ChemE 260 Transient Mass and Energy Balances

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ChemE 260 Transient Mass and Energy Balances

• Dr. William Baratuci
• Senior Lecturer
• Chemical Engineering Department
• University of Washington
• TCD 5 D ECB 4 5

April 20, 2005
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Transient Processes

• If ANY variable associated with the process
  changes over time, then it is a transient
  process.
• Transient processes are very complex and a
  computer is often required to solve the equations
  that describe these processes.
• We will only consider special transient processes
  in which the following assumptions are true.
• Uniform Flow
• The properties and flow rates of all inlet and
  outlet streams are constant or uniform over the
  cross-sectional area for flow and are also
  constant with respect to time.
• Uniform State
• The state of the mass within the system is
  uniform.
• At all times, the properties of the outlet stream
  are exactly the same as the properties of the
  system at that time.
• The properties of the system can change with
  time, but they are always the same as the
  properties of the outlet stream

Baratuci ChemE 260 April 20, 2005
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Transient Mass Balances

• DifferentialMass Balance
• Integrate
• Uniform Flow
• MIMO

Baratuci ChemE 260 April 20, 2005
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Integrating the 1st Law

• The 1st Law for Open Systems

• Ws is work other than flow work, usually shaft
  work.

• Integrate from t1 to t2

Baratuci ChemE 260 April 20, 2005
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Uniform Flow and the 1st Law

• Uniform Flow Assumption

Baratuci ChemE 260 April 20, 2005
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Uniform State and the 1st Law

• Uniform State Assumption

• If changes in kinetic and potential energies are
  negligible

Baratuci ChemE 260 April 20, 2005
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Transient 1st Law for MIMO Processes

• SISO

• MIMO

• If changes in kinetic and potential energies are
  negligible

Baratuci ChemE 260 April 20, 2005
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Next Class

• Problem Session
• After that
• TCD 6 A B and CB 5 1 - 3
• Introduction to the 2nd Law of Thermodynamics
• Heat Engines Thermal Reservoirs

Baratuci ChemE 260 April 20, 2005
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Example Problem

• Steam at a pressure of 1.4 Mpa and 300oC is
  flowing through a pipe. Connected to this pipe
  through a valve is an evacuated tank. The valve
  is opened and the tank fills with steam until the
  pressure in the tank is also 1.4 Mpa. Then, the
  valve is closed. The process is adiabatic and
  changes in kinetic and potential energies are
  negligible. Determin the final temperature of
  the steam in the tank.

Baratuci ChemE 260 April 20, 2005