An evaporator is fed continuously with 25 tonne per hour of a solution consistin
ID: 1941499 • Letter: A
Question
An evaporator is fed continuously with 25 tonne per hour of a solution consisting of 10% NaOH (sodium hydroxide), 10% NaCl (salt) and 80% water (by weight). During evaporation, water is boiled off and some of the salt precipitates as crystals. The precipitated salt crystals are settled and removed from the remaining liquor. The concentrated liquor leaving the evaporator contains 50% NaOH, 2% NaCl and 48% water.Using a basis of 25 tonne of solution (10% NaOH, 10% NaCl, 80% water) entering the evaporator per hour:
(ii) Calculate the amount of concentrated liquor produced per hour.
[3 marks]
(iii) Calculate the amount of water evaporated per hour.
[3 marks]
(iv) Calculate the amount of salt precipitated per hour.
[3 marks]
(v) If 100 tonne of solution enters the evaporator per hour, how much concentrated liquor is produced per hour?
Explanation / Answer
An evaporator is fed with10000 kg/hr of a solution containing 1% solute by weight. It is to be concentrated to 1.5% solute by weight. The feed is at a temperature of 37oC. The water is evaporated by heating with steam available at a pressure of 1.34 atm absolute, corresponding to a temperature of 108.3oC. The operating pressure in the vapor space is 1 atm absolute. Boiling point elevation and other effects can be neglected. The condensate leaves at the condensing temperature. All the physical properties of the solution may be taken to be same as that of water. What is the quantity of steam required per hour? What is the quantity of steam required per hour? Data: Enthalpy of feed = 38.1 kcal/kg Enthalpy of solution inside the evaporator (at 100oC) = 644 kcal/kg Enthalpy of vapor at 100oC = 644 kcal/kg Latent heat of vaporization of steam = 540 kcal/kg