CHE 312 Lecture 2

CHE 312
Chemical Engineering
Thermodynamics II
Credit Hours
30
By
Dr. Qazi Nasir
Lecture # 1
CHE 312 Chemical Engineering

Thermodynamics II
Production of Power from heat
Except nuclear energy, the sun is the source of
all mechanical energy
e.g. Solar photovoltaic cell, Solar radiation
Kinetic energy of atmospheric wind are used to
produce power by large windmills
Potential energy of tides is another possible
source of power. e.g. hydroelectric power is
generated by conversion of potential energy of
water into work.

Thermodynamics II
All these processes are design and improved so the

efficiency reaches 100%, however its not.
e.g. efficiency of conventional fossil-fuel-steampower plants exceeds 35%, whereas, combinedcycle plants with dual power generation have
efficiencies greater than 50%
Direct conversion of chemical energy into electrical
energy without the intermediate generation of heat,
is the electrochemical cell. e.g. a battery, fuel cell.
The resulting efficiency can be as great as 85%.
In conventional power plant the molecular energy of
fuel is released by combustion into mechanical
energy.

Thermodynamics II
Nuclear power plant the fission processes
releases energy of the nucleus of the atom as
heat, which is partially converted into work.
The thermodynamic analysis of heat engines,
applies equally well to conventional (fossil-fuel)
and nuclear power plants
Steam power plants are large scale heat
engine in which working
fluid (H20) is in
steady-state flow successively through pump, a
boiler, a turbine and a condenser in cyclic
process.

Thermodynamics II
Heat Engines
Devices or machines that produce work from heat in

a cyclic process. e.g. steam power plant which the
working fluid (steam) periodically returns to its
original state.
Liquid water in pumped into boiler at high pressure
Heat from a fuel in transferred into boiler to the
water, converting it to a high-temperature steam
Energy is transferred as from steam to turbine
Exhaust steam from the turbine is condensed by
transfer of heat to surroundings, producing liquid
water, thus completing the cycle

Thermodynamics II
Heat Engines
All heat engines cycles are absorption of heat into
the system at high temperature, rejections of
heat to the surroundings at lower temperature
and production of work.

Thermodynamics II
Example : A central power plant rated at 800,000

kW, generates steam at 585 K and discards heat to a
river at 295 K, If the thermal efficiency of the plant is
70% of the maximum possible value, how much heat
is discarded to the river at rated power ?
Answer:
Steam
generation
temperature (TC)
temperature
max = 1 295/585 = 0.4957

(0.4957)=0.3470
(TH)
and
= (0.7)
Heat discarded can be calculated as

|Qc| = (1-)/ |W| = (1-0.347/0.347)(800,000)
= 1505,500 kW
River

Thermodynamics II
Heat Engines
Heat engine operating in a completely reversible manner is

called Carnot engine (ideal engine) proposed by N.L.S. Carnot
in 1824.
Step 1: The system at cold reservoir TC undergoes a reversible
adiabatic process that cause the temperature to rise to that of
hot reservoir at TH .
Step 2: The system in contact with the hot reservoir at TH,
undergoes reversible isothermal process during which heat is
absorbed from the hot reservoir
Step 3: The system undergoes a reversible adiabatic process in
the opposite direction of step 1 that brings the temperature back
to cold reservoir at TC
Step 4: The system in contact with reservoir TC, undergoes
reversible isothermal process in the opposite direction of step 2
and returns to its initial state with rejection of heat
to the

Thermodynamics II
Steam power plant
The Carnot-engine cycle, consist of two

isothermal steps connected by two adiabatic
steps

Thermodynamics II
Steam power plant
Saturated
liquid
C
Saturated
vapor
Carnot cycle
TS diagram
Step 1> 2 : vaporization process (boiler), where saturated
liquid absorb heat at the constant temperature TH to produced
saturated vapor
Step 2> 3: Adiabatic expansion of saturated vapor into two
phase region to produce mixture of saturated liquid and vapor
at TC.
Step 3>4: Partial condensation process wherein heat is
rejected (TC)
Step 4>1: Takes the cycle back to its origin, producing
saturated-liquid water at point 1

Thermodynamics II
Steam power plant
C
Saturated
liquid
Saturated
vapor
Rankine cycle
TS diagram
Step 1>2 : A constant-pressure heating process in
boiler. Consist of three sections: heating of
subcooled
liquid
water
to
its
saturation
temperature, vaporization at constant temperature
and pressure, superheating of the vapor to a
temperature well above its saturation temperature.

Thermodynamics II
TS diagram
Step 2>3 : Reversible, adiabatic expansion of
vapor in a turbine to the pressure of condenser.
The step normally crosses the saturation curve,
producing a wet exhaust. However, superheating
in step 1>2 shifts the vertical line far enough to
right so the moisture content is not too large.
Step 3>4: A constant-pressure, constant
temperature pressure process in a condenser to
produce saturated liquid at point 4.
Step 4>1: Reversible, adiabatic pumping of the
saturated liquid to the pressure of the boiler,
producing compressed (subcooled liquid).

Thermodynamics II
Turbines (Expanders)
The expansion of a gas in a nozzle to produce a
high-velocity stream and in a process convert
internal energy into kinetic energy.
The kinetic energy is in turn converted into shaft
work when the stream impinges on blades
attached to a rotating shaft
The stream which provides the motive force as in
a power plant, the device is called a turbine
When high pressure gas, e.g. ammonia or
ethylene in a chemical or petrochemical plant is
used as working fluid, the device is call expander

Thermodynamics II

Thermodynamics II
The General Energy Balance
Energy is like mass, is conserved, the rate of
change of energy within the control volume
equals the net rate of energy transfer into the
volume.
Streams flowing into and out of the control
volume have associated with energy in its
internal, potential, and kinetic forms, which all
contributed to energy change of the system
(1)

Thermodynamics II
Since there is a little change in elevation, in
any properly designed turbine, and heat
transfer is negligible and inlet and exit pipes
are sized to make fluid velocities roughly
equal.
Normally, T1 and P1 and discharge pressure P2
are known.
H1 are known and H2 and Ws remains as
unknown

Thermodynamics II
Adiabatic expansion process in a turbine or

expander

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CHE 312

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

Production of Power from heat

All these processes are design and improved so the

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

Devices or machines that produce work from heat in

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

Example : A central power plant rated at 800,000

max = 1 295/585 = 0.4957

Heat discarded can be calculated as

CHE 312 Chemical Engineering

Heat engine operating in a completely reversible manner is

CHE 312 Chemical Engineering

The Carnot-engine cycle, consist of two

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

CHE 312 Chemical Engineering

Adiabatic expansion process in a turbine or

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