Example 3.3: It is necessary that complete combustion of propane yield a temperature of 1600 °C. The propane supply temperature is 25 °C. Determine the air-to-fuel ratio on a molar basis that must be supplied to the burner to attain the desired temperature. Heat of combustion of propane is 530605.6 cal/mole. Latent heat of vaporization of water at 25 °C is 10500 cal/mole. Mean molal heat capacity of the compounds between 25 °C and 1600 °C is given as below:
C3H8 = 6.601 cal/(mol·°C), O2 = 8.269 cal/(mol·°C), CO2 = 12.75 cal/(mol·°C),
N2 = 7.844 cal/(mol·°C), H2O = 9.95 cal/(mol·°C), air = 7.929 cal/(mol·°C)
Solution: Basis of calculations: 1 mol of gas,
Feed temperature, TFeed = 298.15 °K,
Product temperature, TProducts = 1873.15 °K.
Enthalpy of feed, SHFeed = 0
Heat added to the system, q = 0
Standard heat of reaction, ΔH298
ΔH298 = -530605.6 + 4(10500) = -488605.6 cal
Enthalpy of products, SHP = SHF - ΔH298 - q = SnpCpm(TP - TF) = 488605.6 cal
Excess air, X
(225.67 + 7.929X)(1873.15 - 298.15) = 488605.6
X = 10.664 mol
Stoichiometric air = 5(4.764) = 23.82 mol
Total air requirement = 10.66 + 23.82 = 34.48 mol