ChemE 260 Transient Mass and Energy Balances April 20, 2005 Dr. William Baratuci Senior Lecturer Chemical Engineering Department University of Washington TCD 5: D & E CB 4: 5

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

Transient Mass Balances Differential Mass Balance: –Integrate : –Uniform Flow: –MIMO : Baratuci ChemE 260 April 20, 2005

Integrating the 1 st Law Integrate from t 1 to t 2 : The 1 st Law for Open Systems: –W s is work other than flow work, usually shaft work. Baratuci ChemE 260 April 20, 2005

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

Uniform State and the 1 st Law Uniform State Assumption: If changes in kinetic and potential energies are negligible: Baratuci ChemE 260 April 20, 2005

Transient 1 st Law for MIMO Processes SISO: MIMO: If changes in kinetic and potential energies are negligible: Baratuci ChemE 260 April 20, 2005

Next Class … Problem Session After that… TCD 6: A & B and CB 5: –Introduction to the 2 nd Law of Thermodynamics –Heat Engines & Thermal Reservoirs Baratuci ChemE 260 April 20, 2005

Example Problem Steam at a pressure of 1.4 Mpa and 300 o C 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