Sizikov S.A. «Effective drying equipment for bulk materials (sand, undersized crushed materials, crushed stone )»

Drying of bulk materials is the most common
process in almost all industries.

Dryers are used to obtain product with low residual
moisture. The dryers’ operation is based on moisture
removal from the material due to water phase transformation
under heat. This type of drying called ‘heat drying’ is
most widely used in industrial production. Basic methods
of heat transfer to dehydrated material are: conductive
(heat transfer from hot surface), convective (heat transfer
by gaseous medium), radiant-heat drying (radiant heat exchange),
dielectric (placing wet material in the high radio
frequency electromagnetic field), as well as cold or sublimation
drying. In large-tonnage production with capacity
of 1 up to 500 tons of dried bulk per hour, the most commonly
used are continuous convective dryers due to their
relatively simple design and efficiency as compared to the
dryers based on other methods of drying. Convective dryers
for bulk (granular) materials vary in construction as
follows: tower dryers, air tube dryers, rotary dryers, flash
dryers, fluidized-bed dryers and vibrofluidized-bed dryers.
Each of the above mentioned dryers has certain advantages
and disadvantages and appropriate application area.
In order to make a correct choice of convective dryer, brief
and comprehensible analysis of convective bulk dryers’
efficiency is provided below. This integrated assessment
of dryers’ effectiveness includes costs related to fuel consumption
(dryer performance), electric power consumed to
transit material and gaseous medium, capital expenditures
on the dryer and dust-cleaning system, overall dimensions
and amount of metal required to build the equipment.

1. Brief description of convective drying
process

Convective drying process is a direct interaction
of wet material with gaseous drying medium (hot air).Temperature difference between the surface of material’s
particles and heat carrying medium causes evaporation
of moisture into gaseous medium. At that occurs
vapor mass transfer into gaseous medium caused by the
difference between partial pressures of moisture over
wet surface of particles and in surrounding gas medium.
In other words, heat and mass exchange between hot
gases and moisture takes place on a surface of particles.
When moisture evaporates from the surface and is discharged
as vapor, moisture concentration gradient arises,
and it causes internal transit of moisture from deep
layers of material to evaporation surface. Speed of moisture
transition and, therefore, speed of drying depends
on a character and type of bonds between moisture and
solid substance of material.

According to P.A. Rebinder’s classification, there
are three types of bonds between moisture and material:
chemical, physical-and-chemical and physical-and-mechanical.
Chemically bound water is a part of crystal lattice
of the substance, it can not be eliminated by drying, and
therefore, is not taken into consideration in drying process.
Physical-chemical forces keep water that is bound by adsorption
and osmotic ties in material. Physical-mechanical
forces keep the water in macro- and micro-capillary of material
and structural moisture.

Hereinafter, we are going to consider only finegrained
materials with unbound, free moisture on surface
of particles, where water is kept by capillary tension (sand,
scoria, undersized crushed granite and dolomite, crushed
stone and etc.)

Almost every hard material in contact with humid air
is able to absorb moisture or discharge it to the medium.
Direction of moisture transfer is determined by the sign
of pressure difference of vapor in material (Pm) and in the
medium (P). When Рm-Р<0 the sorption process (surface
adsorption of moisture from air, i.e. moistening of material)
occurs, i.e. the material absorbs (sorbs) water vapors.
This state of material is called hygroscopic. When Рm-Р>0,
desorption process occurs (separation of moisture from
material’s surface, and transfer of vapors to the air by convective
diffusion), i.e. drying of material is performed. The
state of material when it discharges (desorbs) water vapors
is called wet condition of material. Value of pressure difference
determines the value of moving force and intensity
of moisture transfer process. Рm=Р and tm=t. Moisture content
when hygrothermal balance is established between
air and material is called balanced.

Drying process is characterized by interrelation of
heat and mass transfer: elimination of moisture and simultaneous
heating of wet material. Interrelation of processes
is caused by the fact that mass exchange physical parameters
of medium depend on temperature, and thermal
parameters depend on moisture concentration in material,
and vice versa in heat exchange processes.

Efficiency of mass exchange processes can be evaluated
using mass exchange equation, by convective diffusion
over evaporation surface:

1

1

1

1

The formulae mentioned above are true for constant
parameters of temperature, moisture content and air
speed. In real operating dryers the considerable change
of initial parameters of drying medium takes place: increase
of moisture content in the air and reduction of its
temperature as a result of gas interaction with material
as it passes through the dryer. Therefore, instead of ¥ in
the equations 1-7, the average integral gradient value
of moving force is used that takes into consideration
changes in concentrations and temperatures along the
drying path.

