This document discusses evaporation and different types of evaporators used in evaporation processes. It begins by defining evaporation as the process of vaporizing a solvent from a solution below the boiling point of the solvent, leaving a concentrated product. It then discusses various factors that influence the evaporation rate, such as temperature, surface area, vapor pressure, and concentration of the solution. The document goes on to classify different types of evaporators, including natural circulation evaporators, forced circulation evaporators, and multiple effect evaporators. It provides details on the design and working of pan evaporators, tube evaporators, film evaporators, and forced circulation evaporators.

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By Studying this topic Students will able to understand:
▰ To have idea about basic principle behind evaporation.
▰ To learn diverse elements influencing evaporation process.
▰ To learn practical applicability of evaporation process by studying
different types of evaporator.

3. Evaporation is the process of vaporization of the solvent from the surface of
the solution below the boiling point of solvent leaving concentrated product.
The vapours are formed at the surface of the solution and removed.
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Evaporation Other processes
Final product is
concentrated liquid
Drying: Final product is solid
Evaporating liquid is
only one component
Distillation: Evaporating liquid
is combination of two or more
component, it is compulsory to
separate each component
Crystal are not
generated
Crystallization: Final product is
crystal

5. ▰ A. Temperature :
▰ Higher the temperature greater the kinetic energy and rate of
evaporation.
▰ A molecule of the solvent requires kinetic energy to move from surface or
bulk into the vapours. When the temperature of the molecule is increased
all the molecule will have sufficient kinetic energy to move at faster rate
and convert into vapour and rate of evaporation will be increased.
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6. ▰ Temperature and Time of Evaporation :
▰ For thermo stable product greater is the time of exposure more will
be evaporation.
▰ Exposure of the higher temperature for shorter period time will result in
less degradation as compared to the lower temperature and longer
period of time. Film evaporators are more preferred than pan
evaporators as film has higher temperature and less exposure time
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7. ▰ Surface area :
▰ Greater the surface are more contact time of liquid with
heating media, more will be evaporation. Surface area is
more in tubular evaporators as compared to pan
evaporator.
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8. ▰ Vapor Pressure :
▰ Lower the external pressure, lower the boiling of the solvent higher is the
rate of evaporation.
▰ A liquid with high vapor pressure at low temperature boil and evaporates
quickly.
▰ If vaporized solvent is accumulated in the surrounding environment then
rate of evaporation is slow as the surrounding space is saturated with the
air.
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9. ▰ Temperature and Moisture content of the Feed liquor :
▰ Certain drugs undergo hydrolysis in the presence of moisture
at elevated temperature. Hence the product is initially kept at
lower temperature and then slowly temperature is raised to
avoid the degradation.
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10. ▰ Effect of the Concentration :
▰ As the process of evaporation continuous the solvent is removed and
the product become more and more concentrated over a period time.
▰ The amount of solid in the product increases the boiling point of the
product also increases.
▰ Viscous product deposits at the heating surface and the rate of heat transfer
decreases. This can be prevented by constant stirring. 10

11. ▰ Type of the product required :
▰ Requirement of the final product decides the equipment used for
evaporation.
▰ For dry concentrated products pan evaporators are used, for viscous
liquids film evaporators are used and for porous products vacuum
evaporators are preferred.
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12. ▰ Film and Deposits :
▰ Concentrated product and precipitates tend to deposit at the inner
lining of the evaporator surface thereby decrease the rate of heat
transfer and reduction of evaporating surface. Stirring of the
concentrate can prevent film formation and deposit.
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Classification of Evaporator
Natural Circulation
e.g.
Evaporating Pan
e.g.
Short Tube
Forced Circulation Film Evaporator
Long Tube Wiped Film
Multiple Effect
Evaporator

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Classification of Evaporator
Apparatus Heated
by Direct Fire
Evaporator with
Jacketed Pan
Evaporators with
Tubular Heating Area
With Horizontal Tubes
With Vertical
Tubes
Natural Circulation Forced Circulation
Multiple Effect Evaporators

