Considered
as a piece of process plant, the evaporator has two principal functions,
to exchange heat and to separate the vapour that is formed from the liquid.
Important practical
considerations in evaporators are the:
 maximum
allowable temperature, which may be substantially below 100°C.
promotion
of circulation of the liquid across the heat transfer surfaces, to attain
reasonably high heat transfer coefficients
and to prevent any local overheating,
viscosity
of the fluid which will often increase substantially as the concentration
of the dissolved materials increases,
tendency
to foam which makes separation of liquid and vapour difficult.
THE SINGLE EFFECT EVAPORATOR
The
typical evaporator is made up of three functional sections: the heat exchanger,
the evaporating section, where the liquid boils and evaporates, and the
separator in which the vapour leaves the liquid and passes off to the
condenser or to other equipment. In many evaporators, all three sections
are contained in a single vertical cylinder. In the centre of the cylinder
there is a steam heating section, with pipes passing through it in which
the evaporating liquors rise. At the top of the cylinder, there are baffles,
which allow the vapours to escape but check liquid droplets that may accompany
the vapours from the liquid surface. A diagram of this type of evaporator,
which may be called the conventional evaporator, is given in Fig. 8.1.
Figure 8.1 Evaporator
In the heat exchanger
section, called a calandria in this type of evaporator, steam condenses
in the outer jacket and the liquid being evaporated boils on the inside
of the tubes and in the space above the upper tube plate. The resistance
to heat flow is imposed by the steam and liquid film coefficients and
by the material of the tube walls. The circulation of the liquid greatly
affects evaporation rates, but circulation rates and patterns are very
difficult to predict in any detail. Values of overall heat transfer coefficients
that have been reported for evaporators are of the order of 1800-5000
J m-2 s-1 °C-1 for the evaporation
of distilled water in a vertical-tube evaporator with heat supplied by
condensing steam. However, with dissolved solids in increasing quantities
as evaporation proceeds leading to increased viscosity and poorer circulation,
heat transfer coefficients in practice may be much lower than this.
As
evaporation proceeds, the remaining liquors become more concentrated and
because of this the boiling temperatures rise. The rise in the temperature
of boiling reduces the available temperature drop, assuming no change
in the heat source. And so the total rate of heat transfer will drop accordingly.
Also, with increasing solute concentration, the viscosity of the liquid
will increase, often quite substantially, and this affects circulation
and the heat transfer coefficients leading again to lower rates of boiling.
Yet another complication is that measured, overall, heat transfer coefficients
have been found to vary with the actual temperature drop, so that the
design of an evaporator on theoretical grounds is inevitably subject to
wide margins of uncertainty.
Perhaps
because of this uncertainty, many evaporator designs have tended to follow
traditional patterns of which the calandria type of Fig. 8.1 is a typical
example.
Vacuum Evaporation
For the evaporation of liquids that are adversely affected by high temperatures,
it may be necessary to reduce the temperature of boiling by operating
under reduced pressure. The relationship between vapour pressure and boiling
temperature, for water, is shown in Fig.
7.2. When the vapour pressure of the liquid reaches the pressure of
its surroundings, the liquid boils. The reduced pressures required to
boil the liquor at lower temperatures are obtained by mechanical or steam
jet ejector vacuum pumps, combined generally with condensers for the vapours
from the evaporator. Mechanical vacuum pumps are generally cheaper in
running costs but more expensive in terms of capital than are steam jet
ejectors. The condensed liquid can either be pumped from the system or
discharged through a tall barometric column in which a static column of
liquid balances the atmospheric pressure. Vacuum pumps are then left to
deal with the non-condensibles, which of course are much less in volume
but still have to be discharged to the atmosphere.
Heat Transfer in Evaporators
Heat transfer in evaporators is governed by the equations for heat transfer
to boiling liquids and by the convection and conduction equations. The
heat must be provided from a source at a suitable temperature and this
is condensing steam in most cases. The steam comes either directly from
a boiler or from a previous stage of evaporation in another evaporator.
Major objections to other forms of heating, such as direct firing or electric
resistance heaters, arise because of the need to avoid local high temperatures
and because of the high costs in the case of electricity. In some cases
the temperatures of condensing steam may be too high for the product and
hot water may be used. Low-pressure steam can also be used but the large
volumes create design problems.
Calculations
on evaporators can be carried out combining mass and energy balances with
the principles of heat transfer.
 EXAMPLE
8.1. Single effect evaporator: steam usage and heat transfer surface
A single effect evaporator is required to concentrate a solution from
10% solids to 30% solids at the rate of 250 kg of feed per hour. If the
pressure in the evaporator is 77 kPa absolute, and if steam is available
at 200 kPa gauge, calculate the quantity of steam required per hour and
the area of heat transfer surface if the overall heat transfer coefficient
is 1700 J m-2 s-1 °C-1.
