- 1.Khalid nawaz cell no: 0300-70244 Institute Of Chemical
Engineering And Technology Lahore Pakistan
2. Evaporation Evaporation is a unit operation that consists
ofthe elimination of water of a fluid food by means of vaporization
or boiling. Evaporation is the removal of solvent as a vapor from
asolution 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 3. Evaporator The equipment used to remove
water from the foodproduct is called evaporator Evaporators are
used to separate materials based on differences in their boiling
temperatures. Its purpose is to concentrate nonvolatile solutes
such as organic compounds, inorganic salts, acids or bases. Typical
solutes include phosphoric acid, caustic soda, sodium chloride,
sodium sulfate, gelatin, syrups and urea. 4. Why we need
evaporation Reduces transportation cost Storage costs Prepare for
the next Unit operation drying, crystallisation etc. Reduces
deteriorative chemical reactions Better microbiological stability
Recovery of solvent 5. Driving force&Result Driving force:
temperature difference in between steam chest temperature and
product temperature Result: volatile solvent is removed from the
feed Solution(high volatile solvent+non volatile solute)
Concentrate(non volatile solute) 6. Examples Concentration of milk
to produce condensed milk Concentration of juices Concentration of
NaOH , NaCl from aqueous solutionsto produce salt. Ether recovery
from fat extraction 7. Principles Steam heat is used for transfer
of heat for subsequentvessels. Steam has a very high heat content
Heat is given up at constant temperature. It can be used at high
pressure to generate electric power and low-pressure exhaust steam
is used for process heating. Evaporation is a process of vaporizing
large quantities of volatile liquid to get a concentrated product.
Evaporation is a surface phenomenon, i.e., mass transfer takes
place from the surface. 8. Basic Parts of an Evaporator
Heat-exchanger Vacuum Vapour separator
Condenserhttp://avibert.blogspot.co m 9. Vaccum for non
condensableEvaporatorCoolant InVapor out Condensor unitCoolant
outVapor Separator Feed in Steam in (Saturated vapor) Heat
Exchanger Condensate out (Saturated Liquid)Product out 10.
Evaporator An evaporator is used to evaporate a volatilesolvent,
usually water, from a solution. Its purpose is to concentrate
non-volatile solutes such as organic compounds, inorganic salts,
acids or bases. Typical solutes include phosphoric acid, caustic
soda, sodium chloride, sodium sulphate, gelatine, syrups and urea.
In many applications, evaporation results in the precipitation of
solutes in the form of crystals, which are usually separated from
the solution with cyclones, settlers, wash columns, elutriating
legs, filters or centrifuges. Examples of precipitates are sodium
chloride, sodium sulfate, sodium carbonate and calcium sulphate.
The desired product can be the concentrated solution, the
precipitated solids, or both. 11. Evaporator Type Batch Pan Rising
Film Falling Film Multiple Effect Evaporators
http://avibert.blogspot.co m 12. Heat Transfer Coefficient, h A
coefficient which indicates the amount of heat flowthat is
exchanged across a unit area of a medium or system in a unit amount
of time with a unit of temperature difference between the boundary
of the system. S.I. unit W m-2 K-1.http://avibert.blogspot.co 13.
Overall Heat Transfer Coefficient, U In cases of combined heat
transfer for a heat exchanger, there are two values for h.
Convective heat transfer coefficient for the fluid film inside the
tubes Convective heat transfer coefficient for the fluid film
outside the tubes. The thermal conductivity (k) and thickness(Dx)
of the tube wall must also be accounted for. So an additional term
(Uo), called the overall heat transfer coefficient, must be used
instead. http://avibert.blogspot.co 14. http://avibert.blogspot.co
15. 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 heatsensitive
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 16. Cont..
Used for concentration of Jams and Jellies, alsofor some
pharmaceutical products 17. 3) Calandria type Evaporator: Commonly
it is known as short tube or single effect evaporator.Features:
Vertical type of evaporator. Tubes sheets extending across the body
and central down take. Material which has to be evaporated is
introduced in tubes. The tube may be about 1.2m long and 5 cm in
diameter, but the size varies with the nature of the substances.
