Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING PULP
Field of the invention
The present invention relates to a method for producing pulp. More
particularly, it
concerns a displacement batch cooking process comprising a steam-phase
prehydroly-
sis step, wherein the recovery of the by-products is improved.
Background of the invention
The prehydrolysis-sulfate (Kraft) cooking for the production of special pulps
having a
high content of alpha cellulose was developed in the 1930's, see e.g. Rydholm,
S. E.,
Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968.
The ba-
sic idea is to remove as much hemicellulose as possible from cellulose fibers
in con-
nection with delignification, so as to obtain a high content of alpha
cellulose. This is
essential because the various end uses of such pulps, dissolving pulp for
instance, do
not tolerate short-chained hemicellulose molecules with a randomly grafted
molecular
structure.
A separate prehydrolysis step permits the desired adjustment of the hydrolysis
of he-
micelluloses by varying the hydrolysis conditions. In the prehydrolysis-kraft
cooking
process the necessary delignification is not carried out until a separate
second cooking
step. The prehydrolysis is carried out either as a water or steam phase
prehydrolysis,
or in the presence of a catalyst. In the former processes, organic acids
liberated from
wood during the process perform a major part of the hydrolysis, whereas in the
latter
process, small amounts of mineral acid or sulfur dioxide may be added to
"assist" the
prehydrolysis. In the prehydrolysis stage carried out in a steam phase, direct
steam is
introduced to the chip column in the digester.
Traditionally after prehydrolyzing the cellulosic material in a reactor, the
hydrolysate
and the prehydrolyzed cellulosic material are neutralized in the reactor with
alkaline
neutralizing liquor so as to produce neutralized hydrolysate and neutralized
prehydro-
lyzed cellulosic material. There is hydrolysate both in the free liquid
outside the chips
and also trapped and immobilized inside the chips. If desired, as much as
possible of
the hydrolysate can be recovered before the neutralization step in order to be
able to
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utilize the carbohydrates released in the prehydrolysis. A separate washing
stage, in
which the digester is first filled up with a washing liquid and then the
liquid contain-
ing the carbohydrates is removed from the digester, can be used between the
prehy-
drolysis and cooking stages. This is although time-consuming, unfavourable to
the
energy balance and produces a very dilute carbohydrate solution.
WO 2007/090925 describes an improved method for treating lignocellulosic
material,
wherein the digester and its contents are first heated with direct steam to a
predeter-
mined hydrolysis temperature and then a volume of washing liquid is introduced
into
the digester from one end and which washing liquid is removed from the other
end of
the digester, which is opposite to the introduction end.
According to the process described in patent publication WO 2007/090926 the
hydro-
lysate is recovered by utilizing trickle-bed type down-flow of hydrolysate. In
this me-
thod the first fraction of the trickled-down hydrolysate is collected as a
product frac-
tion and the second fraction is discharged from the digester to a hot
hydrolysate sto-
rage tank to be used as the first trickle flow liquid in the next batch. By
the trickle-bed
type recovery it is obtained concentrated hydrolysate, but the recovery step
is too slow
and therefore it is disadvantageous to the pulp quality. Additionally, the
treatment is
uneven to the contents of the digester; it is obvious that channeling will
occur during
the treatment, the liquid goes there where it is easiest.
After the removal of the hydrolysate the process may continue by a
neutralization-
cooking process known in the art.
The object of the present invention is to optimize the recovery of the
hydrolysate in a
prehydrolysis sulfate cooking process. By the present method the disadvantages
of the
prior processes can be overcome or at least diminished remarkably. An even
treatment
to the whole contents of the digester is obtained, a high-grade dissolving
pulp or paper
grade pulp is obtained and the recovered hydrolysate can be obtained
essentially pure
of sulfur. Essentially pure of sulfur means that according to the invention it
is possible
that no sulfur containing liquids are added to the recovered hydrolysate i.e.
according
to one embodiment only sulfur originating from the wood is present. The
further
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processing of the carbohydrates is thus easier. Also the energy-efficiency of
the
process is taken into account.
Summary of the invention
The displacement batch pulping process for producing pulp according to the
present
invention comprises the following steps; chip filling, heating of the chips,
prehydro-
lyzing of the chips, recovering of the hydrolysate including neutralizing of
the digester
contents, preparing the digester contents for kraft cooking by increasing the
pH i.e. by
adding alkaline cooking liquid and cooking to a required H-factor. In the
recovery step
of the present invention, after the target P-factor in the prehydrolysis stage
is reached,
hot washing liquid is introduced into the digester from the bottom thereof.