15. ▰ It is mainly used for the small scale processing. It can be operate by
natural circulation.
▰ Principle :
▰ Evaporating pan is surrounded with the jacket through which saturated
steam is supplied.
▰ Latent heat of vaporization from the steam is transferred to the pan
further it is gained by the evaporating solution through conduction and
convection mechanism.
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16. ▰ With increasing temperature vapors are liberated from the surface
and the bulk. Finally the evaporating solution is converted to
concentrated product.
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18. ▰ Semi-circular pan is made up of inner kettle and outer steam jacket. Semi-
circular shape gives the more surface area for heating as well as vapour
removal will be faster.
▰ The pan is made up of stainless steel, aluminium or copper.
▰ Metals with good conductivity are preferred for inner pan.
▰ Outer jacket is made up of insulators to avoid the loss of heat in the
surrounding. Cast iron can be used as outer jacket.
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19. ▰ Jacket and the pan may vary in the depth to diameter proportion as
shallow pans are more preferred for evaporation. These are having
limited heating surface but evaporation will be more rapid.
▰ Shallow pans are more expensive than semicircular as these are thicker
to withstand the pressure of the steam.
▰ Evaporating pan with small size can be made up of single sheet of metal
for large size two or more sheets are welded together.
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20. Steam enters from top of the jacket and condensate is removed from
the bottom, provision is made to remove the non-condensable gases
from vent at the top of jacket.
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21. ▰ The solution to be evaporated is kept in the pan. Saturated steam is
supplied in the jacket. Steam gives its heat to the solution.
▰ Heat is transferred by conduction and convection mechanism.
▰ solvent is converted into vapor and product is get concentrated.
▰ Condensed steam is removed from the bottom and non-condensable gases
are removed from the top.
▰ Concentrated product can be removed from the bottom outlet. 21

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For small scale operation pan can be tilted to remove the product.
Stirring of the solution during operation helps to uniform distribution of
heat as well as rapid removal of the formed vapors.
Advantages :
1. Simple and cheap in construction.
2. Easy to handle, clean and maintain.

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1. It is not suitable for thermolabile product.
2. Solvent recovery is difficult.
3. Overall heat transfer coefficient is less because of natural circulation so its
time consuming process.
4. Some products tend to deposit on the pan surface are more prone to
degradation as well as these residue reduces the heat transfer.
5. It is limited to the aqueous solution only.

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6. Sufficient ventilation should be provided in the room where operation is
carried out because vapors are directly escaped in the surrounding
environment which leads to saturation of the surrounding ultimately
reducing the rate of evaporation.
7. Vapor pressure in the system cannot be lowered to reduce the boiling
point of the solvent.
Application : Concentration of the thermostable aqueous extracts.

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Horizontal tube evaporator is the only tube evaporator in which heating
media is inside the tubes.
Principle :
Tubes are immersed inside the pool of the liquid to be evaporated and steam
is passed inside the tubes. Heat is transferred from the steam to the liquid.
Liquid is heated and liberated vapors are removed from the top, product
concentrate is removed from the bottom.

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• It consists of vertical cylindrical body with dome shaped top and bottom.
• These evaporators are usually of iron or steel plate with a diameter of
approximately 2 meters and 3 meters height.
• The lower part of the cylinder contains heating compartment and upper part
is having vapor storage.
• Heating compartment contains horizontal tubes.
• The diameter of the tubes is usually 2 to 3 inches. These are made up of
stainless steel, aluminium, nickel.

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• Tubes are held inside the plate for support.
• The length of the tubes is such a way that these are project 1 inch outside
the plate at both the ends.
• The tubes are further secured by the use of gasket to avoid slippage.
• Steam compartment and condensate are place on the either side of the
heating compartment at opposite ends.
• Steam is introduced from one end and condensate is removed from
another end.

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• Feed liquor is introduced at any convenient point such a way that it
covers all the tubes. Vapors are removed from the top and concentrated
product is removed from the bottom.

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• Feed liquor is introduced in the heating compartment till it covers all the
tubes.
• Steam is introduced from steam compartment.
• Heat form the steam is supplied to the tubes and ultimately to the product.
• The condensate and the non-condensable gases are removed from the
opposite compartment.
• Liquid is heated by the steam and vapors are removed from the top.

