Transcript Chapter 1: Introduction
CLB 20703 Chemical Engineering Thermodynamics
Chapter 1: Basic Concepts in Thermodynamics
Objective of Chapter 1
Introduce the students towards some of the fundamental concepts and definitions that are used in the study of Engineering Thermodynamics.
Introduction
Stems from the Greek words Therme (Heat) and Dynamis (Force or Power)
In early 19 th century
: consideration of the motive power of Heat and the capacity of hot bodies to produce Work
Today
: deals generally with Energy and with relationships among the properties of matter
What is Thermodynamics?
Thermodynamics is the with Heat and Work Science that deals and those properties of substances that bear a relation to Heat and Work.
Thermodynamics is the effects of Work , Heat study and of the Energy on a System. Thermodynamics is only concerned with large scale observation.
System in Thermodynamics
Open System
: Exchange of Energy AND Matter/Mass.
Closed System
: Exchange of Energy but
NOT
Matter/Mass.
Isolated System
:
NOT AT ALL
exchange either Energy or Matter/Mass.
Property in Thermodynamics
Thermodynamic State or Condition defined by a few measurable is
Macroscopic Properties .
All properties of a System has fixed values. If one property value change, the state will change to a different one.
Thermodynamic Properties can be classified into : a.) Extensive Properties.
b.) Intensive Properties.
Property in Thermodynamics
a.)
Extensive Properties
: Those properties that depends on the amount of material present.
E.g : Mass and Volume.
b.)
Intensive Properties
: Those properties that independent on the amount of material present.
E.g. : Temperature, Pressure, Density and Specific Volume.
State And Equilibrium
Equilibrium indicate the State Of Balance .
A System that is in equilibrium experiences NO changes when it is isolated from its surroundings.
A System is NOT in Thermodynamic Equilibrium unless the conditions of all the relevant types of equilibrium are satisfied.
Types of Equilibrium a.) Thermal c.) Phase b.) Mechanical d.) Chemical
State And Equilibrium
a.)
Thermal Equilibrium
.
The Temperature is the SAME throughout entire system, i.e. no temperature difference.
b.)
Mechanical Equilibrium
.
If there is
NO
change in Pressure at any point of the System with time.
State And Equilibrium
c.)
Phase Equilibrium
.
Mass of each phase reaches an equilibrium level and stays there.
d.)
Chemical Equilibrium
.
Chemical composition of substance
DOES NOT
change with time, i.e. no chemical reactions occur.
State And Equilibrium
Quasi-static/Quasi Equilibrium Process .
“Process that proceeds in the manner that the System remains infinitesimally/approx. close to Equilibrium State at
ALL TIMES
.” Is a slow and Ideal process that allow the System to adjust itself internally in order that properties in one part of the system do not change any faster than those at other parts.
Act as a standard.
State Postulate
The state properties .
of a System is described by its Once a sufficient number of properties are specified, the rest of the properties assume certain values automatically, i.e. specifying a certain number of properties is sufficient to fix a state .
The number of properties required to fix the state of a System is given by the
State Postulate.
State Postulate
State Postulate : “The state of a simple compressible system is completely specified by
TWO
independent, Intensive Properties .” Simple Compressible System is when a System is
not
under the influence of external force fields such as electrical, magnetic, gravitational, motion and surface tension effects
State Postulate
These external force fields are negligible for most engineering problems.
The State Postulate requires
TWO
Independent Properties specified to fix the state.
Two properties are independent if one property can be varied while the other one is held constant. Good examples are Temperature and Specific/Molar Volume
Process, Path And Cycle
Process : “ Any change that a System undergoes from one equilibrium state to another equilibrium state.” Path : “Consist of series of states through which a System passes during a process.”
Process, Path And Cycle
Cycle : “Occurs when the System
RETURNS
initial state at the
END
of the process.” to its This is the case when the Initial and Final states are the
SAME
or overlaps.
Some
important
terminologies : a.) Isobaric – Constant
Pressure
.
b.) Isothermal – Constant
Temperature
.
c.) Isochoric – Constant
Volume
.
d.) Adiabatic – Constant
Heat .
