Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR REDUCING MOISTURE CONTENT
FIELD OF THE INVENTION
This invention relates to a method and a system
for reducing the moisture content of a moist material and,
more particularly, pertains to a drying system and method
wherein the treating medium is drawn through the moist
material to be treated.
BACKGROUND OF THE INVENTION
Over the years various systems and methods have
been developed for removing liquid components from moist
materials such as sewage sludge, bagasse, de-inked paper
siudge, biological sludge, wood pulp, bark, woodchips,
slurry, peat and pasty products of different types. For
instance, United States Patent No. 4,768,292 issued to
Manzei on September 6, 1988 and United States Patent No.
5,653,872 issued to Cohan on August 5, 1997 each disclose a
thermal drying system in which the sewage sludge to be
treated is reduced into pellet masses and then exposed to a
heated gaseous treating medium to drive off a portion of
the moisture contents of the sewage sludge. The heated
gaseous medium flows through the sewage sludge by operation
of a blower fan.
More particularly, the thermal drying svstem
described in United States Patent No. 4,768,292 generally
includes a pair of upper and lower porous conveyors
vertically superposed and staggered so that the pellet
masses will drop from the upper conveyor to the lower
conveyor which is driven in a direction opposite to that of
the upper conveyor. The hot gaseous medium travels
successively and alternately from the layer of pellet
masses through perforations defined in the supporting
surfaces of the conveyors and from the perforations through
the layer of pellet masses to provide an homogeneous and
decreasing level of moisture.
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Although the above thermal drying systems are
effective for removing a portion of the moisture content of
the moist material, they require a high level of energy as
they all necessitate that the gaseous treating medium be
heated at relatively high temperatures.
Accordingly, there is a need for a drying method
which is economical while maintaining a high efficiency for
removing liquids and/or moisture from the moist material to
be treated. Another disadvantage of such thermal drying
systems is that they dry an exterior layer of the pellets
before the core thereof, thereby forming a barrier to the
subsequent extraction of the moisture from the core of the
pellets.
In another art, as illustrated by United States
Patent No. 4,219,942 issued to Coliva on September 2, 1980,
it is known to draw heated air through a damp fabric to dry
the same and thus generate a shrinking and stabilizing
effect. However, this drying system also requires a lot of
energy as the drying process is still carried out by a
thermal effect due to a flow of hot air.
Therefore, it would be highly valuable to
introduce a drying system which could operate at ambient
and low temperatures.
SUMMARY OF THE INVENTION
It is therefore an aim of the present invention
to provide a drying system and method which requires a
relatively low level of energy.
It is also an aim of the present invention to
provide a drying system which could operate at ambient and
relatively low temperatures.
It is also an aim of the present invention to
provide a drying system in which the material to be treated
is dried from a central portion to an outer layer thereof.
It is also an aim of the present invention to
provide a drying system in which different residual
moisture contents of the treated material may be obtained.
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It is still an aim of the present invention to
provide such a drying system which is adapted to minimize
the time required to dry the moist material.
It is a further aim of the present invention to
provide a method suitable for recycling waste materials
into useful products such as fertilizers, soil
conditioners, absorbent materials or combustion agents.
Therefore, in accordance with the present
invention, there is provided a method for reducing moisture
content of a moist material having an external surface,
comprising the steps of increasing the exterior surface of
the moist material, and drawing a relatively dry gaseous
fluid through the moist material.
Also in accordance with the present invention,
the method includes the step of separating the moist
material into individual pieces prior to drawing a gaseous
drying medium therethrough.
Typically, the gaseous drying medium flows by
suction from an upstream side to a downstream side of each
individual piece at a relatively high rate such as to
sufficiently reduce the pressure on the downstream side of
each individual piece and so causing moisture located in a
central portion thereof to migrate near the external
exposed surfaces of each individual piece substantially on
the downstream side thereof.
Also in accordance with the present invention,
there is provided a pelletizer for transforming a pasty
material into a plurality of pellets. The pelletizer
comprises a perforated plate means defining a plurality of
openings, means for forcing the pasty material to pass
through the perforated plate means from an inner surface
thereof to an outer surface thereof, and cutting means
adjacent the outer surface of the perforated plate means
for transversally cutting the pasty material emanating from
the perforated plate means.
