One kilogram of propane initially at 8 bar and 50oC undergoes a process to 3 bar

Question

One kilogram of propane initially at 8 bar and 50oC undergoes a process to 3 bar, 20oC while being rapidly expanded in a piston-cylinder assembly. Heat transfer between the propane and its surroundings occurs at an average temperature of 35oC. The work done by the propane is measured as 42.4 kJ. Kinetic and potential energy effects can be ignored. Determine whether it is possible for the work measurement to be correct. Problem 6.50, Chapter 6, Fundamentals of Engineering Thermodynamics, 7th Edition.

Explanation / Answer

W=42.4 kJ, P1=8, T1 = 50 P2=3, T2= 20 Tb = 35 degree C =35+273 =308 K an energy balance reduce to ?U=Q-W from table A-15 Q= m(u2-u1)+W = 1.0 kg(1364.13-1399.05) kJ/kg + 42.4 kJ =7.48 kJ an entropy balance take form ?S =Q/Tb +s s=m(s2 - s1) - Q/Tb from table A-15 s=1.0(5.7103-5.3986)- 7.48/Tb = 0.3117- 7.48/Tb for Tb greater than equal to 308 K, s>0 so the value of work is correct

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