Evaporation and water vapour ppt.xxxxxxx

Evaporation

1. Evaporation

2. •unit operation commonly employed to
remove water from dilute liquid foods to
obtain concentrated liquid products.
•Evaporation differs from dehydration
•differs from distillation

3. •http://rpaulsingh.com/animations/evap_singl
e.html
•http://rpaulsingh.com/animations/evap_multi
.html

4. •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.

5. •Food products are noted for their heat
sensitivity.
•Evaporation processes must involve reducing the
temperature for boiling as well as the time of
heating, to avoid excessive product degradation.
•In addition, fouling of the heat-exchange surface
can seriously reduce the rate of heat transfer.
•Frequent cleaning of heat-exchange surfaces
requires shutdown of the equipment, thus
decreasing the processing capacity.

6. •Liquid foods that foam during vaporization
cause product losses as a result of escape
through vapor outlets.

7. •Solute concentration increase results in
…………. In boiling point
•A) increase
•B) decrease
•C) No change

8. BOILING-POINT ELEVATION
•Boiling-point elevation of a solution (liquid food) is
defined as the increase in boiling point over that of
pure water, at a given pressure.
•A simple method to estimate boiling-point elevation is
the use of Dühring’s rule.
•The Dühring rule states that a linear relationship exists
between the boiling-point temperature of the solution
and the boiling point temperature of water at the
same pressure.
•The linear relationship does not hold over a wide range
of temperatures, but over moderate temperature
ranges, it is quite acceptable.

10. •The boiling-point elevation merits
consideration since the temperature
difference between steam and product
decreases as the boiling point of the liquid
increases due to concentration.
•The reduced temperature differential causes a
reduction in rate of heat transfer between
steam and product.

11. Equipment Used in Vacuum
Evaporation
•A single-effect vacuum evaporator has the following
components:
•A heat exchanger, known as a calandria, by means of which
the necessary sensible and latent heat is supplied to the feed
to bring about the evaporation of some of the liquid.
•Saturated steam is the usual heating medium but hot water
and other thermal fluids are sometimes used.
•Tubular and plate exchangers of various designs are widely
used.
•A device to separate the vapour from the concentrated liquid
phase. In vacuum evaporators, mechanical devices such as
chambers fitted with baffles or meshes and cyclone
separators are used to reduce entrainment losses.

12. •A condenser to convert the vapour back to a liquid
and a pump, steam ejector or barometric leg to
remove the condensate, thus creating and
maintaining the partial vacuum in the system.
•Most evaporators are constructed in stainless steel
except where there are extreme corrosion problems.

13. Types of evaporators
1. Vacuum Pans
•hemispherical pan equipped with a steam jacket and sealed lid,
connected to a vacuum system,
•is the simplest type of vacuum evaporator in use in industry.
•The heat transfer area per unit volume is small and so the time
required to reach the desired solids content can run into hours.
•Heating occurs by natural convection. However, an impeller stirrer
may be introduced to increase circulation and reduce fouling.
• They are useful for frequent changes of product and for low or
variable throughputs.
•They are used in jam manufacture, the preparation of sauces,
soups and gravies and in tomato pulp concentration.

14. Short Tube Vacuum Evaporators
•consists of a calandria made up of a bundle of short vertical tubes
surrounded by a steam jacket, located near the bottom of a large
vessel
•The liquid being concentrated normally covers the calandria.
•Steam condensing on the outside of the tubes heats the liquid
causing it to rise by natural convection.
•Some of the water evaporates and flows to the condenser.
•The liquid circulates down through the larger, cooler tube in the
centre of the bundle, known as the downcomer.
•This type of evaporator is suitable for low to moderate viscosity
liquids, which are not very heat-sensitive.
•With viscous liquids heat transfer rates are low, hence residence
times are relatively long and there is a high risk of fouling.

16. •In another design of a short tube evaporator the calandria is
external to separator chamber.
•The liquid circulates by natural convection within the heat
exchanger and also through the separation chamber.
•The liquid enters the separation chamber tangentially.
•A swirling flow pattern develops in the chamber, generating
centrifugal force, which assists in separating the vapour from
the liquid.
•The vigorous circulation of the liquid results in relatively high
rates of heat transfer.
•It also helps to break up any foam which forms.

17. •A pump may be introduced to assist in
circulating more viscous liquids.
•This is known as forced circulation.
•The choice of pump will depend on the
viscosity of the liquid.

