Note: Descriptions are shown in the official language in which they were submitted.
CA 02453335 2004-11-10
TAPERED SCREt=N ASSEMBLY FOR A CELLULOSE PULP
DIGESTER
BACKGROUND AND SUMMARY DF THE IN1I1C TION
In the art of chemical pulping of comminuted cellulosic fibrous
material, for example wood chips, the cellulose material is typically
treated with cooking chemicals under pressure and temperature in one
or mare cylindrical vessels, known as digesters. This treatment can be
performed continuously or in a batch mode_ 1n the continuous mode,
chips are continuously fed into one end of a continuous digester,
treated, and continuously discharged from the other end. In the batch
method, one or more batch digesters are. filled with chips and cooking
chemical, capped and then treatment commences. Once the treatment
is finished the contents of the batch digester are discharged. In either
batch or continuous digesters, a slurry of comminuted cellulosic fibrous
2a material and caoktng chemical is treated in one or more cylindrical
vessels.
In both continuous and batch digesters, in order to uniformly
distribute both temperature and cooking chemical, Cooking liquor is
typically circulated through the slurry of chips and liquor, typically
refereed to as "the chip column". This circulation is typically effected by
some ~torm of screen, located along the internal surtace of the
cylindrical vessel, a pump, a heater, and a return conduit. The screen
retains the material within the digester as the liquor is removed,
augmented with other I'~quors andlor a portion thereof removed,
pressurized, heated, and then retuned to the slurry in the vicinity of the
screen or elsewhere. The proper operation of the digester and the
production of uniform product having the
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best properties, for example, strength, are highly dependent upon the
efficiency of this liquid circulation process.
The radial removal of liquor typically produces radial
compression of the chip column in the vicirdty of the screen assembly.
in addition, the weight of the column of chips above the chips near the
screen introduces another source of compression of the chips-
Furthermore, the vertices! movement of free liquor in the chip Column,
either upward or downward van vary the compression load, or
compaction, of the chip column. It is known in the art that this radial
and vertical compression can interfere with the uniform movement of
the chip column, which is so 0ssential for the uniform treatment of the
chips. For this n3ason, conventional digesters and screen assemblies
are designed so that the diameter of the flow path increases just below
the screen. This increase in diameter or "step-out" relieves the
compression in the chip column and permits more uniform movement
of the column- This step-out typically consists of a radial increase of
about 8 inches to 2 feet. United States Patent No. 5,985,09Ea discloses
several nova! methods of accommodating this "column relief' while
maintaining a relatively uniform vessel shell diameter.
The radial compression of the chip column against the surface
of the screen, due to the radial flow of liquid, also add in reducing
pluggage of the screen surface. For instance, the normal pressure
load on the screen surface in conjunction with the downward
movement of the chip column acts to scour or "rub" the surface of the
screen. This "rubbing" action helps to keep the apertures of the screen
free of obstructions, for example, chips, pulp, or other debris. For
instance, far vertically-oriented bay type screens, the vertical rubbing
action helps to dislodge any chips that may accumulate between the
scn~n bars.
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However, excessive radial compression can interfere with the
uniform removal of liquid through the screen. As the radial flow
increases, the compression induced by the flow in the chip column can
compact the chip column making it difficult to pass liquid through the
column. Therefore, the flow of liquid required to uniformly distribute
chemical and temperature is typically limited. Thus, though some radial
compression which produces a normal pressure on the surface of the
screen is desired, this radial compression cannot exceed the compression
that reduces the radial flow of liquid or hinders the axial flow of the chip
column.
Typically, prior art screen assemblies comprise right cylindrical
screen surfaces of reDatively uniform diameter. These screen surfaces
may comprise or consist of perforated plate, having slots or holes, or
parallel-bar type construction having parallel apertures between the bars.
These bars typically have a vertical orientation, but that may have various
orientations including horizontal or at some oblique angle, for example, at
a 45° angle to the vertical.
~y analyzing the distribution of the forces within the chip column
that are produced by the flow of liquid within the column it has been found
according to the invention that by designing these screen assemblies so
that they are not uniformly cylindrical, but slightly divergent, the radial
compression loading in the chip column and on the screen surface can be
reduced, and the volume and rate of liquid that can flow through the chip
column and be removed through the screen can be increased. For
example, screen surfaces having a slight increase in diameter in the
direction of chip flow can reduce the compression load in the chip column
and improve the performance of the screen assembly, and the digester in
general.
