Propane is burned completely with excess oxygen. The product gas contains 24.8 m

Question

Propane is burned completely with excess oxygen. The product gas contains 24.8 mole% CO2, 6.12% CO, 40.8% H2O, and 28.6% O2.

a) Calculate the percentage excess O2 fed to the furnace.

b) A student wrote the stiochemtric equation of the combustion of propane to form CO2 and Co as

2C3H8 + 17/2O2 ---------> 3CO2 + 3CO +8H20

According to this equation, CO2 and CO should be a ratio of 1/1 in the reaction products, but in the product gas of part (a) they are in a ratio of 24.8/6.12. Is the result possible? (Hint:Yes) Explain how.

Explanation / Answer

C3H8 + O2 --> CO2 + H2O

balance equation

C3H8 + 5O2 --> 3CO2 + 4H2O

Now calculate theoretical oxygen

Assume 100 mol of outlet gas

24.8 mol of CO2, 6.12 mol of CO, 40.8 mol of H2O and 28.6 mol of O2

Total mol of C = 24.8+6.12 = 30.92 mol of C

Since 3 mol of C are found in 1 mol of propane

30.92/3 = 10.31 mol of C3H8 were initially

Now, theoretical air is given as

1 mol of propnae : 5 mol of oxygen

therefore 10.31 * 5 = 51.53 mol of O2 are needed (theoretical)

NOTE that we need to know how much O2 is left after CO is combusted completely (because the definition is based on 100% combusition)

Therefore

CO+1/2O2 --> CO2

6.12 mol of CO will require 1/2*6.12 = 3.06 mol

So actually 28.6 mol of O2 will not be the excess, but 28.6-3.06 = 25.54 mol of O2 will be in theory left.

Then, calculate % exces

% excess = (O2 input - O2 required ) / O2 required * 100%

% excess = 25.54/51.53*100 = 49.5 or 50% excess

b)

This is actually not a good idea, there is taking place two reactions

Full combustion:

C3H8 + 5O2 --> 3CO2 + 4H2O

And incomplete combstion

C3H8 + 7/2O2 --> 3CO + 4H2O

If you force the first equation, you will have a ratio of 3:3 always, which is a very unique case in which 50% of the reactant reacts to full combustion and 50% of the reactant goes to an incomplete combustion.

The most common case is to have a 90% - 10% ratio or something like that. This is still posible, but is very extreme case.

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