Note: Descriptions are shown in the official language in which they were submitted.
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FEED PRECONDITIONING FOR CHEMICAL PULPING
Background of the Invention
The present invention relates to so-called "chemical pulping" of
lignocellulosic feed material and in particular, to method and apparatus
for improving the yield of chemical pulping.
The term "chemical pulping" as used herein, should be
understood in the same sense as that used in the art, namely, to
processes such as the Kraft, soda, soda AQ, Kraft AQ, sulphite,
bisulfite, and other similar processes, whereby chemical reagents
remove the lignin from the fiber structure of the feed material. For
convenience, the present specification will focus on the Kraft process,
but it will be understood by those familiar with this field of technology,
that the concepts and features described and claimed herein, can readily
be incorporated into the other types of chemical pulping processes.
Chemical pulping has long been used in connection with the
paper making industry. Moreover, perhaps 90% of all better grade
papers used throughout the world, are made predominantly of chemical
pulp. For this reason, it can well be appreciated that any improvements
by which the yield is increased, would have very favorable economic
and environmental consequences.
In general, chemical pulping processes rely on the release of
wood fibers by dissolution of the lignin which binds the fibers together.
Because lignin and other non-cellulosic portions of the wood chips are
removed in the process, chemical pulping processes typically provide
yields of only 40-50% based on the dried chips, which is considerably
less than so-called thermo-mechanical pulping (TMP), implemented in
rotating disc refiners.
U.S. Patent 4,869,783 discloses one technique for improving the
yield of the chemical pulp process, whereby wood chips are partially
defiberized in a high compression screw device associated with the feed
end of the pulping process. The fibers in the chips are substantially
separated from one another but sufficient inter fiber bonding is
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maintained to preserve chip integrity and thereby provide chips having
an open porous fibrous network. The chips are subjected to chemical
pulping to remove a majority of the lignin in the chips. Although highly
compressing the chips in the feed portion of the pulping process would
indeed appear to improve the bleached yield considerably (e.g., from
about 45% to 50% on a comparative basis) with the pulp brightness
also generally showing improvement, the strength properties of the pulp
deteriorate significantly. Moreover, the present inventor believes that
the strength deterioration of the fibers is the main reason why the high
compression of feed chips as described in the '783 patent has not seen
commercial fruition notwithstanding the impressive data on improved
yield and brightness relative to conventional processes. Specifically, it
is believed that the loss in strength arises from the shattering and
fracturing of fibers prior to feeding the chemical pulp digester.
The process described in International App. No. PCT/US98/14710
published 18 February, 1999, and entitled, "Method of Pretreating
Lignocellulose Fiber-Containing Material" overcomes the significant
strength deterioration associated with the process described in the '783
patent, by preceding the high compression of the chips with a
conditioning step, whereby the chips are exposed to elevated
temperature and pressure, preferably in an environment of saturated
steam with no pressure barrier between the conditioning step and the
compression zone. The conditioning of the chips prior to high
compression, results in considerably less brittle fracture and therefore
a larger percentage of the original fiber retains acceptable size and
strength during pulping and bleaching, rather than being washed away
as fines for disposal or going forward with the fiber to the digester.
Thus, relative to conventional chemical pulping, the process and
apparatus of said published application produces higher yield,
comparable if not better brightness, and comparable strength to that of
conventional pulping. This indeed represents a noteworthy advance in
the state of the art of chemical pulping. The disclosure of International
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' PCT/US00/04640 3
App. PCTlUS98l14710 is hereby incorporated by reference.
Bath of these feed end techniques require high mechanical
compression (at a ratio of at feast 3 or 4 to 1 ) before the feed material
is introduced into the chemical digestion and recovery system of the
plant. For convenience this compression wilt be referred to as
"compressive pretreatment". The present invention advances the state
of the art even farther, by improving the processes described in the
'783 patent and said International application.
W097l28305 describes a process_and apparatus for pretreating
pulp raw material, to be subsequently prepared in a chemical pulping
process, and for preparing cellulose pulp from a fibrous starting
material. According to the process the starting material is delignified to
yield a chemical cellulose pulp, and the obtained pulp is bleached when
desirable. According to the invention the starting material is crushed in
cooking liquor prior to delignification in order to open its fiber structure.