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• This process continues till satisfactory concentrated product is obtained.
Final product is removed from the central outlet.
Advantages :
1. Ability to carry large volume of the liquid inside body.
2. Low cost for per unit heating surface and partial repairs.
Disadvantages:
1. It is used only when rigorous boiling can be obtained with natural
circulation

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2. Unsuitable for salting or scaling liquids
3. Smaller capacity than other evaporators
4. Liquid distribution hard to maintain
Application :
It is widely used in the evaporation of sugar syrup, beef extract, phosphoric
acid, zinc chloride and the liquids which do not have tendency of scaling and
deposit.

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• Heated feed liquor is fed from the bottom in the tubes surrounded by
steam.
• Liquid inside the tubes starts boiling.
• Vapors forms core at the center of the tube, will try to escape through the
pipe and starts rising upward.
• During the upward movement vapors carries slug of liquid.

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• This slug is forced by the high velocity liquid at the inner wall of the tube
to form thin film.
• As the velocity of the liquid is high thin film is formed which increases
the heat transfer coefficient.
• Finally vapors strike on the separator, where the entrapped liquid gets
separated from vapor.

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• Cylindrical body of the evaporator contains tubes are 7m long with the
internal diameter of 5cm.
• These tubes are supported by the plates.
• Steam inlet is arranged at any convenient point so as to have counter flow
with the feed liquor.
• Provision is made to remove condensate and vent.
• Separator is attached cylindrical body at the top to separate vapor and
entrapped feed liquor

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• Heated feed liquor passed from the bottom of the evaporators and it
enters inside the narrow long tubes.
• Tubes are surrounded by saturated steam and feed liquor is already
heated so the temperature gradient is less.
• Feed liquor boils, core of the vapor forms at the center of the tube and try
to escape away from the liquid.
• It starts rising upward and forms slug by joining bubbles.

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• Slug pushes liquid above, which forms a thin film at the wall of evaporator.
• Heat transfer coefficient is high in the thin film. Liquid from film starts
vaporizing rapidly.
• Finally mixture of liquid and vapors strikes to the separator and vapors are
removed from the top and concentrated feed liquor is collected from the
receiver.

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1. High velocity of the liquid reduces the boundary layer and heat transfer
coefficient is high.
2. Time of contact between feed liquor and evaporator is very short even
though preheated feed liquor is passed so thermolabile materials can be
handled.
3. Evaporation rate is high irrespective of feed liquor volume and heating
time as film has large surface area.

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4. The mixture of vapor and solvent strikes to the separator the separation
efficiency is higher.
5. Tubes are not submerged in the feed liquor; elevation of boiling point is
avoided as hydrostatic head is not created.
6. Narrow tubes having large area for heat transfer.
7. Low hold up and small floor space is required.
8. Foam forming liquids can be handled.

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1. Complicated design, cost is high
2. Difficult to clean and maintain
3. Optimization of feed liquor rate is difficult, too high feed liquor rate
effects insufficient concentration and too low feed liquor rate creates dry
patches inside the tubes as constant film is not maintained.
Application :
Can be used for the concentration of thermosensitive products such as
vitamins, bile salts and fish extract.

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• The feed liquor is circulated through the pump inside the
calandria, where it is superheated at an elevated boiling point.
• Feed rises upward.
• When feed enters in the vapour head, the pressure in the liquid is
suddenly reduced resulting in some of the liquid being flashed,
during flashing some amount of liquid is rapidly boiled off and
vapours are formed.

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• The mixture of liquid and vapour is separated in the cyclone
separator.
• Vapours are escaped from the top and concentrated product is
removed from the bottom.

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• Evaporator is cylindrical with conical bottom and vapor head is
present at the top.
• Conical bottom provide gradual, low pressure drop transitions
from the circulating piping to the tube bundle which is
particularly important for the establishment of uniform feed to
the tubes.

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• The calandria are long and narrower than the standard short tube
evaporator. The inner diameter of the tube is 0.1 meter and length
is 2.5 meter.
• Circulating piping interconnects the calandria and the pump.
Upper part of the calandria enters into the vapor head.
• Vapor head is required to have an adequate space above the
liquor level to allow the entrained liquor droplets in the vapor and
return by gravity to the circulating slurry.

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• The large diameters of vapor head result in low vapor velocities
which minimize entrainment. Deflector is typically installed in the
upper portion of the vapor head to reduce entrainment.
• Vapor head is attached to the separator which separated vapor
and entrained feed.
• Entrained feed is connected to the return pipe for recirculation or
final collection.

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• Steam compartment is at the bottom through which stem is
introduced and it covers the calandria from outside.
• Condensate and non-condensable gases are collected from the
bottom of the evaporator.

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• Feed liquor is passed at high velocity through the tubes which are
surrounded by superheated steam.
• A high circulation rate is provided for adequate tube velocity to
achieve good heat transfer.
• Feed rises upward and simultaneously heated under high
pressure, boiling of the liquid is avoided.
• Sufficient liquid height is maintained above the tubes to suppress
mass boiling in the inlet and prevent boiling at the tube surface.

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• This is will prevent precipitation in the tubes which would lead to
fouling of the heat transfer surface.
• At the top when it reaches to the vapor head, feed undergo
flashing due to sudden reduction in pressure.
• At this point maximum vapors are formed.
• Flashed mixture of vapor and feed strikes to the deflector where
feed is thrown at downward.

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• Vapors formed are moving to the separator. From the separator
vapors are removed from the top and entrained feed is removed
from the bottom.
• Concentrated feed from the top is removed by recirculation pipe
and can be recirculated again by the pump.
• Fully concentrated product is collected from the bottom.

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• Highly viscous solutions, solution containing suspended particles
and heat sensitive materials can be handled.
• Optimum heat transfer capability due to faster liquid circulation.

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• High liquid hold up
• High equipment cost.

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• Manufacturing process of sodium chloride, urea, citric acid,
sodium sulphate and magnesium chloride involves the use of
forced circulation evaporator.

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• In case of multiple effect evaporator attempts are made to utilize
the latent heat of the vapors formed.
• Two or three evaporators are connected together with piping so
that feed liquor of the first evaporator is heated by the saturated
steam.
• Vapors generated from the first evaporator act as heating media
for the second evaporator and vapor gets condensed.
• This phenomenon is repeated for the next evaporator.

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• The evaporator A1 is heated by the steam, the liquor in the
evaporators boils and vapors generated during this process are
supplied to the calandria of the evaporator A2.
• In turn vapor liberated from A2 are transferred to calandria of the
evaporator A3. Latent heat of vaporization from A1 is recovered
in A2 and that of A2 is recovered in A3.
• Evaporator A1 is working at higher pressure than A2 and A3 so
the liquor form A1 will be efficiently transfer to A2 and finally A3.

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• If Liquor in the A3 is not sufficiently concentrated as the
temperature of A3 is less than the A1, It can be circulated back in
A1 with the help of pump.
• Condensate is removed from the bottom and Vapor removed from
the top are removed by the cyclone separator at top and
entrainment are removed from the bottom.

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• Forward feed : Often many of the later effects are under vacuum.
Under these conditions, the liquid feed progress is simplest if it
passes from effect one to effect two, to effect three, and so on, as
in these circumstances the feed will flow without pumping. It
means that the most concentrated liquids will occur in the last
effect.

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• Backward feed : Feed may pass in the reverse direction, starting
in the last effect and proceeding to the first, but in this case the
liquid has to be pumped from one effect to the next against the
pressure drops. The concentrated viscous liquids can be handled
at the highest temperatures in the first effects it usually offers
larger evaporation capacity than forward feed systems.

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• Mixed feed : Feed is introduced in intermediate effect, flows in
forward feed to the end of the series and is then pumped back to
the first effect for final concentration. This permits the final
evaporation to be done at the highest temperature.

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• Energy losses are less as vapor heat is utilized and economy of the
process id higher.

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• Limitation on the total number of effects than can be arranged so
that each has suitable temperature gradient.
• Complicated arrangement.

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• Manufacturing of the active pharmaceutical ingredients and
excipients: in the process of manufacturing of the active
pharmaceutical ingredients evaporation process may be used.
Excipients such as sugar syrup, zinc chloride etc. can be
manufactured by using evaporators.
• Concentration of extracts: Extracts from natural origin can be
concentrated such as liquorice extract, cascara extract, fish
extract, coffee extract etc.

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