Transcript The Scope of Thermodynamics - Dicky Dermawan

ITK-233
Termodinamika Teknik Kimia I
3 SKS
Basic Principles & First Law
DICKY DERMAWAN
www.dickydermawan.net78.net
[email protected]
Score & Grading
 20 4 all homework & quiz
 25 4 1st midterm exam
 25 4 2nd midterm exam
 30 4 final term examination
• A 4 74.5 ++
• B 4 59.5 ++
• C 4 49.5 ++
• D 4 39.5 ++
The Scope of Thermodynamics
Thermodynamics deals with transformations of energy of all
kinds from one form to another.
The general restrictions within which all such transformations
are observed to occur are known as the first and second law
of thermodynamics. These laws cannot be proved in the
mathematical sense. Rather, their validity rests upon
experience.
The universal applicability of thermodynamics is shown by the
fact that it is employed alike by physicists, chemists, and
engineer. The basic principles are always the same, but the
applications differ.
Chemical Engineering Thermodynamics
The chemical engineer must be able to cope a wide variety of
problems viz. the determination of heat & work
requirements for physical & chemical processes & the
determination of equilibrium conditions for chemical
reactions and for the transfer of chemical species between
phases.
Thermodynamic consideration alone are not sufficient to
allow calculation of the rates of chemical or physical
processes, because rates depend on both driving force &
resistance. Driving forces are thermodynamic variables,
resistances are not.
Chemical Engineering Thermodynamics
Thermodynamics is a macroscopic-property formulation. It
cannot reveal the microscopic (molecular) mechanisms of
physical or chemical processes. But, on the other hand,
knowledge of the microscopic behavior of matter can be
useful in the calculation of the thermodynamic properties.
Such property values are essential to the practical application
of thermodynamics; numerical results of thermodynamic
are accurate only to the extent that the required data are
accurate.
The chemical engineer must deal with many chemical species
& their mixtures, and experimental data are often
unavailable.
Applying Thermodynamics




The application of thermodynamics to any real
problem starts with the identification of a particular
body of matters as the focus of attention. This quantity
of matter is called the system, and its thermodynamic
state is defined by a few measureable macroscopic
properties:
Force
Temperature
Volume: specific volume, molar volume, density
Pressure: gauge pressure, absolute pressure
Pressure
The reading on a mercury manometer at 70 oF (open to
the atmosphere at one end) is 25,62 in. The local
acceleration of gravity is 32,243 ft/s2. Atmospheric
pressure is 19,86 inHg. What is the absolute pressure
in (psia) being measured? The density of mercury at
70 oF is 13,543 g/cm3.
Unit Conversion
Convert of physical parameters below:
500 oF = ….. K = ….. oC = ….. R
R = 8,314 J/mol.K = ….. Btu/lbmol.R = …..
cmHg.cm3/mol.K = ….. kPa.m3/mol.K
4500 kW = ….. Btu/jam = …..lbf.ft/detik = …..
kgf.m/detik
Thermodynamics Variable: Work, Energy & Heat
Work W is done whenever a force acts through a distance. The
quantity of work done is defined by the equation:
F is the component of the force acting in the direction of the
displacement dl.
dW  F dl
In engineering thermodynamics an important type of work is
that which accompanies a change of volume of a fluid, i.e.
compression or expansion.
V2
W   P dV
V1
2
A gas is confined in a 0.47-m-diameter cylinder by piston, on
which rest a weight. The mass of the piston and weight
together is 150 kg. The local acceleration of gravity is 9.813
m s-2, and atmospheric pressure is 101.57 kPa.
a. What is the force in newton exerted on the gas by
atmosphere, the piston and the weight, assuming no
friction between the piston and cylinder
b. What is the pressure of the gas in kPa
c. If the gas in the cylinder is heated, it expands, pushing the
piston & weight upward. If the piston & weight are raised
15 cm, what is the work done by the gas in kJ
d. What is the change in potential energy in kJ of the piston
& weight?
Work & Energy
Work done on a body in accelerating it from an initial velocity
u1 to a final velocity of u2 is equal to the change of kinetic
energy of the body mu2/2
Work done on a body in raising it through the distance z2-z1 is
equal to the change in the quantity of potential energy mgz
If the work done on a body in accelerating it or in elevating it
can subsequently be recovered, then the body by virtue of
its velocity or elevation must contain the ability or capacity
to do this work.
