Note: Descriptions are shown in the official language in which they were submitted.
CA 02273146 1999-06-15
DISSOLVED SOLIDS CONTROL IN PULP PRODUCTION
BACKGROUND AND SUMMARY OF THE INVENTION
According to conventional knowledge in the art of kraft pulping of
cellulose, the level of dissolved organic: materials (DOM) -- which mainly
comprise dissolved hemi-cellulose, and lignin, but also dissolved
cellulose, extractives, and other materials extracted from wood by the
cooking process -- is known to have a detrimental affect in the later stages
of the cooking process by impeding the delignification process due to
consumption of active cooking chemical in the liquor before it can react
with the residual or native lignin. in wood. The effect of DOM
concentration at other parts of cooking, besides the later stages, is
according
to conventional knowledge believed insignificant. The impeding action of
DOM during the later stages of the cook is minimized in some state-of-the-
art continuous cooking processes, particularly utilizing an EMCC~ digester
from Kamyr, Inc. of Glens Falls, New 'York, since the counter-current flow
of liquor (including white liquor) at the end of the cook reduces the
concentration of DOM both at the end of the "bulk delignification" phase,
and throughout the so-called "residual delignification" phase.
According to the present invention, it has been found that not only does
DOM have an adverse affect on cooking at the end of the cooking phase,
but that the presence of DOM adversely affects the strength of the pulp
produced during any part of the cooking process, that is at the beginning,
middle, or end of the bulk delignification stage. The mechanism by which
DOM affects pulp fibers and thereby adversely affects pulp strength has not
been positively identified, but it is hypothesized that it is due to a reduced
mass transfer rate of alkali extractable organics through fiber walls induced
by DOM surrounding the fibers, and differential extractability of crystalline
regions in the fibers compared to amorphous regions (i.e. nodes). In any
event, it has been demonstrated according to the invention that if the
DOM level (concentration) is minimized throughout the cook, pulp
CA 02273146 1999-06-15
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strength is increased significantly. It has been found, according to the
present invention, that if the level of 170M is close to zero throughout a
kraft cook, tear strength of the pulp is greatly increased, i.e. increased up
to
about 25% (e.g. 27%) at 11 km tensile compared to conventionally
produced kraft pulp. Even reductions of the DOM level to one-half or one
quarter of their normal levels also significantly increase pulp strength.
In state-of-the-art kraft cooks, it is not unusual for the DOM concentration
at some points during the kraft cook to be 130 grams per liter (g/1) or more,
and at 100 g/1 or more at numerous points during the kraft cook (for
example in the bottom circulation, trim circulation, upper and main
extractions and MC circulation in Kamyr, Inc. MCC~ continuous
digesters), even if the DOM level is maintained between about 30-90 g/1 in
the wash circulation (at later cook stages, according to conventional
wisdom). In such conventional situations it is also not unusual for the
lignin component of the DOM level to be over 60 g/1 and in fact even over
100 g/1, and for the hemi-cellulose component of the DOM level to be well
over 20 g/1. It is not known if the dissolved hemicellulose component has
a stronger adverse affect on pulp strength (e.g. by adversely affecting mass
transfer of organics out of the fibers) than lignin, or vice versa, or if the
effect is synergistic, although the dissolved hemi-celluloses are suspected
to have a significant influence.
According to the present invention it has been recognized for the first time
that the DOM concentration throughout a kraft cook should be minimized
in order to positively affect bleachability of the pulp, reduce chemical
consumption, and perhaps most significantly increase pulp strength. By
minimizing DOM levels, one may be able to design smaller continuous
digesters while obtaining the same throughput, and may be able to obtain
some benefits of continuous digesters with batch systems. A number of
these beneficial results can be anticipated by keeping the DOM
CA 02273146 2003-05-14
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concentration at 100 g/1 or less throughout substantially the entire kraft
cook
(i.e., beginning, middle and end of bulk delignification), and preferably
about
50 g/1 or less (the closer to zero DOM one goes, the more positive the
results). It is particularly desirable to keep the lignin component at 50 g/1
or
less (preferably about 25 g/1 or less), and the hemi-cellulose level at 15 g/1
or
less (preferably about 10 g/1 or less).
According to the present invention it has also been found that it is possible
to
passivate the adverse affects on pulp strength of the DOM concentration, at
least to a large extent. According to this aspect of the invention it has been
found that if black liquor is removed and subjected to pressure heat treatment
according to U.S. Patent 4,929,307, e.g. at a temperature of about 170-
350°C
(preferably 240°C) for about 5-90 minutes (preferably about 30-60
minutes)
and then reintroduced, an increase in tear strength of up to about 15% can be
effected. The mechanism by which passivation of the DOM by heat treatment
occurs also is not fully understood, but is consistent with the hypothesis
described above, and its results are real and dramatic on pulp strength.
According to the present invention various methods are provided for
increasing kraft pulp strength taking into account the adverse affects of DOM
thereon, as set forth above, for both continuous and batch systems. Also
according to the present invention increased strength kraft pulp is also
provided, as well as apparatus for achieving the desired results according to
the invention. Further, according to the invention, the H factor can be
significantly reduced, e.g., at least about a 5% drop in H factor to achieve a
given Kappa number. Also, the amount of effective alkali consumed can be
significantly reduced, e.g., by at least about 0.5% on wood (e.g. about 4%) to
achieve a particular Kappa number. Still further, enhanced bleachability can
be achieved, for example, increasing ISO
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brightness at least one unit at a particular full sequence Kappa factor.
According to one aspect of the present: invention, a method of producing
kraft pulp by cooking comminuted cellulosic fibrous material is provided.
The method comprises the steps of continuously, at a plurality of different
stages during kraft cooking of the material to produce pulp: (a) Extracting
liquor containing a level of DOM substantial enough to adversely affect
pulp strength. And, (b) replacing some or all of the extracted liquor with
liquor containing a substantially louver effective DOM level than the
extracted liquor, so as to positively affect pulp strength. Step (b) is
typically
practiced by replacing the withdrawn liquor with liquor selected from the
group consisting essentially of water, substantially DOM free white liquor,
pressure-heat treated black liquor, washer filtrate, cold blow filtrate, and
combinations thereof. For example for at least one stage during cooking,
black liquor may be withdrawn, .and treated under pressure and
temperature conditions (e.g. superatm.ospheric pressure at a temperature
of about 170-350°C for about 5-90 minutes, and at least 20°C
over the
cooking temperature) to significantly passivate the adverse affects of DOM.
The term "effective DOM" as used in the specification and claims means
that portion of the DOM that affects pulp strength, H factor, effective alkali
consumption and/or bleachability. A low effective DOM may be obtained
by passivation (except for effect on bleachability), or by an originally low
DOM concentration.
The method according to the invention can be practiced in a continuous
vertical digester, in which case steps (a) and (b) may be practiced at at
least
two different levels of the digester. There is also typically the further step
(c) of heating the replacement liquor from step (b) to substantially the same
temperature as the withdrawn liquor prior to the replacement liquor being
introduced into contact with the material being cooked. Steps (a) and (b)
can be practiced during impregnation, near the start of the cook, during the
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middle of the cook, and near the end oiE the cook, i. e., during substantially
the entire bulk delignification stage.
According to another aspect of the present invention, a method of kraft
cooking is provided comprising the steps of, near the beginning of the
kraft cook: (a) Extracting liquor containing a level of DOM substantial
enough to adversely affect pulp strength. And, (b) replacing some or all of
the extracted liquor with liquor containing a substantially lower effective
DOM level than the extracted liquor, so as to positively affect pulp
strength.
