Chapter 8 KFUPM Housam Binous CHE 303faculty.kfupm.edu.sa/che/binoushousam/files/CHE 303 Chapter...

Introduction to Chemical Engineering

Thermodynamics

Chapter 8

1KFUPM 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%


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


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


Steam Power Plant


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:


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


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


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 )


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


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.


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).


The Regenerative Cycle


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


By raising the average temperature at which

heat is added in the boiler, one increases the

thermal efficiency of the plant (the

regenerative cycle)


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.


The Otto Engine


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.


The Otto Cycle


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:


The Diesel Cycle


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


Compression ratioExpansion ratio


The Gas-Turbine Engine


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)


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


The Turbojet Engine


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.


The Rocket Engine


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

Chapter 8 KFUPM Housam Binous CHE 303faculty.kfupm.edu.sa/che/binoushousam/files/CHE 303 Chapter...

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