EVAPORATION
Syllabus:
Heat transfer to boiling liquid
in evaporators. Physical and pharmaceutical factors affecting evaporation.
Classification and study of pan,
forced circulation, film, vacuum and multiple effect evaporator.
Entrainment, prevention of foam
and steam traps.
Questions:
1.
Discuss the factors affecting evaporation. (2000) [6]
2.
Classify evaporators with examples. (2000) [5]
3.
With a neat sketch describe the working principle of a
standard type vertical evaporator. (99)
[10]
4.
How in the evaporator outlet vapor is freed of
entrainment? (99) [6]
5.
Sketch different types of long tube vertical
evaporators and discuss their merits an demerits.(98)[6]
6.
What are the different types of feeding arrangements in
triple effect evaporation? Discuss their application. (97) [4]
7.
Write down material and energy balance equations in a
triple effect evaporator. Give expression for economy and capacity. (97) [6]
8.
With the help of a neat diagram give the working of a
multiple effect evaporator. (96)
9.
What are steam traps. (96)
10. With
a neat sketch, describe the working principle of a basket type vertical short
tube evaporator. (95) [10]
11. What
is the principle of entrainment separator? (95) [6]
12. Outline
the various types of evaporators and discuss the advantages and disadvantages
of each type. (94) [16]
13. Derive
the principle and working of ascending and descending film evaporator with the
help of neat sketch. (94) [8]
Definition:
Theoretically,
evaporation means simply vaporization from the surface of the liquid.
N.B. Thus no boiling occurs and the rate of
vaporization from the depends on the diffusion of vapor through the boundary
layers above the liquid. The partial pressure of vapor is the driving force
here. In practice rate of heat transfer by this process is very slow.
Practically,
evaporation is defined as the removal of liquid 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
 |
Description
In
these evaporators the movement of the liquid results from convection currents
set up by the heating process.
The
apparatus consists of a hemispherical, or shallower 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 disengagement 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.
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 this pans.
(g) Pan
evaporation cannot be done under reduced pressure.
(h) Can
only be used for thermolabile products.
EVAPORATING STILLS
 |
They
are called stills because it is essentially a vessel similar to the evaporating
pan, with a cover that connects it to a condenser, so that the liquid is
distilled off.
Often
a quick release system of clamps which allows the cover to be removed easily
for access to the interior of the vessel for cleansing or removal of the
product may be used.
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
Calendria
The lower portion of the
evaporator consist of a nest of tubes with the liquor inside and steam outside.
 |
Tube length: 1 – 2 m
Tube diameter: 40 – 80 mm
Diameter of evaporator: 2.5 m
Number of tubes: 1000
This part of the evaporator is
called the calendria.
·
The liquor is maintained at a level slightly
above the top of the tubes, the space above this being 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.
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
 |
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:
Working Principle
·
The liquor is circulated by means of the pump
and 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 there is a drop in pressure and vapor flashes off from
the superheated liquor.
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.
·
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.
 |
At
the upper end, the mixture of vapor and concentrated liquor enters a separator,
the vapor passing on to a condenser, and the concentrate to a receiver.
Cold
or pre heated liquor is introduced into the tube (i). Heat is transferred to
the liquor from the walls and boiling begins, increasing in vigor (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 (iii).
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.
Long tube evaporators (Falling film evaporators)
 |
Construction and working principle
Construction
is same as climbing film evaporator but is inverted as shown in the figure.
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 out through product outlet
and vapor from vapor outlet.
Advantages of long tube
evaporator
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.
PERFORMANCE OF TUBULAR
EVAPORATORS
 |
The
principal ways of measuring the performance of a steam -heated tubular
evaporator are the capacity and economy.
Capacity is defined as the
number of kgs of water vaporized per hour.
Economy is defined as the
number of kgs of water vapor vaporized per kg of steam fed to the unit.
Evaporator capacity for single effect evaporator
The
rate of heat transfer q through the heating surface of an evaporator is
expressed as:
q
= U A D
t
where q = rate of
heat transfer
U = over
all heat transfer coefficient
A = the
area of heat transfer surface
D t =
the over all temperature drop
Case -I
Feed to the
evaporator is at the boiling temperature corresponding to the absolute pressure
in the vapor space.
Þ
all the heat, q, transferred is available for evaporation
Þ
capacity is proportional to q
Case-2
The
feed is cold
Þ large amount of heat will
be required for increasing the temperature and then latent heat will be taken
for evaporation
Þ capacity is lesser than
that corresponds to q.
Case-3
The feed is at a temperature
above the boiling point in the vapor phase
Þ a portion of the feed
evaporates spontaneously by adiabatic equilibrium with the vapor-space pressure
(flash evaporation).
Þ capacity is higher than
that corresponds to q.
HEAT (ENTHALPY) AND MATERIAL
BALANCES
According
to the highly simplified diagram of an evaporator, in which the heating surface
is represented in the diagram by a simple coil.
|
|
Feed
|
Thick liquor
|
Vapor
|
Steam
|
Condensate
|
|
Flow rate
(lb/hr)
Solute content (fraction)
Enthalpy
(Btu/lb)
|
F
XF.
hF.
|
L
XL.
hL.
|
V
H
|
S
HS.
|
S
hC.
|
Material balance equation:
Total material entering = total
material leaving the unit
F =
L + V ........................(i)
Total solute entering =
total solute leaving the unit
FXF = LXL........................(ii)
Enthalpy balance
Assumption: Condensate leaves at the condensing temperature of the
steam i.e. no further cooling occurs.
