U.S. patent application number 10/994262 was filed with the patent office on 2005-04-28 for pulp cooking with particular alkali profiles.
This patent application is currently assigned to Andritz Inc.. Invention is credited to Henricson, Kaj O., Jiang, Jian E., Kettunen, Auvo K., Stromberg, C. Bertil.
Application Number | 20050087314 10/994262 |
Document ID | / |
Family ID | 22652828 |
Filed Date | 2005-04-28 |
United States Patent
Application |
20050087314 |
Kind Code |
A1 |
Stromberg, C. Bertil ; et
al. |
April 28, 2005 |
Pulp cooking with particular alkali profiles
Abstract
Yield, particularly when treating hardwood chips, can be
improved by at least 1-2% in a kraft cellulose pulping process by
keeping the temperature and effective alkali (EA) low during
impregnation, and by keeping the EA low in at least a first cook
stage. After cooking, the pulp is subjected to cooling low EA
liquor, e.g. to reduce its temperature to below 120.degree. C.
(preferably below 100.degree. C.) with an EA below about 5 g/L
(expressed as NaOH). Continuous treatment in a continuous digester
system is preferred, with the EA below about 20 g/L during
impregnation and the first cook.
Inventors: |
Stromberg, C. Bertil; (Glens
Falls, NY) ; Kettunen, Auvo K.; (Neuvoton, FI)
; Jiang, Jian E.; (Karhula, FI) ; Henricson, Kaj
O.; (Kotka, FI) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
1100 N GLEBE ROAD
8TH FLOOR
ARLINGTON
VA
22201-4714
US
|
Assignee: |
Andritz Inc.
Glens Falls
NY
12801-3686
|
Family ID: |
22652828 |
Appl. No.: |
10/994262 |
Filed: |
November 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10994262 |
Nov 23, 2004 |
|
|
|
09178512 |
Oct 26, 1998 |
|
|
|
Current U.S.
Class: |
162/19 ; 162/237;
162/239; 162/24; 162/90 |
Current CPC
Class: |
D21C 7/10 20130101; D21C
3/26 20130101; D21C 3/02 20130101; D21C 7/14 20130101 |
Class at
Publication: |
162/019 ;
162/024; 162/090; 162/237; 162/239 |
International
Class: |
D21C 003/26; D21B
001/16; D21C 003/02; D21C 007/14 |
Claims
1-24. (canceled)
25. A method of treating comminuted cellulosic fibrous hardwood
chip material to produce cellulose pulp, comprising: (a) treating
the hardwood chip material with a first alkaline liquid having a
first effective alkali concentration and at a temperature less than
120.degree. C.; (b) treating the hardwood chip material with a
second alkaline liquid having a second effective alkali
concentration by adding dilution liquor having a low or
substantially zero alkali concentration, while heating the hardwood
chip material to a temperature above 120.degree. C.; (c) treating
the hardwood chip material with a third alkaline liquid having a
third effective alkali concentration at a temperature greater than
140.degree. C. to delignify the material; and (d) treating the
hardwood chip material with a fourth liquid to cool the material to
a temperature less than 120.degree. C.; wherein the first, second,
third initial effective alkali concentrations are all less than 30
g/L, and wherein the second and third effective alkali
concentrations are about 25 g/L or less.
26. A method as recited in claim 25 wherein (a)-(c) are practiced
with the first, second and third initial alkali concentrations all
less than 25 g/L.
27. A method as recited in claim 25 wherein (a)-(c) are practiced
with the first, second and third initial alkali concentrations all
less than 20 g/L.
28. A method as recited in claim 27 further comprising (e), between
(c) and (d) treating the material with an alkaline liquid having a
fourth effective alkali concentration at a temperature greater than
140.degree. C.
29. A method as recited in claim 28 wherein (e) is practiced at a
fourth initial alkali concentration of greater than 30 g/L.
30. A method as recited in claim 28 wherein (e) is practiced at a
fourth initial alkali concentration of less than about 15 g/L.
31. A method as recited in claim 28 further comprising (f), between
(e) and (d), treating the material at a temperature of greater than
140.degree. C. at a fifth initial alkali concentration of less than
25 g/L.
32. A method as recited in claim 26 wherein (a)-(d) are practiced
using hardwood chips as the comminuted cellulosic fibrous
material.
33. A method as recited in claim 27 wherein (b) is practiced to
heat the material so that its temperature gradually increases, and
so that the second EA concentration gradually increases, but stays
below about 20 g/L.
34. A method as recited in claim 33 wherein (c) is practiced so
that the EA concentration gradually decreases while the temperature
is maintained substantially constant.
35. A method as recited in claim 34 wherein (d) is practiced with a
cooling liquid having an EA of less than 5 g/L so that the EA is
gradually reduced to a final level below 5 g/L while the
temperature is gradually reduced.
36. A method as recited in claim 35 wherein (a) is practiced so
that the EA gradually decreases while the temperature remains
substantially the same.
37. A method as recited in claim 26 wherein (a)-(d) are practiced
continuously.
38. A method as recited in claim 31 wherein (a)-(f) are practiced
continuously in the same vessel.
39. A method as recited in claim 25 wherein (a)-(d) are practiced
so as to increase yield by at least 1% compared to practicing (a)
at a temperature of greater than about 120.degree. C. and an
initial EA of over 30 g/L, and practicing (b) or (c) at an initial
EA of over 30 g/L.
40. A method as recited in claim 27 wherein (a)-(d) are practiced
using hardwood chips as the cellulosic material, and so as to
increase yield by at least 2% compared to practicing (a) at a
temperature of greater than about 120.degree. C. and an initial EA
of over 30 g/L, and practicing (b) or (c) at an initial EA of over
30 g/L.
41. A method of continuously treating hardwood chips to produce
kraft pulp, using a continuous digester system in which a hardwood
chip slurry primarily flows downwardly during treatment,
comprising: (a) impregnating the hardwood chips of the slurry in a
first stage using a first alkaline liquid with an initial EA at the
start of the first stage of about 25 g/L or less, and at a
temperature of between about 90-110.degree. C., the EA gradually
diminishing by at least 10 g/L during the first stage, and so that
at the end of the first stage the EA is about 10 g/L or less; (b)
gradually heating the hardwood chip slurry to a cooking temperature
of about 140-180.degree. C. as the slurry continuously moves
through a second stage substantially contiguous with the first
stage, by treating the slurry with a second alkaline liquid, the EA
of the slurry starting at the beginning of the second stage at less
than 15 g/L, and increasing at least about 5 g/L during the second
stage, but not exceeding about 25 g/L by adding dilution liquor
having a low or substantially zero alkali concentration; (c)
cooking the hardwood chip slurry in a third stage, using a third
alkaline liquid, at a temperature that remains substantially
constant and is between 140-180.degree. C. and at an initial EA at
the start of the third stage of below 25 g/L, and gradually
decreasing by at least about 5 g/L and so that the EA at the end of
the third stage is below 20 g/L; (e) optionally subjecting the
hardwood chips to at least a second cooking in a fourth stage at
approximately the same substantially constant temperature as in the
third stage, using a fourth alkaline liquid; and (d) in a last
stage, using a last alkaline liquid, gradually cooling the hardwood
chips slurry to a temperature less than about 110.degree. C. and so
as to reduce the EA of the slurry at least about 5 g/L from the
beginning to the end of the last stage, and so that the slurry has
a final EA of less than about 5 g/L; wherein (a)-(d) are practiced
so as to increase yield of pulp produced by at least 2% compared to
practicing (a) at a temperature of greater than about 120.degree.