The apparatus of the invention comprises a frame to which two
adjacent first rolls have been fitted, which form a first pair of rolls with
the rolls arranged to distance from each other in such a manner that a
gap clearance is formed between their outer mantles. The rolls are
caused to rotate by means of power transmission, which causes the
raw material to be crushed inside the gap between the rolls where a
liquid pocket is formed, from which liquid is absorbed into the fiber
material being treated,
W097/28305 describes a system for feeding wood or annual
plant chips to continuous digesters for producing paper pulp. The
system comprises two stages: in a first, impregnation stage, the chips
are impregnated with a sufficient amount of chemical reagents to allow
them to be hot-digested by impregnation in alkaline or alkaline-earth
solutions, then drained. In the case of annual plants, the plants are
mechanically mixed with alkaline andlor alkaline-earth reagents and a
minimal quantity of hot water to obtain a wet fiber-tike mixture. In the
AMENDED SHEET
FMPFANGS7FtT 14 MXR 17~f14 DIIC(1Q111'KC7FTT 10 ~AcA X7.1'1
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3A
second stage, the material impregnated above is pumped directly into
a pressurized digester by a hydraulic piston pump. A plug formed in the
pipe between the pump and the digester provides the required seal and
pressure resistance.
Summary of the Invention
The main aspect of the present invention is based on introducing
process fluid into the compressive pretreatment stage, in an
environment of elevated temperature and pressure, thereby initiating
significant chemical pulping action on the chips at a point farther
upstream than was previously known. In a preferred implementation of
the present invention, this process fluid includes recycled process
reagents drawn from process lines farther downstream. In another
aspect of the invention, some of this process fluid is extracted as
pressate during compression of the chips, and the pressate is
reintroduced downstream, into the plant's recovery system. Therefore
there is no net increase in the fluid handling or chemistry requirements
of the pulping equipment, the bleaching plant, and the black liquor
recovery plant. Moreover, even pressate that does not include recycled
process fluid, but does contain at least wood extractives; can be
introduced into the recovery system.
Tha process fluid introduced upstream of the high compression
device can be one or more of (a) steam (b) recirculated black liquor of
any concentration, (c) recircuiated white liquor of any concentration, (d)
a vapor phase extract from the digester, (e) an externally supplied liquor
including sulphite, NaOH, Na2S, etc., or (f) a combination thereof, e.g.,
vapor phase from the digester plus steam. The combination of high
temperature, high pressure, and high compression achieves chemical
AMENDED SHEET
EMPFANGSZEIT 19. MAR. l7:Oy HuSDRUCKSZEIT 19. MAR. 17:17
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impregnation into destructured chips upstream of the digester, to a far
superior degree than was previously possible.
In a particularly effective implementation of the invention, the
feed material is delivered under pressure to a compression device which
has a variable speed motor such that the duration of exposure of the
chips can be controlled therein. In the compression device, the chips
experience an initial processing of consolidation and extraction of fluids
through the walls of the compression device into a pressurized collar or
flash tank. Thereafter the material is further consolidated at a high
compression in the ratio of at least 3 to 1 and up to 8 to 1 or more,
before discharge. The relative duration of the exposure of the material
as between the consolidation phase and the high compression phase,
can be pre-established by, for example, the relative screw shaft lengths
and wall characteristics defining these regions. In general, the total
travel time of the material in the compression device may range from a
few seconds to more than a minute, with generally 50%-80% of the
travel time associated with consolidation, and 20%-50% of the time
associated with compression above a ratio of about 3 to 1. In this
manner, steam and chemicals can penetrate into the chips during partial
defibration, during the initial consolidation phase, even prior to the
continuing defibration in the high compression, plug portion of the
device.
The various aspects of the present invention can thus be defined
according to the following implementations.
In a chemical pulping process wherein feed stock containing
lignocellulosic fibrous material is mechanically compressed to a ratio of
at least about 3/1 before introduction into a chemical digestion system
with optional recovery system where process fluids optionally including
recycled process reagents remove lignin from the fibrous material, the
improvement comprises introducing a portion of a process fluid into the
fibrous material, before the material is mechanically compressed.
