Note: Descriptions are shown in the official language in which they were submitted.
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Compact process for producing prehydrolyzed pulp
FIELD OF THE INVENTION
The present invention relates to a process for the production of pulp in which
hemicellulose is
hydrolyzed into hydrolysate, and lignin is dissolved by a kraft cooking method
for liberating
cellulose fibers. Still more particularly, the present invention relates to a
process for the
production of a pulp which has a high content of alpha cellulose and can be
sold as
dissolving pulp.
BACKGROUND OF THE INVENTION
Traditionally, there are basically two processes for the production of special
pulps having a
high content of alpha cellulose. These include acidic sulfite cooking, and
prehydrolysis-
sulfate (kraft) cooking. The former was developed at the end of the 19th
century, and the
latter in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to
672, lnterscience
Publishers, New York, 1968. The basic idea in both processes is to remove as
much
hemicellulose as possible from cellulose fibers in connection with
delignification, so as to
obtain a high content of alpha cellulose, i.e. polymeric chains having a
relative high
polymerization degree and not short hemicellulose molecules with a randomly
grafted
molecular structure.
In the traditional sulfite process, the removal of hemicelluloses takes place
during the
cooking, simultaneous with dissolving of the lignin. The cooking conditions in
that case are
highly acidic, and the temperature varies from 140 DEG C. to 150 DEG C.,
whereby
hydrolysis is promoted. The result, however, is always a compromise with
delignification. A
drawback is the decrease in the degree of polymerization of the alpha
cellulose and yield
losses, which also limit the potential for hydrolysis. Various improvements
have thus been
suggested, such as modification of the cooking conditions, and even a
prehydrolysis step
followed by an alkaline sulfite cooking stage. The main obstacle in connection
with sulfite
pulping processes is the complicated and costly recovery processes of the
cooking
chemicals.
A separate prehydrolysis step permits the desired adjustment of the hydrolysis
of
hemicelluloses by varying the hydrolysis conditions. In the prehydrolysis-
kraft cooking
process the bulk delignification is not carried out until a separate alkaline
cooking step, even
though some handbooks indicate that as much as 30 kg of lignin per ton of wood
may be
dissolved in the prehydrolysis (i.e. a small part of the total lignin content
as 30 kg per ton of
wood corresponds to some 3% of the wood material). The conditions for
prehydrolysis is
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most often established by heating in a hot steam phase or hot water liquid
environment,
where the natural wood acidity released will usually lower the pH down to
about 3.5, most
often referred to as auto hydrolysis. Sometimes could also additional acid and
a catalyst be
added. The subsequent delignification step has been a conventional kraft
cooking method,
where white liquor has been added to the digester.
Several prehydrolysis-kraft cooking processes have been disclosed and this
technology was
very much in focus during the late 60-ies and early 70-ies.
In the early publication "Continuous Pulping Processes" by Sven Rydholm, 1970,
is
described the experiences from "Continuous Prehydrolysis-kraft Pulping" on
pages 105-119.
On page 106, Fig. 8.1 is disclosed a two vessel continuous cooking system with
a first up
flow prehydrolysis tower followed by a down flow conventional kraft cooking
digester, in
which the up flow tower experienced severe pitch deposits on the extraction
strainers which
clogged after only 3-6 days of operation. Another system is disclosed on page
107, Fig.8.2
with a one vessel hydraulic continuous digester system with a first upper
prehydrolysis zone
and a lower alkaline kraft cooking zone, both zones being separated by a
strainer section.
However, also in this one vessel design was the strainers subjected to severe
pitch deposits
and clogging. The pitch problem also migrated into the chip feeding system
resulting in the
need for an alkali charge in the high pressure feeder to avoid pitch
deposition in the high
pressure feeder. This pitch problem has been seen in almost every installation
of continuous
cooking systems used for Prehydrolysis-kraft Pulping. This causes production
disturbances
and pulp quality variations. Furthermore, the lack of a well defined
prehydrolysis zone in
previous continuous installations has caused variations in the degree of
hydrolyzation, which
in turn led to unacceptable quality variations of the final product.
