Note: Descriptions are shown in the official language in which they were submitted.
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BATCH PROCESS FOR PREPARING PULP
FIELD OF THE INVENTION
The present invention relates to a process for preparing kraal pulp in which
lignocellulosic
material is treated with recycled process liquors, polysulfide and fresh re-
causticized white
liquor for removing lignin therefrom. More particularly, the present invention
relates to the
use of polysulfide white liquor (or another polysulfide source) in the early
stages ofmodern
displacement kraft batch cooking, and the advantage thereof in terms of
improved pulp
yield.
BACKGROUND OF THE INVENTION
In the various kraft pulping processes, lignocellulosic material, most
commonly wood
chips, is generally treated at elevated temperatures with alkaline cooking
liquor containing
sodium hydroxide and sodium hydrogen sulfide, called white liquor. In these
processes a
significant part of the cellulosic material, especially hemicelluloses, are
partly lost due to
reactions with the white liquor. In kraft pulping conditions hemicelluloses
undergo alkaline
peeling reactions, de polymerization, in which the hemicelluloses are
dissolved as organic
acids into the cooking liquor. A known fact is that by a polysulfide
treatment,
hemicelluloses are stabilized and are thereafter more resistant to cooking
liquor in kraft
cooking conditions, which leads to higher yield from the processes
lignocellulosic
substance input.
In kraft pulping processes, fresh cooking liquor is obtained from the recovery
furnace's
inorganic smelt, which is dissolved in water and causticized to produce white
liquor.
Polysulfide-containing white liquor is obtained from causticized white liquor
by various
processes, which partially oxidize the liquor's sodium sulfide into
polysulfide. These
processes typically use oxidation catalysts to polymerize the white liquor's
hydrogen
sulfide.. Also, polysulfide can be produced directly by elemental sulfur
addition into white
liquor. Polysulfide-containing white liquor is called polysulfide white liquor
or orange
liquor.
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The use of polysulfide white liquor has been quite straightforward in
conventional kraft
cooking operations. The normal white liquor has been converted to polysulfide
white
liquor, and that liquor has been used in the same manner as normal white
liquor. Aver the
impregnation into the wood chips in the digester and during the temperature
rise to above
100 °C, the polysulfide has reacted with the carbohydrates making them
less vulnerable to
alkaline degradation. Later during the cook, when temperature has risen over
140 °C, the
rest of the polysulfide has rapidly broken down through thermal decomposition.
As all the
conventional cooking systems have used cooking liquors having initial
temperatures well
below 100 °C and the temperature in the digester has been elevated
slowly or in steps, the
polysulfide has had the possibility to react and facilitate higher pulp yield.
However, the development of the batch cooleing systems has caused a major
deviation from
the conventional cooking technology: White liquor is introduced into the
digester as "hot
1 S white liquor", which has been heated up using the heat of the previously
generated spent
liquor, and steam. For the purpose ofthis disclosure, "hot" is meant to
designate a liquor
having a temperature above its boiling point at ambient pressure, i.e. a
liquor that must be
stored in a pressurized vessel.
The hot white liquor temperature in typical displacement batch cooking is 140 -
180 °C.
From the displacement batch process point of view, polysulfide has been an
impossible
chemical. Polysulfide is unstable at higher temperatures, for instance at
those used in kraft
cooking; the chemical begins to break down at temperatures above 100°C,
and it degrades
completely in a few minutes at 150 °C. Thus at the typical process
temperature ofthe hot
white liquor, polysulfide white liquor is not stable at all. This fact has
disabled the use of
polysulfide in the modern displacement kraft pulping processes developed for
higher
energy and chemical economy and improved pulp quality.
During the early 80's, new energy efficient kraft batch processes using
various liquor
displacements emerged. Characteristic for the displacement kraft batch process
is to recover
hot black liquor from the end of the cooking by displacement and then re-use
the energy in
the subsequent batches. Partly this is done by using the hot spent cooking
liquor as such,
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and partly by heating the white liquor with the heat of the hot spent liquor
leaving the
digester system or collected for re-use at low temperature. This process
arrangement leads
to high temperatures in the input liquor accumulators. A low temperature white
liquor,
typically at 80 °C in the storage tank after re-causticization, cannot
be used anymore, and
the environment for stable polysulfide in the white liquor does not exist
anymore.
