Note: Descriptions are shown in the official language in which they were submitted.
CA 02707330 2010-06-14
TITLE OF THE INVENTION
METHOD AND SYSTEM FOR
HIGH ALPHA DISSOLVING PULP PRODUCTION
BACKGROUND OF THE INVENTION
1) Field of the Invention
[0001] The field of the invention generally relates to pulp processing and,
more
specifically, to an improved method and system for treating effluents from
cold
caustic extraction in connection with a kraft chemical pulping process.
2) Background
[0002] Pulp from wood and plant materials has a large number of commercial
uses. Although one of the most common uses is in paper manufacturing, pulp can
also be used to produce a number of other products including rayon and other
synthetic materials, as well as cellulose acetate and cellulose esters, which
are
used, for example, in the manufacture of filter tow, cloth, packaging films,
and
explosives.
[0003] A number of chemical and mechanical methods exist for processing
wood and plant materials in order to manufacture pulp and paper. The basic
processing steps include preparing the raw material (e.g., debarking and
chipping),
separating the wood fibers by mechanical or chemical means (e.g., grinding,
refining or cooking) to separate the lignin and extractives from cellulose of
the wood
fibers, removing coloring agents by bleaching, and forming the resulting
processed
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pulp into paper or other products. In addition to and in connection with pulp
and
paper manufacturing, paper mills also typically have facilities to produce and
reclaim chemical agents, collect and process by-products to produce energy,
and
remove and treat wastes to minimize environmental impact.
[0004] "Pulping" generally refers to the process for achieving fiber
separation.
Wood and other plant materials comprise cellulose, hemicellulose, lignin and
other
minor components. Lignin is a network of polymers interspersed between
individual fibers, and functions as an intercellular adhesive to cement
individual
wood fibers together. During the pulping process, lignin macromolecules are
fragmented, thereby liberating the individual cellulosic fibers and dissolving
impurities that may cause discoloration and future disintegration of the paper
or
other final product.
[0005] The kraft process is a commonly used pulping process. Paper
produced from kraft pulping process can be used, for example, to make bleached
boxboard and liner board used in the packaging industry. A conventional kraft
process treats wood with an aqueous mixture of sodium hydroxide and sodium
sulfide, known as "white liquor". The treatment breaks the linkage between
lignin
and cellulose, and degrades most of lignin and a portion of hemicellulose
macromolecules into fragments that are soluble in strongly basic solutions.
This
process of liberating lignin from surrounding cellulose is known as
delignification.
The soluble portion is thereafter separated from the cellulose pulp.
[0006] Figure 1 shows a flow diagram of a conventional kraft process 100.
The
process 100 involves feeding wood chips (or other organic pulp-containing raw
materials) 118 and alkaline solutions into a high-pressure reaction vessel
called a
digester to effect deligniflcation, in what is referred to as a "cooking"
stage 121.
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The wood chips are combined with white liquors 111, which may be generated
from
downstream processes or provided from a separate source. Delignification may
take several hours and the degree of delignification is expressed as the
unitless "H
factor", which is generally defined so that cooking for one hour in 100 C is
equivalent to an H factor of 1. Because of the high temperature, the reaction
vessel is often pressurized due to the introduction of steam. Towards the end
of
the cooking step, the reaction vessel is reduced to atmospheric pressure,
thereby
releasing steam and volatiles.
[0007] The white liquor used in the cooking may be, for example, a caustic
solution containing sodium hydroxide (NaOH) and sodium sulfide (Na2S). The
property of the white liquor is often expressed in terms of effective alkali
(EA) and
sulfidity. Effective alkali concentration may be calculated as the weight of
sodium
hydroxide plus one-half the weight of sodium sulfide, and represents the
equivalent
weight of sodium hydroxide per liter of liquor, expressed in gram per liter.
Effective
alkali charge as sodium hydroxide represents the equivalent weight of sodium
hydroxide per oven-dried weight of wood, expressed in percentage. Sulfidity is
the
ratio of one-half the weight of sodium sulfide to the sum of the weight of
sodium
hydroxide and one-half the weight of sodium sulfide, expressed in percentage.
[0008] After cooking, a brown solid cellulosic pulp, also known as "brown
stock," is released from the digester used in the cooking stage 121, and is
then
screened and washed in the washing and screening process 122. Screening
separates the pulp from shives (bundles of wood fibers), knots (uncooked
chips),
dirt and other debris. Materials separated from the pulp are sometimes
referred to
as the "reject" and the pulp as the "accept." Multi-stage cascade operations
are
often utilized to reduce the amount of cellulosic fibers in the reject stream
while
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maintaining high purity in the accept stream. Further fiber recovery may be
achieved through a downstream refiner or reprocess of sieves and knots in the
digester.
[0009] The brown stock may then be subject to several washing stages in
series to separate the spent cooking liquors and dissolved materials from the
cellulose fibers. The spent cooking liquor 112 from the digester employed in
the
cooking stage 121 and the liquor 113 collected from the washing and screening
process 122 are commonly both referred to as "black liquor" because of their
coloration. Black liquor generally contains lignin fragments, carbohydrates
from the
fragmented hemicelluclose and inorganics. Black liquor may be used in addition
to
white liquor in the cooking step, as illustrated for example in Figure 1 by
the arrow
representing black liquor 113 produced in the washing and screening process
122
and transferred to the cooking stage 121. Black liquor 135 from an accumulator
tank (not shown in Figure 1) may also be fed to the digester as part of the
cooking
stage 121, if needed to achieve the appropriate alkaline concentration or for
other
similar purposes.
[0010] The cleaned brown stock pulp 131 from the washing and screening
process 122 may then be blended with white liquor 114 and fed into a reaction
vessel to further separate dissolved materials such as hemicellulose and low
molecular weight cellulose from the longer cellulosic fibers. An exemplary
separation method is the so-called cold caustic extraction ("CCE") method, and
is
represented by CCE reaction stage 123 in Figure 1. The temperature at which
the
extraction is effected may vary but a typical range is less than 60 C.
