Introduction to Chemical Engineering
Thermodynamics
Chapter 8
1KFUPM Housam Binous CHE 303
2KFUPM Housam Binous CHE 303
Solar radiations: evaporation of water to produce salt
Kinetic energy: windmills
Potential energy: Hydroelectric power
Nuclear energy and chemical energy of fuels
Conventional fossil-fuel steam-power plants: because of
2nd law efficiency rarely exceeds 35%
3KFUPM Housam Binous CHE 303
Efficiency greater than 50%:
Advanced-technology gas turbines
Steam-power cycles operating on heat recovered from
hot turbine exhaust gases
Fuel cells and electrochemical cells
4KFUPM Housam Binous CHE 303
Conventional power plants:
Convert part of the heat of combustion into mechanical energy
Apply equally well to fossil-fuel and nuclear power plants
Steam power plant (working fluid is H20)
Internal combustion engine: Otto and Diesel engines, gas turbine
5KFUPM Housam Binous CHE 303
Steam Power Plant
6KFUPM Housam Binous CHE 303
Carnot cycle:
Two isothermal steps connected by two adiabatic steps
Isothermal step at TH, heat |QH| is absorbed by fluid
Isothermal step at TC, heat |QC| is rejected by fluid
Work produced:
Thermal efficiency:
7KFUPM Housam Binous CHE 303
8KFUPM Housam Binous CHE 303
Steam generated in a boiler (heat is absorbed) is expanded
adiabatically in a turbine to produce work
The discharge from the turbine goes through a condenser (heat
is rejected) then it is pumped adiabatically back to the boiler
Net power output= difference between rate of heat input in the
boiler and rate of heat rejection in the condenser
9KFUPM Housam Binous CHE 303
Step 1→2
Vaporization in the boiler (saturated liquid absorb heat at TH)
Step 2→3
Adiabatic expansion (produce mixture of saturated liquid andvapor at TC)
Step 3→4
Partial Condensation (heat is rejected at TC)
Step 4→1
Takes cycle back to point 1 with a pump
10KFUPM Housam Binous CHE 303
Rankine Cycle
Carnot cycle: One encounters several difficulties: Turbine
exhaust has high liquid content causing erosion problems and
the pump must take a mixture of liquid and vapor (point 4 )
11KFUPM Housam Binous CHE 303
12KFUPM Housam Binous CHE 303
Step 1→2Constant-pressure heating process in a boiler. Three sections:
heating of subcooled liquid water, vaporization at constant T
and P and superheating to T well above saturation
temperature
Step 2→3Isentropic expansion. Superheating shifts vertical line so that
moisture content is not too large
13KFUPM Housam Binous CHE 303
Step 3→4Constant P and T process to produce saturated liquid
Step 4→1Reversible adiabatic pumping of saturated liquid to the
pressure of the boiler producing compressed subcooled
liquid. Temperature rise associated with compression is very
small.
14KFUPM Housam Binous CHE 303
15KFUPM Housam Binous CHE 303
Departure from Rankine Cycle solely due to
irreversibilities of work-producing (2-3) and work-
requiring (4-1) steps (lines tend in the direction of
increasing entropy).
16KFUPM Housam Binous CHE 303
The Regenerative Cycle
17KFUPM Housam Binous CHE 303
18KFUPM Housam Binous CHE 303
19KFUPM Housam Binous CHE 303
Higher boiler pressures and temperatures favor high
efficiencies. Seldom use more than 10000 kPa and 600°C
Power plants operate with condenser pressure and
temperature as low as practical
Modern power plant: incorporate feedwater heaters
Water from the condenser rather than being pumped directly
back to condenser is first heated by steam extracted from
turbine
20KFUPM Housam Binous CHE 303
By raising the average temperature at which
heat is added in the boiler, one increases the
thermal efficiency of the plant (the
regenerative cycle)
21KFUPM Housam Binous CHE 303
Internal Combustion Engine
Boiler: heat is transferred through walls, which limits T of heat
absorption
Internal-combustion engine: fuel is burned within the engine itself
and combustion products serve as the working medium (they act
on a piston in a cylinder).
No working medium undergoes a cyclic process. To simplify: one
can imagine that air (ideal gas with constant heat capacities) as the
working fluid. Combustion is equivalent to addition of heat to air.
22KFUPM Housam Binous CHE 303
The Otto Engine
23KFUPM Housam Binous CHE 303
Step 0→1
Piston moving outward draws a fuel/air mixture into the cylinder
Step 1→2
All valves are closed and the fuel/air mixture is compressed adiabatically
Step 2→3
The mixture is ignited and combustion occurs so rapidly that it is at constant volume
Step 3→4
Work is produced. Adiabatic expansion of the high T and P products of the combustion.
Step 4→1
The exhaust valves open and P falls rapidly at nearly constant volume
Step 1→0
Piston pushes the remaining of combustion gases from the cylinder.
24KFUPM Housam Binous CHE 303
The Otto Cycle
25KFUPM Housam Binous CHE 303
Step C→D
Reversible adiabatic compression
Step D→A
Heat is absorbed by the air at constant volume
Step A→B
Reversible adiabatic expansion
Step B→C
Cooling at constant volume
Thermal efficiency increases rapidly with compression ratio, r:
26KFUPM Housam Binous CHE 303
The Diesel Cycle
27KFUPM Housam Binous CHE 303
T at the end of the compression step is high enough that
combustion is initiated spontaneously.
Higher T and P, leads to higher compression ratios and
efficiencies
The fuel is injected slowly at the end of the compression step
and hence the combustion process occurs at approximately
constant P
28KFUPM Housam Binous CHE 303
Compression ratioExpansion ratio
29KFUPM Housam Binous CHE 303
The Gas-Turbine Engine
30KFUPM Housam Binous CHE 303
Turbines are more efficient than reciprocating engines
Two advantages: internal-combustion and turbines
Centrifugal compressor operates on the same shaft as the turbine
Higher T of the combustion gases entering the turbine leads to
higher efficiencies
Must use excess of air (to protect turbine metal blades)
31KFUPM Housam Binous CHE 303
32KFUPM Housam Binous CHE 303
Brayton cycle:
Working fluid is air taken as an ideal gas with constant heat
capacities
4 Steps:
AB: reversible adiabatic compression
BC: addition of heat (the combustion)
CD: an isentropic expansion produces work
DA: a constant pressure cooling completes the cycle
33KFUPM Housam Binous CHE 303
34KFUPM Housam Binous CHE 303
The Turbojet Engine
35KFUPM Housam Binous CHE 303
The expansion provide just enough work to drive the
compressor and the rest of the expansion to the exhaust
pressure is accomplished in the nozzle.
The velocity of the gases with respect to the engine is
increased to a level above the entering air. This provides a
thrust on the engine in the forward direction.
36KFUPM Housam Binous CHE 303
The Rocket Engine
37KFUPM Housam Binous CHE 303
The oxidizing agent is carried with the engine. Can operate
in the outer space (vacuum).
No friction forces to overcome.
Liquid oxygen and liquid hydrogen or kerosene so no need
for large compression energy as they are pumped as
liquids into the combustion chamber
Top Related