Note: Descriptions are shown in the official language in which they were submitted.
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1
PULP COOKING WITH PARTICULAR ALKALI PROFILES
BACKGROUND AND SUMMARY OF THE INVENTION
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 [Na2S]. 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.
The treatment is performed at a temperature of over 100°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.
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
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concentration of NaOH plus one-half the concentration of Na2S expressed in
grams per
liter (g/L) of equivalent NaOH, that is,
EA = [NaOH] +'/2 [NaZS] g/L NaOH
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 Fr4
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.
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 Fr4 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, NY.
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 .
In the 1990s, Marcoccia, et al. introduced the Lo-Solids~ cooking process and
equipment which provided the next dramatic improvement to the kraft cooking
process.
See U.S patents 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and
5,662,775.
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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.
In co-pending U.S. patent application 08/911,366 filed on August 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~ cooking process and equipment described in the
above-
referenced patents.
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
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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
February 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.
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°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°C. (c) Treating the material with a third
alkaline liquid having a
third effective alkali concentration at a temperature greater than
140°C to delignify the
material. And (d) treating the material with a cooling liquid to cool the
material to a
temperature less than 120°C; wherein the first, second, third initial
effective alkali
concentrations are all less than 30 g/L, typically less than 25 gIL,
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°C, e.g. below
90°C.
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°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
CA 02282094 1999-09-13
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°C, the fifth alkaline
liquid having a fifth initial EA concentration. The fifth initial EA
concentration may be less
5 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.
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.
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°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.
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
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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.
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 Fr4 at
the start of the first stage of about 25 g/L or less, and at a temperature of
between about
90-110°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°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°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°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°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.
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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°C adjacent a discharge of pulp
from the digester
system.
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
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last stage. And a pulp discharge from the last stage, adjacent the bottom of
the digester
vessel.
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.
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 DESCR1PT10N OF THE DRAWINGS
FIGURE 1 is a simple block diagram illustrating one exemplary form of a method
of
the present invention;
FIGURE 2 is a block diagram illustrating a laboratory technique that may be
used to
evaluate a method of the present invention;
FIGURE 3 is a graph that displays the alkali and temperature profiles of the
laboratory trials of a method shown in FIGURE 2.
FIGURES 4 and 5 are graphs that display the yield results of the trials shown
in
FIGURES 2 and 3;
FIGURE 6 is a graph which displays another set of alkali and temperature
profiles
for laboratory trials of a method according to the present invention;
FIGURE 7 is a graph that displays the yield results of different components of
the
pulp produced by using the profiles shown in FIGURE 5;
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FIGURE 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
FIGURE 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
FIGURE 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
°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°C,
preferably, between about 90 and 110°C .
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°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 gIL). 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
CA 02282094 1999-09-13
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
5 to a temperature approaching typical kraft or soda cooking temperatures, for
example, to a
temperature of at least 140°C, typically between 140-180°C,
preferably, 140-160°C.
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°C, again, typically between 140-180°C, preferably, 140-
160°C. The initial EA
10 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.
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°C, typically
between 140-180°C, preferably, 140-160°C. More than one
treatment stage 15 may follow
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11
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.
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 °C, typically less than 100°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°C. Typically, slurry
17 is forwarded on
to further processing, such as brown stock washing for chemical recovery and
bleaching, if
desired.
The method described with respect to FIGURE 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.
FIGURE 2 illustrates a schematic diagram of a laboratory procedure used to
evaluate the process disclosed in FIGURE 1. Stages 32, 33, 34 and 35
correspond to
stages 12, 13, 14, and 15, respectively, of FIGURE 1. The cooks were carried
out in a
university laboratory as described in the Achren, 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.
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
CA 02282094 1999-09-13
12
(NaOH) in the cooking stage 2 for all three EA profiles. Three cooks were
prepared from
each combination of Fr4 profiles with a final target kappa number of 24, 18
and 14. Fr4
charges and EA concentrations used are summarized below.
Table 1
Profile Impregnation Impregnation Cooking stage Cooking stage
1 (32) 2 (33) 1 (34) 2 (35)
EA charge on wood EA charge EA charge on Initial EA
on wood 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
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 FIGURE
2 is shown by
the curves of FIGURE 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 FIGURE 3.
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.
The white liquors used in the laboratory cooks were artificially prepared from
technical grade chemicals (NaOH and NaZS). 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.
CA 02282094 1999-09-13
13
The birch chips were steamed for 20 min. at 100°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°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.
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 Fr4
charge. The
impregnation liquor was forced into the digester with the aid of 4 bar NZ
pressure. The
impregnation time in this step (32) was about 40 min. including the time it
took to bring the
digester from 95°C to the cooking temperature 153°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 NZ 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°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.
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).
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°C or 165°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
CA 02282094 1999-09-13
14
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°C and weighed.
A hexeneuronic acid (HexA) content analysis was carried out after application
of the
acid hydrolysis of the pulp (100°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 meqlkg HexA in the pulp.
Selected pulp samples from the C) profile were bleached by an ECF-bleaching
sequence (Do 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
(Do E-D) are given below.
Stage Conditions
D° Retention time 60 min. at 60°C
CIOZ dose (act. CI) 3.6%
end pH 2.2-2.3
E Retention time 90 min. at 60°C
NaOH dose 1-2.5%
end pH 10.9-11.7
D Retention time 180 min. at 80°C
CIOZ dose (act. CI) 3%
end pH 2.4-2.8
Analysis methods used are summarized below.
