1. Evaporation
 Evaporation is the removal of solvent as a
vapor from a solution or slurry
 Evaporation is used for concentration of
aqueous solutions, it involves removal of
water from solution by boiling the liquor
in suitable vessel called evaporator and
withdrawing the vapor

2. Valuable Product
 In evaporation thick liquor is a valuable
product and vapor is usually wasted e.g.
concentration of fruit juices or urea
concentration
 In rare cases, vapor is valuable
product, e.g. Mineral-bearing water
evaporation to get boiler feed water, or
sea water evaporation to get portable
water

3. Evaporation vs
boiling/drying/distillation/crystilization
 Evaporation occurs at liquid-vapor
interface when vapor pressure is less than
saturation pressure of the liquid at a given
temperature
 Boiling occurs at solid-liquid interface
when liquid is brought into contact with a
surface at temp Ts

4. Cont……
 Evaporation differs from drying b/c
residue is always a liquid
 It differs from distillation b/c vapor is a
single component and even if it is a
mixture, no attempt is made to split it
into fractions
 It differs from crystallization b/c we want
to concentrate the solution rather than
forming and building crystals

5. Classification of liquids to be
Evaporated
A) Those which can be heated to high temp
without decomposition and those which
can only be heated to 330 K
B) Those which yield solids on
concentration and those which do not
C) Those which, at a given pressure, boil at
same temp as water, and those which
have much higher boiling point

6. Evaporator
 Exchanger which is used to concentrate a
solution consisting of a non-volatile solute
and a volatile solvent, is called an
evaporator
Evaporator Main Parts
 Feed Inlet, Concentrate outlet, Steam
jacket, steam condensate outlet, sight
glass, vapor head & vapor outlet, heating
section etc

7. Evaporator Types/Classification
 A) based on heating media

8. Cont…..
B) Based on Effect
1. Single effect evaporators
2. Multiple effect evaporators
C) Based on circulation of liquor over
heating surface
1. Natural Circulation Units
2. Forced Circulation Units
3. Film type (wiped film) evaporators

9. Different Evaporators

10. Batch Pan
 Oldest type and used for limited application
 Either jacketed/internal coils/heaters
 Product residence time is many hours
 Boil at low temp and high pressure for heat
sensitive materials
 heat transfer area and coefficients are low
under natural circulation
 Evaporation capacities are low
 Large temp differences not achieved b/c of
rapid fouling of heating surface

11. Cont…..
 Used for concentration of Jams and Jellies,
also for some pharmaceutical products

12. Natural Circulation Units
Circulation is obtained by convection
currents arising from heating surface
A) Tubes are horizontal with steam inside
B) Tubes are vertical with steam outside
a. solution to be evaporated boils outside
the tubes and steam condenses inside of
tubes. These tubes interfere with natural
circulation and minimize liquid agitation
Overall heat transfer coefficients are
lower

13. a. Tubes are horizontal with steam
inside

14. Cont…..
 Cylindrical section above heating portion
is used to separate vapor from liquid
 vapor leaves through de-entraining device
to prevent carry over of liquid droplets
with vapor
 Steam enters through one side chest and
leaves through opposite chest
 Steam condensate out via steam trap

15. Cont….
Advantages
1. Cheap
2. Easy to install
3. Require less space for installation
4. Suitable for liquids that not crystallize
5. Can be used for batch/continuous
operation
Disadvantages
1. Not suitable for viscous liquids b/c of
poor circulation

16. B) Tubes are vertical with steam
outside
 Solution boils inside vertical tubes with
heating media, usually steam, held in steam
chest, through which tubes pass.
 Boiling of liquid in tubes, causes liquid
flow upward through tubes and un-
evaporated liquid flows downward
through central hole

17. Cont…..

18. Cont…..
 These overcome disadvantages of horizontal tube
evaporators
1. natural circulation is promoted(1-3 ft/s)
2. heat transfer coefficients are higher
3. solid built inside tubes is removed by mechanical
cleaning
4.Viscous liquids can be used, but circulation is slow
5. Used in sugar and salt industries
 These are impractical when solution is very
viscous or form foams or is heat sensitive

19. Cont…

20. Thank You
for
Your Attention