When Prehydrolysis-kraft Pulping is implemented in batch systems the pitch
problem is
partially solved by the fact that the strainers in the batch digester is
switching from
withdrawing acidic prehydrolysate to alkaline cooking liquor and later on
black liquor. The
volume of acidic hydrolysate withdrawn after a steam phase prehydrolysis is
also relatively
small in total volume so the exposure in strainers is limited. The latter
alkaline stages will
then also dissolve and wash out any pitch deposits such that they do not build
up over time.
This is not possible to achieve in continuous systems as the strainers are
located in a
stationary process position where the chemical conditions (as of pH, pitch
content etc) do not
change.
In U55589033 is disclosed a batch process for prehydrolyzed kraft pulp sold by
Metso often
in connection with SuperbatchTM cooking. Here is a hot 170 C prehydrolysis
step in a
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gaseous steam phase terminated by a hot neutralization step at 155 C using
heated alkali
and for a duration of only 15 minutes (as shown in example 3). This
neutralization is followed
by a hot black liquor treatment step at 148 C for a duration of 20 minutes and
finally the pulp
is cooked in a kraft cooking stage at 160 C for 54 minutes. Here the degree of
hydrolyzation
could be controlled in a good manner by controlling the duration of each
stage. As the
hydrolysis step is conducted in a steam phase could the following
neutralization and
alkalization of the wood material be obtained rather quickly and thoroughly as
the wood
material has been steamed at high temperature in a steam phase allowing the
alkali to
penetrate the wood material by diffusion. However, this type of well defined
transition zone
between the prehydrolyse in a steam phase and the neutralization is not
favourable in a
continuous system where the wood material is supposed to flow trough reaction
towers in a
plug flow, hence the hydrolysis is instead most often implemented in a liquid
filled stage at
least in final parts.
TM TM
Nowadays dissolving pulp for such end uses as spinning fibers (rayon/Iyocell)
is considered
to be an optional method for producing textiles having less environmental
impact compared
with production of cotton textiles. Dissolving pulp is also a base product for
different additives
and consistency agents and fillers in tyre cord and casings, ether and spongs,
nitrocellulose
and acetate. Hence dissolving pulps may be an alternative product instead of
pulp for regular
paper pulp making.
A common implementation in most prehydrolysis-kraft cooking processes is that
the
prehydrolysis stage has been terminated by withdrawal of the prehydrolysate,
either in form
of a pure acidic prehydrolysate, or in form of a neutralized prehydrolysate.
As indicated
before would any strainers in such process position be subjected to pitch
deposits, both
when the prehydrolysate is kept at its lowest pH level or if the
prehydrolysate is withdrawn in
a transition position where the chip suspension switch from acidic to
alkaline.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an improved prehydrolysis-
kraft process for
the preparation of pulp from lignin-containing cellulosic material. In
accordance with the
invention, these and other objectives have now been accomplished by a process
comprising
prehydrolyzing said cellulosic material in a prehydrolysis stage at a
temperature in the range
of about 120 and 180 C and during at least 20 minutes so as to produce a
prehydrolyzed
cellulosic material and an acidic hydrolysate. Thereafter adding a strong
alkali chock charge
to the mixture of prehydrolyzed cellulosic material and acidic hydrolysate to
such extent that
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the residual alkali concentration after neutralization of the acidic
hydrolysate, preferably
directly after this charge and after thorough mixing, is above 20 g/I EA as
NaOH forming an
alkaline treatment liquor. Thereafter maintaining the prehydrolyzed cellulosic
material in said
alkaline treatment liquor for a sufficient time in an alkaline pre-extraction
stage to reduce the
alkali concentration by at least 10 g/I EA as NaOH but not to a concentration
below 5 g/I EA
as NaOH. In said alkaline pre-extraction stage the dissolved carbohydrates as
well as any
lignin dissolved in the prehydrolysis stage are maintained dissolved during
this alkaline pre-
extraction stage and further carbohydrates and lignin are dissolved from the
cellulosic
material in the alkaline pre-extraction stage. After the alkaline pre-
extraction stage the
cellulosic material is transferred from the alkaline pre-extraction stage to a
kraft cooking
stage. The characterizing part of the invention is that the strong alkali
chock charge is made
using liquor volumes that decrease the temperature of the resulting alkaline
treatment liquor
for the prehydrolysed material by at least 10% in comparison to the
temperature in the
prehydrolysis stage, preferably at least 12 C if the prehydrolyse temperature
is about 120 C
and at least 18 C if the prehydrolyse temperature is about 180 C, in order
to reduce the
alkali consumption during the cooking chemical diffusion process where the
alkali treatment
liquor penetrates to the core of the lignin-containing cellulosic material.
