Chapter 16
Evaporation
The objective of evaporation is to concentrate
a consisting of a nonvolatile solute and a
v
olatile solvent. Evaporation is conducted by v
aporizing a portion of the solvent to
pr
oduce a concentrated solution or thick
liq
uor.
Evaporation differs from distillation in that
the vapor usually is a single component,
and even when the vapor is a mixture ,no a
ttempt is made in the evaporation step to se
parate the vapor into fractions .
Evaporation differs from crystallization
in that emphasis is placed on
concentrating a solution rather than
forming and building crystals.
Normally ,in evaporation the thick liquor is
the valuable product and the vapor is
condensed and discarded.
Liquid characteristics
The practical solution of an evaporation
problem is profoundly affected by the
c
haracter of the liquor to be concentrated.
Some of the most important properties of
evaporating liquids are as follows.
Concentration
As the concentration increases, the solution
becomes more and more individualistic. The
density and viscosity increase with solid
content.
Continued boiling of a saturated solution
causes crystals to form; these must be r
emoved or the tube clog.
The boiling point of the solution may also
rise considerably as the solid content
i
ncreases, so that the boiling temperature
of a concentrated solution may be much
higher than that of water at the same
pressure.
Foaming
A stable foam accompanies the vapor out o
f the evaporator, causing heavy
en
trainment. In extreme cases the entire ma
ss of liquid may boil over into the
vap
or outlet and be lost.
Temperature sensitivity
Many fine chemicals , pharmaceutical
products, and foods are damaged when
heated to moderate temperatures for r
elatively short time .
scale
Some solutions deposit scale on the heating
surfaces. The over-all coefficient then
steadily diminishes, until the evaporator m
ust be shut down and the tubes cleaned.
Single- and multiple-effect operation
Most evaporators are heated by steam
condensing on metal tubes. Nearly always
the material to be evaporated flows inside
the tubes.
Reducing the boiling temperature of
the liquid increasing the temperature d
ifference between the stream and the
boiling liquid and thereby increases
the heat-transfer rate in the
evaporator
When a single evaporator is used, the
vapor from the boiling liquid is condensed
and discarded, the method is called
single-effect evaporation.
If the vapor from one evaporator is fed into t
he stream chest of a second evaporator and
vapor from the second is then sent to a
condenser, the operation become
double-effect.
Additional effects can be added in the s
ame manner.
A series of evaporators between the
stream supply and the condenser is
called
multiple-effect evaporator.
Types of evaporators
The chief types of stream-heated tubular
evaporator in uses today are:
1）Short-tube evaporators
2）Long-tube vertical evaporators
• forced-circulation
• Upward-flow (climbing-film)
• down-ward-flow (falling-film)
Once-through and circulation
evaporators
Evaporators may be operated either as
once-through or circulation units.
In once-through operation the feed liquor p
asses through the tubes only once,
r
eleases the vapor, and leaves the unit as th
icker liquor. All the evaporation is
ac
complished in a single pass
In circulation evaporators a pool of liquid is
held within the equipment. Incoming feed
mixes with the liquid from the pool, and m
