Question

Methane is burned to complete combustion with 30% excess air.

The air enters at 25oC and 1 atm (absolute) at 50%
relative humidity.
a) Calculate the flow of O2 per mole of methane
entering the process.
b) Calculate the flow of N2 per mole of methane
entering the process.
c) Calculate the mole fraction of H2O in the humid
air stream entering the process.
d) Calculate the flow of H2O per mole of methane
entering the process.

Answer 1

a) 2.6 mole of O2 per mole of methane

b) 9.78 mole of N2 per mole of methane

c) Mole fraction of H2O = 0.33

d) 6.12 mole of H2O pero mole of methane

Explanation:

The first thing you have to do is the balanced reaction of the methane combustion:

CH4 +2 O2 -->2 H2O + CO2

You calculate the moles of O2 needed to react with 1 mole of methane and add the excess factor

2 x 1.3 = 2.6 moles of O2

For the N2 moles you calculate it with the air composition (21 % O2, 79% N2)

2.6 moles of O2 * 0.79/0.21 = 9.78 moles of N2

With the total stream of air = 12.38 moles you add the humidity factor

12.38 * 1.5 = 18.57 moles So 6.12 are moles of H20 entering per mole of CH4.

To calculate the mole fraction yo divide the moles of water among the moles of the stream:

6.12/18.57 --> 0.33