This transition from
the acidic prehydrolysis stage to the relative low temperature alkaline pre-
extraction stage
establish a more thorough impregnation of the lignin-containing cellulosic
material to the core
thereof which results in a more homogenous cook with low rejects amounts after
the
subsequent kraft cook stage. Another positive effect is that the cooking
temperature can be
reduced which increases the final alpha cellulose content in the pulp after
the kraft cook.
Lowering of the temperature significantly will also reduce reprecipitation of
hemicelluloses
onto the fibers, since the reprecipitation process is strongly dependent on
temperature.
According to one preferred embodiment is the acidification of said
prehydrolysis established
only by heating and optionally adding water, and without adding any external
acidifiers, only
using the wood acidity released during heating reaching a pH level below 5
during the
prehydrolysis. In this embodiment not using any external acidifiers could the
acidification of
said prehydrolysis be established in a liquid filled phase and preferably is
the temperature at
end of the prehydrolysis in the range of 150-180 C. In such case no external
acidifier is used
is the temperature of the resulting mixture of alkaline treatment liquor and
the prehydrolysed
material below 130 C. Preferably is the resulting mixture of alkaline
treatment liquor and the
prehydrolysed material below 120 C which is well below an optimal kraft
cooking
temperature of about 142 C.
According to another preferred embodiment is the acidification of said
prehydrolysis
established by heating and addition of external acidifiers, reaching a pH
level below 3 during
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the prehydrolysis. In this embodiment using external acidifiers could the
acidification of said
prehydrolysis be established in a liquid filled phase and preferably is the
temperature at end
of the prehydrolysis in the range 120-165 degrees C. In such case external
acidifier is used
is the temperature of the resulting alkaline treatment liquor for the
prehydrolysed material
5 below 125 C. Preferably is the resulting mixture of alkaline treatment
liquor and the
prehydrolysed material below 120 C which is well below an optimal kraft
cooking
temperature of about 142 C.
In both cases of adding or not adding acidifier to the prehydrolysis and if
prehydrolysis is
established in a liquid filled phase, several advantages for reaching the
objective of high yield
alpha cellulose are obtained. As mentioned could as much as 30 kg lignin per
ton of wood be
dissolved during the prehydrolysis, and if in a liquid filled phase could the
lignin easier be
kept in the dissolved state and prevented from condensing onto the fibers.
Condensated
lignin could negatively affect the following diffusion/penetration of cooking
chemicals, as it
has been found to form a layer on the fiber material that obstruct diffusion,
and thus may
impair the delignification rate during the subsequent kraft cook. Lignin
condensation is a well
known effect that occur in depleted alkaline environment, especially in acidic
conditions, and
results in "black cook", i.e. pulp with condensated lignin that is very
difficult to delignify further
thereafter. Lignin condensation will lead to an increased cooking temperature
in order to
reach the target kappa number, which in turn has a negative impact on alpha
cellulose yield.
If condensation of lignin is avoided during the prehydrolysis would the pulp
be much easier to
cook to the desired kappa number at end of the subsequent kraft cook, and with
higher yield
and polymerization degree of cellulose, both favorable for special grades of
dissolving pulp.
