Page 62 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

FUNDAMENTALS OF MATERIAL BALANCES
Solution
Normal carbon dioxide content of air is 0.03 per cent 45
CO 10kg/h
2
air air
0.03 per cent CO 2 0.45 per cent CO 2
Basis: kmol/h, as percentages are by volume.
10
kmol/h CO 2 introduced D D 0.2273
44
Let X be the air flow.
Balance on CO 2 , the tie component
CO 2 in D 0.0003 X C 0.2273
CO 2 out D 0.0045 X
X 0.0045 0.0003 D 0.2273
X D 0.2273/0.0042 D 54 kmol/h
D 54 ð 29 D 1560 kg/h

Example 2.8
In a test on a furnace fired with natural gas (composition 95 per cent methane, 5 per
cent nitrogen) the following flue gas analysis was obtained: carbon dioxide 9.1 per cent,
carbon monoxide 0.2 per cent, oxygen 4.6 per cent, nitrogen 86.1 per cent, all percentages
by volume.
Calculate the percentage excess air flow (percentage above stoichiometric).

Solution

Reaction: CH 4 C 2O 2 ! CO 2 C 2H 2 O
Note: the flue gas analysis is reported on the dry basis, any water formed having been
condensed out.
Nitrogen is the tie component.
Basis: 100 mol, dry flue gas; as the analysis of the flue gas is known, the mols of each
element in the flue gas (flow out) can be easily calculated and related to the flow into the
system.
Let the quantity of fuel (natural gas) per 100 mol dry flue gas be X.
Balance on carbon, mols in fuel D mols in flue gas
0.95 X D 9.1 C 0.2, hence X D 9.79 mol
Balance on nitrogen (composition of air O 2 21 per cent, N 2 79 per cent).
Let Y be the flow of air per 100 mol dry flue gas.
N 2 in air C N 2 in fuel D N 2 in flue gas
0.79 Y C 0.05 ð 9.79 D 86.1, hence Y D 108.4mol