12. Theoretical and excess air
If two reactants are needed for a
reaction the less expensive reactant is fed into system at excess to convert
the total of the expensive reactant. An example of an inexpensive reactant can
be air, which is free. Combustion reactions are run with more air than is
needed to supply oxygen in the right proportion to the fuel.
Theoretical Oxygen: Moles(batch process) or molar flow
rate(continues process) of Oxygen is needed for complete combustion of all fuel
that is fed to the reactor. It could be assumed that all carbon in the fuel can
be oxidized to CO₂ and hydrogen to H₂O.
Theoretical air: Theoretical oxygen
quantity that is contained in the air.
Excess Air: Theoretical air amount that
is exceeded by the amount of air fed
Percent Excess air Formula: [(moles air)fed – (Moles air)theoretical / (moles
air) theoretical] x 100%
It is useful to know the fuel feed rate
and stoichiometry in the equations to help with calculations like theoretical O₂
and air feed rates. If the actual feed rate of air is known it is possible to
calculate percent excess air.
13. Material Balances on Combustion Reactors
Solving material balances for combustion
reactors is the same as for any reactive system but have a few points to keep
in mind like:
1. After drawing flow chart your outlet stream must contain
i)
Unreacted fuel(unless told
otherwise)
ii) Unreacted oxygen
iii) Water and carbon dioxide(also carbon monoxide if said is present by
problem)
iv) Nitrogen since air is not pure
2. Calculate oxygen feed rate from the specified percent oxygen or
percent excess air. Calculate theoretical O₂ from fuel feed rate and the reaction stoichiometry for complete
combustion then calculate oxygen feed rate by multiplying the theoretical
oxygen
3. If one reaction is involved then the three methods of balancing
(molecular, atomic and extent of reaction) are equal. If several reactions
occur at the same tome atomic species are more convenient.
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Some additional Considerations about
chemical processes (what you do not learn out of textbooks)
1. Processes do not always work as designed
2. Variables measured to precision in field measurements have errors
3. Humans make mistakes that influence the process outcome
4. You will not always have all the data you need and may have to make
assumptions
5. In textbooks material balances have a closure of 100%, in practice
there is no such thing as a true steady state
6. Textbooks have only one correct answer whilst real problems may have
the possibility where you can find a variety of solutions.
Reasons for differences between design
values and experimental values:
1. Human errors and data scatter
2. Impurities in feed
3. Incorrect assumptions of steady state
4. Incorrect assumptions that MEK is not reactive.
5. Errors due to approximation in the experimental data analysis
6. Approximations in the design analysis
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