As could be realized from above embodiments using or not using external
acidifiers, is a
higher temperature needed for similar order of prehydrolyse effect if no
acidifier is used, but
for both embodiments it is essential that the transition between the
prehydrolyse stage and
the alkaline pre-extraction stage is made such that an essential lowering of
the temperature
is obtained.
According to a further embodiment of the invention is the step of maintaining
the
prehydrolyzed cellulosic material in said alkaline treatment liquor
implemented for a time
period of from about 10 to 90 minutes. Besides establishing a thorough and
even alkalization
of the wood material after the prehydrolyse stage according to the main
objective it is also
important to optimize the conditions for the alkaline pre-extraction as well
as alkali
impregnation ahead of cook. It has been seen from laboratory tests that only a
minor part of
the total hemicellulose content is dissolved and hydrolysed to oligo-,
monosaccarides and
decomposition products thereof in the prehydrolyse stage, while yet a large
part of the total
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hemicellulose content could be dissolved in a subsequent alkaline pre-
extraction stage. The
total dissolved organic content (DOC) after the prehydrolyse is about 5-10%
and as much as
30% after an extended alkaline pre-extraction stage following the
prehydrolyse, By DOC is
included all organic content from the wood material, including
hemicellulose/carbohydrates
as well as lignin.
According to yet another embodiment is at least 1 m3/ton OD (Owen Dried) of
wood of the
used alkaline treatment liquor removed from said alkaline pre-extraction stage
with its
content of dissolved carbohydrates and lignin before onset of the kraft
cooking stage, and
thereafter adding alkali to the kraft cooking stage. By extracting the used
alkaline treatment
liquor at this late stage, while keeping it alkaline, could the pitch problems
be avoided in any
extraction screen sections in prehydrolysis stage and the total DOC content be
accumulated
to this point in the process. This is especially advantageous for continuous
cooking systems
which hereto had seen severe pitch deposit problems when trying to extract
acidic or neutral
prehydrolysate from extraction screens in such systems. Moreover, extracting
hemicelluloses
rich liquid before the kraft cook stage reduces the carry-over into the kraft
cooking stage,
which in turn reduces the risk for reprecipitation of hemicelluloses to occur
later during the
kraft cook stage, which reprecipitation is dependent on hemicelluloses
concentration and
high temperature.
In a most preferred embodiment of the invention the process could be
implemented in a
continuous digester system using at least one vessel for the prehydrolysis and
one vessel for
the alkaline pre-extraction stage and the kraft cooking stage. In this
embodiment could also
the alkaline pre-extraction stage be implemented in a separate vessel and the
kraft cooking
stage in another separate vessel. As the strong alkali chock charge will swing
the pH
conditions rapidly to the alkaline side after the prehydrolyse not only are
potential pitch
deposit problems avoided which hereto has been a concern for sturdy continuous
processes,
but the risk for redeposition of hemicelluloses triggered by low residual
alkali concentration is
also eliminated. The following alkaline pre-extraction stage is also given
optimum conditions
for a thorough impregnation of the acidic wood material by diffusion of said
alkaline treatment
liquor, which results both in higher degree of alkaline dissolution of DOC as
well as even pH
level to the core of the wood material ahead of the alkaline cooking stage.
Diffusion is a time
dependent process more than a displacement process obtained from by-flushing
alkaline
liquids, and thus the conditions for a continuous process is improved as it
may not be
necessary to implement internal liquid circulations throughout the stage.
When compared with known prehydrolysis-kraft processes, such as US5589033, the
present
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invention offers the following advantages:
A distinct ending of the prehydrolysis stage by both a sharp pH transition to
a sufficient
minimum alkali concentration and lowering of temperature, i.e. the two most
dominant
process conditions for prehydrolysis.
The neutralization of the free liquid occurs rather instantly, but more
favorable conditions for
neutralization of the bound liquid inside the wood material are obtained. The
diffusion is
favored by high temperature but as the alkali consumption rate is far higher
at high
temperature is lower temperature essential for alkali to diffuse into the core
of the wood
material.
