Question

One kmol of gaseous octane (C8H18) is ignited with stoichiometric amount of dry air in a closed, rigid vessel. The pressure of the reactants is 1 atm and the temperature is 25C; while the temperature of the products is 727C. Assume “complete” combustion.

(a) Calculate the pressure (atm) of the products.

(b) Determine the heat transfer (kJ/kg fuel) during the reaction.

Answer: (a) 3.42 atm

Answer 1

The combustion of C8H18 is given as:

C8H18     +      x (O2 + 3.76 N2)           8 CO2    +    9H2O    +     3.76x N2

Balancing O2 on both sides:

x = 8 + 9/2

x = 12.5

Hence balanced combustion equation will be:

C8H18     +      12.5 O2 + 47 N2           8 CO2    +    9H2O    +     47 N2

(a). Total moles on left side = 1 + 12.5 + 47

= 60.5

Total moles on right side = 8 + 9 + 47

= 64

Temperature of reactants = 25 oC = 298 K

Temperature of products = 727 oC = 1000 K

Using, P1 / n1T1 = P2 / n2T2

1 / (60.5 * 298) = P2 / (64 * 1000)

P2 = (64 * 1000) / (60.5 * 298)

P2 = 3.55 atm

Hence, pressure (atm) of the products = 3.55 atm