The wash-
ing liquid can be hot water from the hot water accumulator. It is possible to
add so-
dium hydroxide to the hot water, before introducing it into the digester, if
needed. The
digester circulation pump is started, as the level of the hot washing liquid
in the diges-
ter is over the suction screens and the circulation to the top and the bottom
of the di-
gester is started. The pumping of the hot washing liquid is continued until
all the chips
are "under" the washing liquid. This recovery step washes the hydrolysate and
the
steam condensate produced during the steaming phase (i.e. prehydrolyzing step)
from
the chips to the washing liquid in order to have as high as possible
concentration of
the dissolved sugars/carbohydrates in the used washing liquid prior displacing
it to the
condensate accumulator. At the same time the prehydrolysis is stopped in a
controlled
way and thus the loss of fiber quality is prevented. The condensate i.e.
washing liquid
containing the by-products is displaced from the top of the digester with an
alkaline
liquid, the displacement of the condensate is the final part of the recovery
step. The
pH of the digester contents after the recovery step is finalized can vary
between pH 9 -
12.
In other words the process according to the present invention for recovering
by-
products in a displacement batch pulping process in a digester comprising a
bottom, a
top and suction screens said method comprising prehydrolyzing of chips in a
steam
phase to produce hydrolysate and steam condensate wherein after the
prehydrolysis
stage the process continues by,
- introducing hot washing liquid into the digester
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- while adding the washing liquid, circulating it via the suction screens to
the top and to the bottom of the digester
- continuing the circulation until the prehydrolyzed chips are under the
washing liquid
- recovering the hot washing liquid containing by-products from the di-
gester and neutralizing the digester contents by displacing the washing
liquid with alkaline liquor.
Brief description of the figures
Fig.1 is a schematic flow chart of the process according to one embodiment of
the
present invention, wherein hot water from HOT WATER accumulator is used as the
washing liquid
Detailed description of the invention
The displacement batch pulping process according to the invention is started
by filling
the digester with the lignocellulosic material i.e. with the chips. The chip
flow comes
on a chip belt to the digester plant and from there to the digester(s) using
chip screw
conveyors on top of the digester(s). Low pressure (LP) steam is used to ensure
good
chip packing over the whole digester cross-section. During the chip filling,
air is eva-
cuated through the suction screens. Chip filling is stopped after the digester
level
switch has operated and the capping valve is then closed.
Heating of the chips is started by using low pressure (LP) steam from the top
and bot-
tom of the digester and the heating is continued with medium pressure (MP)
steam,
until the required temperature is reached i.e. 150 - 170 C. The digester is
kept at this
temperature and pressure until the prehydrolysis step is completed, i.e. the
required P-
factor is reached. P-factor is a defined factor to control the prehydrolysis
stage, taking
the temperature and time into account (as e.g. H-factor); Herbert Sixta,
Handbook of
Pulp, Volume 1, Wiley-VCH Verlag, 2006, pages 343-345.
According to the invention the prehydrolysis step is carried out in a steam
phase,
where the acids of the wood are hydrolyzed by the steam and acidic conditions
are
created in the digester. The end-pH of the steam prehydrolysis stage varies
depending
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on the wood species and the prehydrolysis conditions itself. The pH is
typically meas-
ured from the out coming liquid during the hot washing liquid fill and varies
from 2.5
to 4Ø
5 After the P-factor is reached, the recovery step is started by introducing
hot washing
liquid to the digester. The temperature of the hot washing liquid is between
100 -
170 C, preferably between 140 - 160 C and it is pumped into the digester from
the
bottom thereof. According to one embodiment hot water from HOT WATER accumu-
lator as shown in Fig. lis used as the washing liquid. According to another
embodi-
ment sodium hydroxide is added to the hot water prior introducing it into the
digester,
if there is a need to adjust the pH of the recovery step to enhance the
stopping of the
prehydrolysis. When the washing liquid level reaches the suction screens, the
digester
circulation pump is started and the circulation to the top and bottom of the
digester is
started. The suction screens are normally located on the lower half of the
digester,
typically about on the height of one third from the digester bottom. The
pumping of
the washing liquid is continued until the level of the washing liquid reaches
the level
of the chips, i.e. all the chips are under the washing liquid. Typically this
means that
the washing liquid is added in an amount of about 40 to 80 % of the digester
free vo-
lume. During the recovery step the hydrolysate and steam condensate produced
during
the prehydrolysis stage are washed from the chips to the washing liquid in
order to
have as high as possible concentration of the dissolved carbohydrates in the
used
washing liquid prior displacing it to the condensate accumulator.
Additionally, the
prehydrolysis stage is stopped during the recovery step in a controlled way in
order to
prevent the loss of fiber quality (e.g. viscosity and kappa number). The
digester con-
tents are thus neutralized. If hot water without addition of sodium hydroxide
is used as
the washing liquid, the pH of the contents of the digester before the
displacement of
the condensate varies between pH 5 - 7. On the other hand if the washing
liquid con-
tains alkali the pH of the digester contents before the displacement is
typically pH 7 or
more.
