EVAPORATOR
Energetics
Water
can be removed from solutions in ways other than evaporation, including
membrane processes, liquid-liquid extractions, crystallization,
and precipitation. Evaporation can be
distinguished from some other drying methods in that the final product of
evaporation is a concentrated liquid, not a solid. It is also relatively simple
to use and understand since it has been widely used on a large scale. In order
to concentrate a product by water removal, an auxiliary phase is used which
allows for easy transport of the solvent (water) rather than the solute. Water vapor is used as the
auxiliary phase when concentrating non-volatile components, such as proteins and
sugars. Heat is added to the solution and part of the solvent in converted into
vapor. Heat is the main tool in evaporation, and the process occurs more
readily at high temperature and low pressures.
Heat
is needed to provide enough energy for the molecules of the solvent to leave the solution
and move into the air surrounding the solution. The energy needed can be
expressed as an excess thermodynamic potential of the water in the solution. Leading
to one of the biggest problems in industrial evaporation, the process requires
enough energy to remove the water from the solution and to supply the heat of
evaporation. When removing the water, more than 99% of the energy needed goes
towards supplying the heat of evaporation. The need to overcome the surface
tension of the solution also requires energy. The energy requirement of
this process is very high because a phase transition must be caused; the water
must go from a liquid to a vapor.
When
designing evaporators, engineers must quantify the amount of steam needed for
every mass unit of water removed when a concentration is given. An energy
balance must be used based on an assumption that a negligible amount of heat is
lost to the system’s surroundings. The heat that needs to be supplied by the
condensing steam will approximately equal the heat needed to heat and vaporize
the water. Another consideration is the size of the heat exchanger which
affects the heat transfer rate.
q A =
heat transfer area
q =
overall heat transfer rate
How an evaporator
works
The
solution containing the desired product is fed into the evaporator and passes a
heat source. The applied heat converts the water in the solution into vapor.
The vapor is removed from the rest of the solution and is condensed while the
now concentrated solution is either fed into a second evaporator or is removed.
The evaporator as a machine generally consists of four sections. The heating
section contains the heating medium, which can vary. Steam is fed into this
section. The most common medium consists of parallel tubes but others have
plates or coils. The concentrating and separating section removes the vapor
being produced from the solution. The condenser condenses the separated vapor,
then the vacuum or pump provides pressure to increase circulation.
Types of
evaporators used today
Natural/forced
circulation evaporator
Natural
circulation evaporators are based on the natural circulation of the product
caused by the density
differences that arise from heating. In an evaporator using tubing, after the
water begins to boil,
bubbles will rise and cause circulation, facilitating the separation of the
liquid and the vapor at the top of the heating tubes. The amount of evaporation
that takes place depends on the temperature difference between the steam and
the solution. Problems can arise if the tubes are not well-immersed in the
solution. If this occurs, the system will be dried out and circulation
compromised. In order to avoid this, forced circulation can be used by
inserting a pump to increase pressure and circulation. Forced circulation
occurs when hydrostatic head prevents boiling at the heating surface. A pump
can also be used to avoid fouling that is caused by the boiling of liquid on
the tubes; the pump suppresses bubble formation. Other problems are that the
residing time is undefined and the consumption of steam is very high, but at
high temperatures, good circulation is easily achieved.
Falling film
evaporator
This
type of evaporator is generally made of long tubes (4-8 meters in length) which
are surrounded by steam jackets. The uniform distribution of the solution is
important when using this type of evaporator. The solution enters and gains
velocity as it flows downward. This gain in velocity is attributed to the vapor
being evolved against the heating medium, which flows downward as well. This
evaporator is applicable to highly viscous solutions
so it is frequently used in the chemical, food, and fermentation industry.
Plate evaporator
Plate
evaporators have a relatively large surface area. The plates are usually
corrugated and are supported by frame. During evaporation, steam flows through
the channels formed by the free spaces between the plates. The steam
alternately climbs and falls parallel to the concentrated liquid. The steam
follows a co-current, counter-current path in relation to the liquid. The
concentrate and the vapor are both fed into the separation stage where the
vapor is sent to a condenser. Plate evaporators are frequently applied in the
dairy and fermentation industries since they have spatial flexibility. A
negative point of this type of evaporator is that it is limited in its ability
to treat viscous or solid-containing products.
Multiple-effect evaporators
Unlike
single-stage evaporators, these evaporators can be made of up to seven
evaporator stages or effects. The energy consumption for single-effect
evaporators is very high and makes up most of the cost for an evaporation
system. Putting together evaporators saves heat and thus requires less energy.
Adding one evaporator to the original decreases the energy consumption to 50%
of the original amount. Adding another effect reduces it to 33% and so on. A heat
saving % equation can be used to estimate how much one will save by adding
a certain amount of effects.
The
number of effects in a multiple-effect evaporator is usually restricted to
seven because after that, the equipment cost starts catching up to the money
saved from the energy requirement drop.
There
are two types of feeding that can be used when dealing with multiple-effect
evaporators. Forward feeding takes place when the product enters the system
through the first effect, which is at the highest temperature. The product is
then partially concentrated as some of the water is transformed into vapor and
carried away. It is then fed into the second effect which is a little lower in
temperature. The second effect uses the heated vapor created in the first stage
as its heating source (hence the saving in energy expenditure). The combination
of lower temperatures and higher viscosities in subsequent effects provides
good conditions for treating heat-sensitive products like enzymes and proteins.
