Evaporation
Syllabus:
Basic concept of phase equilibria, factors affecting
evaporation, evaporators, film evaporator, single effect and multiple effect
evaporator.
Definition
Theoretically, evaporation means
simply vaporization from the surface of the liquid.
Evaporation is an unit operation
by which a solvent is evaporated from a solution by boiling the liquor in a
suitable vessel and withdrawing the vapor, leaving a concentrated liquid
residue.
Objective of evaporation:
To make a solution more
concentrated. Generally extracts are concentrated in this way.
Factors affecting evaporation:
(i) Temperature:
Heat is
necessary to provide the latent heat of vaporization, and in general, the rate
of evaporation is controlled by the rate of heat transfer. Rate of heat
transfer depends on the temperature gradient.
Many
pharmaceutical agents are thermolabile. So the temperature that will cause the
least possible decomposition should be used.
e.g. Many
glycosides and alkaloids are decomposed at temperature below 1000C.
e.g. Hormones,
enzymes and antibiotics are extremely heat sensitive substances. e.g. Malt
extract (containing enzyme) is prepared
by evaporation under reduced pressure to avoid loss of enzymes.
Some antibiotics
are concentrated by freeze-drying.
(ii) Temperature and time of
evaporation
Exposure to a
relatively high temperature for a short period of time may be less destructive
of active principles than a lower temperature with exposure for a longer
period.
Film evaporators
used a fairly high temperature but the time of exposure is very short.
An evaporating
pan involve prolonged heating.
(iii) Temperature and moisture
content
Some drug
constituents decompose more rapidly in the presence of moisture, especially at
a raised temperature (by hydrolysis). Hence, evaporation should be carried out
at a low controlled temperature, although the final drying can be performed at
higher temperature when little moisture remains.
e.g. Belladonna
Dry Extract is an example of this type.
(iv) Type of product required
Evaporating pans
or stills will produce liquid or dry products, but film evaporators will yield
only liquid products. So a dilute extract
can be first concentrated in a film evaporator and then the concentrated
extract may be died in an evaporating pan.
(v) Effect of concentration
As the liquor
becomes concentrated, the increasing proportion of solids results in elevation
of the boiling point of the solution. This leads to a greater risk of damage to
thermolabile constituents and reduction of the temperature gradient.
In general
concentrated solutions will have increased viscosity, causing thicker boundary
layers, and may deposit solids that may build up on the heating surface that
reduce heat transfer.
All these
problems may be minimized by turbulent flow condition.
EVAPORATORS
Evaporators are classified
according to the form of the movement,
(i) Natural
circulation evaporators.
(ii) Forced
circulation evaporation
(iii) Film
evaporators
(i)Natural Circulation Evaporator
EVAPORATING PAN
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Construction
The apparatus consists of a
hemispherical, or shallow pan, constructed from a suitable material such as
copper or stainless steel and surrounded by a steam jacket.
The hemispherical shape gives the
best surface/ volume ratio for heating, and the largest area for separation of
vapor.
The pan may have a mounting ,
permitting it to be tilted to remove the product, but the shallow form makes
this arrangement somewhat unstable, and an outlet at the bottom, is common.
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:
(a) It is simple and cheap to
construct.
(b) It is easy to use, clean and
maintain.
Disadvantages:
(a) Having
only natural circulation, the overall coefficient of heat transfer will be poor
and solids are likely to deposit on the surface, leading to decomposition of
the product and a further deterioration in heat transfer.
(b) Also
many products give rise to foaming.
(c) The
total liquor is heated over all the time, which may be unsatisfactory with
thermolabile materials.
(d) The
heating surface is limited and decreases proportionally as the size of the pan
increases.
(e) The
pan is open, so the vapor passes to the atmosphere, which can lead to
saturation of the atmosphere.
(f) Only
aqueous liquids can be evaporated in these pans.
(g) Pan
evaporation cannot be done under reduced pressure.
(h) Can
only be used for thermolabile products.
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EVAPORATING STILLS
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:
(a) Simple construction and easy
to clean and maintain.
(b) The vapor is removed by
condensation which
(i) speeds
evaporation
(ii) reduces
inconvenience and
(iii) allows the
equipment to be used for solvents other than water e.g. ethanol.