C. and an initial EA of over 30 g/L, and practicing (b) or (c) at
an initial EA of over 25 g/L.
42. A method as recited in claim 41 wherein (e) is practiced, and
so that the EA increases from the beginning to the end of the
fourth stage by at least 10 g/L.
43. A method as recited in claim 42 wherein (a) is practiced at
least in part by extracting liquor from the slurry at approximately
the interface between the first and second stages; and wherein (b)
is practiced at least in part by adding a combination of heated
cooking and dilution liquor at approximately the interface between
the second and third stages so that the heated liquor flows
substantially countercurrent to the chips slurry; and wherein (c)
is practiced at least in part by extracting liquor at approximately
the interface between the third and fourth stages; and wherein (e)
is practiced at least in part by adding a combination of heated
cooking and dilution liquid below the extraction of approximately
the interface between the third and fourth stages, to flow
substantially countercurrent to the hardwood chips slurry; and
wherein (d) is practiced by introducing dilution liquor at a
temperature below about 110.degree. C. adjacent a discharge of pulp
from the digester system.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
[0001] The term "chemical pulping" applies to the process of
treating comminuted cellulosic fibrous material, for example,
hardwood or softwood chips, with an aqueous solution of chemicals
which dissolve the non-cellulose components of the material, and
some of the cellulose components, to produce a slurry of cellulose
fibers that can be used to produce cellulose paper products. The
commercially significant chemical pulping process in the late
twentieth century is the alkaline process. Two such alkaline
processes are the "kraft" process and the "soda" process. In the
kraft process, the active chemicals with which the wood is treated
are sodium hydroxide [NaOH] and sodium sulfide [Na.sub.2S]. The
aqueous solution of hydroxide and sulfide is referred to as "kraft
white liquor". In the soda process, very little or no sodium
sulfide is present.
[0002] The treatment is performed at a temperature of over
100.degree. C., and the process is typically under superatmospheric
pressure, preferably 5-10 bar. The hydroxide is a strong base and
the process is performed in a highly basic, or alkaline, state, for
example, at a pH typically greater than 12. As the chemistry is
presently understood, the hydroxide dissolves the non-cellulose
compounds of the wood which bind the cellulose fibers together,
that is, the "lignin", and the sulfide acts to protect the
cellulose from degradation by the hydroxide.
[0003] The rate of reaction of the kraft and soda pulping processes
is dependent upon the temperature of the reaction and the
concentration of cooking chemical. The higher the temperature and
the higher the chemical concentration, the more rapid the reaction
of the sodium hydroxide, also known simply as "alkali", with the
wood material. The concentration of cooking chemical is typically
expressed as "active alkali" (AA) or "effective alkali" (EA). In
this application the term "effective alkali as equivalent NaOH"
will be used exclusively to express the concentration of cooking
chemical. EA is typically given by the sum of the concentration of
NaOH plus one-half the concentration of Na.sub.2S expressed in
grams per liter (g/L) of equivalent NaOH, that is,
EA=[NaOH]+1/2[Na.sub.2S] g/L NaOH
[0004] In earlier chemical pulping processes employing the kraft
process, in either batch or continuous mode, the cooking chemical
was introduced essentially in its entirety at the beginning of the
treatment. As the treatment progressed the alkali concentration
diminished as the cooking chemicals were consumed in the pulping
reaction. For example, in what is typically referred to as a
"conventional kraft cook", the initial EA concentration to which
the cellulose is exposed may be 40 g/L or higher. This initial EA
then declines during the treatment such that the final EA at the
completion of the cook may approach 5 g/L or lower.
[0005] In the late 1970s and early 1980s, in the pioneering work
done by the Swedish research firm STFI, the benefits of "leveling
out" the alkali profile throughout the cooking process by
decreasing the initial EA concentration and increasing the final EA
concentration was introduced. Johanson, Mjoberg, Sandstrom and
Teder (Svensk Papperstidning, 87(10):30 (1984)), in discussing this
process, calculate an EA concentration, in a continuous digester
employing counter-current treatment, of between 10-15 g/L initially
and 5-10 g/L at the end of the cook. The EA concentrations rise and
fall during the treatment by introducing white liquor and
extracting spent cooking chemical, known as "kraft black liquor".
This process of "split white liquor addition" and counter-current
treatment, known as "modified kraft cooking", was adopted broadly
throughout the pulping industry in the 1980s. For example, the
process and associated equipment were sold under the trademark MCC
by Ahlstrom Machinery Inc., of Glens Falls, N.Y. Later, the
counter-current process was extended even further by the addition
of white liquor to the counter-current wash zone, known as the
Hi-Heat wash zone, in a process marketed by Ahlstrom Machinery
under the trademark EMCC.
[0006] In the 1990s, Marcoccia, et al. introduced the
Lo-Solids.RTM. cooking process and equipment which provided the
next dramatic improvement to the kraft cooking process. See U.S.
Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562;
and 5,662,775. Marcoccia, et al. recognized that the concentration
of dissolved reaction products, including dissolved lignin,
dissolved cellulose, and dissolved hemicellulose, among other
dissolved compounds, were detrimental not only to the latter stage,
or "residual delignification" stage, of the pulping process, as
proposed by Johanson, et al., but also to the principal stage of
delignification, that is, the stage known as the "bulk
delignification" stage. By the selective removal of spent cooking
liquors and replacement with cooking chemical and dilution liquors,
for example washer filtrate having a lower concentration of
dissolved materials, early or at the beginning of the pulping
process a stronger, cleaner cellulose pulp could be produced.
[0007] In co-pending U.S. patent application Ser. No. 08/911,366
filed on Aug. 7, 1990 (Attorney Docket 10-1216), the benefits of
treatment of the cellulose material in a kraft cook at lower
cooking temperature is disclosed. This process is especially
effective when used in conjunction with the Lo-Solids.RTM. cooking
process and equipment described in the above-referenced
patents.
[0008] In all chemical treatment of wood to produce a cellulose
pulp, the cellulose and non-cellulose constituents are not
segregated in the wood but are typically intermingled with each
other. It is difficult to dissolve the undesirable non-cellulose
constituents without dissolving some of the desirable cellulose. As
a result, in the chemical treatment of wood, though the original
wood may typically consist of 60 to 70% desirable cellulose (and
hemicellulose), typically only about 50% of the usable cellulose is
retained in the final product. (It is to be understood that the
product of the pulping process typically also contains other
tolerable, non-cellulose constituents of the wood, such as some
residual lignin.) Some of the desirable cellulose is dissolved at
the same time as the undesirable non-cellulose. This percentage, by
weight, of the amount of cellulose retained compared to the amount
of wood introduced to the process is referred to as the "yield" of
the process. Note that a 1% increase in yield for a typical 1000
ton-per-day pulp mill, which sells pulp at approximately $500.00
per ton, can mean over a million dollars in revenue per year. Thus,
single-digit increases in yield can have significant impact upon
the profitability of a pulp mill.