Energy is the capacity of a body for doing work
Work
An automobile having a mass of 1,250 kg is traveling at
40 m s-1. What is its kinetic energy in kJ? How much
work must be done to bring it to a stop?
Liquid water at 0oC and atmospheric pressure has a
density of 1.000 g/cm3. At this condition, ice has a
density of 0.917g/cm3.How much work is done at
these conditions by 1 kg of ice as it melts to liquid
water?
Work: Energy in Transit
When work is done, it is done by the surroundings on the
system, or vice versa, and energy is transferred from the
surroundings to the system, or vice versa. It is only during
this transfer that the form of energy known as work exist.
In contrast, kinetic & potential energy reside with the system.
Their value, however, are measured with reference to the
surroundings, i.e. kinetic energy depends on velocity with
respect to the surroundings, and potential energy depends
on eleveation with respect to a datum level.
Change in kinetic and potential energy do not depend on
these reference conditions, provided they are fixed.
Heat & Energy
Heat is another form of energy that always flows from a
higher temperature to a lower temperature. The rate of heat
transfer is proportional to the temperature difference
between the two bodies.
In the thermodynamic sense, heat is never regard as being
stored within a body. Like work, it exist only as energy in
transit between a system and its surroundings.
When energy in the form of heat is added to a body, it is
stored as kinetic & potential energy of the atoms and
molecules making up the body.
Internal Energy U
Kinetic & potential energy are energy which the substance
may posses as a result of its macroscopic position or
motion. They can be regarded as external form of energy
because their numerical values refer to the surroundings.
In contrast, internal energy of a substance refers to the
energy of the molecule making up the substance. Although
absolute values of internal energy are unknown, this is not
a disadvantage in thermodynamic analysis, because only
changes in internal energy is required.
First Law of Thermodynamics
Although energy assumes many forms, the total quantity of
energy is constant, and when energy disappears in one form
it appears simultaneously in other forms.
The first law applies to the system and surroundings, NOT to
the system alone. For closed system when only heat and/or
work is transferred:
 energy of surroundin gs   Q  W
 energy of system
 U  E
When no external energy change:

 U  E   Q  W

U  Q  W
dU  dQ  dW
Thermodynamic State and State Function
State functions are quantities which depend only on present
conditions, however reached. State functions can be expressed
mathematically as functions of thermodynamic coordinates
such as temperature & pressure, their value can always be
identified with points on a graph.
Viz.Nitrogen at 300 K & 1 bar has a definite set properties:
specific volume, a definite viscosity, a definite thermal
conductivity, etc.
Internal energy is a state function, and is therefore, a property
of the system.
Work & heat are not state functions because they depend on
path/process. They cannot be identified with points on a
graph, but rather are represented by areas.
Thermodynamic State and State Function
State Functions
Represent a property of a
system and always has a
value P
2
 dP  P
P1
U2
 dU  U
Not State Functions
Appear only when changes
are caused in a system by a
process, which requires time
 dQ  Q
 dW  W
U1
Work & heat ARE NOT state functions but
the diiference: Q – W IS a state function
Example
A gas is confined in a cylinder by a piston. The initialpressure
of the gas is 7 bar, and the volume is 0.1 m3. The piston is
held in place by latches in the cylinder wall. The whole
apparatus is placed in air at standard atmospheric pressure
of 101.33 kPa.
What is the energy change of the apparatus if the retaining
latches are removed so that the gas suddenly expands to
double its initial value? The piston is again held by latches
at the end of the process. Assume the rate of heat exchange
between the apparatus and the surrounding air slow
compared with the rate at which the process occur.
Intensive vs Extensive Property
Extensive
Intensive
 Depend on the quantity
 Independent of the
involved
 Double when the
quantity of material is
doubled
 V, U,….
quantity involved
 Fix even though the
quatity of material
changed
 P, T
Volume is an extensive property
BUT
specific volume and molar volume are intensive properties
Entalphy H
Entalphy is another thermodynamic function defined as:
H  U  PV
Since U, P & V are all state functions, H must also be a state
function.
dH  dU  d(PV)
H  U  (PV)
Like V & U, H is an extensive property
Specific entalphy, h, is of course, an intensive property
Example
Calculate ΔU and ΔH for 1 kg of water when it is
vaporized at the constant temperature of 100oC and
the constant pressure of 101.33 kPa. The specific
volumes of liquid & vapor water at these conditions
are 0.00104 & 1.673 m3/kg. For this change, heat in
the amount of 2256.9 kJ is added to the water.