[0011] The purified pulp 132 from the reactor used in the CCE reaction
stage
123 is then separated from spent cold caustic solution and dissolved
hemicellulose,
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and washed several times in a second washing and separation unit in a CCE
washing stage 124. The resulting purified brown pulp 133 with relatively high
alpha cellulose content, still containing some lignin, continues to a
downstream
bleaching unit for further delignification. In some pulp production processes,
bleaching is performed before the CCE reaction stage 123 and the CCE washing
stage 124.
[0012] It is desirable in a number of applications, such as the manufacture
of
synthetic materials or pharmaceutical products, to have pulp of very high
purity or
quality. Pulp quality can be evaluated by several parameters. For example, the
percentage of alpha cellulose content expresses the relative purity of the
processed
pulp. The alpha cellulose content can be estimated and calculated based on the
pulp solubility (e.g., S10 and S18 factors described below). The degrees of
delignification and cellulose degradation are measured by Kappa Number ("KN")
and pulp viscosity respectively. A higher pulp viscosity indicates longer
cellulose
chain length and lesser degradation. Standard 236 om-99 of the Technical
Association of Pulp and Paper Industry (TAPPI) specifies a standard method for
determining the Kappa number of pulp. The Kappa number is an indication of the
lignin content or bleachability of pulp. Pulp solubility in 18 wt% sodium
hydroxide
aqueous solutions ("S18") provides an estimate on the amount of residual
hemicellulose. Pulp solubility in 10 wt% sodium hydroxide aqueous solution
("S10")
provides an indication on the total amounts of soluble matters in basic
solutions,
which include the sum of hemicellulose and degraded cellulose. Finally, the
difference between S10 and S18 indicates the amount of alkali soluble
fragmented
cellulose.
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[0013] Conventional techniques can achieve purified pulp with alpha
cellulose
content between 92 and 96 percent, although historically it has been quite
difficult
to reach purities in the upper end of that range, particularly while
maintaining other
required properties of the pulp like high viscosity (i.e., limited cellulose
degradation
resulting from the pulping process).
[0014] In a conventional process, the filtrate 116, also referred to as the
CCE
alkaline filtrate, from the CCE washing and separation stage 124 comprises
both
the spent cold caustic solution and the spent washing liquid from the washing
and
separation stage 124. This filtrate 116 often contains substantial amounts of
high
molecular hemicellulose. When filtrate with high hemicellulose content is
recycled
for use as part of the cooking liquor in the digester of the cooking stage
121,
hemicellulose may precipitate out of the solution and deposit on the
cellulosic
fibers. This can prevent high quality pulp from being achieved. On the other
hand,
certain applications¨such as high quality yarn or synthetic fabrics, materials
for
liquid crystal displays, products made with acetate derivatives, viscose
products
(such as tire cord and special fibers), filter tow segments used in
cigarettes, and
certain food and pharmaceutical applications¨need pulps containing a minimal
amount of redeposited hemicelluloses and a high alpha cellulose content.
[0015] As illustrated in Figure 1, part of the CCE alkaline filtrate 116
has to be
bled to the recovery area 134 in order to control the hemicelluloses
redeposition in
the cooking stage 121. The diverted CCE alkaline filtrate 116 sent to the
recovery
area 134 may be combined with excess black liquor, concentrated and combusted
in a recovery boiler to consume the organics and recover inorganic salts. A
new
alkali source may then be needed to replace the CCE filtrate and black liquor
sent
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to the recovery area 134 in order to maintain proper alkali balance in the
cooking
stage 121.
[0016] The conventional process does not provide an efficient or cost-
effective
means for achieving cellulose of suitable alpha content that may be needed for
a
variety of industrial, pharmaceutical and material uses including those
identified
above.
[0017] There exists a need for a pulp processing method and system that
results in a dissolving pulp with very high alpha cellulose content. There
further
exists a need for a pulp processing method and system that provides an
efficient
and cost effective way for preparing high alphas dissolving pulp by preventing
hemicelluloses redeposition.
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SUMMARY OF THE INVENTION
[0018] In one aspect, an improved method and system for pulp manufacturing
involves, among other things, enriching one or more of black liquor and cold
caustic
extraction (CCE) alkaline filtrate used in the cooking stage with white
liquor.
[0019] According to one or more embodiments, a method and system for pulp
manufacturing used in connection with a kraft process includes a cooking stage
having
the steps of feeding wood chips or other organic pulp-containing materials
into a
digester or similar reaction vessel, performing a sequency of sequential
process
phases: pre-hydrolysis, neutralizing the chips with a white liquor plus a CCE
alkaline
filtrate optionally enriched with white liquor, filling the digester with hot
black liquor
and/or a CCE alkaline filtrate (either or both enriched with a white liquor),
and cooking
for an amount of time effective to result in delignification. These steps may
be
followed with cold displacement and pulp discharge.
[0020] After the cooking stage, further steps may include treating a
resulting
brown stock to yield semi-purified pulp, extracting the semi-purified pulp
with a caustic
solution to yield a purified pulp and a solution containing hemicellulose,
separating the
hemicellulose-containing solution from the purified pulp, washing the purified
pulp and
collecting an alkaline filtrate resulting therefrom, and utilizing a
significant portion of the
alkaline filtrate (optionally concentrated by evaporation or other means) in
the digester.
The overall process may help prevent hemicelluloses deposition, improve the
purity of
high alpha dissolving pulp, and increase the efficiency of the overall pulp
manufacturing system.