[0009] In a paper entitled "Improved Pulp Yield by Optimized
Alkaline Profiles in Kraft Delignification" (Presented at the TAPPI
symposium "Breaking the Pulp Yield Barrier" on Feb. 17-18, 1998),
the inventors, and others under the direction of the inventors,
showed that a low and uniform alkali profile and a low temperature
profile in a kraft cook of birch chips improves the cellulose and
hemicellulose yield, as pursuant to the invention. (Specifically,
the yield of the hemicellulose "xylan" is improved.) This
publication discusses laboratory experiments showing the
theoretical basic aspects of the invention, rather than how the
preferred alkali and temperature profiles can be effected in a
commercial pulp mill.
[0010] The present invention comprises or consists of methods and
apparatus for effecting the desired low and uniform alkali
treatment that has been found desirable according to the present
invention. One embodiment of this invention comprises or consists
of a method of treating comminuted cellulosic fibrous material to
produce cellulose pulp, comprising: (a) Treating the material with
a first alkaline liquid having a first effective alkali
concentration and at a temperature less than 120.degree. C. (b)
Treating the material with a second alkaline liquid having a second
effective alkali concentration while heating the material to a
temperature above 120.degree. C. (c) Treating the material with a
third alkaline liquid having a third effective alkali concentration
at a temperature greater than 140.degree. C. to delignify the
material. And (d) treating the material with a cooling liquid to
cool the material to a temperature less than 120.degree. C.;
wherein the first, second, third initial effective alkali
concentrations are all less than 30 g/L, typically less than 25
g/L, preferably less than 20 g/L as NaOH, and the EA concentration
in (b) and (c) is less than 25 g/L, preferably less than 20 g/L.
The cooling liquid typically has an EA of from 0-5 g/L so that the
EA gradually decreases to a level below 5 g/L as the temperature is
gradually reduced; and the temperature of the cooling liquid is
typically below 110.degree. C., e.g. below 90.degree. C.
[0011] The method may also further include (e), between (c) and (d)
of treating the material with a fourth alkaline liquid at a
temperature greater than 140.degree. C., the fourth alkaline liquid
having a fourth effective alkali concentration. This fourth initial
EA concentration may be less than 30 g/L, for example, less than 25
g/L or less than 20 g/L (e.g. about 15 g/L) as NaOH, but according
to the invention the fourth EA concentration need not be limited to
this lower EA concentration. The fourth initial EA concentration
may also be greater than 30 g/L. The method may also further
include (f), between steps (e) and (d), of treating the material
with a fifth alkaline liquid at a temperature greater than
140.degree. C., the fifth alkaline liquid having a fifth initial EA
concentration. The fifth initial EA concentration may be less than
30 g/L, for example, less than 25 g/L or less than 20 g/L as NaOH,
but according to this invention the fifth EA concentration need not
be limited to this lower EA concentration. The fifth initial EA
concentration may also be greater than 30 g/L.
[0012] The desired EA concentrations are preferably achieved by
introducing dilution liquid to the alkaline liquids prior to
contacting it with the material, in particular, at least
introducing dilution liquor to the second alkaline liquor of (b).
This dilution liquor preferably consists of or comprises washer
filtrate, evaporator or heat exchanger condensate, spent cooking
liquor, fresh water, or combinations thereof. It will be understood
by those skilled in the art that the introduction of dilution to
the cooking chemical may also be effected during the preparation,
storage or transfer of the cooking chemical. For example, it is
within the scope of this invention to reduce the alkali
concentration of the cooking chemical introduced to the material by
diluting the cooking chemical during the recausticization process,
or in any other phase of the liquor preparation process.
[0013] The method of the invention preferably produces a cellulose
pulp having increased yield compared with conventional methods
(e.g. (a) with an initial EA over 30 g/L and a temperature over
120.degree. C., and (b) or (c) with an initial EA over 25 g/L), for
example, yield increases of at least 1%, and preferably at least
about 2% can be produced. This is particularly true of the
application of this process to the treatment of hardwood chips, for
example, birch chips.
[0014] In the method described above, the second and fourth and
fifth alkaline liquors may be obtained by adding to cooking liquor
heated dilution liquor having a low or substantially zero alkali
concentration. Preferably (a) is practiced so that the EA gradually
decreases while the temperature remains substantially the same; and
preferably (a)-(d), or (a)-(f), are practiced continuously, in fact
even in the same upright vessel, though these steps may also be
practiced in more than one vessel. Though this specification will
almost exclusively discuss the implementation of the present
invention for continuous treatment, it would be understood by one
skilled in the art that the present invention can also be
implemented in a non-continuous or "batch" process.
[0015] According to another aspect of the invention a method of
continuously treating hardwood chips, using a continuous digester
system where the chip slurry primarily flows downwardly during
treatment, is provided. The method comprises: (a) Impregnating the
hardwood chips of the slurry in a first stage using a first
alkaline liquid with an initial EA at the start of the first stage
of about 25 g/L or less, and at a temperature of between about
90-110.degree. C., the EA gradually diminishing by at least 10 g/L
during the first stage, and so that at the end of the first stage
it is about 10 g/L or less. (b) Gradually heating the hardwood chip
slurry to a cooking temperature of about 140-180.degree. C. as the
slurry continuously moves through a second stage substantially
contiguous with the first stage, by treating the slurry with a
second alkaline liquid, the EA of the slurry starting at the
beginning of the second stage at less than 15 g/L, and increasing
at least about 5 g/L during the second stage, but not exceeding
about 25 g/L. (c) Cooking the hardwood chip slurry in a third
stage, using a third alkaline liquid, at a temperature that remains
substantially constant and is between 140-180.degree. C. and at an
initial EA at the start of the third stage of below 25 g/L, and
gradually decreasing by at least about 5 g/L and so that the EA at
the end of the third stage is below 20 g/L. (e) Optionally
subjecting the hardwood chips to at least a second cooking in a
fourth stage at approximately the same substantially constant
temperature in the third stage, using a fourth alkaline liquid. (d)
And in a last stage, using a last alkaline liquid, gradually
cooling the hardwood chips slurry to a temperature less than about
110.degree. C. and so as to reduce the EA of the slurry at least
about 5 g/L from the beginning to the end of the last stage, and so
that the slurry has a final EA of less than about 5 g/L; wherein
(a)-(d) are practiced so as to increase yield of pulp produced by
at least 2% compared to practicing (a) at a temperature of greater
than about 120.degree. C. and an initial EA of over 30 g/L, and
practicing (b) or (c) at an initial EA of over about 25 g/L.
[0016] In the above method, (e) is practiced, so that the EA
increases from the beginning to the end of the fourth stage by at
least 5 g/L. Also, (a) may be practiced at least in part by
extracting liquor from the slurry at approximately the interface
between the first and second stages; and wherein (b) may be
practiced at least in part by adding a combination of heated
cooking and dilution liquor at approximately the interface between
the second and third stages so that the heated liquor flows
substantially countercurrent to the chips slurry; and (c) may be
practiced at least in part by extracting liquor at approximately
the interface between the third and fourth stages; and (e) may be
practiced at least in part by adding a combination of heated
cooking and dilution liquid below the extraction at approximately
the end of the fourth stage, to flow substantially countercurrent
to the hardwood chips slurry; and (d) may be practiced at least in
part by introducing dilution liquor at a temperature below about
110.degree. C. adjacent a discharge of pulp from the digester
system.
