Butane (C4H10) is burned with excess air. The exhaust gas from the combustion is analyzed and has the following composition: 4.5 mole% O2, 73.8 mole% N2, 11.824% H2O, 0.236 mole% C4H10, 8.48 mole% CO2, 1.16 mole% CO. Determine: 1. (8 points) The fractional conversion of C4H10 2. (8 points) The ratio of exhaust produced to C4H10 fed 3. (9 points) The percent excess air fed to the process. Draw a flow chart.
The reaction is C4H10+6.5O2----> 4CO2+ 5H2O
Basis : 100 moles of exhaust gases
Exhaust gases contains 73.8 moles and comes from air supplied for combustion. Nitrogen at the inlet= 73.8 moles
79 moles of nitrogen is there in 100 moles of air
73.8 moles of nitrogen is there in 100*73.8/79=93.42 moles of air supplied
air contains 21% oxygen, hence oxygen actually supplied = 93.42*0.21=19.62 moles
moles un reacted = 4.5 moles
Oxygen reacted = 19.62-4.5 moles = 15.12 moles
air supplied = (15.12/0.21)= 72
C4H10 to be reacted = 15.12/6.5 ( 1 mole of C4H10 requires 6.5 moles of oxygen)=2.32
C4H10 remaining =0.236
C4H10 reacted= 2.32-0.236=2.084 moles
Fractional conversion =100*2.084/2.32=89.8%
C4H10 fed /Exhaust= 2.32/100 =0.0232
Excess air : (Excess air/ Stoichiometric air)*100= 100*(93.42-72)/72=29.75%
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