Question

Fifty-thousand pounds per hour of a 20 wt% aqueous solution of NaOH at 120°F is to be fed to an evaporator operating at 3.7 psia, where the solution is concentrated to 40 wt% NaOH. The heating medium is saturated steam at a temperature 40°F higher than the exiting temperature of the caustic solution.

Determine: (each 10 Points) (a) Boiling-point elevation of the solution (b) Saturated-heating-steam temperature and pressure (c) Evaporation rate (d) Heat-transfer rate (e) Heating-steam flow rate (f) Efficiency (g) Heat-transfer area if U = 300 Btu/h-ft 2 - o F

Answer 1

Solution.

a) From the steam tables, at 3.7 psia saturated steam temperature is 122.3 °F. The boiling point of 40% sodium hyroxide is 166 °F. So, the boiling point elevation of the solution is 166-122 = 44 °F.

b) Saturated heating steam temperature is 166+40 = 206 °F, the pressure is 13 psia.

c) As the flow rate of NaOH is 0.2*50000 = 10000 lb/h. The rate of entering water is 50000-10000 = 40000 lb/h. The rate of water in the composition of concentrated NaOH is the evaporation rate is

40000-15000 = 25000 lb/h.

d) The heat transfer rate can be calculated using the enthalpy balance (steam tables for enthalpy used as a reference data):

e) The heating-steam flow rate is as follows:

f) The efficiency is 25000/29100 = 0.859 or 85.9 %;

g) The heat-transfer area is