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| Allgaier’s drum dryer, System Mozer TK, is
suitable for drying materials such as limestone and
diabase. Courtesy Allgaier Process Technology
GmbH |
It can often be difficult to choose the right drying system,
and users are frequently faced with the choice between a rotary
drum dryer or a fluidised bed, vibratory dryer. While the
applications for these two dryer types can overlap somewhat,
there are various selection criteria which favour one dryer
over the other for a given application.
Drum dryers have been used for decades for drying many
different types of goods and are widely used in the building,
minerals and raw materials sectors. Fluidised bed dryers, on
the other hand, are more commonly used during the production of
chemicals, foodstuffs and pharmaceuticals.
Various niche applications have been developed for fluidised
bed dryers, including: spray-granulation dryers, suspension and
paste dryers, and fluidised bed dryers or coolers with heat
exchangers integrated into the fluidised bed.
In the last twenty years fluidised bed dryers have also been
installed in the building material and minerals industries.
Meanwhile, drum dryers have been improved further to allow for
hard-wearing solutions and energy efficient applications,
particularly when used in the minerals industry.
Drum dryers
In drum dryers, the material to be dried is moved by the
rotation of the drum and by the effects of vanes and blades
mounted inside the drum. Past drum dryer models were in most
cases constructed with a slight downward inclination in order
to support the movement of the material.
However, more modern drums are almost always mounted
horizontally. Lifting blades lift the moist material from the
base of the drum and then allow it to fall, thus ensuring good
contact between the moist material and the hot air.
In most drying applications, the material and the hot air
flow parallel in the same direction through the drum. The moist
material is transported through the drum by means of vanes,
moving in both the same direction as the hot air and also at
cross flow to the air.
Drying minerals
In the minerals industry, the drying air is in most cases
heated with the aid of a gas or light-oil burner. The
combustion gases are then mixed with ambient air to achieve
average temperatures between 600-900°C. For materials that
are less sensitive to heat, such as quartz sand, it is possible
for the flame to burn inside the rotating drum.
In the case of heat-sensitive minerals, such as bentonite,
clay, limestone, or recycled plastics or organic waste
materials, combustion chambers are used to ensure that the
combustion gases are mixed with ambient air before they enter
the drum.
The outlay for heating drum dryers is relatively low. Modern
burners have only relatively small fans for the combustion air,
and in many cases it is not necessary to use fans to blow the
heated air into the dryer or to install costly piping.
The moist drying air leaving the dryer is extracted with an
exhaust fan, filtered in the bag filter to remove dust and
exhausted via a chimney into the open air. The piping for the
exhaust air is also relatively simple, since the air needs to
be extracted only at a single point from the dryer housing.
Drum drying rates
Drum dryers are constructed for handling solids throughput
at a rate of 5-10 tph. An advantage of drum dryers is that they
are almost unaffected by variations in the moisture content of
the drying materials, in the throughput or in the grain size of
the material or by lumps or coarse grains in the incoming
material. Even in the event of a power failure, operations can
in most cases be resumed immediately when the power
returns.
Drum dryers are also appropriate for handling fine solids,
but are particularly suitable for coarse and very coarse bulk
goods, and it is not absolutely necessary to adjust the amount
of air when products are changed. Even if the supply of drying
air fails, the material is still moved reliably through the
drum. The dryer controller can be automated relatively
easily.
Abrasive effect
The drum, plus its internal blades and vanes, are made from
thick-walled steel. If the blades and vanes are designed
correctly, even very abrasive materials can be dried in drum
dryers. Care must be taken to ensure that the material to be
dried slides as little as possible on the blades and vanes and
that it falls onto a bed of material in the bottom of the
drum.
In general, dryers have low specific heating-energy
consumption rates if the process can be run with a high inlet
air temperature. High gas temperatures result in low amounts of
drying air and less heat losses with the exhaust air. For this
reason, drum dyers which operate with hot gas temperatures of
up to 900°C are very efficient, and are commonly used in
the minerals industry. The exhaust air temperatures of drum
dryers are generally higher than those of fluidised bed
dryers.
Fluidised bed dryers
In fluidised bed dryers, good fluidisation of the drying
material is a prerequisite for reliable transport and good
ventilation of the material through the dryer. Drum dryers, in
contrast, transport the drying material with the aid of vanes
and blades in the rotating drum.
The fans which blow air into a fluidised bed dryer not only
have to fluidise the material to be dried, but they must also
overcome the pressure loss in the gas distribution plate, which
must be high enough to ensure uniform distribution of the
drying air over the entire cross-section of the dryer.
The electricity consumption of a drum dryer is thus
relatively low and is about two thirds of the value for
fluidised bed dryers.
Transport
The transport of the drying material is achieved by
fluidising this material with an upward flow of drying air. If
this is done correctly, the material assumes a pseudo-fluid
state and flows out of the dryer at the same rate as moist
material enters the dryer. The solid flows in a cross flow to
the upward flow of drying air.
The outlet of the dryer is equipped with an adjustable weir
with which the height of the fluidised bed can be adjusted, and
this affects the retention time of the material in the dryer.
In the ideal case, all particles in a fluidised bed are
continually in contact with the drying air, which results in
the best heat transfer conditions. High specific drying rates
or relatively small dryer dimensions are possible.
Good vibrations
Since this ideal state can often not be achieved in
practical applications, however, fluidised bed dryers are built
as vibratory dryers in order to improve the fluidisation of the
drying material.