According to another aspect of the present invention a method of kraft
cooking is provided comprising the steps of, during impregnation of
cellulosic fibrous material: (a) Extracting liquor containing a level of DOM
substantial enough to adversely affect pulp strength. And, (b) replacing
some or all of the extracted liquor with liquor containing a substantially
lower effective DOM level than the extracted liquor, so as to positively
affect pulp strength.
According to still another aspect of the present invention a method of
kraft cooking pulp is provided comprising the following steps: (a)
Extracting black liquor from contact with the pulp at a given cooking stage.
(b) Pressure heating the black liquor to a temperature sufficient to
significantly passivate the adverse effects on pulp strength of DOM
therein. And, (c) re-introducing the passivated-DOM black liquor back into
contact with the pulp at the given stage.
The invention also comprises the kraft pulp produced by the methods set
forth above. This kraft pulp is different than kraft pulps previously
produced, having a tear strength as much as 25% greater at a specified
tensile for fully refined pulp (e.g. at 9 km tensile, or at 11 km tensile)
(and
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at least about 15% greater) compared to kraft pulp produced under
identical conditions without the DOM maintenance or removal steps
according to the invention, or as much as 15% greater (e.g. at least about
10% greater) where passified black liquor is utilized.
The invention is also applicable to kraft batch cooking of cellulosic fibrous
material utilizing a vessel containing black liquor and a batch digester
containing the material. In such a method of kraft batch cooking according
to the invention there are the steps of: (a) Pressure-heating the black liquor
in the vessel to a temperature sufficient to passivate the adverse effects on
pulp strength of DOM therein. And, (b) feeding the black liquor to the
digester to contact the cellulosic fibrous material therein. Step (a) is
practiced to heat the black liquor at superatmospheric pressure at a
temperature of about 170-350°C for about 5-90 minutes (typically at
least
about 190°C for about 30-60 minutes, and at least 20°C over
cooking
temperature), and step (b) may be practiced to simultaneously feed black
liquor and white liquor to the digester to effect cooking of the cellulosic
fibrous material.
According to another aspect of the present invention an apparatus for kraft
cooking cellulose pulp is provided. The apparatus comprises the following
elements: An upright continuous digester. At least two
withdrawal/extraction screens provided at different levels, and different
cook stages, of the digester. A recirculation line and an extraction line
associated with each of the screens. And, means for providing replacement
liquor to the recirculation line to make up for the liquor extracted in the
extraction line, for each of the recirculation lines. Each recirculatory loop
typically includes a heater, and the digester may be associated with a
separate impregnation vessel in which removal of high DOM
concentration liquor and replacement with lower DOM concentration
liquor also takes place (including in a return line communicating between
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the top of the impregnation vessel and the high pressure feeder).
The invention also relates to a coizimercial method of kraft cooking
comminuted cellulose fibrous material by the step (a) of continuously
passing substantially DOM-free cooking liquor into and out of contact with
the material until completion of the kraft cook thereof, at a rate of at least
100 tons of pulp per day. This method is preferably practiced utilizing a
batch digester having a capacity of at least 8 tons/day (e.g. 8-20), and by
the
further step (b), prior to step (a), of filling the digester with cellulose
material, and the further step (c), after step (a) of discharging kraft pulp
from the digester. The invention also relates to a batch digester system for
practicing this aspect of the invention, each batch digester having a
capacity of at least 8 tons per day (i.e. of commercial size as compared to
laboratory size).
The invention also relates to a modification of a number of different types
of continuous digesters, conventional MCC~ Kamyr, Inc. digesters or
EMCC~ Kamyr, Inc. digesters, to achieve significant dilution of the
effective DOM of the cooking liquor during at least one early or
intermediate stage of the cook. I3y arranging the extraction and
recirculation screens in a particular way, the advantageous results
according to the invention can be achieved in existing digesters merely by
re-routing various fluid flows and introducing low DOM dilution liquor
and/or white liquor at various points, in all conventional types of
continuous digesters including single vessel hydraulic, two vessel
hydraulic, etc.
It is the primary object of the invention to produce increased strength kraft
pulp, and/or also typically reducing H factor and alkali consumption, and
increasing bleachability. This and other objects of the invention will
become clear from an inspection of the detailed description of the
CA 02273146 1999-06-15
_8_
invention and from the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic illustration of one exemplary embodiment of
continuous kraft cooking equipment according to the invention, for
practicing exemplary methods according to the present invention;
Figures 2 and 3 are graphical representations of the strength of pulp
produced according to the present invention compared with kraft pulp
produced under identical conditions only not practicing the invention;
Figure 4 is a schematic view of exemplary equipment for the improved
method of batch kraft cooking according to the invention;
Figure 5 is a schematic side view of another embodiment of exemplary
batch digester according to the present invention;
Figure 6 is a graphical representation of the H factor for producing pulp
according to the invention compared with kraft pulp produced under
identical conditions not practicing the invention;
Figure 7 is a graphical representation of the consumed effective alkali
during the production of pulp according to the present invention
compared with the production of pulp under identical conditions only not
practicing the invention;
Figure 8 is a graphical representation of the effective alkali consumed vs. a
percentage of mill liquor compared to DOM-free liquor;
CA 02273146 1999-06-15
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Figure 9 is a graphical representation comparing brightness response for
pulps produced according to the present invention compared with kraft
pulp produced under identical conditions not practicing the invention;
Figures 10 through 14B are further graphical representations of various
strength aspects of pulp produced according to the present invention;
Figures 12A-B being compared with k:raft pulp produced under identical
conditions only not practicing the invention;
Figure 15 is a graphical representation of DOM concentrations based upon
actual liquor analysis for lab cooks with three different sources of liquor at
various stages during cooking;
Figure 16 is a schematic illustration of an exemplary digester of a two
vessel hydraulic cooking system which practices the present invention;
Figure 17 is a graphical representation of a theoretical investigation
comparing DOM concentration in a conventional MCC~ digester
compared with the digester of Figure 1~5;
Figures 18 through 20 are schematic illustrations of other exemplary
digesters according to the present invention; and
Figures 21 through 25 are graphical representations of theoretical
investigations of varying dilution and extraction parameters using the
digester of Figure 19.
CA 02273146 1999-06-15
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DETAILED DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates a two vessel hydraulic kraft digester system, such as
that sold by Kamyr, Inc. of Glens Falls, New York modified to practice
exemplary methods according to the present invention. Of course any
other existing continuous digester systems also can be modified to practice
the invention, including single vessel hydraulic, single vessel vapor
phase, and double vessel vapor phase digesters.
In the exemplary embodiment illustrated in Figure 1, a conventional
impregnation vessel (IV) 10 is connected to a conventional vertical
continuous digester 11. Comminuted cellulosic fibrous material entrained
in water and cooking liquor is transported from a conventional high
pressure feeder via line 12 to the top of the IV 10, and some of the liquor is
withdrawn in line 13 as is conventional and returned to the high pressure
feeder.
According to the present invention, in order to reduce the concentration
of DOM (as used in this specification and claims, dissolved organic
materials, primarily dissolved hemicel:lulose and lignin, but also dissolved
cellulose, extractives, and other materials extracted from wood by the kraft
cooking process) liquor is withdrawn by pump 14 in line 15 (or from the
top of vessel 10) and treated at stage 16 to remove or passivate DOM, or
selected constituents thereof. The stage 16 may be a precipitation stage (e.g.
by lowering pH below 9), an absorption stage (e.g. a cellulose fiber column,
or activated carbon), or devices for practicing filtration (e.g.
ultrafiltration,
microfiltration, nanofiltration, etc.)solvent extraction, destruction (e.g. by
bombardment with radiation),supercritical extraction, gravity separation,
or evaporation (followed by condensation).