In tube dryer material is fed by a special feeder and
picked up by drying medium supplied from a bottom via
a vertical tube. Material is carried away by a flow of hot
gases and goes with it the whole way through the tube
(pneumatic transport mode). At that material delivers
its moisture to the drying medium. Later the main part
of material is precipitated in cyclone separator (or dust
catcher of other type) from which dried product is discharged.

Tower dryer operates as follows:

Material is loaded by a special feeder into the top
part of tower and falls down free (by gravitation force)
along ramps regularly installed throughout the height
of the tower. At the same time, drying medium (hot gas
from heat-generator) is supplied into the bottom part of
the tower.

Upward movement of drying medium is provided by
smoke exhauster installed after cyclone separator.

Heat carrier moving upward in a counter-current
flow to the falling down material, delivers heat to the material
and absorbs moisture evaporated during material’s
drying, and then removes it from dryer through gas path
into the air.

Rotary dryers (Pic. 3). The material is fed through
special chute into rotating drum and with the assistance
of blades (plates) is lifted to the upper periphery point of
the drum cylinder wherefrom material is poured from the
plates to the «bottom » of the drum and the whole process
is recurred. Due to inclination of the drum, material is
transported through the drum to its discharging end. Drying
medium from a furnace goes through the drum contacting
particles distributed throughout the drum section
by internal blades (plates).

Fluidized bed dryer is a vertical chamber with
round or rectangular cross-section. In lower part of this
chamber a fixed grid is installed. Material is supplied by
feeder into the drying chamber and discharged from
camber by gravity via an overflow lip. Drying medium
is supplied through a grid into a layer of material. Material’s
layer is suspended and thus, expands (the height
of layer constrained by chamber walls is increased),
as a space between particles grows, and particles get
mixed up intensively within the layer. Therefore, the
layer looks like boiling liquid. Such layer is also called
aerated (quasi-liquefied) as it gets a property of fluidity
(like liquid).

Pic.1 Tube dryer: 1. Fan. 2. Furnace. 3. Feed screw. 4. Tube dryer. 5. Feeder. 6. Cyclone separator. 7. Smoke exhauster. 8. Feeder

Pic.2 Tower Dryer: 1. Fan. 2. Furnace. 3. Feed screw. 4. Tower. 5. Feeder. 6. Cyclone separator. 7. Smoke exhauster. 8. Feeder

Pic. 3 Rotary Dryers: 1. Material flow. 2. Medium flow. 3. Feed screw. 4. Fan. 5. Drum. 6. Cyclone separator. 7. Smoke exhauster. 8. Feeder. t1 & t2 – medium temperature (initial and end temperature) V1 & V2 – material temperature (initial and end temperature)

Pic. 4 Fluidized bed dryer: 1. Fan. 2.Furnace. 3. Feed screw. 4. Drying chamber. 5. Gasdistributing grid. 6. Cyclone separator. 7. Smoke exhauster. 8. Feeder 9. Overflow lip. 10. Feeder

Pic. 5 Flash dryer: 1. Fan. 2. Furnace. 3. Supply bin. 4. Pipe. 5. Chamber (liquor glass shape). 6. Cyclone separator. 7. Smoke exhauster. 8. Feeder

Flash dryer chamber section shape reminds of a liquor
glass. The upper part of a chamber can be of cylinder
or parallelepiped shape. Vertical pipe with a heigh not
less than 4 metres is adjoint to a lower, narrow part of the
chamber. This pipe has a horizontal section for input of
material and drying medium. Drying medium is heated in
heat generator to the temperature of 800°C.

Drying medium partially dries and transports the
material along the pipe (pneumatic transport mode) to the
drying chamber where material is finally dried in a flash
drying mode.

Dried material can be discharged from cyclone separator
to where material from the chamber is carried by
exhaust gases via a gas pipeline connecting the chamber
and cyclone separator. Also material can be discharged via
discharge fitting connected to a cone-shaped part of the
chamber in the same way as material is discharged from
FB (fluidized bed) drying chamber.

