Evaporation

Evaporation is a unit operation which

convert liquid into a vapor and leaving
behind a concentrated solution or a
higher boiling point liquid.
 Purpose of Evaporation

To concentrate solution by removing

the vapor from a boiling liquid solution.

In most cases, evaporation refers to

the removal of water
from an aqueous solution.
 Example
concentration of aqueous solutions of
sugar,
sodium chloride,
sodium hydroxide,
glycerol,
glue,
milk, and
orange juice.
 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.

Components of an
evaporator
1. Heat exchanger
2. Vacuum
3. Vapour separator
4. Condenser
 Properties of evaporating liquids that influence
the process of evaporation
1. Concentration: As the concentration increases, the viscosity and
density increases thereby the boiling point of solution increases

2. Foaming: Solutions like organic compounds tend to foam during vaporization

.The foam is carried away along with vapor –heavy entrainment.

3. Scale: Solutions deposit scales on the heating surface. U drastically decreases and
leads to shut down of the evaporators.

4. Temperature sensitivity: Pharmaceuticals products, fine chemicals and foods are

damaged when heated to moderate temperatures for relatively short times. So
special techniques are employed to reduce temperature of the liquid and time of
heating,

5. Material of construction: Evaporators are made of some kind of steel. however,

many solutions attack ferrous metals and are contaminated by them.
Copper, nickel, stainless steels can also be used.
Processing Factors

Concentration in the liquid

- low viscosity: high mass transfer coefficient

- high viscosity: low mass transfer coefficient

-adequate circulation and/or turbulence must be present

to keep the coefficient from becoming too low
Processing Factors

Solubility

- solubility increases with temperature

- crystallization may occur when a hot

concentrated solution is cooled to room
temperature
Processing Factors

Temperature sensitivity
of materials

- food and biological materials may be

temperature sensitive and degrade at
higher temperature or after prolonged
heating.
Processing Factors

Foaming or frothing

- Food solution such as skim milk and some

fatty-acid solution form a foam or froth
during boiling.
Processing Factors

Pressure and
temperature

- high operating pressure: high boiling point

Processing Factors

Scale deposition and materials

of construction

- Some solutions deposit solid materials

called scale on the heating surfaces.

- results in the overall heat-transfer

coefficient decrease and evaporator
must be cleaned.
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

TYPES OF EVAPORATION
EQUIPMENT
Single effect evaporation:

➢ When a single evaporator is used ,the vapor from the

boiling liquid is condensed and discarded. This is called
single effect evaporation.
➢ It is simple but utilizes steam ineffectively.
➢ To evaporate 1 kg of water from the solution we require 1-
1.3 kg of steam.
Multiple effect evaporation:
Increasing the evaporation per kg of steam by using a series
of evaporators between the steam supply and condenser is
called multiple effect evaporation
 Classification of evaporators
(1) Natural circulation evaporators

(a) Long tube vertical falling film evaporator

(b) Long tube vertical climbing up evaporator

(2) Forced circulation evaporators

(a) Forced circulation evaporator with horizontal heating element
(b) Forced circulation evaporator with vertical heating element

(3) Agitated film evaporator

(4) Based on method of operation evaporators also can be classified as

single effect evaporators and multiple effect evaporators.
Once through and circulation evaporators:
➢ In once through operation, the feed liquor passes
through the tubes only once , releases the vapor
and leaves the unit as thick liquor.
➢ Evaporation is done in a single pass.
➢ The ratio of evaporation to feed is limited in single
pass.
➢ These evaporators are well adapted to Multiple
effect operation.
➢ Agitated, falling film evaporators are operated
once through.
➢ These are useful for heat sensitive materials
➢ In circulation evaporators a pool of liquid is held
with in the equipment. Incoming feed mixes with
the liquid in the pool, and the mixture passes
through the tubes. Un evaporated liquid
discharged from the tubes returns to the pool , so
that only part of evaporation occurs in one pass.
➢ All forced circulation evaporators, rising film
evaporators are operated in this manner.
➢ These are adapted to single effect evaporation.
➢ These are not suited for heat sensitive materials.
Long tube vertical climbing up
evaporator :
It has 3 parts:
1.Tubular HE
2.Separator/Vapor space to
remove entrained liquid from
vapor.
3.Return leg to operate it as
circulation unit.
Dia of tubes:25 to 50 mm
L=3 to 10 m
Used for concentrating the
liquids that tend to foam.
Falling film evaporators:

1) Used for concentrating highly heat-

sensitive materials such as orange
juice, food materials etc. which
require short residence times.
2) Operated once through
3) Tubes are large D=50 to 250 mm
L=3 to 10 m
 Long vertical tube evaporator
1. Rising film 2. Falling film

D: 25 to 50 mm
L: 3 to 10 m

D: 50 to 250 mm
L: 3 to 10 m

Applications:
Applications:
Used for concentrating highly heat-
Used for handling of foaming,
sensitive materials such as orange juice,
frothy liquors.
food materials etc. which require short
residence times.
 Forced circulation evaporators
➢ In natural circulation evaporators the liquid enters with velocity 0.3
to 1 m/s and generally the heat transfer coefficients are very low,
particularly with viscous liquids.

