1. Chemical Engineering Thermodynamics
Fall 2004
Dr. Holly J Moore

2. Physical and Chemical Equilibrium for Chemical Engineers
Chapter 1
Introduction to Equilibrium

3. Why Study Equilibrium?
The four basic tools used by chemical and environmental engineers are:
Material Balances
Energy Balances
Equilibrium Relationships
Phase
Chemical
Rate Equations

4. Consider the Synthesis of Ammonia
Recycled Product Ammonia and unreacted feed
Bleed Stream
N2 + 3 H2 ⇋ 2NH3
3 moles H2 1 mole N2
Chiller Condenses most of the ammonia
Reactor partially converts H2 and N2 to NH3
Material Balance
Energy Balance
Equilibrium Relationships
Reaction Rates
Separator
Liquid Ammonia
~15% conversion

5. Why Study Equilibrium?
The four basic tools used by chemical and environmental engineers are:
Material Balances
Energy Balances
Equilibrium Relationships
Phase
Chemical
Rate Equations
This course is about equilibrium !!

6. Equilibrium is like a brick wall between where we are and where we want to go

7. To know the dimensions of our problem, we must know where the equilibrium limits are. This course is about that

8. Equilibrium
A system is at equilibrium if there is no change with time in any of the measurable properties
Some systems come to equilibrium quickly
Combustion is an example

9. Equilibrium
A system is at equilibrium if there is no change with time in any of the measurable properties
Some systems come to equilibrium slowly
Rust is an example

10. Equilibrium
As a practical matter, many of the systems we are interested in as chemical engineers can be assumed to be at equilibrium with very little error

11. Being at equilibrium does not mean the system is static
Liquid water and water vapor are at equilibrium when just as many molecules evaporate as condense

12. Equilibrium is not always desirable
Diamonds are not the equilibrium form of carbon

13. People aren’t at Equilibrium
Carbon dioxide gas
Water
Solids

14. Types of Equilibrium
Stable
Metastable
Unstable
Neutral

15. Types of Equilibrium Stable Metastable Unstable Neutral
If ball is moved from its stable position, it will roll right back

16. Types of Equilibrium Stable Metastable Unstable Neutral
If ball is moved slightly from its stable position, it will roll right back. However, a moderate displacement will cause the ball to roll down the other side of the hill.

17. Types of Equilibrium Stable Metastable Unstable Neutral
If ball is moved from its unstable position, it will roll off the hill, until it finds a stable point

18. Types of Equilibrium Stable Metastable Unstable Neutral
If ball is moved from its stable position, it will remain at the new position

19. Gibbs Free Energy G = H - TS g = h - Ts dGsys = 0
All natural systems try to achieve a minimum Gibbs Free Energy
At equilibrium:
G = H - TS
g = h - Ts
dGsys = 0

20. Units, Conversions and Notation
Both SI and English are used
Extensive properties depend on the size of the system
U, V etc
Usually repesented with capital letters
m is an obvious exception
Intensive properties are independent of the system size
h, v, s, T, P
Usually represented with lower case letters
T and P are obvious exceptions

21. Mass Fractions and Mole Fractions
xi indicates either the mass or mole fraction in a liquid
yi indicates either the mass or mole fraction in a gas
This is obviously a source of confusion – before performing any calculations be sure you understand whether mass fraction or mole fraction is appropriate. Although these parameters are dimensionless, dimensions of kg/kg or mole/mole are sometimes added for clarity

22. PPM and PPB Parts Per Million Part Per Billion
Usually based on volume or mole for gases
Usually based on mass for liquids
mg/kg
Part Per Billion
Uses the same convention

23. Reality, Equations and Tables
It is difficult and expensive to measure physical properties.
Physical property tables are created by:
Finding equations that reproduce experimental data
Using those equations to interpolate or extrapolate to the desired conditions
Much of this course is devoted to finding those equations

24. Phases and Phase Diagrams
An equilibrium phase is one that has a completely uniform composition throughout.

25. Gases
All gases form one phase
They are completely “miscible”

26. Liquids
Liquids can form multiple phases
Oil and water
Oil
Water

27. Solids Homogeneous solids are single phases
Heterogeneous solids are multiple phases
Salt and Pepper are two different phases, even when they are mixed

28. Multiphase solids
Sometimes it’s hard to tell how many solid phases are present
Steel and cast iron consist of two phases
Ferrite and cementite
Copper and nickel alloys consist of one phase
They are solid solutions

29. Different phases may be stable at different conditions
Carbon
Diamond
Graphite
Buckyballs

30. Iron
Molten iron solidifies as a body centered cubic crystal (BCC) called d-ferrite
As it cools, it changed to a face centered cubic crystal (FCC) called austenite
It changes phase again as it cools further, back to BCC, and is called a-ferrite

31. Iron-Iron Carbide Phase Diagram 1 atmFe
1% C
2% C
3% C
4% C
5% C
6% C
6.70% C
a, ferrite
g, austenite
d, ferrite
Cementite (Fe3C

32. Water
You are probably more familiar with the phase diagram of water – or at least you think you are

33. Figure 1.8, page 13
Values at low pressures are not visible when the data is plotted on the arithmetic scale. They become easier to read on a logarithmic scale

34. The appearance of the phase diagram changes dramatically when you change the temperature and pressure range

35. Water Phase Diagram at very high pressures – Note there are 7 separate solid phases!!

36. Water
Most of the time we will only use the water phase diagram at moderate temperatures and pressures
However, it is useful to remember that you can’t just extrapolate water’s behavior to extreme conditions

37. Homework For Friday… Problems 1.3 and 1.5
If you don’t have a text yet, be sure to copy down the homework problems before you go today!!