Dimensions and Units
The
dimensions
require the definition of scales of measure – specific
units
of size Primary units are codified as the International System of Units (
SI
) Multiples and Decimal Fractions of SI units are designated by
prefixes
Other systems of units (e.g. English engineering system) are related to SI units by fixed
conversion factors
Unit second meter kilogram kelvin mole
SI Units and Prefixes
Symbol s m kg K mol
Multiple 10 -15 10 -12 10 -9 10 -6 10 -3 10 -2 10 2 10 3 10 6 10 9 10 12 10 15 Prefix femto pico nano micro milli centi hecto kilo mega giga tera peta Symbol f p n µ m c h k M G T P
Measures of Amount / Size
Three common measures of amount/size: Mass , m Number of moles , n = m/M Total volume , V t Intensive Thermodynamic Variables: Independent of size of system Specific volume , V = V t /m Molar volume , V = V t /n Specific density , ρ = V -1 = m/V t
Force
From Newton’s second law: Force = mass x acceleration (F = ma) Force (F) Equation Unit SI F = ma N or kgms -2 EES F = ma/g c (Ib f ) where g c = 32.174 (Ib m )(ft)(Ib f ) -1 (s) -2
Temperature
Four temperature scales: (i) Kelvin scale (iii) Rankine scale (ii) Celsius scale (iv) Fahrenheit scale Relations among temperature scales:
t
( °C) =
T
(K) – 273.15
T
(R) = 1.8
T
(K)
t
( °F) =
T
(R) – 459.67
t
( °F) = 1.8
t
( °C) + 32
Temperature (
cont’d
)
Pressure
Pressure is defined as the normal force exerted by a fluid per unit area of the surface P = F/A = mg/A Pressure (P) Unit SI EES N m -2 /Pa (Pascal) (psi)
Pressure (
cont’d
)
When using manometers for pressure measurement, P is also expressed as the equivalent height of a fluid column P = F/A = mg/A = (AL ρ)g/A = L ρg
Pressure (
cont’d
)
Absolute Pressure “The ACTUAL pressure at a given position. Measured relative to Absolute Vacuum ( Absolute Zero ).” Gage Pressure “The DIFFERENCE between Absolute Pressure and local Atmospheric Pressure.”
Pressure (
cont’d
)
Vacuum Pressure “Is Pressure BELOW Pressure.” Atmospheric
P abs = P g + P atm P vac = P atm - P abs
Pressure (
cont’d
)
Most Pressure Gauges only give readings of gauge pressures Absolute P = Gauge P + Atmosphere P Absolute P
must
be used in Thermodynamics calculations
Pressure (
cont’d
)
Pressure may vary
WITHIN
the System with elevation as a result of Gravitational Effects.
However, the variation of Pressure as a result of gravity in
MOST
Thermodynamic Systems is relatively
SMALL
and usually
DISREGARDED
.
Energy
Energy can exist in numerous forms : Thermal, Mechanical, Kinetic, Potential, Electric, Magnetic, Chemical, Nuclear, etc.
Thermodynamics provides
NO
information about absolute value of Total Energy of a System.
Thermodynamics only deals with
CHANGE
of Energy which is what really matters in Engineering Problems.
Energy
Total Energy = Macroscopic Energy + Microscopic Energy.
Macroscopic Energy : “A System possesses as a whole with respect to some outside reference frame .” Examples of Macroscopic Energy : Kinetic Energy and Potential Energy.
Energy
Kinetic Energy
E K
1
mu
2 2 Work is done when a body accelerates from velocity of
u 1
to
u 2
, which is equal to the change in Kinetic Energy of the body
W
mu
2 2 2
mu
1 2 2
mu
2 2 Kinetic Energy (E K ) SI Equation E K = mu 2 /2 Unit N m / J (Joule) EES E K = mu 2 /2g c (ft Ib f )
Energy (
cont’d
)
Potential Energy
E P
mzg
Work required to product of force raise a body is the exerted and the change in elevation from
z 1
to
z 2 W
mz
2
g
mz
1
g
mzg
Potential Energy (E P ) SI Equation E P = mzg Unit N m / J (Joule) EES E P = mzg/g c (ft Ib f )
Energy
Microscopic Energy : “Forms of Energy related to molecular structure of a System and the degree of Molecular Activity such as Translational Kinetic Energy ( Motion ), Rotational Kinetic Energy and Vibrational Kinetic Energy ( Attractive/Repulsive ).” Very much independent of outside reference frame.
Energy
The sum Energy, E of all the Energies is the Total of the System.
Total Energy, E in kJ while on a mass basis, e in kJ/kg and on a molar basis in kJ/kmol.
The sum of all the Microscopic forms of Energy is Internal Energy, U .
Heat
Energy can cross the Boundary of a Closed System in two distinct forms :
Work
and
Heat
.
Heat can be defined as : “The form of Energy that is transferred between two Systems ( or a System and its Surroundings ) by virtue of a Temperature Difference .”
Heat
Some phrases related with Heat common use today : in a.) Heat Flow .
The transfer of Thermal Energy.
b.) Heat Addition .
The transfer of Heat into a System.
c.) Heat Rejection .
The transfer of Heat out of a System.
Heat
d.) Body Heat .
The Thermal Energy content of a body.
Other phrases : Heat Absorption, Electrical Heating, Resistance Heating, Frictional Heating, Gas Heating, Heat of Reaction, Liberation of Heat, Specific Heat, Sensible Heat, Latent Heat, Waste Heat, Process Heat, Heat Sink, and Heat Source.
Heat
Heat is Energy in transition and is recognized only as it crosses the Boundary of a System.