In a chemical pulping process wherein feed stock containing
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wo ooisz2~ pcrnJSOOro~s~o
lignocellulosic fibrous material is mechanically compressed at a ratio of
at least about 3/1 before introduction into a chemical digestion system
with optional recovery system, the improvement comprises exposing
the material to an environment of pressure above 30 psi and a
' S temperature above 120°C before and during the mechanical
compression, and in this environment, contacting the material with a
chemical liquor or vapor which initiates dissolution of the lignin from the
fiber in the material.
In a chemical pulping process wherein feed stock containing
lignocelluiosic fibrous material is mechanically compressed before
introduction into a chemical digestion and recovery system, the
saturated steam at a pressure above 30 ps~before the mechanical
compression. During the compression of the material, a pressate
containing wood extractives is produced, and the pressate is 'introduced
into the recovery system downstream of the mechanical compression.
In another aspect, the invention is directed to a modification of
the feed end of a system for chemical pulping of iignocehulosic feed
material. A conditioning chamber has an inlet for receiving feed
material, and means for exposing the feed material to elevated
improvement comprises exposing the material ~;. aor~~vironment of
temperature and pressure for a target time period of exposure. A
compression device maintained at elevated temperature and pressure,
receives the conditioned feed material, consolidates the conditioned
feed material including removal of extractives, compresses the
conditioned material at a ratio of at least 311, and discharges the
compressed material into a feeder for the chemical digester. The
exposure time period in the conditioning chamber, and the travel time
period in the compression device, are both adjustable. The conditioning
chamber is preferably a variable speed transfer conveyor pressurized at
~ ~.o dart
a pressure above 30 psi~with saturated steam, and the compression
device is preferably a variable speed high compression screw
pressurized with saturated steam at a pressure above 30 psit ~ ~'~ ~''
AMENDED SHEET
EMPFANGS7FtT 19. MXR ~7.na ~IIC11R111'KC7GTT 1D AdoD , ,~.,G
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Brief Description of the Drawings
These and other objects and advantages of the invention will
become even more apparent from the following description of the
preferred embodiments, made with reference to the appended drawings,
in which:
Figure 1 is a schematic of the front end of a conventional Kraft
chemical pulping plant with continuous vertical digester, with the feed
portion backfit with a first embodiment of the invention including
inclined impregnator;
Figure 2 is a schematic flow diagram of a Kraft chemical pulping
plant including compressive pretreatment according to the invention,
and downstream processes including digesting, washing, optional
oxygen delignification, bleaching, and liquor recovery;
Figure 3 is a schematic of a variation of the invention as shown
in Figure 1, as would be implemented in a new plant with continuous
vertical digester;
Figure 4 is a schematic of a second embodiment of the invention
including an inclined impregnator, in conjunction with a conventional
continuous inclined digester;
Figure 5 is a schematic of a variation of the embodiment shown
in Figure 4, wherein the high compression device discharges directly
into the vapor phase region of an inclined continuous digester;
Figure 6 is a schematic of a third embodiment, whereby the
invention is used in conjunction with a batch digester;
Figure 7 is a schematic of a third embodiment, whereby the
invention including impregnator is used in conjunction with a batch
digester;
Figure 8 is a schematic of a fourth embodiment, in which the
compression device discharges directly into a vertical impregnator
situated within the inlet to a vertical digester;
Figure 9 is a longitudinal section view of a first embodiment of a
high compression device for implementing the present invention;
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WO 00152256 PCTIUS00104648
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Figure 10 is a longitudinal section view of a second embodiment
of a high compression device adaptod for use with the present
invention.
Figure 1 1 is an electron micrograph illustrating the internal cross
section of partially defibrated softwood chips using pressurized inlet
conditions followed by high mechanical compression; and
Figure 12 is an electron micrograph illustrating the internal cross
section of partially defibrated softwood chips using atmospheric inlet
conditions followed by high mechanical compression.
Description of the Preferred Embodiments
Figure 1 is schematic of one possible hardware implementation
of the present invention, Figure 1 shows how the compressive pre-
treatment disclosed in International App. PCTIUS98114710, can be
implemented in a commonly known vertical continuous digester system
10 such as the Kamyr type digester supplied by Ahlstrom Machinery
inc. The compressive pretreatment portion 12 according to the
invention is shown in phantom.
