A fuel mixture of 40% Ethyl Alcohol gas and 60% n-Butane gas. on a molar basis,
ID: 501968 • Letter: A
Question
A fuel mixture of 40% Ethyl Alcohol gas and 60% n-Butane gas. on a molar basis, at 25 degree C and 100 kPa is combusted with 400% theoretical air at 450 degree C and 100 kPa. For this steady-state process, the products of combustion exit the combustion chamber at 1500 degree C and 100 kPa. (a) Write the theoretical (or stoichiometric) chemical reaction, (b) Write the actual chemical reaction, (c) Determine the mole fraction of the actual products of combustion, and (d) Compute the heat transfer per kmole of fuel.Explanation / Answer
Answer for question (a):
Standard stoichiometric balanced equation for complete combustion of Ethyl alcohol and n-butane,
For Ethyl alcohol................CH3CH2OH + 3 O2 ------------------------------>2 CO2 + 3 H2O...........Eq. 1
For n-butane....................2 CH3CH2CH2CH3 + 13 O2 ------------------------> 8 CO2 + 10 H2O .........Eq.2
Answer for question (b):
From Eq. 1 we know that for complete combustion of mixture of 40% molar ethanol and 60% molar n-butane we need 120% molar oxygen (3 moles oxygen/mole of ethanol) for ethanol and from Eq. 2 we are aware that for complete combustion of 60% n-butane in this mixture it requires 390% molar oxygen (6.5 moles oxygen/mole of n-butane), In total for complete combustion of the above mixture we need 510% molar oxygen (120+390). However only 400% air (not even oxygen) has been used and this leads to incomplete combustion of both ethanol and n-butane. So following is the incomplete combustion products resulting from actual reaction and its balanced oxygen requirqment for both ethanol and n-butane.
For Ethyl alcohol................CH3CH2OH + 2 O2 ------------------------------>2 CO + 3 H2O...............Eq. 3
For n-butane....................2 CH3CH2CH2CH3 + 9 O2 ------------------------> 8 CO + 10 H2O .........Eq. 4
Answer for question (c):
From Eq. 3, molar fraction of incomplete combustion products from ethanol:
Mole fraction of CO: 40% (2/5 x 100)
Mole fraction of H2O: 60% (3/5 x 100)
From Eq. 4 molar fraction of incomplete combustion products from n-butane:
Mole fraction of CO: 44.44% (8/18 x 100)
Mole fraction of H2O: 55.55% (10/18 x 100)
In some instances during incomplete combustion carbon soot also forms if there's no sufficient oxygen in the system.
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