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Typically, the cutting means include a
displaceable perforated plate means disposed downstream of
the perforated plate means and defining a plurality of
openings, and means for imparting a reciprocating motion to
the displaceable plate means such as to alternately align
and offset the openings of the displaceable perforated
plate means with the openings of the perforated plate
means.
Also in accordance with the present invention,
there is provided a pelletizer for transforming a pasty
material into a plurality of pellets, comprising a number
of interchangeable perforated plate means, and means for
forcing the pasty material to pass through one of the
interchangeable perforated plate means selected according
15. to the pasty material to be treated.
Also in accordance with the present invention,
there is provided an apparatus for shredding solid
materials into smaller pieces, comprising a housing
defining a material inlet and a material outlet, at least
two series of spaced- apart crowned discs mounted in an
offset relationship on respective parallel shafts and
rotatably driven in opposite directions for drawing and
cutting the solid material supplied through the material
inlet of the housing.
Also in accordance with the present invention,
there is provided a screw conveyor comprising a
substantially elongated casing defining opposed material
inlet and outlet means and having an arcuate porous bottom
wall, and a rotating screw axially mounted to the elongated
casing for displacing a pasty material having a viscosity
from the material inlet means to the material outlet means
while at the same time compressing the pasty material
against an inner surface of the arcuate porous bottom wall
thereby causing some liquid from the pasty material to flow
therethrough, and means for adjusting the inclination of
the screw conveyor according to the viscosity of the pasty
material to be treated.
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Also in accordance with the present invention,
there is provided a screw conveyor comprising a rotating
screw mounted within an elongated casing for displacing a
pasty material from a material inlet means to a material
outlet means defined at opposed end portions of said
elongated casing, the casing having a flexible arcuate
bottom wall for allowing the screw conveyor to accommodate
various quantity of pasty material.
Typically, the flexible arcuate bottom wall is
releasably mounted to an upper portion of the elongated
casing to permit disengagement thereof when the pressure
exerted thereon reaches a critical point.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the
invention, reference will now be made to the accompanying
drawings, showing by way of illustration a preferred
embodiment thereof and in which:
Fig. 1 is a schematic view of a granulating,
pelletizing, grinding and drying system in accordance with
the present invention;
Fig. 2a is a schematic side elevational view
partly in cross-section of a screw conveyor-compactor
apparatus.
Fig. 2b is a schematic end view of the screw
conveyor-compactor apparatus of Fig. 2a;
Figs. 3a and 3b are respectively schematic
side elevational views partly in cross-section of a mixer
and a cutter-mixer;
Figs. 4a and 4b are schematic elevational views
partly in cross-section of a single pelletizer;
Figs. 4c and 4d are schematic elevational views
partly in cross-section of a multi-pelletizer;
Fig. 5a is a schematic top plan view of a
shredder;
Fig. 5b is a schematic side elevational view
partly in cross-section of the shredder of Fig. 5a;
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Fig. 6a is a schematic side elevational view of a
dryer in accordance with the present invention;
Fig. 6b is a schematic cross-sectional view of
the dryer of Fig.6a.
Fig. 7 is an enlarged cross-sectional view of an
opening defined in a perforated plate of the pelletizer of
Figs. 4a and 4b.
Fig. 8 is a schematic view of a particle
illustrating the aerodynamic suction effect generated by
the dryer of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Now referring to the drawings, and in particular
to Fig. 1, a drying system in accordance with the present
invention and generally designated by numeral 10 will be
described.
The system 10, as will be explained hereinafter,
is adapted to remove liquids and/or moisture from a moist
material such as sewage sludge, bagasse, de-inked paper
sludge, primary sludge, biological sludge, wood pulp,
bleached thermo-chemico mechanical pulp, bark, woodchips,
lumber residues, slurry, peat and pasty products of
different types.
As shown in Fig. 1, depending on the material to
be treated, the drying system 10 essentially comprises a
pelletizer 300 or a shredder 400 and a dryer 500. It is
understood that the shredder 400 is used in connection with
wet solid materials, such as bark, whereas the pelletizer
300 is used to treat fluent materials, such as sludge.