Dry solids content of chips SCAN CM:39:88
Dry solids content of pulp SCAN-C 3:78
Effective alkali in cooking SCAN-N 33:94
liquors
Kappa number SCAN-C1:77
ISO brightness (pulp) SCAN-C 11:75
Viscosity of pulp SCAN-CM 15:88
Hexeneuronic acid (HexA) according to Vuorinen
content et al. /14/
Carbohydrate content of pulpaccording to Laver et
al. /16/
CA 02282094 1999-09-13
Table 2 on the next page contains the cooking conditions and results for the
trials
described with respect to FIGURE 2. Table 3, two pages hence, contains the
carbohydrate composition of the pulps produced in the trials described with
respect to
FIGURE 2. The most representative results according to the invention are
identified as
5 Profile C in the first column of each of Tables 2 and 3. The data in FIGURE
4 is limited to
cooks at 153°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.
CA 02282094 1999-09-13
16
Table 2. Cooking results and key pulping conditions for cooking stage 2 (35)
ProfileCookingCookingInitialResidualKappaTotal RejectsVisoosiry HercAH-
lack
yield Hrighmess
~P.'C~~. EA Fa no Y. Y. milg ISO'/.rt>eq/lcg
min g/L g/L on on
wood wood
A 153 240 5,0 I;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 3Z,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 Z7,1 - 1308
B 153 500 4,6 0,0 18.9 52,9 O,Z 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 - 1672
77
B 153 150 I1,2 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 i5 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 ~ 1,6 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 - 133I
B 165 305 i,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,Z 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 i,9 2,2 16,9 54,1 0,3 1343 29,6 i5 1817
C 153 260 1 5,9 18,3 53,9 0,1 1390 29.9 - 945
l,.i
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 I53 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
CA 02282094 1999-09-13
17
Table 3. Carbohydrate compositions of brown stock pulps
ProfileTaap..ResidualKappaGlucaaXylaaMaanaaTot Tot
hemis sugars
Ligaia
fee
'C g/L numbu% % % % % yield,
EA
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 ?5,4 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 i0,0
C 165 1,2 17,8 75,2 24,5 0,4 24,8 93,7 ~ 1,6
C 165 11,0 17,6 75,6 24,0 0,4 24,4 94,0 .51,0
Gluean,lan,
Xy and
Mannan
contrnt
snd
Tonl
Hemis
have
been
alculated
by
dividing
the
com~pondina
number
with
tool
CA 02282094 1999-09-13
18
FIGURE 3 is a graph of representative effective alkali and temperature
profiles for
the three cooking trials described with respect to FIGURE 2. In FIGURE 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.
FIGURE 4 is a graph of the Total Brownstock Yield versus Residual EA
concentration, that is the EA at the end of stage 35 of FIGURE 2, for the
trials described
with respect to FIGURE 2. In FIGURE 4, the alkali profiles A-C from Table 1
are shown by
graph lines 45-47, respectively; all cooks were at about 153°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).
FIGURE 5 is a graph of the Total Yield as a function of kappa number for the
trials
described with respect to FIGURE 2. In FIGURE 5, the alkali profiles A-C from
Table 1 are
shown by graph lines 49-51, respectively; all cooks were at about
153°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).
FIGURE 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
FIGURE 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 FIGURE 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 FIGURE 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°C.
CA 02282094 1999-09-13
19
For the "High EA" cook 55 in FIGURE 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.
FIGURE 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.
FIGURE 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 FIGURE 8 correspond to steps (a) through
(f) of the
method of the present invention.
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 FIGURE 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~ Feed System, sold by Ahlstrom Machinery Inc. of Glens Falls, NY, as
described in
U.S. patents No. 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°C, for example,
at about 100°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.
While the slurry travels downward in the vessel 105 from the inlet, the
temperature
of the slurry is maintained at about 100°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
CA 02282094 1999-09-13
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
5 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
10 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.
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
15 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.
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
20 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°C, preferably between 140 and 160°C. The slurry is
simultaneously exposed to liquor
CA 02282094 1999-09-13
21
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.
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".
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.
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°C, preferably between about 140 and
160°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°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.
The stage C between screen 116 and screen 123 corresponds to step (c) of the
method of the present invention.
CA 02282094 1999-09-13
22
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.
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°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 Figure 8.
CA 02282094 1999-09-13
23
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.
In another embodiment of the invention shown in Figure 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°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.
Finally, below screen 137 the now essentially-fully cooked pulp encounters
cooler
(e.g. below 110°C preferably below 90°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 FIGURE 8, schematically
shown at
CA 02282094 1999-09-13
24
150 in FIGURE 9). As shown by curves 146 and 147 the temperature of the pulp
preferably is reduced to below 100°C, for example to 80-90°C
while the alkali
concentration is reduced to less than 5 g/L, typically 0 to 4 g/L.
FIGURE 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 FIGURE 9 is similar to the one shown in FIGURE 8, though the
digester in
FIGURE 9 only includes zones A, B, C, F and D, in that order. [In FIGURE 9
structures
that are the same as those in FIGURE 8 are shown by the same reference
numerals, the
illustration in FIGURE 9 being more schematic than in FIGURE 8.] Compared to
the alkali
profile 62 in a conventional mode of operation (initial EA over 30 g/L as seen
in FIGURE
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.
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.
