Evaporation Unit operation

1. Evaporators

2. Evaporation and Evaporators
Evaporation refers to vaporization of liquid or
removal of a portion
of solvent from a solution
• Concentrate the solution
• Concentration of sugar juice, aqueous
solution of ammonium
sulphate,
• Drying – Removal (Vaporization) of entire
solvent out of solution
Different types of evaporators
• Natural circulation evaporators - simpler evaporation
operations
• Short tube vertical evaporators
• Long tube vertical evaporators
• Forced Circulation evaporators – salting, viscous or
scale forming solutions.
• Falling film evaporators
• Rising film evaporators
• Agitated thin film evaporators
• Plate evaporators

3. Horizontal tube evaporators
• Oldest type of evaporator – vertical
cylindrical shell incorporating a horizontal
square tube bundle at lower portion
• 1 – Steam chest, 2 – Tubes, 3 – evaporator
body
• Suitable channels are provided at ends of
tube bundle to introduce steam or to
withdraw condensate.
• Vapour outlet is provided at top cover and
thick liquor outlet is provided at bottom.
• In this construction steam is inside tubes and
and liquor to be concentrated surrounds the
tubes.
• Steam which is admitted in channels flows
through tubes.
• Steam gets condensed by transferring its
latent heat and condensate is removed from
outlet provided at bottom opposite to steam
chest.

4. • Heat given by condensing steam which will be gained by
solution in evaporator and solution boils
• Main advantages are low head room requirements and large
vapor liquid disengaging area.
• Not suited for salting and scaling solutions forming deposits
on outside of tubes.

5. Short tube vertical evaporators
• Short tube bundle enclosed in a shell –
calendria
• Calendria is of annular construction – open
cylindrical region at centre.
• Lower and upper end of calendria is bolted to
the annular region of evaporator.
• Feed is supplied through nozzle above upper
tube sheet, steam is supplied to shell or steam
chest of calendria.
• Feed solution is partially vaporized in tubes
and flows up through tubes in Calendria due to
density difference.
• Vapor liquid disengagement occurs above tube
sheet.
• The liquid flows down through central open
space of calendria called donwtake or
downcomer. (continuous natural recirculation
of solution occurs
• Thick product liquor is withdrawn from
bottom.
• Steam condensate leaves through drain nozzle
connected to steam trap.

6. • A bleed line is provided in shell to release
non-condensables in steam
• Vapor outlet at top and entrainment
separator arrest liquid droplets in vapour
• Concentration of sugar solution
• Not suitable for solutions where
precipitation or sg out occurs
• Propeller installed in downtake pipe to
increase circulation – propeller calendria.

7. Basket type vertical evaporators
• Similar operational features as that of
calendria type evaporators
• Heating element is a tube bundle with
fixed tube sheets welded to a shell piece.
• Tube bundle (basket) is supported by
thick internal brackets.
• Brackets are welded or bolted to
evaporator body and tube bundle
allowing its removal for maintenance.
• Diameter of tube bundle is smaller than
evaporator vessel, thus providing an
annular space used for recirculation.
• Liquid gets heated and boils in tubes
present in basket
• Steam enters through steam chest
located centrally in tube sheet
• Vent line is also connected to upper tube
sheet for purging non –condensables
• Condensate trips down from the bottom
of basket through steam trap.

8. • Vapor generated by
liquid boiling strikes a
deflector plate fixed to
steam pipe facilitating
separation of
entrained liquid
droplets
• Concentrated liquor
leaves through an
outlet pipe at conical
bottom of reservoir.
• Unsuitable with
viscous liquids, due to
weak circulation forces
• Ease of cleaning and
maintenance facilitates
usage of scale forming
solutions.