18. •Short tube evaporator is suitable for
•A) Highly viscous liquids
•B) low to moderate liquids

19. •The heat transfer area per unit volume is ………
in pan evaporators
•A) Small
•B) Large

20. Long Tube Evaporators
•consist of bundles of long tubes, 3–15 m long and 25–50 mm
in diameter, contained within a vertical shell into which steam
is introduced.
•The steam condensing on the outside of the tubes provides
the heat of evaporation.
•There are three patterns of flow of the liquid through such
evaporators.

21. •In the climbing film evaporator the preheated feed is
introduced into the bottom of the tubes.
•Evaporation commences near the base of the tubes. As the
vapour expands, ideally, it causes a thin film of liquid to rise
rapidly up the inner walls of the tubes around a central core of
vapour.
•The liquid becomes more concentrated as it rises.
•At the top, the liquid-vapour mixture enters a cyclone
separator.
•The vapour is drawn off to a condenser and pump, or into the
heating jacket of another calandria, in a multiple-effect system

22. •The concentrated liquid may be removed as product, recycled
through the calandria or fed to another calandria, in a
multiple-effect system.
•The residence time of the liquid in the tubes is relatively
short.
• High rates of heat transfer are attainable in this type of
evaporator, provided there are relatively large temperature
differences between the heating medium and the liquid being
concentrated.
•This type of evaporator is suitable for low viscosity, heat-
sensitive liquids such as milk and fruit juices.

23. •In the falling film evaporator the preheated feed is introduced
at the top of the tube bundle and distributed to the tubes so
that a thin film of the liquid flows down the inner surface of
each tube, evaporating as it descends.
•Uniform distribution of the liquid so that the inner surfaces of
the tubes are uniformly wetted is vital to the successful
operation of this type of evaporator .
•From the bottom of the tubes, the liquid-vapour mixture
passes into a centrifugal separator and from there the liquid
and vapour streams are directed in the same manner as in
the climbing film evaporator.

24. •A climbing-falling film evaporator is also available.
•The feed is first partially concentrated in a climbing
film section and then finished off in a falling film
section.
•High rates of evaporation are attainable in this type
of plant.

25. Plate Evaporators
•In these evaporators the calandria is a plate heat exchanger,
similar to that used in pasteurising and sterilising liquids
•The liquid is pumped through the heat exchanger, passing on
one side of an assembly of plates, while steam passes on the
other side.
•The spacing between plates is greater than that in
pasteurisers to accommodate the vapour produced during
evaporation.
•The liquid usually follows a climbing-falling film flow pattern

26. •The mixture of liquid and vapour leaving the calandria passes into
a cyclone separator.
•The vapour from the separator goes to a condenser or into the
heating jacket of the next stage, in a multiple-effect system.
•The concentrate is collected as product or goes to another stage.
•The advantages of plate evaporators include:
•high liquid velocities leading to high rates of heat transfer, short
residence times and resistance to fouling. They are compact and
easily dismantled for inspection and maintenance. However, they
have relatively high capital costs and low throughputs.

27. Agitated Thin Film Evaporators
•For very viscous materials and/or materials which tend to foul,
heat transfer may be increased by continually wiping the
boundary layer at the heat transfer surface.
•An agitated thin film evaporator consists of a steam jacketed shell
equipped with a centrally located, rotating shaft carrying blades
which wipe the inner surface of the shell.
•The shell may be cylindrical and mounted either vertically or
horizontally.
•Most of the evaporation takes place in the film that forms behind
the rotating blades.
•Relatively high rates of heat transfer are attained and fouling and
foaming are inhibited.

28. Applications for Evaporation
•The purposes for which evaporation is used in the
food industry include:
•to produce concentrated liquid products (for sale to
the consumer or as ingredients to be used in the
manufacture of other consumer products),
• to preconcentrate liquids for further processing and
• to reduce the cost of transport, storage and in some
cases packaging, by reducing the mass and volume of
the liquid.

29. Natural Circulation Evaporators
•short vertical tubes, typically 1–2 m long and
50–100 mm in diameter, are arranged inside
the steam chest
•The whole calandria (tubes and steam chest)
is located in the bottom of the vessel.

30. •The product, when heated, rises through
these tubes by natural circulation while steam
condenses outside the tubes.
•Evaporation takes place inside the tubes, and
the product is concentrated.
•The concentrated liquid falls back to the base
of the vessel through a central annular
section.

31. Rising film evaporator
•http://rpaulsingh.com/animations/evap_rising
.html

32. Falling Film evaporator
•http://rpaulsingh.com/animations/evap_fallin
g.html

33. Rising/Falling-Film Evaporator
•http://rpaulsingh.com/animations/evap_risfall
.html

34. Forced-Circulation Evaporator

35. DESIGN OF A 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.

36. •The temperature of evaporation, T
1, is controlled by
maintaining vacuum inside the heating chamber.
•The vapors leaving the product are conveyed through a
condenser to a vacuum system, usually a steam ejector or a
vacuum pump.
•In a batch system, the feed is heated until the desired
concentration is obtained.
•The concentrated product is then pumped out of the
evaporator system.

