Note: Descriptions are shown in the official language in which they were submitted.
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SCREEN BASKET HAVING DIAGONAL SLOTS FOR TOP
SEPARATOR OF A DIGESTER
BACKGROUND OF THE INVENTION
[0001] The present application is generally directed to making pulp
and is more specifically directed to top separators for pulp digester
systems including digesters, impregnation vessels or other treatment
vessels used in the processing of cellulosic material such as the
production of pulp.
[0002] Wood chips and other cellulosic fibrous material are treated
in treatment vessels, such as digester vessels and impregnation
vessels. These treatment vessels can be continuous or batch and
contain a section near the top of the vessel where hot vapor (such as
steam) or other hot gaseous material is used to heat the cellulosic
material or wood chips entering the vessel, commonly referred to as
vapor phase vessels (digesters) or they can be hydraulically filled
vessels where liquid is used to heat the cellulosic material, commonly
referred to as hydraulic vessels (digesters). In this application the term
"wood chips" refers to all cellulosic fibrous material and is not limited to
true "wood" based material. An impregnation vessel is typically used to
infuse liquids and chemicals into the fibers of wood chips. A digester
vessel chemically separates fibers in the chips and material by
removing lignin. The chips after the removal of lignin are referred to as
pulp. In this application, impregnation and digester vessels are both
treatment vessels and either, or both, could use the top separator
described.
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[0003] The wood chips are treated in a digester vessel with heat,
liquid, and chemicals to convert the chips to pulp. A continuous digester
vessel is typically an upright cylinder with an upper inlet to receive chips
in a continuous flow. The chips flow slowly through the digester vessel,
100 to 300 feet tall (30 to 100 meters) in a generally downward
direction. The digester vessel may be a hydraulic or vapor phase
vessel.
[0004] The wood chips are typically transported, e.g., pumped or
via another pressure transferring means, to a treatment vessel in a
slurry through conduits, e.g., pipes, which feed the wood chips to the
top of the treatment vessel. The liquid content of the slurry used to
transport the chips tends to be substantially higher than the necessary
or desired liquid content of the treatment vessel. Liquid is removed as
the slurry enters the treatment vessel. The removed liquid may be
reintroduced to the wood chips being transported to the treatment
vessel. After removal of a portion of the liquid, the slurry of wood chips
and a reduced amount of liquid are discharged from the top separator
and move down through the treatment vessel.
[0005] A conventional screen basket for a top separator is formed
by parallel bars separated by narrow gaps. Conventional top separators
are described in US Patent (USP) 6,325,889; USP 7,105,076; USP
7,309,401 and USP 7,658,818. The bars extend vertically and form a
circular array of bars, wherein the circle is a horizontal cross-section.
The bars are held together by one or more horizontal wires, horizontal
metal rings or backing bars.
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[0006] The gaps between the bars of a conventional top separator
tend to become blocked with fibers and other debris, such as rocks. As
the gaps between the bars becomes blocked, the volume of the liquid
removed from the slurry is reduced and adversely affects the overall
operation of the treatment vessel. These adverse effects can include
reduced amount of liquid removed by the top separator, reduced
availability of heat energy that can be extracted from the liquid extracted
by the top separator, a greater requirement for externally supplied liquid
to transport the wood chips to the treatment vessel, and non-optimal
treatment of the wood chips in the treatment vessel. There is a long felt
need for a screen basket for a top separator with reduced tendencies
for the slots in the screen basket becoming blocked or clogged.
BRIEF DESCRIPTION OF THE INVENTION
[0007] A novel screen basket has been developed comprising a
solid screen plate, rolled to form a basket, the plate having openings in
the plate of slanted slots having curved or chamfered inlet edges to
minimize chips begin caught on the edges and to deflect chips into the
wood mass. The curved or chamfered inlet slot edges are adjacent the
inside surface of the screen plate and face the wood chip or pulp flow.