Adiabatic Process ( Q = 0 kJ ) : “A process during which there is
NO
Transfer.” Heat Adiabatic comes from the
Adiabatos
Greek word which means
not to be passed
.
Heat
There are two Adiabatic : ways a process can be 1.) The System is well insulated so that only a negligible amount of Heat can pass through the Boundary.
2.) Both the System and the Surroundings are at the
SAME
temperature and therefore,
NO
Driving Force ( Temperature Difference ) for Heat Transfer.
Heat
Symbol : Q.
Unit : Joule ( J ) or kilo Joule ( kJ ).
Heat Transfer per unit mass, q q = Q/m ( kJ/kg ).
Rate of Heat Transfer or Power = Q/∆t Unit : kJ/s or kilo Watt ( kW ).
Heat
Heat is transferred by three mechanisms:
Mechanism
Conduction Convection a.) Free ( Natural ) b.) Forced Radiation
Definition
The transfer of Energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles.
The transfer of Energy between a Solid surface and the adjacent Fluid that is in motion , and it involves the combined effects of Conduction and Fluid Motion.
Fluid motion is caused by bouyancy forces induced by density differences due to the variation of temperature in the fluid.
If the Fluid is forced to flow in a tube or over a surface by external means such as a fan, pump or the wind.
The transfer of Energy due to the Waves ( or Photons ).
emission of Electromagnetic
Heat
Finally, it needs to be emphasized that : Heat always flows from higher temperature to a lower one Rate of heat transfer is proportional to the temperature difference between two bodies Heat is never stored within a body It exists only as energy in transit between a System and its surroundings just like Work.
Work
Energy Transfer If the Energy crossing the boundary of a
CLOSED
system is
NOT
Heat, it must be WORK .
There will be work when there is movement . If there is no movement, no work is done.
Work
Identify weather the situation below are work or not.
A teacher applies a force to a wall and becomes exhausted.
A book falls off a table and free falls to the ground.
A Waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.
A rocket accelerates through space.
Work
Work
Work is force acting through a distance.
W
F
.
s
Unit, kJ
Work
Work done per unit time is
POWER
Power = W/t = ( Fs )/t (unit kJ/s or kW)
Work
W is performed whenever a force acts through a distance
dW
F
dl
W is positive when the displacement is in the same direction as the applied force or vice versa Work (W) Unit SI N m / J (Joule) EES (ft Ib f )
Work (
cont’d
)
W is also performed when there is a change in volume of fluid (compression or expansion)
dW
PA
d V t A dW
P
dV t W
V
2
t V
1
t P
dV t
The minus sign is required because the volume change is negative
Work (
cont’d
)
Work
Moving Boundary Work
.
The work associated with a Moving Boundary is called Boundary Work .
Therefore, Expansion and Compression Work is often called as Moving Boundary Work.
Work
The moving boundary
Work
Work
air air
Compression process - dV –ve,
∴
- W Expansion process - dV +ve,
∴
W
Work
Moving Boundary Work
Total Boundary Work done during the entire process as the piston moves is obtained by adding all the differential works from the Initial to Final state.
δW = Fds = PAds = PdV
Work
Moving Boundary Work Total boundary work from initial state to final state
W
2 1
PdV
Work
Area under curve = PdV=Work
Isochoric Process Constant Volume
Work
Work Isobaric Process
Work
Work
Polytropic Process
A process which occurs with an interchange of both heat and work between the system and its surroundings. The non-adiabatic expansion or compression of a fluid is an example of a polytropic process.
Work
Polytropic Process
Work
Polytropic Process Work,
Work
Isothermal Process
Work
Isothermal Process
Work
,
W
P
1
V
1 ln
V
2
V
1
Work
Other Mechanical Forms Of Work : a.) Gravitational Work : “Work Done by Gravitational Force.” b.) Acceleration Work : “Work Done by ordinary Force.” c.) Shaft Work : “Work Done by Rotating Shaft.” d.) Spring Work : “Work Done by expansion of spring through some displacement.”
Work
Non Mechanical Forms Of Work : a.) Electrical Work : “Work Done as a result of Voltage/Electrical Potential and Electrical Charge.” b.) Magnetic Work : “Work Done by Magnetic Field Strength and Total Magnetic Dipole Moment.” c.) Electrical Polarization Work : “Work Done by Electrical Field Strength and Polarization of the Medium.”
Energy Conservation
Work done on a body is equal to the change in a quantity of Energy
W
E K
mu
2 2
W
E P
mzg
E K
E P
0 Work performed can be recovered by carrying out reverse process and returning the body to its initial condition
Energy Conservation (
cont’d
)
Work is energy in transit , not residing in a body, and can be converted into another form of Energy Work exists only during energy transfer from the surroundings to the system, or the reverse In contrast, Kinetic and Potential Energy reside within the system