1. ChemE 260 Phases and Phase Diagrams for Pure Substances
Dr. William Baratuci
Senior Lecturer
Chemical Engineering Department
University of Washington
TCD 2: A & B CB 2: 1 - 4
March 30, 2005

2. Nomenclature Pure Substance Phases Phase Changes
Uniform chemical composition throughout the system
Phases
Liquids: Multiple liquid phases
Solids: Multiple solid phases
Gases: Only ONE gas phase can exist
Phase Changes
Liquid  Gas: Boiling or Evaporating / Condensing
Liquid  Solid: Melting / Freezing
Gas  Solid: Sublimating / Desublimating
Liquid  Gas : An Isobaric Process Path
Consider the isobaric process on the next slide in which energy is added to a closed system that initially contains liquid water at a T < Tsat
Pure Substance
Dry air is a pure substance
Humid air can be considered to be a pure substance
A tank containing liquid water with humid air above it cannot be considered to be a pure substance !
Phases
Multiple liquid phases can exist in equilibrium: oil and water
Multiple solid phases can exist in equilibrium: diamond and carbon, different types of ice crystals, different types of steels.
Only ONE gas phase can exist at equilibrium. All the molecules always mix.
Phase Changes
What is the difference between boiling and evaporating ??
Sublimation: Did you ever notice that old ice cubes in your home freezer have shrunk ? Do you think they melted ? No. They sublimated !
Phase Diagrams
An isobaric process path is a smooth way to introduce Phase Diagrams
Phase Diagrams are our FRIENDS…they make any process a little easier to understand.
Baratuci
ChemE 260
March 30, 2005

3. Isobaric Heating of a Pure Substance
Quality: x = fraction of the mass in the system that exists in the gas or vapor phase.
1 – Subcooled Liquid: T < Tsat and P*(T) < P and x is undefined
2 – Saturated Liquid: T = Tsat and P*(T) = P and x = 0
3 – Saturated Mixture: T = Tsat and P*(T) = P and 0 < x < 1
4 – Saturated Vapor: T = Tsat and P*(T) = P and x = 1
5 – Superheated Vapor: T > Tsat and P*(T) > P and x is undefined
Baratuci
ChemE 260
March 30, 2005

4. Isobaric Heating Process Path on a TV Diagram5 2 3 4
Elements of the Vapor-Liquid region of a phase diagram
Sat’d Liquid Curve
Sat’d Vapor curve
Critical Point
Two Phase Envelope where vapor and liquid both exist within the system at equilibrium
Subcooled liquid region
Superheated vapor region
Supercritical fluid region
Isobaric Heating Process
Slides up and to the left along an isobar 1
Baratuci
ChemE 260
March 30, 2005

5. Isothermal Heating Process Path on a PV Diagram1 2 3 4
This PV diagram extends down into the solid region
We will focus on the vapor-liquid region in this course.
Same elements in this diagram as in the TV Diagram, but they are located in slightly different positions.
Isobaric Heating Process
Slides to the left along a horizontal isobar 5
Baratuci
ChemE 260
March 30, 2005

6. Nomenclature Latent Heat vs. Sensible Heat Critical Point Triple Point
Latent Heat: As heat is added, T does not change
Sensible Heat: As heat is added, T does change
Critical Point
Above Pc or Tc two phases cannot exist at equilibrium
Only one phase exists: a supercritical fluid
Triple Point
Liquid  Gas  Solid : All exist in the system at equilibrium
Water: 0.01oC and kPa
Latent Heat
Heats associated with phase changes
Sensible Heat
Heat flow which causes a change in temperature
Critical Point
PC , TC and critical molar volume
Above PC the substance is a supercritical fluid
Above TC the substance is a supercritical fluid
Baratuci
ChemE 260
March 30, 2005

7. PT Diagrams Two Types for the Two Types of Substances
Substances that contract when they freeze
Most substances fall into this category
There are very few of these.
Most important one is WATER
Ice floats
Most solids sink because they are more dense than the saturated liquid with which they exist at equilibrium.
Baratuci
ChemE 260
March 30, 2005

8. PT Diagram for Substances That Expand on Freezing
Red lines are saturation curves
Consider an increase in P at constant T for a system on a Saturation Curve
VLE: Vapor condenses into a liquid
Conclusion: liquid is more dense than vapor
SVE: Vapor desublimates into a solid
Conclusion: solid is more dense than vapor
SLE: Solid melts into a liquid
Conclusion: liquid is more dense than solid
The solid is less dense than the liquid because this substance expands on freezing
Baratuci
ChemE 260
March 30, 2005

9. PT Diagram for Substances That Contract on Freezing
SLE: Liquid fuses into a solid
Conclusion: Solid is more dense than solid
The solid is more dense than the liquid because this substance contracts on freezing
Baratuci
ChemE 260
March 30, 2005

10. Next Class Phase Equilibrium and Thermodynamic Data Tables
Properties of Saturated Vapor and Liquid
Properties of Subcooled Liquids
Properties of Superheated Vapors
CB: Steam, R-134a
I will provide you with the ammonia tables if and when you need them
Thermo-CD: Steam, R-134a, Ammonia
For many important substances, vast amounts of property data are available for vapors, liquids and equilibrium mixtures.
Next class we will learn where to find these data tables andhow to use them.
Baratuci
ChemE 260
March 30, 2005