Optionally, and more particularly when the pelletizer 300
is used, a screw compactor-conveyor 100 and a mixer 200 or
cutter mixer 200' may be provided upstream of the
pelletizer 300.
Generally, different products will require
different preparations before drying. For instance, wet
materials having a pasty consistency can be conveyed
directly to the screw compactor-conveyor 100 to remove a
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portion of the water content thereof by compression against
a flexible porous bottom 120 of the screw compactor
conveyor 100, as will be expiained hereinafter. Then, the
extracted liquid is returned to an initial operation for
recycling purposes. Once the above-described preliminary
dewatering operation has been completed, the partly dried
pasty material can be conveyed to the mixer 200 or cutter-
mixer 200' to add various additives if required.
Thereafter, the pasty material is subjected to
pelletization so as to transform the material into pellet-
like mass, thereby increasing the exposed surface of the
pasty material.
If wet solid materials are to be treated, the
screw compactor-conveyor 100, the cutter 200 or cutter
mixer 200' and the pelletizer 300 are omitted and the wet
solid materials are conveyed to the shredder 400 which is
adapted to break the solid materials into smaller pieces
thereby offering more exposed surface for the subsequent
drying operation.
The pasty material emanating from the pelletizer
300 or the wet solid materials leaving the shredder 400 are
conveyed to the dryer 600 where a gas treating medium, such
as ambient air, is drawn by vacuum at high velocity through
the material to be dried in order to obtain a desired
moisture content. The resulting humid air is evacuated in
the room or to the outside of the building. The process may
further include an air dehumidifying step 600, as
schematically depicted in Fig. 1.
The dried materials can be immediately used in a
co-generation energy system (not shown) which can be placed
just beside. The material can also be delivered in bulk for
the industrial, institutional or commercial market as an
absorbing product, a littering product or as a combustible.
Accordingly, it can be said that the drying system 10 of
the present invention contributes to reduce pollution by
recycling waste material or the like.
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Referring now to Figs. 2a and 2b, it can be seen
that the screw conveyor-compactor 100 includes a rotating
helical screw 110 concentrically disposed in an elongated
cylindrical casing or trough 102 defining at a first end
thereof a material inlet 105 and at a second opposed end
thereof a material outlet 145. The casing 102 is provided
with a flexible porous bottom wall 120 through which at
least a portion of the liquid content of a pasty material
may flow as the latter is displaced toward the outlet 145
by the rotating helical screw 110. Indeed, during its
displacement within the screw-compactor 100, the pasty
material is compressed against an inner surface of the
flexible porous bottom wall 120 by the rotating helical
screw 110, thereby causing a portion of the liquid content
of the pasty material to pass through the flexible porous
bottom wall 120. The extracted liquid is directed to a
liquid sump 115 or to any other suitable locations where it
may be recycled.
The flexible porous bottom wall 120 may consist
of a thin perforated metal screen. The flexibility of the
porous bottom wall enables the screw compactor-conveyor 100
to accommodate various quantities of pasty material to be
treated.
The screw compactor-conveyor 100 is mounted at
the first end thereof to a pivot 175 and at the opposed
second end thereof to a telescopic member 160 which could
be retracted or extended to modify the elevation of the
second end of the conveyor with respect to the first end
thereof, as indicated by the arc 180. This permits the
adjustment of the inclination of the screw compactor-
conveyor 100. Depending on the viscosity of the pasty
material to be treated, the compactor-conveyor 100 may be
inclined at different angles to thus remove as much liquid
as possible.
According to a preferred embodiment which is not
illustrated, the porous bottom wall is suspended inwardly
of the elongated cylindrical casing 102 above a bottom
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slotted wall of the latter such that when the pressure
exerted against the porous bottom wall 120 reaches a
critical point, the same will disengage from the casing 102
to release the excess pressure. The extracted liquid will
be evacuated through the slot defined in the bottom wall of
the casing 102.
Referring now to Figs. 3a and 3b, the features of
the mixer 200 and cutter-mixer 200' will be described in
greater detail. As shown in Fig. 3a, the mixer 200
comprises an elongated cylindrical housing 202 defining a
mixing chamber 205 and an inlet 210 adapted to receive the
partially dewatered pasty material from the material outlet
145 of the screw-compactor conveyor 100. A rotating shaft
245 concentrically mounted inside of the housing 202 is
provided at a first end portion thereof with a plurality of
helical blades 220 for regulating the flow of material
conveyed to the mixing chamber 205.
