CHE 312 Lecture 2

CHE 312Chemical

Engineering Thermodynamics II

Credit Hours 3 – 0

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.



• All these processes are design and improved so the efficiency reaches 100%, however its not.

e.g. efficiency of conventional fossil-fuel-steam-power plants exceeds 35%, whereas, combined-cycle 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.



• 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.


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


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.


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 (TH) , River temperature (TC)

ηmax = 1 – 295/585 = 0.4957 and η = (0.7)(0.4957)=0.3470

Heat discarded can be calculated as

|Qc| = (1-η)/η |W| = (1-0.347/0.347)(800,000)

= 1505,500 kW


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 cold reservoir

CHE 312 Chemical Engineering Thermodynamics IISteam power plant

The Carnot-engine cycle, consist of two isothermal steps connected by two adiabatic steps


TS diagram

Step 1> 2 : vaporization process (boiler), where saturated liquid absorb heat at the constant temperature TH to produced saturated vaporStep 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

Carnot cycle

Saturated liquid

Saturated vapor

C


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.

Rankine cycleSaturated liquid

Saturated vapor

C

CHE 312 Chemical Engineering Thermodynamics IITS 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).

CHE 312 Chemical Engineering Thermodynamics IITurbines (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


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)


• 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


Adiabatic expansion process in a turbine or expander

CHE 312 Lecture 2

Documents

Transcript of CHE 312 Lecture 2