2. PURPOSE OF EVAPORATION
To concentrate solution by removing the vapor from a boiling liquid solution .
In the majority of cases, evaporation refers to the removal of water from an
Example: concentration of aqueous solutions of sugar, sodium chloride, sodium
hydroxide, glycerol, glue, milk, and orange juice.
In these cases the concentrated solution is the desired product and the evaporated
water is normally discarded.
In a few cases, water, which contains a small amount of minerals, is evaporated to
give a solids-free water to be used as boiler feed, for special chemical processes.
Evaporation processes to evaporate seawater to provide drinking water have been
developed and used.
3. EVAPORATION VS DISTILLATION VS
EVAPORATION & DISTILLATION
In the distillation method, vaporization takes place at the boiling
point whereas, in evaporation, vaporization takes place below
the boiling point.
Evaporation takes only from the surface of the liquid. In contrast,
distillation is taking place from the whole liquid mass.
At the boiling point of the distillation process, the liquid forms
bubbles and there is no bubble formation in evaporation.
Distillation is a separation or purifying technique, but evaporation
is not necessarily so.
In distillation, the vaporization happens rapidly, whereas the
4. EVAPORATION AND DRYING
The term drying usually refers the removal of
relatively small amounts of water from solid or
nearly solid material, whereas Evaporation is
usually limited to the removal of relatively large
amounts of water from solutions.
In most cases drying involves the removal of
water at temperatures below its boiling point,
whereas , Evaporation means the removal of
water by boiling a solution.
5. WHAT IS EVAPORATION ?
Evaporation is a type of phase transition; it is the process by
which molecules in a liquid state (e.g water) spontaneously
become gaseous (e.g water vapor).
The equipment used for evaporation is known as Evaporator.
6. APPLICATION OF EVAPORATION
MANUFACTURING OF BULK DRUGS
MANUFACTURING OF BIOLOGICAL PRODUCTS
MANUFACTURING OF FOOD PRODUCTS
MANUFACTURING OF DE MINERALISED WATER
7. PROCESSING FACTORS
-Higher the temperature greater will be the evaporation.
2. Vapor pressure:
- Rate of evaporation is directly proportional to the vapor pressure of the
- Lower the pressure ,greater will be the evaporation.
3. Surface area:
- Greater the surface area of the liquid, greater will be the evaporation.
4. Time of evaporation:
- Exposure time is longer, more will be the evaporation.
8. 5. Density:
- The higher the density, slower the liquid evaporates.
- Low concentration of the substance, faster the evaporation.
7. Film and deposits:
- Some solutions deposit solid materials called scale on the heating surfaces.
- results in the overall heat-transfer coefficient decreases and evaporator must
8. Foaming or frothing:
- food solution such as skim milk and some fatty-acid solution form a foam or
froth during boiling.
9. Economic factors:
- The recovery of solvent and utilization of waste heat are also important as they
involves considerable reduction of cost.
9. WHAT IS ECONOMY AND CAPACITY OF THE
Economy of an evaporator is the total mass of
water vaporized per unit mass of steam input to
Capacity of an evaporator is the amount of water
vaporized in the evaporator per unit time.
10. TYPES OF EVAPORATION EQUIPMENT
1. Open kettle or pan
2. Horizontal-tube natural circulation evaporator
3. Vertical-type natural circulation evaporator
4. Long-tube vertical-type evaporator
5. Falling-film-type evaporator
6. Forced-circulation-type evaporator
7. Agitated-film evaporator
8. Open-pan solar evaporator
12. OPEN KETTLE OR STEAM JACKETED
Steam is supplied to the jacketed kettle in which the
aqueous extract is placed. Heat is transferred to the
extract by conduction and convection.
Copper is the excellent material for the kettle
because of its conductivity.
Iron is used for construction of jackets since it has
Aqueous extract to be evaporated is placed in the kettle.
Steam is supplied through inlet. Steam gives out its heat to
the contents and the condensates leaves through the
1. Used for both small scale & large scale operation.
2. Simple in construction and easy to operate.
3. Low maintenance & installation cost.
4. Wide variety of materials.
1. Heat economy is less.
2. Not suitable for heat sensitive materials.
3. Heat decreases on product concentration.
4. Since, open type so vapor passes to atmosphere.
Concentrating aqueous and thermo-stable liquors.