Considering direction of air and material flows, these
dryers can be divided into the following two types:

1. Column type devices, where (column diameter is
3 times less than its length) material and drying medium
move in a longitudinal direction (con-current or countercurrent
flow). These include tube dryers (Pic. 1), tower
dryers (Pic. 2) and rotary dryers (Pic. 3), as well as flash
dryers (Pic. 5);

2. Cross-flow type dryers, where the height of layer is
less than a passage diameter of layer support area, and the
air moves in perpendicular direction to the material layer,while material can move in longitudinal direction (for example,
vibrofluidized bed dryer, Pic. 6) and fluidized bed
dryer with non-movable layer (Pic. 4)

Dryers of 1st type in their turn can be divided into
dryers with mutual con-current movement of heating medium
and material (Fig.1 and 3b), i.e. in the same direction,
and dryers with counter-current (Fig.2 and 3a) of medium
and material. In counter-current flow process, wet material
enters the dryer from air output side, i.e. meets with
low potential air and therefore, drying process is slow at
the beginning. In con-current process, drying process is intensive
in the beginning, but then slows down as a result of
drying potential drop (¥). In counter-current process, material
contacts the higher potential ¥ at the outlet, than in
con-current process and at the discharge end of the dryer.
In counter-current process the final moisture of the material
can be lower than in con-current process, because
material’s equilibrium moisture content is lower in hot gas
with low moisture load, as compared to the gas moistened
at a discharge end of dryer. Material’s initial temperature
(ti) increases during the first drying stage up to the wetbulb
temperature ™ and then approaches the air temperature:
in counter-current process it is the temperature
of gas in inlet, and in con-current process it is the temperature
of gas in outlet (please refer to Pic. 3). Temperature of
exhaust gases in both cases is assumed to be the same and
equal to 120°C

To summarize the above, the conclusion can be made
that column type dryers are less effective because drying
medium’s moisture content continuously increases in
proportion to amount of moisture evaporated by material
as it goes through the dryer. Therefore, drying potential
decreases with an increase of moisture content in drying
medium as it goes through the dryer. Besides, for this reason
exhaust gases have quite high temperature (~120°С).
Above mentioned circumstances cause reduction of dryer’s
heat efficiency, increase of load on dust-cleaning equipment
and growth in power consumption of forced-draught
equipment.

Let’s review dryers with cross flow. Fluidized bed
(FB) is the perfect material’s mixing and perfect gas
displacement device due to intensive mixing of particles
and gas contained within the layer. At that thanks to intensive
mixing of particles, temperature of material is
practically constant throughout the layer and equal to
the layer’s temperature tl , except a narrow zone (about
30 mm from the grid) where the dry hot gases with initial
parameters of the drying medium enter the layer.
Besides, the temperature of gas leaving the layer is close
to layer temperature (the difference is 2-3°С) and thus,
to the material temperature and therefore, partial pressure
of vapors in gas is almost equal to partial pressure
of vapors on the surface of particles. When drying sand
in FB to 0,3ä0,5% residual moisture, temperature of exhaust
gases and sand (layer) is about 80-90°С, while heat
consumption to evaporate 1kg of liquid is less than 950
kcal/kg of liquid. This is 30% higher than performance
(for the sand with the same parameters) of other convective
dryers reviewed here.

Pic. 6 Vibrofluidized bed dryer (VF B) with longitudinal jigger and heat medium blow through the layer of material: 1. Fan. 2. Furnace. 3. Frame. 4. Vibration generator. 5. Springs. 6. Jigger. 7. Grid. 8. Gas collector. 9. Cyclone separator. 10. Smoke exhauster. 11. Feeder

Pic. 7 Change of gas temperature (t) and material temperature (tm) throughout the length of moving VF B

Vrbrofluidized bed dryer (VFB) (Pic. 6) is classified
as cross flow dryer. This is atray vibrating conveyor (jigger)
with double bottom, upper of which is a fixed grid.

Material is loaded onto the grid. Particles are conveyed
by the grid vibration. Height of conveying layer is
usually about 100ä200 mm. Hot air is supplied from under
the grid. It goes through the grid and the layer of material
and then smoke exhauster removes it via cyclone separator
to the atmosphere. Material’s bulk layer becomes loosen by
vibration, and its particles mixing intensively thus creating
favorable conditions for the drying medium to go through.
In other words, gas is not required to create the aerated
layer as in fluidized bed dryers.

The maximum blow speed of gaseous heat medium
through the VFB is set to be less than speed of free-flowing
particles that are allowed to be removed from the material.
Structure-wise VFB belongs to systems with perfect material
mixing and perfect gas displacement, i.e. it possesses
all the advantages of heat and mass exchange processes
noticed in FB. Besides, vibration creates pulsating mode
for gas blow through the layer, thus increasing efficiency
of heat transfer from gas to particles (heat transfer coefficient
increases – refer to equation 2). In the end, this leads
to 20% increase of bulk material drying efficiency in VFB
as compared to FB drying.

The other important advantage of VFB dryers is
a possibility to perform drying by the cross flow of heat
medium through the layer that moves in longitudinal direction
(Pic. 7). Pic. 7 shows a scheme of heat medium and
bulk material motions, as well as corresponding changes of
their moisture content and temperatures throughout the
length of the dryer during drying process. Labels on Pic. 7
are as follows: t и tm – temperature of gas and material in the layer, accordingly; tо и te – initial and end temperature
of gas; tmо and tme – initial and end temperature of material;
Í – length of layer in the dryer.

