Complete combustion of propane produces gases at 1600
degrees C. Propane is fed to the
combustion chamber at 25 degrees C.
Determine the amount of air that is provided to the burner. Mean molal heat capacity values between 25
and 1600 degrees C for propane, oxygen, nitrogen, carbon dioxide, water, and
air are provided.
Solution:
Let us consider one mole of propane as the basis of
calculations. If we write the
combustion reaction for propane, we can see that it requires 5 moles of oxygen. Thus producing 3 moles of carbon dioxide and
4 moles of water. Heat of reaction can
be calculated using heat of formation for the components involved. It is provided as negative 530,605.6
calories per mol. Remembering that
water would be in the vapor form at 1600 degree C, thus available heat of
reaction is reduced by latent heat of vaporization per mole of water
produced. Note that there are 4 moles
of water produced. Thus heat of
reaction is calculated to be negative 530,605.6 plus 4 times 10,500 equals
negative 488,605.6 calories.
If we use X moles (excess) of air to produce the desired
temperature, then feed comprises of 5 moles of oxygen, 5 times 3.76 moles of
nitrogen, and X moles of excess air.
The product comprises of 3 moles of carbon dioxide, 4 moles of water,
18.82 moles of nitrogen (5 times 3.76 moles of nitrogen), and X moles of excess
air. Weighted specific heat capacity is
equal to 225.67 plus 7.929X. Please
note that enthalpy of feed at 298.15 K is zero.
Now, we can use energy balance equation. Enthalpy of products is given as enthalpy of
feed minus heat of reaction minus q, where delta HR is negative for exothermic
reactions and q is heat lost to the environment.
Plugging these values, we can find that 10.664 moles of air
would be sufficient to produce the desired temperature of 1600 degrees C.
Total air that needs to be provided = excess amount plus
stoichiometric amount = 0,that is equal to 10.664 plus 5 times 4.764 equaling
to 34.484 moles. The factor 4.764
represent ratio of 1 mole of air to mole of oxygen = 1/21 = 4.764.