The portion of the shaft 245 extending through
the mixing chamber 205 is provided with a plurality of
radially extending blades 235 and/or cutter blades 236. The
shaft 245 may be driven by a motor (not shown) coupled to a
conventional pulley and belt arrangement 215 which is in
turn connected to the shaft 245. The blades 235 and the
cutter-blades 236 are preferably individually adjustable in
both angle and length. The cutter-blades 236 serve to cut
the bigger pieces or particles of the pasty material,
whereas the blades 235 are adapted to pulverize and mix the
pasty material and to convey the material towards an outlet
240 defined at a second end of the housing 202. It is
understood that depending on their angle, the blades 235
will essentially serve to pulverize and mix the pasty
material or to advance the same through the mixing chamber
205.
Straight blades 238 are used for mixing while
angled blades 237 are used to move the pasty material
towards the outlet 240. The number of blades which are
angled, the angle of the blades and the length of the
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blades are predetermined parameters determined by the
material which is being processed, including its viscosity,
and the nature of the desired finished product. As
mentioned above, the blades 235 may be individually
adjustable in both their angle and length. It is also
conceivable that the entire shaft 245 may be removable and
replaceable with other shafts having preconfigured blade
arrangements.
Generally, pasty materials requiring a relatively
greater amount of mixing are maintained for a longer period
of time within the mixing chamber 205 by adjusting the
blades such that their angle with respect to the axis of
the shaft 245 is relatively small. Alternatively or
conjointly, the number of blades which are angled may be
decreased to reduce the advance speed of the pasty material
within the mixing chamber 205. Longer blade lengths are
employed when the material to be processed is less viscous.
As shown in Fig. 3a, the blades 235 are disposed closely
together along shaft 245 but in arrays set at 90 degrees to
one another. According to the illustrated embodiment, every
third blade 237 has an angle. The non-angle blades 238
pulverize the material as the shaft 245 rotates, preferably
at about 400 R.P.M.. The pulverizing action has the
advantage that less wetting agent is required. This result
in a further advantage in that by reducing the required
quantity of wetting agent, the amount of drying needed to
reduce the moisture content of the pasty material to a
desired level is simultaneously decreased.
Fig. 3b illustrates a cutter-mixer 200'
comprising a first housing 252 defining a material inlet
265 and a material outlet 280 communicating with a material
inlet 290 defined in a second housing 254 directly disposed
under the first housing 252 and defining at an opposed end
thereof a material outlet 295. The first and second housing
252 and 254 respectively support a first rotating shaft 256
and a second rotating shaft 258. The first rotating shaft
256 is operational to displace the pasty material from the
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material inlet 265 to the material outlet 280 of the first
housing 252, whereas the second rotating shaft is adapted
to displace the pasty material entering the material inlet
290 from the material outlet 280 and moving to the material
outlet 295 of the second housing 254. According to a
preferred embodiment, the shafts 256 and 258 are
independently driven by means of conventional belt and
pulley arrangements 270 coupled to respective motors (not
shown).
The first shaft 256 is provided at a first end
portion thereof with a helical blade 259 for regulating the
flow of material conveyed to a cutting chamber 255 defined
by the housing 252. The portion of the shaft 256 extending
within the cutting chamber 255 is provided with a plurality
of radially extending cutter-blades 260 adapted to shred
the pasty material into fine particles.
As to the shaft 258, it is provided at a first
end portion thereof with a helical blade 261 for regulating
the flow of pasty material conveyed to a mixing chamber 275
defined by the housing 254. The portion of the shaft 258
extending within the mixing chamber is provided with angled
and straight blades 262 and 263 for respectively displacing
and mixing the pasty material, as described hereinbefore
with reference to Fig. 3a.
It is noted that the above described mixer 200
and cutter-mixer 200' are particularly useful when the
proportions of the components forming the substance to be
treated are very different. The material leaving the mixer
200 or the cutter-mixer 200' has a smooth and even
consistency and is thus ready to be introduced into the
pelletizer 300.