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[0020a] Accordingly, there is provided a method for processing pulp-
containing
organic material that has undergone pre-hydrolysis in a reaction vessel, as
part of a
kraft process for producing dissolving pulp, comprising: adding a first
quantity of white
liquor to the reaction vessel as a first neutralization fluid to only
partially fill available
space within the reaction vessel with the white liquor, while maintaining the
vessel at a
temperature of between 120 C and 180 C, the white liquor constituting a
first alkaline
solution; after adding the first quantity of white liquor, adding a second
alkaline solution
other than white liquor or only partially containing white liquor, and
containing a filtrate
containing hemicellulose, to the reaction vessel to constitute, along with the
first
quantity of white liquor, a complete neutralization fluid present in the
reaction vessel;
displacing the neutralization fluid with one or more cooking fluids suitable
for carrying
out kraft cooking; cooking the pulp-containing organic material in the
reaction vessel;
and discharging the cooked pulp from the reaction vessel.
[0020b] There is also provided a method for pulp processing used in a kraft
process for producing dissolving pulp, comprising: adding pulp-containing
organic
materials to a digester; performing pre-hydrolysis on the pulp-containing
organic
material in the digester; adding a first quantity of white liquor to a base of
the digester
as a neutralization fluid to partially fill remaining space in the digester,
in order to
elevate a pH level within the digester and reduce pH shock of additional
fluids for
neutralization; after adding the first quantity of white liquor, adding a
solution including
a first quantity of cold caustic extraction alkaline filtrate to fill the
digester from the base
in order to carry out neutralization of the contents of the digester using the
combination
of the first quantity of white liquor and the added solution, the cold caustic
extraction
alkaline filtrate containing hemicellulose; displacing the neutralization
fluid with one or
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CA 02707330 2016-12-13
more cooking fluids comprising at least a hot black liquor enriched with an
additional
quantity of white liquor, followed by a second quantity of cold caustic
extraction
alkaline filtrate; cooking the pulp-containing organic material in the
digester; and
discharging the cooked pulp from the digester.
[0020c] There is also provided a method used in connection with a kraft
process
for producing dissolving pulp, comprising: placing lignocellulose material in
a reaction
vessel and performing pre-hydrolysis; adding neutralization fluid to the
reaction vessel,
the neutralization fluid comprising (i) a first quantity of white liquor as a
pad having an
effective alkali level of between 100 and 130 grams NaOH per liter to
partially fill
available space in the reaction vessel from the bottom, and (ii) a first
quantity of a
different solution introduced after the first quantity of white liquor and
including an
alkaline filtrate having an effective alkali level of between 60 and 68 grams
NaOH per
liter, the first quantity of white liquor comprising between 10% and 30% of
the total
effective alkali charge in carrying out neutralization with the neutralization
fluid;
displacing the neutralization fluid in the reaction vessel with a cooking
fluid comprising
a hot black liquor having an effective alkali level of between 30 and 50 grams
NaOH
per liter and a cold caustic extraction alkaline filtrate having an effective
alkali level of
between 50 and 75 grams NaOH per liter, the cold caustic extraction alkaline
filtrate
containing hemicellulose; cooking the pulp-containing organic material in the
reaction
vessel; and discharging the cooked pulp from the reaction vessel, the cooked
pulp
having a residual hemicelluloses content of 3.1% or less as measured in terms
of 318
solubility.
[0021] Further embodiments, alternatives and variations are also described
herein or illustrated in the accompanying figure.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a general process flow diagram of a conventional pre-
hydrolysis kraft pulping process used in connection with pulp production, as
generally known in the art.
[0023] FIG. 2 is a diagram of a conventional system and related process for
washing and cleaning pulp in connection with a cold caustic extraction
process.
[0024] FIG. 3 is a diagram of a conventional system and related process for
a
cooking as may be used in a pre-hydrolysis kraft pulping process.
[0025] FIG. 4 is a general process flow diagram of a system and related
process for pulp production process in accordance with one embodiment as
disclosed herein.
[0026] FIG. 5 is a diagram of a system and related process for a cooking
stage
used in connection with a pulp production process, in accordance with one
embodiment as disclosed herein.
=
[0027] FIGS. 6A and 6B are cross-sectional diagrams of a digester
illustrating,
among other things, typical liquor and material levels as used in a convention
process for the neutralization stage.
[0028] FIGS. 7A, 7B and 7C are cross-sectional diagrams of a digester
illustrating, among other things, liquor and material mixtures and levels
during the
neutralization stage in accordance with one embodiment as disclosed herein.
[0029] FIGS. 8 and 9 are cross-sectional diagrams of a digester
illustrating,
among other things, liquor and material mixtures and levels during hot black
filling
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and final liquor displacement in accordance with one embodiment as disclosed
herein.
[0030] FIG. 10 is a process flow diagram of a preferred cooking process as
may be used in a cold caustic extraction pulp manufacturing process, in
accordance with one or more embodiments as disclosed herein.
[0031] FIG. 11 is a diagram showing a datasheet used to calculate and
register
the liquor volumes "in" and "out" in the bench (lab) scale digester and
process
conditions in general accordance with the process flow of FIG. 10.
[0032] FIG. 12 is a graph charting the pH and effective alkali
concentrations of
the neutralisate out of various samples in connection with the process of FIG.
11.
[0033] FIGS. 13A and 13B are graphs summarizing various process conditions
and results according to various examples of processes.
[0034] FIG. 14 is a graph of S18 versus kappa number fora process according
to one embodiment as disclosed herein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] According to one or more embodiments, a method and system for pulp
processing used in connection with a kraft process involves combining a first
caustic solution, such as white liquor, with a quantity of wood or other
organic
material containing raw pulp in an appropriate tank or reaction vessel (i.e.,
a
digester) for cooking at a suitable temperature of, e.g., between 140 and 180
C to
yield a brown stock. Washing and screening of the brown stock results in semi-
purified pulp as well as derivatives (such as black liquor) that are fed back
to the
digester. The semi-purified pulp may be extracted with another caustic
solution
(which again may be white liquor) at a suitable temperature of, e.g., below 50
C to
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yield a purified pulp. Through additional washing, a hemicellulose-containing
solution may be separated from the purified pulp, resulting in another caustic
solution in the form of a cold caustic extraction (CCE) alkaline filtrate that
can be
separately collected and stored. This CCE alkaline filtrate may be
concentrated by,
e.g., evaporation or other means, and used by itself or in combination with
the first
caustic solution in the digester to treat the organic materials and re-start
the cycle.
