2- Evaporation

The objective of evaporation is to concentrate a solution consisting of a nonvolatile solute and a volatile
solvent. In the great majority of evaporations the solvent is water. Evaporation is conducted by
vaporizing a portion of the solvent to produce a concentrated solution of thick liquor. Evaporation differs
from drying in that the residue is a liquid—sometimes a highly viscous one—rather than a solid; it differs
from distillation in that the vapor usually is a single component, and even when the vapor is a mixture, no
attempt is made in the evaporation step to separate the vapor into fractions; it differs from crystallization
in that it is based on concentrating a solution rather than forming and building crystals. In certain
situations, e.g., in the evaporation of sea water to produce common salt, the line between evaporation
and crystallization is far from sharp. Evaporation sometimes produces a slurry of crystals in a saturated
mother liquor.
Normally, in evaporation the thick liquor is the valuable product and the vapor is condensed and
discarded, in one specific situation, however, the reverse is true. Mineral-carrying water often is
evaporated to give a solid-free product for special process requirements, or for human consumption. This
technique is often called water distillation, but technically it is evaporation. Large-scale evaporation
processes have been developed and used for recovering potable water from seawater. Here the
condensed water is the desired product. Only a fraction of the total water in the feed is recovered, and
the remainder is returned to the sea.
Liquid characteristics
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The practical solution of an evaporation problem is profoundly affected by the character of the liquor to
be concentrated. It is the wide variation in liquor characteristics (which demands judgment and
experience in designing and operating evaporators) that broadens this operation from simple heat
transfer to a separate art. Some of the most important properties of evaporating liquids are as follows.
Concentration
Although the thin liquor fed to an evaporator may be sufficiently dilute to have many of the physical
properties of water, as the concentration increases, the solution becomes more and more individualistic.
The density and viscosity increase with solid content until either the solution becomes saturated or the
liquor becomes too sluggish for adequate heat transfer. Continued boiling of a saturated solution causes
crystals to form; these must be removed or the tubes block. The boiling point of the solution may also
rise considerably as the solid content increases, so that the boiling temperature of a concentrated
solution may be much higher than that of water at the same pressure.
Foaming
Some materials, especially organic substances, foam during vaporization. A stable foam accompanies
the vapor out of the evaporator, causing relatively entrainment. In extreme cases the entire mass of
liquid may boil over into the vapor outlet and be lost.
Temperature sensitivity

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Many fine chemicals, pharmaceutical products, and foods are damaged when heated to moderate
temperatures for relatively short times. In concentrating such materials special techniques are needed to
reduce both the temperature of the liquid and the time of heating.
Scale
Some solutions deposit scale on the heating surfaces. The overall coefficient then steadily diminishes,
until the evaporator must be shut down and the tubes cleaned. When the scale is hard and insoluble, the
cleaning is difficult and expensive.
Many other liquid characteristics must be considered by the designer of an evaporator. Some of these
are specific heat, heat of concentration, freezing point, gas liberation on boiling, toxicity, explosion
hazards, radioactivity, and necessity for sterile operation. Because of the variation in liquor properties,
many different evaporator designs have been developed. The choice for any specific problem depends
primarily on the characteristics of the liquid.
Materials of construction
Whenever possible, evaporators are made of some kind of steel. Many solutions, however, attack
ferrous metals, or are contaminated by them.
Special materials such as copper, nickel, stainless steel, aluminum, impervious graphite, and lead are
then used. Since these materials are expensive, high heat transfer rates become especially desirable to
minimize the first cost of the equipment.

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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. Usually the steam is at a low pressure, below 3 atm; often the boiling
liquid is under a moderate vacuum, up to about 0.05 atm. Reducing the boiling temperature of the liquid
increases the temperature difference between the steam 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. This
method is called single-effect evaporation, and although it is simple, it utilizes steam ineffectively. If the
vapor from one evaporator is fed into the steam chest of a second evaporator and the vapor from the
second is then sent to a condenser, the operation becomes double-effect. The heat in the original steam
is reused in the second effect, and the evaporation achieved by a unit mass of steam fed to the first
effect is approximately doubled. Additional effects can be added in the same manner. The general
method of increasing the evaporation per pound of steam by using a series of evaporators between the
steam supply and the condenser is called multiple-effect evaporation. Figure 17 represents the two
types.