However, in many cases, the screw-conveyor
compactor 100 and the mixer 200 or cutter mixer 200' are
not required and thus the material to be treated may be
directly conveyed to the pelletizer 300.
As seen in Figs. 4a and 4b, the pelletizer 300
includes a housing 305 defining an inlet 320 for receiving
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the pasty material to be pelletized. The pasty material is
deposited in mass on a bottom arcuate perforated plate 315.
The arcuate perforated plate 315 defines a plurality of
openings through which the material is forced in order to
form pellet like-masses. It will be appreciated that, at
this stage, the material is rather viscous, and therefore,
does not readily flow on it's own. Preferably, the material
is forced through the openings of the perforated plate 315
by means of a series of rigid blades 350 mounted to
respective vanes 360 extending radially from a drive shaft
325 axially disposed with respect to the perforated plate
315, as shown in Fig. 4a.
Each rigid blade 350 is provided with a curved
distal end portion which is oriented in a direction
opposite to the rotation of the drive shaft 325, as
illustrated by the arrow 370. The rigid blades 350 are
disposed and configured such that the curved distal ends
thereof are continuously in contact with the inner
cylindrical wall of the pelletizer 300, thereby forcing the
material immediately preceding the rigid blades.350 through
the openings of the arcuate perforated plates 315 which
forms the bottom portion of the inner cylindrical wall.
More particularly, as the drive shaft 325 rotates in the
direction indicated by arrow 370, the blades 350 are swept
along the inner surface of the perforated plate 315 causing
a pressure on the material, forcing the material through
the openings of the perforated plate 315.
The rigid blades 350 are interchangeable with
other rigid blades of varying curvatures for adapting the
sweeping force necessary to press different kinds of
materials through the openings of the arcuate perforated
plate 315. For instance, rigid blades 350 having a more
open angle would be used for pelletizing a rough and heavy
material, as it would applied a higher pressure on the
material to thus efficiently force the same through the
openings of the arcuate perforated plate 315.
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The size of the thus produced pellet-like mass,
i.e. the diameter, and/or cross-sectional shape is
generally attributable to the size and the shape of the
openings of the perforated plate 315, although some
shrinkage may occur during the subsequent drying step. The
axial dimension of the pellet-like mass can be controlled
by imparting a reciprocating motion to an additional
similar perforated plate 317 disposed underneath the
perforated plate 315. The additional perforated plate 317
may have a longitudinally axially reciprocating motion
parallel to the drive shaft 325. The reciprocating motion
may be imparted by any conventional motor means such as
hydraulic cylinders. The reciprocating motion of the
additional perforated plate 317 will cut the pellet-like
masses when the openings of the additional perforated plate
317 are in offset relation with the openings of the arcuate
perforated plate 315. Accordingly, the reciprocating cycle
will determine the length of the pellet-like mass. Thereby,
it will be possible to obtain a pellet-like mass all having
substantially the same axial dimensions.
As the rigid blades 350 are continuously driven
by the drive shaft 325 and as the material is continuously
supplied to the pelletizer 300, the thus produced pellet-
like mass has constant dimensions.
It is noted that the number, the shape and the
rotational speed of the drive shaft 325 as well as the
number, the diameter, the disposition and the shape of the
openings of the perforated plate 315 are adjusted to suit
the application.
It has been found that openings having conical
configuration produce additional compression and friction
on certain types of fluent material. Fig. 7 is an enlarged
view of such a cone configurated opening 319 of the
perforated plate 315. The angle 0 determines the amount of
compression obtained on the material to be pelletized.
Accordingly, perforated plates 315 with conical openings
319 having different angles 0 may be provided for fluent
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materia].s having different densities in order to obtaizi a
desired density in the pelletized material. For instance,
experirnental results have demonstrated that for chicken
feathers mixed wi.th appropriate binding agents, an angle @
cf approximately 380 degrees doubles the density of the
pelletized material compared to what is normally obtained
with conventional cylindrical openings. Further
experimentation has demonstrated that for biological
sludge, an angle 9 of approximately 270 degrees triplea the
l0 density of the pelletized material compared to what is
normally obtained with conventional cylindrical openings.
