Question

The pressure of an evaporator is 25.6 Kpa (3.72 psia) and a solution of 30% NaOH is being boiled. Determine the boiling temperature of the NaOH solution and the boiling point rise (BPR) of the solution over that of water at the same pressure.

Answer 1

Solution:

From the steam tables in Appendix A.2, the boiling point of water at 25.6 kPa is 65.6 °C. (Please check the steam table)

From Fig. 8.4-2 for 65.6 °C (150 °F) and 30% NaOH, the boiling point of the NaOH solution is 79.5 °C (175 °F).

The boiling-point rise is the temperature that is rised above the water at same conditions,

   i.e.    79.5 - 65.6 = 13.9 °C (25°F).FIGURE 8.4-2. Duhring lines for aqueous solutions of sodium hydroxide.