The curved inlet slot edges may be rounded, sloped, chamfered, or
inclined. For example, inlets may have a generous radius of curvature
equal to one third to two thirds the thickness of the plate. The curved or
chamfered inlets may be only on the lower side surface of a slot or on
the upper and lower slot side surfaces. A curved or chamfered inlet only
on the lower side surface is suitable for a continuous digester in which
the wood chip or pulp flow is generally downward and chips tend to
impinge on the inlet edge of the lower sides of slots. Curved or
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chamfered inlets on both the upper and lower side surfaces of slots are
suitable for both continuous and batch digesters. In addition, the lower
side surface of the slot may be horizontal in cross-section or be inclined
upward from the inside surface of the plate to the outer surface. Such a
horizontal or upwardly inclined lower slot surface tends to deflect chips
in the slot out of the slot and into the pulp stream.
[0008] A screen basket for a top separator for a cellulosic material
treatment vessel, the screen basket comprising: a cylindrical plate
having a vertical joint connecting opposite edges of the plate, and rows
of slots extending through the plate, wherein each slot has a curved or
chamfered inlet corner edge adjacent an inside surface of the plate and
each slot is oblique to a vertical axis of the basket. The curved corner
inlet slot edge may be at least one of rounded, chamfered, sloped or
inclined. The curved or chamfered inlet corner edge has a radius of
curvature in a range of one third to two thirds a thickness of the plate.
The curved or chamfered corner edge is only at one of a lower edge or
upper edge of each slot.
[0009] The slots may be each offset along an axis from the inside
surface to an outside surface of the plate by an angle of between 5
degrees to 45 degrees, or 5 degrees to 30 degrees, or 5 degrees to 15
degrees or any variant therein. The slots may be oblique to the vertical
axis at an angle of 1 degree to 75 degrees, or 30 degrees to 60
degrees, or preferably 40 degrees to 50 degrees or 45 degrees. The
rows of slots may be uniform in height and orientation of the slots or the
slots in each row may be uniform in shape and dimensions, while the
dimensions of the slots differ from row to row.
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[0010] A method has been conceived for extracting a liquid from a
top separator of a treatment vessel, the method comprising: feeding a
slurry of cellulosic material and a liquid in the top separator screen
basket of a vessel, wherein the slurry of cellulosic material and liquid
flows into the screen basket; extracting a portion of the liquid in the
slurry of cellulosic material and liquid through a screen basket
assembly, wherein the screen basket formed of a screen basket plate
within the top separator and is formed around a conveyor screw device,
and the screen basket plate includes slots having curved or chamfered
inlet corner edges adjacent an inside surface of the screen basket plate
and facing a flow of slurry of cellulosic material and liquid, and
deflecting cellulosic material flowing through the top separator with the
curved or chamfered inlet corner edges to avoid the material from
becoming caught in the slots of the screen basket.
[0011] A top separator has been conceived for use in a treatment
vessel comprising: a rolled plate formed into a cylindrical screen; rows
of diagonal slots formed in the plate and oriented horizontally; a single
welded joint extending vertically between abutting edges of the plate; a
conveyor screw within interior to the cylindrical screen, wherein a gap
between an outer edge of the screw and an interior surface of the
cylindrical screen is thirty thousandths of an inch; a housing supporting
the cylindrical screen and forming a liquid chamber between the
housing and screen, and a vertical shaft supporting the screw and
extending vertically through the cylindrical screen.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGURE 1 is a front view of a conventional continuous
digester shown schematically and partially cut away.
[0013] FIGURE 2 is a front view, of a conventional top separator,
shown in cross-section of the digester shown schematically in Figure 1.
[0014] FIGURE 3 is a front view of a novel screen basket for the top
separator, showing the screen basket in cross-section.
[0015] FIGURES 4, 5 and 6 show the interior surface of a portion of
the novel screen basket, the portion of the basket in cross-section, and
the outer surface of the portion of the screen basket, respectively.
[0016] FIGURE 7 shows a cross-section of a slot in the novel
screen basket.
[0017] FIGURE 8 shows a cross-section of an alternative slot for
the novel screen basket.
DETAILED DESCRIPTION OF THE INVENTION
[0018] FIGURE 1 is a side view of a continuous vertical digester
vessel 10 for processing wood chips into pulp. Though a vertical
continuous digester is shown, the screen basket and screen slots
described herein are applicable to other types of digesters,
impregnation or other treatment vessels. The digester shown in
FIGURE 1 is typically referred to as a vapor phase digester. It is
understood by one skilled in the art that a top separator (as described in
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this invention) is and can be used in both vapor phase any hydraulic
phase treatment vessels, treatment vessels such as impregnation
vessels or digesters or even in systems where the separator is located
outside of the treatment vessel such as described in US 5,413,677.
The digester vessel 10 may comprise a cylindrical wall 12 that typically
forms a column of, for example, 100 feet (30 meters) tall. The top and
bottom of the digester vessel are capped. A top separator 14 may be
mounted to the top cap 16 of the vessel. A discharge outlet 18 may be
mounted to the bottom cap 20.
[0019] The wood chips 21 enter the digester vessel as a slurry
through the top separator 14. The slurry is formed of wood chips and
liquid such as cooking chemicals and water (many times referred to as
liquor). The slurry flows to the top separator through a conduit 22, which
may be coupled to pumps, high pressure feeders, a chip supply bin or
pretreatment vessel. The slurry flowing into the inlet of the top separator
14 has a liquor to chip ratio that is high relative to the desired ratio of
liquor to chips for the process preformed in the digester vessel. The top
separator extracts liquor from the slurry entering the treatment vessel.
The extracted liquor flows through an outlet conduit 24 and may be
added to inlet conduit 22 feeding the slurry to the top separator.
[0020] Wood chips 21 are continually added to a continuous
treatment vessel while the chips already in the treatment vessel are
processed and pulp 26 is discharged from the bottom of the vessel.
Heat energy and pressure may be added to the vessel such as by
injecting steam 28 into the top of the vessel. As the chips and liquid
cooking chemicals move down through the treatment vessel, heat,
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pressure and chemical reactions within the vessel dissolve or break-
down the lignin binding fibers together in the chips. The removal of the
lignin releases the fibers from being bound within the chips and
converts the chips to pulp.
[0021] The chips may form a pile (dotted line 23 represents the top
of the pile) in the vessel that is at least partially immersed in the liquid
of
cooking chemicals, generally referred to as cooking liquor (see dotted
line 25 representing liquid surface). In a vapor phase vessel 10, a vapor
filled region may be in the upper portion of the vessel that is above the
chip pile and surface of the cooking liquor. In a hydraulic vessel, the
liquor may fill the vessel such that there is no vapor filled area in the
vessel.
[0022] Screen assemblies 18 may be at one or more elevations in
the digester vessel 10. The screen assemblies are conventionally
formed of screen plates 30 in an annular array and mounted to the wall
of the vessel. Each screen plate is typically a rectangular metal plate
having a slight arch to conform to the curvature of the cylindrical vessel.
Slots in the screen plates may be vertical or slanted. Cooking liquor is
extracted from the wood chips in the vessel through the slots of the
screen plates. The fibrous pulp 26 and remaining spent cooking liquor is
discharged as pulp from the outlet 18 at the bottom 20 of the digester
vessel.
[0023] The wood chips enter bottom or top of the top separator 14
depending on whether the separator is inverted. An inverted top
separator, such as shown in Figures 1 and 2, may be used in a vapor
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phase digester vessel. As the slurry of chip enter the top separator, a
screw conveyor 32 moves the chips through the separator. The screw
conveyor may be driven turned by a gear and motor arrangement 34
coupled by a vertical shaft 35 (Fig. 2) for the screw conveyor. A screen
basket 36 forms a cylindrical wall around the outer edges of the screw
conveyor. The screen basket has slots to allow liquor from the slurry to
flow into an annular chamber 38 and flow into the liquor outlet conduit
24. As the wood chips move through the separator, the chips are
discharged (see arrows 40) from the top separator and flow down to the
chip pile 23 in the vessel 10.