(c) A receiver and vacuum pump can be fitted to the condenser,
permitting operation under reduced pressure and, hence, at lower temperature.
Disadvantages:
(a) Natural
convection only
(b) All the
liquor is heated all the time
(c) The heating
surface is limited.
Uses:
(i) Aqueous
and other solvents may be evaporated
(ii) Thermolabile
materials can be evaporated under reduced pressure.
(iii) Removing
the still head it is convenient for evaporating extracts to dryness.
SHORT TUBE EVAPORATOR (Basket
type vertical short tube evaporator)
Construction and Principle
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:
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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 feed is introduced through the feed inlet
and the liquor is maintained at a level slightly above the top of the tubes (of
calendra), the space above this is left for the disengagement of vapor from the
boiling liquor.
·
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).
·
This sets up a circulation, with boiling liquor
rising up the smaller tubes of the calendria and returning down the larger
central downtake.
·
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.
The vigorous circulation reduces boundary layers and
keeps solids in suspension, so 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.
3.
The head (pressure) of the liquor increases pressure at
the bottom of the vessel and, in large evaporators where the liquor depth may
be of the order of 2 m; this may give rise to a pressure of about 0.25 bar,
leading to elevation of the boiling point by 5 to 60C.
FORCED CIRCULATION EVAPORATORS
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Forced circulation evaporators are
natural circulation evaporators with some added form of mechanical agitation.
Different forms of forced circulation evaporators can be designed.
·
An evaporating pan, in which the contents are
agitated by a stirring rod or pole could be described as a forced circulation
evaporator.
·
A mechanically operated propeller or paddle
agitator can be introduced into an evaporating pan or still.
·
Propeller or paddle agitator can be introduced
into the downtake of a short-tube evaporator.
·
A typical forced circulation evaporator can be
shown as follows:
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
·
Rapid liquid movement improves heat transfer,
especially with viscous liquids or materials that deposit solids or foam
readily.
·
The forced circulation overcomes the effect of
greater viscosity of liquids when evaporated under reduced pressure.
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·
Rapid evaporation rate makes this method
suitable for thermolabile materials, e.g. it is used in practice for the
concentration of insulin and liver extracts.
FILM EVAPORATORS
Film evaporators spread the
material as a film over the heated surface, and the vapor escapes the film.
Long tube evaporators
(Climbing film evaporators)
Construction and working principle
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.
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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 (fig.-i). Heat is transferred to the liquor from the
walls and boiling begins, increasing in vigor (fig.-ii). 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
(fig.-iii).
As more vapor is formed, the slug
of liquid is blown up the tube (fig.-iv), 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.
Long tube evaporators (Falling film evaporators)
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Construction and working principle
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 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 of long tube
evaporator(s)
Since the movement of the film is
assisted by gravity, more viscous liquid can be handled by falling film
evaporator.
(i) Very
high film velocity reduces boundary layers to a minimum giving improved heat
transfer.
(ii) The
use of long narrow tubes provides large surface area for heat transfer.
(iii) Because
of increased heat transfer efficiency, a small temperature gradient is
necessary with less risk of damage to thermolabile materials.
(iv) Although
the tubes are long, they are not submerged, as in the short-tube evaporator; so
that there is no elevation of boiling point due to hydrostatic head.
Disadvantages
(i) Expense
to manufacture and install the instrument is high.
(ii) Difficult
to clean and maintain.
(iii) From
the operational point of view the feed rate is critical. If too high, the
liquor may be concentrated insufficiently, where as, if the feed rate is to
low, the film cannot be maintained and dry patches may form on the tube wall.
Multiple effect evaporator
Triple-effect evaporator: ps, p1, p2,
p3 vapor pressures, Ts, T1, T2, T3 temperatures
where ps> p1> p2 >p3
.
In a single effect evaporator
steam is supplied for heating the liquor. The total heat is not transferred
form the steam. So the rest of the heat is wasted. To use that heat efficiently,
connections are made so that the vapor from one effect serves as the heating
medium for the next effect.
(i) The
dilute feed (liquor) enters the first effect, where it is partly concentrated;
it flows to the second effect for additional concentration and then to the
third effect for final concentration. This liquor is pumped out of the third
effect.