Good examples of the development of displacement batch processes are given in,
e. g.,
Fagerlund, U.S. Pat 4,578,149 and Ostman, U.S. Pat 4,764,251. A fixrther
characteristic
feature of a modern displacement batch process is to combine energy efficiency
and
efficient use of residual and fresh cooking chemicals to achieve, in addition
to energy
efficiency, high pulp strength and good delignification through hydroxyl and
hydrosulfide
concentration and temperature profiling at different stages of the cooking
cycle (Hiljanen,
Tikka, EP-B 520 452, Tikka, U.S. Pat 5,183,535).
The strive for higher pulp yield is easy to understand. It is always more
profitable to
produce more pulp from a certain wood amount. The pulping industry has faced
increasing
environmental pressure to radically cut down environment-polluting effluents.
Reduction of
the organic effluents from pulp bleaching requires kraft cooking to be
extended to yield
much lower residual lignin concentration in the pulp than before. Extended
cooking means
more carbohydrate losses. In order to enable extended cooking while improving
energy
economy and pulp quality, one popular option for the industry has been to use
modern
displacement batch cooking technology. In spite of all the advantages of this
technology, it
has not been possible to take the pulp yield advantage by using polysulfide
white liquor.
The present invention overcomes these problems and enables an efficient use
ofpolysulfide
by a novel displacement batch cooking process.
The use of anthraquinone (A~) as a cooking catalyst is well known. It acts by
reducing
carbohydrate end groups, stabilising them against alkaline peeling and
producing
anthrahydroquinone, which is alkali soluble. Anthrahydroquinone reduces
lignin, making it
more reactive. In this process, anthraquinone is formed again and may react
with
carbohydrates.
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During operation of displacement batch cooking processes, it has been observed
that the
properties of the black liquor originating from the early stages of the cook
differ from those
of black liquor from a traditional cook. Recycled black liquor originating
from the early
stages of the cooking sequence may complicate the evaporation of black liquor.
A
particular problem is fouling of the surfaces of heat exchangers in the
evaporation plant,
leading to a decrease in heat transfer. These evaporator fouling problems are
typically
related to calcium. In the early stage of alkaline cooking, calcium-containing
material
dissolves into the black liquor from the lignocellulosic material. In a
traditional cook, the
cook proceeds with heating, the temperature increases and no essential liquor
exchange
occurs. Then, a major part of the dissolved calcium-containing material in the
cooking
liquor is broken down, calcium carbonate is formed and as a result, a major
part of the
calcium is resorbed onto the lignocellulosic material in the digester.
Following such a
cooking process, evaporation of the black liquor can normally be carried out
without
problems caused by precipitation of calcium. In contrast, the evaporation
problems with
black liquors originating from the early stages of cooking are typically
related to calcium-
containing material dissolved in the early stages of a cook. The dissolved
calcium-
containing material has not been degraded, and the amount of calcium bound to
the
dissolved material in the black liquor is high. As disclosed in publication EP
1242 674, this
problem may be solved by re-introducing liquor having a high calcium
precipitating
potential into the digester as a displacement liquor at the end of a cook. At
this point, the
digester temperature is suiEciently high to cause calcium liberation. The
calcium is then
removed together with the pulp, and does not enter the evaporation and
regeneration cycle.
SUNIl~~IARY OF THE INVENTION
In accordance with the present invention, a method for producing pulp using
polysulfide
white liquor has been provided. The method involves a kraft batch process
which
comprises the addition ofpolysulfide white liquor in the impregnation and
pretreatment in
the very 'front-end' of the cooking process, and a finalizing delignification
stage with white
liquor and optional cooking catalyst addition, which is facilitated by the
modified 'front-
end' process chemistry resulting in improved pulp yield. A volume of
polysulfide white
liquor is added as the first portion of liquor to enter the digester, or
admixed with the
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impregnation liquor prior to this being pumped into the digester. For the
purpose ofthis
disclosure, "impregnation liquor" means a liquor stored at a temperature below
its
atmospheric boiling point, used for a first immersion of the lignocellulosic
raw material in a
batch pulping process.
In the present invention, lignocellulosic material entering a displacement
batch process is
thus pretreated with polysulfide- containing process liquors in conditions
favourable for
pre-treatment and polysulfide stability. A set of displacement batch process
arrangements,
capable of taking advantage of the polysulfide white liquor, are presented.
The required
dose of fresh polysulfide white liquor is introduced in the initial stage of
the process. The
impregnation stage provides for both impregnation and polysulflde pre-
treatment reactions.