In using this system, an increase in the heating surface area of subsequent
effects is required. Another way to proceed is by using backward feeding. In
this process, the dilute products is fed into the last effect with has the
lowest temperature and is transferred from effect to effect with the
temperature increasing. The final concentrate is collected in the hottest
effect which provides an advantage in that the product is highly viscous in the
last stages so the heat transfer is considerably better.
Applications
The
goal of evaporation is to concentrate a target liquid, and this needs to be
achieved for many different targets today. One of the most important
applications of evaporation is that on the food and drink industry. Many foods
that are made to last for a considerable amount of time or food that needs a
certain consistency, like coffee, need to go through an evaporation step during
processing. It is also used as a drying process and can be applied in this way
to laboratories where preservation of long-term activity or stabilization is
needed (for enzymes
for example). Evaporation is also used in order to recover expensive solvents
such as hexane which would otherwise be wasted. Cutting down waste handling
cost is another major application of evaporation for large companies. Legally,
all producers of waste must dispose of the waste in a methods that abides by
environmental guidelines; these methods are costly. If up to 98% of a wastes
can be vaporized, industry can greatly reduce the amount of money that would
otherwise be allocated towards waste handling.
Problems
Technical
problems can arise during evaporations, especially when the process is applied
to the food industry. Some evaporators are sensitive to differences in
viscosity and consistency of the dilute solution. These evaporators could work
inefficiently because of a loss of circulation. The pump of an evaporator may
need to be changed if the evaporator needs to be used to concentrate a highly
viscous solution. Fouling
also occurs when hard deposits form on the surfaces of the heating mediums in
the evaporators. In foods, proteins and polysaccharides can create such
deposits that reduce the efficiency of heat transfer. Foaming can also create a
problem since dealing with the excess foam can be costly in time and
efficiency. Antifoam agents are to be used, but only a few can be used when
food is being processed. Corrosion can also occur when acidic solutions such as
citrus juices are concentrated. The surface damage caused can shorten the
long-life of evaporators. Quality and flavor of food can also suffer during
evaporation. Overall, when choosing an evaporator, the qualities of the product
solution need to be taken into heavy consideration.
Evaporators
Evaporators are thermal concentrators. Evaporators are
for concentrating dilute liquids, for increasing solids content & for
reducing volume by evaporating water.
Evaporators are based on steam economy i.e. Kg of solvent evaporated per Kg of steam used. Reusing evaporated solvent as heating medium, higher steam economies are achieved. This method of increasing utilization of steam is used in multi-effect evaporator. In multi-effect evaporator vapour from the first effect is passed over to the next effect in which boiling takes place at lower temperature & pressure.
Thermo-compression evaporator is another alternative to increase steam economy. Vapor of solvent evaporated is compressed & is used as heating medium in the same evaporator.
Selection of evaporator depends on the various properties of feed & product such as crystallization, foaming, corrosion, salting & scaling.
Evaporators are based on steam economy i.e. Kg of solvent evaporated per Kg of steam used. Reusing evaporated solvent as heating medium, higher steam economies are achieved. This method of increasing utilization of steam is used in multi-effect evaporator. In multi-effect evaporator vapour from the first effect is passed over to the next effect in which boiling takes place at lower temperature & pressure.
Thermo-compression evaporator is another alternative to increase steam economy. Vapor of solvent evaporated is compressed & is used as heating medium in the same evaporator.
Selection of evaporator depends on the various properties of feed & product such as crystallization, foaming, corrosion, salting & scaling.
Types of Evaporators:
|
Falling Film
Evaporators
- The feed gets distributed and thin film is formed
- Hold up and residence time is short hence liquid does not get
overheated
In Falling film evaporators, feed solution forms a film
in the evaporator tubes & gets evaporated. These are best suited for heat
sensitive materials such as fruit juices. The hold up time is short &
liquid doesn't get overheated. Falling film evaporators are not suitable for
crystallizing liquids. They are not suitable for salting & severely scaling
liquids or solutions.
Rising Film
Evaporators
- Useful for moderately heat sensitive product
- Compact and simple design
- Most suitable for corrosive liquids
In rising film feed, solution forms a rising film in the
tubes. These are unsuited to salting or severely scaling liquids. These
evaporators are compact, simple in construction. They are suitable for
corrosive liquids.
Forced Circulation
Evaporators
- Can handle salting, scaling and fouling liquids
- Best suitable for crystalline products, corrosive viscous
solutions
Forced circulation evaporators can handle salting,
scaling & fouling liquids. They are best suitable for crystalline products,
corrosive & viscous solutions.
These evaporators have relatively high hold up or residence time thus not suitable for heat sensitive liquids. They suffer frequently from plugging of tubes, salting in tubes & poor circulation.
These evaporators have relatively high hold up or residence time thus not suitable for heat sensitive liquids. They suffer frequently from plugging of tubes, salting in tubes & poor circulation.
Short Tube Vertical
Evaporators
- Reclusively less expensive
- Facilitate easy de-scaling
- Can be widely used in sugar industry.
They are relatively inexpensive & easy de-scaling. This type is seldom used as a crystallizing evaporator. By installing a propeller, propeller calendria type evaporator can be used for crystalline products. They suffer from mild scaling but can be cleaned mechanically.
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