Most batch cooking processes are coming from a state of the art where heat
economy is very
much in focus, starting with the RDH-process and the more modern batch cooking
process
Superbatch. Heat is recovered by using spent hot and warm liquors from end of
cooking
stage in pretreatment stages ahead of cooking. Like in US5589033 are high
cooking
temperatures often used (160 C). There is thus a general approach in batch
cooking to try to
maintain the high temperatures during the process in order to improve heat
economy. In
modern continuous kraft cooking has the optimum cooking temperature been
lowered since
the late 1950-ies when cooking temperature most often was about 160 C or even
up to
170 C for hardwood, and now in the late 1990-ies the typical cooking
temperature for
hardwood is about 142 C.
In this process according to the invention is a relatively cold alkali charge
used for
interrupting the prehydrolysis. This is referred to as a cold alkali charge
not being heated
before addition. Suitable alkali charge for use herein contains caustic soda,
and the preferred
agent is alkaline kraft cooking liquor, i.e., white liquor. The effective
alkali concentration, i.e.
EA, is most often given in % and corresponds to; 1/2 Na25 + NaOH. Such white
liquor
typically holds a temperature of some 80-90 C when delivered from the recovery
island, and
a white liquor of this temperature is included by definition in the used cold
alkali charge.
Preferably could this white liquor be cooled down even further in a heat
exchanger in order to
reach the desired final temperature after addition at end of the
prehydrolysis. Yet, other
alkaline filtrates could be added, preferably those having a high alkali
content and a low
temperature.
The lignin-containing cellulosic materials to be used in the present process
are suitably
softwood, hardwood, or annual plants.
According to the present invention, prehydrolysis-kraft pulp can be obtained
with a high yield
of alpha cellulose with a high polymerization degree.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of the cooking process according to
U55589033;
FIG. 2 is a schematic representation of the cooking process according to the
invention;
FIG. 3a is a schematic representation of the pH through a chip of wood after
the
prehydrolysis stage, End of PR, and when exposed to the alkaline treatment
liquor, Initial of
Ex;
FIG. 3b shows the alkaline consumption rate as a function of temperature;
FIG. 4 show the different pH level established inside a wood chip after a
prehydrolysis when
exposed to alkaline treatment liquor during a substantial time using cold or
hot alkaline
treatment liquor;
FIG. 5 show the kappa number after a kraft cook inside a wood chip after the
two different
treatments as shown in figure 4;
FIG. 6 shows a principal set up for a continuous cooking system using the
inventive process,
here using a prehydrolysis tower and two subsequent vessels for alkaline
treatment and
cook;
FIG. 7 shows a principal set up for a continuous cooking system using the
inventive process,
here using a prehydrolysis tower and one subsequent vessels for alkaline
treatment and
cook;
FIG. 8a-8d shows the inventive process implemented in a batch digester when
ending the
prehydrolysis and starting the subsequent alkaline treatment;
DETAILED DESCRIPTION OF THE INVENTION
As a comparison is in FIG. 1 shown the cooking steps of U55589033. The chips
are first
treated in the prehydrolysis step Pr where chips are heated by steam to 170 C
for 25
minutes. Thereafter is heated white liquor added in order to establish a
neutralization step Ne
and the acidic prehydrolysate RECAc is withdrawn from the process. The
neutralization step
is established at 155 C for 15 minutes. Even though the white liquor is heated
is the
temperature decreased some 8%. After the neutralization step is the
neutralization liquid
displaced by adding hot black liquor BLHoT, and this establish an alkaline
black liquor
impregnation step BL held at 148 C for 20 minutes. Thereafter the black liquor
is withdrawn
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and a new charge of white liquor is added ahead of the following cooking step
Co which is
held at 160 C for 54 minutes. In commercial batch cooking systems like
SuperbatchTM sold
by Metso is the white liquor used heated both in a heat exchange with hot
spent cooking
liquor as well as steam in order not keep the temperature at high level,
before being used as
the neutralizing liquid.
