Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W092/07994 ~09 4 ~ 1 9 PCT/US91/07402
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PULPING PROCESS
Backqround of the Invention
i. Technical Field
The present invention relates generally to the field
of chemical cooking processes, and more specifically to a
pulp cooking process in which fibrous material is cooked at
elevated temperatures and pressures in the presence of
chemicals and subsequently further treated in the digester
to separate fibers from the bonding agents in the material
and to whiten the pulp.
ii. Technical Background
In typical cellulose chemical pulping operations, wood
fibers are separated from lignin by cooking wood chips at
elevated temperatures and pressures in the presence of
chemicals. A certain level of delignification can be
achieved in these cooking vessels, known as digesters, with
the pulp resulting therefrom being passed from the
digesters to subsequent operations. The subsequent
operations often include various stages of washing to
remove residual chemicals and other pulp constituents; as
well as various stages of bleaching wherein the pulp is
subjected to still further chemical treatments to further
remove lignin and increase the brightness of the resulting
pulp. The bleaching operations take place in separate
bleaching vessels and are followed again by various stages
of washing, to remove bleach chemicals from the pulp. The
washing stages are frequently performed in drum type
washers wherein the slurry is washed on a drum in a
continuous operation. Bleach plants and the associated
washing stages require vast expanses of real estate for
equipment, and significant capital investment.
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In recent years, enhanced environmental awareness has
subjected the pulp and paper industry to close
scrutinization in that the various pulping, washing, and
bleaching stages can result in the discharge of spent
aqueous solutions potentially harmful to the environment.
Typical bleaching processes included the use of chlorine,
and bleaching effluents can contain residual chlorine,
which is perceived as undesirable in the environment.
Chlorine recovery processes are difficult in that the
corrosive nature of the effluents is harmful to common
metals, and requires expensive equipment for chemical
recovery. Additionally, such recovery processes are
extremely complex and expensive, while yielding no
financial return on the capital outlay required for their
construction.
In typical batch cooking processes, upon completion of
the cook, a digester blow valve is opened and the high
temperature, high pressure liquid in the digester flashes,
thereby emptying the digester. In recent years,
displacement heating and cooking techniques have become
popular wherein, before or after completion of the cook,
liquor displacements have been utilized to conserve energy
and chemicals.
In a known process, as taught in U.S. patent
4,578,149, at the completion of the cook, hot spent cooking
liquors in the digester are displaced with effluent from
subsequent washing stages, with the hot spent cooking
liquors being made available for preheating and pretreating
subsequent chip charges in a digester. This cooking
process has lower steam requirements, is more acceptable
environmentally, produces higher quality pulp and recovers
more of the energy and chemicals put into the process than
conventional batch processes. After a displacement at the
end of the cook, cooked pulp chips are emptied from the
digester by the application of pressure at the top of the
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digester, with the pulp then being passed to subsequent
conventional treatments such as further pulp washing,
bleaching, additional washing, and the like.
~ .S. patent no. 5,015,333 teaches a proces~
in which the equivalent of one or several independent
stages of washing can be achieved by the utilization of
displacement washing techniques in the digester.
Summary of the Invention
It is therefore one of the principle objects of the
present invention to further expand the process steps which
take place in a cellulose pulp digester, to minimize
equipment and space required for subsequent treatment of
the fibrous material after it leaves the digester.
Another object of the present invention is to provide
a cellulose pulping process operable in a batch process
which can complete various washing and bleaching stages
without having to transport the pulp from a batch digester
to subsequent process apparatus.
A further object of the present invention is to
provide a pulping process wherein real estate requirements
for equipment are minimized, making pulping plants of
smaller eYr~nce while producing acceptable quality pulp.
A still further object of the present invention is to
provide a pulp bleaching process which does not require the
use of chlorine and is environmentally preferred to
previously known processes utilizing chlorine.
Yet another object of the present invention is to
provide a pulping process which reduces total process time
from beginning to end as compared with conventional
equivalent processes.