In other embodiments, the CCE alkaline filtrate is returned in significant
portion to
the digester, but without undergoing concentration.
[0036] According to an aspect of one or more embodiments, wood chips or
other pulp-containing organics are reacted with a caustic solution in a
reaction
vessel as part of a cooking stage. The cooking stage preferably involves
feeding
wood chips or other organic pulp-containing materials into a digester or
similar
reaction vessel, performing pre-hydrolysis, neutralizing the mixture with a
white
liquor plus a CCE alkaline filtrate optionally enriched with a white liquor,
filling the
digester with hot black liquor and CCE alkaline filtrate (either or both
preferably
being enriched with white liquor), and cooking for an amount of time effective
to
result in delignification. These steps may be followed with cold displacement
and
pulp discharge.
[0037] The discharged pulp mixture generally contains liberated cellulosic
fibers. These fibers may be further extracted with another caustic solution to
dissolve hemicellulose. The spent caustic solution together with dissolved
hemicellulose may be separated from the extracted pulp, and the pulp subject
to
further washing to remove residual caustic solution and hemicellulose. The
washing liquids and the spent caustic solution containing hemicellulose are
combined and optionally concentrated to form a concentrated CCE filtrate. The
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concentrated or unconcentrated CCE filtrate, as the case may be, may then be
used singularly or in combination with another caustic solution to treat wood
in the
reaction vessel.
[0038] In this manner, potentially the entire amount of the alkaline
filtrate
generated in the washing and cleaning step may be returned and used as an
alkali
source in the pre-hydrolysis kraft (PHK) cooking process, thereby helping
prevent
hemicelluloses deposition and improving the purity of high alpha dissolving
pulp.
All steps outlined above may be carried out with traditional equipment.
[0039] For comparative purposes, Figures 2 and 3 show certain relevant
aspects of a pre-existing process in accordance with the general pulp
manufacturing technique illustrated in Figure 1. Shown in Figure 2 is a pre-
existing
system and related process for washing and cleaning pulp, and shown in Figure
3
is a pre-existing system and related process for a cooking, all as may be used
in a
pre-hydrolysis kraft pulping process. With reference first to Figure 2, a
system 200
and related process for washing and cleaning pulp involves transporting a
purified
pulp 232 from the brown stock washing and screening (i.e., stage 122 in Figure
1)
via a suitable conveyance to the CCE reactor 210 (i.e., stage 123 in figure
1), along
with a mixture of white liquor 215 that is cooled, CCE alkaline filtrate 226,
or
possibly other fluids or solutions which may be temporarily stored in one or
more
mixing tanks 271,272. From the CCE reactor 210, the pulp mixture 233 may be
provided to a battery of twin roll press units 251-254, which are used as part
of the
washing and cleaning of the pulp. After treatment using the twin roll press
units
251-254, the treated pulp 260 may then be further treated or mixed with
sulphuric
acid (H2SO4) 261 and/or other liquid and passed downstream to a bleaching
process. In connection with the washing process, CCE alkaline filtrate 216
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extracted from the twin roll press units 251-254 may be collected and used for
various purposes, including returned and recycled upstream for use in the
cooking
stage.
[0040] As previously noted, a portion of the CCE alkaline filtrate 216,
usually
much less than half, is typically bled off to a recovery area or otherwise
removed.
[0041] Figure 3 illustrates a system 300 and related process for a cooking
as
conventionally known in which the CCE alkaline filtrate may optionally be
used. In
Figure 3, one or more digesters 310a, 310b are fed wood chips or other
cellulose-
containing organic material, and are the basic reaction vessels used in the
cooking
process. The system 300 also includes a white liquor tank 320, a displacement
liquor tank 330, and one or more hot black liquor accumulator tanks 340a,
340b.
White liquor 319 from an external source may be pumped into the white liquor
tank
320, from which it may be drawn and used as a neutralization liquor 322 in the
digesters 310a, 310b. The displacement liquor tank 330 holds a solution that
may
comprise diluted black liquor or a mixture including black liquor, as may be
obtained
for example as a by-product from the brown stock washing stage, as indicated
by
the incoming arrow 325.
[0042] The white liquor 319 or CCE filtrate 316 may be pumped through
several heat exchangers to the suction side of the pump associated with white
liquor tank 320. Another pump sends white liquor or CCE filtrate for the
neutralization stage to the discharge side of the pump associated with the
displacement liquor tank 330. During hot black liquor fill, the liquor from
hot black
liquor accumulator tank 340a is pumped through heat exchanger 353 and
eventually to the digesters 310a, 310b via cooking liquor pipeline 324. After
the hot
black liquor fill comes the while liquor fill (or CCE filtrate) through the
same pump
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and same line as the hot black liquor fill. When the cooking is finished, the
displacement liquor 327a, 327b is fed to the digesters 310a, 310b and used in
the
at the end of the cooking stage. The hottest part of the displacement is sent
to the
first hot black liquor accumulator tank 340a to be used in the next cook, and
the
cooler part is sent to the second hot black liquor accumulator tank 340b. From
the
second hot black liquor accumulator tank 340b the liquor is sent to an
evaporation
plant through the heat exchangers and a liquor filter, and from there to a
recovery
boiler where the organics are burned to produce steam while the inorganics are
recovered.
[0043] In general, when high purity pulp is not being produced a cold
caustic
extraction stage may not be needed and while liquor may be fed directly to the
digesters 310a, 310b. When cold caustic extraction is employed, the CCE
filtrate is
generally pumped back into the digesters 310a, 310b.
