Page 63 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

46
CHEMICAL ENGINEERING
Stoichiometric air; from the reaction equation 1 mol methane requires 2 mol oxygen,
100
so, stoichiometric air D 9.79 ð 0.95 ð 2 ð D 88.6mol
21
air supplied stoichiometric air
Percentage excess air D ð 100
stoichiometric air
108.4 88.6
D D 22 per cent
88.6

2.12. EXCESS REAGENT
In industrial reactions the components are seldom fed to the reactor in exact stoichiometric
proportions. A reagent may be supplied in excess to promote the desired reaction; to
maximise the use of an expensive reagent; or to ensure complete reaction of a reagent,
as in combustion.
The percentage excess reagent is defined by the following equation:

quantity supplied stoichiometric
Per cent excess D ð 100 2.7
stoichiometric quantity

It is necessary to state clearly to which reagent the excess refers. This is often termed the
limiting reagent.

Example 2.9

To ensure complete combustion, 20 per cent excess air is supplied to a furnace burning
natural gas. The gas composition (by volume) is methane 95 per cent, ethane 5 per cent.
Calculate the mols of air required per mol of fuel.

Solution

Basis: 100 mol gas, as the analysis is volume percentage.

Reactions: CH 4 C 2O 2 ! CO 2 C 2H 2 O
1
C 2 H 6 C 3 O 2 ! 2CO 2 C 3H 2 O
2
1
Stoichiometric mols O 2 required D 95 ð 2 C 5 ð 3 D 207.5
2
120
With 20 per cent excess, mols O 2 required D 207.5 ð D 249
100
100
Mols air (21 per cent O 2 ) D 249 ð D 1185.7
21
1185.7
Air per mol fuel D D 11.86 mol
100