Department of Chemical Engineering
Separations Processes
FISA Prep
Evaporation Questions
Question 1
1.25 kg/s of a solution is concentrated from 10 to 50 per cent solids in a triple-effect evaporator
using steam at 393 K, and a vacuum such that the boiling point in the last effect is 325 K. If the
feed is initially at 297 K and backward feed is used, what is the steam consumption, the
temperature distribution in the system and the heat transfer area in each effect, each effect being
identical?
For the purpose of calculation, it may be assumed that the specific heat capacity is 4.18 kJ/kgK,
that there is no boiling point rise, and that the latent heat of vaporisation is constant at 2330 kJ/kg
over the temperature range in the system. The overall heat transfer coefficients may be taken as
2.5, 2.0 and 1.6 kW/m2 K in the first, second and third effects, respectively.
Question 2:
A double-effect forward-feed evaporator is required to give a product which contains 50 per cent
by mass of solids. Each effect has 10 m2 of heating surface and the heat transfer coefficients are
2.8 and 1.7 kW/m2 K in the first and second effects respectively. Dry and saturated steam is
available at 375 kN/m2 and the condenser operates at 13.5 kN/m2. The concentrated solution
exhibits a boiling-point rise of 3 deg K. What is the maximum permissible feed rate if the feed
contains 10 per cent solids and is at 310 K? The latent heat is 2330 kJ/kg and the specific heat
capacity is 4.18 kJ/kg under all the above conditions.
Ashleen Marshall
FISA Prep Evaporation 2021
Question 3
An evaporator, working at atmospheric pressure, is used to concentrate a solution from 5 per cent
to 20 per cent solids at the rate of 1.25 kg/s. The solution, which has a specific heat capacity of
4.18 kJ/kg K, is fed to the evaporator at 295 K and boils at 380 K. Dry saturated steam at 240
kN/m2 is fed to the calandria, and the condensate leaves at the temperature of the condensing
stream. If the heat transfer coefficient is 2.3 kW/m2 K, what is the required area of heat transfer
surface and how much steam is required? The latent heat of vaporisation of the solution may be
taken as being the same as that of water.
Ashleen Marshall
FISA Prep Evaporation 2021