[0044] In typical cooking processes, the digesters 310a, 310b are filled
with
wood chips or similar organic material and then subjected to a pre-hydrolysis
process. After pre-hydrolysis, a neutralization liquor 322 is provided to the
digesters 310a, 310b, which is then displaced in sequence by an appropriate
cooking liquor. The temperature of the digesters 310a, 310b is then raised to
a
cooking temperature at which they are maintained for a sufficient period of
time for
delignification to occur. When cooking is complete, a blow valve in each
digester
310a, 310b is opened, and the delignified pulp from the digester is then
discharged
into a blow tank (not shown). Towards the end of a cooking cycle, the digester
is
kept pressurized while a displacement liquid is introduced to displace the hot
black
or spent liquors, which are released out of the digester 310a, 310b while
still
roughly at the temperature used for cooking. In a typical process, the
displacement
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fluid constitutes a filtrate obtained from washing the brown stock pulp. The
displaced hot black liquor is collected in one or more high temperature
accumulators 340a, 340b for subsequent reuse. After the displacement process,
the displacement liquid and remaining spent black liquor, which are cooler
than the
normal cooking temperature, may optionally also be stored in a low temperature
accumulator and sent to the recovery area. The digesters 310a, 310b are
eventually drained to remove the delignified pulp.
[0045] Figure 4 is a general process flow diagram of a process 400 for pulp
production process in accordance with one embodiment as disclosed herein, in
which the cooking process is modified and improved over the conventional
technique. The process 400 in Figure 4 begins with a cooking stage 421 in
which,
generally similar to a conventional kraft process, wood chips or other pulp-
containing organic materials 418 are fed into a digester capable of
withstanding
high pressure. The digester may be of any suitable volume such as, for
example,
approximately 360 cubic meters. The particular choice of wood type or other
plant
or organic materials may depend upon the desired end products. For example,
soft
woods such as pine, fir and spruce may be used for some derivatization
processes
to obtain products with high viscosity, like cellulose ethers (which may be
used, for
example, as additives in food, paint, oil recovery fluids or muds, paper,
cosmetics,
pharmaceuticals, adhesives, printing, agriculture, ceramics, textiles,
detergents and
building materials). Hardwoods, such as eucalyptus and acacia may be preferred
for those applications that not require a pulp with very high viscosity.
[0046] In one embodiment, and as described in further detail below, the
digester is heated during the pre-hydrolysis portion of the cooking stage 421
to a
first pre-determined temperature with steam or other appropriate means. This
pre-
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determined temperature may, for example, be between 110 to 130 C and, more
specifically, may be approximately 120 C. The heating in this particular
example is
effected over a period of time between 15 to 60 minutes (e.g., 30 minutes),
although other heating times may be used depending upon the particulars of the
equipment and the nature of the organic materials being heated.
[0047] The digester is preferably then further heated by steam or other
means
to a second temperature above the first pre-determined temperature for a pre-
hydrolysis stage. This second pre-hydrolysis temperature is preferably around
165
C, although again the precise temperature may depend upon a number of
variables including the equipment and organic materials. The heating for pre-
hydrolysis may be effected over a period of 30 to 120 minutes (e.g., 60
minutes),
although again the heating time may vary as needed. Once the pre-hydrolysis
temperature is attained, the digester is held at that temperature for a
suitable period
of time, e.g., 35 to 45 minutes, or any other time sufficient to complete pre-
hydrolysis.
[0048] In a preferred embodiment, a neutralization solution is added to
digester
as part of the cooking stage 421. The neutralization solution may be composed
of
a white liquor 411, an alkaline filtrate 417, or a mixture thereof. A white
liquor may
take the form of, e.g., a mixture of sodium hydroxide and sodium sulfide. In a
preferred embodiment, the white liquor has between 85 to 150 grams per liter
effective alkali as sodium hydroxide (NaOH), more preferably between 95 to 125
grams NaOH per liter effective alkali, and most preferably between 100 to 110
grams NaOH per liter of effective alkali. The sulfidity of the white liquor
may have a
range between 10% and 40%, preferably between 15 and 35%, and most
preferably between 20 and 30%.
-16-
CA 02707330 2010-06-14
[0049] The concentration of effective NaOH in the black liquor 435 used for
hot
liquor fill before enrichment with white liquor may be between 15 to 35 grams
per
liter and is preferably in the range of 20 to 30 grams per liter, or in the
alkaline
filtrate 417 after enrichment with white liquor may be between 35 to 75 grams
per
liter and is preferably in the range of 40 to 50 grams per liter, although it
may vary
according to the particular process.
[0050] The neutralization solution may be added to the digester in one
portion
or else may be added to the digester in several portions. In one embodiment,
the
neutralizing solution comprising of both a white liquor and alkaline filtrate
is added
in two portions, whereby the white liquor is first provided to the digester as
a white
liquor pad 461 followed by addition of the CCE alkaline filtrate 417. In one
embodiment, the neutralization solution is added at a temperature between 120
to
160 C, and more preferably between 140 to 150 C. The white liquor may
comprise between 20% and 40% of the total effective alkali charge in the
neutralization step, and more preferably may comprise between 25% and 30% of
the total effective alkali charge in neutralization.
[0051] A cooking liquor then may displace the neutralization liquor in
digester
and is used for cooking the wood in the digester. The cooking liquor may be
added
to the digester in several portions. In one embodiment, the cooking solution
comprising of both a hot black liquor and a white liquor or CCE alkaline
filtrate
added in two portions,. The range and preferred range of sodium hydroxide and
sodium sulfide in the black liquor, white liquor and CCE filtrate solutions
may be
the same as those for the neutralization phase.