There is space below containing a steam coil to give extra heating
capacity and large enough to afford circulation of liquid. 18.
Disadvantages:*Large floor space and weight *Poor heat transfer at
low temperature differences. *Not use for thermo-labile products
19. 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 20. a. Tubes are horizontal with steam inside 21. Cont..
Cylindrical section above heating portion is usedto separate vapor
from liquid vapor leaves through de-entraining device toprevent
carry over of liquid droplets with vapor Steam enters through one
side chest and leavesthrough opposite chest Steam condensate out
via steam trap 22. The major use is for making distilled water for
boilerfeed. Horizontal tube evaporators are used in the
pharmaceutical industry, pulp and paper industry. They are
relatively low cost. They have very low headroom. Horizontal tube
evaporators are not suitable for salting or scaling liquids, and
they have smaller capacity than other evaporators.11.01.201426 23.
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 24. outside
Solution boils inside vertical tubes with heatingmedia, 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 25. Cont..
26. Cont.. These overcome disadvantages of horizontal
tubeevaporators 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 formfoams or is
heat sensitive 27. Cont 28. Climbing film Evaporators: It is also
known as long tube or rising film evaporator. The theory of
climbing film evaporators working is that the ascending force of
this steam produced during the boiling causes liquid and vapors to
flow upwards in parallel flow. At the same time the production of
vapor increases and the product is pressed as a thin film on the
walls of the tubes, and the liquid rises upwards. This co-current
upward movement against gravity has the beneficial effect of
creating a high degree of turbulence in the liquid. This is
advantageous during evaporation of highly viscous products and
products that have a tendency to foul the heating surface. Usually
there must be a rather high temperature difference between the
heating and boiling sides of this type of evaporator. Otherwise the
energy of the vapor flow is not sufficient to convey the liquid and
to produce the rising film. The length of the boiling tubes will
typically not exceed 23 ft (7m). 29. Advantages: High heat transfer
rates at high temperature differences Ease of cleaning Relatively
inexpensive Disadvantages: Large floor space and weight Poor heat
transfer at low temperature differences Not use for thermolabile
products 30. Horizontal Film Evaporator: Also known as falling film
evaporator. This type of evaporator is generally made of long tubes
(4-8 meters in length) which are surrounded by steam jackets. The
uniform distribution of the solution is important when using this
type of evaporator. The solution enters and gains velocity as it
flows downward. This gain in velocity is attributed to the vapor
being evolved against the heating medium, which flows downward as
well. This evaporator is applicable to highly viscous solutions so
it is frequently used in the chemical, food, and fermentation
Falling Film Evaporator with boiling inside the tubes. The feed to
the evaporator is fed to the top of the Calandria through efficient
liquid distributors. Steam is given on the shell side. The
concentrate is collected at the bottom. The same operation is
repeated in multiple effects to achieve steam economy. 31. )
Rising-falling film Evaporator: Rising and falling film
evaporatorsare some time combined in to a same unit. When a high
ratio of evaporation to feed is required and concentrated liquid is
viscous, a tube bundle can be divided in to two sections with the
first section functions as a rising film evaporator and the second
section serves as falling film evaporator. 32. Advantages: Low cost
and small floor space Suitable for viscous material Large units for
handling high evaporation load Disadvantages: High headroom
required Recirculation is frequently required 33. Multiple-effect
Evaporator Water is boiled in a sequence of vessels, each held at
alower pressure than the last. Because the boiling point of water
decreases aspressure decreases, the vapor boiled off in one vessel
can be used to heat the next Generally the first vessel (at the
highest pressure)requires an external source of
heathttp://avibert.blogspot.co 34. Construction: A multiple effect
evaporator system for concentrating a process liquid comprises: (a)
a plurality of evaporator effects arranged in series, each effect
including a process liquid inlet and a process liquid outlet; a
heating fluid inlet and heating fluid outlet;(b) heat exchange
means in each effect for passing said process liquid in heat
exchange relationship with heating fluid for evaporating water out
of said process liquid; and wherein evaporated water from one
effect serves as heating fluid for an adjacent effect; and (c) an
evaporative condenser provided with liquid inlet means for
receiving process liquid from one of said evaporator effects, and
liquid outlet means for transmitting said process liquid to another
of said evaporator effects; and means for receiving heating fluid
vapor and for passing said heating fluid vapor in heat exchange
relationship with cooled process liquid in a cooling circuit, for
condensing said heating fluid vapor. 35. Equipment description (1)
Thermal recompression unit, (2) Steam for heating (3) Feed in (4)
Calandria (5) Feed out (6) Vapour Separator (7) Pre-heater (8)
Condenser (9)Cooling water in, (10) Cooling water return 36.