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 Figure 17 Single –effect evaporator and multiple-effect evaporator

https://www.youtube.com/watch?app=desktop&v=VBaz3NIIJ9o
https://www.youtube.com/watch?v=kHMlLDsJqXE
Types of Evaporators
The chief types of steam-heated tubular evaporators in use today may be operated either as once-
through or as circulation units.
The essential parts a typical long-tube vertical evaporator are:
(l) A tubular exchanger with steam in the shell and liquid to be concentrated in the tubes,
(2) A separator or vapor space for removing entrained liquid from the vapor, and
(3) When operated as a circulation unit, a return leg for the liquid from the separator to the bottom of the
exchanger. a pool of liquid is held within the equipment.

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(4) Inlets are provided for feed liquid and steam, and outlets are provided for vapor, thick liquor, steam
condensate, and non-condensable gases from the steam.

Once-through Evaporators:
Falling-film Evaporators
Falling-film evaporators are always operated once through; Agitated-film can also be operated in this
way.
the feed liquor passes through the tubes only once, releases the vapor, and leaves the unit as thick
liquor. All the evaporation is accomplished in a single pass. These evaporators are well adapted to
multiple-effect operation.
Once-through evaporators (Figure 18) are especially useful for heat-sensitive materials. By operating
under high vacuum. With a single rapid passage through the tubes the thick liquor is at the evaporation
temperature but at a short time and can be quickly cooled as soon as it leaves the evaporator.
Concentration of highly heat sensitive materials such as orange juice requires a minimum time of
exposure to a heated surface. The liquid enters at the top, flows downstream inside the heated tubes as
a film, and leaves from the bottom. The tubes are large, 2 to 10 inch in diameter (1 inch = 2.45 cm).
Vapor evolved from the liquid is usually carried downward with the liquid and leaves from the bottom of
the unit. In appearance these evaporators resemble long, vertical, tubular exchangers with a liquid vapor
separator at the bottom and a distributor for the liquid at the top.

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Falling-film evaporators are also well adapted to concentrating viscous liquids.

Fig. 18 Falling-film evaporator

Agitated-film evaporator
This is a modified falling-film evaporator with a single jacketed tube containing an internal agitator. Feed
enters at the top of the jacketed section and is spread out into a thin, highly turbulent film by the vertical
blades of the agitator. Concentrate leaves from the bottom of the jacketed section; vapor rises from the
vaporizing zone into an unjacketed separator, which is somewhat larger in diameter than the evaporating
tube. Fig. 19 shows an agitated film evaporator. The chief advantage of an agitated-Film evaporator is its
ability to give high rates of heat transfer with viscous liquids.

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 Fig. 19 Agitated film evaporator.
Circulation Evaporators
A pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool, and the
mixture passes through the tubes more than one time. Unevaporated liquid discharged from the tubes
returns to the pool. All forced-circulation evaporators are operated in this way, climbing-film evaporators
are usually circulation units (Figure 20). The thick liquor from a circulation evaporator is withdrawn from
the pool.

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All the liquor in the pool must therefore be at the maximum concentration. Since the liquid entering the
tubes may contain several parts of thick liquor for each part of feed, its concentration, density, viscosity,
and boiling point are nearly at the maximum. Accordingly, the heat-transfer coefficient tends to be low.
Circulation evaporators are not well suited to concentrating heat-sensitive liquids. With a reasonably
good vacuum, the temperature of the bulk of the liquid may be nondestructive, but the liquid is repeatedly
exposed to contact with hot tubes. Some of the liquid, therefore, may be heated to an excessively high
temperature. Although the average residence time of the liquid in the heating zone may be short, part of
the liquid is retained in the evaporator for a considerable time. Circulation evaporators are well adapted
to single-effect evaporation. They may operate either with natural circulation, with the flow through the
tubes induced by density differences, or with forced circulation, with flow provided by a pump.

Fig. 20 Climbing film, long – tube vertical evaporator.

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The tubes are typically 1 to 2 in. in diameter and 12 to 32 ft. long (1 foot = 30 cm). Liquid and vapor flow
upward inside the tubes as a result of the boiling action; separated liquid returns to the bottom of the
tubes by gravity. Dilute feed, often at about room temperature, enters the system and mixes with liquid
returning from the separator.
The mixture enters the bottom of the tubes, on the outside of which steam is condensing. For a short
distance the feed to the tubes flows upward as liquid, receiving heat from the steam. Bubbles are then
formed in the liquid as boiling begins, increasing the linear velocity and the rate of heat transfer. Near the
top of the tubes the bubbles grow rapidly. In this zone bubbles of vapor alternating with slugs of liquid
rise very quickly through the tubes and emerge at high velocity from the top. From the tubes the mixture
of liquid and vapor enters the separator. The diameter of the separator is larger than that of the
exchanger, so that the linear velocity of the vapor is greatly reduced.

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