PVT Behaviour prepared by Muhammad Awais Sharif..

1. PVT Behavior
Prepared by:
Awais Sharif
Comsats Lahore

2. P-V-T Behavior of Pure Substances
PT Diagram
• A typical P-T diagram
showing the
relationship between
pressure and
temperature of a pure
substance is shown
below:

3. PT Diagram
• The three lines 1-2, 2-3
and 2-C display
conditions of P and T at
which two phases may
co-exist in equilibrium,
and are boundaries for
the single-phase regions
of solid, liquid and
vapor (gas).

4. Graph Explanation
• Line 1-2 is known as the sublimation curve, and it
separates the solid from the gas regions.
• Line 2-3 is known as the fusion curve, and it
separates the solid and liquid regions.
• Line 2-C is known as the vaporization curve, and
it separates the liquid and the gas regions. All
three lines meet at Point 2, known as the Triple
Point. This is a point where all 3 phases can co-
exist in equilibrium.

5. Critical Pressure and Critical
Temperature
• The pressure and temperature corresponding to
this point(Critical Point) are known as the critical
pressure PC and critical temperature TC
respectively. These are the highest pressure and
temperature at which a pure substance can exist
in vapor-liquid equilibrium.
• The shaded area shows the area existing at
pressure and temperature greater than P and T.
This region is called the fluid region.

6. Explanation
• The gas region is sometimes divided into two
parts, as indicated by the dotted vertical line
through temperature TC.
• A vapor region is the region to the left of this line
and represent a gas that can be condensed either
by compression at constant temperature or by
cooling at constant pressure.
• The region everywhere to the right of this line,
including the fluid region, is termed supercritical.

7. P-V Diagram for Pure Substance
• The P-T Diagram does not provide any
information about volume.
• It merely displays the phase boundaries on as
a function of pressure and temperature.
• On the P-V Diagram, the triple point appears
as a horizontal line, where all 3 phases co-exist
at a single temperature and pressure.

8. P-V Diagram

9. P-V Diagram
• Isotherms are lines of
constant temperature
and these are
superimposed on the P-
V Diagram as shown in
the Figure.

10. Explanation
• Point C is the critical point. VC is the critical
volume at this point.
• The isotherm labeled T > TC does not cross a
phase boundary.
• The lines labeled T1 and T2 are isotherms for
subcritical temperatures, and they consist of 3
segments.

11. Continue..
• The horizontal segment of each isotherm
represents all possible mixtures of liquid and
vapour in equilibrium, ranging from 100%
liquid at the left end (curve B-C) to 100%
vapour at the right end (curve D-C).
• Curve B-C represents saturated liquid at their
boiling points, and curve D-C represent
saturated vapours at their condensation
points.

12. PV Diagram (Continue)
• Subcooled liquid and superheated vapour
regions lie to the left and right, respectively.
• Subcooled liquid exists at temperatures below
the boiling point for the given pressure.
• Superheated vapour exists at temperatures
above the boiling point for the given pressure

13. PV Diagram (Continue)
• Isotherms in the subcooled liquid region are
very steep, because liquid volumes change
little with large changes in pressure.
• The horizontal segments of the isotherms in
the 2-phase region become progressively
shorter at higher temperatures, being
ultimately reduced to a point at C, the critical
point.