Problem 2.1 (1)
An insulated and nonconducting container filled with 10 kg of
water at 20oC is fitted with a stirrer. The stirrer is made to turn
by gravity acting on a weight of mass 25 kg. The weight falls
slowly through a distance of 10 m in driving the stirrer.
Assuming that all work done on the weight is transferred to
the water and that the local acceleration of gravity is 9.8 ms-2,
determine:
a. The amount of work done on the water
b. The internal-energy change of the water
c. The final temperature of the water
d. The amount of heat that must be removed from water to
return it to its initial temperature
Problem 2.1 (2)
An insulated and nonconducting container filled with 10 kg of
water at 20oC is fitted with a stirrer. The stirrer is made to turn
by gravity acting on a weight of mass 25 kg. The weight falls
slowly through a distance of 10 m in driving the stirrer.
Assuming that all work done on the weight is transferred to
the water and that the local acceleration of gravity is 9.8 ms-2,
determine the total energy change if the universe because of:
a. The process of lowering the weight
b. The process of cooling the water back to its initial
temperature
c. Both processes together
Problem 2.4
Liquid water at 100oC and 1 bar has an internal
energy (on an arbitrary scale) of 419.0 kJ/kg & a
specific volume of 1.044 cm3/g.
a.What is its enthalpy?
b. The water is brought to the vapor state at 200oC
and 800 kPa, where its entalphy is 2838.6 kJ/kg
and its specific volume is 260.79 cm3/g.
Calculate ΔH & ΔU for the process.
First Law Expression for Steady-state Flow Process
E K  u 2  u  12 u 2
1
2
2
1
2
2
1
EP  z2g  z1g  gz
W1  P1A1
V1
 P1V1
A1
W2  P2 A 2
V2
 P2 V2
A2
W  Ws  W2  W1
u 2
U 
 gz  Q  Ws  P2 V2  P1V1
2
u 2
H 
 gz  Q  Ws
2
When no external energy change:
H  Q  Ws
Example
Air at 1 bar & 25oC enters a compressor at low velocity,
discharges at 3 bar, & enters a nozzle in which it
expands to a final velocity of 600 ms-1 at the initial
conditions of pressure & temperature. If the work of
compression is 240 kJ/kg of air, how much heat
must be removed during compression?
Problem 2.10
Liquid water at 70 oF flows in a straight horizontal pipe in
which there is no exchange of either work or heat with the
surroundings. Its velocity is 30 ft/s in a pipe with an
internal diameter of 1 in until it flows into a section where
the pipe diameter abruptly increases.
a. What is the entalphy change of water if the downstream
diameter is 1.5 in?
b. What is the entalphy change of water if the downstream
diameter is 3 in?
c. What is the maximum change in entalphy for an
enlargement in the pipe?
Problem 2.11
Water flows through a horizontal coil heated from the
outside by high-temperature flue gases. As it passes
through the coil the water changes state from 2 atm
& 180 oF to 1 atm & 250 oF. Its entering velocity is
10 ft/s & its its exit velocity is 600 ft/s.
Determine the heat transferred through the coil
perunit mass of water.
Entalphy of the inlet and outlet water streams are:
Inlet: 148 Btu/lbm
Outlet: 1168.8 Btu/lbm
Steam Table &
Unit Conversion
Complete of steam table as
following:
P, kPa
T, oC
x, %
, m3/kg
h, kJ/kg
s, kJ/kg.K
300
70
…
…
…
…
…
200
…
…
1500
…
1200
…
…
0.142
…
…
800
…
0
…
…
…
…
4000
…
…
…
6.0689
Example
To operate a generator, the utility unit generates amount mechanic
energy by operating a turbine. The turbine uses steam as working
fluid. Assume that turbine works adiabatically and reversibly. Steam
enters the turbine at 1.2 MPa and 250 oC and leave the turbine at
saturated condition with pressure and vapor quality of 300 kPa and
96.92%, respectively. Electrical power produced by generator is 10
MW at mechanical efficiency of 80%.
a.
Calculate the change of enthalpy and internal energy of steam
(in kJ/kg)!
b.
Calculate the required mass flow rate of steam (in ton/hour)!
c.
Determine the steam’s temperature leave a turbine (in oC)!
