Thermodynamics One for ChemE I Chapter 1.pdf

College of Biological and Chemical Engineering
Department of Chemical Engineering
Chapter One
Basic Concepts of Thermodynamics
Thermodynamics

1.Basic Concepts of Thermodynamics
Objectives:
• Define Basic Concepts
• Review SI and British Unit Systems
• Explain system, state, state postulate, equilibrium,
process, and cycle.
• Review concepts of T, T. scales, absolute and
gage pressures.

1.1 Definition of Thermodynamics
What is Thermodynamics?
 The science of energy.
 Feel energy, but difficult to define.
 The ability to cause changes.
 Thermodynamics stems from the Greek words thermo (heat)
and dynamics (power).
 The early efforts to convert heat into power.
 Today, it includes all aspects of energy and energy
transformations.

Encountered in many engineering systems and other aspects
of life such as:
 Automobile engines
 Turbines, compressors, pumps
 Fossil and nuclear fueled power stations
 Combustion systems
 Cryogenic systems, gas separation, and liquefaction
 Heating, ventilating, and air-conditioning systems
 Vapor compression and absorption refrigeration
1.2 Application areas of thermodynamics

 Heat pumps
 Cooling of electronic equipment
 Alternative energy systems
 Fuel cells
 Thermoelectric and thermionic devices
 Solar activated heating, cooling, and power generation
 Geothermal systems
 Ocean thermal, wave, and tidal power generation
 Wind power
 Biomedical applications
Cont.……

Figure 1.1: some application areas of thermodynamics
Cont.……

A. Zeroth Law of Thermodynamics
 If two bodies are in thermal equilibrium with a third body,
they are also in thermal equilibrium with each other.
 Example, hot gas inside an electric bulb, the electrical
filament and the glass wall of the bulb .
 Let A be a test body . Then all bodies in thermal equilibrium
with it are in thermal equilibrium with each other ;i.e., they
have common property. It’s called temperature.
 Temperature is commonly measured with liquid-in-glass .
1.3 Laws of Thermodynamics

 Mercury, alcohol, or some other fluid
 Celsius scale
 Kelvin scale
 The lower limit of temperature, called absolute zero on the
Kelvin scale, occurs at -273.15oC.
Toc =TK – 273.15
 Rankin scale and
 Fahrenheit scale
T(R) = 1.8TK
T(oF) = T(R) – 459.67
Cont.……

Pressure
 A normal force exerted by a fluid per unit area (valid for gas
and L ).
 SI unit Pascal or N/m2 ,1 Pa =1 N/m2, kPa, MPa, bar, standard
atmosphere, kilogram-force per square centimeter
• 1 bar = 105 Pa = 0.1 MPa = 100 kPa
• 1 atm = 101,325 Pa = 101.325 kPa = 1.01325 bars
Cont.……

• 1 kgf/cm2 = 9.807 N/cm2 = 9.807 *104 N/m2 = 9.807 * 104 Pa
= 0.9807 bar
= 0.9679 atm
 Note that the pressure units bar, atm, and kgf/cm2 are almost
equivalent to each other
 English system, (lbf/in2, or psi),
 1 atm = 14.696 psi.
 1 kgf/cm2 = 14.223 psi.
Cont.……

 Pressure is also used for solids, which is force acting
perpendicular to the surface per unit area.
Absolute pressure: The actual pressure at a given position
 it is measured relative to absolute vacuum (i.e., absolute zero
pressure).
 Most pressure-measuring devices, however, are calibrated to
read zero in the atmosphere (Fig. 1.20), and so they indicate
the difference between the absolute pressure and the local
atmospheric pressure.
Cont.……

 This difference is called the gage pressure.
 Pressures below atmospheric pressure are called vacuum
pressures and are measured by vacuum gages.
 Absolute, gage, and vacuum pressures are all positive
quantities and are related to each other by
Pgage = Pabs – Patm
Pvac = Patm – Pabs
Cont.……

