Page 90 - Coulson Richardson's Chemical Engineering Vol.6 Chemical Engineering Design 4th Edition

FUNDAMENTALS OF ENERGY BALANCES
From a plot of the integral heats of solution versus concentration,
H Ž soln 2.22 mol/mol D 27.0kJ/molNaOH 73
42.2 mol/mol D 42.9kJ/molNaOH
Heat liberated in the dilution per mol NaOH

D 42.9 27.0 D 15.9kJ
Heat released per batch D mol NaOH per batch ð 15.9

3
2500 ð 10 ð 0.05
3
D ð 15.9 D 49.7 ð 10 kJ
40
Heat transferred to cooling water, neglecting heat losses,
49.7MJper batch

In Example 3.5 the temperature of the feeds and final solution have been taken as the
Ž
same as the standard temperature for the heat of solution, 25 C, to simplify the calculation.
Heats of solution are analogous to heats of reaction, and examples of heat balances on
processes where the temperatures are different from the standard temperature are given
in the discussion of heats of reaction, Section 3.10.

3.9. ENTHALPY-CONCENTRATION DIAGRAMS
The variation of enthalpy for binary mixtures is conveniently represented on a diagram.
An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of
ammonia and water versus concentration; with pressure and temperature as parameters.
It covers the phase changes from solid to liquid to vapour, and the enthalpy values given
include the latent heats for the phase transitions.
The enthalpy is per kg of the mixture (ammonia C water)
Ž
Reference states: enthalpy ammonia at 77 C D zero
Ž
enthalpy water at 0 C D zero
Enthalpy-concentration diagrams greatly facilitate the calculation of energy balances
involving concentration and phase changes; this is illustrated in Example 3.6.

Example 3.6
Ž
Calculate the maximum temperature when liquid ammonia at 40 C is dissolved in water
Ž
at 20 C to form a 10 per cent solution.

Solution
The maximum temperature will occur if there are no heat losses (adiabatic process). As
no heat or material is removed, the problem can be solved graphically in the enthalpy-
concentration diagram (Figure 3.3). The mixing operation is represented on the diagram