A
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2 ~ s~e8a1n9d other objects are achieved in the present
invention by utilizing a batch cooking process wherein at
the completion of a batch cook at elevated pressures and
temperatures, liquor displacement techniques are used to
wash the chips and remove residual liquors therefrom, and
to further delignify the chips through the use of
in-digester displacements with chemicals for pulp
bleaching. In a preferred process, bleaching is performed
with chemicals other than those containing chlorine, and
acceptable brightness pulp is achieved.
Further objects and advantages of the present
invention will become apparent from the following detailed
description and the accompanying drawings.
Brief DescriPtion of the Drawings
Figures 1 through 10 illustrate a preferred pracess
sequence for carrying out the expanded treatment of pulp in
a digester according to the present invention.
Figure 1 illustrates the chip filling step;
Figure 2 illustrates a cool liquor fill;
Figure 3 illustrates a warm liquor fill;
Figure 4 illustrates a hot liquor fill;
Figure 5 illustrates the time-to-temperature and
time-at-temperature steps in the digesting process;
Figure 6 illustrates the end of cook displacement
step;
Figure 7 illustrates a post cook oxidized green liquor
treatment;
Figure 8 illustrates a peroxide treatment step;
Figure 9 illustrates a peroxyacetic acid treatment;
and
Figure 10 illustrates the pulp discharge step.
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Detailed Description of the Preferred Embodiment
A process to meet the objectives outlined previously herein
shoul~ h2ve familiar unit operations and be compatlble with
subsequent and prior process steps. In the aforementioned U.S.
patent 4,578,149, a batch cookin~ pr~cess is known utilkin~ familiar
~quipment and resultin~ in energy conservation, extended cooking
capabilities, effectivQ reuse and recycle of chemical- alk~li charges
resu(tin~ in acceptable deli~nification levels requirin~ lesser fresh
chemical tiquor charges, and an in-di~ester brown s~ck washin~
stage.
The present invention entails the continu~tion of process steps
in the digester, wherein additional displacements are made uti~izing
various additional chsmicals and/or spent liquors to further dissolve
li~nin and bri~h~en the pulp. Previously, these process steps h~ve
been performed outside the digester, in additional equipment, in less
effi~ient manner. The chemi~als sRlected for the further deli~nification
and brightening prefera~ly include rea~ents of n~n-ohlorine types,
includin~ but not limited to hydrosen peroxide, oxygen, sodium
polysulfide, sodium sulfite, sodi~m borohydride, sodium hydrosulfite,
ozone, and peroxyacetic acid. Other rea~ents rnay be useful ~or the
same purpose, so lon~ as the general select;on criteria ~re met. The
group of ~ppropriate bleaching agents includes bleaching agents that
will dissolv~ nin and brighten pulp, that are compatible with process
metallur~y, that ar~ compatib~e with practical ch~mical recovery and
matcriai balance, and preferably the by-products of which are
relatively innocuous to the environment i~ accidentally lost or spilled.
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The process can be carried out in a varie~ of manners wherein
additional in-digester disp~acemen~s are utilized to affec~ both pulping
and bl~achin~ operations, with necessary washin~, so as to reduce
downstream processing required fot sirnilar delignification results, as
compared with common and conventional processes.
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The following examples of the present invention
represent trials done in a pilot plant operation.
Example 1
Southern yellow pine was cooked in a conventional
kraft displacement heating process according to Rapid
Displacement Heating (RDH) techniques as marketed by Beloit
Corporation. This process is essentially that as disclosed
in U.S. patent 4,578,149. The RDH cook continued to a
target Kappa level of 16, after which a conventional RDH
displacement occurred. Following displacement, rather than
discharging the pulp as is conventionally known in the RDH
process, a solution of hydrogen peroxide was displaced
through the chip column at 80~ C for a period of 30
minutes. A noticeably whiter pulp was obtained, measuring
31.9% I.S.O. versus 30.3% I.S.O. in the conventional RDH
pulp at 16 Kappa.