9. Long tube vertical evaporators
• Widely used natural circulation evaporator,
long tube bundle fitted with a shell
• Shell is projected into a larger diameter or
vapour head space at top
• Feed enters tube bundle at bottom, flows
through tube once while vigorous boiling,
discharges into vapour head
• Vapor impinges on a deflector plate above
free top end of tube bundle, liquid
droplets are removed from vapour
• Concentrated liquor leaves vapour
chamber through a pipe and is withdrawn.
• Heat Transfer advantages – liquid side heat
transfer coefficient controls the rate of
heat transfer
• Resistance on condensing steam side is
very small
• Promote liquid side heat transfer
coefficient which can be done by
increasing liquid Reynold’s number

10. • As feed enters long vertical tube, it gains heat and reaches
to its boiling point – occurs over a small portion at bottom
of tube.
• Due to boiling two phase mixture flows at higher velocity
due to its larger specific volume
• With continuous boiling more vapour is generated and
velocity of mixture is increased tremendously
• Large increase in heat transfer coefficient and heat transfer
rate – high capacity for LTV evaporator
• Tubes are of diameter 25 to 50mm with length of 5 m to
10m
• Baffles are provided in shell side for more uniform flow of
steam fed to shell
• Stem condensate drips through steam trap
• LTV evaporator is used in paper and pulp industries for
concentrating black liquor.

11. Forced circulation evaporators
• Solutions of high viscosities or solutions of scale forming
tendencies – forced circulation evaporator is used.
• Increasing flow velocity of liquor increasing heat transfer
coefficient through circulation through a centrifugal pump.
• Velocity of circulation – 2 to 6 m/s
• Smaller diameter tubes are used of order of 50mm
• Centrifugal pump forces liquor through tubes at high
velocity
• It is heated due to heat transfer from condensing steam on
shell side.
• Boiling takes place in tubes gradually and becomes
superheated as it enters separator due to reduction in static
head pressure.

13. • Deflector plate separates vapour from liquid solution,
vapours are removed from the top.
• Liquid is returned to centrifugal pump.
• Part of liquid solution leaving separating space is withdrawn
as concentrated liquor
• Makeup feed is continuously introduced at pump inlet.
• High heat transfer coefficients are obtained even with
viscous material
• Positive head circulation and close control of flow
• Due to high velocities it prevents formation of scales
• Moderately heat sensitive liquids can be handled due to
high velocities.

14. Falling film evaporators
• In falling film evaporators,
liquid flows down the
inner walls of tubes in a
vertical tube bundle.
• Tubes are heated by
condensing steam or any
other hot liquid on shell
side
• As feed liquor flows down
tube wall, water vaporizes
and liquid concentration
increases gradually
• Thick liquor is withdrawn
from bottom and vapour
goes into a separator
where entrained liquid
droplets are separated

15. • Liquid distribution is most important factor – tubes has large
number of rectangular notches
• Notched part projects above tube sheet and feed enters at a
point above tube sheet and forms a pool of liquid in it
• Feed liquor enters tubes through the notches and forms a
falling film in each of these tubes.
• Number of tubes in bundle is determined by liquid rate
where a continuous film needs to be formed
• Maintaining a liquid rate is essential to form a liquid film.
• High heat transfer rate – Suitable of heat sensitive materials
– viscosity must not be too high
• Concentration of fruit juices

16. Rising film evaporators
• Similar to LTV
evaporators it has a
vertical tube bundle &
vapour chamber
• Liquid gains sensible
heat from lower part of
tube and starts boiling
• Heat transfer rate is
sufficiently high and
tube is long, volume
fraction of vapours in
two phase mixture
increases
• Bubbles form slugs
rising through tubes at
high velocity.

17. • Liquid remain sandwiched between vapour slugs
• Thin liquid film is dragged along tube wall by rising slugs
generating rising film
• High heat transfer coefficient is very high
• Deflector plate placed above upper tube sheet of
evaporator helps in reducing entrainment of liquid
droplets.

18. Agitated thin film evaporators
• If viscosity of liquid is
very high or viscosity
increases sharply with
concentration of feed –
Agitated thin film
evaporator is used
• Contain a vertical steam
jacketed cylinder along
inner surface of which
feed liquid flows as a
film
• Four vertical blades
mounted on a central
shaft agitating the film

19. • The blades maintain a close clearance with the wall.
• Action of agitator increases heat transfer coefficient
reducing fouling, keeping film thickness very small.
• Vapor generated flows through the core and leaves through
a vapour pipe
• This section has larger diameter so that vapour velocity is
reduced and entrained liquid droplets return to evaporation
zone.
• Agitated thin film evaporators can concentrate liquids of
high viscosity offering low residence time and liquid hold up
and producing uniform concentrate
• Highly viscous and heat sensitive materials can be
evaporated in this type of evaporator.

