
Concept explainers
a)
The total entropy change and exergy destruction associated with the process using ideal gas approximation.
a)

Answer to Problem 91RP
The entropy generated is
The energy destroyed is
Explanation of Solution
Write the entropy balance equation to obtain the expression of entropy generation in terms of
Here, mass of
Write the expression to obtain the energy destroyed during a process
Here, initial temperature is
Conclusion:
Refer Table A-2b, “Ideal gas specific heats of various common gases”, obtain the specific heat at constant pressure of
Obtain the value of
Here, constant pressure specific heat is
The partial pressure of
Substitute
Obtain the value of
The partial pressure of
Substitute
Substitute
Thus, the entropy generated is
Substitute 303 K for
Thus, the energy destroyed is
b)
The total entropy change and exergy destruction associated with the process using Kay’s rule.
b)

Answer to Problem 91RP
The entropy generation is
The energy destroyed is
Explanation of Solution
Write the expression to obtain the pseudo-critical temperature of the mixture
Here, mole fraction of
Write the expression to obtain the pseudo-critical pressure of the mixture
Here, critical pressure of
Write the expression to obtain the initial reduced temperature
Here, mixing critical temperature is
Write the expression to obtain the initial reduced pressure
Here, mixing critical pressure is
Write the expression to obtain the final reduced temperature
Write the expression to obtain the specific change in entropy of a system
Here, universal gas constant is
Write the expression to obtain the entropy generation
Write the expression to obtain the energy destroyed during a process
Here, initial temperature is
Conclusion:
From the Table of critical properties, obtain the critical temperature and pressure for
Substitute 0.25 for
Substitute 0.25 for
Substitute 180 K for
Substitute 8 MPa for
Refer Figure A-30, “Generalized entropy departure chart”, obtain the value of initial entropy departure
Substitute 210 K for
Refer Figure A-30, “Generalized entropy departure chart”, obtain the value of final entropy departure
Substitute
Substitute
Thus, the entropy generation is
Substitute 303 K for
Thus, the energy destroyed is
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Chapter 13 Solutions
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