Exam~le 2
Again southern yellow pine was cooked in a
conventional RDH kraft process in conventional manner to a
target Kappa of 16. Following the conventional RDH
displacement, oxidized green liquor (sodium polysulfide at
low alkalinity) was displaced through the chip column
followed by two more subsequent displacements and
treatments with moderate amounts of hydrogen peroxide. The
resulting pulp was found to have a Kappa number of 7.5,
with a digester discharge brightness of 55% I.S.O.
Example 3
A cook similar to that of Example 2 was performed,
after which 2% hydrogen peroxide was used in 2 separate
post cooking treatments. A Kappa of approximately 10 was
achieved with a brightness of 40.7% I.S.O. after the
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treatments. As a final treatment, an aqueous solution of
peroxyacetic acid was displaced through the chip column in
a pH4 buffer solution of acetic acid sodium acetate. The
pulp discharged from this treatment had a Kappa of
approximately 6 and a brightness of 50.8% I.S.O.
Additionally, several other bleaching sequences using
peroxide (P) and peroxyacetic acid (A) were compared with
similar sequences using chlorine dioxide (D), which is
analogous to peroxyacetic acid, but having the undesirable
chlorine atoms. The following results were obtained:
Sequence Conventional RDH Sample 4 Sample 5
(EOP)-A-P-A 84.8 86.3 ----
A-P-A ---- 82.2 86.9
(EOP)-D-P-D 90.6 89.7 ____
It can be seen that market pulp brightness can be
obtained from the more environmentally friendly sequences
containing only hydrogen peroxide and peroxyacetic acid
comparable to similar sequences using chlorine dioxide.
However, the advantages of in-digester treatment are
obtained, including lower installed bleach plant costs and
fewer process steps; and the undesirable use of chlorine
based reagents is eliminated. While higher chemical
utilization may result due to less washing between stages,
these disadvantages are offset since all chemicals used are
recycled to the chemical recovery cycle. If desired,
additional washings by displacement may be used between the
various process steps, including the use of chemicals to
neutralize or otherwise prepare the digester contents
between successive chemical treatments, to make more
effective use of reactants displ_sed through the chip
column. An effective process for performing such washing
or preparation is to precede a treatment displacement with
spent chemical of that treatment from a previous treatment,
perhaps in a different digester.
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The in-digester bleaching following RDH cooking
processes results in shorter process times for a single
chip charge. Whereas a conventional process from chip
charge at the digester prior to cooking to the time
bleached pulp goes to high density storage requires
approximately 15 hours, using the present in-digester
bleaching process reduces the total elapsed time to
approximately 9 hours. While more process time is required
in the digester for a single charge, additional digester
capacity can be provided in a pulp mill at lower installed
cost than is necessary for typical bleaching and washing
sequences. Time is saved due to the inherent efficiencies
in the displacement processes, as compared to the
conventional washing and bleaching process which require
thickening, mixing, dilution, and reacting in multiple
repetitive steps.
Referring now more specifically to the drawings,
various steps in a preferred process according to the
present invention will be described.
In Figure 1, a suitable process equipment layout 10 is
illustrated for carrying-out the present invention. A
plurality of digesters 12 are provided for cooking wood
chips according to displacement heating techniques and for
carrying out the subsequent reaction steps with the pulped
chips according to the present invention. The process
layouts in Figures 1 through 10 illustrate numerous
essential vessels, tanks, and the like, some of which
operate at super atmospheric pressure and others of which
operate at atmospheric pressure. Those vessels operating
under pressure are illustrated by circular tanks, and
square vessels are representative of atmospheric vessels.
Various alternative processes are illustrated by optional
tanks and vessels shown in dotted lines and to be described
more specifically with respect to later drawings.
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In Figure 1, the chip filling step of the pulping
process is illustrated wherein chips from a chip storage
location 14 are transferred to any one of the digesters 12
for beginning the pulping process. The chips may have been
pretreated by known processes such as steaming or the like,
depending on the chips being used and the desired pulping
results. Normally, steam packers or the like, also not
part of the present invention, will be used to fill the
digester.