[0017] According to another aspect of the present invention there
is provided a continuous digester system comprising: At least one
substantially upright digester vessel having first, second, third,
fourth, and last consecutive stages, each stage substantially
contiguous with the previous stage and having an interface
therewith. An inlet adjacent the top of the first stage. A first
liquor extraction device at approximately the interface between the
first and second stages, including an extraction screen. A first
withdrawal and recirculating system at approximately the interface
between the second and third stages, including a first
recirculation screen, pump, heater, at least one cooking and
dilution liquor addition conduit, and a recirculation pipe at
approximately the level of the first recirculation screen. A second
extraction screen at approximately the interface between the third
and fourth stages. A second withdrawal and recirculating system at
approximately the interface between the fourth and next consecutive
stage, including a second recirculation screen, pump, heater, at
least one cooking and dilution liquor addition conduit, and a
second recirculation pipe at approximately the level of the second
recirculation screen. Cooling liquor introducing devices adjacent
the bottom of the digester vessel, at the bottom of the last stage.
And a pulp discharge from the last stage, adjacent the bottom of
the digester vessel.
[0018] In the digester system the second recirculation screen may
be substantially at the interface between the fourth and last
stages, or a fifth stage (with associated recirculation system as
described with respect to the above systems) provided at
approximately the interface between the fourth and fifth stages.
All of the first through last stages may be in a single upright
vessel (i.e. the at least one vessel consisting essentially of one
vessel). The first through last stages may also be performed in
more than one vessel. For example, the first stage or the first and
second stages may be performed in a first vessel, for example, in a
pretreatment or impregnation vessel, and the rest of the stages
performed in a second vessel, or digester.
[0019] It is the primary object of the present invention to provide
increased yield in the kraft pulping of cellulose, including
hardwood chips. This and other objects will become clear from a
detailed description of the invention, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a simple block diagram illustrating one exemplary
form of a method of the present invention;
[0021] FIG. 2 is a block diagram illustrating a laboratory
technique that may be used to evaluate a method of the present
invention;
[0022] FIG. 3 is a graph that displays the alkali and temperature
profiles of the laboratory trials of a method shown in FIG. 2.
[0023] FIGS. 4 and 5 are graphs that display the yield results of
the trials shown in FIGS. 2 and 3;
[0024] FIG. 6 is a graph which displays another set of alkali and
temperature profiles for laboratory trials of a method according to
the present invention;
[0025] FIG. 7 is a graph that displays the yield results of
different components of the pulp produced by using the profiles
shown in FIG. 5;
[0026] FIG. 8 shows an exemplary apparatus for practicing a method
of the present invention, along with graphical displays of
representative alkali and temperature profiles at various locations
in the apparatus; and
[0027] FIG. 9 schematically shows a continuous digester, and an
actual alkali profile of the continuous digester operating
according to a method of the present invention, compared to
conventional operation.
DETAILED DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic illustration 10 of the method of the
present invention. The method comprises or consists of a series of
treatments of a slurry of comminuted cellulosic fibrous material,
for example, hardwood chips 11. In the first stage 12, the slurry
is treated with a first alkaline liquid 18 at a temperature less
than 120.degree. C. The initial alkalinity of the liquid 18,
expressed as effective alkali (EA) is typically less than 30 g/L as
NaOH, for example, less than 25 g/L, preferably, between 15 and 25
g/L (or any narrower range within this broad range, e.g. 18-22
g/L). This lower EA is preferably achieved by introducing
low-EA-containing dilution liquid to stage 12, for example, by
adding dilution liquid to the alkaline liquid introduced at 18 or
by adding dilution liquid directly to the slurry by a conduit 18a.
Dilution liquid may comprise or consist of washer filtrate,
evaporator or heat exchanger condensate, weak black liquor, fresh
water, or combinations thereof. The dilution of the cooking liquor
may also be effected during preparation, storage or transfer of the
cooking liquor. The temperature in stage 12 is typically between 80
and 120.degree. C., preferably, between about 90 and 110.degree.
C.
[0029] After treatment at 12, the slurry passes to a second
treatment stage 13 in which the slurry is treated with a second
alkaline liquid 19 while the slurry is heated to a temperature
greater than 120.degree. C. Again, the liquid 19 typically has an
EA concentration of less than 30 g/L as NaOH, for example, less
than 25 g/L, preferably, between 15 and 25 g/L (or any narrower
range within this broad range, e.g. 18-22 g/L). Again, this lower
EA is preferably achieved by introducing low-EA-containing dilution
liquid to stage 13, for example, by adding dilution liquid to the
alkaline liquid introduced at 19 or adding dilution liquid directly
to the slurry by a conduit 19. Again, the dilution liquid may
comprise or consist of washer filtrate, evaporator or heat
exchanger condensate, weak black liquor, fresh water, or
combinations thereof. The heating that occurs during stage 13 is
typically achieved by circulating heated liquor through the slurry.
The temperature of the slurry is typically raised to a temperature
approaching typical kraft or soda cooking temperatures, for
example, to a temperature of at least 140.degree. C., typically
between 140-180.degree. C., preferably, 140-160.degree. C.
[0030] Following the treatment and heating stage 13, the slurry is
then treated with a third alkaline liquid 20 while the temperature
of the slurry is maintained at the temperature above 140.degree.
C., again, typically between 140-180.degree. C., preferably,
140-160.degree. C. The initial EA of the treatment liquid 20 is
again kept to a relatively low concentration of less than 30 g/L as
NaOH, for example, less than 25 g/L, preferably, between 15 and 25
g/L (or any narrower range within this broad range, e.g. 18-22
g/L). This lower EA is preferably achieved by introducing
low-EA-containing dilution liquid to stage 14, for example, by
adding dilution liquid (e.g. the types described below) to the
alkaline liquid introduced at 20 or adding dilution liquid directly
to the slurry by a conduit 20a. During stage 14, typically referred
to as the "bulk delignification" stage, the principal
delignification reaction takes place.
[0031] Stage 14 may be followed by a further delignification stage
15 in which the slurry is treated with alkaline liquid 21. Unlike
the earlier stages, the liquid 21 introduced to stage 15 may
contain a broad range of EA concentrations. For example, a low EA
in stage 15, for example, an initial EA of less than 30 g/L as
NaOH, or less than 25 g/L, or preferably, between 15 and 25 g/L,
can result in the decreased dissolution of hemicellulose and result
in a pulp having greater hemicellulose content. On the other hand,
a higher initial EA in stage 15 can result in more hemicellulose
dissolution and less hemicellulose in the resulting pulp. Since the
content of hemicellulose in the pulp affects the properties of the
resulting paper, the EA of stage 15 can be varied to produce the
desired properties in the resulting pulp. Stage 15 is typically
maintained at a temperature above 140.degree. C., typically between
140-180.degree. C., preferably, 140-160.degree. C. More than one
treatment stage 15 may follow stage 14. In addition, stage 15 may
be omitted, as indicated by the dotted lines, and stage 14 may be
followed immediately by stage 16.