[0052] In one or more embodiments, the cooking solution includes one or
both
of the following elements: (i) a black liquor 435 with an effective alkali
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CA 02707330 2010-06-14
concentration of 15 to 35 grams per liter as NaOH, optionally enhanced with an
added amount of white liquor 462 with an effective alkali concentration of 95
to 125
grams per liter as NaOH to achieve an effective alkali concentration of 40 to
50
grams per liter as NaOH or else enhanced with an added amount of recycled CCE
filtrate 417 (optionally concentrated to increase alkali level or enriched
with white
liquor); and (ii) a CCE alkaline filtrate 417 derived from a downstream cold
caustic
extraction washing stage 424 with an effective alkali concentration of 55 to
75
grams per liter as NaOH, after enrichment or enhancement with added white
liquor
463, and optionally concentrated by evaporation or other similar means.
[0053] The digester may be heated to the cooking temperature with steam or
other means. The cooking temperature may be in the range between 140 and 180
C, and is preferably in the range between 145 to 160 C. The heating can be
over
a period of 10 to 30 minutes or other suitable period. The digester is then
held at
the cooking temperature for a suitable period for the cooking process, such as
between 15 to 120 minutes. The temperature range and the cooking time are
chosen for target H factor, which is preferably in the range of between 130
and 250.
[0054] As a result of the cooking stage 421, a brown stock 412 is produced.
The brown stock 412 is provided to a washing and screening process 422,
similar
to a conventional kraft procedure, whereupon the brown stock 412 is screened
through the use of different types of sieves or screens and centrifugal
cleaning.
The brown stock 412 is then washed with a washer in the screening and washing
process 422. The washer may be of any commercial type, including horizontal
belt
washers, rotary drum washers, vacuum filters, wash presses, compaction baffle
filters, atmospheric diffusers and pressure diffusers. The washing unit may
use
counter current flow between the stages so that pulp moves in the opposite
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CA 02707330 2015-06-09
direction to the washing waters. In one embodiment, pressurized water is used
to
wash the brown stock 412. In another embodiment, a diluted caustic solution is
used
to wash the brown stock 412. The diluted caustic solution may, for example,
have an
effective alkali concentration of less than 5 grams NaOH per liter, more
preferably of
less than 1 gram NaOH per liter. The spent washing liquor is collected and
used as
black liquor 413 elsewhere in the process 400. In one embodiment, the black
liquor
413 is used as part of the displacement liquor provided to the digester at the
end of
the cooking stage 421.
[0055] The semi-purified pulp from the washing and screening process 422 is
then pumped as a slurry to a reactor which is employed in cold caustic
extraction
("CCE") stage 423, again similar to the conventional method, in which the semi-
purified pulp is mixed with a second caustic solution 414 (which may be the
same or
different from the first caustic solution 411) to effect further separation of
hemicellulose
from the desired cellulosic fibers. Cold caustic extraction is a process well
known in
the art. Examples of cold caustic treatment processes and systems are
described in
greater detail, for instance, in Ali et al., U.S. Patent Publication No.
2004/0020854, and
Svenson et al., U.S. Patent Publication No. 2005/0203291.
[0056] The caustic solution 414 used in the blending and extraction
procedures of
the CCE extraction process 423 may comprise freshly prepared sodium hydroxide
solutions, recovery from the downstream process, or by-products in a pulp or
paper
mill operation, e.g., concentrated CCE filtrate, white liquor and the like.
Other basic
solutions, such as ammonium hydroxide and potassium hydroxide, may also be
employed. Cold alkali extraction may be performed with
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CA 02707330 2010-06-14
additional chemicals added such as hydrogen peroxide, sodium hypochlorite,
sodium borohydride, and surfactants.
[0057] After the desired dwell time, the pulp is separated from the spent
cold
caustic solution in a following washing process 424. The spent cold caustic
solution contains extracted hemicellulose. The pulp is washed in CCE washing
unit. Exemplary washers include horizontal belt washers, rotary drum washers,
vacuum filters, wash presses, compaction baffle filters, atmospheric diffusers
and
pressure diffusers. The washing liquid may comprise, for example, pure water
or
diluted caustic solution with an effective alkali concentration of, e.g.,
below 1 gram
NaOH per liter. The spent washing liquid is collected in a conventional manner
and
can be combined with spent cold caustic solution to form another caustic
solution
416 which, in one aspect, comprises an alkaline filtrate resulting from the
washing
process 424. The extracted and washed pulp 433 is, in the meantime,
transported
to the next stage for bleaching.
[0058] The CCE alkaline filtrate 416 may be provided in whole or part to a
concentrating process, and may, for example, be fed into an evaporation system
for
concentration, although in other embodiments the CCE alkaline filtrate 416 is
not
subject to a concentration process. A typical evaporation system may contain
several units or effects installed in series. The liquid moves through each
effect
and becomes more concentrated at the outlet of the effect. Vacuum may be
applied to facilitate the evaporation and concentration of solutions. In
connection
with the concentrating process , a weak black liquor may also be concentrated
into
a strong black liquor by, e.g., evaporation using one or more effects in
sequential
arrangement, gradually increasing the concentration of the weak black liquor
during
the process. The strong black liquor may be stored in an accumulation tank and
- 20 -
CA 02707330 2015-06-09
used in the recovery boiler generating steam and power, thus increasing
efficiency
through the reuse or recycling of output by-products. One technique for
concentrating
CCE alkaline filtrate for reuse in the cooking stage is described in
corresponding U.S.
Patent No. 8,535,480 entitled "Method and System for Pulp Processing Using
Cold
Caustic Extraction with Alkaline Filtrate Reuse".
[0059] The concentrated alkaline filtrate solution 417 may be reused, in
whole or
part, in the cooking stage 421 as either part of neutralization liquor and/or
as part of
the cooking liquor. As noted earlier, the CCE alkaline filtrate 416 may be
combined
with a white liquor 463 for use as part of the cooking liquor. In certain
embodiments,
the concentrated CCE alkaline filtrate solution 417 may be used without
enrichment
from white liquor.