➢ By increasing the velocity of the liquid flow (2 to 6 m/s)

through the tubes heat transfer coefficients increases enormously
and it also prevents the scale formation on heating surfaces.

These are two types.

(i) Forced circulation evaporator with horizontal heating element

(ii) Forced circulation evaporator with vertical heating element
Forced circulation evaporators
 With horizontal heating element

With vertical heating element

Advantages:

➢High transfer coefficients obtained even with viscous solutions.

➢Whenever we are dealing with concentration of highly viscous and scale
forming solutions forced circulation evaporators prevents the scale formation
on heating surfaces
➢Residence times are low so that heat sensitive materials can be used.

Disadvantage:

The main disadvantage of forced circulation evaporators is high pumping cost.

Applications:

➢These types of evaporators are widely used in industries for salting,

viscous and scale forming solutions.
Agitated-film evaporator

Resistance to heat transfer lies on the liquid side.

By mechanical agitation of liquid we can reduce the
resistance
It is a modified falling film evaporator with a single
jacked tube containing an internal agitator
Advantages:

It gives high heat transfer coefficients even with very

high viscous and heat sensitive liquids such as
gelatin, rubber latex, antibiotics and fruit juices.

Disadvantages:

➢But these are very costly and will be having smaller

capacities.
➢Maintenance is difficult because of many moving parts.
Agitated-film evaporator
Energy balances for Single-effect Evaporator
 Energy balances for Single-effect Evaporator
mV, λV
Vapor

Overall material balance

mf, xf, CPf, Tf
Feed mf = mP + mv
P, T
Solute balance
Saturated mS, TS, λS
Steam mf xf = mP xP

mC Energy balance
Condensate mS λS = mf CPf (T – Tf) + mV λV

Enthalpy balance
mS λS = (mP HP + mV HV) – mf Hf

mP, xP
Thick
product
 Performance of Evaporator

Kg of vapor evaporated
Capacity = = mV
Time
Kg of vapor evaporated mV
Economy = =
Kg of steam used mS
 Methods of improving Evaporator economy

1. The vapors, which contain latent heat, are generally discarded in an

evaporator, thereby wasting energy.
2. But thermal energy in the vapor evolved from a boiling solution can
be utilized to vaporize more water.

The following techniques are used to utilize the thermal energy that is
available in the vapors coming out from the evaporator.

1. Multiple effect evaporation

2. Vapor recompression
 1. Multiple-effect evaporation
Salient features
➢ The vapors, which contain latent heat, are generally discarded in an evaporator,
thereby wasting energy.

➢ But it can be used as steam supply to another unit operating under lower
pressure and temperature.

➢ The vapor from the second unit can be further used as a steam supply to a third
unit operating at a still lower pressure and temperature.

➢ Each unit in such a series is called an effect and the method of re-using the
latent heat is called multiple-effect evaporation.

➢ In the case of multiple effect evaporators the economy increases at the cost of
capacity.
➢ Operating cost is same, but the capital cost, repair and maintenance cost
increases with increase in number of effects.
Methods of feeding

Forward feed
Backward feed
Mixed feed
Parallel feed
 Forward feed Backward feed

This arrangement is simplest and ➢ This method requires a pump

no need of any pump to transfer between each pair of effects
liquid from effect to effect as the since the flow is from lower
liquid flows in the direction of pressure to the higher pressure.
decreasing pressure. ➢ If the liquid is very viscous then we
have to adopt this arrangement for
better capacity.
 Mixed feed Parallel feed

This arrangement is ➢ The fresh feed is fed to each effect

combination of forward and simultaneously and the thick liquor is taken
backward feed adopted for out from the same effect separately.
best overall performance.
➢ In this arrangement there is no transfer of
liquid from one effect to another effect.
 2. Vapor recompression
In this method, the vapors from the evaporator are compressed to a
saturation pressure of steam to upgrade the vapors to the conditioning of
original steam to permit the use as heating media.

These are two types.

(a) Mechanical recompression or
(b) Thermal recompression.
Mechanical Recompression:
In this method the vapor evolved from the evaporator is compressed to some
what higher pressure by positive displacement (or) centrifugal compressor and
fed to a heater as a steam
Thermal Recompression:
In this method vapor is compressed by means of steam jet ejector. Here the high
pressure steam is used to draw and compress the major part of vapors from the
evaporator
➢Thermal recompression is better suited than mechanical recompression to
vacuum operation.
➢Jets are cheaper and easier to maintain than compressors.
➢Disadvantages of thermal recompression include low mechanical efficiency
of jets
 Factors affecting the capacity and economy

q = U A ΔT = UA (T – Tf)

Capacity
Single-effect Multiple-effect
1. Temp. of the feed 1. Temp. of the feed
2. Boiling point elevation 2. Boiling point elevation
3. Liquid head and friction 3. Liquid head and friction
4. Overall heat transfer
coefficient
(hi , ho)
Economy
1. No. of effects
2. Temp. of the feed