20. • Due to their compact construction and flexibility widely
used
• Construction is same as that of plate heat exchangers
• Liquid flows as a thin film over one surface of a plate
while other side remains in contact with heating
medium.
• Common configuration involve rising film, falling film or a
combination of both
• Film flow remains turbulent because of high vapour
velocity – high heat transfer coefficient is achieved.
• Useful for evaporating heat sensitive solutions
Plate evaporators

21. Capacity
Capacity is defined as numberof kilograms of water vaporized
per hour.
Economy
Economy of an evaporator is defined as number of kilograms of
water evaporated per kilogram of steam fed to evaporator
In a single effect evaporator the amount of water evaporated
per kg of steam fed is always less than one
The latent heat of vaporization of water decreases as pressure
increases tends to make the ratio of water vapor produced per
kg of steam less than unity
Increase in economy of evaporator is achieved by reusing the
vapor produced. The methods for increasing economy are
1. Using multiple effect evaporation system
2. Vapor recompression.

22. Material and Energy Balance Calculations
Overall Material Balance
F = L + V
Feed = Thick Liquor + Vapour
Solid Balance
F xF = L xL
• Heat Transferred to solution in evaporator by
condensing steam
• This heat content is used to heat the feed solution from
Feed temperature to its boiling temperature and for
evaporation of water from solution
Q = Qs = S λS = F Cpf (T-Tf)+ V λv
Q = S λS = U A ∆T
∆T = Ts – T = Condensing steam temperature - Boiling
point of solution
Overall enthalpy Balance for evaporator
F HF + S λS = V HV + L HL λS = Specific enthalpy of saturated steam – Specific enthalpy of
saturated condensate

23. • An evaporator operating at atmospheric pressure is
desired to concentrate feed solution from 5% to 20 %
solute at a rate of 5000 kg/h. Dry saturated steam at
a pressure corresponding to saturation temperature
of 399 K is used. The feed is at 298 K and boiling
point elevation is 5K. Overall heat transfer coefficient
is 2350 W/m2 K. Calculate the economy of
evaporator and area of heat transfer.
Data: Latent heat of condensation of steam at 399 K =
2185 kJ/ kg.
Latent heat of vaporization of water at 101.325 kPa
and 373 K = 2257 kJ/kg.
Specific heat of feed = 4.187 kJ/kg K

24. Boiling point elevation
Material and Energy Balance Calculations
Overall Material Balance
F = L + V
Feed = Thick Liquor + Vapour
Solid Balance
F xF = L xL
Overall enthalpy Balance for evaporator
F HF + S λS = V HV + L HL
• Heat Transferred to solution in evaporator by
condensing steam
• This heat content is used to heat the feed solution from
Feed temperature to its boiling temperature and for
evaporation of water from solution
Q = Qs = S λS = F Cpf (T-Tf)+ V λv
Q = S λS = U A ∆T
∆T = Ts – T = Condensing steam temperature - Boiling
point of solution
λS = Specific enthalpy of saturated steam – Specific enthalpy of
saturated condensate
Evaporators produce concentrated solutions having
boiling point higher than that of water
• This difference is referred as BPE.
• BPE is required before evaporator design

25. Heat Transfer coefficient
• Heat transfer coefficient for condensation of steam in shell
is very high
• Liquid side coefficient controls rate of heat transfer
• As velocity of entering liquid is very low heat transfer
coefficient is also low.
• This increases greatly as liquid starts boiling after climbing
through tubes.
• For high liquid velocities no evaporation occurs heat transfer
coefficient can be determined by equation
𝒉𝒊𝒅𝒊
𝒌
= 𝟎. 𝟎𝟐𝟑𝑹𝒆𝟎.𝟖
𝑷𝒓𝟎.𝟒
𝒅 = 𝒊𝒏𝒏𝒆𝒓 𝒅𝒊𝒂𝒎𝒆𝒕𝒆𝒓 𝒐𝒇 𝒕𝒖𝒃𝒆
𝒌 = 𝒕𝒉𝒆𝒓𝒎𝒂𝒍 𝒄𝒐𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒊𝒕𝒚 𝒐𝒇 𝒍𝒊𝒒𝒖𝒊𝒅
𝑹𝒆 𝒂𝒏𝒅 𝑷𝒓 = 𝑹𝒆𝒚𝒏𝒐𝒍𝒅 & 𝑷𝒓𝒂𝒏𝒅𝒕𝒍 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒍𝒊𝒒𝒖𝒊𝒅

