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Understanding Heat Capacities and Phase Changes in Chemical Engineering

Learn about heat capacity, specific heat, phase changes, ideal gases, and hypothetical process paths in chemical engineering. Explore the significance of heat capacity and phase changes in various substances.

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Understanding Heat Capacities and Phase Changes in Chemical Engineering

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  1. ChemE 260 Heat Capacities, Phase Changes &Hypothetical Process Paths Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 3: C, D & ECB 2: 11 + Supplement April 6, 2005

  2. Heat Capacity • Definition • Amount of energy (J, Btu) that must be added to 1 mole or lbmole of a substance to increase its temperature by 1 degree ( oC , K, oF or oR ). • Units: J/mole-K Btu/lbmole-oF • Specific Heat • Amount of energy (J, Btu) that must be added to 1 gram , kilogram or lbm of a substance to increase its temperature by 1 degree ( oC , K, oF or oR ). • Units: kJ/kg-K Btu/lbm-oF • Constant Pressure Heat Capacity / Specific Heat: / • Definition: • Constant Volume Heat Capacity / Specific Heat: / • Definition: • Heat Capacity Ratio : Baratuci ChemE 260 April 6, 2005

  3. Why is Heat Capacity Useful ? • Isobaric Processes • Isochoric Processes Baratuci ChemE 260 April 6, 2005

  4. Ideal Gases • is a function of T only, not P • is a function of T only, not P • Therefore: are valid for ALL processes for ideal gases • IG Heat Capacities: • NIST: Shomate Equation:where: Baratuci ChemE 260 April 6, 2005

  5. Solids & Incompressible Liquids •  constant : • is very small for most solids and many liquids(from from the critical point). • Therefore: Baratuci ChemE 260 April 6, 2005

  6. Gibbs Phase Rule • oFreedom = C – P + 2 • oFree = number of intensive variables you can independently specify. • C = number of chemical species in the system • P = number of phases within the system. • Consider a pure substance in a single phase • C = 1, P = 1 … oFree = 2 • Therefore, we must specify both T & P to completely determine the state of subcooled liquids and superheated vapors ! Baratuci ChemE 260 April 6, 2005

  7. Hypothetical Process Paths • Actual Process Path • The series of states that a system actually passes through during a process • Hypothetical Process Path • A convenient path connecting the initial and final states of a process that makes it easy to evaluate changes in state variables that occur during the process • Goal: Construct an HPP in which just T or P changes or the phase changes in each step that makes up the path. • Good news: • We will use thermodynamic tables (only) for real gases in this course • We will use the IG heat capacity (Shomate Eqn) for ideal gases • If a liquid is not in the thermodynamic tables in the NIST Webbook, we will consider it to be incompressible • We will consider all solids to be incompressible Baratuci ChemE 260 April 6, 2005

  8. Phase Changes • Latent Heat of Vaporization: • Best choice is to look up the value • Estimate the value • Clausius-Clapeyron Equation: • Vapor Pressure • Best choice is to look up the value • Estimate the value • Antoine Equation • Get A, B & C for each chemical from NIST Baratuci ChemE 260 April 6, 2005

  9. Clausius-Clapeyron Plot Baratuci ChemE 260 April 6, 2005

  10. Next Class • Problem Session • After that… • Work • Heat • TCD Ch 4: A & B • CB Ch 3: 1 – 4+p150-155 Baratuci ChemE 260 April 6, 2005

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