Wood chips are deposited in an atmospheric chip bin 14 and
delivered via a chip metering valve 16 with pressurized discharge, to
a horizontal presteaming conveyor 18. The pressure therein is below
psi (gauge) when operated in a conventional manner. However, in
accordance with the invention, a pressurized conveying screw 20
having an inlet intersecting the vertical drop feg 22, exposes the chips
to an environm nt of saturated steam, at a higher pressure, in the range
t. o - 9.Z be~~
25 of 30 - 150~psi or more, depending on the optimization of process
conditions. This pressurized conveyor 20 is driven by a variable speed
motor 24 such that the travel time through the conveyor can be
controlled as an independent variable.
' The conditioned chips are then introduced via feed column 26
30 into a specially modified MSD Pressafiner or RT Pressafiner iavailable
from Andritz inc., Muncy, PA) with a pressurized inlet yr similar
AMENDED SHEET
EMPFANGSZEIT 19. MAR. 17~ C~ AuSDRUCKSZFIT 19. MAR. 17~ 16
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compression device 28 which destructures the fibers and 'at the same
time presses out liquid 30 and drives out air that may be present
naturally or otherwise introduced, as will be described below.
Therefore, the chips upon emerging from the discharge 32 of the
compression device are receptive to the rapid and deep take-up of fluids
(liquid or vapor) that may be present at the discharge. For this reason,
an inclined wet impregnatorlconveyor 34 !such as is available from
Andritz Inc. as Model 60130) is provided extending from the discharge
of the compression device, back to the drop leg 22. All or some of the
impregnator preferably contains process liquid, most commonly
recirculated bleak iiquor~ in the case of Kraft pulping.
One suitable compression device 28 shown in Figure 9 has a
variable speed motor 36, an inlet section 38 in which the screw core or
shaft 46 has a substantially uniform diameter, a consolidation or
extraction section 42, through which the screw core gradually increases
in diameter, forming an annulus of decreasing flow cross-section along
the substantially cylindrical, perforated wall 44, followed by a plug
section 46 which typically has a substantially cylindrical core without
screw flights, and preferably includes adjustability 48 of the flow cross-
section andlor flow restrictor elements. In this particular embodiment,
the screw flight outer diameter 50 remains substantially constant and
the flights are not interrupted, but other variations having the desire
functionality according to the present invention, are within the ordinary
skill of practitioners in this field of technology.
In Figure 9 the pressurized conveyor 20 is shown in section,
feeding into a Topwinder type forced feeder 52 (available from Andritz
Inc.) which is particularly effective for feeding non-chip material such
as bamboo, bagasse, sawdust, and similar materials, which cannot be
introduced at a sufficient volummetric rate by relying on gravity alone.
Accordingly. the enhanced feeder 52 for the inlet to the compression
device is optional. The material after entering the inlet 38, flows to the
right, is consolidated as it passes through the reducing cross-section.
AMENDED SHEET
EMPFANfiSZEIT 19. MAR. l7:Oy RuSDRUCKSZEIT 19. MAR. 17:16
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Because the material is at the same elevated pressure conditions as in
the pressurized conveyor 20, a pressurized jacket 54 surrounds the
y perforated wall 44 in the consolidation section 42. Fluid is extracted,
either by flashing or simply by draining and is ultimately, withdrawn
through drain pipes 30 or valve controlled flashing. The material
undergoes a particular travel time in a consolidation along distance D1,
where consolidation and extraction is occurring at an increasing rate, in
distinction from the travel time along distance D2, where a plug Is
formed and the material experiences very high compression i.e., a ratio
above 311, pre~8rabiy in the range 4/1 to 8/1, and even more especially
5l1 or more. As noted above, during both the time of travel along D 1
tbetween the inlet and formation of the chip seal or plug) and travel
time along D2 (when the material is in compressed plug form), a
significant degree of fiber destructuring occurs in an environment of
e.g., high pressure saturated steam. Defibering is most intense along
t
02. Moreover, if as described below, additional reagents are introduced
anywhere upstream of the device discharge 32, for example, atX~ in the
tfi
pressurized conveyor, orX3 in the Topwinder feeder or at the inlet to the
compression device, the initial stages of lignin dissolution can begin to
a significantly greater extent than is possible in a compression screw
device that has a substantially atmospheric environment, because the
fibers are more open and reactive. Reagents added upstream can also
be used to initiate milder reactions prior to cooking in the digesters,
such as subjecting the chips to black liquor with low hydroxyl content
and high hydrosulfide content in the case of kraft pulping'.
