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ChemE 260 Heat Capacities, Phase Changes & Hypothetical Process Paths

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

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ChemE 260 Heat Capacities, Phase Changes & Hypothetical Process Paths

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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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