[0024] FIGURE 2 is a schematic diagram of the top separator 14,
showing the screen basket 36 in cross-section. The screen basket 36
includes a cylindrical shell. A support structure for the basket may
include a cylindrical housing 42 that mounts to the top cap 16 of the
digester vessel and supports the screen basket. The housing 42 may
also define the chamber 38 for liquor extracted through the screen
basket.
[0025] The screen basket has cylindrical inner wall that is adjacent
the outer edges 44 of the screw conveyor. The gap 46 between the
edges 44 of the screw conveyor and the wall of the screen basket
narrow, such as less than thirty thousandths of an inch. Due to the
narrow gap 46 and the rotation of the screw, the edges of the screw
move across the surface of the inner wall of the screen basket. The
screen basket may have structural rigidity and be built to tight
tolerances to ensure that gap 46 remains constant as the edges 44 of
the conveyor swipe across the interior surface of the screen basket.
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[0026] FIGURE 3 is a side view of an exemplary screen basket 36
which is a metal plate formed into a cylindrical shell. The screen basket
may have a diameter of 36 to 96 inches and a vertical length of 48 to
120 inches. These dimensions are exemplary.
[0027] Slots 48 extend through the plate of the screen basket and
are sized to allow liquor to pass and block fibers and the wood chips.
The slots are slanted with respect to vertical and horizontal orientations.
The slots are arranged in rows 50 that may be horizontal rows
extending completely around the cylinder of the screen basket.
[0028] The screen basket 36 may be formed of a metal plate rolled
to form a cylinder, wherein the edges of the plate are welded along a
vertical joint 51. Rather than a metal material, the screen basket may be
formed of another material suitable for use in a treatment vessel which
typically houses an environment having acidic and alkaline chemicals.
Due to the narrow diameter of the screen basket and the proximity of
the screw conveyor, the thickness of the plate forming the screen
basket will tend to be thicker than the thickness of screen plates 30
used elsewhere in the treatment vessel such as extraction screens
within a digester vessel as shown in Figure 1. The relatively greater
thickness and single vertical weld of the screen basket assists in
minimizing damage to the interior surface of the screen basket due to
continuous mechanical action of the conveyor screw device.
[0029] The screen basket differs in several respects from a
conventional screen plate 30 mounted to the sidewall of a digester
vessel in that the screen basket of this invention (used for example in
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the top separator of a treatment vessel) requires substantial structural
rigidity in view of the narrow gap 46 and the proximity of the edges of
the screw conveyor. Further, the screen basket may be formed of a
single metal plate rolled into a cylinder, whereas a screen plate 30 is
assembled with other plates to form the screen assembly. Each plate 30
need have a relatively small arc which conforms to the wall of the
vessel. Because of the close tolerances of the gap between the
conveyor screw device and the screen basket surface, the screen
basket may be formed as a precision piece having narrow machining
tolerances.
[0030] The slots 48 may be arranged in rows 50 on the screen
basket. Each slot may have, for example, a length of three (3) to
fourteen (14) inches (8mm to 36mm). The vertical height of each row
50 may be 2 to 10 inches (5mm to 25mm). The number of slots in each
row is dependent on the circumference of the screen basket. The
number of slots in each row and the dimensions, e.g., vertical height,
each row may be uniform in the screen basket or vary from row to row.
Within any row 50, the slots sizes (slot width, relief angle and diagonal
angle relative to the horizontal) may remain constant from slot to slot.
The slot size (slot width, relief angle and diagonal angle relative to the
horizontal) may vary from row to row. The number of rows may depend
on the desired open area in the screen basket to allow for the desired
flow of liquor extracted from the chip slurry in the top separator.
[0031] To help support the screen basket, rows of horizontal rings
52, ribs or other stiffeners may be attach to the basket at the horizontal
solid ring region between the rows of slots in the basket. Flanges 54, 56
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at the top and bottom of the screen basket provided supports for
mounting the screen basket to the housing 42 and the top cap of the
digester or treatment vessel. The flanges may be metal rings that hold
the top and bottom of the screen plate in a cylindrical shape.
