Page 58 - Chemical engineering design

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FUNDAMENTALS OF MATERIAL BALANCES
Physical systems, no reaction
If there is no chemical reaction the number of independent components is equal to the
number of distinct chemical species present.
Consider the production of a nitration acid by mixing 70 per cent nitric and 98 per cent
sulphuric acid. The number of distinct chemical species is 3; water, sulphuric acid, nitric
acid.
H SO /H O H O
2
2
4
2
Mixer HNO 3 Nitration
HNO /H O H SO 4 acid
2
3
2
Chemical systems, reaction
If the process involves chemical reaction the number of independent components will
not necessarily be equal to the number of chemical species, as some may be related by
the chemical equation. In this situation the number of independent components can be
calculated by the following relationship:
Number of independent components D Number of chemical species
Number of independent
chemical equations 2.2

Example 2.5
If nitration acid is made up using oleum in place of the 98 per cent sulphuric acid, there
will be four distinct chemical species: sulphuric acid, sulphur trioxide, nitric acid, water.
The sulphur trioxide will react with the water producing sulphuric acid so there are only
three independent components
Oleum
H SO /H O/SO 3 H O
2
2
4
2
HNO 3 Nitration
HNO /H O H SO 4 acid
2
2
3
Reaction equation SO 3 C H 2 O ! H 2 SO 4
No. of chemical species 4
No. of reactions 1
No. of independent equations 3
2.9. CONSTRAINTS ON FLOWS AND COMPOSITIONS

It is obvious, but worth emphasising, that the sum of the individual component flows
in any stream cannot exceed the total stream flow. Also, that the sum of the individual
molar or weight fractions must equal 1. Hence, the composition of a stream is completely
defined if all but one of the component concentrations are given.