Replacement liquor (e.g. after stage 16) may or may not be is added to the
CA 02273146 2003-05-14
4
-11-
line 13 by pump 14' in line 17, depending upon whether impregnation is
practiced co-currently or counter-currently. The replacement liquor added in
line 17, instead of extracted liquor treated in stage 16, may be dilution
liquor,
e.g. fresh (i.e. substantially DOM-free) white liquor, water, washer filtrate
(e.g.
brown stock washer filtrate), cold blow filtrate, or combinations thereof. If
it is
desired to enhance the sulfidity of the liquor being circulated in the lines
12,
13, black liquor may be added in line 17, but the black liquor must be treated
so as to effect passivation of the DOM therein, as will be described
hereafter.
In any event, the liquor withdrawn at 15 has a relatively high DOM
concentration, while that added in 17 has a much lower effective DOM level,
so that pulp strength is positively affected.
In the impregnation vessel 10 itself the DOM is also controlled preferably
utilizing a conventional screen 18, pump 19, and reintroduction conduit 20.
To the liquid recirculated in conduit 20 is added - as indicated by line 21 -
dilution liquid, to dilute the concentration of the DOM. Also the dilution
liquid
includes at least some white liquor. That is the liquor reintroduced in
conduit
20 will have a substantially lower effective DOM level than the liquor
withdrawn through the screen 18, and will include at least some white liquor.
A treatment stage 16' - like stage 16 - also may be provided in conduit 20 as
shown in dotted line in Figure 1.
From the bottom of the IV 10, the slurry of comminuted cellulosic fibrous
material passes through line 22 to the top of the digester 11, and as is
known,
some of the liquid of the slurry is withdrawn in line 23, white liquor is
added
thereto at 24, and passes through a heater (typically an indirect heater) 25,
and then is reintroduced to the bottom of the IV 10 via line 26 and/or
introduced close to the start of the conduit 22 as indicated at 27 in Figure
1.
CA 02273146 1999-06-15
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In existing continuous digesters, usually liquid is withdrawn at various
levels of the digester, heated, and then reintroduced at the same level as
withdrawn, however under normal circumstances liquor is not extracted
from the system and replaced with fresh reduced-DOM liquor. In existing
continuous digesters, black liquor is extracted at a central location in the
digester, and the black liquor is not reintroduced, but rather it is sent to
flash tanks, and then ultimately passed to a recovery boiler or the like. In
contra-distinction to existing continuous digester, the continuous digester
11 according to the present invention actually extracts liquor at a number
of different stages and heights and replaces the extracted liquor with liquor
having a lower DOM concentration. This is done near the beginning of the
cook, in the middle of the cook, and near the end of the cook. By utilizing
the digester 11 illustrated in Figure 1, and practicing the method according
to the invention, the pulp discharged in line 28 has increased strength
compared to conventional kraft pulp treated under otherwise identical
conditions in an existing continuous d igester.
The digester 11 includes a first set of withdrawal screens 30 adjacent the top
thereof, near the beginning of the cook, a second set of screens 31 near the
middle of the cook and third and fourth sets of screens 32, 33 near the end
of the cook. The screens 30-33 are connected to pumps 34-37, respectively,
which pass through recirculation 1i es 38-41, respectively, optionally
including heaters 42-45, respectively, these recirculation loops per se being
conventional. However according to the present invention part of the
withdrawn liquid is extracted, in the lines 46-49, respectively, as by passing
the line 46 to a series of flash tanks 50, as shown in association with the
first set of screens 30 in Figure. 1.
To make up for the extracted liquor,, which has a relatively high DOM
concentration, and to lower the DOM level, replacement (dilution) liquor
CA 02273146 2003-05-14
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Is added, as indicated by lines 51 through 54, respectively, the liquor added
in
the lines 51 through 54 having a significantly lower effective DOM
concentration than the liquor extracted in lines 46-49, so as to positively
affect
pulp strength. The liquor added in lines 51 through 54 may be the same as
the dilution liquors described above with respect to line 17. the heaters 42-
45
heat the replacement liquor, as well as any recirculated liquor, to
substantially
the same temperature as (typically slightly above) the withdrawn liquor. Any
number of screens 30-33 may be provided in digester 11.
Prior to transporting the extracted liquor to a remote site and replacing it
with
replacement liquor, the extracted liquor and the replacement liquor can be
passed into heat exchange relationship with each other, as indicated
schematically by reference numeral 56 in Figure 1. Further, the extracted
liquor can be treated to remove or passify the DOM therein, and then be
immediately reintroduced as the replacement liquor (with other, dilution,
liquor
added thereto if desired). This is schematically illustrated by reference
numeral 57 in Figure 1 wherein the extracted liquor in line 48 is treated at
station 57 (line stage 16) to remove DOM, and then reintroduced at 53. White
liquor is also added thereto as indicated in Figure 1, as a matter of fact at
each of the stages associate with the screens 30-33 in Figure 1 white liquor
can be added (to lines 51-54, respectively).
Another option for the treatment block 57 - schematically illustrated in
Figure
1 - is black; liquor pressure heating. From the screens 32 liquor that may be
considered "black liquor" is withdrawn, and a portion extracted in line 48.
The
pressure heating in stage 57 may take place according to U.S. Patent No.
4,929,307. Typically, in stage 57 the black liquor would be heated to between
about 170-350°C (preferably above 190°C, e.g. at about
240°C( at
superatmospheric pressure for about 5-90 minutes (preferably about 30-60
CA 02273146 2001-02-15
14
minutes), at least 20°C over cooking temperature. This results in
significant passivation of the DOM, and the black liquor may then be
returned as indicated by line 53. The treatment stage illustrated
schematically at 58 in Figure 1, associated with the last set of
withdrawal/extraction screens 33, is like stage 16. A stage like 58 may be
provided, or omitted, at any level of the digester 11 where there is
extraction instead of ad d ing dilution liquor. White liquor may be added at
58 too, and then the now DOM-depleted liquor is returned in line 54.
Whether treated extracted liquor or dilution liquor is utilized, according to
the invention it is desirable to keep the total DOM concentration of the
cooking liquor at 100 g% 1 or below during substantially the entire kraft
cook (bulk delignification), preferably below about 50 g/1; and also to keep
the lignin concentration a t 50 g/ 1 or below (preferably about 25 g/ 1 or
less),
and the hemi-cellulose concentration at 15 g/1 or less (preferably about 10
g/1 or below). The exact: commercially optimum concentration is not yet
known, and may differ depending upon wood species being cooked.
Figures 2 and 3 illustrate the results of actual laboratory testing pursuant
to
the present invention. Figure 2 shows tear-tensile curves for three
different laboratory kraft cooks all prepared from the same wood furnish.
The tear factor is a measure of the inherent fiber and pulp strength.
In Figure 2 curve A is pulp prepared utilizing conventional pulp mill
liquor samples (from an MCC~ commercial full scale pulping process) as
the cooking liquor. Curve B is obtained from a cook where the cooking
liquor is the same as in curve A except that the liquor samples were heated
at about 190°C for one hour, at superatmospheric pressure, prior to use
in
the cook. Curve C is a cook which used synthetic white liquor as the
cooking liquor, which synthetic white liquor was essentially DOM-free,
(i.e. less than 50 g/1). The cooks for curves A and B were performed such
CA 02273146 1999-06-15
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that the alkali, temperature (about 160°C), and DOM profiles were
identical
to those of the full-scale pulping process from which the liquor samples
were obtained. For curve C the alkali and temperature profiles were
identical to those in curves A and B, but no DOM was present.
Figure 2 clearly illustrates that as a result of low DOM liquor contacting the
chips during the entire kraft cook, there is approximately a 27% increase in
tear strength at 11 km tensile. Passivation of the DOM utilizing pressure
heating of black liquor, pursuant to curve B according to the invention,
also resulted in a substantial strength increase compared to the standard
curve A, in this case approximately a 15% increase in tear strength at 11
km tensile.