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Conical divergent screening surtaces are not unknown in the art
of chemical pulping. For example, continuous hydraulic digesters
typically include a canicat screen surface at the very top of the digester,
in the vicinity of where the slurry of cellulose is introduced, to remove
excess Liquid from the slurry and return it the digester feed system_ 1n
two vessel digester systems those screens, which are typically referred
to as "bottom circulation" or "BC" screens, are conical in shape but do
not provide the function of the screens of the present invention. Since
BC screens are typically located above the chip pile they do not
experience the compressive loading that screens located lower down in
the digester do. Also, the BC screens typically do not interfere with
the movement of the chip slurry though the digester. Conical BC
screens would not be used in the cooking or extraction zones as are
the screens of the present invention.
One embodiment of this invention comprises or consists of a
cylindrical screen assembly for removing liquid from a slurry of
comminuted celluiosic fibrous material in a cylindrical vessel havivg a
diameter that diverges in the direction of the movement of the slurry.
The screen asserr~biy can have an angle of divergence of between
about 0.5 and 45°. However, it is believed that using angles of
divergence greater than approximately 15° will diminish the rubbing
action of the chip column on the screens that is desirable to prevent
screen plugging. Through screens having larger angles of divergence
can be used, it is preferred that the angle of divergence of the screen
be limited to between about 0_6 and 10°, preferably, about 0.5 to
5°, to
ensure that at least some fiorm of normal rubbing force is exerted on
the screen surface.
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According to another aspect of the invention an assembly, per se,
for use in screening liquid is provided. The assembly comprises: An
annular screen assembly for separating liquid from solid material, the
screen assembly having a screen surface with a top, a bottom, and a first
5 internal diameter and a second internal diameter, and the screen
assembly having an external diameter defining an annular volume
exteriorly of the screen surface. The screen surface may comprise a
substantially continuous cylindrical screen surface, or have a wide variety
of other configurations as is conventional for screen surfaces per se,
particularly for screens in chemical pulp digesters, and has a substantially
constant opening size, and percentage of open area, in the flow direction.
According to another aspect of the present invention a method of
treating a liquid slurry of comminuted cellulosic fibrous material under
cooking conditions in a substantially vertical continuous digester having at
least one substantially annular screen surface, and having a top and a
bottom, to produce chemical pulp, is provided. The method comprises
the steps of substantially continuously: (a) Introducing the slurry of
comminuted cellulosic fibrous material into the digester adjacent the top
thereof, to flow downwardly in the digester in a first flow path, having a
first diameter. (b) Screening the slurry to remove liquid therefrom using
the at least one screen surface, having a substantially constant screen
surface opening size, and percentage of open area, in the first flow path.
(c) During step (b) causing the slurry of comminuted ceilulosic fibrous
material to transition from the first flow path to a diverging second flow
path [preferably having an initial second diameter, equal to or greater
than the first diameter]. And, (d) removing the chemicaP pulp from
adjacent the bottom of the digester.
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The method also preferably comprises the further step (e), after
step (c) and before step (d), of causing the down.wardly moving slurry
to move in a third flow path having a diameter substantially equal to
or larger than the second diameter. There is also preferably the further
step of repeating steps {b), (c), and (e), at least once prior to step (d),
and there is the further step of heating the liquid removed in the
practice of step (c), and reintroducing the heated liquid into the
digester adjacent where it was removed. As is conventional, some of
the liquid flow may be removed, and/or other liquid added, prior to
return to the digester.
The invention also comprises a comminuted cel~.ulosic fibrous
material treatment vessel assembly. The vessel includes the following
components: A substantially vertical vessel having a top, a bottom, an
inlet and an outlet, and through which comminuted ceYlulosic fibrous
material flows in a flow direction, the vessel having a substantially
cylindrical wall, and preferably with at least one diameter-changing
transition between the inlet and outlet. And, a screen assembly (e.g.
provided at or just past the transition), the screen assembly comprising
an annular screen surface diverging in the flow direction of the
comminuted cellulosic fibrous material, and engaging (contacting) the
slurry, so as to reduce the radial compression of material thereon [and
also preferably to increase the volume and rate of liquid that can flow
through. the material and be removed through the screen surface
compared to a right cylindrical surface of the same construction], the
screen assembly having screen surface openings with a substantially
constant screen surface opening size [and preferably percentage of
open area] in the flow direction.
Typically the outlet is adjacent the bottom of the vessel and the
inlet is adjacent the top so that the screen surface diverges
downwardly.