(ii) In
the first effect raw steam is fed in which the vapor pressure in the evaporator
is the highest, p1. the second effect has the intermediate vapor
pressure; i.e. p1>p2>p3. This pressure
gradient is maintained by drawing the vapor through a vacuum pump and
condensing after the final effect.
(iii) Depending
on the lowering of vapor pressure boiling point of the liquids of 2nd and 3rd
effect will also be lowered; i.e. T1 > T2 > T3.
(iv) In
the 2nd effect vapor from the 1st effect (T1 ) is heating the liquor
(having temperature T2). So there is a temperature gradient (T1
– T2); consequently the liquor will be heated.
Similar
heating will be there in the 3rd effect also.
Methods of feeding
Forward feed
Advantages:
1. Feed
moves from high pressure (in effect-2) to low pressure (in effect – 4)
chambers, so pumping of liquor is not required.
2. Product
is obtained at lowest temperature.
3. This
method is suitable for scale-forming liquids because concentrated product is
subjected to lowest temperature.
Disadvantages
It is not suitable for cold feed because, the steam input in
effect-1 raises the temperature of the feed, and a small amount of heat is
supplied as latent heat of vaporization. Therefore, amount of vapor produced
will be less than the amount of steam supplied. Lower amount of vapor in
effect-1 produces lower amount of vapor in the subsequent effects. Therefore,
the overall economy is lower.
Backward feed
In backward-feed the feed enters in the last effect and
moves towards first effect (i.e IV®III®II®I).
Advantages
It is suitable for cold feed, because the heat used for
increasing the temperature in IV effect is already used for heating 3 times.
This will give more economy.
The method is suitable for viscous products, because highly
concentrated product is at highest temperature, hence lower viscosity (®
higher heat transfer ® higher capacity)
Disadvantages
The liquid moves from low-pressure (IV) to high-pressure
chambers (III ®
II ®
I) pumping is requierd.
Mixed feed method
The feed enters in the
intermediate effect, moves forward and then backward to effect-I (III®IV®II®I).
Advantages
·
Liquid moves from high pressure (III) to low
pressure (IV), hence no pump is required. Liquid moves from IV®II®I
requires pump.
·
Product is obtained from highest temperature (I)
hence lowest viscosity.
Parallel feed
It is suitable where the feed has
to be concentrated slightly.
ECONOMY OF
MULTIPLE EFFECT EVAPORATOR
Assumptions: (a) Feed is at boiling point and
(b) Loss of heat
is negligible
In effect-1
1 Kg of steam
transfers its heat to feed. Since feed is at boiling point so the total amount
of heat is used as latent heat of vaporization. Therefore, 1 kg steam will
produce 1 kg vapor.
In effect – 2
1 Kg vapor from
effect-1 will transfer heat to the liquor of effect -2. Here also 1 kg vapor
produce 1 kg vapor from the liquor.
In effect – 3
1 Kg vapor from
effect-II will produce 1 kg vapor in effect-3.
Therefore, 1 kg
steam will produce 3 kg vapor.
Now, economy of
a single effect evaporator =
= 1
= 1
And economy of
a triple effect evaporator = 
= 3
= 3
So for N number
of effects economy will be N times that of a single effect evaporator.
CAPACITY OF
MULTIPLE EFFECT EVAPORATORS
Capacity =
total evaporation per hour
= vapor production rate
Capacity is
also expressed in terms of total heat transferred because latent heats are
nearly constant all over the ranges of pressure ordinarily involved.
The heat transferred in the three
effects can be represented by the following equations:
q1 = U1A1Dt1.
q2 = U2A2Dt2.
q3 = U3A3Dt3.
Total capacity
will be found by adding these equations, giving:
q = q1 + q2 + q3.
= U1A1Dt1 + U2A2Dt2
+ U3A3Dt3.
Assuming that
all effects have equal areas A1 » A2 » A3
= A(let) and that average
coefficient Uavg can be applied
to the system. The eqn (i) can be written as:
q = Uavg A (Dt1 + Dt2 + Dt3)
However the sum
of individual temperatures drops equals the total over-all temperature drop
between the temperature of the steam and the temperature in the condenser;
therefore
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q
= Uavg A Dt
This is exactly
the same as that of single effect.
Conclusion
It follows from
this that if the number of effects of an evaporation system is varied and if
the total temperature difference is kept constant, the total capacity of the
system remains substantially unchanged.