Another compression screw device 28' usable with the present
invention, is shown in Figure 10. The structural aspects of this device
are disclosed and described in international application
PCT/US92100939 (published as W0921137101. The additional
modifications include a variable speed drive, pressurized inlet and high
compression ratio. The respective retention times in region D1, where
the material is consolidated under steam pressure, and the retention
AMENDED SHEET
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time for D2, where the material is in the form of a compressed chip
plug, are closely analogous to those described with respect to Figure 9.
The device of Figure 10 shows an interrupted flight section 56 where
the compressed material can be "worked" to some extent thereby
5 increasing the defibering action as well as producing further pressate,
before passing through the discharge section.
With further reference to Figure 1, a high pressure feeder column
58 with return pump 70 following the drop leg 22 delivers a slurry of
chips in black liquor or an alternative liquor, at a pressure of 5 to 10
10 bar, along line 60 to the inlet 62 at the top of the continuous vertical
digester 64. It is common in the conventional portion of the system
represented in Figure 1, for a black liquor liquid level 66 to be present
in the drop leg immediately above the inlet 68 to the high pressure
feeder column 58.
Typically, the inlet 62 at the top of the digester includes a
pressurized drainer 72 or top separator which extracts much of the
liquid in the slurry for return via pump 70 to the high pressure feeder
58. The separated but wet chips drop into the digester proper for
chemical pulping action with liquor of increasing intensity as the chips
move downwardly through the cooking zone of the digester column.
Due to improved and more selective pulping available with the
present invention, the cooking conditions of temperature, pressure and
possibly reagent concentration in digester 64 can be reduced to achieve
a predetermined degree of lignin dissolution. This in turn will result in
an increase in yield due to less degradation of the cellulose and
hemicellulose components. Conversely, the cooking conditions can be
maintained, however, with a reduction in cook time and concomitant
increase in production capacity.
By introducing process fluid in the compressive pretreatment
equipment (as shown in phantom) in accordance with the invention,
particularly into the pressurized conditioning chamber 20 atX, and/or via
X2 at the discharge 32 of the compressive destructuring device 28,
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significant benefits can be achieved.
These benefits will be more fully appreciated when understood
in the context of the balance of the process plant as described with
respect to Figure 2. The process begins with the atmospheric
presteaming bin 14 as shown in Figure 1 and is followed by the
compressive pretreatment stage 12 which, in the preferred embodiment,
comprises pressurized conditioning and compressive destructuring of
the chips. The pretreated chips are then introduced into the chemical
pulping equipment (i.e., digester 64), and thereafter washed 74. The
"cooking" in the digester is performed with increasingly strong
concentrations of white liquor via lines) 76. In some variations of the
kraft cooking process, a higher temperature black liquor is first
introduced prior to the addition of white liquor. After removal of the
lignin to the desired Kappa value, the process liquid has darkened
considerably and is thereafter referred to as black liquor. The black
liquor is washed from the chips and the diluted black liquor is passed
through a drain line 78 to the black liquor seal tank 80. The washed
chips are then sent to a bleaching plant 82 where the chips are exposed
to successive stages of chlorination, extraction and chlorine dioxide
bleaching, thereby producing essentially bleached white pulp as the
desired end product of the process. Several variations of the bleaching
process are also practiced including oxygen pre-delignification, ozone
pre-treatment, extraction with oxygen and peroxide addition (EOP),
chlorine substitution with chlorine dioxide, etc. In some applications,
no elemental chlorine is used (ECF) and occasionally no chlorine of any
type is used (TCF).
The liquor from the black liquor seal tank eventually passes
through an evaporator array 84 and the concentrated black liquor,
containing approximately 65% solids, is delivered to a recovery boiler
86. For simplification in Figure 2, optional black liquor oxidation
treatment is not shown. In the boiler, the lignin, wood extractives, and
some hemicellulosic material are burned, producing gases and some
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particulates which may require scrubbing (i.e., electrostatic precipitator)
before discharge through a stack. The smelt, consisting essentially of
inorganic chemicals, passes through a chemical recovery process 88.
