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.