An impregnation liquor circulation collects the displaced impregnation liquor
which is re-
used to fill and pressurize the digester after the first fresh liquor fill. As
a result, the
polysulfide concentration and dosage remain at a high level, and only the
liquor balance
excess is left over to be sent to chemicals recovery or re-use later in the
cooking sequence.
The displacement cooking process sequence then continues with hot fills of
black and white
liquors, cooking and final displacement.
;:An optional feature in the present invention is a new way to use a cooking
catalyst, such as
anthraquinone (AQ). As the generation of polysulfide white liquor decreases
the
concentration ofhydrosulfide ions, the delignification rate is slower,
reducing cooking
capacity. The cooking catalyst can be used to maintain a normal kraft cooking
rate and
produci~on level. In the present invention, the catalyst may be introduced
into the digester
together with the hot white liquor. By this means it will not escape due to
displacements,
and on the other hand, it an~ives in the hot cooking stage where it is most
needed. As a
result, the technical and economical benefits of the displacement batch
cooking are
retained, while the advantages of polysulfide and cooking catalyst can be
fully exploited.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 shows a block diagram of a kraft displacement batch cooking system
according to
the invention. The top of the figure defines the required tanks, leaving or
entering streams
denoted by letters. The cooking stage sequence is displayed therebelow,
chronologically
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starting from the top and the vertically indicated streams representing the
liquors passing
through the digester during the various stages. The letters denoting the
various entering and
leaving streams correspond to the source or destination in the tank scheme.
Figure 2 shows a further embodiment of a kra,ft displacement batch cooking
system
according to the invention.
DISCLOSURE OF THE INVENTION
In the following, the method ofthe invention is disclosed in detail with
reference to the
attached Figure 1. It is understood, that liquor is charged from the digester
bottom and
displaced gases and liquor leave from the top.
The kraft batch cook is started by charging the digester with wood chips, or
with another
lignocellulosic material. Ail:er the lignocellulosic material charge, the
digester is filled in a
stepwise sequence using a volume of fresh polysulfide white liquor Xl from
polysulfide
white liquor tank 6, and impregnation liquor A1 from tank 4, topping the
digester full of
liquor. The volume ofpolysulfide white liquor may be added as a separate first
portion,
forming a raising pad of liquor as further liquor is pumped in from below.
Alternatively, the
polysulfide white liquor is mixed into the impregnation liquor.
An overflow, A2, conducted to the impregnation liquor tank 4, is preferred in
order to
remove air and the diluted first front of liquor. Ailer the flow A2 is
stopped, the digester is
pressurized and impregnation is completed during an appropriate period oftime.
The
cooking process is then continued by pumping in a volume of hot black liquor B
1 from hot
black liquor accumulator 1. The cooler impregnation liquor A3, displaced by
hot black
liquor, is conducted to impregnation liquor tank 4 to be ro-used in subsequent
batches.
Pumping a volume C of hot white liquor from tank 3 into the digester then
continues the
cooking sequence. Together with white liquor C, an amount of cooking catalyst
may be
dosed into the digester. The liquor D1, displaced by hot liquor above about
the atmospheric
boiling point, in conducted to hot black liquor accumulator 2.
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At the end of the sequence described above, the digester is close to the final
cooking
temperature. The final heating-up is carried out using direct or indirect
steam heating and
digester re-circulation. After the desired cooking time when the
delignification has
proceeded to the desired final reaction degree, the spent liquor is ready to
be displaced with
wash filtrate E1 from tank 5. A volume A4 from impregnation liquor tank 4 may
be used as
a first portion of displacement liquor in order to deactivate the potential
scale-forming
calcium therein, as disclosed in EP 1242 674.
In the final displacement, the f rst displaced volume B2 displaced by wash
filtrate is
conducted to black liquor accumulator 1. The second portion D2 of displaced
black liquor,
which is diluted by the wash filtrate but is still above atmospheric boiling
point, is
conducted to the hot black liquor accumulator 2. After completed final
displacement, the
digester contents are discharged for further processing of the pulp. The above
cooking
sequence may then be repeated
The hot black liquor from accumulator 2 is conducted to the evaporation plant,
first being
used to heat white liquor due for tank 3, and/or water, by means ofheat
exchange.
Tank 6 is~provided for storage of polysulfide white liquor, supplied from a
plant for
polysulfide generation.
DESCRIPTION OF PREFERABLE EMBODIMENTS
In the following, reference is made to the figure described in the preceding
section.
