Evaporation

© Ramaiah University of Applied Sciences
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Faculty of Pharmacy © Ramaiah University of Applied Sciences
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Faculty of Pharmacy
Evaporation
Burhanuddin Madriwala
M.Pharm – II SEM
Department of Pharmaceutical Chemistry
M.S Ramaiah University of Applied Sciences

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Contents
• Introduction
• Types of evaporators
• Factors affecting evaporation
• Summary
• References

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What is Evaporation ?
• Theoretically, it is a vaporization of liquid below
its boiling point form the surface of solution.
Thus, no boiling occurs & rate of vaporization
depends on the diffusion of vapours through the
boundary layers above the liquid.
• Practically, it is a removal of water or organic
solvents from a solution by boiling it in a
suitable vessel & withdrawing the vapours
leaving a concentrated product & so heat is
necessary to give latent heat of vaporization &
the rate of evaporation is controlled by the rate
of heat transfer.

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Mechanism
• Heat applied to the liquid or solution
increases the motion of molecules on
the surface whereby they overcome the
surface tension & evaporates because
surface molecules have less cohesive
force than the bulk.

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Factors affecting evaporation
• Temperature – rate of evaporation is directly proportional to temperature.
• Surface area – larger the surface of vessel exposed, higher the rate of
evaporation.
• Agitation – to increase the evaporation, agitation is required.
• Aqueous vapor pressure – evaporation decreases as the pressure increase.
• Atmospheric pressure – higher pressure will prevent the evaporation of
liquid.
• Type of product required – selection of evaporation method depends on type
of product to be obtained.
• Economic factors

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Types of Evaporators
• Natural circulation evaporators
- Evaporating Pans
- Evaporating stills
- Short tube Evaporators
• Forced circulation evaporators
• Film evaporators
- Wiped film evaporator
- Long tube evaporator : Climbing film evaporator
Falling film evaporator

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Evaporating Pans
• Principle – evaporation occurs as a result of convection currents set up by the
heating process. Convection currents are movement of heat through gas or liquid as
hotter parts rises & cooler parts sinks.
• Construction – pan consist of
1. hemispherical shallow made of copper, stainless steel, aluminium or enamelled
iron.
Hemispherical shape gives proper surface/volume ratio for heating & largest area for
vaporization.
2. steam jacket
3. boiler
4. concentrate collector

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• Working – dilute solution is taken in pan. Steam is introduced into the jacket to heat
the pan. The evaporation is takes place due to convection currents set up in the
liquid. The concentrated liquid is collected through the outlet placed at the bottom.
• Advantages
 Simple & easy handling
 Inexpensive
 Stirring is done easily
• Disadvantages
 Decomposition of products due heating process.
 Formation of foam on the surface preventing evaporation.
 Deposition of solids at the bottom requires stirring.
 Only used to evaporate aqueous & thermostable extracts.
 Not used for evaporation of organic solvents.

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Evaporating Stills
• Principle – as that involved in evaporating
pan.
• Construction – consist of jacketed
evaporating pan with a cylindrical cover
which connects it to a condenser. The whole
assembly is called Still. The cover is clamped
with evaporating pan.
• Working – the dilute liquid is fed into the
still & cover is clamped. Steam is introduced
into the jacket. The liquid is evaporated &
condensed in the condenser & collected.

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• Advantages
 Easy to clean & maintain.
 Used for organic solvents also.
 Evaporation of acids
• Disadvantages
 Heating surface is limited.
 Degradation of extracts.

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Short tube evaporator
• Principle – evaporation occurs as a result of convection currents causing liquid to
boil in short tubes.
• Construction – the evaporator is a cylindrical vessel. The lower portion of vessel
consist of nest of tubes with liquor inside & steam outside. The assembly is called
Calandria.
Specifications of Calandria: -
tube length – 1 to 2 m
tube diameter - 40 to 80 mm
diameter of evaporator – 2.5 m
no. of tubes - 1000
The feed inlet is at the top of calandria. The product outlet is placed at the bottom of
evaporator. Steam inlet & outlet is placed on the sides of calandria.

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• Working - the liquid boils in the tubes due to the steam provided & the vapours &
liquid shoot up in the tubes similarly when the liquid shoot up in the test tube on
vigorous heating.
• Advantages
 Use of calandria increases the heating by 10 to 15 folds compared to the external
jacket.
 Rate of heat transfer increases.
 Condenser & receiver can be attached to run evaporation under vacuum with non-
aqueous solvents.
• Disadvantages
 Continual heating of liquid occurs as it is filled to the point above the level of
calandria.
 Complicated design & difficulty in handling

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Forced circulation evaporators
• Construction – evaporator consists of short
tubes calandria & long cylindrical vessel for
separating vapour & liquid.
• Liquor inlet is attached at the side of vessel.
• Pump is fitted between calandria & vessel.
• Tangential inlet for liquid under high
pressure is placed at the neck of vessel.
• vapour outlet is placed at the top of body &
it may be passed through a condenser to
collect the condensed liquid.

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• Working – feed is introduced through the liquor inlet. Pump will force the liquid
through the calandria. Steam heats the liquid inside calandria. As it is under
pressure in the tubes, the boiling point is elevated & no boiling occurs. As the liquor
leaves the tube & enters the vessel through tangential inlet, there is a drop in
pressure & vapours flashes off from the superheated liquor. The concentrated liquid
is pumped out through the product outlet & vapour is collected through vapour
outlet.
• Advantages
 rapid liquid movement improves heat transfer especially for viscous liquids or
materials that deposit solids or foam.
 Suitable to operate under reduced pressure.
 Overcome the effect of higher viscosity of liquids.
• Disadvantages
 corrosion – erosion can occur due to high circulation velocities.
 Salt deposit detach & accumulate at the bottom.

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Wiped film evaporator
• Film evaporators spread the material as a film over heated surface & liquid
escapes from the film.
• Construction – also called Rotary film evaporator consisting of single, short tube of
wide diameter (narrow cylindrical vessel) 1-2 m in length. The vessel is
surrounded by a jacket & through the vessel there is a bladed rotor.
• Working – the liquor is introduced at the top of vessel & spread as film over the
heated wall by the action of rotor. Evaporation occurs as the liquid passes down
the wall, vapour is taken to a condenser & concentrated liquor withdrawn at the
bottom of vessel.

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Climbing film evaporator
• Construction – heating unit consist of steam jacketed tubes having length to
diameter ratio of 140 : 1. Thus a large evaporator have tubes 50 mm in diameter & 7
m in length.
• Working – cold or preheated liquor is introduced into the tubes. Heat is transferred
to the liquor from the walls & boiling begins. The smaller bubbles unite to form
larger ones upon sufficient vapour formation filling the width of tube & trapping
slug of liquid above bubble. As more vapor is formed, the slug of liquid is blown up
the tube, tube is filled with vapor while the liquid continues to vaporize rapidly. The
vapor escapes the tube & because friction between vapor & liquid causes the film to
dragged up to the tube & so it is called climbing film evaporator.

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Falling film evaporator
• Construction – the assembly
remains the same only the
difference is the separator is
attached below the tubes.
• Working – the liquor is introduced
at the top of evaporator tubes &
liquor comes down due to gravity.
The concentrate & vapor leaves
the bottom. They are separated in
the chamber where the
concentrate is taken out through
product outlet & vapor through
vapor outlet.

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Application of Evaporation
• Manufacture of pharmaceuticals.
• Preparation of liquid extracts, soft & dry extracts.
• Concentration of blood plasma & serum.
• Used in manufacture of drugs containing antibiotics, hormones, enzymes etc.
• Purification of vitamins.
• Concentration of proteins & other biological products.
• Stripping of solvents from plant or other herbal extracts.
• Removal of water or other solvents from fermentation broths.
• Concentration of penicillin & other products.
• Refining petroleum products.
• Removal of excess moisture from the drugs.

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Summary
• Evaporation is a process of removal of water or solvent from the material.
• Different factors affect rate of evaporation.
• Evaporators are used for various methods.
• Different types of evaporators are used.
• Evaporation is a basic process & has wide applications.

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References
• Hickey, A. and Ganderton, D., 2001. Pharmaceutical process engineering. 5th ed.
New York: M. Dekker.
• McCabe, W., Smith, J. and Harriott, P., 2005. Unit operations of chemical
engineering. 4th ed. Boston, MA: McGraw-Hill.

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