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

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

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

Isobaric Heating Process Path on a TV Diagram 5 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

Isothermal Heating Process Path on a PV Diagram 1 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

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 0.612 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

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

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

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

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