Example
Water at 200oF is pumped from a storage tank at the
rate of 50 gpm. The motor for the pump supplies
work at the rate of 2 hp. The water passes through a
heat exchanger where it gives up heat at the rate of
40,000 Btu/min, & is delivered to a second storage
tank at an elevation 50 ft above the first tank. What
is the temperature of the water delivered to the
second tank?
Concept of Process Reversibility
A process is reversible when its direction can be
reversed at any point by an infinitesimal change in
external conditions.
The reversible process is ideal in that it can never be
fully realized; it represents a limit to the
performance of actual processes. Results for
reversible processes in combination with appropriate
efficiencies yield reasonable approximations of the
work for actual processes.
Vocabulary: Irreversible process, dissipative process
Irreversible Process
An irreversible process is a process that cannot return both the
system and the surroundings to their original conditions.
That is, the system & 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.
There are many factors that make a process irreversible. Four
of the most common causes of irreversibility are friction,
unrestrained expansion of a fluid, heat transfer through a
finite temperature difference, and mixing of two different
substances. These factors are present in real, irreversible
processes and prevent these processes from being reversible.
Constant Volume Process & Constant Pressure Process
2
Constant Volume
U  Q   PdV
Constant Pressure
1
2
 Q  U
 P dV  0
1
Heat capacity:
 U 
CV  

 T  V
T2
Q  U   C V dT
T1
U  Q  PV 
 Q  H
H  U  PV 
Heat capacity:
 H 
CP  

 T  P
T2
Q  H   C P dT
T1
Problem 2.17
The internal energy Ut of an amount of gas is given by
the equation:
t
t
U  1.5PV
Where P is in (psia) and Vt is in (ft)3. The gas
undergoes a mechanically reversible process from an
initial sate at 1500 psia and 500 R. During the
processVt is constant and equal to 10 ft3 and P
increases by 50 percent.
Determine the value for Q and Ht in Btu for the
process
Heat Capacity
Five moles of nitrogen at 80oC is contained in a rigid vessel. How
much heat must be added to the system to raise its temperature to
300oC if the vessel has a negligible heat capacity?
If the mass of the vessel is 100 kg and if its heat capacity is 0.5
J/goC, how much heat is required?
Three moles of nitrogen at 230oC is contained in a piston/cylinder
arrangement. How much heat must be extracted from this system,
which is kept at constant pressure, to cool it to 80oC if the heat
capacity of the piston & cylinder is neglected?
Take CV = 20.8 and CP = 29.1 J/moloC for nitrogen gas.
Illustration: Calculation of Work
A cylinder with a piston is used to compress carbon
dioxide from 1 to 20 bar. Assuming that the process
can be carried out reversibly and isothermally at 300
K, calculate the work required per mole, given that at
300 K the P-V-T properties of carbon dioxide follow
the equation P( - b) = RT with b = -0,00011 m3/mol
Heat Capacity
Five moles of nitrogen at 100oF is contained in a rigid vessel. How
much heat must be added to the system to raise its temperature to
400oF if the vessel has a negligible heat capacity?
If the mass of the vessel is 250 lbm and has a heat capacity of 0.12
Btu/lbmoF how much heat is required?
Three moles of nitrogen at 450oF is contained in a piston/cylinder
arrangement. How much heat must be extracted from this system,
which is kept at constant pressure, to cool it to 100oF if the heat
capacity of the piston & cylinder is neglected?
Take CV = 5 and CP = 7 Btu/lbmol oF for nitrogen gas.
Problem 2.19
The path followed by a gas during a particular mechanically
reversible process is described by the equation:
P  aV t  c
Where a & c are constants. In the initial state, P1 = 60 bar and
Vt1 =0.002 m3. In the final state, P2 = 20 bar and Vt2 = 0.004
m3. During the process, heat in the amount of 5000 J is
transferred to the gas. Determine W and ΔUt for the process.
U
Heat in the amount of 7.5 kJ is added to a closed
system while its internal energy decreases by 12
kJ. How much energy is transferred as work?
For a process causing the same change of state but for
which the work is zero, how much heat is
transferred?
H
1.5 kg of saturated steam at 2 bars is confined in a
cylinder by piston. Assume no mechanical friction.
Amount work 750 kJ given to that steam, so
pressure and specific volume of steam to be 10 bars
and 0.042. Determine:
Change of enthalpy and internal energy of steam (in
kJ/kg)
 Temperature at final compression (in oC)
 Amount of heat transferred during process (kJ)