Eg. Cooking pickels, liquorice extract etc.
16. HORIZONTAL TUBE EVAPORATOR
Steam is passed through the horizontal tubes, which are immersed in a
pool of liquid to be evaporated. Heat transfers through the tubes and
the solvent evaporates. Concentrated liquid is collected.
Large cylindrical body with doomed shaped at the top and bottom,
made of cast iron or plate steel. Stainless steel tubes are used in steam
Feed is introduced into the evaporator until the steam compartment is
immersed. The horizontal tubes receives the heat and conduct it to the
liquid. The feed absorbes heat and solvent gets evaporated.
Concentrated liquid is collected.
Best suited for non-viscous solution. E.g. Cascara extract.
17. VERTICAL TUBE EVAPORATOR
Liquid is passed throgh the vertical tubes and steam is supplied
from outside tubes. Heat transfer takes place through the tubes
and the liquids inside tube gets heated. The solvent evaporates,
vapor escapes from the top and concentrated liquid is
collected from bottom.
Consist of long cylindrical body made up of cast iron with dome
shaped top and bottom. Calandria are fitted at the bottom.
Calandria consist of number of vertical tubes with diameter 0.05-
0.075 meters & length of 1-2 meters.
About 100 such tubes are fitted in the body of 2.5 mtr. Inlets are
provided for steam and feed. Outlets are provided for vapor,
concentrated products, non-condensed gases and
1. Increases the heating surface 10-15 times than steam jacketed
2. Vigorous circulation enhances rate of heat transfer.
3. More units can be joined.
1. Liquid to be maintained above calandria.
2. Complicated- increased installation cost.
3. Pressure has to maintain.
4. Cleaning and maintenance is difficult.
Manufacture of cascara extract, sugar, salt, caustic soda etc.
19. CLIMBING FILM EVAPORATOR ( RISING FILM
Tubes are heated externally by steam. The
preheated heat enters from the bottom and flows
up through the the heated tubes. Liquid near walls
becomes vapour and forms small bubbles. Larger
bubbles flow up with slag and strikes deflector.
Deflector throws the concentrate, down to the
Heating unit consists of steam jacketed tubes. Long
and narrow tubes are held between the plates.
Deflector is placed at the top of the vapour head.
Inlets are provided for steam and feed. Outlets are
provided for vapour, concentrated product, non
condensed gases and condensate.
Insulin, liver extracts, vitamins, foaming liquids, corrosive
solutions can be concentrated
1. Large area for heat transfer
2. Enhanced heat transfer
3. Suitable for heat sensitive
4. Used for foam forming
5. Instrument needs less space
1. Expensive, construction is
2. Cleaning and maintenance
is quite difficult
3. Large head space required
4. Not for viscous, salting and
22. FALLING FILM EVAPORATOR
Feed enters from the top and flows down the
walls of the tube. The liquid becomes vapour and
forms small bubbles. They tend to fuse to form
layers of bubbles. Concentration takes place
during downward journey. Vapour and liquid are
separated at the bottom.
It resembles climbing film evaporator, but it is
inverted. Feed inlet is from the top of the steam
compartment. The outlet of the product is at
bottom and is connected to a cyclone separator.
1. Separate volatile and non volatile liquids
2. Concentration of yeast extracts
3. Manufacture of gelatin
4. Extracts of tea and coffee
1. Suitable for high viscous
2. Liquid hold up time is less
3. Liquid is not overheated
4. Highly acidic and corrosive
feeds can be concentrated
1. Not for suspensions, salting
and scaling liquids
2. Poor feed distribution in
3. Feed ratio is high
25. FORCED CIRCULATION
Liquid is circulated through the tubes at high pressure by
means of pump. Hence boiling does not takes place as
boiling point is elevated. Forced circulation creates
agitation. When liquid leaves the tube and enters the
vapour head, pressure falls suddenly. This leads to flashing
of superheated liquor. Thus evaporation is effected.