[0032] Stage 16 is a cooling stage in which the treatment is
terminated by introducing cooler liquid 22, typically having a much
lower EA concentration, to the slurry discharged from stage 14 or
15. Typically, liquid 22 is introduced to cool the slurry to a
temperature less than 120.degree. C., typically less than
100.degree. C. Cooling liquid 22 is typically washer filtrate,
evaporator or heat exchanger condensate, weak black liquor, fresh
water, or combinations thereof. The liquid 22 typically has an EA
less than 10 g/L, for example, between 0 and 5 g/L as NaOH (or any
narrower range within this broad range). The slurry 17 discharged
from stage 16 typically comprises or consists of delignified chips
or pulp having little or no EA concentration and a temperature less
than 100.degree. C. Typically, slurry 17 is forwarded on to further
processing, such as brown stock washing for chemical recovery and
bleaching, if desired.
[0033] The method described with respect to FIG. 1 may be effected
in conventional cooking devices, including in a batch digester or a
continuous digester. One preferred apparatus for effecting
continuous treatment is described below. The application to a batch
process is effected by varying the concentration of cooking liquors
and temperature via the liquor circulation common to conventional
batch digesters.
[0034] FIG. 2 illustrates a schematic diagram of a laboratory
procedure used to evaluate the process disclosed in FIG. 1. Stages
32, 33, 34 and 35 correspond to stages 12, 13, 14, and 15,
respectively, of FIG. 1. The cooks were carried out in a university
laboratory as described in the Achrn, et al. article referenced
above. Specifically, the hardwood material used was fresh birch
chipped and screened at a commercial pulp mill. Before cooking, the
chips were screened for the thickness fraction 2-6 mm to be used in
the study. All visually observed pieces or barks and knots were
removed prior to pulping.
[0035] The cooks were divided into four stages, i.e., two
impregnation stages, and two cooking stages. Three EA profiles of
A)-C) were applied in the impregnation stages and the first cooking
stage. Initial EA concentration was adjusted within the range of
4-32 g/L (NaOH) in the cooking stage 2 for all three EA profiles.
Three cooks were prepared from each combination of EA profiles with
a final target kappa number of 24, 18 and 14. EA charges and EA
concentrations used are summarized below.
1TABLE 1 Impregnation Impregnation Cooking Cooking 1 (32) 2 (33)
stage 1 (34) stage 2 (35) EA charge EA charge EA charge Initial EA
Profile on wood % on wood % on wood % concentration, g/L A 10 7 3
5, 8, 12, 18, 25, 32 B 10 7 0 4, 8, 11, 18, 25 C 10 4 3 5, 8, 11,
18, 24
[0036] In the above Table, EA represented as a charge on wood is a
weight percent of the chemical charged per weight of the wood
treated or pulp produced. Note that the EA of the liquid with which
the chips were treated in the tests indicated by FIG. 2 is shown by
the curves of FIG. 3. In this case, a charge of 10% EA on wood
corresponds to an EA of about 22-23 g/L as NaOH, as indicated by
the initial peak of FIG. 3.
[0037] The profile in Table 1 most representative of the invention
is Profile C; however, Profiles A and B are not prior art, merely
less representative of the results achieved according to the
invention.
[0038] The white liquors used in the laboratory cooks were
artificially prepared from technical grade chemicals (NaOH and
Na.sub.2S). The white liquor used in impregnation step 1 (32)
contained 100 g/L. (as NaOH) and its sulphidity was 50%. The same
EA but a lower sulphidity (35%) was used in impregnation step 2
(33) and cooking stage 2 (35) as the white liquor used in cooking
stage 1 (34) contained 200 g/L and had a 35% sulphidity. The birch
black liquor (13 g/L) used in the first and second impregnation
stages was from the commercial pulp mill. The black liquor used in
cooking stage 2 (35) was obtained from cooking stage 1 (34). The
digester used for the steaming as well as for the two-stage
impregnation and cooking stage 1 (34) was a forced circulation unit
with a volume of 25-L. the wood charge was 400 g OD chips. Cooking
stage 2 (35) was carried out in 1-L autoclaves which were kept
rotating in an oil bath. The pulp charge was 100 g OD obtained from
cooking stage 1.
[0039] The birch chips were steamed for 20 min. at 100.degree. C.
and at atmospheric pressure. The steamed chips were then pretreated
in impregnation step (32) with a preheated mixture of black and
white liquor. The impregnation liquor was forced into the digester
with the aid of 4 bar N2 pressure. The impregnation step 1 (32)
lasted for 60 min. at 95.degree. C. and the liquor-to-wood ratio
was 4.5:1. At the end of impregnation step 1 (32) the spent liquor
was drained off and analyzed. The spent liquor contained 5-6
g/L.
[0040] In the impregnation step 2 (33) the liquor consisted of mill
black liquor and water (a:1 v/v), with the appropriate amount of
white liquor to give the desired EA charge. The impregnation liquor
was forced into the digester with the aid of 4 bar N2 pressure. The
impregnation time in this step (32) was about 40 min. including the
time it took to bring the digester from 95.quadrature.C to the
cooking temperature 153.degree. C. At the cooking temperature a
liquor sample was taken and cooking stage 1 (34) began by forcing
white liquor into the digester with the aid of 10 bar N2 pressure.
The liquor-to-wood ratio was 4.6:1 for A) and C)-profiles and 4.5:1
for B). the chips were then delignified for 60 min. for A) and B)
and 80 min. for C), all at 153.degree. C. After cooking stage 1
(34) the liquor was drained off, analyzed, and saved for use in the
subsequent cooking stage 2 (35). The pulp was defibrated in a
Wennberg disintegrator with a minimum amount of water, centrifuged
and homogenized.
[0041] After impregnation step 2 (33) the spent liquor contained
about 10 g/L EA (expressed as NaOH) from both profile A) and B),
and approximately 4 g/L from profile C). After cooking stage 1 (34)
the spent liquor from profile A) contained about 8 g/L, from
profile B) about 4 g/L and from profile C) about 2 g/L. The kappa
number after cooking stage 1 (33) was 44.9 for profile A), 57.0 for
profile B) and 55.3 for profile C).
[0042] The pulps cooked according to profiles A), B) and C) were
divided into portions of 100 g (OD) and delignification was
continued in cooking stage 2 (35) in a 1-L autoclave. The cooking
liquor comprised 0.6 L black liquor from the respective cooking
stage 1 (34) and of varying amounts of water and white liquor to
give the desired EA concentration. The consistency in cooking stage
2 (35) was 10% and cooking temperature was either 153.degree. C. or
165.degree. C. and the cooking time varied to reach the desired
kappa level. At the end of a cook, the autoclave was cooled for 20
min. in tap water. Spent liquor was drained and analyzed. The pulp
was rinsed in water, centrifuged and put in a plastic jar with 2 L
of water. After 16 hours of soaking, the pulp was defibrated in a
2-L disintegrator, washed and screened with a Sommerville screen,
centrifuged, homogenized and weighed. The reject from the
Somerville screen was dried at 105.degree. C. and weighed.
