O’zbekiston respublikasi oliy ta’lim vazirligi toshkent kimyo texnologiya instituti

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 water if the solution is to be discharged at a temperature of 25°C. The temperature of the solutions fed to the vessel can be taken to be 25°C.

88 CHAPTER 3 FUNDAMENTALS OF ENERGY BALANCES

Solution
Integral heat of solution of NaOH H₂O, at 25°C

mols H2O/mol NaOH	2ΔHsoln 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:
50percentw=w¼50=1850=40¼2:22molH₂O=molNaOH
5percentw=w¼95=185=40¼42:2molH₂O=molNaOH
From a plot of the integral heats of solution versus concentration,
Δ^H°_soln2:22mol=mol¼27:0kJ=molNaOH
42:2mol=mol¼42:9kJ=molNaOH
Heat liberated in the dilution per mol NaOH
¼42:927:0¼15:9kJ

Heat released per batch ¼ mol NaOH per batch 15.9
2500
¼ 1040³ 0:0515:9¼49:710₃kJ
Heat transferred to cooling water, neglecting heat losses,
49:7 MJ per batch

In Example 3.5, the temperature of the feeds and final solution have been taken as the 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 wherethe temperatures are differentfromthe standard temperature are given in the discussion of heats of reaction, Section 3.10.