37. •Heat and mass balances conducted on the
evaporator system allow determination of
various design and operating variables.
•Such variables may include mass flow rates,
final concentration of product, and heat-
exchanger area.

38. •Evaporation occurs from
•A) surface of liquid
•B) interior of liquid

39. •The following expressions
can be obtained by
conducting a mass balance
on flow streams and
product solids, respectively.
•m
f =m
v + m
p
•where m f is the mass flow
rate of dilute liquid feed
(kg/s), m v is the mass flow
rate of vapor (kg/s), and m
p is the mass flow rate of
concentrated product
(kg/s),

42. •T he last term, m
sH
cs , represents the total
enthalpy associated with the condensate
leaving the evaporator.
•Since an indirect type of heat exchanger is
used in evaporator systems, the rate of mass
flow of incoming steam is the same as the rate
of mass flow of condensate leaving the
evaporator.

43. •The enthalpy H cs is obtained from the steam Table
as enthalpy of saturated liquid evaluated at
temperature T
s
•For the heat exchanger, the following expression
gives the rate of heat transfer:
•q = UA(T
s
-T
1
) = m
s
H
vs
- m
s
H
cs

•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
)

44. •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
•In single effect evaporator steam economy is less
than 1

45. •Apple juice is being concentrated in a natural-circulation
single-effect evaporator. At steady-state conditions, dilute
juice is the feed introduced at a rate of 0.67 kg/s. The
concentration of the dilute juice is 11% total solids. The juiceis
concentrated to 75% total solids. The specific heats of dilute
apple juice and concentrate are 3.9 and 2.3 kJ/(kg °C),
respectively. The steam pressure is measured to be 304.42
kPa. The inlet feed temperature is 43.3°C. The product inside
the evaporator boils at 62.2°C. The overall heat-transfer
coefficient is assumed to be 943 W/(m 2 °C). Assume
negligible boiling-point elevation.
•Calculate the mass fl ow rate of concentrated product, steam
requirements, steam economy, and the heat-transfer area.

46. •Temperature of steam at 304.42 kPa = 134°C
•Enthalpy for saturated vapor H vs (at T s
134°C) 2725.9 kJ/kg
•Enthalpy for saturated liquid H cs (at T s
134°C) 563.41 kJ/kg
•Enthalpy for saturated vapor H v1 (at T 1
62.2°C) 2613.4 kJ/kg

47. •Rate of evaporation is______
a) directly proportional to temperature of
liquid
b) inversely proportional to temperature of
liquid
c) independent of temperature of liquid
d) directly proportional to humidity of
surrounding air

48. •Rate of evaporation increases as…………………
a) exposed surface area of liquid increases
b) exposed surface area of liquid decreases
c) movement of air above surface of liquid
decreases
d) atmospheric pressure increases

49. •A single effect evaporator is to be used to concentrate a
food solution containing 15% (by mass)dissolved solids to
50% solids. The feed stream enters the evaporator at 291 K
with a feed rate of 1.0 kg s−1. Steam is available at a
pressure of 2.4 bar and an absolute pressure of 0.07 bar is
maintained in the evaporator. Assuming that the properties
of the solution are the same as those of water, and taking
the overall heat transfer coefficient to be 2300 W m−2K−1,
calculate the rate of steam consumption and the necessary
heat transfer surface area

50. DESIGN OF A MULTIPLE-EFFECT
EVAPORATOR

55. VAPOR RECOMPRESSION
SYSTEMS
•energy requirements of the total system are
decreased by using exit vapors as the heating
medium in subsequent effects.
•Two additional systems that employ vapor
recompression assist in reduction of energy
requirements.
•These systems are thermal recompression and
mechanical vapor recompression.

56. Thermal Recompression
•Thermal recompression involves the use of a steam jet
booster to recompress part of the exit vapors
• Through recompression, the pressure and
temperature of exit vapors are increased.
•These systems are usually applied to single-effect
evaporators or to the first effect of multiple-effect
evaporators.
•Application of this system requires that steam be
available at high pressure, and lowpressure steam is
needed for the evaporation process.

58. Mechanical Vapor Recompression
•Vapor compression is accomplished
mechanically, using a compressor driven by an
electric motor, a steam turbine, or a gas engine.
•A steam-turbine-driven compressor is most
suitable for mechanical recompression if high
pressure steam is available.
•Availability of electricity at low cost would favor
the use of an electric motor.