[0032] FIGURES 4 to 7 show screen basket in detail. Figure 4
shows an outer surface 60 of a portion of the screen basket. Figure 5 is
a cross-sectional view of the screen basket taken along lines 5-5 shown
in Figure 3. Figure 6 shows an interior surface 62 of the screen basket.
Figure 7 is a cross-section al view of the screen basket taken along
lines 7-7 in Figure 4.
[0033] The slots 48 in the rows 50 of slots in the screen basket are
configured to minimize the clogging of cellulosic material in the screens.
Each slot may have a width (throat) of 3 to 9mm and a length of 25 to
100 mm. The slots may be wide at the outer surface 60 of the screen
basket and narrow to the interior surface 62. The slots may taper from
the inside to outer surfaces of the basket at relief angle 63 of 5 to 450
,
or 5 to 30 , or 5 to 15 or any variant therein. Further, the slots may
be offset such that the opening 64 of each slot is axially offset from the
exit 66. The offset may be generally aligned with the swiping motion of
the edges of the conveyor screw. This offset is selected to enhance the
effect of pumping the liquor through the slots due to the swiping motion
of the conveyor screw. The offset may orient the axis of the slot at an
angle, e.g., 10 to 25 degrees, or 15 degrees. The offset is opposite to
the swiping direction of the edges of the screw conveyor. The offset
reduces the tendency of fibers entering the slots by pointing the axis of
the slot away from the direction of chip flow through the top separator.
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Similarly, the slots 48 may be oriented at an angle 68 of 45 degrees or
in a range of 40 to 50 degrees or zero (vertical) to 75 degrees. The
angle of the slots may be arranged to slant the slots in the opposite
direction as the rotation of the screw conveyor.
[0034] FIGURE 8 shows in cross-section another exemplary slot 70
in a screen basket for a top separator, wherein the slot has a curved or
chamfered edge 72 at the inlet opening 74. The slot extends through
the plate 76 of the screen basket. The thickness (T) of the plate may be
9 to 10 mm, 8 to 11 mm or 7 to 12 mm. The throat (x), which is the
narrowest portion of the slot, may have a width of 3mm to 8mm, such as
6mm.
[0035] The slot may expand from the inlet opening 74 to the outlet
opening 90 at a relief angle (6) of 15 to 45 degrees, such as 30
degrees. The axis (see flow arrow 80) of the slot may be offset in a
direction opposite to the movement (see arrow 82) of the edges of the
screw conveyor. The offset may be at an angle such as 30 degrees
formed by the angle (w) of one. of the sides slot and one-half the relief
angle for the slot.
[0036] The upper and lower edges of the slots may be curved, e.g.,
uniform radius of curvature, or chamfered. Avoiding sharp angles in the
slots reduces the tendency of fibers being cut or caught at the edges of
the slots. The edges of the slots at the opening 74 may be rounded,
chamfered, sloped or inclined. For example, the openings 74 may have
a generous radius of curvature equal to one third to two thirds the
thickness of the plate.
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[0037] The curved or chamfered edge 72 may be only on the side
of a slot facing the direction of movement 82 of the edges of the screw
conveyor. For a top separator having an upper inlet, the slots may have
a curved or chamfered edge at the lower edge of the slot opening. For
an inverted top separator, the curved or chamfered edge may be at the
upper edge of the slot openings. The curved or chamfered edge 72
may be is less susceptible to catching fibers (cellulosic material) in the
slurry flowing through the top separator. The curved or chamfered inlet
on the slot tends to deflect cellulosic material into the flow and away
from the slot. The curvature of the slot inlet may be defined by a radius
of the curvature. The radius may be, for example, one-third to two-third
of the thickness of the plate.
[0038] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be limited
to the disclosed embodiment, but on the contrary, is intended to cover
various modifications and equivalent arrangements included within the
spirit and scope of the appended claims.
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