Question

Methane is a clean gaseous fuel used in chemical industries worldwide. 1000 kmol/hr of methane is combusted in a burner to provide heat. The combustion reaction is:

CH4 + O2 à CO2 + H2O

Air is used for combustion and it can be assumed to be composed of 20% oxygen and 80% nitrogen. Nitrogen is considered inert and does not undergo any oxidation reaction.

The inlet pressure for all stream is atmospheric.

Use a conversion reactor in VMGSim simulation package to simulate burner. Assume that the heat loss to surrounding is negligible (very well insulated burner). Pressure drop across the burner is assumed to be 10 kPa. Assume adiabatic conditions with methane conversion of 95%. Follow the simulation guideline (to be provided) and having converged the simulation, export the results to WORD. The followings can be easily determined from the results:

1- Properties of exhaust gases such as average specific heat, enthalpy, entropy, thermal conductivity, viscosity etc.

2- Composition of exhaust gases from the burner in mole% and mass%

3- The flow rate of each component in the exhaust gas in Kg/h

4- Temperature of exhaust gases

5- Pressure of exhaust gases

6- Volumetric flow rate of gases

Now repeat the simulation assuming the you use 50% excess air. You need to increase the mole fraction in the inlet stream to burner. Keeping all other conditions the same, compare the results of the two simulation and answer the following questions in the report. You may add these to the report (exported to WORD).

1- Is the final temperature lower or higher? Why?

2- Is there a change in reaction enthalpy? Explain.

3- Is there any change in specific heats?

Answer 1

Given data:

Methane flowrate = 1000 kmol/hr

Air inlet composition, N2 = 80% and O2 = 20%

Total conversion percentage = 95%

Total del.p = 10 kpa

Pressure = 1 atm.

Assumption for the simulation:

Inlet temperature = 25 ° C

Let us write down the balanced reaction,

CH4 + 2 (O2 + 4N2) = CO2 + 2H2O (Since, Air contains 20 % O2 and 80% N2, so 20/20 O2 and 80/20 N2)

CH4 + 10 Air = CO2 + 2H2O

therefore, 1 kmol of methane requires 10 kmol of air for the reaction to complete with 100 % conversion, so here only 95% conversion, thus the required mole flow of air = 1000 * 9.5 = 9500 kmol of air.

The results I got from the simulation is,

Results:

1. Properties of exhaust gases such as average specific heat, enthalpy, entropy, thermal conductivity, viscosity etc.

Mass specific heat = 1.682 kJ/kg °C

Mass enthalpy =-258.8 kJ/kg

mass entropy = 8.484 kJ/kg °C

thermal conductivity = 0.07906 W/m K

viscosity = 0.07944 cP

2- Composition of exhaust gases from the burner in mole% and mass% =

mole%

CH4 =0.48 %

O2 = 0%

H2O = 18.1%

CO2 = 9.05%

N2 = 72.38%

Mass %:

CH4 =0.28 %

O2 = 0%

H2O = 11.81%

CO2 = 14.43%

N2 = 73.48%

3- The flow rate of each component in the exhaust gas in Kg/h

Methane   802.145 kg/h

Nitrogen   212898.80 kg/h

Oxygen   0 kg/h

CO2   41809.21 kg/h

H2O   34228.69 kg/h

4- Temperature of exhaust gases

Temperature = 1943 ° C

5- Pressure of exhaust gases

Pressure = 0.9133 bar

6- Volumetric flow rate of gases

Volume flow of gase = 2118495.88 m3/hr