Figure 1.20: Absolute, gage, and vacuum pressures
Cont.……

 Like other pressure gages, the gage used to measure the air
pressure in an automobile tire reads the gage pressure.
 Therefore, the common reading of 32 psi (2.25 kgf/cm2)
indicates a pressure of 32 psi above the atmospheric
pressure.
 At a location where the atmospheric pressure is 14.3 psi,
for example, the absolute pressure in the tire is 32 + 14.3 =
46.3 psi.
Cont.……

B. First Law of Thermodynamics
 An expression of the conservation of energy principle
 Energy can change from one form to another but the total
amount of energy remains constant.
 Energy cannot be created or destroyed.
 Energy balance is expressed as:
Ein - Eout = ΔE.
Cont.……

Figure 1.2: Energy cannot be created or destroyed;
it can only change forms (1st law)
Cont.……

Figure 1.3: conservation of energy principle for the
human body
Cont.……

C. Second Law of Thermodynamics
 Asserts that energy has quality as well as quantity
 Actual processes occur in the direction of decreasing
quality of energy
 Assume, a cup of hot coffee left on a table eventually cools,
but cooled coffee in the same room never gets hot by itself
 The high-temperature energy of the coffee is degraded
(transformed into a less useful form at a lower
temperature).
Cont.……

Figure 1.4: Heat flows in the direction
of decreasing temperature
Cont.……

Based on the behavior of particles it is divided into classical and
statistical thermodynamics.
 Classical thermodynamics utilizes macroscopic approach.
 This approach does not require knowledge of the behavior of
individual
 The average behavior of many atoms/molecules using
instruments.
Types of Thermodynamics

 Statistical thermodynamics utilizes microscopic approach
 Based on the average behavior of large groups predicts the
behavior of individual particle
 Understanding and prediction of macroscopic phenomena and
calculation of macroscopic properties from the properties of
individual molecules
Cont.……

 Thermodynamic system: a quantity of matter or a region in
space chosen for study.
 Surrounding: The mass or region outside the system is called
the surroundings.
 Boundary: The real or imaginary surface that separates the
system from its surroundings.
 The boundary of a system can be fixed or movable.
1.5 Basic Concept and Terminologies

Figure 1.5: thermodynamic system, its boundary
and surrounding
Cont.……

 Closed or Open System
I) Closed System
 No mass can cross its boundary but energy, in the form
of heat or work, can cross the boundary.
 The volume of a closed system does not have to be
fixed.
1.6 Types of Systems

Figure 1.6: Closed system Figure 1.7: closed system with a
moving boundary
Cont.……

II) Open System (control volume)
 A properly selected region in space.
 It usually encloses a device that involves mass flow such as a
compressor, turbine, or nozzle
 Flow is best studied by selecting the region within the device as the CV.
 Both mass and energy can cross the boundary of a CV.
 The boundaries of a CV are called a control surface, and they can be
real or imaginary.
 A CV can be fixed in size and shape, or it may involve a moving
boundary
Cont.……

 An isolated system is a general system of fixed mass where
no heat or work may cross the boundaries
Figure 1.10: Isolated system
Cont.……

 A property is a macroscopic characteristic of a system in
equilibrium such as P, T, volume V, m, viscosity, thermal
conductivity, etc.
 Properties are either intensive or extensive.
 Intensive properties: are those that are independent of the
mass and size of a system.
 Some intensive properties are T, P, density, etc.
1.7 Properties of a System

 Extensive properties: values depend on the size or extent of
the system.
 Total mass, total volume ,etc. are some examples of extensive
properties.
Cont.……

 Consider a system not undergoing any change.
 At this point, all the properties can be measured or calculated
throughout the entire system, which gives us a set of properties
that completely describes the condition, or the state, of the
system.
 At a given state, all the properties of a system have fixed
values.
 If the value of even one property changes, the state will change
to a different one.
 In Fig. 1.13 a system is shown at two different states.
1.8 State and Equilibrium

Figure 1.13: A system at two different states
Cont.……

Equilibrium:
 Implies a state of balance.
 No unbalanced potentials (or driving forces) within the
system.
 A system in equilibrium experiences no changes when it is
isolated from its surroundings.
 There are many types of equilibrium, and a system is not in
thermodynamic equilibrium unless the conditions of all the
relevant types of equilibrium are satisfied.
Cont.……

