Evaporation presentation

Evaporation
Dr.Zulcaif Ahmad
Hajvery University (Faculty of Pharmacy) 1

TABLE OF CONTENTS
• EVAPORATION
Defination
Mechanism
Factors affecting the Evaporation
• EVAPORATION UNDER REDUCEDE PRESSURE
• EQUIPMENT USED FOR EVAPORATION
• APPLICATION OF EVAPORATION
• REFERENCES

EVAPORATION
Defination:-
Theoretically,
• “Evaporation means simply vaporization from
the surface of a liquid. Vaporization of a liquid
below its boiling point is called evaporation.”
• Thus, no boiling occurs and the rate of
vaporization depends on the diffusion of
vapour through the boundary layers above the
liquid.

• Thus a practical definition of evaporation is
• “The removal of liquid from a solution by
boiling the solution in a suitable vessel and
withdrawing the vapour, leaving a
concentrated liquid residue.”
• This means that heat will be necessary to
provide the latent heat of vaporization and, in
general the rate of evaporation is controlled
by the rate of heat transfer.
• Evaporators are designed, therefore, to give
maximum heat transfer to the liquid, with the
largest possible area, a suitable temperature
gradient.

Mechanism
• When heat applied in solution the motion of
molecules increase and molecules present in
the surface overcome the surface tension of
the liquid and it evaporates because surface
molecules have less cohesive force than
others.

Factors affecting the Evaporation
• There are seven factors to affect evaporation.
1. Temperature
2. Surface area
3. Agitation
4. Atmospheric aqueous vapour pressure
5. Atmospheric pressure on the liquid under
evaporation
6. Type of product required
7. Economic factors

1) Temperature: The rate of evaporation is
directly proportional to the temperature.
2) Surface area: The rate of evaporation is
directly proportional to the surface area of
the vessel exposed to evaporation.
3) Agitation: is necessary for evaporation.
4) Atmospheric aqueous vapour pressure: The
rate of evaporation is inversely proportional
to the atmospheric aqueous vapour pressure.

5) Atmospheric pressure on the liquid under
evaporation: The rate of evaporation is
inversely proportional to the atmospheric
pressure on the liquid under evaporation.
6) Type of product required: The selection of
the method and apparatus to be used for
evaporation depends upon type of product
required.
7) Economic factors: When selecting the
method and apparatus the economic factors
are important

EVAPORATION UNDER REDUCEDE
PRESSURE
• At reduced pressure evaporation increases.
Advantage:
1. Evaporation occurs at low temperature, with
less risk of damage to heat sensitive materials.
2. A lower operating temperature gives higher
temperature gradients, without the need of
excessive steam pressures.
3. The lower the operating, temperature the lower
the steam pressure that can be used.

EQUIPMENT USED FOR EVAPORATION
• Equipments which are used for evaporation are
called Evaporators.
Types of Evaporator:
• Evaporators are divided mainly into three groups.
1. Natural circulation evaporator
 Types:
i) Evaporating pans
ii) Evaporating stills
iii) Short tube evaporator.

2) Forced circulation evaporator.
3) Film evaporator
Types:
i) Wiped Film evaporator
ii)Long Tube Evaporator
a) Climbing film evaporator
b) Falling film evaporator

1) NATURAL CIRCULATION
EVAPORATORS
• Working principle:
• The movement of the liquid results from
convection currents set up by the heating
process.
• Convection currents: The process in which
heat moves through a gas or liquid as the
hotter parts rises and the cooler part sinks.

Evaporating Pans
• “On a manufacturing scale, liquid extracts containing water
are evaporated in open pans called evaporating pans.”
Construction:
• The evaporating pan consists of
 Hemispherical shallow made of
Copper
Stainless steel
Alluminium
Enameled iron
 Steam jacket
• The hemispherical shape gives the best surfacevolume
ratio for heating and the largest area for the
disengagement of vapour.

• Working:
• The dilute solution is
taken in the pan. Steam is
introduced through the
steam inlet into the jacket
to heat the pan. In these
evaporators the
movement of the liquid
results from convection
currents set up by the
heating process. The
concentrated liquid is
collected through the
outlet placed at the
bottom of the pan.

Advantages
1) They are simple, easy and cheap to construct.
2) They are easy to use and clean.
3) Stirring of the evaporating liquids can be
done easily.

