Page 89 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

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CHEMICAL ENGINEERING
Heats of mixing and heats of solution are determined experimentally and are available
in the handbooks for the more commonly used solutions.
If no values are available, judgement must be used to decide if the heat of mixing for
the system is likely to be significant.
For organic solutions the heat of mixing is usually small compared with the other heat
quantities, and can usually be neglected when carrying out a heat balance to determine
the process heating or cooling requirements.
The heats of solution of organic and inorganic compounds in water can be large,
particularly for the strong mineral acids and alkalies.

3.8.1. Integral heats of solution

Heats of solution are dependent on concentration. The integral heat of solution at any
given concentration is the cumulative heat released, or absorbed, in preparing the solution
from pure solvent and solute. The integral heat of solution at infinite dilution is called
the standard integral heat of solution.
Tables of the integral heat of solution over a range of concentration, and plots of the
integral heat of solution as a function of concentration, are given in the handbooks for
many of the materials for which the heat of solution is likely to be significant in process
design calculations.
The integral heat of solution can be used to calculate the heating or cooling required
in the preparation of solutions, as illustrated in Example 3.5.

Example 3.5

A solution of NaOH in water is prepared by diluting a concentrated solution in an agitated,
jacketed, vessel. The strength of the concentrated solution is 50 per cent w/w and 2500 kg
of 5 per cent w/w solution is required per batch. Calculate the heat removed by the cooling
Ž
water if the solution is to be discharged at a temperature of 25 C. The temperature of the
Ž
solutions fed to the vessel can be taken to be 25 C.

Solution
Ž
Integral heat of solution of NaOH H 2 O, at 25 C
mols H 2 O/mol NaOH H Ž soln kJ/mol NaOH
2 22.9
4 34.4
5 37.7
10 42.5
infinite 42.9
Conversion of weight per cent to mol/mol:
50 per cent w/w D 50/18 ł 50/40 D 2.22 mol H 2 O/mol NaOH
5 per cent w/w D 95/18 ł 5/40 D 42.2mol H 2 O/mol NaOH