A gas, leaving an incinerator stack, contains 7 percent
oxygen by volume on wet basis. Toluene
is fed to the incinerator at a rate of 184 pounds per hour. Air is provided to burn this waste. How much excess air is being used under
these conditions? It is found that
carbon monoxide in the flue gas leaving the stack is 500 parts per million or
ppm. What is the combustion efficiency
under these conditions? If the oxygen
measurement was reported on dry basis, how much excess air was being fed to the
incinerator?
Solution:
Toluene is fed at 184 pounds or 2 moles per hour. Note that molecular weight of toluene is
72. If we write the combustion reaction
for toluene, we can see that for every mole of toluene 9 moles of oxygen are
required. In the combustion reaction 7
moles of oxygen and 4 moles of water are produced. If X is the amount of oxygen fed to the incinerator, then product
stream is found to be 4.764X plus 4.
This value is obtained by adding all components in the product stream.
It contains zero mole of toluene, X minus 18 moles of oxygen, 14 moles of
carbon dioxide, 8 moles of water, and 3.764 times X moles of nitrogen.
Now mole fraction of oxygen is ratio of moles of oxygen to
total moles in the product. X minus 18
over 4.764 times X plus 4 is equal to 0.07.
This can be solved to give X equal to 27.39 moles. Now one can easily find excess oxygen as
27.39 minus 18 over 18 that equals 0.522 or 52.2 percent.
Mole fraction of carbon dioxide can be seen to 0.104. But carbon monoxide is 500 ppm or 5 times 10
to the power minus 4. Combustion
efficiency is given conveniently by ratio of yCO2 minus yCO
and yCO2. This value is
found to be 0.9952 or 99.52 percent.
For second case, where the reporting was done on dry basis,
water is not included. So, the number
of moles in the product stream is reduced by amount of water, 8 moles in this
case.
If we do calculations similar to one for wet basis, then X
is found to be 26.56. Excess oxygen is
found to be 47.5 percent as compared to 52.2 percent (for wet basis). Finally, combustion efficiency is 99.56
percent as compared to 99.52 percent.