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The screen surface preferably diverges at a substantially constant angie-
to the vertical of between about 0.5-10°, most preferably between about
0.5-5°. The screen surface may comprise a first screen surface, and the
vessel may further comprise a second annular screen surface
substantially immediately downstream of the first screen surface in the
direction of flow, the second surface also diverging in the flow direction (at
the same angles as indicated above). Preferably the annular screen
surfaces are continuous, however they can be "checkerboard" in
configuration, or have other known confgurations. The screen surfaces
may have any conventional construction, such as perforated plate, bar,
etc. For example, the screen surface comprises a perforated metal
surface with perforations of substantially uniform size and density. As
another example the screen surface comprises a plurality of bars spaced
from each other in a direction substantially parallel to the flow direction,
the spacing between the bars being substantially constant both from bar
to bar and along the entire lengths thereof in the flow direction.
The invention also relates to a method of treating a liquid slurry of
comminuted ceUulosic material in a substantially vertical vessel (and
preferably having at least one diameter transition). The method
comprises the steps of: (a) liltroducing the slurry into the vessel to flow
substantially vertically therein in a flow direction. (b) while the slurry is
flowing in the flow direction screening (e.g. at or just downstream of the
diameter transition) the slurry to remove liquid therefrom while causing
the (squid to diverge in the flow direction at an angle of.between about 0.5-
10° using the at least one screen surface, having a substantially
constant
screen surface opening size land preferably percentage of open~area] inw--
the first flow path. And, (c) downstream of step (b) in the flow direction,
removing the slurry from the vessel. Steps (a) through (c) may be
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practiced substantially continuously, and so that the~fiow direction is ~~
substantially downward. Also there may be the further step of repeating
step (b) at least once prior to the practice of step (c).
It is the primary object of the present invention to provide a
simplified screen assembly for a comminuted ceilulosic fibrous material
treatment vessel which allows for increased liquid removal and improved
material movement thraugh the vessel. This and other objects of the
invention wilt become clear from an inspection of the detailed description
of the drawings and from the appended claims.
BRIEF DESCRIPTION OF THE DRAInIINGS
FIGURE 1 is a schematic side view of a prior art continuous
digester having conventional right cylindrical screen assemblies;
FIGURE 2 is a detail side cross-sectional view at one of the prior
art right cylindrical screen assemblies of the digester of FIGURE 1;
FIGURE 3 is a view like that of FIGURE ~ only for a screen
assembly according to the invention;
FIGURES 4A and 4B are schematic plan views of two different
versions of a portion of an exemplary perforated plate screen surface,
according to the invention, developed linearly; and
FIGURE 5 is a schematic plan view of a portion of an exemplary
bar screen surface according to the invention.
DETAILED DESCRIPTION OF THE DRA1NINGS
FIGURE 1 illustrates a typical prior art continuous digester 10
exhibiting right cylindrical screening surfaces associated with each screen
assembly. (Though a vertical cantinuous digester is shown, it is to be
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understood that the present invention is applicable to any type of
cylindrical digester, continuous or batch.] A slurry of comminuted
cellulosic fibrous material and cooking chemical is introduced at the top
of the digester 11 and a slurry of fully-cooked pulp and spent cooking
liquor is discharged at the bottom 12. The digester 10 comprises a
cylindrical shell,13, and numerous right cylindrical screen assemblies
14,15,16 and 17. The typical geometry of right cylindrical screen 16 is
illustrated in more detail in FIGU1ZE 2.
FIGURE 2 illustrates a typical prior art screen assembly 16
having an upper screen 18 and a lower screen 19. 'The screens 18, 19
may be of various construction, such as perforated plate, for example,
plates having circular holes or milled slots, or they may be constructed
of parallel bars having parallel apertures between the bars. The slots or
apertures may be positioned in various orientations such as vertically,
horizontally, or any oblique angle; for example, the parallel bars may
be oriented at a 45-degree angle from the vertical.
Behind each screen 18,19 typically is a annular cavity 20, 21, for
collecting the liquid withdrawn through each screen 18,19. Beneath
each annular cavity 20, 21, are smaller annular cavities 22, 23,
commonly referred to as "internal headers'°, for collecting the liquid
from cavities 20, 21, and discharging it to liquor removal conduits 24,
25. Though these cavities are shown as being located internal to the
shell 13, they may also be located external to the shell, that is, "external
headers" may be used. Cavities 20, 22 and cavities 21, 23, typically
communicate via apertures having specially-desigmed dimensions, that
is, orifice holes, in order to promote uniform removal of liquid
through each screen, as is conventional. Conduits 24, 25 typically join
to form a single conduit 26 which communicates with a re-circulation
pump 31. Beneath each
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screen assembly 16 the diameter of the shell 13 is increased at step
cut 27. This step-out helps to relieve the compressive forces formed in
the chip column due to the vertical compression of the weight of the
chips and the radial compression of the liquor removed through the
5 screens. This radial increase may range from about 1 to 36 inches, but
is typically between about 8 and 2~l inches.
