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Evaporator
• Evaporation occurs at the
liquid–vapor interface when
the vapor pressure is less
than the saturation pressure
of the liquid at a given
temperature.
Evaporation is a special case of heat transfer to a
boiling liquid.
Evaporation is a unit operation in which solvent
(water) is removed by means of vaporization or
boiling
Evaporation is the removal of solvent as a vapour
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 vapour.
Objectives of Evaporation:
To concentrate a non-volatile solute (solute has
negligible volatility) such as organic compounds,
inorganic salts, acids or bases from a solvent.
Common Solutes:
Caustic soda, Caustic potash, Sodium sulfate,
Sodium chloride, Phosphoric acid and Urea
Common Solvent:
Water
Examples of Evaporation:
1. Concentration of cane sugar juice in a sugar
plant
2. Concentration of an aqueous solution of
ammonium sulphate in a fertilizer plant
3. Concentration of dilute recycled sodium
hydroxide in an alumina plant and many
others
Distillation: Solution containing more than one
volatile compound is vaporized (in a reboiler)
and the components are separated thereafter in
distillation column.
Drying: Entire solvent is vaporized out from a
solution leaving a solid residue as the product,
the operation is called drying.
Evaporation: Evaporation of solution is an
essential step in the operation of a
crystallization unit. In crystallization, the
solution is evaporated to make it supersaturated.
Crystal grow in the supersaturated solution.
Evaporation Crystallization
Vaporizing solvent is
the main function
Crystalline product
and crystal growth is
main function
Food, pulp and paper, pharmaceuticals, fine
chemicals, organic and inorganic chemicals,
polymer etc.
Evaporator
Invention of Evaporators
Norbert Rillieux is famous for his invention
of the multiple effect pan evaporator for
sugar refining process in 1881.
Rillieux was born in New Orleans, Louisiana
in 1806.
He used the steam generated from one pan
to heat the sugar juice in the next pan for
energy efficient means of water evaporation.
Equipment, in which
evaporation is performed, is
known as evaporator.
The evaporators used in
chemical process industries are
heated by steam and have
tubular surface.
An adequate number of tubes are provided through
which the solution is circulated and the tubes are
heated by steam.
In general the steam is the saturated steam and thus
it condenses on the outer tube surface in order to
heat the tube.
Classification of Evaporators
The velocity of circulation of the solution through the
tubes should be reasonably high so that (i) a high
inside heat transfer coefficient is attained and (ii)
formation of deposits or scales on the inner surface
is reduced.
Circulation may be caused by density gradient of the
solution in the vertical tubes or by an external
mechanical means like a pump.
Accordingly, most evaporators are broadly
classified as:
(1) Natural circulation and
(2) Forced circulation
Classification of Evaporators
Evaporation can be divided into three categories
on the basis of boiling phenomena
(i) Pool boiling: In this phenomena bulk or pool of
liquid boils. Examples are kettle boiling, natural
circulation boiling units, thermo siphon
reboilers in distillation
(ii) Convection heating and boiling: example is
forced circulation boiling units
(i) Film evaporation: In film evaporation, a thin
liquid film is maintained on the heating surface
Evaporators can be classified as:
Solar Evaporator Batch Pan Evaporator
Natural Circulation
Evaporator
Forced circulation
Evaporator
Horizontal Tube Evaporator Basket Type Evaporator
Long Tube Vertical
Evaporator
Short Tube Vertical
Evaporator
Rising Film Evaporator Falling Film Evaporator
Agitated Thin Film
Evaporator
Horizontal Spray Film
Evaporator
Plate Type Evaporator Vapor Compression
evaporator
Classification of Evaporators
Evaporators are classified by the number
of effects.
In a single-effect evaporator, steam provides
energy for vaporization and the vapor product is
condensed and removed from the system.
In a double-effect evaporator, the vapor product
off the first effect is used to provide energy for a
second vaporization unit.
Classification of Evaporators
This cascading of effects can continue for many
stages. Multiple-effect evaporators can remove
much larger amounts of solvent than is not
possible in a single effect.