http://avibert.blogspot.co 37. Working: Multiple effect evaporator
Due to heat transfer, the liquid temperature increases &
reaches the B.P. during this process, vapour well be generated from
the liquid feed. So, formed vapour displaces air in the upper part
of 1st evaporator. Moreover, the vapour also displaces the air in
the steam space of the 2nd evaporator. After complete displacement
of air by vapour in the steam compartment of 2nd evaporator, the
second valve is closed. The vapour of 1st evaporator transmits its
heat to the liquid of 2nd evaporator & gets condensed.
Condensate is removed through the second condensate valve. These
steps continue in the 3rd evaporator also. 38. Working: As the
liquid in 1st evaporator gains temperature the difference
intemperature between the liquid & steam decreases, hence, the
rate of condensation decreases. As a result, the pressure in the
vapour space of 1st evaporator gradually increases to P1 by
increasing temperature to T1 , which is the B.P. of the liquid in
first evaporator & decreasing the temperature difference
(t0-t1). A similar change takes place in the 2nd evaporator &
the liquid reaches the B.P. similarly, the process will be repeated
in 3rd evaporator. Finally 3 evaporators come to a steady state
with the liquid boiling in all the 3 bodies. 39. Working: As
boiling proceed, liquid level in 1st evaporator comes down. Feed is
introduced through the feed valve to maintain the liquid level
constant. Similarly evaporation of liquid takes place in 2nd &
3rd evaporators. To maintain the liquid levels constant, feed
valves F2 & F3 are used for 2nd & 3rd evaporator
respectively. This process is continued until the liquid in all the
evaporators reaches the desired viscosity. Now the product valves
are opened to collect the thick liquid. Thus in this evaporators,
there is continuous supply of feed, continuous supply of steam
& continuous withdrawal of liquid from all 3 evaporators.
Hence, evaporators work continuously. 40. Efficiency of multiple
effect evaporator It is the quantity of vapour produced per unit
steamadmitted. Feed is admitted at its B.P. so it does not require
any more heat to raise its temp. Hence, the supplied steam is
condensed to give heat of condensation. This heat will then
transferred to the liquid. The heat transferred now serves as
latent heat of vaporization, i.e. liquid undergoes vaporization by
receiving heat. Loss of heat by means is negligible. 41. Feeding of
Multiple Effect Evaporators There are three types of feeding
methods:Forward feeding Backward feeding Parallel feeding 1)Forward
feeding: The pressure in the second effect must be reduced below
that in the first. In some cases, the first effect may be at a
pressure above atmospheric; or the first effect may be at
atmospheric pressure and the second and subsequent effects have
therefore to be under increasingly lower pressures. 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. This is called forward feed 42.