[0043] A hexeneuronic acid (HexA) content analysis was carried out
after application of the acid hydrolysis of the pulp (100.degree.
C., 4 hours at pH 3). The kappa number was determined before and
after the hydrolysis, and the HexA content was calculated by
considering that 1 kappa unit corresponds to 10 meq/kg HexA in the
pulp.
[0044] Selected pulp samples from the C) profile were bleached by
an ECF-bleaching sequence (D.sub.o-E-D) in plastic bags immersed
into a warm water bath. During bleaching the pulp was mixed
manually. The pulp consistency was 10% in all stages. The retention
time was 60 min., 90 and 180 min. respectively. Conditions used in
the ECF sequence (D.sub.o-E-D) are given below.
2 Stage Conditions D.sub.o Retention time 60 min. at 60.degree. C.
ClO.sub.2 dose (act. Cl) 3.6% end pH 2.2-2.3 E Retention time 90
min. at 60.degree. C. NaOH dose 1-2.5% end pH 10.9-11.7 D Retention
time 180 min. at 80.degree. C. ClO.sub.2 dose (act. Cl) 3% end pH
2.4-2.8
[0045] Analysis methods used are summarized below.
3 Dry solids content of chips SCAN CM:39:88 Dry solids content of
pulp SCAN-C 3:78 Effective alkali in cooking liquors SCAN-N 33:94
Kappa number SCAN-C1:77 ISO brightness (pulp) SCAN-C 11:75
Viscosity of pulp SCAN-CM 15:88 Hexeneuronic acid (HexA) content
according to Vuorinen et al. /14/ Carbohydrate content of pulp
according to Laver et al. /16/
[0046] Table 2 on the next page contains the cooking conditions and
results for the trials described with respect to FIG. 2. Table 3,
two pages hence, contains the carbohydrate composition of the pulps
produced in the trials described with respect to FIG. 2. The most
representative results according to the invention are identified as
Profile C in the first column of each of Tables 2 and 3. The data
in FIG. 4 is limited to cooks at 153.degree. C. and the yield data
has been corrected by calculating the equivalent yield for a kappa
number 18. That is, data in Table 2 having higher and lower kappa
numbers compared to kappa 18 was adjusted by 0.2% yield per kappa
unit.
4TABLE 2 Cooking results and key pulping conditions for cooking
stage 2 (35) Cooking Cooking Initial Residual Kappa Total yield
Rejects Viscosity Brightness HexA Profile temp. .degree. C. time,
min EA g/L EA g/L no % on wood % on wood mL/g ISO % meq/kg H-factor
A 153 240 5.0 1.2 18.6 51.1 0.7 1349 33.5 67 872 A 153 120 8.4 4.4
21.1 50.7 1.1 1343 32.5 -- 436 A 153 180 8.4 4.0 18.1 50.9 0.6 1317
34.8 68 654 A 153 150 11.8 7.1 17.9 -- -- 1329 35.0 69 545 A 153
135 17.6 12.2 16.2 49.3 0.8 1341 39.7 64 491 A 153 120 24.2 19.0
15.6 47.7 0.6 1347 42.4 -- 436 A 153 80 32.4 27.4 21.9 47.7 1.2
1439 36.7 -- 291 B 153 360 4.6 0.3 20.0 53.0 0.3 1421 27.1 -- 1308
B 153 500 4.6 0.0 18.9 52.9 0.2 1391 26.4 70 1817 B 153 200 7.9 3.6
21.5 53.8 0.4 1461 30.3 -- 727 B 153 280 7.9 3.0 19.2 53.4 0.1 1403
31.1 -- 1018 B 153 460 7.9 1.8 17.6 51.8 0.5 1309 29.6 77 1672 B
153 150 11.2 6.7 22.4 53.0 0.8 1420 29.6 -- 545 B 153 290 11.2 4.9
17.6 51.7 0.1 1316 30.7 75 1054 B 153 460 11.2 4.0 17.1 50.2 0.3
1251 29.5 -- 1672 B 153 180 17.6 11.2 17.2 51.6 0.2 1357 33.0 --
654 B 153 300 17.6 9.8 16.5 50.2 0.0 1226 32.1 73 1090 B 153 140
24.3 18.0 18.0 50.0 0.3 1409 32.9 -- 509 B 153 200 24.3 16.8 16.2
49.0 0.1 1306 34.2 -- 727 B 165 130 7.6 3.1 21.6 52.6 0.6 1404 29.5
-- 1331 B 165 305 7.6 1.2 17.5 50.6 0.2 1293 29.9 73 3122 B 165 260
17.6 8.3 15.2 48.5 0.1 1082 31.9 50 2662 B 165 95 24.3 18.3 18.5
49.8 0.7 1380 33.6 61 973 C 153 340 4.2 0.8 18.9 54.8 0.3 1506 26.3
-- 1236 C 153 540 4.2 0.3 17.6 54.4 0.5 1412 26.4 63 1962 C 153 280
7.9 3.2 18.3 54.3 0.2 1434 28.2 -- 1018 C 153 500 7.9 2.2 16.9 54.1
0.3 1343 29.6 75 1817 C 153 260 11.4 5.9 18.3 53.9 0.1 1390 29.9 --
945 C 153 460 11.4 3.6 16.4 52.6 0.7 1303 30.5 73 1672 C 153 150
17.5 11.8 20.3 53.1 0.4 1476 30.4 -- 545 C 153 300 17.5 10.2 16.5
51.4 0.3 1267 31.2 74 1090 C 153 130 24.6 19.5 18.4 51.4 1.3 1475
33.1 -- 472 C 153 200 24.6 17.2 16.3 50.7 0.3 1338 32.9 66 727 C
165 280 7.5 1.2 17.8 53.0 0.2 1246 28.8 -- 2866 C 165 360 7.5 0.7
17.5 52.4 0.0 1222 27.9 70 3685 C 165 125 16.3 11.0 17.6 52.3 0.2
1362 34.0 -- 1280 C 165 310 16.3 6.9 14.9 49.8 0.1 1006 31.3 51
3173 C 165 95 23.8 18.5 19.6 51.7 0.6 1453 34.4 -- 973 C 165 240
23.8 12.4 14.1 48.6 0.0 956 32.7 41 2457
[0047]
5TABLE 3 Carbohydrate compositions of brown stock pulps Temp..
Residual Kappa Lignin-free Profile .degree. C. g/L EA number Glucan
% Xylan % Mannan % Tot. hemis % Tot. sugars % yield, % A 153 1.2
18.6 74.0 25.7 0.3 26.0 94.3 49.7 A 153 7.1 17.9 74.6 25.2 0.3 25.5
93.5 -- B 153 1.8 17.6 74.2 25.5 0.3 25.8 93.4 50.5 B 153 18.0 18.0
76.6 23.2 0.3 23.5 93.2 48.7 C 153 0.8 18.9 74.3 25.4 0.3 25.7 91.4
53.3 C 153 3.2 18.3 74.1 25.7 0.3 25.9 92.9 52.8 C 153 5.9 18.3
74.2 25.5 0.3 25.8 94.0 52.5 C 153 10.2 16.5 75.4 24.3 0.3 24.7
95.0 50.1 C 153 19.5 18.4 77.2 22.5 0.3 22.8 94.2 50.0 C 165 1.2
17.8 75.2 24.5 0.4 24.8 93.7 51.6 C 165 11.0 17.6 75.6 24.0 0.4
24.4 94.0 51.0 Glucan, Xylan, and Mannan content and Total Hemis
have been calculated by dividing the corresponding number with
total sugars.