26. • Fouling inside the tubes is a major problem requiring
periodic cleaning
• Scale thickness and hence fouling resistance grows with
time
• The change in overall heat transfer coefficient with time can
be correlated by the equation.
𝟏
𝑼𝟐 =
𝟏
𝑼𝜽
𝟐 + 𝜷𝜽
𝑼𝜽 = 𝒐𝒗𝒆𝒓𝒂𝒍𝒍 𝒉𝒆𝒂𝒕 𝒕𝒓𝒂𝒏𝒔𝒇𝒆𝒓 𝒄𝒐𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒕 𝒇𝒐𝒓 𝒄𝒍𝒆𝒂𝒏 𝒕𝒖𝒃𝒆
𝑼 = 𝒐𝒗𝒆𝒂𝒍𝒍 𝒉𝒆𝒂𝒕 𝒕𝒓𝒂𝒏𝒔𝒇𝒆𝒓 𝒄𝒐𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒕 𝒂𝒕 𝒂𝒏𝒚 𝒕𝒊𝒎𝒆 𝜽
𝜽 = 𝒕𝒊𝒎𝒆 𝒇𝒐𝒓 𝒘𝒉𝒊𝒄𝒉 𝒆𝒗𝒂𝒑𝒐𝒓𝒂𝒕𝒐𝒓 𝒊𝒔 𝒊𝒏 𝒐𝒑𝒆𝒓𝒂𝒕𝒊𝒐𝒏
𝜷 𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕 𝒇𝒐𝒓 𝒂 𝒑𝒂𝒓𝒕𝒊𝒄𝒖𝒍𝒂𝒓 𝒍𝒊𝒒𝒖𝒊𝒅

27. Multiple Effect Evaporator
• Advantage with multieffect evaporators – higher steam
economy
• Different modes of feeding – Forward feeding, Backward
feeding, Mixed Feed, Parallel feeding
Comparison of Forward & Backward Feed
Modes
• Forward feeding mode is relatively simple – no need of
intermediate pumps to transfer liquid from one effect to
another
• Feed flows from one effect to another as pressure in next
effect is lower
• Backward feeding arrangement allows some advantages
over forward feeding arrangements.

28. • Thick liquor is highly viscous
• Forward feeding – last effect performs final concentration
job at lowest temperature
• Viscosity in last effect will be high with low values of heat
transfer coefficient
• For backward feeding arrangement, liquor of highest
concentration will be in first effect – steam temperature is
highest
• Viscosity of black liquor is substantially reduced reducing
value of heat transfer coefficient - capacity will be higher
• Advantage of using backward feed if final liquor is highly
viscous but not very heat sensitive
Justification # 1: Advantages of Backward Feeding

29. • Feed is cold – feed admitted to first unit consumes extra
amount of steam to raise temperature of feed to boiling
point
• Steam supplies only to rise sensible heat of feed
• Potential to evaporate equal amount of liquid in each of
subsequent effects cannot be accomplished.
• For backward feeding using cold feed absorbs heat from
waste steam leaving last unit
• Purpose of preheating liquid is served in this mode of
feeding
• Net result is that there is net increase in capacity.
Justification # 2: Advantages of Backward Feeding

30. • Feed is hot – Forward feed arrangement is expected to give
a higher steam economy
• This produces flash steam when flowing into next effect
operating at lower pressure.
• By vapor flashing some evaporation occurs starting from
second effect
• In backward feed on contrary some temperature rise occurs
in each effect.
Justification # 3: Advantages of Backward Feeding