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CH-107

Lecture

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CHEMICAL
ENGINEERING
THERMODYNAMICS-I
Dr
...
M Smith, 7th Edition
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• “Thermodynamics, an Engineering
Approch” by Yunus A Cengel
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CHEMICAL ENGINEERING THERMODYNAMICS-I

2

Faculty of Chemical Engineering, NEDUET

Lecture Outline
Pure Substance
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Phase Rule
PVT Phase Diagram
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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phases

PURE SUBSTANCE
 A substance that has a fixed chemical composition throughout
is called a pure substance
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 A mixture of various chemical elements or compounds also
qualifies as a pure substance as long as the mixture is
homogeneous
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 A mixture of two or more phases of a pure substance is still a
pure substance as long as the chemical composition of all
phases is the same
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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phases

DIFFERENT STATES IN PHASES

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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phase

PHASE CHANGE PROCESSES OF PURE
SUBSTANCE
 Compressed liquid: Liquid, compressed
liquid or a subcooled liquid are all same
not about to vaporize
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 Saturated vapour: Vapour that is about to
condense is called a saturated vapour
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 Midway between saturated liquid and
saturated vapour
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e
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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phase

SATURATION TEMPERATURE AND PRESSURE
 At a given pressure, the temperature at which a pure
substance changes phase is called the saturation
temperatureTsat
 Likewise, at a given temperature, the pressure at which a
pure substance changes phase is called the saturation
pressure Psat
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325 kPa,Tsat is 99
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Conversely,
at a temperature of 99
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325 kPa
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0 C,Psat is101
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)

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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phase

Phase Diagram of Water
The variations of properties during phasechange processes are best studied and understood
with the help of property diagrams such as the T-v,
P-v, and P-T diagrams for pure substances
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 The sublimation line
separates the solid
and vapour regions,
the vaporization line
separates the liquid
and vapour regions,
and the melting (or
fusion) line
separatesThe solid
and liquid regions
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 For water, the triple-point temperature and pressure are 0
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6117 kPa , respectively
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 The temperature, pressure, and specific volume of a substance at
the critical point are called, respectively, the critical temperature
Tcr, critical pressure Pcr, and critical specific volumeVcr
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06MPa,
Tcr=373
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003106m3/kg
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Water boils at 100C at 1
atm pressure

Find F
Water in solid phase?
Liquid water in equilibrium
with its vapours?
3 phase are in equilibrium?

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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phase

T-V Diagram
T-v diagram for
the heating
process of water
at constant
pressure
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CHEMICAL ENGINEERING THERMODYNAMICS-I

16

Faculty of Chemical Engineering, NEDUET

Phase

T-V Diagram
 First, water starts boiling at a much higher temperature (179
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(P α B
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 Second, the specific volume of the saturated liquid is larger and the
specific volume of the saturated vapor is smaller than the
corresponding values at 1 atm pressure
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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

Phase

P-V Diagram
In this case temperature is being
held constant, our substance is
undergoing an isothermal
compression process
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 If we keep compressing
isothermally, we will end up at
point F, where the gas will be
saturated and the first droplet of
liquid will appear
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We keep compressing by decreasing
volume path F-G the pressure of the
system remains constant, this
condition continues until all the
vapour has become liquid
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Once we have only liquid, if we keep
compressing we will observe a rapid
increase in pressure but small
reduction in volume because liquid is
mainly incompressible
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Table A–5: Saturation properties of water under pressure
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Enthalpy of vaporization, hfg (Latent
heat of vaporization): The amount of
energy needed to vaporize a unit mass
of saturated liquid at a given
temperature or pressure
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CHEMICAL ENGINEERING THERMODYNAMICS-I

22

Faculty of Chemical Engineering, NEDUET

Phase

Saturated Liquid–Vapor Mixture

Quality, x : The ratio of the mass of vapor to the total mass of the mixture
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liquid, 1: sat
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The properties of the saturated liquid are the same whether it exists alone or in
a mixture with saturated vapor
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The relative
amounts of
liquid and
vapor phases
in a saturated
A two-phase system can be
mixture are
treated as a homogeneous
specified by
mixture for convenience
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CHEMICAL ENGINEERING THERMODYNAMICS-I

23

Faculty of Chemical Engineering, NEDUET

Phase

Saturated Liquid–Vapor Mixture
Consider a tank that contains a saturated liquid-vapor
mixture
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CHEMICAL ENGINEERING THERMODYNAMICS-I

Faculty of Chemical Engineering, NEDUET

y

Phase

v, u, or h

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