Chemical Engineering Thermodynamics-I

Engr. Prof. Dr. Mahmood Saleem

Institute of Chemical Engineering & TechnologyUniversity of the Punjab, Lahore

4th Semester2011

Recommended Books1. Smith J.M., Van Ness H.C., Abbott M.M.,

“Chemical Engineering Thermodynamics”, 6th Ed. McGraw-Hill International Edition.

2. J.R. Elliott, and Carl T. Lira, Introductory Chemical Engineering Thermodynamics, Prentice Hall, (1999).

3. T.D. Eastop, A. McConkey, “Applied Thermodynamics for Engineering Technologists” 5th Ed. Published by Educational Low Price Books Scheme.

ASSESSMENT PLANACTIVITY WEEK MARKS

PROBLEM SHEET 1

ISSUE: W6SUBMIT: W8

10

MID TERM EXAM

W9 35

PROBLEM SHEET 2

ISSUE : W13SUBMIT: W15

15

END TERM EXAM

W17 40

GRADING CRITERIA• ASSIGNMENTS

– COMPLETENESS– NEATNESS– PRESENTATION– ON-TIME

• EXAMINATIONS– APPROACH– ACCURACY– NEATNESS– COMPLETENESS– READABILITY

Course Outlinehttp://pu.edu.pk/program/show/2501/B.Sc.-Engineering.html

• Fundamental Concepts– Basic Definitions in thermodynamics– Laws of thermodynamics– Volumetric properties- Equations of state– Property relations for pure substances

• Thermodynamics applied to practical situations:– Concepts of equilibrium

• Phase Equilibrium• Reaction Equilibrium

– Concept of Fugacity– Concept of Exergy Fluid flow processes– Power generation (Internal Combustion Engines, Gas Turbines, and

Steam power Plant)– Refrigeration,Air conditioning and water cooling– Liquefaction of gases

1- Basic Definitions• Thermodynamics• System and

Surroundings• Heat and Work• Intensive properties• Extensive properties• State function and Path

function

• Temperature• Pressure• Volume• Internal Energy• Enthalpy• Entropy• Helmholts Energy• Gibbs Free Energy

Submit an assignment within next 2 weeks (hand written) 7<pages>10

What is ThermoWhat is Thermo……? ? • ENERGY Science• Energy ConversionWhat is Energy…?• Cause Changes - Work, Heat, etc.• Energy is property of matter!• Therefore, ENERGY and

PROPERTY interrelations

Thermo VocabularyThermo Vocabulary• System, Surroundings, and Boundary• Closed System - Control Mass• Open System - Control Volume • Control Surface• Adiabatic System - NO Heat Transfer• Isolated System - NO Energy Transfer

Forms of EnergyForms of Energy• TOTAL Energy

– thermal, kinetic, potential, electric, magnetic, nuclear, ….

• Macroscopic vs. Microscopic• INTERNAL Energy (U)

- sum of ALL microscopic energies

System PROPERTIESSystem PROPERTIES• Extensive

– m, V, U, E• Intensive

– T, P, ρ, u, e

State, Equilibrium, ProcessesState, Equilibrium, Processes• State or Condition of a System• Equilibrium - System in balance,

NO Changes– thermal/temperature balance - no HEAT

transfer/adiabatic– mechanical/pressure balance - no WORK

transfer– phase - no change of phases– chemical - no change of spices

Pressure Scales

Vacuumpressure

Gaugepressure

Absolutepressure

Standard atm. pressure

101.325 kPa(abs)14.696 psi(a)760 mmHg(a)29.92 inHg(a)

Local atm. pressure

Localatm.

pressure

Temperature Measurement• Temperature:

measure of hotness or coldnessindicating the direction in which heat (energy) will spontaneously flow, i.e., from a hotter body (one at a higher temperature) to a colder one (one at a lower temperature).

• Temperature is not the equivalent of the system energy

• Historical background

Temperature Measurement Devices

• thermocouples,• resistive temperature devices (RTDs and

thermistors), • infrared radiators, • bimetallic devices, • liquid expansion devices, and • change-of-state devices.

Temperature Standards• Temperature scale (ITS-90)• Definition of the degree

Kelvin K, defined as the 1/273.16 fraction of the thermodynamic temperature of the

triple point of water (TPW).

• Fixed (reference) points• Interpolation between the fixed points

Thermo LawsThermo Laws

0th Law: TA=TC and TB=TC then TA=TB

1ST Law - Energy Conservation2ND Law - Process Direction3RD LAW – pure crystalline substance at 0K has 0 entropy.

In ConclusionIn Conclusion• Be imaginative• Try to differentiate• Make sense of what you study• Work and think hard and smart

Thermodynamics is a science of Thermodynamics is a science of ENERGY and MATTER and their relationsENERGY and MATTER and their relations

Laws of Thermodynamics

Laws of Thermodynamics• 1st Law of thermodynamics

– Definition– Mathematical formulation– Applications

• 2nd Law of thermodynamics– Definition– Mathematical formulation– Applications

• 3rd Law of thermodynamics

Concepts of Equilibrium• Equilibrium• Mathematical formulation• Applications

– Phase equilibrium– Reaction equilibrium

ThermodynamicsThermodynamics• a system:

Some portion of the universe that you wish to study

The surroundings:The adjacent part of the universe outside the

system

Changes in a system are associated with the transfer of energy

Natural systems tend toward states of minimum energy

Energy StatesEnergy States

• Unstable: falling or rollingStable: at rest in lowest energy state

Metastable: in low-energy perch

Figure 1.1. Stability states. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Gibbs Free EnergyGibbs Free EnergyGibbs free energy is a measure of chemical

energyAll chemical systems tend naturally toward states

of minimum Gibbs free energy

G = H - TSWhere:

G = Gibbs Free EnergyH = Enthalpy (heat content)T = Temperature in KelvinsS = Entropy (can think of as randomness)

ThermodynamicsThermodynamicsa Phase: a mechanically separable portion of a

system• Mineral• Liquid• Vapor

a Reaction: some change in the nature or types of phases in a system

reactions are written in the form:reactants = products

ThermodynamicsThermodynamicsThe change in some property, such as G for a

reaction of the type:2 A + 3 B = C + 4 D

ΔG = Σ (n G)products - Σ(n G)reactants

= GC + 4GD - 2GA - 3GB

ThermodynamicsThermodynamicsFor a phase we can determine V, T, P, etc., but not G

or HWe can only determine changes in G or H as we change

some other parameters of the system

Example: measure ΔH for a reaction by calorimetry - the heat given off or absorbed as a reaction proceeds

Arbitrary reference state and assign an equally arbitrary value of H to it:

Choose 298.15 K and 0.1 MPa (lab conditions)...and assign H = 0 for pure elements (in their natural

state - gas, liquid, solid) at that reference

ThermodynamicsThermodynamicsIn our calorimeter we can then determine ΔH for the

reaction:Si (metal) + O2 (gas) = SiO2 ΔH = -910,648 J/mol

= molar enthalpy of formation of quartz (at 298, 0.1)

It serves quite well for a standard value of H for the phase

Entropy has a more universal reference state: entropy of every substance = 0 at 0K, so we use that (and adjust for temperature)

Then we can use G = H - TS to determine G of quartz= -856,288 J/mol

ThermodynamicsThermodynamicsFor other temperatures and pressures we can use the

equation:dG = VdP – SdT (ignoring ΔX for now)

where V = volume and S = entropy (both molar)

We can use this equation to calculate G for any phase at any T and P by integrating

zzG G VdP SdTT P T PT

T

P

P

2 1 11

2

1

2

2− = −

ThermodynamicsThermodynamics

If V and S are constants, our equation reduces to:GT2 P2

- GT1 P1= V(P2 - P1) - S (T2 - T1)

which arn’t bad!

ThermodynamicsThermodynamicsIn Worked Example 1 we used

GT2 P2- GT1 P1

= V(P2 - P1) - S (T2 - T1)and G298, 0.1 = -856,288 J/mol to calculate G for quartz at several temperatures and pressures

Low quartz Eq. 1 SUPCRTP (MPa) T (C) G (J) eq. 1 G(J) V (cm3) S (J/K)

0.1 25 -856,288 -856,648 22.69 41.36500 25 -844,946 -845,362 22.44 40.730.1 500 -875,982 -890,601 23.26 96.99500 500 -864,640 -879,014 23.07 96.36

Agreement is quite good (< 2% for change of 500o and 500 MPa or 17 km)

ThermodynamicsThermodynamicsSummary thus far:

G is a measure of relative chemical stability for a phaseWe can determine G for any phase by measuring H and S for the reaction creating the phase from the elementsWe can then determine G at any T and P mathematically

Most accurate if know how V and S vary with P and T• dV/dP is the coefficient of isothermal compressibility• dq/dT is the heat capacity (Cp)

Use? If we know G for various phases, we can determine which is most stable

Why is melt more stable than solids at high T?Is diamond or graphite stable at 150 km depth?What will be the effect of increased P on melting?

Does the liquid or solid have the larger volume?

High pressure favors low volume, so which phase should be stable at high P?

Does liquid or solid have a higher entropy?

High temperature favors randomness, so which phase should be stable at higher T?

We can thus predict that the slope of solid-liquid equilibrium should be positive and that increased pressure raises the melting point.

Figure 1.2. Schematic P-T phase diagram of a melting reaction.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Does the liquid or solid have the lowest G at point A?

What about at point B?

The phase assemblage with the lowest G under a specific set of conditions is the most stable

Figure 1-2. Schematic P-T phase diagram of a melting reaction.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Free Energy vs. TemperatureFree Energy vs. Temperature

dG = VdP - SdT at constant pressure: dG/dT= -S

Because S must be (+) G for a phase decreases as T increases

Would the slope for the liquid be steeper or shallower than that for the solid?

Figure 5.3. Relationship between Gibbs free energy and temperature for a solid at constant pressure. Teq is the equilibrium temperature. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Free Energy vs. TemperatureFree Energy vs. TemperatureSlope of GLiq > Gsol since

Ssolid < SliquidA: Solid more stable than

liquid (low T)B: Liquid more stable than

solid (high T)– Slope δP/δT = -S– Slope S < Slope L

Equilibrium at Teq

– GLiq = GSol

Figure 1.3. Relationship between Gibbs free energy and temperature for the solid and liquid forms of a substance at constant pressure. Teq is the equilibrium temperature. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Now consider a reaction, we can then use the equation:

dΔG = ΔVdP - ΔSdT (again ignoring ΔX)

For a reaction of melting (like ice → water)ΔV is the volume change involved in the reaction (Vwater - Vice)similarly ΔS and ΔG are the entropy and free energy changes

dΔG is then the change in ΔG as T and P are variedΔG is (+) for S → L at point A (GS < GL)ΔG is (-) for S → L at point B (GS > GL)ΔG = 0 for S → L at point x (GS = GL)

ΔG for any reaction = 0 at equilibrium

Pick any two points on the equilibrium curveΔG = ? at eachTherefore dΔG from point X to point Y = 0 - 0 = 0

dΔG = 0 = ΔVdP - ΔSdT

X

Y

dPdT

ΔS= ΔV