Types of Equilibrium
 Thermal equilibrium: if the temperature is the same throughout the
entire system
 Mechanical equilibrium: is related to pressure, and a system is in
mechanical equilibrium if there is no change in pressure at any point
of the system with time.
• Phase equilibrium: If a system involves two phases, it is in phase
equilibrium when the mass of each phase reaches an equilibrium level
and stays there.
• Chemical equilibrium: if its chemical composition does not change
with time, that is, no chemical reactions occur.
Cont.……

A system is said to be in thermodynamic equilibrium
if it maintains :
 thermal equilibrium(uniform temperature).
 mechanical (uniform pressure) equilibrium,
 phase (the mass of two phases, e.g. ice and liquid
water) equilibrium and
 chemical equilibrium.
Cont.……

The State Postulate
 The state of a simple compressible system is completely
specified by two independent, intensive properties.
 A system is called a simple compressible system in the
absence of electrical, magnetic, gravitational, motion, and
surface tension effects.
 These effects are due to external force fields and are
negligible for most engineering problems.
Cont.……

 The state postulate requires that the two properties specified be
independent to fix the state.
 Two properties are independent if one property can be varied
while the other one is held constant.
 Temperature and specific volume, for example, are always
independent properties, and together they can fix the state of a
simple compressible system.
 Temperature and pressure, however, are independent properties
for single-phase systems, but are dependent properties for
multiphase systems.
Cont.……

1.9.1 Process
 Any change that a system undergoes from one equilibrium
state to another.
 The series of states through which a system passes during
a process is called the path of the process.
To describe a process completely, one should specify :
 the initial and final states of the process,
 the path it follows,
 interactions with the surroundings.
1.9 Process and Cycles

Figure 1.16: A process between states 1 and 2
Cont.……

Adiabatic Process
 It is one in which there is no heat transfer into or out of the
system.
 The system can be considered to be perfectly insulated.
Cyclic process
 A system is said to have undergone a cycle if it returns to its
initial state at the end of the process.
 That is, for a cycle the initial and final states are identical.
Cont.……

Figure 1.18: cyclic process
Cont.……

 One thermodynamic property held constant.
 Isothermal, isobaric, isochoric and isentropic.
 An isothermal process: is a process during which the
temperature T remains constant.
 An isobaric process: is a process during which the pressure
P remains constant.
 An isochoric (isometric) process: is a process during which
the specific volume v remains constant.
 An isentropic process: is a process during which the
entropy remains constant.
Cont.……

Reversible process
 A process that, once having taken place, can be reversed,
and in so doing leaves no change.
 The system and surroundings are returned to their original
condition.
 In reality, there are no truly reversible processes
 It helps the analysis simpler, and to determine maximum
theoretical efficiencies.
 It is a starting point on which to base engineering study
and calculation.
Cont.……

Irreversible Process
 It is a process that cannot return both the system and the
surroundings to their original conditions.
 That is, the system and the surroundings would not return
to their original conditions if the process was reversed.
 For example, an automobile engine does not give back the
fuel it took to drive up a hill as it coasts back down the hill.
Cont.……

Common causes of irreversibility are:
 Friction
 Unrestrained expansion of a fluid,
 Heat transfer through a finite temperature difference,
 Mixing of two different substances.
 These factors are present in real, irreversible processes
and prevent these processes from being reversible.
Cont.……

Figure 1.17: Quasi-equilibrium and non
quasi equilibrium
Cont.……

Cont.……
• An example of quasi-static expansion of a mixture of
hydrogen and oxygen gas, where the volume of the system
changes so slowly that the pressure remains uniform
throughout the system at each instant of time during the
process.
• For an adiabatic expansion/compression of a gas, the non-
uniformity of pressure will render this process non-quasi-
equilibrium. If the piston is pushed in very rapidly, the gas
molecules near the piston face will not have sufficient time to
escape, and they will pile up in front of the piston.

Thermodynamics One for ChemE I Chapter 1.pdf

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