There are three zones of wet material drying. First
zone corresponds to material warming up from initial temperature
tо to wet-bulb temperature tmт. Second zone
corresponds to a period of constant drying speed, where
the material’s temperature is constant and equals to wetbulb
temperature tm = tmт = const. Third zone corresponds
to the period of drying speed reduction, when material’s
temperature increases to tme, as material gets dry and some
«dried out» spots appear on the surface of the particles, and
evaporation speed drops. When material is fully dry its
temperature tm achieves tme.

It must be mentioned that when drying fine-grained
materials in VFB, such as sand, the time of material’s
layer warming up in the initial drying period (I) is insignificant
(less than 5 seconds), as well as the time of period
III, which is less than 25% of the whole time that material
spends in the dryer, which is 60-80 sec for sand dried
by 800-900°С gases from initial sand moisture of 8ä10%
to 0,2ä0,3%. Since VFB is the perfect material mixing
system, temperature of material is the same throughout
the layer’s height, i.e in the bottom, as well as in the upper
monolayer. Temperature of gas in the layer and of
gas leaving the layer is almost equal to temperature of
the layer (te>tem approx. 2-3°С). In other words, the temperature
of material is tm=tmeŸte during the whole drying
period. Drying medium goes through the grid, contacts
material throughout the layer’s length at the same initial
temperature tо and moisture content Хо, while temperature
and moisture of material is changing continuously as
the layer moves along the grid.

Therefore, VFB drying process is highly effective,
since moving forces of layer and medium heat and mass
exchange processes are maximal at VFB inlet (gradients ¥t
and ¥Х are maximal in the equations 2 and 6). Also the heat
and mass exchange processes are effective inside VFB, as
a result of intensive mixing of particles in VFB and high
porosity of the layer. Therefore, heat-exchange surface F
value reaches its maximum, which is almost equal to the
surface of all particles contained in the layer volume. At
that heat exchange inside the layer is performed not only
by convection, but also by contact between the particles
heated at the grid and moved upward with the less heated
particles.

It must be mentioned that due to high efficiency of
heat and mass exchange processes in VFB, temperature
of gases leaving the layer is just 2-3°С higher than the
temperature of material’s layer, i.e. all over the length
Í of layer te = tme . At that during practically the whole
periods I and II temperature of exhaust gases is almost
equal to the wet-bulb temperature te = tmт., while moisture
content in exhaust gases is close to saturation limit. In
the drying period III temperature of exhaust gases grows
together with increase of temperature of material in the
layer, and at the final stage of the process te = tme. Drying
process is complete after that. Note that during sand drying process, temperature at 75% of the dryer length
is the temperature of exhaust gases fully saturated with
moisture that have the wet-bulb temperature. Only this
one parameter shows high heat efficiency of the drying
process that can not be achieved in other types of dryers
reviewed here.

Summary

The above analysis of bulk materials drying efficiency
in convective dryers with different designs and
principles of operation shows that the lowest loss of heat
associated with exhaust gases is achieved in dryers with
cross flow of gaseous medium through the moving layer.
This process is most efficient in vibro-fluidized bed dryers,
where the highly efficient heat transfer from gas to
material is provided due to intensive mixing and highly
developed surface of heat exchange, as well as effect of
pulsation in gas blown through the layer. This provides
the best heat efficiency of the process as compared to
other types of dryers. Moreover, vibration not only reduces
resistance of material to gas blow through it, but
also conveys the material in the least power-consuming
manner. The minimal loss of fine particles entrained by
gas is achieved in VFB dryers. Low heat consumption reduces
the volume of exhaust gases. Low hydrostatic pressure
in VFB and insignificant loss of fine particles – altogether
it more than once reduces the costs of gas-cleaning
equipment, force-draught equipment and their power
consumption. It must be mentioned that even higher heat
efficiency of VFB dryers can be achieved for such materials
as sand, if this material is dried to 1,5-2% by hot air and
then further drying of material to 0,3ä0,5% is performed
by atmospheric air utilizing the heat accumulated by material
in the previous drying stage. This approach is long
used in practice.

Thus, at present time and in the nearest future
the best convective dryers for bulk materials are vibro-
fluidized bed dryers due to their maximum heat
efficiency.

Pic. 8 VF B dryer manufactured by «Ventilex»

Pic. 9 VF B dryer manufactured by OOO PKB «Stroytechnika»


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