of
the curve changes and data shows considerable deviation from ideal behavior. At
30.98 C carbon dioxide remains gas up to 73 atmospheric pressure. At 73
atmospheric pressure, liquid carbon dioxide appears for the first time. The temperature
30.98 C is called Critical temperature (Tc) of
carbon dioxide. This is the highest temperature at which liquid carbon dioxide
is observed. Above this temperature it is gas. Volume of one mole of the gas at
critical temperature is called Critical Volume (Vc)
and pressure at this temperature is called critical pressure (Pc).
The critical temperature, pressure and volume are called critical constants. Further
increase in pressure simply compresses the liquid carbon dioxide and the curve
represents the compressibility of the liquid. The steep lime represents the
isotherm of liquid. Even a slight compression results in steep rise in pressure
indicating very low compressibility of the liquid. Below 30.98 C, the behavior of
the gas on compression is quite different. At 21.5 C, carbon dioxide remains as
a gas only up to point B. At point B, liquid of a particular volume appears. Further
compression does not change the pressure. Liquid and gaseous carbon dioxide
coexist and further application of more gas until the point C is reached. At point
C, all the gas has been condensed and further application of pressure merely
compresses the liquid as shown by steep line. A slight compression from volume
V2 to V3 results in steep rise in pressure from p2
to p3. Below 30.98 C (critical temperature) each curve shows
that similar trend. Only length of the horizontal line increases at lower
temperatures. At critical point horizontal portion of the isotherm merges into
one point. Thus we see that a point like A represents gaseous state. A point
like D represents liquid state and a point under the dome shaped area
represents existence of liquid and gaseous carbon dioxide in equilibrium. All
the gases upon compression at constant temperature (isothermal compression)
show the same behavior as shown by carbon dioxide. Also above discussion shows
that gases should be cooled below their critical temperature for liquification.
Critical temperature of a gas is highest temperature at which liquifaction of
the gas first occurs. Liquefaction of so called permanent gases (i.e., gases which show continuous
positive deviation in Z value) requires cooling as well as considerable
compression. Compression brings the molecules in close vicinity and cooling
slows down the movement of molecules therefore, intermolecular interactions may
hold the closely and slowly moving molecules together and the gas liquefies.
It is
possible to change a gas into liquid or a liquid into gas by a process in which
always a single phase is present. For example in above figure, we can move from
point A to F vertically by increasing the temperature, then we can reach t he
point G by compressing the gas at the constant temperature along this isotherm
(isotherm at 31.1 C). The pressure will increase. Now we can move vertically
down towards D by lowering the temperature. As soon as we cress the point H on
the critical isotherm we get liquid. We end up with liquid but in this series
of changes we do not pass through two-phase region. If process is carried out
at the critical temperature, substance always remains in one phase.
Thus there
is continuity between the gaseous and liquid state. The term fluid is used for
either a liquid or a gas to recognize this continuity. Thus a liquid can be
viewed as a very dense gas. Liquid and gas can be distinguished only when the
fluid is below its critical temperature and its pressure and volume lie under
the dome, since in that situation liquid and gas are in equilibrium and a
surface separating the two phases is visible. In the absence of this surface
there is no fundamental way of distinguishing between two states. At critical
temperature, liquid passes into gaseous state imperceptibly and continuously;
the surface separating two phases disappears. A gas below the critical temperature
can be liquefied by applying pressure, and is called vapour of the substance. Carbon
dioxide gas below its critical temperature is called carbon dioxide vapour.
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