Heating unit consists of steam jacketed tubes. Inlets are
provided for steam and feed. Outlets are provided for
vapour, concentrated products, non condensed gases
and condensate. Pump is connected near the inlet.
1. Insulin and liver extract
2. Crystallizing operations
1. Heat transfer coefficient is
2. Salting, scaling are not
3. Suitable for high viscous
1. Equipment is expensive
2. More power supply is
30. The feed enters at TF
Saturated steam at TS enters the heat- exchange section
Condensed steam leaves as condensate or drips
The solution in the evaporator is assumed to be completely
Hence, the concentrated product and the solution in the
evaporator have the same composition
Temperature T1 is the boiling point of the solution
The temperature of the vapor is also T1, since it is in
equilibrium with the boiling solution
The pressure is P1, which is the vapor pressure of the solution
If the solution to be evaporated is assumed to be dilute and
like water, then 1 kg of steam condensing will evaporate
approximately 1 kg of vapor (if the feed entering has TF near
the boiling point)
Single-effect evaporators are often used when the required
capacity of operation is relatively small and/or the cost of
steam is relatively cheap compared to the evaporator cost
However, for large-capacity operation, using more than one
effect will markedly reduce steam costs
31. FORWARD FEED MULTIPLE-EFFECT
A single-effect evaporator as shown in Fig. 8.2-2 is
wasteful of energy
The latent heat of the vapor leaving is not used
but is discarded
Much of this latent heat, however, can be
recovered and reused by employing a multiple -
A simplified diagram of a forward-feed triple-
effect evaporation system is shown in Fig. 8.2-3.
33. If the feed to the first effect is near the boiling point at the pressure
in the first effect, 1 kg of steam will evaporate almost 1 kg of water
The first effect operates at a temperature that is high enough that
the evaporated water serves as the heating medium to the
Here, again, almost another kg of water is evaporated, which can
then be used as the heating medium to the third effect
As a very rough approximation, almost 3 kg of water will be
evaporated for 1 kg of steam in a three-effect evaporator
Hence, the steam economy, which is kg vapor evaporated/kg
steam used, is increased
This also holds approximately for more than three effects
However, the increased steam economy of a multiple-effect
evaporator is gained at the expense of the original first cost of
34. In forward-feed operation as shown in Fig. 8.2-3, the fresh feed
is added to the first effect and flows to the next in the same
direction as the vapor flow
This method of operation is used when the feed is hot or when
the final concentrated product might be damaged at high
The boiling temperatures decrease from effect to effect. This
means that if the first effect is at P1 = 1 atm abs pressure, the
last effect wilt be under vacuum at a pressure P3
35. BACKWARD FEED MULTIPLE-EFFECT
In the backward-feed operation shown in Fig. 8.2-4 for a triple-effect
evaporator, the fresh feed enters the last and coldest effect and
continues on until the concentrated product leaves the first effect.
FIGURE 8.2-4. Simplified diagram of backward-feed triple-effect
36. This method of reverse feed is advantageous when the
fresh feed is cold, since a smaller amount of liquid must be
heated to the higher temperatures in the second and first
However, liquid pumps must be used in each effect, since
the flow is from low to high pressure
This reverse-feed method is also used when the
concentrated product is highly viscous
The high temperatures in the early effects reduce the
viscosity and give reasonable heat-transfer coefficients
37. PARALLEL FEED MULTIPLE-EFFECT
Parallel-feed in multiple-effect evaporators
involves the adding of fresh feed and withdrawal
of concentrated product from each effect
The vapor from each effect is still used to heat the
This method of operation is mainly used when the
feed is almost saturated and solid crystals are the
product, as in the evaporation of brine to make
40. MULTIPLE EFFECT EVAPORATOR
Suitable for large scale and continuous process
Upto 5 evaporator can be attached
Monitoring of Evaporators
Concentration of salt solution.
41. TYPICAL TYPES OF EQUIPMENT USED FOR BIOLOGICAL
1. Long-tube vertical evaporator: condensed milk
2. Falling film evaporator: fruit juices
3. Agitated-film evaporator: rubber latex, gelatin,
antibiotics, fruit juices
4. Heat-pump cycle evaporator: fruit juices, milk,