Properties of Pure Substances - ABE 210 Lecture 3

Objectives

• Introduce the concept of a pure substance.
• Illustrate the phase-change processes.
• Discuss the P-v, T-v, and P-T property diagrams and P-v-T surfaces of pure substances.
• Evaluate thermodynamic properties of pure substances from property tables.

Pure Substances

• Definition: A pure substance is any material with a fixed chemical composition throughout.
• Examples:
  • Homogeneous mixtures like air (a mixture of various gases) are considered pure substances.

Phases of a Pure Substance

• States of Matter: Three primary phases of a pure substance are:
  • Solid
  • Liquid
  • Gas

Phase Diagrams

• p-v-T Surface:
  • Displays the relationship between pressure (P), specific volume (v), and temperature (T) for a pure substance.
  • Different states of matter are visible in the diagrams:
  • Solid and Liquid
  • Liquid and Vapor
  • Solid and Vapor
  • Triple State (where all three phases coexist)
  • Critical State: Is a unique phase where properties of the liquid and gas merge, seen at the critical temperature.

Property Diagrams Explained

• T-v Diagram:

  • Illustrates temperature (T) vs. specific volume (v).
  • States such as saturated liquid and saturated vapor are indicated along with heat addition.

• P-v Diagram:

  • Shows how pressure (P) relates to specific volume (v).
  • Contains regions for:
  • Saturated Liquid
  • Saturated Vapor
  • Superheated vapor

Thermodynamic Properties and Fluid Tables

• Fluid property tables provide essential data for evaluating properties of pure substances:
  • Specific Volume (v)
  • Internal Energy (u)
  • Enthalpy (h)
  • Entropy (s)

Example Problems

• Example A:

  • A reservoir holds 10 kg of saturated vapor water at 80 °C. Find the pressure and tank volume using fluid property tables.

• Example B:

  • Saturated water vapor (m = 1 kg) is condensed at 44 kPa. Determine the volume change and energy transferred.

Saturated Liquid-Vapor Mixture

• Volume Relationships:
  • Total Volume: V = Vf + Vg where (Vf) is the volume of the saturated liquid and (Vg) is the volume of the saturated vapor.
  • The quality (x), defined as the ratio of the mass of vapor to the total mass, can be calculated as: x = \frac{m_g}{m}
  • The specific volume can be calculated as: v = Vf + x(vg - v_f)

Example Calculation

• Example: For 2 kg of refrigerant 134a in a 100-L tank at $P = 200$ kPa, determine temperature, quality, enthalpy, and vapor volume.

Summary of Property Determination

• For thermodynamic analysis:
  • Identify which two independent properties are known. These could include pressure (P), temperature (T), specific volume (v), internal energy (u), enthalpy (h), entropy (s), or quality (x).
  • Depending on the known properties, follow the appropriate tables and calculations to find other desired properties.