Page 52 - Chemical engineering design

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FUNDAMENTALS OF MATERIAL BALANCES
For a steady-state process the accumulation term will be zero. Except in nuclear processes,
mass is neither generated nor consumed; but if a chemical reaction takes place a particular
chemical species may be formed or consumed in the process. If there is no chemical
reaction the steady-state balance reduces to
Material out D Material in
A balance equation can be written for each separately identifiable species present, elements,
compounds or radicals; and for the total material.

Example 2.1

2000 kg of a 5 per cent slurry of calcium hydroxide in water is to be prepared by diluting
a 20 per cent slurry. Calculate the quantities required. The percentages are by weight.

Solution

Let the unknown quantities of the 20% slurry and water be X and Y respectively.
Material balance on Ca(OH) 2
In Out
20 5
X D 2000 ð a
100 100
Balance on water
100 20 100 5
X C Y D 2000 b
100 100
From equation a X D 500 kg.
Substituting into equation b gives Y D 1500 kg
Check material balance on total quantity:
X C Y D 2000

500 C 1500 D 2000, correct

2.4. UNITS USED TO EXPRESS COMPOSITIONS
When specifying a composition as a percentage it is important to state clearly the basis:
weight, molar or volume.
The abbreviations w/w and v/v are used to designate weight basis and volume basis.

Example 2.2

Technical grade hydrochloric acid has a strength of 28 per cent w/w, express this as a
mol fraction.