A 0.8 m3 rigid tank contains carbon dioxide gas at 250?K and 100 kPa. A 500-W el

Question

A 0.8 m3 rigid tank contains carbon dioxide gas at 250?K and 100 kPa. A 500-W electric
resistance heater placed in the tank is now turned on and kept on for 40 min after which thepressure of CO2 is measured to be 175 kPa. Assuming the surrounding to be at 300 K and usingconstant specific heats determine: a) the final temperature of CO2,(437 ?K) b) the net amount of heat transfer from the tank (976 kJ), c) entropy generation for the process. (3.92 kJ/K)

I am unable to get any of the following answers to the sub-questions.

Explanation / Answer

Approximating the gas as ideal gas and noting that the container is rigid=> constant volume process;

P1T2=P2T1;

T2= 175*250/100; = 437.5 K ---------------->ANSWER;

calculating mass of co2 m= PV/RT * M (molecular mass of co2 is 44)

100000*0.8*44/8.314*250 =1.693 Kg;

Win (electrical) = 500*2400(sec) = 1200 KJ;

Applying First Law ; Ein- Eout =Esys;

Win(electrical)- Qout = U = m.Cv.T =>

1.693*0.7056*187.5= 1200- Qout ; So Qout=976.1067 KJ-----------------------.>ANSWER

For entropy change, s= Cvln(Tf/Ti) +R ln (Vf/Vi) KJ/kg-k;

as final and initial volume are same , second term becomes zero;

s= 0.7056*ln(437.5/250)* 1.693= 0.6685 KJ/K ;------------->ANSWER;

(the answer u have given will only be possible if cv >1 , which not possible at that temperature..so that answer might be wrong)

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