Forward feeding:In the case of a forward feet operation, the raw
feed is introduced in the first effect and is passed from effect to
effect parallel to steam flow. The product is withdrawn from the
last effect. This procedure is highly advantageous if the feed is
hot. The method is also used if the concentrated product may be
damaged or may deposit scale at high temperature 43. Backward feed:
Alternatively, 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. This is called backward feed and because 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, but it may be
disadvantageous from the viewpoint of product quality. 44. Backward
feeding:In the backward operation, the raw feed enters the last
(coldest) effect and the discharge from this effect becomes a feed
for the next to last effect. This technique of evaporations is
advantageous, in case the feed is cold, as much less liquid must be
heated to the higher temperature existing in the early effects. The
procedure is also used if the product is viscous and high
temperatures are required to keep the viscosity low enough to
produce good heat transfer coefficients. 45. Parallel feeding:
Parallel feed : 46. A hot saturated solution of the feed is
directly fed into each of the three effects in parallel without
transferring the material from one to another. Thisis commonly used
in the concentration of the salt solution, where the solute
crystallizes on concentration without increasing the viscosity.
Operations :-The equipment is at room temp. & at atm. Pressure
at the beginning. The liquid feed is introduced into all the 3
evaporators up to the level of upper tube sheets. 47. Multiple
effect evaporator Advantages Suitable for large scale & for
continuous operation. Highly economical when compared to single
effect. Multiple effects, or stages, are now used to minimize
theenergy input required to evaporate or boil off undesirable water
content. The total evaporation achieved in these systems is
approximately the number of effects times the energy input to the
first effect. 48. Typical materials of construction for a number of
evaporator applications are shown below: PRODUCTMATERIAL OF
CONSTRUCTIONMost dairy and food products304/316 stainless steelMost
fruit juices316 stainless steelSugar productsCarbon steel
/304/316Foods containing high salt (NaCl)Titanium/Monel High alloy
stainless steels Duplex stainless steelsCaustic soda < 40%Stress
relieved carbon steelCaustic soda high
concentrationNickelHydrochloric acidGraphite/Rubber lined carbon
steel 49. Design criteria and processing factors: Must prevent
entrainment due to product loss Contamination of the vapour phase
(polution) Condensation of vapour onto surfaces (corrosion and
fouling) Overhead mist or spray may cause troublesome deposits
Vortices increase pump head requirements and therefore equipment
Configuration Short circuiting a big problem as it presents the
problem of cavitation (there must be a net positive suction head)
50. Other factors: Liquid concentration -> relates to viscosity
and heat transfer Temperature and Pressure Boiling temperature is
inversely proportional to pressure. Boiling points may increase as
solution get concentrated (boiling point rise) Foaming -> will
determine the height of your freeboard in the Design Solubility of
materials -> May be the limit to the concentration that you can
achieve. Scale deposits -> decrease your heat transfer
coefficient. 51. Criteria for selection of Multiple Effect
Evaporator plant: During the design of evaporation plants,
numerous, sometimescontradictory, requirements have to be
considered. They determine which type of construction and
arrangement is chosen, and the resulting process and economic data.
The most important requirements are as follows: Capacity and
operational data, including quantities, concentrations,
temperatures, annual operating hours, change of product, controls
automation, etc. Product characteristics, including heat
sensitivity, viscosity and flow properties, foaming tendency,
fouling and precipitation, boiling behaviour, etc. Required
operating media, such as steam, cooling water, electric power,
cleaning agents, spare parts, etc. 52. Capital and other financial
costs Personnel costs for operation and maintenance Standards and
conditions for manufacture, delivery, acceptance,etc. Choice of
materials of construction and surface finishes Site conditions,
such as available space, climate (for outdoor sites), connections
for energy and product, service platforms, etc. Legal regulations
covering safety, accident prevention, sound emissions,
environmental requirements, and others, depending upon the specific
project. 53. Possible CausesPossible RemediesDrive motor trips The
product may be backing up in the evaporator body due to choking.
Open discharge lines to check and eliminate cause of choking. The
evaporator body may be heavily scaled. Dismantle the unit and check
for scaling. The feed rate is excessive. Control the feed rate.
There might be a problem with the drive motor. Check for
malfunctioning of the drive motor.System starts vibrating There may
be heavy scaling. Go through the cleaning procedure. The product is
backing up in the body due to choking. Check and eliminate the
cause of product backup. Uneven heating of evaporator shell causing
thermal stresses on its body. Check the steam pressure of
thermocompressor.