ixture passes through the tubes.
Unevaporated liquid discharged from the
tubes returns to the pool
All short-tube and forced-circulation
evaporators are operated in this way.
Short-tube evaporators
In the older types of evaporators the
ubes are “short”.
In the short-tube vertical evaporator
shown in Fig
t
vapor
Central downcomer
feed
Stream
inter
condensate
concentrate
The stream condenses outside the tubes.
The tube bundle contains a large central
downcomer, the cross-sectional area of
which is 25 to 40 percent of the total
cross-sectional area of the tubes
Most of the boiling takes place in the
smaller tubes, so that the liquid rises
through these tubes and returns
through the downcomer.
In this evaporator the driving force for
flow of liquid through the tubes is the
difference in density between the
l
iquid in the downcomer and the
mixture of liquid and vapor in the
tubes.
Circulation is by natural convection but
at much less rapid rate than in longtube natural-circulation evaporators;
the heat-transfer coefficients,
therefore, are fairly high with thin
liquids but low when the liquid is
viscous.
Long-tube evaporators with upward flow
A typical long-tube vertical evaporator
with upward flow of the liquid is shown
in Fig.
Vapor out
Stream in
feed
condensate
Forced-circulation evaporators
Higher coefficients are obtained in force
d-circulation evaporators, an example
of which is shown in fig.
Vapor out
Deflector
plate
Stream in
Concentrate
out
Condensate
feed
Here a centrifugal pump forces liquid
through the tubes at an entering velocity of
2 to 5.5m/s
The tubes are under sufficient static head
to ensure that there is no boiling in the
tubes; the liquid becomes super heated as
the static head is reduced during flow from
the heater to the vapor space, and it
“flashes” into a mixture of vapor and
spray in there
Film evaporators
Concentration of highly heat-sensitive m
aterials such as orange juice requires a
minimum time of exposure to a
h
eated surface
This can be done in once-through film
evaporators
Vapor out
Concentrate
out
Stream in
Condensate
Feed in
There are two types of film evaporators:
• climbing-film evaporator
• falling-film evaporator
• The chief problem in a fall-film
evaporator is that of distributing the
liquid uniformly as a film inside tubes.
Feed in
Stream in
Vapor
out
condensate
Concentrate out
This is done by inserts the metal plate in
the tube ends to cause the liquid to flow
evenly into each tube, or by spraying the
feed on the inside surface of each tube
with the “spider” distributors
Still another way is to use an
individual spray nozzle inside each
tube
For good heat transfer the Reynolds number
of the falling film should be greater than 2000
at all point in the tube.
During evaporation the amount of liquid
is continuously reduced as it flows dow
nward.
Too great a reduction can lead to dry spots
near the bottom of the tube.
Falling-film evaporators can be used in
concentrating sensitive products. They are
also well adapted to concentrating viscous
liquids.
Performance of tubular evaporators
The principal measures of the performance
of a steam-heated tubular evaporator are
capacity and the economy
• Capacity
is defined as the number of kilograms
of water vaporized per hour.
• Economy
is the number of kilograms vaporized
per kilogram of stream fed to the unit.
Evaporator Capacity
The rate of heat transfer q through the
heating surface of an evaporator is the
roduct of three factors:

The area of the heat-transfer surface A

The overall heat-transfer coefficient U

The overall temperature drop Δt
p
q  UAt
(16-1)

If the feed to the evaporator is at the
boiling temperature corresponding to
the absolute pressure in the vapor
space.
All the heat transferred through the
heating surface is available for
evaporation.
The capacity is proportional to q

If the feed is cold, the energy is requir
ed for heating it to its boiling point.
The capacity for a given value of q is
reduced accordingly , as heat used to
heat the feed is not available for
evaporation.

If the feed is at the temperature above the
boiling point, a portion of the feed
evaporates spontaneously.
The capacity is greater than that
corresponding to q. this process is called
flash evaporation.
Temperature difference
The actual temperature drop across the
eating surface depends on :
h

The solution being evaporated

The difference in pressure between the
stream chest and the vapor space above
the boiling liquid, and depth of liquid over
heating surface.

The friction loss in the tubes
In actual evaporators, however, the boiling
point of a solution is affected by two factors:

The boiling point elevation

And liquid head
Boiling-point elevation and
Dühring’s rule
For a given pressure the boiling point o
f the aqueous solutions is higher
t
han that of pure water.
The increase in boiling point over that
of water is known as the boiling-point
elevation (BPE) of the solution.
(BPE) is best found from an empirical
rule known as Dühring’s rule. Which
states that the boiling point of a given
solution is a linear function of the
b
oiling point of pure water at the
s
ame pressure.
If the boiling point of the solution is
p
lotted against that of water at the
s
ame pressure, a straight line results.
Different lines are obtained for different
concentrations
Effect of liquid head and friction on
temperature drop
If the depth of liquid in an evaporator is
a
ppreciable, the boiling point corresponding to
the pressure in the vapor space is that of the
surface layer of liquid only.
At the distance Z m below the surface
is under a pressure of the vapor space
plus a head of Z m of liquid.
In an evaporator, therefore, the average
boiling point of the liquid in the tubes is
higher than the boiling point in the vapor
space.
This increase in boiling point lowers
the average temperature drop between
the steam and the liquid and reduces t
he capacity.
The true temperature drop, corrected for
both boiling elevation and static head, is
represented by the average temperature
drop between the saturation temperature o
f stream and the variable liquid
t
emperature.
Heat-transfer coefficient
The overall coefficient is strongly influenced
by the design and method of operation of t
he evaporator.
In most evaporators the fouling factor
of the condensing steam and resistance
of the tube wall are very small, and
they are usually neglected.
Steam-film coefficients
The steam-film coefficient is high. Since
the presence of non-condensable gas
s
eriously reduces the film coefficient.
Provision must be made to vent
noncondensables from the steam chest
and to prevent leakage of air inward.
Liquid-side coefficients
The liquid-side coefficient depends to a l
arge extent on the velocity of the liquid
over the heated surface.
The resistance of the liquid side controls the
overall rate of heat transfer to the boiling
liquid for viscous fluid.
Forced circulation gives high liquid-side
coefficient.
Because of the difficulty of measuring
the high individual film coefficients in
an evaporators, experimental results
are usually expressed in terms of
overall coefficients
If one resistance( say, that of the liquid
film) is controlling ,large changes in
the other resistances have almost no
effect on the overall coefficient.
Typical overall coefficients for various types
of evaporators are given in table
type
Overall coefficient
W/m2ºC
Long-tube vertical evaporator
Natural circulation
1000-2500
Force circulation
2000-5000
Evaporator Economy
The chief factor influencing the economy of a
n evaporator system is the number of
ef
fects.
The economy also is influenced by the
temperature of the feed.
Quantitatively, evaporator economy is
entirely a matter of enthalpy balance.
Enthalpy balances for single-effect
evaporator
In single-effect evaporator, the latent
heat of condensation of the team is t
ransferred through a heating surface t
o vaporize water from a boiling
s
olution.
Two enthalpy balances are needed,
one for the team and one for the vapor
or liquid side.
Fig. shows a single-effect Evaporator.
• The rate of steam and of condensate is
ms
• Feed is mf and that of the
concentrate is m
• The rate of vapor flow to the condense
r is mf -m
Vapor out
Stream in
feed
condensate
It is assumed that there is no leakage or
entrainment,
That the flow of noncondensable is
negligible, and that heat losses from
the evaporator need not be considered.
Both the superheat of the steam and the
subcooling of the condensate are small,
however, and it is acceptable to neglect t
hem in making an enthalpy balance.
Under these assumptions the difference
between the enthalpy of the steam and
that of the condensate is simply λs .
• The enthalpy balance for the steam side
is
qs  ms ( H s  Hc )  ms s
(16-2)
• The enthalpy balance for the liquor side i
s
q  (mf  m)Hv  mf H f  mH
(16-3)
• In the absence of heat losses, the heat