It is noted that the longitudinal dimensions of the conical
= and cylindr4cal portions of the opening 319, as depicted by
11 and 12 in Fig. 7, are substantially similar. The
thickness of the perforated plate 315 is at least equal to
1/8 inch (0,3175 cm).
it is also contemplated to provide perforated
plates having oval configured openings as well as slanted
openings.
Preferably, the components of the pelletizer 300,
including the housing 305, the vanes 360 and the xigid
blades 350, are fabricated from materials which are
substantially non-degenerative when subjected to high
levels of production and which could not be altered by the
material to be processed. For instance, when pellets for
= human or animal consumption are processed, resistant metals
such as ni-hard or hardened metala are preferred. Other
proved materials like rubber, hard metal, synthetic
products, fiberglass and various plastics such as PVC may
also be.used.
Figs. 4c and 4d illustrate a multi-pelletizer
300' which is provided with two coaxial drive shafts 325'
;,o increase the production capacity thereof. Each drive
shaft 325' is provided with at least four radial vanes 364'
which are each adapted to support a blade 350'. The drive
shafts 325' are driven in opposite directions to force, at
regular intervals, the material supplied to the multi-
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pelletizer 300' through the adjacent perforated plates 315,
respectively.
The pelletizing step gives cohesion to the fluent
material and considerably increases the external exposed
surface thereof, thereby facilitating and accelerating the
subsequent drying process. The pelletizing step also
contributes to make the fluent or pasty material physically
uniform. Furthermore, the agglomeration of the pasty
material resulting from the pelletizing step prevents the
formation of dust at later stages of the process. This is
important in that, in the past, dust has been the source of
explosion of certain types of conventional thermal dryers.
It is also noted that additional products, such
as nutrients, coloring and/or odor neutralizing agents,
may be added to the pasty material during the pelletizing
step. For instance, nutrients and seeds may be added
directly to a de-inked sludge to produce a single
application soil modifying product.
Referring now to Figs. 5a and 5b, it can be seen
that the shredder 400, which is adapted to shred wet solid
material into smaller particles, generally includes a
housing 402 defining a material inlet 420 through which wet
solid material may be introduced. The shredder 400 further
includes a set of spaced-apart small diameter crowned discs
450 and a set of spaced-apart large diameter crowned discs
460 respectively mounted along the longitudinal axis of
separate parallel shafts 425.
The small and large diameter crowned discs 450
and 460 are in an offset relationship such that each small
crowned discs 450 is provided between two adjacent large
diameter crowned discs 460. The inverse rotational movement
of the two sets of crowned discs 450 and 460 draws the wet
solid material to the shredding zone where the material is
shredded and forced through a series of fixed semi-circular
crowned bars 415 for further cutting and shearing the
material before being discharged through outlet 470.
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The semi-circular crowned bars 415 are regularly
spaced-apart along an axis parallel to the rotating axis of
the shafts 425 and are offset relative to the set of large
diameter crowned discs 460. Accordingly, there is a fixed
semi-circular crowned bar 415 between the bottom portion of
each pair of consecutive large diameter crowned discs 460.
The space between adjacent semi-circular crowned bars 415
allows the material to pass therethrcugh. The teeth of the
crowned discs 450 and 460 have different shapes ar_d the
disc rotation epeed can vary depending of the size and the
type of materials to be shredded. The two shafts 425 can be
either dri-ven by two separated gear motors or by a single
gear motor 470 with a conventional assembly of nulleys and
gears, as shown in Fig. 5a.
Preferably, the components of the shredder 400,
including the rigid discs 450 and 460, and the serni-
circular crowned bars 415, are fabricated from a resistant
metal such as ni-hard or stainless steel. Other proven
mater=als like rubber, hard metal, synthetic products,
fiberglase and various plastics may also be used.
A conveyor may be provided underneath the outlet
410 of the shredder 400 for contir_uously conveying the
shredded material to the dryer 500. zt is noted that, at
this stage, the maximum dimensicne of shredded material
should be about three inches (7,62 cm) long, three inches
(7,62 cm) wide and 3/8 inch (0,9525 cm) thick to ensure an
efficient drying thereof . =
Figs. 6a and 6b illustrate a dryer 500 which is
adapted to receive the pelletized material leaving the
pelletizer 300 or the shredded material leaving the
shredder 400. More specifically, the dryer 500 includes an
upper belt conveyor 502, an intermediate belt conveyor 504
and a J.ower belt conveyor 506 disposed in superpoeed
relationship internally of a housing 510 defining a
material inlet 512 and a material outlet 514.