Single Effect Evaporators
Single effect Evaporator
In single effect evaporator,
the steam is fed to the
evaporator which
condenses on the tube
surface and the heat is
transferred to the solution.
The saturated vapor comes
out from the evaporator
and this vapor either may
be vented out or
condensed. The
concentrated solution is
taken out from the
evaporator.
Conventional Evaporator
Evaporator is made up of
three functional sections:
(1) Heat exchanger,
(2) Evaporating section,
where the liquid boils
and evaporates, and
(3) Separator in which the
vapour leaves the
liquid and passes off
to the condenser or to
other equipment
Single effect evaporator
• In many evaporators, all three
sections are contained in a single
vertical cylinder.
• In the centre of the cylinder there is
a steam heating section, with pipes
passing through it in which the
evaporating liquors rise.
• At the top of the cylinder, there are
baffles, which allow the vapours to
escape but check liquid droplets that
may accompany the vapours from
the liquid surface.
• A diagram of this type of evaporator,
which may be called the
conventional evaporator.
• In the heat exchanger section, called
a calandria in this type of evaporator,
steam condenses in the outer jacket
and the liquid being evaporated boils
on the inside of the tubes and in the
space above the upper tube plate.
• The resistance to heat flow is imposed
by the steam and liquid film
coefficients and by the material of the
tube walls.
• The circulation of the liquid greatly
affects evaporation rates, but
circulation rates and patterns are very
difficult to predict.
Values of overall heat transfer coefficients that have
been reported for evaporators are of the order of
1800-5000 J m-2 s-1 °C-1 for the evaporation of
distilled water in a vertical-tube evaporator with
heat supplied by condensing steam.
Multi Effect Evaporators
Multi effect Evaporator
The saturated vapor
coming out from the
evaporator-1 is used as
steam in the second
evaporator.
Partially concentrated
solution works as a feed to
the second evaporator.
This arrangement is known
as double effect evaporator
in forward feed scheme.
Multi Effect Evaporators
Note: The vapour leaving evaporator-2 is at the
boiling temperature of the liquid leaving the first
effect.
In order to transfer this heat from the
condensing vapor from the evaporator-1 to the
boiling liquid in evaporator-2, the liquid in
evaporator-2 must boil at a temperature
considerable less than the condensation
temperature of the vaporization, in order to
ensure reasonable driving force for heat
transfer.
Multi Effect Evaporators
A method of achieving this is to maintain a
suitable lower pressure in the second effect so
that the liquid boils at a lower temperature.
Therefore, if the evaporator-1 operates at
atmospheric pressure, the evaporator-2 should
be operated at same suitable vacuum.
The benefit of the use of multiple effect
evaporators is that in this arrangement multiple
reuse of heat supplied to the first effect is
possible and results in improved steam
economy.
Solar Evaporator
In this evaporator, solar energy is the heating source.
Production of sodium chloride from seawater or brine by
concentration in large ponds has been practiced all around
the world.
Crystallization is done in large open tanks.
The process depends upon solar radiation intensity,
weather, humidity and wind velocity.
It is the cheapest evaporation process because solar
energy is free of cost.
When small quantities of solutions are to be concentrated,
batch pan evaporator is the choice.
Batch Pan Evaporator
Production of jams and jellies, fruit juice concentration,
production of some pharmaceutical products is done in
these unit.
The pan can be provided with a jacket or coil for circulating
heating medium.
This unit can be provided agitation.
These are small units having restricted heat transfer area.
In these units high temperature difference cannot be used
due to possibility of degradation of product and fouling of
the heat transfer surface.
These are useful for small capacity batch operation.
Natural Circulation Evaporator
As the name indicates, the circulation of the solution is
natural and the density difference derives it.
The solution gets heat up and partially vaporized as it
flows up the tubes.
The heated liquid flows up because of the density
difference.
Vapor-liquid disengagement occurs above the tube.
Thick liquor comes down from this down comer and
withdrawn from the bottom.
The natural-circulation evaporators may be used if the
solution is quite dilute.
Natural Circulation Evaporator
In the dilute solution the natural circulation will be at
sufficient speed.
It may also be used when the solution does not have
suspended solid particles.
As the solution stays in the tube for larger time, the solution
should not be heat sensitive.
The Calandria type or short-tube evaporators have short
tubes as compared to the long tube evaporators.
The short-tube evaporation uses circulation and solution
flows many times in the evaporators. However, in case of
the long tube evaporator the flow is once through.
Natural Circulation Evaporator
Calandria type Evaporator
Short tube Vertical Evaporators
It is called as a calandria or standard evaporator.
It consists of short tubes 1-2 m in length and 50-70 mm
diameter attached in two tube sheets.
The entire assembly is called the calandria of the
evaporator.
The tube bundle has a large down comer at the center that
helps for circulation of liquid in the evaporator.
The driving force for the fluid flow is the density difference
between the liquid in the down comer and in the tubes.
In the tubes there is a two-phase mixture.
Short tube Vertical Evaporators
A short tube vertical evaporator has a short tube bundle
enclosed in a shell. This is called a calendria.
The calendria is of annular construction, i.e. there is an open
region at the center.
The liquid flows down through the central open space of the
calendria called down take or down comer.
Thus, a continuous natural recirculation of the solution occurs.
Thick product liquor is withdrawn from the bottom.
Their STV evaporators are also known as standard
evaporates.
Natural Circulation Evaporator
Long-tubes vertical Evaporators
Horizontal tube Evaporators
In horizontal tube evaporator, the tubes re horizontal.
Example is kettle reboilers in distillation column.
In horizontal standard evaporator, the process liquid is on the
outer surface of the tube and the heating medium is inside the
tubes.
The unit is relatively cheaper and offers moderate to high heat
transfer coefficient.
Horizontal tube unit is not suitable where fouling problem is
serious, because scale build up reduces the performance
drastically.
It is used as re boilers for distillation column and can be used
for boiler feed water preparation.
Basket Evaporators
In basket evaporator the tube bundle can be removed from
the main body.
Circulation of liquid occurs in the space between the shell
and the tube bundle.
Its advantage is that cleaning of tube is easy because the
bundle is removable.
In these units thermal expansion problem does not arise.
The concentrated liquor leaves through an outlet pipe at the
conical bottom of the evaporator.
A basket evaporator uses tubes similar to those of the
calandria type.
Falling Film Evaporators
Highly heat sensitive materials are processed in falling
film evaporators.
They are generally once-through evaporator, in which the
liquid enters at the top, flows downstream inside the
heater tubes as a film and leaves from the bottom.
The tubes are heated by condensing steam over the tube.
As the liquid flows down, the water evaporates and the
liquid gets concentrated.
To have a film inside of the tube, the tube diameter is kept
high whereas the height low to keep the residence time
low for the flowing liquid.
Falling Film Evaporators
Therefore, these evaporators, with non-circulation and
short resistance time, handle heat sensitive material,
which are very difficult to process by other method.
The main problem in falling film evaporator is the
distribution of the liquid uniformly as a thin film inside the
tube.
Falling Film Evaporators
Forced Circulation Evaporator
Natural circulation evaporators have many limitations (as
mentioned earlier) through they are economical as compared
to forced circulation evaporator.
A forced circulation evaporator has a tubular exchanger for
heating the solution without boiling.
The superheated solution flashes in the chamber, where the
solution gets concentrated.
In forced circulation evaporator horizontal or vertical both type
of design is in- practice.
The forced circulation evaporators are used for handling
viscous or heat sensitive solution.
Forced Circulation Evaporator
Vertical tube forced-circulation Evaporators
Agitated Thin Film Evaporators
Agitated Thin Film Evaporators
Plate Evaporators
Plate Evaporators
Characteristics of different types of EvaporatorsEvaporator Typical products handled comments
Calandria Salt, glycerin from spent
soap lye
Suitable for batch or
continuous operation in single
or multiple effects
Forced
circulation
Salting or scale-forming
materials depending on
steam-chest
configuration; caustic
soda solution, sodium
sulphate, tomato juice to
30% concentration etc.