The final neutralization of the digester contents if any is achieved during
the dis-
placement of the condensate. The displacement of the condensate is started by
stop-
ping the digester circulation pump and starting to pump alkali through a
cooking liq-
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nor heater to the bottom of the digester. The condensate which contains the
dissolved
carbohydrates (i.e. by-products) is displaced from the top part of the
digester to a con-
densate accumulator as the alkali pumping is continued. The amount of alkali
needed
for neutralization is dependent on the wood material. Any suitable alkali can
be used
in the displacement of the condensate and/or in the neutralization of digester
contents.
According to a preferred embodiment, in case essentially sulfur-free
condensate is
needed, the condensate is displaced with sodium hydroxide. It is also possible
to ob-
tain essentially sulfur-free condensate by using white liquor as the alkaline
source, but
in that case it is important to make sure that no white liquor (WL) goes to
the conden-
sate tank. If the sulfur content of the condensate is not a limiting factor,
the displace-
ment can be performed with white liquor and it is not so critical if some
white liquor
goes to the condensate tank. Typically, in the displacement operation, first
alkaline
liquid is conducted into the digester from the bottom thereof in an amount,
which is
needed to neutralize the digester contents and is less than the digester free
volume.
Thereafter the displacement is continued with hot black liquor and this stage
is actual-
ly the first cooking stage. The outcoming hydrolysate is directed to the
condensate
accumulator and as the first displacement alkali starts to come out the
outcoming liq-
uid is conducted into the hot black liquor (HBL) accumulator. After the
displacement
of the condensate i.e. in the end of the recovery step the pH of the digester
contents
typically varies between pH 9 - 12.
After the recovery step the pH of the digester contents is increased to over
10, typical-
ly to the pH between 11 and 12, in order to continue with alkaline kraft
cooking step.
Thus, after the required volume of alkali (depending on the wood species) has
been
pumped, the process continues by pumping hot black liquor (HBL) from the HBL
accumulator through cooking liquor heater. The displaced liquid is sent to the
HBL-
accumulator while the HBL displacement continues.
The alkaline Kraft cooking is carried out with alkali rich HBL or by using a
hot white
liquor after the HBL stage. Typically the need for extra heating is minimal as
the di-
gester is filled with hot liquor. If heating is required, direct steam nozzles
in the circu-
lation line are used for that purpose. Cooking continues to the required H-
factor.
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After reaching the target H-factor the cooking is stopped by introducing cool
dis-
placement liquor to the bottom of the digester. This process step can be
called as ter-
minal displacement. The displacement liquor (DPL) is filtrate from the first
brown
stock washing stage. The DPL displaces the used cooking liquor from the
digester to
the HBL accumulator. By this way the energy and chemicals can be stored/
transferred
from one cook to another. This is also an efficient way to terminate the cook.
The temperature of the digester contents is below the boiling point after the
displace-
ment is finished and the emptying of the digester is carried out with a pump.
Dilution
liquor is added to the digester in order to improve discharge. The dilution
liquor is
taken from the displacement liquor tank. The temperature is low and thus no
strong
gases are formed. An atmospheric discharge tank is used and the vent gases are
con-
nected to gas handling system. New cooking cycle can be started after
finishing the
discharge step, i.e. the digester is empty.
In order to operate the process according to the invention the tank farm area
(see fig-
ure 1) typically comprises three accumulators (hot water, condensate and hot
black
liquor i.e. HBL accumulator) and four atmospheric tanks (displacement i.e. DPL
tank,
discharge tank, digester and NaOH/white liquor (WL) tank).
In the process according to figure 1, the HBL accumulator is used to storage
the alka-
line Kraft liquors used in hot liquor fillings and it acts as a receiving tank
for used
cooking liquor from the terminal displacement. Excess liquor from this tank is
sent to
evaporation through heat exchanger and a liquor filter.
According to the embodiment shown in Fig. 1, the incoming water is first
heated with
hot black liquor from HBL accumulator and then with condensate from condensate
accumulator prior directing to the hot water (HW) accumulator, the heat
exchangers
are shown in figure 1. The fibres are separated with a liquor filter. The HBL
accumu-
lator has a circulation line where the HBL temperature is controlled with MP-
stream
when no hot fill is in operation.
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The condensate accumulator is used as a receiving tank for the prehydrolysis
stage
condensate. The circulation line is used to control the pH in the accumulator.
Addition
of NaOH is possible if required. The condensate is cooled with the aid of heat
ex-
changer before sending to further processing.
The hot water accumulator is a storage tank for hot water coming from the heat
ex-
hangers. The circulation line of the hot water accumulator is equipped with a
possibili-
ty to heat the water with MP-steam to the required temperature. The hot water
is used
in the recovery step as described above.
The displacement liquor tank is used to store washing filtrate for terminal
displace-
ment and for discharge dilution.
As shown in figure 1 there are also storage tanks for NaOH and/or white
liquor.
NaOH can be used to neutralize the condensate in the circulation line of the
conden-
sate accumulator. NaOH is needed in the digester after the prehydrolysis
stage. NaOH/
white liquor can also be used to control the alkalinity of the HBL used in the
cooking
stage.
