Download - Lecture#2 Ch3
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he fascinating world
of
Thermodynamics
Instructor
Dr. Madhusree Kundu
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Thermodynamics
Lecture # 3
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Pure Substance
The pure substance is one that has ahomogeneous and invariable chemicalcomposition.
A pure substance may exist in manyphases, but the chemical composition is
same in all the phases.
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In our analysis, we will deal only withsimple compressible substances andsubstances whose surface effects,
magnetic effects, and electric effects arenegligible.
Example: liquid water, water vapor, ice, a
mixture of liquid water and water vaporare all pure substances.
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Depending on the nature of substance,
applied pressure and temperature, thechange in molar volume of the
substances may be large, the
substances are compressible and wherethe change in molar volume is less
then those are incompressible.
Liquids and solids are generally
incompressible, gases are compressible
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Properties of a puresubstance
What we need to study about a pure substance?
The phases in which a pure substance
may exist.
The number of independentproperties of pure substance.
Methods of presenting thermodynamic
properties.
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Properties of a puresubstance
Example:We need to know properties ofwater to determine pipe diameter throughwhich it has to flow.
Example:We need to know properties ofsteam in order to design a boiler.
Understanding the properties andbehavior of substances is essential fordesigning and sizing various equipment.
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Phase Rule The number of intensive parameters
that can be varied independently toestablish the intensive state of a system,
is called the degree of freedom, F of
the system and is given by the
celebrated phase rule of J. Willard
Gibbs (1875).
F=N- +2, where N is the number of
components, is the number of
coexisting phases.
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The intensive state of a system containing
N chemical species, and phases in
equilibrium is characterized by the
intensive variables pressure(P),
temperature T and (N-1) mole fractionsfor each phase. These are called phase
rule variables
The number of phase rule variables are
2+(N-1) .
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The masses of the phases are not phase
rule variables. The masses of the phases
have nothing to do with the intensive state
of the system.
Example:Liquid water is in equilibrium
with its vapour, Find the degree of
freedom of the system?
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This result is in agreement with the well
known fact that at a given pressure water ishaving only one boiling point. Either
temperature or pressure can be varied
independently when water is in equilibriumwith its vapour
Applying phase rule F=N- +2. Here
N=1, =2, Hence F=1.
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Example:Liquid water is in equilibrium
with a mixture of water vapour and
nitrogen. Determine F
Applying phase rule F=N- +2. Here
N=2, =2, Hence F=2.
T and P may be independently varied, but
once they are fixed, The system described
can exist in equilibrium only at a
particular composition of of the vapour
phase.
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Vapor-Liquid-Solid Phase
Equilibrium in a PureSubstance
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Constant pressure change from liquid to
vapor phase for pure substance
InitialConditions:T= 20OCP= 100 kP
Tincreasesconsiderably
increasesslightlyPremains const.(100 kP)
Tremains const.(99.6 OC)
increases verymuchPremains const.(100 kP)
Tincreasesincreases
Premainsconst.
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Phase Equilibrium in a PureSubstance
Saturation temperature means thetemperature at which vaporization takes
place at a given pressure. This pressure
is called the saturation pressurefor thegiven temperature.
Example: For water at 99.6OC the
saturation pressure is 100 kP, and forwater at 100 kP the saturation
temperature is 99.6OC.
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Vapor-pressure curve of a puresubstance
A point on this
diagram represents
a state of thesubstance. Any point
lying on the curve is
a saturated state.
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0
200
400
600
5 50 100 150 200
Tsat,oC
Psat,kPa
475.8kPa
101.35kPa
Temp,
Tsat,oC
Saturation
pressure,Psat, kPa
0.01 0.6113
5 0.4
10 0.87
30 4.25
50 12.35
100 101.3
150 475.8
200 1554
250 2973
Generating vapor-pressure curve of water
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Phase Equilibrium in a PureSubstance
At saturation temperature and pressure, If a
substance exists as vapor, it is calledsaturated vapor.
At saturation temperature and pressure,
If a substance exists as liquid, it is called
saturated liquid.
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If for given pressure, the temperature of
liquid is lower than saturation temperature,
it is called a subcooled liquid(T < TS) or a
compressed liquid(P > PS).
If for given pressure, the temperature ofvapor is greater than saturation
temperature, it is called a superheated
vapor.The substances called gases are actually
highly superheated vapors.
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Quality has meaning only forsubstance at saturated state.
A substance exists as part liquid and
part vapor at saturation temperature.
Quality, x= mass of vapor/ total mass
Quality is an intensive property.
Example:For saturated vapor,
quality, x = 100%
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Quality, x
Only defined when the system containstwo-phase mixture of a single substance.
x= m
mg
Liquid, mf
Vapor, mg
gf mmm
Saturated
Saturated
P, Tsat
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A piston cylinder contains 5 kg of water
liquid-vapor mixture, in which 3 kg is inliquid phase. Find the quality. (How muchpercent of the mixture is in vapor phase?)
x=
Example
m
mg
Liquid,
mf=3kg
Vapor, mg
gf mmm kg3 fm5kg,m
2kg3kg5kg
mmm fg
40%or4.05
2
kg
kg
m=5kg
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A piston cylinder contains 5kg of water-watervapor mixture, in which 80% is in vapor phase.How much liquid is in the mixture?
Example
mmx g /
Liquid,
mf=1kg
Vapor, mg
gf mmm
5kg,m
xm-m
mmm gf
m=5kg
0.2x-1
1kg0.8)-(15kg
x)-m(1
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Generating the T-vdiagram
P=1 atm
T=20oC
State 1
heat
W1
W2W3
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Generating the T-vdiagram
P=1MPa
T=20oC
State 1
heat
W1
W2W3
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Temperature-Volume Diagram for
water (showing liquid & vapor phases)
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T-vdiagram of a pure substance
Tc
vc
Subcritical
isobars
Supercritical
isobar
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Liquid+vapor region
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Compressed liquid region
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Superheated vapor region
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Supercritical fluid region
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Supercritical fluid region
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Important PointsThe boiling temperature of a pure substance
increases with pressure. The specific volumes vfandvgof the saturated liquid and vapor also change withpressure.The normal boiling temperatureof a pure substanceis the temperature at which it boils under a pressure
of exactly 1 atm.A series of isobars can plotted on a T-v diagram. Ifthe locus of all saturated liquid and vapor states areconnected, a dome enclosing the region of two phases(liquid+vapor) results.
Each pure substance has one isobar that becomeshorizontal (flat) at only one point: critical point. Thispoint coincides with the top of the two-phase dome.
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Important Points
The T-v diagram contains three single phase regions(liquid, vapor, supercritical fluid), a two-phase(liquid+vapor) region, and two important curves - thesaturated liquid and saturated vapor curves. The numberof regions and curves will increase when we considersolids.The saturated liquid and vapor curves meet at the criticalpoint, which represents the highest temperature andpressure in which vapor and liquid phases can coexist.The single phase liquid region is also called subcooledliquid because, at a given pressure, temperature will bebelow the boiling temperature.The single phase vapor region is also called superheatedvapor because, at a given pressure, temperature willalways be above the boiling temperature.