As is conventional, FIGURE 2 illustrates the return system
associated with an exemplary screen assembly 16. Some of the
screen assemblies will have merely extraction, or liquid removal, but
1 D typically two or more of the screen assemblies in the digester have the
pump 31 connected to the conduit 26 to withdraw Ilquid in the conduit
26, with potentially some Ilquor added as indicated schematically at 32
in FIGURE 2, andlor some liquor withdrawn as indicated schematically
at 33 in FIGURE 2. The added liquid in 32 may be white liquor, or
make-up f~quor (e.g. filtrate) having lower dissolved organic material
content than the withdrawn liquor in sine 33, or it may have any other
compositions known in th~ art
From the pump 31 the liquid is pumped typiCatly through a
heater 34, and the heated liquid is reintroduced into the digester using
2t) an internal conduit 35 so that the withdrawn liquor is returned near the
area where it was removed (typically just above the screen 18). There
are a wide variety of different oan~entional structures for this purpose.
FIGURE 3 illustrates a typical digester screen assembly
according to the present invention. Several of the features shown in
FIGURE 3 are similar or identical to those shown in FIGURES 9 and 2;
these features are distinguished from the earlier ones by the prefixed
numeral "1 ".
Shell 113 contains a screen assembly 116. The screen
assembly 116 is shown as a double screen 118, 119 but it is to be
understood that
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the screen assembly 116 may comprise or consist of one, two, or more---
screens. Screens 118,119 may be of the various types of constructions
as described for screens 18, 19 above, but unlike screens '! 8, 19, screens
118, 119 are tapered such that they diverge in the direction of the slurry
flow, shown by the arrows F. This divergence is typically at least by about
0.5 degrees from the vertical and is preferably at most approximately 10
degrees from the vertical, and is preferably substantially uniform and
continuous. For example, the upper screen 118 has an upper internal
diameter Do and a lower internal diameter D, greater than Do, preferably
with a substantially constant taper between them. Also, screen 119 has
an upper internal diameter essentially equal to D, and a lower internal
diameter D2, greater than D,, preferably with a substantially constant
taper between them.
Though the blank plates 40, 41, 42, are shown as right-cylindrical
cylinders, these may also be divergent conical sections. As an
alternative, only one of the two screens 118, 119 may diverge while the
other may be essentially cylindrical. That is, D, may be essentially equal
to DO and D2 be greater than D,; or D, may be greater than Do and DZ be
essentially equal to D,.
As described above, the divergent flow path provided by the
screens 118, 119 sufficiently reduces the compression in the chip column
due to the radial removal of liquid that the potential for hang-up of the chip
column is reduced or the volume of liquid that can be removed increased
compared to conventional right-cylindrical screens.
Screen assembly 116 typically includes annular cavities 120, 121
and internal headers 122, 123 which discharge to conduits 124, 125 as is-
conventional. Cavities 120, 121 and cavities 122, 123 typically
communicate via multiple orifices (not shown). As in FIGURES 1- and 2,~
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the column compaction may be relieved by introducing a diameter
step increase 127, or the like, below the screen 119.
'Though the screens 118,119 are each shown as having a
continuous cylindrical screen surface, 43, it is to be understood that the
screen surface 43 may not be continuous or cylindrical. For example,
the screen surface 43 may also comprise multiple individual circular
screens, or the screen surface 43 may comprise alternating screen
surfaces and blank plates, commonly referred to as a °'checker board
pattern'°. More than one such screen assembly 11~ can ~be -- and almost
always is -- used in the same vessel 113. 'The conduit 126 of the screen
assembly 116 can also include a recirculation system like the
components 31-35 in FIGURE 2.
In a method according to the present invention, utilizing the
apparatus of FIGURE 3, a slurry of comminuted cellulosic fibrous
material which flows in the direction F in the vertical vessel 113 having
at least one diameter transition (at 40), comprises the following steps:
Introducing the slurry into the vessel 113 to flow substantially
vertically therein in a flow direction F. At or just downstream of the
diameter transition (40), screening the slurry (using the diverging
screens 116, and/or 119) to remove liquid therefrom (which is
ultimately withdrawn in the conduit 126) while causing the slurry to
diverge in the flow direction at an angle of between about 0.5-10° (the
same angle as the angle of divergence of the screen surfaces 43). And,
downstream of the liquid as indicated in FIGURE 3, removing the
slurry from the vessel. The screening step may be repeated at least
once prior to the removal of the slurry from the vessel. Also the
removed liquid in conduit 126 may be heated and reintroduced (and
some liquid withdrawn therefrom and/or other liquid inserted into the
flow) as indicated by the elements 31-35 of FIGURE 2.
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It is highly desirable that the screen surfaces used in the practice- -
of the invention have substantially constant opening size, and percentage
of open area, in the flow direction of the slurry. 'That is, rather than using
bars with an increasing slot spacing in the direction of flow (and therefore
necessarily an increase in percentage of open area), such as is provided
in the strainer of U S Patent 3,385,753, if bars are used according to the
invention the slot spacing is kept substantially constant in the flow
direction. When perforated screen surfaces are used, the size and
spacing of the openings are kept substantially constant, so that, again,
the percentage of open area in the screen is kept substantially constant in
the flow direction. This feature of the invention can best be seen with
respect to the two exemplary embodiments illustrated schematically in
FIGURES 4 and 5.
FIGURES 4A and 4B are each a schematic representation
(exaggerated in proportion for clarity of illustration) of a linear
development of a portion of a screen assembly 143 that may be used
according to the invention, which has a screen surface 50 in the form of a
perforated metal plate surface. The screen surface 50 has a plurality of
perforations 51 (typically slots, as in FIGURE 40., or circular holes, as in
FIGURE 4B, although other shapes could be used) spaced from each
other a distance 52. Whatever the size of the perforations 51, and their
spacings 52, the size and spacings are kept substantially constant in the
flow direction F so that the percentage of screen open area is kept the
same as the slurry moves in the direction F.. Thus, it is simply the gradual
increase in diameter of the screen surface 50 (when in annular
configuration) that allows relief of compaction;-arrd tf~e- consequent-w ----
increase in screen surface area (without the_need~ for-a change .in the _ _...
percentage of open area, or screen opening size) that allows more liquid
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removal, according to the invention. That is, the surface 5D has ---~--
perforations 51 of substantially uniform size and density.
The same results achieved for the embodiments of FIGURES 4A
and 4B are achievable according to the invention for any other
conventional type of screen assembly. For example, with respect to
FIGURE 5, a section of a bar screen assembly 243 that may be utilized
according to the invention is schematically illustrated, again greatly
exaggerated in proportion for clarity of illustration. The bar screen
assembly 243 includes a plurality of metal bars 54 which are typically held
together (when in annular configuration) by two or more rings 55, 56. The
rings 55, 56 may be provided at any locations along the bars 54, but in
FIGURE 5 are shown at the top and bottom thereof. Each set of bars 54
has a spacing 57 in a direction substantially parallel to~ the flow direction
F. The spacings 57 are substantially equal to each other, and
substantially uniform in the direction F. Also, the amount of open area of
the screen preferably remains the same.
The uniform spacings 57 may be provided by using bars having a
substantially trapezoidal (rather than substantially rectangular) shape in
plan. Again, compaction is relieved solely by the increase in diameter of
the screen assembly 243 in the flow direction F. The percentage of
screen open area may be maintained the same by providing appropriate
perforated screen surface sections between the bar sections illustrated in
FIGURE 5, or by providing openings 58 In selected ones of the bars 54 at
the substantially most remote portions thereof in the direction. of flow F.
..In
this way an increase in the amount of liquid that can be removed is
achieved by the increase in the screen surface area in the direction F
without the need for increasing the spacing 57 size, or percentage of
open area.
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In all of FIGURES 4A, 4B, and 5 embodiments a checkerboard
configuration, or other blanked screen portions, or the like, may be
provided where desired, just as discussed above with respect to
FIGURE 3.
5 As another alternative for the screen assembly surface
according to the invention, a screen surtace with slanted apertures (for
example milled sluts at a 80-60 degree angle, e.g. about ~5 degrees, to
the vertical) may be used, such as described in international
application, publication number WO 9fi126315.
10 It will thus be seen that according to the present invention an
advantageous digester, screen assembly, and method of treating a
liquid slurry to produce chemical pulp, have been provided. The
invention reduces ttte potential for the liquor removal screens to
interfere with the movement of the cellulose material slurry while
15 increasing the volume of liquid that can be removed. It is to be
understood that though the discussion above generally refers to the
vessels in which the present invention can be used as digesters, this
invention can be applied to any treatment vessel for treating a slurry of
Comminuted cellulosic fibrous material that requires that liquid be
2D removed from the slurry. These include impregnation vessels,
pretreatment vessels, washing vessels, and bleaching vessels.
While the invention has been herein shown and described in
what is presently conceived to be the most practical and preferred
embodiment thereof, it will be apparent to those of ordinary skill in the
art that many modifications may be made thereof within the scope of
the invention, which scope is to be accorded the broadest interpretation
of the
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appended claims so as to encompass all equi~ralent structures and
methods.
