Question

4 kg of carbon dioxide at 40°C and 1.4 bar are mixed with 8 kg of nitrogen at 160°C and 1.0 bar to form a mixture at a final pressure of 0.7 bar. The process occurs adiabatically in a steady flow apparatus. Calculate : 

(i) The final temperature of the mixture ; 

(ii) The change in entropy. 

Take value of c_p : for CO₂ = 0.85 kJ/kg K and N₂ = 1.04 kJ/kg K.

Answer 1

(i) Final temperature, T₂ : 

In this process, W = 0, Q = 0 

∴ The steady flow equation may be written as

H₁ = H₂

(ii) Change in entropy :

nCO₂ = \(\cfrac4{44}\) = 0.0909

nN₂ = \(\cfrac8{28}\) = 0.2857

n = nCO₂ + nN₂  = 0.0909 + 0.2857 = 0.3766

\(\cfrac{p(CO_2)_2}{p_2}\) = xCO₂

[p₂ = pressure of the mixture]

∴ Change in entropy, ∆S

= 4(0.2046 + 0.3984) + 8(– 0.0871 + 0.1885) = 3.2232 kJ/K 

Change in entropy = 3.2232 kJ/K.