Figure 3 illustrates further laboratory work comparing conventional kraft
cooks with cooks according to the invention. The cooks represented by
curves D through G were prepared utilizing identical alkali and
temperature profiles, for the same wood furnish, but with varying
concentrations of DOM for the entire kraft cook. The DOM concentration
for curve D, which was a standard MCC~ kraft cook (mill liquor) was the
highest, and the DOM concentration for curve G was the lowest
(essentially DOM-free). The DOM concentration for curve E was about 25%
lower than the DOM concentration for curve D, while the DOM
concentration for curve F was about 50% lower than the DOM
concentration for curve D. As can be seen, there was a substantial increase
in tear strength inversely proportional to the amount of DOM present
during the complete cook.
Cooking according to the invention i.s preferably practiced to achieve a
pulp strength (e.g. tear strength at a specified tensile for fully refined
pulp,
e.g. 9 or 11 km) increase of at least about 10%, and preferably at least about
15%, compared to otherwise identical conditions but where DOM is not
CA 02273146 1999-06-15
-16-
specially handled.
While with respect to Figure 1 the invention was described primarily with
respect to continuous kraft cooking, the principles according to the
invention are also applicable to batch k:raft cooking.
Figure 4 schematically illustrates conventional equipment that may be
used in the practice of the Beloit RDI~T"' batch cooking process, or for the
Sunds Super BatchTM process. The system is illustrated schematically in
Figure 4 includes a batch digester 60 having withdrawal screen 61, a source
of chips 62, first, second and third accumulators 63, 64, 65, respectively, a
source of white liquor 66, a filtrate tank: 67, a blow tank 68, and a number
of
valuing mechanisms, the primary valuing mechanism illustrated
schematically at 69. In a typical conventional operating cycle for the Beloit
RDHTM process, the digester 60 is filled with chips from source 62 and
steamed as required. Warm black liquor is then fed to the digester 60. The
warm black liquor typically has high sulfidity and low alkalinity, and a
temperature of about 110-125°C, and is provided by one of the
accumulators (e.g. 63). Any excess warm black liquor may pass to a liquor
tank and ultimately to evaporators, and then to be passed to chemical
recovery. After impregnation, the warm black liquor in digester 60 is
returned to accumulator 63, and then the digester 60 is filled with hot black
and white liquor. The hot black liquor may be from accumulator 65, and
the hot white liquor from accumulator 63, ultimately from source 66.
Typically the white liquor is at a temperature of about 155°C, while
the hot
black liquor is at a temperature of about 150-165°C. The chips in the
digester 60 are then cooked for the predetermined time at temperature to
achieve the desired H factor, and then the hot liquor is displaced with
filtrate direct to the accumulator 65, the filtrate being provided from tank
67. The chips are cold blown by compressed air, or by pumping, from the
vessel 60 to the blow tank 68.
CA 02273146 1999-06-15
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During the typical RDHTM process, white liquor is continuously preheated
with liquor from the hot black liquor accumulator and then is stored in
the hot white liquor accumulator 64. The black liquor passes to the warm
weak black liquor accumulator 63, and the warm black liquor passes
through a heat exchanger to make hot water and is stored in an
atmospheric tank before being pumped to the evaporators.
With regard to Figure 4, the only significant difference between the
invention and the process described above is the heating of the black
liquor, which may take place directly in accumulator 65, in such as way as
to effect significant passivation of they DOM therein. For example this is
accomplished by heating the black liquor to at least 20°C above cooking
temperature, e.g. under superatmospheric pressure to at least 170°C for
about 5-90 minutes, and preferably at or above 190°C (e.g.
240°C) for about
5-90 minutes. Figure 4 schematically illustrates this additional heat being
applied at 71; the heat may be from any desired source. During this
pressure heating of the black liquor, off-gases rich in organic sulfur
compounds are produced and withdrawn as indicated at 72. Typically, as
known per se, the DMS (dimethyl sulfide) produced in line 72 is converted
to methane and hydrogen sulfide, and the methane can be used as a fuel
supplement (for example to provide the heat in line 71) while the
hydrogen sulfide can be used to pre-impregnate the chips at source 62 prior
to pulping, can be converted to elementary sulfur and removed or used to
form polysulfide, can be absorbed into white liquor to produce a high
sulfidity liquor, etc. If the heat treatment in accumulator 65 is to about 20-
40°C above cooking temperature, black liquor can be utilized to
facilitate
impregnation during kraft cooking.
Alternatively, according to the invention, in the Figure 4 embodiment, the
valuing mechanism 69 may be associated with a treatment stage, like stage
CA 02273146 1999-06-15
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16 in Figure 1, to remove DOM from cooking liquor being withdrawn
from screen 61 and recirculated to the digester 60 during batch cooking.
Figure 5 schematically illustrates an exemplary commercial (i.e. producing
at least 8, e.g. 8-20, tons of pulp per day) batch digester system 74
according
to the present invention. A laboratory size version of the solid line
embodiment of system 74 as seen in Figure 5 was used to obtain plot C
from Figure 2, and has been in use for many years. The system 74 includes
a batch digester 75 having a top 76 and bottom 77, with a chips inlet 78 at
the top and outlet 79 at the bottom, with a chips column 80 established
therein during cooking. A screen 81 i;; provided at one level therein (e.g.
adjacent the bottom 77) connected to a withdrawal line 82 and pump 83,
leading to a heater 84. From the heater 84 the heated liquid is recirculated
through line 85 back to the digester 75, introduced at a level therein
different than the level of screen 81 (e.g. near the top 76).
Prior to the heater 84, a significant portion (e.g. to provide about three
turnovers of liquid per hour) of the withdrawn lignin in line 82 is
extracted at line 86. This relatively high DOM concentration liquor is
replaced by substantially DOM free (at least greatly reduced DOM
concentration compared to that in line 86) liquor at 87. The substantially
DOM-free liquor added at 87 may have an alkali concentration that is
varied as desired to effect an appropriate kraft cook. A varying alkali
concentration may be used to simul;~te a continuous kraft cook in the
batch vessel 75. Valves 88, 89 may be provided to shut down or initiate
liquor flows, and/or to substitute or supplement the desired treatment
using the system shown in dotted line in Figure 5.
In accordance with the invention, instead of, or supplemental to, the
extraction and dilution lines 86, 87, the desired level of DOM and its
components (e.g. <50 g/1 DOM, <25 g/1 lignin, and < 10 g/1 hemi-
CA 02273146 1999-06-15
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cellulose) may be achieved by treating the extracted liquor for DOM, for
example by passing the high DOM level liquor in line 90 to a treatment
stage 91 -- like the stage 16 in Figure 1 -- where DOM, or selected
constituents thereof, are removed to greatly reduce their concentrations in
the liquor. Makeup white liquor (not shown) can be added too, the liquor
reheated in heater 92, and then returned via line 93 to the digester 75
instead of using lines 90 and 93, lines 86 and 87 can be connected up to
treatment unit 91, as schematically illustrated by dotted lines 95, 96 in
Figure 5.
Other laboratory test data showing advantageous results that can be
achieved according to the present invention are illustrated in Figures 6
through 15. In this laboratory test data, procedures were utilized which
simulate continuous digester operation by sequentially circulating heated
pulping liquor through a vessel containing a stationary volume of wood
chips. Different stages of a continuous digester were simulated by varying
the time, temperature and chemical concentrations used in the
circulations. The simulations used actual mill liquor when the
corresponding stage of a continuous digester was reached in the lab cook.
The effect of minimizing DOM in pulping liquors upon required pulping
conditions (that is, time and temperature) is illustrated in Figure 6. Figure
6 compares the relationship between Kappa number and H factor for
laboratory cooks using mill black liquor and substantially DOM-free white
liquor. The wood furnished for the cooks represented in Figure 6 was a
typical north-western United States soft wood composed of a mixture of
cedar, spruce, pine and fir. The H factor is a standard parameter which
characterizes the cooking time and temperature as a single variable and is
described, for example, in Rydholm Pulping Processes, 1965, page 618.
Line 98 in Figure 6 shows the relationship of Kappa number to H factor for
CA 02273146 1999-06-15
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a lab cook using mill liquor (collected at a mill and then used in a
laboratory batch digester). A lower line, 99, indicates the relationship of
Kappa number to H factor for a lab cook using substantially DOM-free
white liquor manufactured in the lab. Lines 98, 99 indicate that for a given
Kappa number, the H factor is substantially lower when the DOM is lower,
for example, for Kappa number 30 in Figure 6, there being approximately a
100 H factor units difference. This means that for the same furnish with
the same chemical charge if lower DOM cooking liquor is utilized, a less
severe cook (that is, less time and lower temperature) than for a
conventional kraft cook is required. For example, by extracting liquor
containing a level of DOM substantial enough to adversely affect the H
factor, and replacing some or all o:E the extracted liquor with liquor
containing a substantially lower effecaive DOM level than the extracted
liquor so as to significantly reduce the H factor; preferably the steps are
practiced to decrease the H factor at least about 5% to achieve a given
Kappa number, and the steps are practiced to keep the effective DOM
concentration at about 50 g/1 or less during the majority of the kraft cook.
As illustrated in Figure 7, when utilizing reduced DOM concentration
according to the present invention, the effective alkali (EA) consumed is
reduced. EA is an indication of the amount of cooking chemicals,
particularly NaOH and Na2S used in a cook. The results obtained in Figure
7 were obtained utilizing the same furnish as in Figure 6, and the two
graph lines 100, 101 were obtained at the same conditions. Line 100
indicates the results when the cooking liquor was conventional mill
liquor, while line 101 shows the results when the cooking liquor was
substantially DOM-free white liquor. At a Kappa number of 30, the DOM-
free cook consumed approximately 30% less alkali (i.e. 5% less EA on
wood) than the conventional mill liquor cook. Thus, by extracting liquor
containing a level of DOM substantial enough to adversely affect the
amount of effective alkali consumed to reach a particular Kappa number,
CA 02273146 2003-05-14
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and replacing some or all of the extracted liquor with a liquor containing a
substantially lower effective DOM level, the amount of effective alkali
consumed to reach a particular Kappa number may be significantly reduced,
e.g., the amount of alkali consumed may be decreased by at least about 0.5%
on wood (e.g. about 4% on wood) to achieve a particular Kappa number.
Both the beneficial H factor and EA consumption results illustrated in Figures
6 and 7 may be achieved by replacing extracted relatively-high DOM liquor
with water, substantially DOM-free white liquor, pressure heat treated black
liquor, filtrate, and combinations thereof.
FIGURE 8 provides a further graphical representation of effective alkali
consumption compared to the percentage of mill liquor to substantially DOM-
free white liquor. Line 101A indicates that for the same relative Kappa
number, the effective alkali consumed decreases with decreasing percent mill
liquor (that is, increasing percent substantially DOM-free white liquor).
Table
1 below shows the actual lab results which were used to make the line 101A
of Figure 8.
Table 1
Effective Alkali Consumption
Cook NumberA3208 A3219 A3216 A3239 A3217
DescriptionMill Liq 75% mill 50% mill 25% mill Lab Liq
Total EA 15.8 16.5 14.9 15.7 14.0
consumed,
Kappa, 30.7 30.6 28.0 29.8 30.8
screened
Reduction or elimination of DOM in pulping liquor also improves the ease with
which the resulting pulp is bleached, that is, its bleachabillity.
CA 02273146 1999-06-15
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Figure 9 illustrates actual laboratory test results showing how the
brightness of a bleached cedar-spruce-pine-fir pulp increases with the
increase of bleaching chemical dosage. The parameter plotted on the X-axis
of the graph of Figure 9, the "full sequence Kappa factor", is a ratio of
equivalent chlorine dosage to the incoming Kappa number of the pulp.
That is, it is a somewhat normalized ratio of chlorine used to initial lignin
content of the brownstock pulp. Figure 9 thus shows how pulp brightness
responds to the amount of bleaching chemical used.
The curves 102, 103, 104 and 105 of Figure 9 are, respectively, substantially
DOM-free white liquor (102), conventional mill liquor (103), a mill-cooked
pulp (not a laboratory pulp using mill liquor) (104), and mill heat treated
black liquor which was heat-treated (105). These graphical representations
clearly indicate that the best bleachability is achieved when substantially
DOM-free liquor is used for the cooking liquor. Thus, by extracting liquor
containing a level of DOM substantial enough to adversely effect the
bleachability of the pulp, and replacing some or all of the extracted liquor
with liquor containing a substantially lower effective DOM, the
bleachability of the pulp produced may be significantly increased, for
example, at least one ISO brightness unit at a particular full sequence
Kappa factor. Alternatively, this data indicates that a specific ISO
brightness
can be achieved while using a reduced bleaching chemical charge.
However, graph line 105 indicates that while heat treated black liquor may
improve delignification (see Figure 2), the residual lignin may not be as
easily removed. Thus, the treated black liquor may not be desirable for use
as a dilution liquor where increased bleachability is desired, but rather
water, substantially DOM-free white liquor, and filtrate (as well as
combinations thereof) would be more suitable as dilution liquors.
However, the heat-treated liquor may be used for pulp that is not bleached,
i.e., unbleached grades.
CA 02273146 2001-02-15
23
As earlier discussed, reducing the DOM concentration of pulping liquors
appears to have the most dramatic effect upon pulp strength. This is
further supported by data graphically illustrated in Figures 10 through 14B.
All of this data is for thc~ same cedar-spruce-pine-fir furnish as discussed
above with respect to Figures 6 through 9, and this data indicates that
under the same cooking .conditions the tear strength significantly increases
as the amount of DOM increases. For example, Figure 10 indicates that the
tear strength at 11 km increases (see line 106) as the amount of mill liquor
decreases (and thus the amount of substantially DOM-free white liquor
increases) for the laboratory cooks illustrated there. Figure 11 indicates the
same basic relationship by graph line 107, which plots percentage mill
liquor versus tear at 600 CSF.
Table 2 below shows the tear strength at two tensile strengths for lab cooks
performed with various liquors, with a tear for a mill-produced pulp
shown for comparison. The data from cooks 2 and 3 in Table 2 indicate a
twenty percent (20%) increase for tear at 10 km tensile for the lab cook with
substantially DOM-free white liquor compared with a lab cook using mill
liquor, and a twelve percent (12%) increase is indicated for tear at 11 km
tensile. Lab cooks 4, 5, and 6 in Table 2 show the result of replacing DOM-
free liquor in specific parts of the cook with corresponding mill liquor. For
example, in cook 4 the liquor from the bottom circulation, BC, line
replaced the lab-made liquor in the BC stage of the lab cook. Similarly, in
cook 5 BC and modified cook, MC, mill liquor was used in the lab cook in
the BC and MC stages, while substantially DOM-free liquor was used in the
other stages. The data irc Table 2 indicate that minimization of DOM is
critical throughout the cook, not simply in later stages, and fully supports
the analysis provided above with respect to Figures 2 and 3.
CA 02273146 1999-06-15
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Table 2
Effect of Dissolved Organics on Pulp Tear Strength for Hemlock Furnish
Cooking InstructionsTear C~ 10 km Tear Q 11 km
1) Mill Cook 123 N/A
2) Lab Cook w/Mill (A) 174 156
Liquor
Average 17 _150
173.5 153
3) Lab Cook (A) 207 174
w / Lab Liduor ~B) 206, _l,~
Average 20E~.5 172
4) Lab Cook 183 159
5) Lab Cook 181 157
w/ Mill BC and MC
Liquor
6) Lab Cook 187 N/A
w/Mill Wash Circulation
Liquor
Figures 12A - 14B illustrate the effect of DOM upon bleached pulp strength.
Figure 12A shows the tear and tensilE~ strength for unbleached pulp, line
108 showing pulp produced by substantially DOM-free lab liquor, line 109
from pressure-heat treated black liquor, and line 110 from conventional
mill liquor. Figure 12B shows the tear versus tensile relationship after the
pulps graphically illustrated in Figure 12A were bleached utilizing the
laboratory bleach sequence of DEoD(nD). Line 111 shows the substantially
DOM-free-white-liquor-produced, bleached pulp; line 112, the pressure-
heat-treated-mill-liquor-produced pulp; and line 113, a conventional mill-
liquor-produced, bleached pulp, while,, for comparison, line 114 shows the
strength of the mill pulp taken from the decker, after bleaching. Figure 12B
shows that not only is the substantially DOM-free cooked pulp stronger
than the mill liquor pulp, but this relative strength is maintained after
bleaching. The heat treated liquor cooked pulp also maintains higher
CA 02273146 1999-06-15
-25-
strength than the mill liquor cooked pulp after bleaching, but the
difference in strength after bleaching is minimal.
Figures 13A and 13B plot the results of testing of the same cooks/bleaches
as Figures 12A and 12B only tear factor is plotted against Canadian
standard freeness (CSF). Line 115 is substantially DOM-free pulp; line 116;
pressure-heat-treated-mill- liquor-produced pulp; line 117, mill-liquor
produced pulp; line 118, bleached, substantially DOM-free-produced pulp;
line 119, pressure-heat-treated-liquor-produced, bleached pulp; line 120,
bleached, mill-liquor-produced pulp; and line 121, taken at the mill decker.
Figures 14A and 14B are plots of same cooks/bleaches as in Figures 12A
and 12B only plotting tensile vs. frE~eness. Line 122 is for mill-liquor-
produced pulp; line 123, for pressure-heat-treated-mill-liquor produced
pulp; line 124, for substantially DOM-free produced pulp; line 125, for mill-
liquor-produced, bleached pulp; line 126, for substantially DOM-free
liquor-cooked, bleached pulp; line 127, at the decker; and line 128, for
pressure-heat-treated-mill-liquor-cooked, bleached pulp. Figures 14A and
14B show that tensile declines for both heat-treated-liquor-cooked pulp
and substantially DOM-free-liquor-cooked pulp, however Figure 14B
shows that the bleaching reduces the relative tensile strength of the heat-
treated liquor pulp below that of the L>OM-free liquor cooked pulp. Again,
as noted above, the heat-treated-liquor process may be suitable for
unbleached pulps.
The laboratory cooks discussed above all simulated the pulping sequence
of a Kamyr, Inc. MCC~ continuous digester. Each lab cook has a
corresponding impregnation stage, co-current cooking stage, counter-
current MCC~ cooking stage, and a counter-current wash stage. Typical
DOM concentrations based upon actual liquor analysis are shown in
Figure 15 for lab cooks with three sources of liquor. The line 130 is for mill
CA 02273146 2003-05-14
-26-
liquor; line 131, for 50% mill liquor and 50% substantially DOM-free lab white
liquor; and the X's 132, for 100% substantially DOM-free lab white liquor. In
Figure 15, note that at time = 0, the beginning of impregnation, all lab
liquors
used were DOM-free. This was done because there was no reliable method
of sampling the liquor at this stage of the cook in the mill. Thus, the DOM
concentrations of the mill and 50/50 liquor cooks at the end of impregnation
are lower than expected for this set of data, and more representative
concentrations are extrapolated and shown in parenthesis in Figure 15.
Figure 15 does show how each of the concentrations follow a consistent trend
throughout the cook, the concentrations gradually increasing until the
extraction stage and then gradually decreasing during the countercurrent
MCC~ and wash stages. Even with a substantially DOM-free source of
liquor, of course, DOM is released into the liquor as cooking proceeds.
Figure 16 illustrates an exemplary continuous digester system 133 that
utilizes the teachings of the present invention to produce pulp of increased
strength. System 133 comprises a conventional two-vessel Kamyr, Inc.
continuous hydraulic digester with MCC. cooking, the impregnation vessel not
being shown in Figure 16, but the continuous digester 134 being illustrated.
Figure 16 illustrates a retrofit of the conventional MCC~ digester 134 in
order
to practice the lower DOM cooking techniques according to the present
invention.
The digester 134 includes an inlet line 137 at the top thereof and an outlet
136 at the bottom thereof for produced pulp. A slurry of comminuted cellulose
fibrous material (wood chips) is supplied from the impregnation vessel in
inlet
line 137 to the inlet 135. A top screen assembly 138 withdraws some liquor
from the introduced slurry in line 139 which is fed back to the BC heaters and
the impregnation vessel. Below the top screen assembly 138 is an extraction
screen assembly 140 including a line 141 therefrom leading to a
CA 02273146 1999-06-15
-27-
first flash tank 142, typically of a series of flash tanks. Below the
extraction
screen assembly 140 is a cooking screen assembly 143 which has two lines
extending therefrom, one line 144 providing extraction (merging with the
line 141), and the other line 145 leading to a pump 145'. A valve 146 may
be provided at the junction between the lines 144, 145 to vary the amount
of liquor passing in each line. The liquor in line 145 passes through a
heater 147 and a line 148 to return to the interior of the digester 134 via
pipe 151 opening up at about the level of the cooking screen assembly 143.
A branch line 149 also may introduce recirculated liquid in pipe 150 at
about the level of the extraction screens 140. Below the cooking screen
assembly 143 is the wash screen assembly 152, with a withdrawal line 153
leading to the pump 154, passing liquor through heater 155 to line 156 to be
returned to the interior of the digester 134 via pipe 157 at about the level
of
the screen 152.
For the system 133, the mill has presently increased the digester's
production rate beyond the production rate it was designed for, and
production is presently limited by the volume of liquor that can be
extracted. This limitation can be circumvented by utilizing the techniques
according to the invention, as specifically illustrated in Figure 16. Since
the
amount of extraction in line 141 is limited, this will be augmented
according to the present invention by supplying extraction also from line
144. For example, the rate of extraction will be, utilizing the invention,
typically about 2 tons of liquor per ton of pulp. In effect, 1 ton of liquor
per
ton of pulp extracted at line 144 is replaced with dilution liquor (wash
liquor) from the source 158. This is accomplished in Figure 16 by passing
the wash liquor from source 158 (e.g. filtrate water) through a pump 159,
and valve 160, the majority of the wash liquor (e.g. 1.S tons liquor per ton
of pulp) being introduced in line 161 to the bottom of the digester, while
the rest (e.g. 1 ton of liquor per ton of pulp) passing in line 162 into the
line
145 to provide the dilution liquor. Also, substantially DOM-free white
CA 02273146 1999-06-15
-28-
liquor from source 163 may be added in line 164 to the line 145 prior to
heater 147, and recirculation back to the digester through pipes 150 and/or
151. Of course, white liquor may also be added to the wash circulation in
line 153 (see line 165) to effect EMC:C~ cooking. The flow arrows 166
illustrate the co-current zone in digester 134. As a result of the
modifications illustrated in Figure 16, the counter-current flow in the
MCC~ cooking zone 167 will contain cleaner, DOM-reduced, liquor with
improved results in pulp strength, and in this case also an increase in the
digester 134 production rate.
The effect of the modifications illustrated in Figure 16 upon DOM
concentration has been investigated using a dynamic computer model of a
Kamyr, Inc. continuous digester. Preliminary results of this theoretical
investigation are illustrated schematically in Figure 17. Figure 17 compares
variation in DOM concentration in a conventional MCC~ digester with
the digester illustrated in Figure 16, the conventional MCC~ digester
results being illustrated by line 168, and the digester of Figure 16 results
by
line 169. As can be seen in Figure 17, the DOM concentration at the screen
assembly 143 drops dramatically with the addition of DOM-reduced
dilution, also reducing the DOM in the counter-current flow back up to the
extraction screen assembly 140. Furthermore, the downstream, counter-
current wash liquor contains less DOM since less DOM is being carried
forward with the pulp. Graph lines '.L70, 171, part of the lines 168, 169,
indicate that in the counter-current cooking zone the DOM always
increases in the direction of liquor flow. That is, the counter-current flow
is cooking and accumulating DOM as it passes through the down-flowing
chip mass.
Figures 16 and 17 thus illustrate the dramatic impact of only a single
extraction- dilution upon the DOM profile in a continuous digester, which
DOM reduction may have a corresponding dramatic effect upon resulting
CA 02273146 2003-05-14
-29-
pulp strength.
Figure 18 illustrates another mill variation implementing techniques according
to the invention. This also indicates a digester 134 that is part of a two-
vessel
hydraulic digester. Since many of the components illustrated in Figures 16
and 18 are the same, they are indicated by the same reference numerals.
Only the modifications from one to the other will be described in detail.
In the Figure 18 embodiment, an even more dramatic DOM reduction will
occur. In this embodiment, the screens 140, 143 are reversed compared to
the Figure 16 embodiment, and also another screen assembly 173 is provided
between the screen assemblies 138, 143. The screen assembly 173 is a trim
screen assembly; according to the invention the withdrawal conduit 174
therefrom provides extraction to the flash tank 142.
In the embodiment of Figure 18, as one particular operational examples, two
tons of liquor per ton of pulp will be extracted in line 174, and four tons of
liquor per ton of pulp in line 141. Dilution liquor will be added in line 162
and
substantially DOM-free white liquor in line 164. This will result in the flows
176, 177 illustrated in Figure 18, the digester 134 thus being characterized
as
co-current, counter-current, co-current, counter-current flow (which may be
called alternate-flow continuous cooking).
Figure 19 illustrates another digester system 179 according to the present
invention. In this two-vessel system, the impregnation vessel 180 is
illustrated, having an inlet 181 at the top thereof and an outlet line 182 at
the
bottom. Liquid withdrawn at 183 is recirculated to the conventional high
pressure feeder, while white liquor is added at line 184. Liquor withdrawn at
185 may be passed to an introduction point between the first flash tank 186
and second flash tank 187. The slurry from the outlet line 182 is introduced
at
188
CA 02273146 2003-05-14
-30-
into the top of the digester 189, having a "stilling well" arrangement 190,
from
which liquor is withdrawn at 191 and recirculated to the bottom of the
impregnation vessel 180. The liquor is heated in heater 192 when
recirculated.
Digester 189 also has a trim screen assembly 194 with the withdrawal 195
therefrom in this case merging with the recirculating liquid in line 191.
Cooking screen assembly 196 is provided below the trim screen assembly
194, with liquid withdrawn in line 197 passing through the valve198 into a
line
199, and optionally some of the liquid passing from valve 198 being directed
in line 200 to the flash tank 186. The liquid in line 199 is diluted with
lower
DOM liquor, such as the substantially DOM-free white liquor 201 and the
filtrate 202, before passing through heater 203 and being reintroduced into
the digester 189 by the conduit 204 at about the level of the screen assembly
196. The extraction screen assembly 206 has a withdrawal line 207
therefrom which leads to the flash tank 186. The wash screen assembly 208
includes recirculation line 209 to which white liquor at 210 may be added
before the liquor passes through heater 211, and then is reintroduced by a
conduit 212 at about the level of the wash screen assembly 208. Filtrate
providing wash liquor is added at 213, while the produced pulp is withdrawn in
line 193.
Note that the system 179 has the potential to extract from line 197, through
valve 198 into conduit 200. The dilution liquid in the form of filtrate also
is
preferably added at 214 to the line 182, while substantially DOM-free white
liquor is added at 214'.
Figure 20 illustrates a one vessel hydraulic digester that is modified
according
to the teachings of the present invention, this modification also including
two
sets of cooking screens, as is conventional. This increases the potential for
the introduction of extraction/dilution at two more
CA 02273146 2003-05-14
-31-
locations.
The single vessel hydraulic digester system 215 includes the conventional
components of chips bin 216, steaming vessel 217, high pressure transfer
device (feeder) 218, line 219 for adding cellulose fibrous material slurry to
the
top 220 of the continuous digester 221, and a withdrawal 222 for produced
pulp at the bottom of the digester 221. Some of the liquid has been
withdrawn in line 223 and recirculated back to the high-pressure feeder 218.
The cooking screens are below the line 223, e.g. the first cooking screen
assembly 224 and the second cooking screen assembly 225.
Associated with the first cooking screen assembly 224 is a first means for
recirculating the first portion of liquid withdrawn from the cooking screen
assembly 224 into the interior of the digester 221, including line 226, pump
227, and heater 228, with reintroduction conduit 229 at about the level of the
screen assembly 224. A valve 230 may be provided for extraction prior to the
heater 228, into line 231, while dilution liquid, such as white liquor (e.g.
10%
of the total white liquor utilized) is added by a line 232 just prior to the
heater
228.
Second means for recirculating some withdrawn liquor, and extracting other
withdrawn liquor, is provided for the second cooking screen assembly 225.
This second system comprises the conduit 235, pump 236, heater 237, valve
238, and reintroduction conduit 239. One portion of the liquid is augmented
with dilution liquid in conduit 242 while dilution liquid in the form of white
liquor
is added in line 241, and while some liquor is extracted in line 240. In this
way, the DOM concentration is greatly reduced in the cooking zone adjacent
the screen assemblies 224, 225.
Located below the second cooking screen assembly 225 is extraction screen
assembly 245 having a line 246 extending therefrom to a valve 247.
CA 02273146 2003-05-14
- 32 -
From the valve 247 one line 248 goes to the first flash tank 249 of a recovery
system which typically includes a second flash tank 250. Some of the liquor
in line 246 may be recirculated by directing valve 247 into line 251.
The digester 221 further comprises a third screen assembly 253 located
below the extraction screen assembly 245, and including a valve 254
branching out into a withdrawal conduit 255 and an extraction conduit 256.
That is, depending upon the positions of the valves 247, 254, liquid may flow
from line 246 to line 255, or from line 256 to line 248.
The line 255 is connected by pump 257 to heater 260 and return conduit 261
at about the level of the third screen assembly 253. Dilution liquor is added
to
the line 255 before the heater 260, white liquor (e.g. about 15% of the white
liquor used for cooking) being added via line 258, and dilution liquid, such
as
wash filtrate, from source 243 being added via line 259.
The digester 221 also includes a wash screen assembly 263 including a
withdrawal conduit 264 to which white liquor from source 233 may be added
(e.g. 15% of the total white liquor for the process) via line 265. A pump 266,
heater 267, and return conduit 268 for re-introducing withdrawn liquid at
about the level of the screen assembly 263, are also provided. Wash filtrate
is also added below the screen assembly 263 by conduit 269 connected to
wash filtrate source 243.
In one exemplary operation according to the invention, 55% of the white liquor
used for treatment of the pulp is added in line 271 to impregnate the chips as
they are handled by the high pressure transfer device 218 and sluiced into the
line 219, 5% is added to the high pressure feeder 218 via line 272, 10% is
added, collectively, in lines 232, 241 (e.g. 5% each), and 15% is added in
each of the lines 258, 265.
CA 02273146 1999-06-15
-33-
Utilizing the single vessel hydraulic continuous digester assembly 215 of
Figure 20, a low level of DOM will be maintained, and additionally, there
are numerous modes of operation. For example, at least each of the
following three modes of operation may be provided:
(A) Extended modified continuous cooking with
extraction/dilution at the lower cooking screens: In this mode, the
digester 221 operates with conventional extraction in line 246, and
with extended modified continuous cooking, white liquor being
added in 232, 258, 265. Extraction also occurs in line 240 with a
corresponding dilution liquor added at 242 from the wash filtrate
243, resulting in a DOM-reduced liquor flow either counter- current
or co-current between the extraction screen assembly 245 and the
lower cooking screen assembly 225. Whether the flow is counter-
current or co-current depends upon the values of the extractions at
240, 246.
(B) Extended modified continuous cooking with
extraction/dilution at modified continuous cooking circulation: In
this mode, all of the flows just described with respect to (A) are
utilized and in addition an extraction occurs in line 256, valves 247,
254 being controlled to allow a portion of the liquid from the third
screen assembly 253 (the modified continuous cooking screen
assembly) to pass to line 248. Dilution liquid to make up for this
extraction is added at 259, resulting in yet another reduced DOM,
counter-current liquid flow between the screen assemblies 245, 253.
(C) Displacement impregnation and extraction dilution in
upper cooking screens: This mode may be used alone or with a
conventional modified continuous cooking process, or in addition
CA 02273146 1999-06-15
-34-
to the modes (A) and (B) above. This mode includes extraction at
the upper screen assembly 224, as indicated by a line 231, under the
control of valve 230, and dilution with white liquor in line 232.
Additional dilution can be provided from line 259 (not shown in
Figure 20). This results in displacement impregnation, which
occurs when a counter-current flow at the inlet to the digester is
induced not by an extraction, but by the liquor content of the
incoming chips. Low liquor content of the chips will cause the
hydraulically-filled digester 221 to force dilution flow back up into
the inlet 220 which results in a counter-current flow of reduced
DOM liquor.
The system 215 illustrated in Figure 20 is not limited to the modes A-C
described above, but those modes are only exemplary of the numerous
modified forms the flow can take to utilize the low DOM principles
according to the present invention to produce a pulp of increased strength.
Note that all of the embodiments of Figures 16 and 18 through 20 may be
retrofit to existing mills, and exact det<~ils of how the various equipment is
utilized will depend upon the particular mill in which the technology is
employed. All will result in the benefits of reduced DOM described above,
e.g. enhanced strength, enhanced bleachability, reduced effective alkali
consumption, and/or lower H factor. This is best demonstrated for the
configuration of Figure 19 with respect to Figures 21-25.
In Figure 19, 185 is considered the first extraction, 200 the second
extraction, 207 the third extraction, 214 the first dilution, 202 the second
dilution, and 213 the third dilution.
Figure 21 shows a computer simulation comparison of the DOM profiles
for a standard EMCC~ cook and a similar cook according to the invention
CA 02273146 1999-06-15
-35-
using the system of Figure 19 with extended co-current cooking. In a
standard EMCC~ cook, extraction is From conventional extraction screen
and white liquor is added to the conventional cooking circulation and
wash circulation, with the liquor flow from the top of the digester to the
conventional extraction screens being co-current, while the flow for the
remainder of the digester is counter-current. According to the extended
cocurrent mode of FIGURE 21, the third extraction 207 is the primary
extraction so that co-current cooking takes place all the way to screen
assembly 206. Figure 21 shows the conventional EMCC~ cook by graph
line 275, and the cook according to the extended co-current cooking mode
by graph line 276. In the computer model generating Figure 21, the
tonnage rate was 1200 ADMT/D and the distribution of white liquor was
60% in the impregnation 184, 5% in the BC line 214', 15% in the MCC~
circulation 201, and 20% in the wash circulation 210. At 213 1.5 tons of
liquor per ton of pulp washer filtrate was added as counter-current was
liquid.
As can be seen from Figure 21, although the DOM concentration is
initially reduced in the cooking zone, the DOM concentration is greater in
the counter-current stage. Therefore, little improvement in DOM
concentration is provided with this form of extended co-current cooking
(276). While the computer model does have some limitations, Figure 21
does show that DOM concentration can be varied throughout the cook.
Figure 22 illustrates the theoretical effect of adding white liquor at 201 and
low DOM dilution liquor at 202 in Figure 19. In Figure 22, 1.0 tons of liquor
per ton of pulp washer filtrate is added at 202, along with 0.6 t/tp white
liquor. A corresponding liquor flow of 1.6 t/tp is extracted at 200. As seen
by graph line 277, compared to graph. line 276 of Figure 21, the resulting
DOM concentration drops dramatically between the screens 196, 206.
CA 02273146 1999-06-15
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Figure 23 shows the effect of varying the distribution of washer filtrate to
dilution at 202 and 213. In this case the total washer filtrate of 1.5 + 1.0 =
2.5
t/tp is distributed at 213 and at 202. Graph line 778 shows a simulation for
1 /3 of the dilution liquor being added at 202; 279, 1 /2 at 202; and 280, 2/3
at
202 (the rest at 213 in each case). Thus, it is clear that DOM profile varies
significantly with varying dilution flow, and the more dilution is added to
the cooking zone, the more the DOM decreases there (though increasing in
the wash zone).
Figure 24 illustrates the theoretical effect of varying the extraction at 200.
Graph line 281 predicts the DOM profile where the extraction at 200 is 1.35
t/tp; line 282, where the extraction at 200 is 1.85 t/tp; and line 283, where
the extraction at 200 is 2.6 t/tp. In each case the total 2.5 t/tp dilution is
split
evenly between 202 and 213, and an additional 0.6 t/tp white liquor is
added at 201. Figure 24 clearly shows that the theoretical DOM
concentration in the cooking zone decrease with increased extraction at
200, and is essentially unchanged throughout the counter-current zone.
Therefore, this extraction can be varied to accommodate extraction-screen
pressure drop without affecting the DOM profile very much.
Figure 25 shows the effect of extracting from 185 (the top of the
impregnation vessel 180) to create a zone of counter-current impregnation
while employing extended co-current cooking with dilution. In this case
the reference co-current impregnation vessel data are identical to those
shown in Figure 22. The extraction flow 185 is 1.1 t/tp; the extracted liquor
is not replaced by washer filtrate, but by white liquor at 184. In the
previous
models of Figures 21-24, 60% of the white liquor added was added at 184
and 5% at 214'; in Figure 25, these are :reversed, 5% at 184, and 60% at 214'.
Graph line 284 shows the results for co-current impregnation vessel flow,
while line 285 shows the results for. counter-current flow (60% white
liquor at 214'). Thus, this demonstrates that the theoretical DOM
CA 02273146 1999-06-15
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concentration decreases both in the vessel 180 and in the cooking zone,
and is comparable in the counter-current cooking zone. Thus, lower DOM
concentrations are possible due to extraction in the vessel 180 in addition
to extraction and dilution in the digester 189.
It will thus be seen that according to the present invention, a method and
apparatus have been provided which enhances the strength of kraft pulp
by removing, minimizing (e. g. by dilution), or passifying DOM during any
part of a kraft cook and/or enhancing other pulp or process parameters.
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 appended
claims so as to encompass all equivalent structures, methods, and
products.