Figure 2 illustrates the second step of the
conventional Beloit Corporation RDH process which is
referred to as the cool liquor fill. Cool black liquor
from the cool black liquor accumulator 16 is pumped into
the digester at the bottom thereof, in such a manner as to
overfill the digester with the cool black liquor flowing
from the digester at the top thereof, and back to the cool
black liquor accumulator 16. This cool liquor fill expels
air from the digester and immerses the chips in liquor.
The direct contact of the chips with the liquor preheats
the chips, and the residual chemicals remaining in the cool
black liquor begin initial reactions for the
delignification of the chips. During this fill with cool
black liquor, to enhance the precooking reactions, the cool
black liquor or a portion of it can be fortified with the
addition of cool spent green polysulfide liquor from a
green polysulfide liquor storage vessel 18. Additionally,
oxidized green liquor from an oxidized green liquor storage
tank 20 can be added.
Figure 3 illustrates the conventional warm liquor fill
associated with the RDH process, wherein warm black liquor
from an accumulator 22 is pumped to the digester containing
chips and cool liquor, with the cool black liquor being
thus displaced by subsequent quantities of warm black
liquor and being transferred back to the cool black liquor
accumulator 16. The warm black liquor provided to the
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digester can be fortified with the addition of
anthraquinone from an optional storage tank 24.
For the purpose of generating hot water and/or
reducing the volume of warm black liquor in the accumulator
22, the warm black liquor can be passed through a heat
exchanger 26 in heat exchange relationship with cold water
supplied at 28 for generating hot water removed at 30 and
additional volumes of cool black liquor being passed to the
cool black liquor accumulator 16.
Figure 4 illustrates the hot liquor fill of the RDH
process, in which hot white liquor from a hot white liquor
storage vessel 32 is pumped into the digester, perhaps with
the addition of hot black liquor from a hot black liquor
accumulator 32. The hot white liquor is heated by passing
cool white liquor from a cool white liquor tank 36 through
a heat exchanger 38 in heat exchange relationship with hot
black liquor from the hot black liquor accumulator 34. The
hot black liquor leaving the heat exchanger 36 is passed to
the warm black liquor accumulator 16. As the hot white
liquor is pumped into the digester, warm black liquor in
the digester is displaced and is passed to the warm black
liquor accumulator 22. Upon complete displacement of the
warm black liquor from the digester, some hot liquor may
pass from the digester, which is then transferred to the
hot black liquor accumulator 34.
Figure 5 illustrates the cooking sequence wherein
liquor in the digester is circulated through the digester
by means of a pump 40. In the process illustrated, liquor
is withdrawn from the digester at a point intermediate the
top and bottom ends by the pump 40, and is supplied to
inlets at the top and bo~tom of the digester
simultaneously. An external source of heat, such as steam
(not shown) may be used to raise the liquor to the desired
temperature level. After the desired temperature is
reached, the temperature is held for a specified period of
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time until the desired level of delignification has been
reached. These time-to-temperature and time-at-temperature
operations are well-known for various wood species,
processes and desired results.
Figure 6 illustrates the end of the cooking stage
wherein cool spent green polysulfide liquor from the
storage vessel 18 is pumped into the bottom of the digester
to displace therefrom the cooking liquor present at the end
of the cook. The liquors displaced from the digester are
passed to the hot black liquor accumulator, the warm black
accumulator, or back to the cool spent polysulfide storage
vessel, as appropriate.
Figure 7 illustrates a post-cook green polysulfide
liquor treatment in the digester. Green polysulfide liquor
from the storage unit 18 is pumped into the bottom of the
digester after having had added thereto oxidized green
liquor from the oxidized green liquor storage tank 20.
Displaced liquors are passed to the cool black liquor
storage accumulator.
Figure 8 illustrates a post cook peroxide treatment
wherein cool spent hydrogen peroxide liquor from a cool
spent hydrogen peroxide storage vessel 42 is mixed with
fresh hydrogen peroxide from a storage tank 44, the mixture
then being pumped into the bottom of the digester to
displace therefrom spent polysulfide liquor which is passed
to the polysulfide liquor storage vessel 18.
Figure 9 illustrates an alternative peroxyacetic acid
treatment wherein peroxyacetic acid from a storage tank 46
is combined with displacement liquor from a storage tank 48
and is pumped into the bottom of the digester, to displace
hydrogen peroxide to the hydrogen peroxide storage vessel
42.
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Figure lO illustrates a suitable pulp discharge cycle
wherein, upon completion of treatment, pulp from the
digester is removed to a dump tank 50 and then to washing
stages 52 which may include the addition of hot water at
54. Evacuation of the digester to the dump tank can be by
any of several means, including the application of fluid
pressure at the top of the digester, either liquid or
gaseous, with the pulp and remaining liquid in the digester
being passed from the bottom of the digester to the dump
tank 50. The digester also can be evacuated by-pumping the
contents from the digester, either in a continuous pumping
manner, as known previously, or through intermittent cyclic
pumping as taught in CAnA~;An patent application no. 2,025,454
which has matured into CAnA~;An patent no. 2,025,454. Wa~hing
can be performed in any of the known conventional belt or drum
vacuum and pressure washers, or a final wash may be performed
consistent with the teaching of the aforementioned ~.S. patent
no. 5,015,333. Liquor from the wAch;n~ stages is passed to a
brown stock washer filtrate tank 56 and to the displacement
liquor storage tank 48 for subsequent use.
It has been found that various reagents may be
utilized to continue pulp processing in the digester, while
the pulp is maintained in a substantially stationary
column. Following the final disp}acement in the
conventional R~H process, bleaching chemicals can-be
displaced through the column of pulp to continue the
removal of lignin and the whitening and brightening of the
pulp. Effective bleaching reactions occur without the
previously required separate steps of mixing, blending,
thickening, and reacting. Since time is not needed to
transfer the pUlp between various stages, continuing the
bleaching and washing processes in the digester saves
overall process time from beginning to end, for a given
volume of pulp. While additional digester time is required
for any given chip charge, the reduced overall process time
minimizes, to some degree, the increased time required in
the digester.
r.~
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It will be recognized that in a typical pulping
operation, the various digesters in the plurality of
digesters 12 will be operating at different process stages
at different times. However, the various accumulators and
storage vessels and tanks will receive from and/or supply
liquor and chemicals to each of the various digesters, as
required. This provides the capability to enhance and make
more efficient the various treatments with selected
reagents by preparing the chip column for treatment prior
to the addition of the reagents. For example, if a
peroxide treatment in one digester is required, the actual
peroxide treatment displacement can be preceded by a
displacement using spent peroxide liquor obtained from the
peroxide treatment in a different digester. By preceding
the actual treatment displacement with a displacement of
the spent liquor, process efficiencies are obtained in that
initial reactions are begun with the spent liquors, thereby
making the fresh liquor treatment in the following
displacement more effective. More complete use of residual
reagents in spent liquors is realized. This enables
certain treatments to be performed using displacement
volumes less than the digester volume, as the necessary
treatment will occur as the layer of treatment liquor
progresses upwardly through the digester as it is displaced
therethrough by the addition of other liquids subsequently.
Thus, while some processes may require liquor volumes as
great as or greater than the digester volume, others may be
performed by displacement through the digester of liquor
volumes substantially less than the digester volume, and at
a given time in the digester, a plurality of layers of
different liquors may be present.
It has been found that various bleaching sequences not
using chlorine produce similar advantageous bleached pulp
results, without the use of chlorine. These
environmentally more acceptable reagents are preferred for
the bleaching displacements.
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While a general concept for expanded pulp treatment in
a digester has been shown and described in detail herein,
it should be recognized by those skilled in the art that
various changes may be made without departing from the
scope of the present invention.