Upon exiting the chemical recovery process as regenerated white liquor,
the liquid can be returned for introduction into the digester or similar
chemical pulping unit.
Various points along the process shown in Figure 2 are labelled
with an alphabetic identifier. Points B1, B2, B3 and B4, represent
various locations in which the used liquor is processed and recovered
in the kraft process. Point W represents a line containing white liquor.
Point V represents a vapor phase of cooking (white) liquor that often is
deliberately maintained near the inlet of the digester.
In accordance with the one aspect of the present invention, a
flow of one or more black liquor B, white liquor W, or vapor phase V
from the digester, can be introduced from a downstream line, into the
compressive pretreatment stage 12. Furthermore, steam S or external
liquor L with chemical reagent, can be similarly introduced in the
compressive pretreatment stage.
With reference again to Figure 1, line X~ represents the
introduction of any one or combination of the process fluids B, W, V,
S or L, into the pressurized conditioning chamber 20 (e.g., pressurized
conveyor) of the compressive pretreatment stage. Fluid line X2
represents a different point of introduction of the same or different one
or combination of process fluids B, W, V, S or L, at the discharge 32 of
the high compression device (e.g., Pressafiner).
It is evident that to the extent process fluid is introduced at any
point upstream of the compression zone of the compression unit 28, a
pressate will be extracted. In a preferred embodiment wherein, for
example, the process fluid through line X, is black liquor, such as from
point B2 of Figure 2, the pressate 30 from the compression unit will be
a concentrated low volume stream of black liquor and wood extractives.
In another aspect of the invention, this pressate is fed downstream to
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the black liquor seal tank 80 for subsequent combustion in the recovery
boiler 86. A large portion of wood extractives are thus selectively
removed prior to cooking. There is no liquid effluent treatment costs
and therefore the incorporation of the invention as shown in Figure 1
will not necessitate expansion in effluent treatment when backfit to an
existing chemical pulping plant.
The trend today in Kraft pulping is toward low alkalinity during
the initial phase of the cook to help maintain a higher yield. Defibration
of the lignocellulose material under pressurized inlet conditions of device
28 according to the present invention (e.g., as shown in Figure 9)
produces structurally intact material with high levels of exposed lignin
and fiber wall material, permitting improved chemical diffusion and more
selective lignin dissolution during chemical pulping, and thus improved
yield. The introduction of vapor phase chemicals (V) or recycled black
liquor B (or low alkalinity white liquor W) to the pressurized inlet of the
compression unit, such as shown with process fluid stream X3 (or X~),
will help further reduce the total alkalinity required during cooking due
to improved penetration during partial defibration in the compression
zone 46. Thus, there will be less carbohydrate degradation and yield
loss.
With the preferred embodiment of pressurized conditioning
followed by compressive destructuring according to the present
invention, the chips are not damaged during pretreatment and can be
subjected to conventional cooking conditions without deterioration in
the yield, strength, or brightness of the fibers. For example, the chips
can be cooked to low Kappa number values, or at least to the same
values as conventionally cooked chips. This is of particular significance
with mills which use TCF bleaching, in which a low initial Kappa number
is necessary. Inspection of chips which have been conditioned under
high pressure and then highly compressed, reveals an open chip
structure in which the chips are quite pliable when rolled between the
figures, without internal fracturing of the fiber structure. This is in
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contrast to chips inspected after atmospheric presteaming followed by
high compression, in which the fibers tend to crumble when rolled
between the fingers. The accompanying electron micrographs illustrate
the internal cross section of partially defibrated softwood chips, firstly
using pressurized inlet conditions (Figure 1 1 ), and secondly using
atmospheric inlet conditions (Figure 12). The figures clearly indicate
atmospheric preheating was not sufficient to prevent fiber shattering
during compression in the high compression screw device. The
damaged internal structure of the atmospherically heated wood chips
explains why the chips crumble when rolled between the fingers, unlike
that of the chips compressed under pressurized inlet conditions.