In contrast is in FIG. 2 shown the cooking steps according the inventive
process. Here is
shown a first steaming step ST for the chips but this step may be avoided of
the subsequent
prehydrolysis is implemented in a steam phase. The chips are thereafter
treated in the
prehydrolysis step Pr where chips are heated by steam at a temperature of
between about
120 and 180 C and during at least 20 minutes so as to produce a
prehydrolyzed cellulosic
material and an acidic hydrolysate. Addition of liquid such as water H20 is an
option, which
may be preferable if a liquid prehydrolysis is sought for, for example in a
continuous cooking
system. Another option is to add an acidifier Ac if a lower temperature is
sought for in the
prehydrolysis.
According to the inventive process is a distinct ending of the prehydrolysis
implemented by
adding a strong and cold alkali chock charge WI-COLD with a volume and at a
temperature that
will reduce the temperature of the cellulosic material by at least 10% in
comparison to the
temperature in the prehydrolysis stage, preferably at least 12 C if the
prehydrolyse
temperature is about 120 C and at least 18 C if the prehydrolyse temperature
is about 180
C. This will establish an alkaline treatment liquor which after this charge
establishes a
residual effective alkali concentration above 20 g/I EA as NaOH. Thereafter
the
prehydrolyzed material is maintained in an alkaline pre extraction stage Ex
for a sufficient
time in the alkaline pre-extraction stage to reduce the alkali concentration
by at least 10 g/I
EA as NaOH but not to a concentration below 5 g/I EA as NaOH. In this pre-
extraction stage
the dissolved carbohydrates as well as any lignin dissolved in the
prehydrolysis are
maintained dissolved during this alkaline pre-extraction stage and further
carbohydrates and
lignin are dissolved from the cellulosic material. Thereafter the cellulosic
material is
transferred from the alkaline pre-extraction stage Ex to a kraft cooking stage
Co. Before
transfer to the kraft cooking stage is preferably a large part of the spent
alkaline treatment
liquor withdrawn to recovery and a fresh charge of alkali WL is added to start
of cook. The
kraft cook may be implemented in any kind of known kraft cooking method for
batch or
continuous cooking such as, Compact Cooking, Lo-Solids cooking, ITC-cooking,
MCC
cooking, EAPC cooking as examples. The kraft cook is then finished by a wash
stage Wa,
which may be implemented in any kind of known wash equipment, such as a
countercurrent
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wash zone in bottom of a digester or using a pressure diffuser wash or filter
wash after the
cook.
In figure 3a is disclosed schematically the pH profile trough a wood chip as
exposed to the
alkali chock charge after a prehydrolysis. The pH level at the core of the
chips is as low as
5 established in the prehydrolysis while the outer surface of the chip is
exposed to the alkaline
treatment liquor established.
In figure 3b is disclosed the reaction rate, i.e. consumption rate, of alkali
during a
delignification process as a function of temperature. Here is disclosed the
rule of thumb for
kraft cooking where the reaction rate is doubled for each increase in
temperature by 8 C. If
10 one starts with a typical cooking temperature of 140 C this temperature
corresponds to a
reaction rate which establishes a base reference at 100%. If the cooking
temperature is
reduced in steps by 8 C to 132, 126, 118 and 110 C the reaction rate would
decrease in
steps to 50%, 25%, 12.5% and 6.25% respectively. Hence, it is of outmost
importance to
reduce the temperature if the core of the chip should be soaked with strong
alkali, reducing
consumption of alkali during diffusion due to lignin delignification
reactions. Too high alkali
consumption rate during the diffusion process may lead to alkali shortage in
the core of the
lignin containing cellulose material resulting in increased amounts of rejects
after the
subsequent kraft cook stage.
In figure 4 is shown the schematic difference in established pH level inside a
wood chip if a
diffusion of alkali is made into prehydrolysed wood material using either hot
or cold alkaline
treatment liquor, i.e. THoT and TCOLD respectively. As could be seen in the
right hand picture is
the pH level reached after treatment in the cold alkaline liquor, TooLD, the
dotted line, much
higher in the core of the cellulose material than if hot alkaline treatment
liquor THoT was used.
The reason is due to the reduced alkali consumption rate during the diffusion
process.