[0048] FIG. 3 is a graph of representative effective alkali and
temperature profiles for the three cooking trials described with
respect to FIG. 2. In FIG. 3, the alkali profile A from Table 1
above is shown by graph line 40, while the profiles B and C from
Table 1 are shown by graph lines 41, 42, respectively. The
temperature graph for all profiles is shown by graph line 43.
[0049] FIG. 4 is a graph of the Total Brownstock Yield versus
Residual EA concentration, that is the EA at the end of stage 35 of
FIG. 2, for the trials described with respect to FIG. 2. In FIG. 4,
the alkali profiles A-C from Table 1 are shown by graph lines
45-47, respectively; all cooks were at about 153.degree. C.
Clearly, the pulp produced by the most representative practice of
the method of the present invention (graph line 47) produces a
higher total yield than the less representative, referenced,
methods (graph lines 45, 46).
[0050] FIG. 5 is a graph of the Total Yield as a function of kappa
number for the trials described with respect to FIG. 2. In FIG. 5,
the alkali profiles A-C from Table 1 are shown by graph lines
49-51, respectively; all cooks were at about 153.degree. C. Again,
the yield at kappa number for the pulps produced by the most
representative practice of the method of the present invention (51)
are greater than the pulps produced from the less representative,
referenced, methods (49, 50).
[0051] FIG. 6 illustrates another alkali and temperature profile
for a series of laboratory cooks for hardwood, according to the
invention and the prior art. The data shown in FIG. 6 compare the
alkali profile according to the present invention 53 to a
"conventional" cook 54 and to a "High EA" cook 55. [The temperature
profile for each is shown by 56.] The resulting yield data for
these trials are shown in FIG. 7. The low EA profile 53 according
to the present invention produces a hardwood pulp have a greater
total yield, cellulose yield, and xylan yield, than either the
conventional cook 54 or the high EA cook 55. In the conventional
cook 54, as can be seen from FIG. 6, the initial EA concentration
in the cooking liquor was about 44 g/L as NaOH, and the temperature
in the representative stage leading up to cooking was over
120.degree. C.
[0052] For the "High EA" cook 55 in FIG. 6, the initial EA
concentration was 28 g/L, while for the cook 53 of the invention
the initial EA concentration was about 22 g/L.
[0053] FIG. 8 illustrates a continuous digester system 100 that can
be used to practice the method of the present invention and
comprising apparatus according to the invention. FIG. 8 also shows
the respective alkali concentration profile 101 and temperature
profile 102 of the treated slurry as it passes through the vessel
105. The zones lettered A through F on the left-hand side of FIG. 8
correspond to steps (a) through (f) of the method of the present
invention.
[0054] Typically, the slurry of comminuted cellulosic fibrous
material 103, for example wood chips, is introduced to the top 104
of the digester vessel 105. Vessel 105 may be a single vessel or
may be part of a multiple-vessel system, for example, the slurry
103 may have been treated in an initial pretreatment or
impregnation vessel. For example, step A or steps A and B shown in
FIG. 8 may be performed in a first vessel, for example, an
impregnation vessel, and steps C-F may be performed in a second
vessel, for example, a digester. The slurry 103 may be fed by a
conventional feed system or preferably by a Lo-Level.RTM. Feed
System, sold by Ahistrom Machinery Inc. of Glens Falls, N.Y., as
described in U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025;
5,736,006; 5,753,075; 5,766,418; and 5,795,438. The fully-treated
pulp slurry 107 is discharged from the bottom 106 of vessel 105.
The slurry 107 enters the vessel at a temperature of about 80 to
120.degree. C., for example, at about 100.degree. C. as shown, and
at an initial EA concentration of less than 20 g/L as NaOH, for
example, at about 18 g/L as shown. Excess liquor is removed from
the slurry introduced to the vessel by means of separator 108 and
returned to the feed system or previous vessel via conduit 109.
[0055] While the slurry travels downward in the vessel 105 from the
inlet, the temperature of the slurry is maintained at about
100.degree. C., as shown by profile 110, while the EA concentration
decreases, as shown by profile 101, as alkali reacts with the
constituents of the wood. When the slurry encounters first
extraction screen 111, at approximately the interface between
stages A and B, some liquor is removed from the slurry and passed
via conduit 112 to a Chemical Recovery System, as is conventional,
or is used elsewhere as needed, for example, as a source of heat in
a heat exchanger. Some of the liquor removed by screen 111 may be
re-circulated by pump 113, heater 114, and conduit 115 to be
reintroduced to the vessel 105 in the vicinity of the screen 111.
This recirculated liquor may be augmented with additional liquors,
including cooking liquors (for example, kraft white liquor, green
liquor or black liquor), dilution liquids or liquids containing
yield or strength enhancing additives, such as anthraquinone or
polysulfide or their equivalents or derivatives, or combinations
thereof. However, as shown by the dashed lines, this recirculation
is not necessary to perfect the present invention. Upon reaching
screen 111 the alkali content of the slurry decreases to less than
10 g/L, typically between 3-10 g/L as shown by curve 101. The
treatment stage A between the top of the vessel 104 and screen
assembly 111 corresponds to step (a) of the method of the present
invention.
[0056] Screen assembly 111 may be a double screen assembly with one
set of screens and associated liquor removal conduit located above
a second screen and its associated liquor removal conduit. The
upper screen assembly of screen 111 may be used to remove liquor as
in conduit 112 and the lower screen assembly may be used to remove
and recirculate liquor as described above with respect to
structures 113, 114 and 115.
[0057] After passing screen 111, the slurry enters a heating zone
or stage B between screen 111 and a first recirculating screen 116.
Though the arrows 117 indicate that this heating zone is a
counter-current heating zone, this zone may alternatively be a
co-current heating zone. The removal of liquid via conduit 112
causes an upward flow of hot liquor 117. This hot liquid is
introduced via a first circulation conduit/reintroduction pipe 121
associated with screen 116. As the slurry passes screen 116 liquor
is removed and recirculated by means of pump 119, heater 120, and
conduit 121. The liquor removed and circulated is augmented by
cooking liquor and dilution liquor introduced via conduit 118,
though cooking and dilution liquor may be introduced via separate
conduits. As the slurry passes below screen 111, the
counter-current flow of hot, alkali-laden liquor 117 heats the
slurry as shown by curve 102 to cooking temperature, for example,
to a temperature above 140.degree. C., preferably between 140 and
160.degree. C. The slurry is simultaneously exposed to liquor
having increasing alkali content as shown by curve 122. As the
alkali passes upward in stage B, it is gradually consumed such that
the alkali decreases as shown by curve 122.