Thus, with the preferred embodiment of the present invention,
conventional yield can be increased in a first increment by the use of
pressurized conditioning immediately preceding the compressive
destructuring. This results from better penetration of chemicals
between the fibers at the discharge of the compression device. A
further increment of yield and/or efficiency can be achieved by a
reduction in the severity of cooking (via decreased temperature,
pressure, alkali concentration) to achieve a given Kappa value.
Conversely, because according to the invention the partially defibrated
chip structure is more rigid the material can tolerate extensive cooking
to lower Kappa values without loss in pulp strength.
As a result of implementing the preferred embodiment, the plant
can be optimized as between utilization of the conventional size and
footprint of digesting equipment to achieve a higher than standard yield,
or else for a given desired throughput, the size and/or footprint of the
equipment can be reduced. In particular, smaller digester units can be
utilized, to achieve current levels of throughput.
Further operational techniques and details of a typical Kraft
pulping plant may be found in the brochure entitled "Continuous
Progress in Cooking", available from the Alhstrom Machinery Inc.,
~1997, and from said U.S. Patent 4,869,783, the disclosures of which
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WO 00/52256 PCT/US00/04640
are hereby incorporated by reference. According to the process
described in the '783 patent, the cooking is terminated upon reaching
Kappa levels greater than 40 (typically about 45 to 70) and is then
followed by other processes, such as oxygen delignification, to reach
5 the desired pre-bleaching Kappa of 15 to 25. With the present
invention, similar Kappa levels above 40 can be achieved in the brown
stock (i.e., the washed pulp from the digester) but significantly, levels
in the range of 15-35 with acceptable strength and brightness can also
be achieved, for consistency with the operation of a wide variety of
10 brown stock treatment (with or without oxygen delignification).
Figure 3 illustrates an alternative system configuration 100, to
that of Figure 1. In this embodiment, which would be suitable for use
in a new plant, the rotary valve 16 with pressurized discharge deposits
chips directly into the pressurized conveyor 20. A stream of process
15 fluid X, is introduced near the inlet to the pressurized conveyor. The
pressurized conveyor discharges immediately into the inlet of the
compression device 28, which in turn discharges into a conventional
drop leg 22' associated with feeding into the high pressure feeder
column 58. By eliminating conveyor 18 (Figure 1 ) this configuration
uses only one (pressurized) conveyor 20 and is thus more compact.
Process liquid along line X2 can be introduced as a vapor or liquid at the
discharge of the compression device 28.
Figure 4 shows another system embodiment 200 of the
invention, as used in conjunction with an M&D continuous digester 202
(available from Andritz Inc.), or other digester operating on similar
principles. As is well known in the relevant field of technology, these
digesters have buckets or the like which move along line 204 whereby
the material is introduced into a vapor cooking phase in the buckets in
the upper portion 206, the buckets travel downwardly through a liquid
phase 208 of the cooking fluid, and rise upwardly through the vapor
phase again, for discharge through a rotary valve 210 with pressurized
inlet. The atmospheric chip bin 14 delivers chips through a rotary valve
+850-52~-5229 RLIX YRLE & RISTRS 484 P12 MRP '~ '~' "~ "
19-03-2001 CA 02363158 2001-08-31 US ~~~~~4~40
WO 00152256 PCTlUS00/04640
96
16 with pressurized discharge, into a pressurized conveyor 20, which
in turn is followed by a compression device 28. The compression
t device discharges into an inclined wet conveyor or impregnator 34 at .
atmospheric conditions. The discharge from the impregnator is
horizontally conveyed to a rotary valve 210 with pressurized discharge,
which in turn feeds the pressurized vapor phase inlet of the digester in
a conventional manner. Process fluid can as in the previous
wh Z~er x ~t ~ 0p'~iow~lty
embodiments, be introduced at X,
w
Figure 5 shows alternative configuration 300 for use with an
M&D type digester 202, wherein the chips are conveyed from the
atmospheric chip bin 14 through a rotary valve 16 with ,pressurized
discharge, into a pressurized conveyor 20 which in turn feeds the
compression device 28. Tht discharge 32 of the compression device
is directly exposed to the vapor phase 206 of the cooking fluid in the
upper, inlet portion of the digester. .
f
Figure 6 shows a feed system configuration 400 that can be
utilised upstream of the feed chute 402 to batch type digester. Chips
from the atmospheric bin 14 are fed through the rotary valve 18 into a
pressurized conveyor 20 and then through the chip compression device
28, which in turn discharges to a transfer conveyor 404 for charging
into the batch digester. Figure 7 shows a further modification 500
wherein the compression device 28 discharges into an inclined
impregnator 34 prior to transfer for charging at 402 into the batch
digester. It can be seen in both Figures 6 and 7 that process flow lines
x, andlor X2 can be effectively utilized, as with the previously-described
embodiments.