An even pH profile is of outmost importance for the subsequent cook which is
shown in figure
5, where schematically the kappa number through the cooked wood chips is
disclosed with
the dotted line for hot respectively cold alkaline treatment. As could be seen
in the left hand
picture is the kappa number higher in the core, i.e. is undercooked UC, while
the surface has
much lower kappa number, i.e. is overcooked OC. This result in high reject
amount from core
parts and alkali degradation of the cellulose at the surface. In the right
hand picture is shown
a more even delignification with less difference between surface and core of
wood material
due to leveled out pH profile. Both left and right hand pictures resulting in
same average
kappa number HAv but the cook conducted after treating with cold alkaline will
result in a pulp
containing higher alpha cellulose.
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In figure 6 is disclosed a three vessel continuous cooking system for
prehydrolysis and
cooking. The chips are first fed to a chip bin 1 and subsequent steaming
vessel 2 during
addition of steam ST for purging the chips from bound air. From the steaming
vessel the
steamed chips falls into a liquid filled chute above a high pressure sluice
feeder 3, which
pressurize the steamed chips and feed the formed slurry of chips in a feed
flow 4 to the
prehydrolysis vessel 10. Here the prehydrolysis vessel is in form of a steam-
liquid phase
digester having an inverted top separator 11 withdrawing a part of the
transport liquid from
line 4 back to start of feeding via A. As indicated is steam ST added to top
of vessel 10, and
optionally could also acid be added from source Ac. In bottom of the
prehydrolysis vessel 10
is the cold and strong alkali charge added from source WL. This could be done
by mixing in
the alkali to the return flow B. The prehydrolysed wood material in its now
alkaline treatment
liquor is fed in line 14 to a succeeding pre extraction vessel 20, here in
form of a hydraulically
filled vessel with a downward feeding top separator 21. After the necessary
treatment time in
the pre extraction vessel the alkaline wood material is fed to steam-liquid
phase digester 30
via line 24, and excess transport fluid is withdrawn by an inverted top
separator 31 and sent
to C, which is added to bottom of the pre extraction vessel 20 as part of the
transfer
circulation. Here could a substantial part of the used alkaline treatment
liquor be withdrawn to
recovery from this return flow C. The kraft cook is established in the
digester 30 and finally
the prehydrolysed and cooked pulp is washed in a pressure diffuser 40.
In figure 7 is disclosed a two vessel continuous cooking system for
prehydrolysis and
cooking. The difference here in relation to figure 6 is that the pre
extraction vessel 20 is
implemented as a first stage in the vessel 30, between the top separator 31
and a screen
section from where a substantial part of the used alkaline treatment liquor is
withdrawn to
recovery REC1. Fresh white liquor for the subsequent cooking phase is added
via a central
pipe at the level of this withdrawal screen.
In figures 8a to 8d are shown how the inventive method may be implemented in a
batch
digester in a 4 step sequence. Here is a steam prehydrolysing phase shown in
figure 8a,
where the steam phase is filling the vessel at the prehydrolyse temperature
THyD. At ending
of the prehydrolysis phase is cold white liquor added to the bottom and as
shown in figure 8b
which added white liquor catch the acidic condensate on the wood material as a
layer PC in
front of the rising cold white liquor level. In figure 8c is shown the later
phase of the
displacement of the cold white liquor trough the vessel, and here is also a
larger volume of a
mixture MX of cold white liquor and acidic condensate formed between the
acidic
condensate PC and the rising cold white liquor. When the liquid level reaches
an upper
screen, it could be circulated back to the bottom as shown in figure 8d, and
at least a part of
CA 02837277 2013-11-25
WO 2012/158075
PCT/SE2011/050610
12
the acidic condensate PC could be returned, either in the mixed fraction MX or
also as a part
of the acidic condensate. However, some or all of the acidic condensate PC may
be sent to
recovery as the purer fraction PC, while only the acidic condensate as
contained in the
mixture MX may be circulated. The switching from withdrawal to recirculation
could be
controlled by a pH sensor and/or a temperature sensor..