[0058] According to the present invention, the greatest alkali
concentration introduced at the screen 116 is less than 30 g/L,
preferably less than 20 g/L as shown. This lower concentration is
established by diluting the cooking liquor introduced by adding
dilution liquor to it. The nature of the dilution liquor is as
described previously and is preferably washer filtrate from a
downstream washer, often referred to as "cold blow filtrate".
[0059] The low alkali concentration below screen 111 is typically
below 10 g/L, preferably between 5 and 10 g/L. This effective
alkali concentration immediately below screen 111, though shown as
being slightly higher than the concentrations above the screen 111,
may be higher or lower than or essentially equal to the EA
concentration above the screen 111. The difference shown is that of
only one typical alkali concentration that may be used according to
this invention. The stage B between screen 111 and screen 116
corresponds to step (b) of the method of the present invention.
[0060] The flow of liquor 117 and the introduction of steam to
heater 120 is preferably controlled so that after passing screen
116, the slurry is at or near cooking temperature, that is, at at
least 140.degree. C., preferably between about 140 and 160.degree.
C. The "bulk delignification" occurs in the zone between screens
116 and 123 (a second extraction screen). As the slurry flows below
screen 116, the free liquid in the slurry flows in the same
direction as the flow of the cellulose material, that is,
co-currently, as shown by arrows 125. Upon reaching screen 123,
liquor is removed from the slurry via conduit 124 and passed to
recovery or other uses. The temperature of the slurry between
screens 116 and 123 is preferably maintained relatively constant,
for example, at about 150.degree. C., as shown by curve 126. The
alkali concentration in this zone gradually decreases from a peak
alkali at screen 116 as the alkali is consumed in the pulping
reaction, as shown by curve 127. The alkali concentration is
typically decreased to at least 10 g/L, preferably between 5 and 10
g/L as shown by curve 127.
[0061] The stage C between screen 116 and screen 123 corresponds to
step (c) of the method of the present invention.
[0062] In one embodiment of this invention, the cooking phase is
terminated at or below screen 123 by cooler dilution liquor
introduced and drawn upward by the removal of liquid in conduit
124. Such is the case in older digesters, known as "cold blow"
digesters where the extraction, as from screen 123, was located in
the bottom of the vessel and followed by cold blow dilution. For
example, such a configuration can be seen by removing zones E and F
from digester 105 and following screen 123 by a cooling dilution
stage or zone D. However, in a preferred embodiment of this
invention, at least one additional counter-current cooking zone is
present below screen 123, for example, zones E and/or F.
[0063] In the preferred embodiment, for example, in a configuration
indicative of how a new digester would be built, after passing
screen 123, the slurry passes into a counter-current cooking zone
between screens 123 and 128 (a second recirculation screen). The
removal of liquid via conduit 124 produces a counter-current flow
of free liquor as shown by arrows 129. As in zone B above, the
down-flowing slurry in stage E is exposed to a gradually increasing
concentration of alkali as shown by curve 130. The temperature in
this zone is maintained at cooking temperature, for example, at
approximately 150.degree. C., as shown by curve 131. This alkali is
introduced by conduit 135 associated with screen 128. As the slurry
flows downward passed screen 128, some liquor is removed and
recirculated via pump 133, heater 134 and second
conduit/recirculation pipe 135. This circulated liquor is
preferably augmented with cooking liquor and dilution liquid via
conduit 132, similar to that described with respect to conduit 118.
Again, the flow of cooking liquor and dilution via conduit 132 is
controlled so that the alkali concentration between screens 123 and
128 is regulated as desired to produce the most optimum
carbohydrate content in the resulting pulp. Providing a higher
alkali concentration, for example greater than 15 g/L, dissolves
more hemicellulose so that less hemicellulose is present in the
resulting pulp. Conversely, providing a lower alkali concentration
in this zone, for example, less than 15 g/L, causes less
hemicellulose dissolution and more hemicellulose present in the
resulting pulp. Again, the cooking process can be terminated after
screen 128 by introducing a cooling dilution step as shown by zone
D in FIG. 8.
[0064] As discussed above with respect to screen 111, the
difference in alkali concentration above and below screen 123 is
representative only. According to the invention, these alkali
concentration may be different or relatively the same depending
upon the desired characteristics of the treatment and the species
processed. The zone between screen 123 and screen 128 corresponds
to step e) of the method of the present invention.
[0065] In another embodiment of the invention shown in FIG. 8, the
counter-current zone E is followed immediately by counter-current
zone F. In this case, the slurry passing screen 128 encounters
another counter-current flow of liquor 136. Again, as described
with respect to zone E above, the down-flowing slurry is exposed to
a gradually increasing concentration of alkali as shown by curve
138. Also, as before, the temperature in this zone is maintained at
cooking temperature, for example, at approximately 150.degree. C.,
as shown by curve 139. The alkali is introduced by conduit 142
associated with screen 137. As the slurry passes screen 137, some
liquor is removed and recirculated via pump 140, heater 141 and
conduit 142. This circulated liquor is preferably augmented with
cooking liquor and dilution liquor via conduit 143, similar to that
described with respect to conduits 118 and 132. Again, the flow of
cooking liquor and dilution liquor via conduit 143 is controlled so
that the alkali concentration between screens 123 and 128 can be
controlled as desired to produce the most optimum carbohydrate
content in the resulting pulp, as described with respect to conduit
132 above. Again, the difference in alkali concentration from above
screen 128 to below screen 123 is only one representation of the
alkali profiles that can be achieved. These concentrations may be
different or relatively the same depending upon the desired
treatment and the species being treated.
[0066] Finally, below screen 137 the now essentially-fully cooked
pulp encounters cooler (e.g. below 110.degree. C. preferably below
90.degree. C.), low-alkali-containing dilution liquor introduced
via one or more conduits 144 and 145, typically by way of a
conventional ring header. This cooler liquor terminates the pulping
reaction, cools the pulp, and lowers its alkali concentration so
that it can be discharged via conduit 107, typically with the aid
of a conventional rotating discharge device (not shown in FIG. 8,
schematically shown at 150 in FIG. 9). As shown by curves 146 and
147 the temperature of the pulp preferably is reduced to below
100.degree. C., for example to 80-90.degree. C. while the alkali
concentration is reduced to less than 5 g/L, typically 0 to 4
g/L.
[0067] FIG. 9 illustrates a comparison of actual mill alkali
concentration data for a continuous digester 200 operated according
to the present invention, as shown by graph line 61 and a digester
operated conventionally, as shown by graph line 62. The digester
200 shown in FIG. 9 is similar to the one shown in FIG. 8, though
the digester in FIG. 9 only includes zones A, B, C, F and D, in
that order. [In FIG. 9 structures that are the same as those in
FIG. 8 are shown by the same reference numerals, the illustration
in FIG. 9 being more schematic than in FIG. 8.] Compared to the
alkali profile 62 in a conventional mode of operation (initial EA
over 30 g/L as seen in FIG. 9), the alkali concentration profile 61
according to the present invention, particularly the initial EA, is
much lower during impregnation A and heating B to cooking
temperature and much higher in the final stage, or residual
delignification stage F, of the cook.
[0068] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *