This presentation discussed about evaporation process and the properties of the liquid that can affect the evaporation process. Evaporation is a process of movement of heat and mass fraction in different form in liquid. Balance of the heat and mass occured in evaporation are presented in single and multiple effect evaporator. Aroma recovery also a major phenomena in food industry. A number of different equipment and technologies are used in evaporation of liquid food products. The scope of appointment of this phenomena and applications are vast.

1. PRESENTATION ON
EVAPORATION: Properties of Liquids, Heat and Mass Balance in
Single Effect And Multiple Effect Evaporator, Aroma Recovery,
Equipment And Application
ADVANCED FOOD PROCESS ENGINEERING

2. Content:
 Evaporation: introduction
 Properties of Liquids
 Heat and Mass Balance In Single Effect And Multiple Effect Evaporator
 Aroma Recovery
 Equipment
 Application

3. Introduction
 Evaporation, or concentration by boiling, is the partial removal of water from liquid foods by boiling off water
vapour.
 Evaporation is used to pre-concentrate foods (for example fruit juice, milk and coffee) prior to drying, freezing
or sterilization and hence to reduce their weight and volume.
 Evaporation is a food preservation technique in which dilute liquid foods and solutions are concentrated by the
evaporation of water, with the aim of increasing microbiological stability and shelf life.
 A second major reason for the concentration of liquids is the reduction in transport and storage costs which can
be achieved by reducing the product bulk volume
 Most food liquids have relatively low solids contents. For example, whole milk contains approximately 12.5%
total solids, fruit juice 12%, sugar solution after extraction from sugar beet 15%.
 The equipment used to remove this water from the food product is called an evaporator.

4. Factors Influencing The Rate of Heat Transfer:
 Temperature difference between the steam and boiling liquid.
 Deposits on heat transfer surfaces- The ‘fouling’ of evaporator surfaces reduces the rate of heat transfer.
 Boundary films- film of stationary liquid at the evaporator wall is often the main resistance to heat transfer.
Factors Influencing The Economics of Evaporation:
 Product losses are caused by foaming, due to proteins and carbohydrates in the food, which causes
inefficient separation of vapour and concentrate, and entrainment, in which a fine mist of
concentrate is produced during the violent boiling, and is carried out of the evaporator by the
vapour.
 The economics of evaporation are therefore substantially improved by attention to the design and
operation of equipment and careful planning of energy use.
 Energy can be saved by re-using heat- vapour recompression, preheating, multiple effect evaporation.

5. Properties of Liquid:
 All liquids evaporates.
 All liquids experience surface tension, an imbalance of forces on surface of liquids.
 If volume is limited, evaporation eventually reaches dynamic equilibrium, and a constant vapour
pressure mainteained.
 The rheological properties of liquid food- are altered based on thermal properties such as density, heat
capacity, thermal conductivity.
 Important factor for determining properties of liquid food are shear flow properties based on shear
stress and shear rate.
 the viscosity of many liquids changes during heating, cooling, concentration etc. its important property
in many areas of food processing.
 The characteristics of the liquid food have a profound effect on the performance of the evaporation
process. As water is removed, the liquid becomes increasingly concentrated, resulting in reduced heat
transfer.
 The boiling point rises as the liquid concentrates, resulting in a smaller differential of temperature
between the heating medium and the product. This causes reduced rate of heat transfer.

6. Single Effect Evaporator:
 In a single-effect evaporator, dilute liquid
feed is pumped into the heating chamber,
where it is heated indirectly with steam.
Steam is introduced into the heat
exchanger, where it condenses to give up
its heat of vaporization to the feed, and
exits the system as condensate.
 In a single-effect evaporator it takes 1.1–
1.3 kg of steam to evaporate 1.0 kg of
water. This is known as the specific
steam consumption of an evaporator
Fig.- schematic diagram of single effect evaporator

7. Heat and Mass Balance of Single Effect Evaporator:
Mass balance mf =mv+mp
where mf is the mass flow rate of dilute liquid feed (kg/s),
mv is the mass flow rate of vapor (kg/s), and mp is the mass flow rate of
concentrated product (kg/s),
xfmf=xpmp
where xf is the solid fraction in the feed stream and xp is the solid fraction
in the product stream.
enthalpy balance
mf Hf + ms Hvs =mv Hv1 + mp Hp1 +ms Hcs
Hf =cpf (Tf +0°C)
Hp1 cpp(T1 0°C)
where c pp is the specifi c heat content of concentrated product
(kJ/[kg °C]).
Fig.- block diagram of single effect evaporator process

8.  For the heat exchanger, the following expression gives the rate of heat transfer:
 q=UA(Ts T1) = m sHvs -m sHcs
 where q is the rate of heat transfer (W), U is the overall heat transfer coefficient (W/[m 2 K]),
and A is the area of the heat exchanger (m 2 ).
 Steam economy is a term often used in expressing the operating performance of an evaporator
system. This term is a ratio of rate of mass of water vapor produced from the liquid feed per
unit rate of steam consumed.
 Steam economy= m v/m s

9. Multiple Effect Evaporator:
 In a triple-effect evaporator, dilute liquid feed
is pumped into the evaporator chamber of the
first effect. Steam enters the heat exchanger
and condenses, thus discharging its heat to the
product.
 The condensate is discarded. The vapors
produced from the first effect are used as the
heating medium in the second effect, where the
feed is the partially concentrated product from
the first effect.
 The vapors produced from the second effect
are used in the third effect as heating medium,
and the final product with the desired fi nal
concentration is pumped out of the evaporator
chamber of the third effect. The vapors
produced in the third effect are conveyed to a
condenser and a vacuum system.
Fig.- schematic diagram of multiple effect evaporator

10.  Design expressions for multiple-effect evaporators
can be obtained in the same manner as for a
single-effect evaporator.
 Conducting mass balance analysis on the fl
ow streams,
m f = m v1 + m v2 m v3 + m p
 Using mass balance on the solids fraction in
the fl ow streams,
xf mf = xp mp
 where x f is the solid fraction in the feed
stream to be consistent with the fi rst effect
(dimensionless) and x p is the solid fraction in
the product stream from the third effect
(dimensionless).
Heat and Mass Balance of Multiple Effect Evaporator:
Fig.- block diagram of multiple effect evaporator process

11.  We write enthalpy balances around each effect separately.
mf Hf +ms Hvs = mv1 Hv1 +mf1 Hf1 + msHcs
mf1 Hf1 + mv1 Hv1 = mv2 Hv2 +mf2 Hf2 +m v1Hc1
m f2Hf2 +m v2Hv2 = m v3Hv3 +m pHp3 +m v2Hc2
 The heat transfer across heat exchangers of various effects can be expressed by the
following three expressions:
q1 = U1A1(Ts - T1) = ms Hvs + ms Hcs
q2 = U2A2(T1 - T2) = m v1 H v1 + m v1 Hc1
q3 = U3A3(T2 - T3) = m v2 H v2 + m v2 Hc2
Steam economy = m v1 + m v2 + m v3 / m s

12. Aroma Recovery Process:
 An aroma recovery process removes the volatile aroma
from a food and concentrates it to a useful aroma-rich
product. In the aroma removal step the aroma is
transferred from the food to a second phase.
 This second phase can be a gas, a liquid, or a solid. A
gaseous phase can be created by partly evaporating
the food itself (evaporation) or by addition of an inert
gas (stripping). A liquid phase which is only partially
miscible with the food material can often be found (for
example, pentane or liquid carbon dioxide).combinations
for aroma removal.
 To complete the aroma recovery process, the dilute aroma
must be concentrated (or rectified), typically to 1/100 to
1/150 the volume of the original food.
 This concentrated aroma solution can be added back to the
final product or stored separately for later use.
Fig.- General aroma recovery process

13. Aroma Recovery Process:

14. Types of Evaporators:
1. Falling film evaporator
2. Agitated thin film evaporator
3. Forced circulation evaporators
4. Climbing and falling film plate
evaporator
5. Rising/falling film evaporators
6. Rising film/long tube vertical
evaporator
7. Multiple effect evaporator

15. Falling film evaporator
• Falling film evaporators are made up of 13 ft to 26 ft
tubes which is enclosed with the steam jackets.
• Falling film evaporators require uniform distribution of
solution. The solution in this evaporator enters from top
from where distributor distributes it evenly to every
tube.
• In this evaporator, as the solution flows downward it
gains velocity due to the gravitational pull.
• As the vapour developed gradually in response to the
heating medium, the velocity increases as well. Falling
film evaporators are used for high viscous solutions
therefore they are usually used for sugar, chemical,
fermentation and food industries.
• Application of Falling Film Evaporator:
Falling film evaporator are used for sugar solution, black
liquor, concentration of dairy products and phosphoric acid

16. Forced Circulation Evaporator
• also known as natural circulation evaporator and these
evaporators are generally based on natural circulation which is
the result of the difference in density which arises due to the
heating.
• The main problem with forced circulation evaporators arises
when the tubes of evaporator do not completely immersed in
solution and this will create compromised circulation and dried
out system. To overcome this problem, circulation are achieved
with the help of inserting a pump for increasing the circulation
as well as pressure. This type of circulation generally takes
place when the hydrostatic head restrict the boiling at heating
surface.
• Application of Forced Circulation Evaporators:
Forced circulation evaporators are used for sodium sulphate, citric
acid, urea, sodium chloride, caustic potash, ammonium sulphate
and magnesium chloride.

17. Climbing and Falling-Film Plate
Evaporator
 These evaporators have generally larger surface area.
 The plates are generally supported with the help of
frame and are corrugated. At the time of evaporation the
steam will flow through the free space channels which
are in between plates, steam climbs and then fall parallel
to the liquid which is concentrated. In relation to liquid,
the steam will follow counter current path.
 The vapour as well as the concentrated liquid is then
sent to separation stage in which vapour is sent to the
condenser. Due to the spatial flexibility this evaporators
are generally used for fermentation and dairy industries.
The disadvantage of this evaporator is that their ability is
limited in treating solid containing and viscous products.
 Application:
Climbing and falling film plate evaporator are used in dairy
industries

18. Rising / Falling Film Evaporator
• In this evaporator boiling occur in the tubes
because the heating is done outside the tubes
generally by steam.
• Therefore submergence is not required. These
evaporators are generally efficient but the major
disadvantage is that scaling occurs quickly in
the internal surface of tubes.
• So, this evaporator is suitable for non salting
and clear solution. Tubes in this evaporator are
generally long which can be more than 4 meters.
Deciding the size of the evaporator is difficult
because it need precise calculation of level in
process liquid in tubes.

19. Agitated Thin Film Evaporator
• These evaporators are able in the separation of volatile component from the
components which are less volatile with the help of mechanical agitation and
indirect heat transfer of flowing product film in control condition.
• This separation is done in vacuum condition for maximizing the change in
temperature with maintaining temperature which is favorable for product.
There are some technical issues occur at the time of evaporation.
• There are few evaporators which can be sensitive to viscosity difference as
well as consistency of dilute solution, these evaporators results in inefficient
work due to the circulation loss. When high viscous solution is needed to be
concentrate then evaporator’s pump are changed.
• Fouling in evaporator takes place when stiff deposits create surface on the
heating medium of evaporators, it occurs in case of proteins, food and they
ultimately decrease the heat transfer efficiency.
Application of Agitated Thin Film Evaporator
Agitated thin film evaporators are commonly used for purifying organic chemicals which include insecticides, natural
oils, herbicides, fatty acids. Pharmaceuticals and food concentration which include biological solution, fruit purees and
vegetable purees, vegetables and plant extracts etc.

20. Rising Film / Long Tube Vertical
Evaporator
• in this evaporator boiling occur in the tubes
because the heating is done outside the tubes
generally by steam.
• Therefore submergence is not required. These
evaporators are generally efficient but the major
disadvantage is that scaling occurs quickly in the
internal surface of tubes.
• So, this evaporator is suitable for non salting and
clear solution. Tubes in this evaporator are
generally long which can be more than 4 meters.
Deciding the size of the evaporator is difficult
because it need precise calculation of level in
process liquid in tubes.

21. Multiple Effect Evaporator
• There is a heat saving percent which is used for the estimation of how much energy is saved with the addition of a
particular number of effect. Generally in multiple effect evaporators, the number of effects are kept to seven because at
seven effect the equipment cost will reach to cost saving.
• These evaporators are manufactured of seven evaporators
effects/ stages.
• The requirement of energy for single effect evaporators are
huge and are responsible for most of the cost of evaporation
system. By putting seven evaporators effects/ stages together
can save the heat requirement therefore need less energy.
• By providing one evaporator to the existing original one
results in reduction of energy consumption by 50 percent,
and further addition of a evaporator will reduce the energy
consumption by 33 percent

22. Advantages and limitations of various methods of multiple effect evaporation
Arrangement of effects Advantages Limitations
Forward feed Least expensive, simple to operate, no feed pumps
required between effects, lower temperatures with
subsequent effects and therefore less risk of heat
damage to more viscous product
Reduced heat transfer rate as the feed becomes more
viscous, rate of evaporation falls with each effect, best
quality steam used on initial feed which is easiest to
evaporate. Feed must be introduced at boiling point to
prevent loss of economy (if steam supplies sensible heat,
less vapour is available for subsequent effects)
Reverse feed No feed pump initially, best quality steam used on
the most difficult material to concentrate, better
economy and heat transfer rate as effects are not
subject to variation in feed temperature and feed
meets hotter surfaces as it becomes more
concentrated thus partly offsetting increase in
Viscosity
Interstage pumps necessary, higher risk of heat damage to
viscous products as liquor moves more slowly over hotter
surfaces, risk of fouling.
Mixed feed Simplicity of forward feed and economy of
backward feed, useful for very viscous foods
More complex and expensive
Parallel For crystal production, allows greater control over
crystallization and prevents the need to pump
crystal slurries.
Most complex and expensive of the
arrangements, extraction pumps required
for each effect

23. Comparison of residence times and hat
transfer coefficients in evaporators

24. Applications of evaporation in Food Industry:
 Concentrated Liquid Products
1. Evaporated (Unsweetened Condensed) Milk - used in the manufacture of ice cream and yoghurt.
Concentrated whey and buttermilk are used in the manufacture of margarine and spreads.
2. Sweetened Condensed Milk- Sweetened condensed milk is used in the manufacture of other products
such as ice cream and chocolate
3. Concentrated fruit and vegetable juices are also produced for sale to the consumer
 Evaporation as a Preparatory Step to Further Processing
1. Instant Coffee - Beverages, such as coffee and tea, are also available in powder form, so called instant
drinks
2. Granulated Sugar - Vacuum evaporation is used in the production of granulated sugar from sugar cane
and sugar beet
 The Use of Evaporation to Reduce Transport, Storage and Packaging Costs
1. Concentrated Fruit and Vegetable Juices - The frozen concentrate is then shipped to several other sites
2. where it is diluted, packaged and sold as chilled fruit juice. Orange juice is the main fruit juice processed
in this way

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