[0060] Concentrated alkaline filtrate solution 417 that is not reused in
the cooking
stage 421 may be used for other purposes. For example, it may optionally be
diverted
for other purposes, such as use on an adjacent production line (as white
liquor). The
concentrated alkaline filtrate solution 417 may also allow the use of higher
liquor
concentrations in the cooking stage 421, thus preventing re-deposition of hemi-
celluloses on the fibers.
[0061] Figure 5 is a diagram of a system 500 and related process for a
cooking
stage used in connection with a pulp production process, in accordance with
one
embodiment as disclosed herein. In Figure 5, one or more digesters 510 (in
this
example, eight digesters) are, similar to the conventional process, fed wood
chips or
other pulp-containing organic material, and serve as the basic reaction
vessels
- 21 -
CA 02707330 2010-06-14
used in the cooking process. The system 500 also includes, among other things,
a
white liquor/CCE filtrate holding tank 520, a displacement liquor tank 530,
one or
more hot black liquor accumulator tanks 540a, 540b, and one or more blow tanks
560. White liquor 519 from a suitable source may be heated by fluid heaters
551,
552 and pumped into the white liquor/CCE filtrate holding tank 520, where it
may
be re-circulated and stored for later use, and from which it may be drawn and
used
as a neutralization liquor 522 in the digesters 510. CCE filtrate 516 may
likewise be
heated and pumped into the white liquor/CCE filtrate holding tank 520 for
later use.
The displacement liquor tank 530 holds a solution that may comprise diluted
black
liquor or a mixture including black liquor, which may be, for example, a by-
product
from the washing stage 424, as indicated by the incoming arrow 525.
[0062] At the end of the cooking process, cold liquor (75 ¨85 C) from the
displacement liquor tank 530 is sent to the digester 510 in order to end the
cooking
reaction. The first part of the liquor displaced from the digester 510 is
relatively hot
(140¨ 160 C) and is sent to the first hot black liquor accumulator tank 540a
for use
in the next cook. The colder liquor displaced next from the digester 510 is
cooler
(about 120 ¨140 C) and is sent to the second hot black liquor accumulator tank
540b. From the second hot black liquor accumulator tank 540b, the hot black
liquor
536 is pumped through heat exchangers to a liquor filter 570. The black liquor
is
cooled down while at the same time its heat is used to warm up the white
liquor or
CCE filtrate circulating through the heat exchangers 551, 552. From there, the
filtered black liquor is sent to an evaporation plant for further processing.
[0063] In a preferred cooking process illustrated in Figure 5, the
digesters 510
are filled with wood chips or similar organic material. Pre-hydrolysis is
carried out
with steam, after which a neutralization white liquor 517 in the form of a
white liquor
- 22 -
CA 02707330 2010-06-14
"pad" is provided to the digesters 510 followed by introduction of a CCE
alkaline
filtrate 516 as part of the neutralization fluid 522. The neutralization fluid
is then
displaced by an appropriate cooking liquor. The cooking liquor may include (i)
a
CCE filtrate 524 from the white liquor/CCE filtrate holding tank 520
especially
prepared for cooking; (ii) a black liquor 535 from the black liquor
accumulator tank
540a, optionally enhanced with an added amount of white liquor (or CCE
filtrate)
562 and, in this example, circulated through fluid heater 553 for controlling
its
temperature; and/or (iii) a CCE alkaline filtrate 516 derived from a
downstream cold
caustic extraction washing stage 424 (see Figure 4), either concentrated or
not
through evaporation or other similar means, and optionally enhanced or
enriched
with added white liquor 519 to produce a white liquor-enriched concentrated
CCE
alkaline filtrate. The CCE alkaline filtrate 516 is pumped into the white
liquor
holding tank 520 through heat exchangers 551, 552 to be used in the
neutralization
phase as neutralization fluid 522 or in the cooking phase a cooking CCE
filtrate
524. Preferred concentrations of the various cooking liquors are described
elsewhere herein.
[0064] Once the cooking liquor(s) are added to the digesters 510, the
temperature of the digesters 510 is raised to a cooking temperature at which
the
digesters are maintained for a sufficient period of time for delignification
to occur.
When cooking is complete, a blow valve in each digester 510 is opened, and the
delignified pulp from the digester 510 is then discharged into one of the blow
tanks
560. Towards the end of a cooking cycle, the digester is kept pressurized
while a
displacement liquor from the displacement liquor tank 530 is introduced to
displace
the hot black or spent liquors, which are released out of the digesters 510
while still
roughly at the temperature used for cooking. The displacement liquor, as
noted,
- 23 -
CA 02707330 2010-06-14
generally comprises a black liquor or similar filtrate obtained from washing
the pulp
or delignified fibers during pulp production of prior batches. The displaced
hot
black liquor is collected in one or more high temperature accumulators 540 for
subsequent reuse.
[0065] The digesters 510 are eventually drained to remove the delignified
pulp.
The hot black liquor previously drained from the digester 510 may be reused
(and
mixed with other solutions or filtrates, such as hot white liquor).
[0066] Various aspects of the overall cooking process may be explained by
further reference to Figures 6 ¨ 9. Figures 6A and 6B are cross-sectional
diagrams
of a digester (such as any of digesters 510 illustrated in Figure 5)
depicting, among
other things, a typical liquor and material level as used in a pre-existing
process for
a neutralization step. Figures 7A ¨ 7C, 8 and 9 are also cross-sectional
diagrams
of a digester depicting liquor and material mixtures during neutralization
prior to
cooking in accordance with one or more embodiments as disclosed herein. First
as
shown in Figure 6A, a digester 610 during the neutralization step of a known
cooking process may be filled after pre-hydrolysis with a substantial amount
of CCE
filtrate (liquor) 616 representing a significant percentage (e.g., 60%) of the
total
volume of the digester 610. For example, for a digester with a capacity of 360
cubic meters and a charge of 72 tons of wood (dry-weight) and 11 tons of
dissolved
solids, about 214 cubic meters of CCE filtrate 616 may be used as part of the
neutralization phase. During this step, the CCE filtrate concentration may be
approximately 51.3 grams NaOH per liter, with an effective alkali (EA) charge
on
wood of 13.2% as NaOH. After pre-hydrolysis, the digester 610 may be at
roughly
165 C with a relative pressure of 7 bar (i.e., pressure relative to local
atmospheric
pressure). At this point, the wood chips or other pulp-containing organic
material
-24 -
CA 02707330 2010-06-14
should be almost air free, with steam present inside the voids within the
chips or
similar organic material. Almost all chip water is in liquid form.
