Chapter 16

Evaporation
The objective of evaporation

• To concentrate a solution ,consisting of a

nonvolatile solute & a volatile solvent.

• Evaporation is conducted by vaporizing a

portion of the solvent to concentrate
solution or thick liquor.
 Evaporation Vs crystallization:
 emphasis on concentrating to form
supersaturated solutions till
formation of crystals.
• Normally ,in evaporation the thick
• liquor is the valuable product & the
vapor is condensed and discarded.
Liquid characteristics

Important properties of
evaporating
liquids are as follows.
As the concentration increases;

•The density and viscosity increase

• Saturation causes Elevation in B.P.

(Concentrated
solution may have higher B.P. than that of pure
water at the same pressure.)
• Concentration may cause heavy
entrainment.

• A stable foam formation may occur

Temperature sensitivity

• Many chemicals , pharmaceuticals

,&
foods products are damaged during
Evaporation
• Thick solutions deposit or form a
scale
on the heating surfaces needs
periodical cleaning
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 difference 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 the 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 same manner.

A series of evaporators between

the stream supply and the
condenser is called
multiple-effect
evaporator.
Types of evaporators

The main types of tubular

Evaporators –
1 ） Short-tube evaporators
2 ） Long-tube vertical evaporators
• Forced-circulation
• Upward-flow (climbing-film)
• Down-ward-flow (falling-film)
Once-through &
circulation
evaporators
Evaporators may be operated either
as once-through or circulation
units.
In once-through operation the feed
liquor passes through the tubes
only once, releases the vapor,
and leaves the unit as thicker liquor.
Evaporation is accomplished in a
single pass
Short-tube evaporators

In the oldest type of

evaporators the tubes are
“short”.

In the short-tube vertical

evaporator shown in Fig
 vapor

Central downcomer

feed
Stream
inter

condensate

concentrate
Long-tube evaporators with
upward flow

A typical long-tube vertical

evaporator
with upward flow of the liquid is
shown in Fig.
 Vapor out

Co
n cen
Stream in tra
te
o ut

feed

condensate
Forced-circulation
evaporators

Higher coefficients are obtained

in forced-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
Film evaporators

Concentration of highly heat-

sensitive materials such as
orange juice requires a
minimum time of exposure to a
heated 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
 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 downward.
 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 product of three factors:
 The area of the heat-transfer surface
A
 The overall heat-transfer coefficient U
 The overall temperature drop Δt
q UAt
 Ifthe feed to the evaporator is
at the boiling temperature
corresponding to the absolute
pressure in the vapor space then
All the heat transferred through
the heating surface is available
for evaporation.
The capacity is proportional to q
 Ifthe feed is cold, the energy is
required 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.
 Ifthe 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 heating surface depends on :
 The solution being evaporated
 Thedifference 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 of the aqueous solutions
is higher than 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 linear function of the
boiling point of
pure water at the same
pressure”.
If the boiling point of the
solution is plotted against
that of water at the same
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 appreciable, 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 the capacity.
The true temperature drop,
corrected for both boiling
elevation and static head, is
represented by the average
temperature drop between the
saturation temperature of stream
and the variable liquid
temperature.
Heat-transfer coefficient

The overall coefficient is strongly

influenced by the design and
method of operation of the
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 seriously
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 large 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 an evaporator system is
the number of effects.

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 transferred
through a heating surface to
vaporize water from a boiling
solution.
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
condenser is mf -m
 Vapor out

Co
n cen
Stream in tra
te
o ut

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
them 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  H c ) ms s
(16-2)
• The enthalpy balance for the
liquor side is

q (m f  m) H v  m f 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 (m f  m) H v  m f 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 heat effect.
Some of solutions when mixed
or dilute evolve 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 single-effect
evaporator can be
calculated from the specific
heats and temperatures of the
solutions.
The heat-transfer rate q on the
liquor side includes:

• qf, the heat transferred to the thin

liquor to change its temperature
from tf to the boiling temperature
t
• qv, the heat to accomplish the
evaporation
If the specific heat of the thin
liquor is assumed constant over
the temperature range , 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 thick liquor is negligible,
λv=λ, the latent heat of
vaporization of water at the
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 m f c pf (t  t f )  (m f  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 of dilution

If the heat of dilution of the

liquor being concentrated is too
large to be neglected, an
enthalpy-concentration diagram
is used 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 to which the raw steam is
fed and in which 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 than that in the effect
from which it receives steam
and higher than that of the
effect to which it supplies
vapor.
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
concentrations, flow rates,
pressures, and
temperatures 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 same vapor condenses in
the second effect.
The heat transmitted in the
second effect, however, is
given by the equation

q2  A2U 2 t2 (16-9)

As has just been shown, q1 and
q2 are nearly equal, and
therefore

1 1t1  A2U 2 t 2
AU
(16-
10)
This same reasoning may be
extended to show that,
roughly

1 1 t1  A2U 2 t 2  A3U 3 t3

AU
(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 capacity than
forward feed when the thick
liquor is viscous.

It may gives a lower economy

than forward feed when the
feed liquor is cold.