Available with: (1) horizontal
steam-chest with external
vapor separator (less used
now) (2) Vertical steam –chest
with external separator (3)
vertical steam-chest with
integral vapor head. Operates
with either submerged or
partially filled tubes in single or
multiple effect
Falling film Low to medium viscosity
materials, heat sensitive
products, fruit juices and
pharmaceuticals
Single or multiple effects; can
be operated on single pass or
with partial recycle of
concentrated products
Characteristics of different types of Evaporators
Evaporator Typical products handled comments
Natural
circulation
(Thermosyphon)
Foaming liquids, less
viscous materials, black
liquor from the pulp
industry, spent soap lye,
electroplating solutions,
spin bath liquid
External separator
provides some holding
time adjustment, integral
vapor head type with
down comer gives
minimum hold up
Agitated film Handles the full range of
feed viscosities, gelatin,
fruit puree, glue
Available (1) vertical with
integral vapor separator
(2) vertical with external
separator, co-current
flow (3) horizontal with
tapered shell counter
current flow
Rising Film Caprolactum; ammonium
nitrate, fruit juices, for
crystal producing solutions
with suspended solids
Allows single pass
operation with high liquid
and vapor velocities;
minimum liquid hold up
Characteristics of different types of Evaporators
Evaporator Typical products handled comments
Plate type Fruit juices, extracts,
gelatin, condensed and
whole milk
Liquid and vapor flow
essentially as in rising
and falling film
evaporators without
liquid distribution
problems
Typical overall heat transfer coefficient in evaporator
Methods of Feeding of Evaporators
Evaporator trains may receive their feed in several
different ways. The feed order is NOT related to the
numbering of effects. Effects are always numbered
according to decreasing pressure (steam flow).
1. Forward feed
2. Backward Feed
3. Mixed Feed
4. Parallel Feed
Methods of Feeding of Evaporators
Forward feed arrangement in triple-effect evaporator
(dotted line: recycle stream)
Forward Feed arrangements follow the pattern I, II, III.
These require a single feed pump (reduced fixed costs).
They typically have reduced economy (higher operating
costs) since the cold feed must be raised to the highest
operating temperature.
These also tend to have the most concentrated liquour,
which tends to be the most viscous, in the lowest
temperature effects, so their may be difficulties getting a
good overall heat transfer coefficient.
Forward Feed
Methods of Feeding of Evaporators
Backward feed arrangement in triple-effect evaporator
(dotted line: recycle stream)
Backward Feed arrangements go III, II, I.
These need multiple pumps to work against the pressure
drop of the system; however, since the feed is gradually
heated they usually have better economies.
This arrangement also reduces the viscosity differences
through the system and so is better for viscous solutions.
Backward Feed
Methods of Feeding of Evaporators
Mixed feed arrangement in triple-effect evaporator
(dotted line: recycle stream)
Mixed Feed arrangements offer a compromise, with the
feed entering in the middle of the system (i.e. II, III, I).
The final evaporation is done at the highest temperature so
economies are still better than forward feed, but fewer
pumps are required than in a backward feed arrangement.
Mixed Feed
Methods of Feeding of Evaporators
Parallel feed arrangement in triple-effect evaporator
Parallel Feed systems split the feed stream and feed a
portion to each effect.
This is most common in crystallizing evaporators where
the product is likely to be a slurry.
Parallel Feed
Advantages and limitations of different modes of feed
supply to multiple effect evaporator
Mode of
Feed supply
Advantages Limitations
Forward
Feed
Simple to operate; less
expensive; the liquor flows from
one effect to the next driven by
the pressure differential between
successive effects and hence no
pump is required to transferring
the liquor; less chance of
deterioration of heat sensitive
materials because the more
concentrated liquor is vaporized
at a lower temperature
Reduced rate of heat transfer
in the second and higher
effects; feed should not be
below the boiling point
because this reduces steam
economy by consuming
external steam to supply
sensible heat.
Backward
Feed
The most concentrated liquor is
in contact with the highest
temperature steam and thus
lower viscosity and higher heat
transfer rate in the first effect as
a result.
Inter-effect pumps are
necessary; higher risk of
damage of the viscous
product subjected to a higher
temperature; risk of fouling.
Advantages and limitations of different modes of feed
supply to multiple effect evaporator
Mode of
Feed supply
Advantages Limitations
Mixed feed Combines the simplicity of
forward feed and economy of
backward feed; useful for
concentration of a highly viscous
feed.
More complex, piping
instrumentation which make
arrangement more
expensive.
Parallel feed More suitable for use with
crystallizers; allows better
control.
More complex arrangement;
pumps generally required for
each effect.
There is wide variation in characteristics of liquor to be
concentrated that requires judgment and experience in
designing and operating evaporators.
Some of the properties of evaporating liquids that influence the
process of evaporation are:
1) Concentration:
Initially, the solution may be quite dilute and the properties of
the solution may be taken as the properties of solvent.
As the concentration increases, the solution becomes viscous
and heat transfer resistance increases.
The crystal may grow on the heating coil or on the heating
surface. The boiling points of the solution also rise
considerably.
Solid or solute contact increases and the boiling temperature
of the concentrated solution became higher than that of the
solvent as the same pressure (i.e. elevation in boiling point).
2) Foaming:
Many of the materials like organic substance may foam
during vaporization.
If the foam is stable, it may come out along the vapor known
as entrainment (carry over of a portion of liquid by rising
vapour is called as entrainment).
Heat transfer coefficient changes abruptly for such systems.
3) Scale:
Many solution have tendency to deposit the scale on the
heating surface, which may increase the heat transfer
resistance that results in reduction of heat transfer
coefficient and hence heat transfer rate.
These scales produce extra thermal resistance of
significant value. Therefore, scaling in the equipment
should not be ignored thus de-scaling becomes an
important and routine matter.
It is therefore necessary to clean the tubes at definite
intervals.
4) Temperature sensitivity:
The products of many chemical, food, pharmaceutical
industries etc. are very temperature sensitive and they
may get damaged during evaporation (when heated to
moderate temperature even for short times).
For concentrating such materials special techniques are to
be used that reduce temperature and time of heating.
5) Materials of Construction:
The material of the equipment must be chosen considering
the solution properties so that the solution should neither be
contaminated nor react with the equipment material.
Generally evaporators are made of mild steel whenever
contamination and corrosion is a problem.
Special materials such as copper, nickel, stainless steels may
be used.
Other liquid characteristics that must be considered in design
are specific heat, freezing point, toxicity, explosion hazards
and radioactivity.
Factors related to process are
• Viscosity
• Thermal sensitivity
• Heat transfer coefficient
• Vapour-liquid separator
• Fouling
• Crystallization
• Evaporator load
• Temperature driving force
• Foaming properties
Criteria for selection of evaporator
The selection of evaporator is done on the basis of
1. Factor related to process
2. Factor related to mechanical design
Viscosity:
It is the highly influencing parameter.
The higher the viscosity the lower is heat transfer
coefficient and larger the heat transfer surface area.
Fouling:
In most evaporators fouling is due to sedimentation,
crystallization, chemical reaction, corrosion, polymerization
etc.
Fouling tendency is considerably reduced as velocity
increases
Crystallization:
For solutions which have a tendency to crystallize during
evaporation, tabular heating surface is the best choice.
Thermal sensitivity:
Food, dairy, brewery and pharmaceutical products are
temperature sensitive; Film type evaporators are best for
such operations; usually operated under vacuum.
Evaporator load:
When heat load is high or heat transfer coefficients are
low, large heating surface area is required. In such
operations multi effect evaporation is a good choice.
Heat transfer coefficient:
Heat transfer coefficient is a function of fluid velocity,
viscosity, density, specific heat and thermal conductivity.
Higher heat transfer coefficient needs smaller heat transfer
surface area for a given duty.
Fouling of the heating surfaces, reduces heat transfer
coefficient and increases pumping cost.
Heat transfer coefficient:
• Film type evaporators are attractive in such cases.
• The thin film offers less resistance to vapour-liquid
separation.
• Separation of entrained liquid is carried out in the vapour
head to avoid excessive entrainment, product loss and
environmental pollution.
Foaming:
• If foam is formed during evaporation.
• It can cause large material loss due to entrainment.
• Either antifoaming agents has to be used or if it is not
possible special arrangement has to be made for
vapour liquid separation.
Selection guide of evaporator
Performance Measures:
There are three main measures of evaporator performance:
1.Capacity (kg vaporized / time)
2.Economy (kg vaporized / kg steam input)
3.Steam Consumption (kg / hr)
Steam Consumption = Capacity/Economy
Capacity: Capacity of an evaporator is defined as the
number of kilogram of water vaporized or evaporated per
hour.
Evaporator economy: Economy of an evaporator is
defined as the number of kilogram of water evaporated per
kilogram of steam fed to the evaporator. It is also called as
steam economy.
Performance Measures:
Economy calculations are determined using enthalpy
balances.
In single effect evaporator the amount of water
evaporated per kg of steam fed is always less than one
and hence economy is less than one.
The methods of increasing the economy are:
(i) Use of multiple effect evaporation system
(ii) Vapour recompression
In multiple effect evaporation system, the vapour produced
in first effect is fed to the steam chest of second effect as
heating medium in which boiling takes place at low
pressure and temperature and so on. Thus in triple effect
evaporator, 1 kg of steam fed to first effect evaporates
approximately 2.5 kg of water
Another method to increase the economy of an evaporator
is to use principle of thermo compression.
Here the vapour from the evaporator is compressed to
increase its temperature so that it will condense at a
temperature higher enough to permit its use as heating
media in the same evaporator.
Boiling Point Elevation
As evaporation proceeds, the liquor remaining in the
evaporator becomes more concentrated and its boiling point
will rise.
The extent of the boiling-point elevation depends upon the
nature of the material being evaporated and upon the
concentration changes that are produced.
In actual practice, boiling point elevation and liquid head
affect the boiling point of a solution.
As vapour pressure of most aqueous solutions is less than
that of pure water at a given temperature; the boiling point of
the solutions is higher than that of pure water at a given
pressure.
Boiling Point Elevation
The difference between the boiling point of a solution and
that of pure water at any given pressure is known as boiling
point rise/elevation of a solution will be higher than that of the
water at a given pressure is known as Boiling Point
Rise/Elevation(BPE) or vapour pressure lowering.
Boiling Point Elevation
The boiling point of a solution is a colligative property -- it
depends on the concentration of solute in the solution, but
not on what the solute and solvent are.
Boiling point elevation is small for dilute solutions and
large for concentrated solution of inorganic salts.
Boiling point elevation of strong solution can be obtained
from an empirical rule known as Duhring’s rule. It states
that the boiling point of a given solution is linear function of
the boiling point of pure water at the same pressure.
Hence when the boiling point of the solution plotted
against the boiling point of the water, straight line results.
Duhring plot for boiling point
of sodium chloride solutions
Vapour pressure/temperature
curve for water
Duhring plot for boiling point
of sodium hydroxide solutions
To use a Duhring plot:
1. For a particular system pressure, determine the boiling
temperature of pure water. This can be done from a
vapor pressure equation or steam table.
2. Enter the plot from the bottom (the water boiling point),
trace up to the diagonal line representing the NaOH
fraction, then trace left to read the solution boiling point
from the vertical axis.
3. The boiling point elevation is the difference between
the two temperatures.
Vacuum Evaporation
For the evaporation of liquids that are adversely affected
by high temperatures, it may be necessary to reduce the
temperature of boiling by operating under reduced
pressure.
The reduced pressures required to boil the liquor at lower
temperatures are obtained by mechanical or steam jet
ejector vacuum pumps, combined generally with
condensers for the vapours from the evaporator.
Vacuum Evaporation
Mechanical vacuum pumps are generally cheaper in
running costs but more expensive in terms of capital than
are steam jet ejectors.
The condensed liquid can either be pumped from the
system or discharged through a tall barometric column in
which a static column of liquid balances the atmospheric
pressure.
Vacuum pumps are then left to deal with the non-
condensibles, which of course are much less in volume but
still have to be discharged to the atmosphere.