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Unit 2: CHC 2050 (Chemical Engineering Thermodynamics)

1. Properties of pure substances: a) Pure Substance

• A substance that has homogenous chemical composition throughout its system is

called pure substance. For example: Water, helium, carbon dioxide, nitrogen etc.

• A pure substance can exist in two or more phases. For example, ice and water (solid

and liquid) or water and steam (liquid and gas). However, in case of air, the pure

substance cannot exist because air is a mixture of multicomponent (Oxygen and

Nitrogen).

Water

(Pure substance)

b) Phases of a Pure Substance:

Air

(Not a pure substance

because the composition of

liquid air is different from the

water

• There are three principle phases – solid, liquid, and gas, but a substance can have several

other phases within the principle phase. For examples: solid carbon (diamond and

graphite) and iron (three solid phases).

• Solids: Solid contains the strongest molecular bonds of its molecules and hence they

are closely packed three dimensional crystals or structure. The molecules within solid

do not move relative to each other. Find the three examples of solid substance.

• Liquid: The spacing within liquid molecules is more as compared to solid molecules.

The molecules of liquid can float in groups. Liquid molecules contain intermediate

bond strength or lower bond strength than solid. Find the types of bond exists within

liquid molecules.

• Gas: The molecules of gas are far apart from each other and do not contains ordered

structure. The molecules move randomly as individual or in group and collide with

each other. The molecules of gas contain weak Vander Waal force of attraction are

at higher energy levels, they must release large amounts of energy to condense or

freeze.

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c) Phase – Change Processes of Pure Substances

• Consider water at room temperature (i.e., 20 °C) and 1 atmospheric pressure in a piston-

cylinder device. The water is in liquid phase, and it is called as subcooled liquid (not

about to vaporize).

Compressed liquid

• If we add heat to water, its temperature will increase; let us say until 50 °C. Due to the

increase in the temperature, the specific volume “v” will increase. Consequently, the

piston will move slightly upward and hence maintains the constant pressure (i.e., 1 atm).

Compressed liquid

• Now, if we continue to add heat to the water, the temperature will increase further

increase until 100 °C. At this point, any additional addition of the heat will vaporize the

water. This specific point where water starts to vaporize is called saturated liquid.

Liquid

Liquid

Liquid

Increase in volume

Saturated liquid

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• If we continue to add heat to the water, more and more water vapor will be formed,

while, the temperature and the pressure remain constant (i.e., T = 100 °C and P = 1 atm).

• The state between saturated liquid (only liquid) and saturated vapor (only vapor)

where two phases exist is called saturated liquid-vapor mixture.

Saturated liquid-vapor

mixture

.

• The only property that changes is the specific volume “v”. These conditions will remain

the same until the last drop of liquid is vaporized. At this point, the entire cylinder is

filled with vapor at 100°C. This state is called “saturated vapor”.

• After the saturated vapor phase, any addition of heat will increase the temperature

of the vapor, this state is called “superheated vapor”

Figure 1: T-v diagram representing phase change for water at constant pressure

(Note: This concept can be also applied to the pure substance other than water)

Vapor

Liquid

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d) Saturation Temperature and Saturation Pressure

Saturation Temperature: At a given pressure, the temperature at which a pure substance

changes phase is called the saturation temperature (Tsat).

Saturation Pressure: At a given temperature, the pressure at which a pure substance

changes phase is called the saturation pressure (Psat).

2. Types of Thermodynamic diagrams:

• T-v Diagram (Temperature-Specific volume diagram):

o Consider a cylinder-piston system filled with the water at 20 ⁰C and 1 atm. For an

increase in the pressure of water, the process changes from subcooled liquid to the

superheated vapor follows a path that looks like the process for P = 1 atm (or 0.1

MPa) in Figure 2., the only difference is that the width of the mixture region will be

shorter and shorter as we go on increasing the pressure.

Figure 2: T-v diagram

o Then, at a certain pressure, the mixture region will be represented only by one point.

This point is called the “critical point”. It is defined as “the point at which the

saturated liquid and saturated vapor states are identical”.

o At the critical point, the properties of a substance are called critical properties (i.e.,

critical temperature (Tcr), critical pressure (Pcr) and critical specific volume (vcr) etc.).

o If we connect all the points representing saturated liquid, we will obtain the

saturated liquid line.

o If we connect all the points representing saturated vapor, we will obtain the

saturated vapor line.

o The intersection of the two lines is the critical point as shown in Figure 3.

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Figure 3: T-v diagram with saturate liquid and vapor lines

• P-v Diagram (Pressure-Specific volume diagram):

o If we consider the cylinder-piston system filled with the water, but with some weights

above the piston, and if we remove the weights one by one to decrease the pressure,

and we allow a heat transfer to obtain an isothermal process, we will obtain one of

the curves of the P-v diagram.

Figure 4: P-v diagram with saturate liquid and vapor lines o The P-v diagram can be extended to include the solid phase, the solid- liquid

and the solid-vapor saturation regions.

o Some substances, like water, expands when it freezes, and the rest (the

majority of the substances) contracts during freezing process, Hence, two

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configurations for the P-v diagram with solid phase are possible as shown in

Figure 5.

Figure 5: P-v diagram for a substance that contracts during freezing (left) and for

a substance that expends during freezing (right)

• P-T diagram and Triple Point

o Under certain conditions, water can exist at the same time as ice (or solid),

liquid and vapor state. These conditions at which all three phase of water co-

exist is called “triple point”.

o The critical point is also possible for the compounds other than water.

o On a P-T diagram, these conditions are represented by a point as shown

below, (Water triples point: at T = 0.01°C (or 273.16 K) and P = 0.6113 kPa)

Figure 6: P-T diagram and the triple point

• The P-T diagram is often called the phase diagram since all three phases are

separated by three lines.

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P-T-v Diagram:

Figure 7: P-T-V diagram for a substance that contracts during freezing

(left) and for a substance that expends during freezing (right)

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3. Properties of Single-phase system

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Solved Practice Problem