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The superposed belt conveyors 502, 504 and 506
are driven in cpposite consecutive directions for conveying
the mater:al to be dried from the rnaterial inlet 512 to the
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material outlet 514. Each belt conveyor 502, 504 and 506 is
continuously or intermittently driven by means of an
individual drive mechanism (not shown) thereby allowing
each conveyor to be independently driven at speeds varying
as required. Alternatively, the superposed belt conveyors
502, 504 and 506 may be driven by a single drive mechanism
(not shown) in the same continuous or intermittent modes.
Depending on the application, the conveyors 502, 504 and
506 are driven at speeds varying from about 1 to 15 ft/min.
The intermediate belt conveyor 504 is
longitudinally offset with respect to the upper and lower
belt conveyors 502 and 506 to enable the material to drop
by gravity directly from the upper belt conveyor 502 to the
intermediate belt conveyor 504 and from the intermediate
belt conveyor 504 to the lower belt conveyor 506.
Each belt conveyor 502, 504 and 506 includes a
porous supporting surface 516, such as a perforated belt or
perforated metal plates, for allowing a gas treating medium
to flow therethrough. For instance, the porous supporting
surfaces 516 may consist of woven metallic or synthetic
perforated belts. The percentage of openings and the shape
and size defined in the supporting surfaces 516 of the
conveyors 502, 504 and 506 are selected according to the
application.
As shown in Fig. 6a, a spreading bar 520 is
disposed at the entry of the dryer 500 to uniformly
distribute the material M to be dried on the porous
supporting surface 516 of the upper belt conveyor 502. Even
distribution of the material M is important to ensure a
good exposure of the external surface of each shredded
particle or pellet-like mass to the gas treating medium.
The superposed belt conveyors 502, 504 and 506
are each disposed in respective enclosures 522a, 522b and
522c having lateral inlet 524a, 524b, 524c and outlet 526a,
526b, 526c defined along opposed longitudinal sides thereof
for allowing independent circulation of a gas treating
medium, such as air, through each belt conveyor 502, 504,
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CA 02289444 1999-10-29
WO 98/51139 PCT/CA98/00461
and 506. Suction fans 528a, 528b and 528c are respectively
installed at each outlet 526a, 526b, .526c to cause a
transversal flow of air to pass through the material M and
then through the porous supporting surface 516 before being
expulsed outside of the dryer 500 by the suction fans 528a,
528b and 528c. Accordingly, the air flow rate may be varied
for each conveyor 502, 504 and 506. Air seals 530 between
the vertical walls of each enclosure 522a, 522b, 522c and
the supporting surface of the belt conveyor disposed inside
of the enclosure ensure that the air flow passes vertically
down and through the material M and then through the
supporting surface to provide effective usage of air.
The suction fans 528a, 528b and 528c are adapted
to draw air at relatively high velocity, i.e. up to 440
M/min., through the material M thereby causing the moisture
in the central portion of each pellet-like mass or shredded
particles to migrate to near the external exposed surface
thereof and mostly to the undersurface thereof where it can
be easily evaporated. This is an important advantage in
that the pellet-like mass or shredded particles may be
dried from the center thereof instead of from their
exterior surface. The vacuum effect considerably increases
the speed at which the liquid migrates to the external
exposed surface of the material M and thus contributes to
reduce the time required to dry a given particle.
Furthermore, the friction of the air passing through the
material increases the temperature of the air allowing for
increased humidity absorption by the air.
By drawing air at high velocity through the
pellet-like masses or the shredded particles, a negative
pressure is created under the material to be dried( i.e.
the pellet-like mass or the shredded particles), whereby
the liquid in the central portion of the pellet-like mass
or shredded particles is drawn downwardly toward the
periphery thereof. More particularly, it is the resistance
of the air to flow through the pellet-like mass or the
shredded particles that reduces the pressure on the
-18-
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JV. 1JVv ~,ya;, -. tY=y rs~ ~:s~~44ti~:~lU
i v=. :'l='J~ u1l J=f..:J-1 ....~ y ==
== I T C. V V U J=1 J c~. : V :'J/ l downstream sides (i.e. undersurf aces) of
the pellet-like
masses or shredded particlea. The abova-deecribed
aerodynamic suction effect is schematically illustrated in
Fig. 8, wherein the arrows A represent the flow of air
around a given particle M and the arrows L depict the
migration oIL the liquid content of the particle M.
The aerodynamic suction effect is an important
element of the present drying processl Ir.deed, because of
this aerodynamic suction effect, it is poesible to used air
at ambient temperature for treating the material to be
dried and thus minimize the energy required to operate the
drying system. More particularly, satisfactory results have
been obtained at temperature as low as SQC. However, in =
order to obtain an efficient aerodynamic suction effect,
the flow of air passing through one square inch(6,45 cmZ) of
supporting surface of the belt conveyors 502, 504 and 506
must at least always be equal to 2 CFM (cubic feet per
minute) (0,0566m'/tnin.). Therefore, a relatively large
volume of air must be circulated to obtain the required
vacuum effect.
Preferably, the suction fans 528a, 528b and 528c
have adjustable blades to enable fine tuning of the air
flow to specific conditior.s.
It is r_eted that the width of the intermediate
belt conveyor 504 may be smaller than that of the uppex
conveyor 502 and that the width of the lower conveyor 506 =
may be smaller than that of the intermediate conveyor 504
in order to maintain the negative pressure under the
material to be dried substantially constant for each
conveyor 502, 504 and 506 although the material may be
shrunlc during the drying process.
It is important that the speeds of each conveyor
502, 504 and 506 be individually adjusted to control the
thickness of the layer of material disposed on each
conveyor and zo control the residual moisture content of
the material.
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BNSDOCID: <E1 9892128800>
~=. i JJ CA 02289444 1999-10-29~0~ *'}'J ~'i
.I JL _ . . = ~
I~ is also contempiated to provide an external
heat source at the air inlets 524a, 524b ar.d 524c of the
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BNSDOCIO: <E1 9892128800>
CA 02289444 1999-10-29
WO 98/51139 PCT/CA98/00461
dryer 500 for slightly raising the temperature of the air,
especially when the same contains a high level of humidity.
For instance, increasing the absorbing capacity of
moisture-saturated air at 20 C by 20% requires raising the
air temperature by only 2,77 C.
In operation, the pelletized or shredded material
to be dried M is conveyed to the material inlet 512 of the
dryer 500, whereby it falls down by gravity on the
underlying upper belt conveyor 502. As the particles of
material M are displaced by the upper belt conveyor 502,
the suction fan 528a disposed at the outlet 526a of the
enclosure 522a, containing the upper belt conveyor 502, is
operated so as to draw a flow of air sideways through the
material and then downwardly through the upper belt
conveyor 502. The drawn air is then vented in the room or
outside the building or can be directed into a
dehumidifying unit 600 which can be incorporated in the
process. The drawn air may also be recirculated.
Once the particles of material M have reached the
opposite end of the upper belt conveyor 502, they fall on
the intermediate belt conveyor 504 which is driven in the
opposite direction at a speed different from that of the
upper belt conveyor 502. The material is then further dried
by the flow of air drawn therethrough by the suction fan
528b disposed at the outlet 526b of the enclosure 522b
containing the intermediate belt conveyor 504.
Finally, when the particles of material M reach
the opposite end of the intermediate belt conveyor 504,
they fall onto the lower belt conveyor 506 where they are
further dried as per the two preceding steps. The dried
particles exiting the dryer 500.may be conveyed to storage
facilities or directly be used as a source of combustible.
Although zhe above dryer 500 includes three
superposed conveyors 502, 504 and 506, it is understood
that more or fewer conveyors may be provided depending on
the intended application. It is also noted that only one
suction fan could be provided in combination with an
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CA 02289444 1999-10-29
WO 98/51139 PCT/CA98/00461
appropriate valve arrangement for regulating the flow of
air through the enclosures 522a, 522b and 522c containing
superposed conveyors 502, 504 and 506, respectively.
Alternatively, more than one suction fan may be provided
for each level of conveyor. Finally, it is noted that the
air exhausting from one level of conveyor may be
recirculated to the same level or to another level in order
to draw more moisture.
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