Disadvantage
1) The whole of the liquid is heated all the time
which may lead to decomposition of the
components.
2) On the evaporating surface foam is rapidly
formed which decreases evaporation. Solids may
be deposited at the bottom which make the
stirring necessary.
3) This pan can only be used for evaporating
aqueous and thermostable liquid extracts.
4) They can not be used for evaporating extracts
containing organic solvents like alcohol etc.

ii) Evaporating still
• Construction
• It consists of a jacketed-evaporating pan with
a cylindrical cover that connects it to a
condenser. The over all assembly is called still.
The cover is clamped with the evaporating
pan.

Working :
• The dilute liquid is fed
into the still, the cover is
clamped. Steam is
introduced into the
jacket. The liquid is
evaporated and
condensed in the
condenser and collected.
The product (i.e.
concentrated liquid) is
collected through the
product outlet.

Advantages:
• Easy to clean and maintain.
• Allow the equipment to be used for slovents
other than water. E.g. ethanol.
Disadvantages:
• All the liquor is heated all the time.
• The heating surface is limited.

iii) Short tube evaporator
Construction:
• The evaporator is a cylindrical vessel. The lower
portion of the vessel consists of a nest of tubes with
the liquor inside and steam outside– this assembly is
called calendra.
• The specifications of calendria are as follows:
• Tube length: 1 – 2 m
• Tube diameter: 40 – 80 mm
• Diameter of evaporator: 2.5 m
• Number of tubes: 1000
• The feed inlet is at the top of the calendra. The product
outlet is placed at the bottom of the evaporator. Steam
inlet and outlet is placed from the side of the calendria.

Working:
• The liquor in the tubes is
heated by the steam and
begins to boil, when the
mixture of liquid and
vapor will shoot up the
tubes (in a similar manner
to that of a liquid that is
allowed to boil to
vigorously in a test-tube).
• The product is collected
through the product
outlet.

• Advantages
• 1. Use of tubular calendria increases the heating area,
possibly by a factor of 10 to 15 compared to that of an
external jacket.
• 2. Increasing the rate of heat transfer.
• 3. Condenser and receiver can be attached to run the
evaporation under vacuum with nonaqueous solvents.
• Disadvantages
• 1. Since the evaporator is filled to a point above the level of
the calendria, a considerable amount of liquid is heated for a
long time. The effect of this continual heating can be reduced
to some extent by removing concentrated liquor slowly from
the outlet at the bottom of the vessel.
• 2. Complicated design, difficult for cleaning and
maintenance.

2) Forced circulation evaporators
Construction:
• The evaporator consists of a short tube calendria and a
large cylindrical vessel (body of the evaporator) for
separation of vapor and liquid takes place.
• The liquor inlet is provided at the side of the cylindrical
vessel.
• A pump is fitted in between the calendria and the body
of the evaporator.
• A tangential inlet for liquid under high pressure is
placed at neck of the body of the evaporator.
• The vapor outlet is placed at the top of the body and it
may be passed through a condenser to collect the
condensed liquid.

Working principle:
• Feed is introduced through the
liquor inlet. Pump will force the
liquid through the calendria.
Steam heats the liquid inside
the calendria. As it is under
pressure in the tubes the
boiling point is elevated and no
boiling takes place. As the
liquor leaves the tubes and
enters the body of the
evaporator through the
tangential inlet there is a drop
in pressure and vapor flashes
off from the superheated
liquor. The concentrated liquid
is pumped out through the
product outlet and the vapor is
collected through the vapor
outlet.

Advantages
1) These have an great advantage over natural
circulation evaporators in that the rapid liquid
movement improves heat transfer, especially
with viscous liquids or materials that deposit
solids or foam readily.
2) The equipment is suitable for operation under
reduced pressure due to the capacity of the
process to overcome the effect of greater
viscosity of liquids.
3) These evaporators possess rapid evaporation
rate.

4) These evaporators are mainly used for
thermolabile materials.
5) These can also be used in practice for the
concentration of insulin and liver extracts.
Disadvantage:
• Corrosion-erosion can occur, due to high
circulation velocities.
• Salt deposits detach and accumulate at the
bottom.

