10.3 Phase Changes and Latent Heat

Phase changes are crucial in chemical processes, involving energy absorption or release without temperature change. This concept, known as latent heat, plays a vital role in various industrial applications like refrigeration and steam generation.

Understanding how to calculate energy during phase changes is essential for engineers. The relationship between pressure and phase transitions, governed by the Clausius-Clapeyron equation, impacts process design and operation in numerous chemical engineering applications.

Phase Changes and Latent Heat

Latent heat in phase changes

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• Latent heat absorbs or releases energy during phase change without temperature change
• Types of latent heat encompass fusion for melting/freezing, vaporization for boiling/condensation, and sublimation for solid to gas transitions
• Energy storage during phase transitions enables temperature stabilization and facilitates heat transfer in industrial processes (refrigeration, steam generation)

Energy calculations for phase changes

• Energy calculation utilizes formula $Q = m \times L$ where Q represents energy (J), m denotes mass (kg), and L signifies specific latent heat (J/kg)
• Specific latent heat values vary across substances and phase changes, found in reference tables or handbooks
• Energy balance applications include heating/cooling processes with phase changes, distillation columns, and condensers/evaporators

Pressure effects on phase transitions

• Pressure-temperature relationship governed by Clausius-Clapeyron equation, visualized through phase diagrams
• Increased pressure raises boiling point while decreased pressure lowers it (pressure cookers, vacuum distillation)
• Melting point generally less affected by pressure changes than boiling point
• Most substances exhibit increased melting point with higher pressure, water being an exception with lowered melting point

Energy balances with phase changes

• Energy balance equation $\Delta H = Q - W$ incorporates sensible heat changes, latent heat changes, and work done by or on the system
• Process steps involve heating/cooling of single-phase systems, phase transitions, and mixing of streams with different phases
• Calculation methods employ enthalpy-concentration diagrams and steam tables for water/steam systems
• Common processes include distillation, evaporation, crystallization, and condensation