Combustion Reactions (Thermochemistry)

Solution Preview

Please see the attached file.

Problems

1. Gaseous Propane at 25ºC is burned with moist air at 400 K in a steady state, steady flow process. The combustion process is adiabatic, and the exiting temperature is measured to 1200 K. A sample of the products is tested and found to have a dewpoint temperature of 70ºC. Determine the percentage of theoretical air used and the relative humidity of this air. Assume the combustion is complete and that the pressure is 100 KPa throughout the process.

2. A mixture of 80% ethane and 20% methane on a mole basis is throttled from10MPa, 65ºC, to 100 KPa and is fed to a combustion chamber where it undergoes complete combustion with air, which enters at 100 KPa , 600 K. The amount of air is such that the products of combustion exit at 100 KPa , 1200 K. Assuming that the combustion process is adiabatic and that all components behave as ideal gases except the fuel mixture, which behaves according to the generalized tables or charts, with pseudocritical constants, determine :
a) the percentage of theoretical air used in the process
b) the dew-point temperature of the products

Solution:

1) Notations:

Nx = number of kmols of component (x)
Mx = molar mass of component (x) (kg/kmol)
rx = the molar (volumetric) percentage of component "x" in a mixture
Q = molar heat exchange (kJ/kmol)
 = universal constant of gases,  = 8.314 kJ/kmol.K
Cp = molar specific heat at constant pressure (kJ/kmol.K)
cp = mass specific heat at constant pressure (kJ/kg.K)
 = air excess coefficient
 = volumetric ratio nitrogen/oxygen of air composition,  = 3.762
d = moisture content in the air, d = kg vapors / kg dry air
 = relative humidity of air (mixture)

The stoichiometric oxidation reaction of propane with dry air is:
( 1)
while the actual reaction with dry air in excess is
( 2)
Now, we consider a humid air coming into reaction. If (d) is the moisture content, we can ...