Assume that the temperature of the feed is 18°C and that the boiling
point of the solution under the pressure of 77 kPa absolute is 91°C.
Assume, also, that the specific heat of the solution is the same as for
water, that is 4.186 x 103 J kg-1°C-1,
and the latent heat of vaporization of the solution is the same as that
for water under the same conditions.
From
steam tables (Appendix 8), the condensing
temperature of steam at 200 kPa (gauge)[300 kPa absolute] is 134°C
and latent heat 2164 kJ kg-1; the condensing temperature at
77 kPa (abs.) is 91°C and latent heat is 2281 kJ kg-1.
Mass balance
(kg h-1)
 |
|
Solids
|
Liquids
|
Total
|
| |
Feed |
25
|
225
|
250
|
| |
Product |
25
|
58
|
83
|
| |
Evaporation |
|
|
167
|
Heat balance
Heat available per
kg of steam
= latent heat + sensible heat in cooling to 91°C
= 2.164 x 106 + 4.186 x 103(134 - 91)
= 2.164 x 106 + 1.8 x 105
= 2.34 x 106 J
Heat required by
the solution
= latent heat + sensible heat in heating from 18°C to 91°C
= 2281 x 103 x 167 + 250 x 4.186 x 103 x (91 - 18)
= 3.81 x 108 + 7.6 x 107
= 4.57 x 108 J h-1
Now, heat from steam
= heat required by the solution,
Therefore quantity of steam required per hour = (4.57 x 108)/(2.34
x 106)
= 195 kg h-1
Quantity of steam/kg of water evaporated = 195/167
= 1.17 kg steam/kg water.
Heat-transfer
area
Temperature of condensing
steam = 134°C.
Temperature difference across the evaporator = (134 - 91) = 43°C.
Writing the heat transfer equation for q in joules/sec,
q = UA DT
(4.57 x 108)/3600 = 1700 x A x 43
A = 1.74 m2
Area of heat transfer
surface = 1.74 m2
(It
has been assumed that the sensible heat in the condensed (cooling from
134°C to 91°C) steam is recovered, and this might in practice
be done in a feed heater. If it is not recovered usefully, then the sensible
heat component, about 8%, should be omitted from the heat available, and
the remainder of the working adjusted accordingly).
Condensers
In evaporators that are working under reduced pressure, a condenser, to
remove the bulk of the volume of the vapours by condensing them to a liquid,
often precedes the vacuum pump. Condensers for the vapour may be either
surface or jet condensers. Surface condensers provide sufficient heat
transfer surface, pipes for example, through which the condensing vapour
transfers latent heat of vaporization to cooling water circulating through
the pipes. In a jet condenser, the vapours are mixed with a stream of
condenser water sufficient in quantity to transfer latent heat from the
vapours.
EXAMPLE
8.2. Water required in a jet condenser for an evaporator
How much water would be required in a jet condenser to condense the vapours
from an evaporator evaporating 5000 kg h-1 of water under a
pressure of 15 cm of mercury? The condensing water is available at 18°C
and the highest allowable temperature for water discharged from the condenser
is 35°C.
Heat balance
The
pressure in the evaporator is 15 cm mercury = Zrg
= 0.15 x 13.6 x 1000 x 9.81 = 20 kPa.
From Steam Tables, the condensing temperature of water under pressure
of 20 kPa is 60°C and the corresponding latent heat of vaporization
is 2358 kJ kg-1.
Heat removed from
condensate
= 2358 x 103 + (60 - 35) x 4.186 x 103
= 2.46 x 106 J kg-1
Heat taken by cooling water
= (35 - 18) x 4.186 x 103
= 7.1 x 104 J kg-1
Quantity of heat
removed from condensate per hour
= 5000 x 2.46 x 106 J
Therefore quantity of cooling water per hour
= (5000 x 2.46 x 106)/7.1 x 104
= 1.7 x 105 kg
EXAMPLE
8.3. Heat exchange area for a surface condenser for an evaporator
What heat exchange area would be required for a surface condenser working
under the same conditions as the jet condenser in Example 8.2, assuming
a U value of 2270 J m-2 s-1 °C-1,
and disregarding any sub-cooling of the liquid.
The
temperature differences are small so that the arithmetic mean temperature
can be used for the heat exchanger (condenser).
Mean temperature difference
= (60 - 18)/2 + (60 - 35)/2
= 33.5°C.
The data are available
from the previous Example, and remembering to put time in hours.
Quantity of heat required by condensate = UA DT
5000 x 2.46 x 106 = 2270 x A x 33.5 x 3600
and so A
= 45 m2
Heat transfer area required =
45 m2
This would be a large surface condenser so that a jet condenser is often
preferred.
 Evaporation
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Single Effect Evaporator