In the process of the present invention, a volume of polysulfide white liquor,
corresponding
to 25-90 % of the total active alkali dosage and resulting in 0,5% to 5%
polysulfide dosage
on abs. dry wood is added into the digester before the impregnation fill is
completed.
Impregnation is carried out using temperature ofbetween 70 °C and 120
°C, preferably
between 85 - 95 °C, and a time ofbetween about 20 to about 120 minutes,
preferably
between about 30 to about 60 minutes, more preferably between about 25 to
about 40
minutes.
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In a preferable embodiment, the polysulfide white liquor is added as a
separate portion
before the introduction of further liquor.
In a preferable embodiment, the volume ofpolysulfide white liquor corresponds
to 50-75
of the total active alkali dosage for the relevant batch.
In accordance with a particular embodiment of the process of the present
invention, a
cooking catalyst is used. Preferable catalysts are anthraquinone,
anthraquinone sulfonate,
and hydroanthraquinone. Any other appropriate redox-catalyst as anticipated by
those
skilled in the art may be used.
In accordance with an alternative embodiment of the process ofthe present
invention, the
following fractions displaced from the digester:
- hot cooking spent liquor fraction B2 and
- fractions Dl and D2, which are of lower temperature and poorer in chemicals,
are collected in a single accumulator to be used in the hot black liquor
pretreatment stage.
In this configuration, hot black liquor accumulators 1 and 2 of Figure 1 are
combined. The
black liquor flow to the evaporation plant is drawn from the single hot black
liquor
accumulator, the stream being used to heat, by means of heat exchange, white
liquor
heading for white liquor accumulator 3, and/or water, as shown for tank 2 in
Fig 1.
Ifhigh dosages ofpolysulfide white liquor are used in the impregnation stage
ofthe
process, longer impregnation times than in the prior art and/or a two
accumulator system
and/or impregnation liquor transfer to hot black liquor tank 1 may be used in
order to avoid
too high a residual alkali concentration in the feed of the evaporation plant.
Thus, in
accordance with a further embodiment of the process of the present invention,
shown in
Figure 2, a volume of impregnation liquor from impregnation lank 4 is
conducted to hot
black liquor tank 1 to be used in hot black liquor treatment, B 1. The stream
from
impregnation liquor tank 4 to hot black liquor tank 1 may be heated by means
of heat
exchange with the stream from hot black liquor accumulator 2 to the
evaporation plant.
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In addition to the white liquor charge prior to the cooking stage, one or more
additional
white liquor charges may be carried out during the cook.
The principal advantage of the process of the present invention is that the
'front-end' of the
kraft displacement batch cooking system is rendered more effective from a
chemical point
of view, and in regard to process arrangements. The polysulfide charge used
reflects the
improvement in pulp yield from the digested lignocellulosic material.
The following examples are illustrative of the invention. The following
abbreviations are
used:
WBL Warm impregnation black liquor
DWBL Displaced WBL (from the digester)
HBL Hot black liquor
RHBL Displaced HBL (from the digester)
DCBL Displaced cooking black liquor
WF Wash filtrate
PSWL Polysulfide white liquor
Production of a normal 'reference' pine kraft pulp using the displacement
kraft batch
technique:
4,0 kg pine (Pious sylvesfris) chips {oven dry basis) were metered into a chip
basket
positioned in a 20-litre jacketed displacement batch digester. The lid of the
digester was
closed. Impregnation black liquor, consisting of wash filtrate and white
liquor (WBL, 80-
90°C, 25 gEA/1) was pumped in during app. 10 minutes followed by
impregnation at 90°C
under 2,5 bar overpressure. The duration of the impregnation stage was 40
minutes (WBL-
fill and impregnation in toi:al). Alter impregnation, a hot black liquor pre-
treatment stage
was commenced by introducing hot black liquor (HBL 1, 150°C, app. 20
gEAll) to the
bottom of the digester, displacing the spent impregnation liquor (DWBL) out
from the top
of the digester. The hot black liquor pre-treatment stage duration was 30
minutes. After hot
black liquor pre-treatment stage, hot white liquor (121 gEA {NaOI~/l,
sulfidity 37%)
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charge was introduced into the digester bottom displacing the corresponding
volume of
spent hot black liquor (RI-IBL) out of the digester. A 45 minute heat up
sequence with
cooking liquor circulation raised the temperature from 150°C to the
cooking temperature of
167°C. At 250 H-factors, a white liquor charge was introduced into the
digester bottom
5 displacing the corresponding volume of spent hot black liquor out of the
digester (DCBL
1). After the cooking time fulfilled the target H-factor, terminal
displacement was started
by introducing cool DWBL and wash filtrate (WF) in a sequence into the
digester bottom
displacing the hot spent black liquor out of the digester top {DCBL 2). The
first displaced
portion the hot black liquor with the hot black liquor displaced earlier
during the cooking
10 sequence (DCBL 2 + DCBL 1) covered the volume needed for the next cook's
hot black
liquor (HBL 1 ). The terminal displacement stage duration was approximately 40
minutes.