transferred from the steam to the
tubes equals that transferred from the
tubes to the liquor.
Thus , by combing Eqs. (16-2) and(16-3)
q  mss  (mf  m)Hv  mf H f  mH
(16-4)
The liquor-side enthalpies depend upon
the characteristics of the solution being
concentrated.
Most of solutions when mixed or dilute at
constant temperature do not give much h
eat effect.
Some of solutions when mixed or dilute e
volve considerable heat effect.
An equivalent amount of heat is
required, in addition to the latent heat
of vaporization, when dilute solutions
of these substances are concentrated
to high densities.
Enthalpy balance with negligible
heat of dilution
For solutions having negligible heats of
dilution, the enthalpy balances over a s
ingle-effect evaporator can be
c
alculated from the specific heats and t
emperatures of the solutions.
The heat-transfer rate q on the liquor side
includes:
• qf, the heat transferred to the thin liquor t
o change its temperature from tf to the b
oiling temperature t
• qv, the heat to accomplish the evaporation
If the specific heat of the thin liquor is a
ssumed constant over the temperature r
ange , then
q f  m f c pf (t  t f )
(16-5)
and
q f  (m f  m)v
(16-6)
If the boiling-point elevation of the t
hick liquor is negligible, λv=λ, the lat
ent heat of vaporization of water at th
e pressure in the vapor space.
When the boiling-point elevation is
appreciable, λv differs slightly fromλ.
In practice, however, it is nearly
always sufficiently accurate to use λ.
The final equation for the enthalpy
balance can be gotten from Eqs. (16-5)
and (16-6)when the heat of dilution is
negligible.
q  mf cpf (t  t f )  (mf  m)
(16-7)
Equation (16-7) states that the heat
from the condensing steam is utilized
To vaporize water from the solution
To heat the feed to the boiling point
If the feed enters above the boiling
point in the evaporator, part of the
evaporation is from flash.
Enthalpy balance with appreciable heat o
f dilution
If the heat of dilution of the liquor being
concentrated is too large to be neglected,
an enthalpy-concentration diagram is u
sed for the values of H in Eq. (16-4)
Figure is an enthalpy-concentration
diagram for solution of sodium
hydroxide and water.
Single-effect calculations
The use of material balances , enthalpy
balances, and the capacity equation
(16-1) in the design of single-effect
evaporation is shown in example16.1
Multiple-effect evaporators
• Figure shows a triple-effect system
feed
Steam in
Connections are made so that the vapor
from one effect serves as the heating
medium for the next.
A condenser and air ejector establish a
vacuum in third effect in the series
and withdraw noncondensables from
the system.
The first effect in the series is the effect t
o which the raw steam is fed and in w
hich the pressure in the vapor space.
The last effect is that in which the
vapor-space pressure is minimum.
In this manner the pressure difference
between the steam and the condenser
is spread across two or more effects in
the multiple-effect system.
The pressure in each effect is lower t
han that in the effect from which it r
eceives steam and higher than that o
f the effect to which it supplies
v
apor.
Each effect has a temperature drop
across its heating surface
corresponding to the pressure drop in
that effect.
In figure, dilute feed 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.
Thick liquor is pumped out of the third
effect.
In steady operation all internal concentr
ations, flow rates, pressures, and
t
emperatures are kept constant.
The heating surface in the first effect
will transmit per hour an amount of
heat given by the equation
q1  AU
1 1t1
(16-8)
If the part of this heat that goes to heat
the feed to the boiling point is negligible
for the Moment.
The temperature of the condensate
leaving the second effect is very near
the temperature t1 of the vapors from
the boiling liquid in the first effect.
In steady operation the heat that was
expanded in creating vapor in the first
effect must be neglected when this s
ame vapor condenses in the second
effect.
The heat transmitted in the second
effect, however, is given by the
equation
q2  A2U2t2
(16-9)
As has just been shown, q1 and q2 are
nearly equal, and therefore
AU
1 1t1  A2U 2 t2
(16-10)
This same reasoning may be extended
to show that, roughly
AU
1 1t1  A2U2 t2  AU
3 3t3
(16-11)
In ordinary practice the heating areas in
all the effects of a multiple-effect
evaporator are equal.
Therefore, from Eq. (16-11)it follows
that since q1=q2=q3=q
q
U1t1  U 2 t2  U 3 t3 
A
(16-11)
Methods of feeding
Forward feed
The usual method of feeding in a
multiple-effect evaporator system is
forward feed.
The feed is pumped into the first effect
and send it in turn through the other
effects. As shown in fig.
The concentration of the liquid increases
from the first effect to the last.
The transfer from effect to effect can be
done with pumps, since the flow is in
the direction of decreasing pressure.
Backward feed
Another common method is backward
feed. As shown in figure.
Dilute liquid is fed to the last effect and
then pumped through the successive
effects to the first.
This method requires a pump between
each pair of effects, since the floe is
from low pressure to high pressure.
Backward feed often gives a higher
c
apacity than forward feed when the t
hick liquor is viscous.
It may gives a lower economy than
forward feed when the feed liquor is
cold.