Figure 8 shows yet another system implementation fi00, whereby
the chip material from the feed bin 14 is conditioned at high pressure
at Z0, then introduced to the compression device 28, which provides
the add'ttional function of the feeder device for the vertical digester fi02.
In particular, the compression device 28 discharges into a vertical
impregnator 604 where the compressed chips, upon expansion as they
AMENDED SHEET
FMPFAN~C7F1T 14 IUisQ t7~nQ nnenanr~re~GTT ~n ~e~e ~~.~c
+800-52'7-j629 RL I X YfaLE a F2I STRS 484 PI3 MRr? ~'a
19-03-2001 US 000004640
CA 02363158 2001-08-31
wo oo~sziss pcriusoom4~o
17
discharge from the compressing unit at 32, are exposed to the liquid
506 and are thereby impregnated immediately as they are conveyed
upwardly by screw 608 before dropping down into the main region 610
of the vertical digester.
It can be appreciated that with all the foregoing systems, the
preferred implementation includes a variable speed drive 24 on the
pressurized conveyor 20, thereby providing control of the .exposure of
the chips to the elevated temperature and pressure conditions, for a
controlled time period, preferably in the range of 10-1800 seconds.
i 0 Furthermore, it would be within the ordinary skill of those in this field
to provide pressurized steam to maintain saturats~ conditions in the
( ~.0- 9~s- ba~' ( a ~7- t 1!'
range generally of 30 - 150 ps~ preferably 40-100 ps~ The
compression device 28 in all emboli ~mtents preferably includes a variable
speed motor 36 to provide an independent control on the time interval
during which the chips are processed in the device. For example, with
respect to Figure 9, the two time intervals during travel over D1 and D2
from inlet 38 to chip plug and in the plug mode respectively, are
regulated by the speed of the screw and screw design 'itself. A range
of 5-15 seconds would be especially beneficial where a process fluid
line such as X3 of Figure 1, is connected to the inlet 38 of compression
device Ior at any point upstream of the discharge end 32?. Thus, even
if a particularly strong liquor is introduced, the time period of exposure
can be controlled, at one or both of the conditioning chamber 20 or
compression device 28. Moreover, as shown at 90 in Figure 3 the
invention can be optimized by continually measuring and controlling the
relative mixing of process fluids or with other fluid such as L or S
chemical reagents such as B, W, or V for delivery via any of lines X,, Xz.
or X3~
It should also be appreciated that there is no need for a pressure
barrier or differential between the pressurized conveyor 20 and the
compression device 28. The physical configuration of these two
functional units need not be limited to what is shown herein, i.e., a
AMENDED SHEET
FMPFANGC7FTT 10 MaR t'l.na enenanr~re~GTT ~o ~~~p ~ ~~,»
19-03-2001 US 000004640
CA 02363158 2001-08-31
WO Q0/52Z56 PC'TNS00104b40
18
horizontal conveyor with discharge into a horizontal compression unit.
The functions of these two units could be combined into a single unit
y having for example, a conditioning chamber leading into a compression
chamber, both of which are maintained at elevated temperature ~ d,~ ray'
pressure, preferably a saturated steam environment above 40 ps~ As
noted above, the pressurized environment, as distinct from the
atmospheric pressure in the conveyor and compression device as is
typical in conventional atmospheric inlet screw devices, not only opens
the chips up for more efficient lignin removal, but importantly maintains
--- 10 the elongation of the chips arid--minimizes brittle fractures and -
shattering. This advantage contributes significantly to the ability of the
present invention, to permit cooking to desired Kappa number levels
without loss in pulp strength properties.
AMENDED SHEET
EMPFA~IGSZEIT 19. MAR. 1709 awbRIICKC7FtT 14 h~I~R 17~ t~