3) FILM EVAPORATORS
• Film evaporators spread the material as a film
over the heated surface, and the vapor
escapes the film.
• Following are the types of film evaporators.
i) Wiped Film evaporator
ii)Long Tube Evaporator
a) Climbing film evaporator
b) Falling film evaporator

i) Wiped film evaporators
• Construction:
• A form of film evaporator coming into increasing use is
the wiped film evaporator or rotary film evaporator,
which contains of a single, short tube of wide diameter,
better described as a narrow cylindrical vessel,1 or 2
meters in length.
• A section across the evaporator is shown here where it
will be seen that the vessel is surrounded by a heated
jacked. Through the vessel is a bladed rotor, with a
clearance of the order of 1mm between the tips of the
rotor blades and the wall of the vessel.

Working:
• The liquor is introduced at the
top of the vessel and spread
as a film over the heated wall
by the action of the rotor.
• Evaporation occurs as the
liquor passes down the wall,
vapour is taken to a
condenser and the
concentrated liquor
withdrawn at the bottom of
the vessel.
• The evaporator is therefore a
form of single tube, falling
film evaporator in which the
film is formed and agitated
mechanically.

ii) Long tube evaporators
(Climbing film evaporators)
Construction:
• The heating unit consists of steam-jacketed
tubes, having a length to diameter ratio of about
140 to 1, so that a large evaporator may have
tubes 50 mm in diameter and about 7 m in
length.
• The liquor to be evaporated is introduced into the
bottom of the tube, a film of liquid forms on the
walls and rises up the tubes, hence it is
called climbing film evaporator.

Working:
• At the upper end, the mixture of vapor
and concentrated liquor enters a
separator, the vapor passes to a
condenser, and the concentrated liquid
to a receiver.
• Cold or pre heated liquor is introduced
into the tube.
• Heat is transferred to the liquor from
the walls and boiling begins.
• Ultimately sufficient vapor has been
formed for the smaller bubbles to unite
to a large bubble,
• filling the width of the tube and
trapping a ‘slug’ of liquid above the
bubble.
• As more vapor is formed, the slug of
liquid is blown up the tube, the tube is
filled with vapor, while the liquid
continues to vaporize rapidly, the vapor
escaping up the tube and, because of
friction between the vapor and liquid,
the film also is dragged up the tube upto
a distance of 5 to 6 metres.

ii) Long tube evaporators
(Falling film evaporators)
Construction:
• The heating unit consists of steam-jacketed
tubes, having a length to diameter ratio of
about 140 to 1, so that a large evaporator may
have tubes 50 mm in diameter and about 7 m
in length.

Working principle
• The liquor to be
evaporated is introduced
at the top of the
evaporator tubes and the
liquor comes down due to
gravity.
• The concentrate and
vapor leaves the bottom.
They are separated in a
chamber where the
concentrate is taken out
through product outlet
and vapor from vapor
outlet.

Advantages:
• Because of obtaining good heat transfer the
method being especially useful with liquids that
are too viscous to be processed in units in which
the film is formed naturally.
Disadvantages:
• A major disadvantage of falling film evaporators is
the potential instability of the falling film
• Expense to manufacture and install the
instrument is high.
• Difficult to clean and maintain.

APPLICATION OF EVAPORATION
• Evaporation is one of the most important
processes in the manufacture of
pharmaceuticals.
• It is used in the preparation of
1. Liquid extracts, soft extracts & dry extracts.
2. In the concentration of blood plasma &
serum.
3. It is also used in the manufacture of drugs
containing, antibiotics, enzymes, hormones &
many other substances.

4) Used in purification of vitamins.
5) Concentration of proteins.
6) Concentration of biological products.
7) Stripping of solvents from vegetable & plant
or herbal extracts.
8) Removal of water & solvents from
fermentation broths.
9) Concentration of penicillin & related products

Refrences
• Cooper and Gunn’s, Tutorial Pharmacy Edited
by S.J.Carter, Sixth Edition, Part two,
Pharmaceutical Technology, Unit # 14,Page
number, 164-172.
• http://www.sspindia.com/prd/forced-eve-
img1.jpg
• http://www.avta-us.com/images/lab_horiz.jpg
• http://www.sws.uiuc.edu/atmos/statecli/instr
uments/panevap.jpg

Evaporation presentation

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