An evaporator is used to concentrate 12 000 kg/h of a 15% solution of NaOH entering at 60oC to a product of 50% solid. The pressure of the saturated steam used is 300 kPa and the pressure in the vapour space of the evaporator is 11.7 kPa. The overall heat transfer coefficient is 1 560 W/m2K. Calculate the steam used and the steam economy in kg vaporised/kg steam.
F = 4536 kg/h
TF = 60 ºC
xF = 0.2
S = ?
For the total balance,
For the balance on the solute alone,
L = 1814 kg/h of liquid
Substituting into total balance and solving,
V = 2722 kg/h of vapor
To determine T1 = Tsat + BPR of the 50 % concentrate product, first we obtain Tsat of pure water from steam table. At 11.7 kPa,
Tsat = 48.9 ºC.