Vibration ensures that even coarse particles which cannot be
fluidised by the flow of air are transported better. But there
are limits to this. Depending on the range of grain sizes and
the apparent density of the material to be dried, the use of
fluidised bed dryers is recommended only up to a grain size of
about 6mm, with an absolute cut-off at grains of 8mm in
size.
Coarse particles can accumulate in the dryer if these
restrictions are not adhered to, as the grains cannot flow over
the adjustable weir at the outlet of the dryer. If no
adjustable weir is used (in order to prevent the accumulation
of coarse particles), it is no longer possible to adjust the
retention time of the material being dried by varying the
height of the weir, or, in other cases, the poorly fluidised
solid material is no longer ventilated correctly.
Optimal conditions
In general, the air distribution plates should be designed
with a sufficiently high pressure loss in order to ensure good
distribution of the drying air over the complete area of the
dryer.
Therefore the fans for the drying air must generate a high
pressure, otherwise the air will take the path of least
resistance through the drying material; resulting in a poor air
flow, particularly in places where the material is moist. For
this reason, fluidised bed dryers have a higher specific
consumption of electricity (about 150% of the energy
consumption of a drum dryer).
Wherever possible, fluidised bed dryers should always be
operated with solid materials comprising fine grains, since
they are designed for this. If, for example, in the case of a
product change, material with a considerably larger grain size
enters the dryer, the fluidisation may collapse and efficient
drying may no longer be possible.
Thanks to the fact that each particle of the drying material
is continuously surrounded by air, sensitive moist solids or
granulates from mixer granulators can be dried without damage
in a fluidised bed. Due to the continuous lifting and dropping
of the material, drum dryers cause a higher abrasion of the
particles.
Just like drum dryers, fluidised bed dryers are normally
regulated such that the temperature of the drying air leaving
the dryer remains constant. Depending on the temperature of
this air, a control loop adjusts the heat input to the incoming
drying air and thus reacts to varying amounts of material or to
varying moisture contents.
Because the temperature of the exhaust air is closely
related to the temperature of the material at the end of the
dryer, and since this determines the residual moisture of the
material, it is possible to ensure constant residual moistures
of the material being dried.
However, the specific fuel consumption per tonne of the
drying material increases if the drying system is not operated
at its design parameters. This may occur owing to a
continuously low amount of material in the dryer, or if the
material contains a low moisture content.
Since the amount of air entering a fluidised bed dryer must
be kept constant in order to maintain the fluidisation of the
material, the same amount of heat is lost with the exhaust air
even if the dryer is operated at a reduced capacity. It is not
possible to reduce the amount of air in order to maintain a
high temperature of the drying air, as it is possible in drum
dryers.
Fluidised bed dryers are particularly suitable for use in
the chemicals industry and for the processing and recycling of
materials which are easily damaged by high temperatures.
Which dryer?
Owing to the suitability of drum and fluidised bed dryers in
a number of applications, the criteria which should ultimately
determine which dryer you buy are:
● expected stability of the production conditions
(throughput, moisture content, grain size)
● prices of the energy sources (electricity or
gas)
● infrastructure at the installation location
● level of training and organisation of the operating
personnel
● the services available at site of installation
The investments for a complete drum dryer or fluidised bed
system, including the electrical control system, are often
comparable. In some cases, the criteria are met by both
systems. In most cases, therefore, the decision must be made on
the basis of two or three of the main criteria which are of
particular importance to the operator or the investor.
Contributor: Dr.-Ing. Mathias Trojosky,
head of technology drying department, Allgaier Process
Technology GmbH, Germany
Saving energy during drying
In cases where the drying material has to be cooled
immediately, the use of a double-shell dryer for drying and
cooling in a single unit is recommended. In a double-shell
dryer the material is dried in the inner tube of the drum. The
warm, dry material then flows from the inner tube into the
outer shell, where it is moved in the opposite direction by
suitably designed vanes and blades to be cooled by the
incoming, cold ambient air.
With the System Mozer-TK, the dried goods can be cooled to
about 55-60°C, which is sufficient for most applications in
the building materials and minerals industry, and in particular
in the production of mortar. If lower temperatures are
required, a separate cooling drum, followed by a water-cooled
contact cooler or a fluidised bed cooler can be used.
A particularly energy efficient drying and cooling drum is
the System Mozer-TK Plus, which is used primarily for the
drying and cooling of quartz sand. In contrast to the System
Mozer-TK, the material is not cooled with ambient air. Instead,
a certain amount of moist sand is mixed with the warm sand
after this has passed through the inner tube of the drum.
The evaporation of the water in the moist sand cools the
mixture, and the heat stored in the dry sand helps to dry the
moist sand. In the TK Plus, the moist material is fed in from
both sides.
Depending on the moisture content of the sand to be dried,
10-20% of moist sand is mixed with 80-90% of dry sand. This
system permits the fuel consumption for drying to be reduced by
about 15%.
In fluidised bed dryers, energy can be recovered by feeding
the warm, filtered exhaust air from the cooling zone back into
the drum as pre-heated drying air – particularly in cases
where it is possible to achieve a good balance between the
amount of drying air and the amount of cooling air. In the
ideal case, all of the heat resulting from cooling of the dried
material can be used for drying.
The energy consumption is then reduced to the amount of fuel
which would be needed, without recycling of the exhaust air, to
heat the ambient air to the temperature of the exhaust air from
the cooler which is recycled.
In all variants of combined drying/cooling systems, the
initial cooling of the dried material leaving the dryer results
from the evaporation of the residual moisture. Further drying
in the first part of the cooling zone results from the heat
still stored in the dried material – rather like the drum
dryer/cooler TK Plus.