Therefore, heat entering = heat
leaving
or more specifically,
(Heat in feed) +
(heat in steam) = (heat in thick liquor) + (heat in vapor)
+
(heat in condensate) + (heat lost by radiation)
If heat lost by radiation is
negligible then heat balance equation takes the following form:
FhF + SHS =
VH + LhL + ShC..................(iii)
N.B.
Generally the following
parameters are either given or found from steam tables and the unknowns to be
estimate are as follows:
|
Feed
|
Liquor
|
Vapor
|
Steam
|
Condensate
|
|
F
XF
hF
|
L
XL
hL
|
V=?
H
|
S=?
Hs
|
S=?
hC
|
We may have to determine q =
? and or A=?
V can be determined from eqn.
(i). V = F – L
S can be obtained from eqn. (i)
& (iii)
q can be determined from eqn. q =
S (HS – hC)
A can be determined from eqn. q =
UADT or 
where U and DT are
given.
where U and DT are
given.
Capacity of the evaporator
unit = vapor flow rate = V lb/hr (or kg/hr)
Economy
of the evaporator unit = 
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
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 it
the total temperature difference is kept constant, the total capacity of the
system remains substantially unchanged.
 |
ENTRAINMENT
SEPARATORS
When a bubble of vapor rises to the
surface of liquid and bursts, the liquid film that forms the top of the bubble
is usually sprayed as very fine droplets along with the stream of vapor.
This droplets greatly vary in size.
Some of them drop back quickly into the
liquid from which they came; some settle more slowly; and some will not settle
at all, at any vapor velocity (that is practicable to maintain). Such finely
divided liquid carried along with the stream of vapor is called entrainment.
Entrainment may
cause :
(a)
serious losses from the liquid being evaporated,
(b)
and contamination of the condensate, if desired for
other purpose.
Methods for reducing entrainment
1.
A certain amount of separation is made first by making
the diameter of the vapor head of the evaporator such that the rising
velocity of the vapor is kept down to a reasonable value.
2.
The vapor space is made higher. More of the
medium-sized droplets can settle back into the liquid in the time that is
available before the vapor leaves the evaporator.
3.
Baffles are placed over the liquid in the vapor
space in the evaporators. The liquid droplets will impinge on the surface of
the baffle, they coalesce into a sheet and are not easily picked up again by
even extremely high vapor velocities.
4.
If the mixture of vapor and entrained liquid were given
a rotary motion, centrifugal force would tend to throw the droplets out against
the side of the vessel, where they would coalesce and run down as sheet of
liquid. The figure describes such an entrainment separator.
The vapor is fed into the
entrainment separator by a tangential tube so that it starts a whirling motion
at once (hence called cyclone separator). The presence of a spiral vane ensures
this whirling motion so that most of the entrained liquid is thrown out against
the wall of the separator, where it runs down, to be returned to the
evaporator.
In
the lower part of the separator the vapors turn through 1800 to rise
through the central vapor-off take pipe, and this again projects some
particles of liquid down into the bottom of the separator.
FOAMING
Foam
is the formation of a stable blanket of bubbles that lies on the surface of the
boiling liquid. The cause of foaming is not known but it depends on:
(a) the
formation at the surface of the liquid of a layer whose surface tension is
different from that of the bulk of the liquid and,
(b) the
presence of finely divided solids or colloidal material that stabilizes the
surface layer.
Methods to tackle foaming
(i) The
liquid may be carried at a level below the top of the heating surface,
so that the bubbles of foam come in contact with a hot surface and thereby
burst.
(ii) Steam
jets are sometimes directed against the layer of foam to break it.
(iii) The
liquid carrying foam at high velocity may be ejected against baffles, where the bubbles of the foam
are broken mechanically. This happens in forced-circulated evaporators.
(iv) The
best method is the addition of small amount of sulphonated castor oil,
cottonseed oil, or other vegetable oils or some of the silicone oils.
They often control or completely eliminate foam.
STEAM TRAPS
These
are devices those are used for removing condensate from evaporators. The
function of a steam trap is to allow condensate to drain but to prevent steam
from blowing out of the space drained.
Steam traps may
be classified as:
(a) expansion
traps
(b) bucket
traps and
(c) tilt traps.
Expansion traps
 |
This trap consists of a closed
metal cartridge. To one end of this metal cartridge is connected a collapsible
corrugated tube. The left hand end of the tube is sealed, and to it is attached
the stem of a valve. The space between the cartridge and the corrugated tube
filled with oil.
The
collection of condensate against the valve, losing heat by radiation, cools the
cartridge, the oil contracts, the valve opens, and the condensate is blown out.
When the condensate is all discharged and steam enters the trap again, the
cartridge expands and the trap closes.
Advantage: This device is very simple and has no moving parts.
Uses: It is best suited for small capacities.
 |
Bucket traps
The
condensate that enters this trap collects in the bucket until a definite weight
is reached. The bucket then goes down, pulls the valve rod, and opens the valve
at the top of the trap. This allows the condensate to blow out. When sufficient
water has been blown out the bucket floats and closes the valve.
Noncondensed
gases may be removed by opening the petcock in the cover plate from time to
time.
Tilt traps
This
trap is complicated.
SCALE FORMATION
Scales are the
solid deposits, which accumulate on the heat transfer surface during
evaporation. The scale has very low thermal conductivity. As evaporation
proceeds, there is gradual increase resistance to heat transfer due to
deposition of scales.
Inorganic
substances such as calcium sulphate, calcium hydroxide, sodium carbonate,
sodium sulphate and calcium salts of organic acids have scaling tendency.
The scale formation can be minimized by:
(a)
Maintaining high circulation velocities in the tubes
e.g. in forced circulation.
(b)
Arranging the evaporators in forward feed arrangement so that highly concentrated solution is
subjected to low temperature (effect - I).
(c)
Periodic removal of scales should be done.