After the terminal displacement, the pulp was disintegrated, washed with
deionised water,
screened and analysed. The cooking procedure was repeated and thus equilibrium
in the
cooking process was obtained and the cooks si;arted to repeat themselves
corresponding to
the industrial batch cooking system.
The experimental set up to produce pulp according to the invention is
described below.
,. 4,0 kg pine (Panes sylvesfris) chips (oven dry basis) were.metered into a
chip basket
positioned in a 20-litre jacketed displacement batch digester. The lid of the
digester was
closed. The chips were steamed until the digester temperature was
100°C, approximately 5
minutes. Impregnation liquors were introduced as sequence as follows:
initially polysulfide
white liquor black liquor (PSWL, volume according to the charge, app. 4,5-51);
subsequently re-circulated spent impregnation liquor (DWBL) to hydraulically
fill the
digester. The impregnation fill sequence took app. 11-13 minutes and was
followed by
impregnation at 90°C under 2,5 bar overpressure. The duration of the
impregnation stage
was 40 minutes (fill sequence and impregnation in total). After impregnation,
a hot black
liquor pre-treatment stage was carried out by introducing hot black liquor
(ILL l, 150°C,
app. 20 gEA/1) to the bottom of the digester of the digester, displacing the
spent
impregnation liquor from the top of the digester (DWBL). The hot black liquor
pre-
treatment stage duration was 30 minutes. Ailer the hot black liquor pre-
treatment stage, hot
white liquor {l21 gEA (NaOH)!1, sulfidity 37%) charge was introduced,
displacing the
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corresponding volume of spent hot black liquor from the digester (RHBL).
Together with
the WL, a cooking catalyst was charged mixed with a small volume of hot black
liquor. A
45 minute heat-up sequence with cooking liquor circulation raised the
temperature from
150°C to the cooking temperature of 167 °C. At 250 H-factors, a
further white liquor
charge was introduced into the digester bottom displacing the corresponding
volume of
spent hot black liquor from the digester (DCBL 1). After the cooking time
fulfilled the
target H-factor, terminal displacement was started by introducing a volume of
cool DWBL
and wash filtrate (WF) in a sequence into the digester bottom displacing the
hot spent black
liquor out of the digester top (DCBL 2). The first displaced portion of hot
black liquor
together with the hot black liquor displaced earlier during the cooking
sequence (DCBL 2 +
DCBL 1) covered the 171 volume needed for the next cook's hot black liquor
(HBL 1).
After the terminal displacement, the pulp was disintegrated, washed with
deionised water,
screened and analysed. The cooking procedure was repeated, equilibrium in the
cooking
process was reached and the cooks started to repeat themselves corresponding
to the
industrial batch cooking system.
Example 1. Demonstration of yield improvement using polysulfide white liquor
addition
The reference cook follows the same addition and temperature sequences as the
cooks
according to the invention. All cooks were digested to the same target kappa
number. A
clear yield improvement correlating with polysulfide addition can be observed
PS-charge, % Total yield, % Rejects, % on
Cooking model on wood Kappa number on wood wood
Reference Nil 25,9 44,0 0,26
SB-PSAQ 1,21 25,9 45,8 0,36
SB-PSAQ 1,58 25,8 46,8 0,24
Example 2. The effect of anthraquinone to preserve the rate of delignification
during
cooking
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It is seen, that the reference cook requires a significantly higher H-factor
to reach the target
kappa number.
PSWL Reject,
in imp,PS, % Yield,%
% on
Method % on wood H-factorEoC, Kappa#on wood
g/1 wood
Reference - - 1285 16 26,9 43,6 1,01
PS pad, AQ 70 % 1,45 1106 19,1 27,1 44,3 0,15
0%
PS pad, AQ 70 % 1,45 1030 19,9 26,7 44,5 0,16
0,05%
PS pad, AQ 70 % 1,45 1096 18,1 27,3 45,8 0,1
0,1%