[0067] When pumping neutralization liquor to the digester 610 at a typical
temperature of 130 C, the steam inside the chips or other organic material
condenses, and liquor is sucked inside the chips or other organic material due
to
lower pressure created by the condensation. During this process, a certain
amount
of liquid is added from steam and also lost from de-gassing. For example, with
the
amounts described above, approximately 11.9 cubic meters of water from steam
may be added, and about 1.6 cubic meters of water lost from de-gassing 625. In
total, about 224 net cubic meters of liquid, in terms of free liquid
(neutralization
liquor and steam water), may be added during this part of the cooking process.
After pre-hydrolysis and neutralization, the digester 610 may typically
contain
approximately 203 cubic meters of free liquid, with roughly 109 cubic meters
of
liquor still bound in the pre-hydrolized chips, which corresponds to 1.31
m3/BDt
(cubic meters of liquor per bone dry metric ton of chips) or 3.15 m3/ADt
(cubic
meters of liquor per air dry metric ton of chips). Thus, a total content of
312 cubic
meters of liquid may be present as either free liquid or bound in the chips.
At this
point, the digester 610 may hold 72 metric tons of wood, 36 metric tons of
water
absorbed within the wood, and 11 metric tons of dissolved solids of various
sorts.
The density of the liquid after neutralization in this example would be about
1.13
t/m3 (i.e., tons per cubic meter).
[0068] As shown now in Figure 6B, the neutralization liquor added will fill
in the
voids inside the chips (discounted chip water) and the void space around the
chips.
Thus, taking the current example, the 214 cubic meters of added neutralization
liquor would be distributed as roughly 56.8 cubic meters filling in the void
space
- 25 -
CA 02707330 2010-06-14
inside the chips (8.3 cubic meters in the cone 607 of the digester 610 and
48.5
cubic meters in the cylindrical part 608 of the digester 610), and 157.2 cubic
meters
filling the void space around the chips (22.8 cubic meters in the cone 607 of
the
digester 610 and 134.4 cubic meters in the cylindrical part 608 of the
digester 610).
This assumes a volume for the cone 607 of 40 cubic meters and a height of the
cylindrical part 608 of 9.6 meters. In this case, the chip amount in the
digester cone
607 can be approximated as 9.3 BDt (bone dry metric tons) with a bound liquid
volume of 12.3 cubic meters, bound water volume of 4 cubic meters, free liquor
around the chips of 22.8 cubic meters, and total volume taken in the cone 607
of
31.1 cubic meters (that is, 22.8 + 12.3 ¨ 4.0 cubic meters). A small band of
condensate 613 of approximately 0.6 ¨ 0.7 meters in height collects or forms
at the
surface of the liquid mixture, where the steam and liquid meet.
[0069] A white liquor "pad" or enrichment step in the cooking process can
be
used to replace part of the CCE alkaline filtrate used in the beginning of the
neutralization step in order to reduce or avoid hemicelluloses re-deposition
on the
wood fibers. Thus, after pre-hydrolysis as first part of the neutralization
phase, an
amount of white liquor is added preferably in quantity sufficient to fill the
voids
inside the wood chips or other pulp-containing organic material, followed by
an
infusion of CCE filtrate. Preferably, for each metric ton of wood chips,
approximately 0.35 to 0.55 cubic meters, and more preferably 0.40 to 0.44
cubic
meters, of white liquor are added after pre-hydrolysis in order to fill voids
inside the
wood chips and improve the ultimate alpha content of the pulp being produced.
The remainder of the fluid added for neutralization takes the form of CCE
alkaline
filtrate, as per the conventional process, or optionally may involve using a
concentrated CCE alkaline filtrate. Although these steps raise the alkali
level in the
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CA 02707330 2010-06-14
digester, it has been found by the inventors that hemicelluloses redeposition
is
inhibited and higher alpha content is achievable while keeping other process
attributes, such as viscosity, kappa number and/or effective alkali
consumption,
within acceptable ranges.
[0070]
Referring back to the prior example, for instance, a volume of 30 cubic
meters of white liquor may be added to the digester 610 containing 72 tons of
wood
chips after pre-hydrolysis, as illustrated by Figure 7A. As shown therein, the
white
liquor pad 715 together with the lower portion of the wood chips or similar
organic
material approximately fills the cone 607 of the digester 610. Then, a volume
of
82.9 cubic meters of CCE filtrate (either enriched or a concentrated CCE
alkaline
filtrate) may be added to the digester 610 to displace the white liquor pad,
with the
result that effectively all of the white liquor will be consumed to fill the
voids inside
the wood chips. This is followed by the introduction of an additional volume
of
130.6 cubic meters of CCE filtrate (preferably a concentrated CCE alkaline
filtrate)
to the digester 610 to complete the neutralization process. Figure 7B
illustrates the
contents of the digester 610 after the introduction of the 30 cubic meters of
white
liquor pad and the 82.9 cubic meters of CCE filtrate 716. As shown, the
combination of white liquor pad and initial CCE filtrate cover about 41%
percent
(roughly 33.9 bone dry metric tons) of the wood mass in the digester 610, as
reflected in Figure 7B by the lower portion 718 of wood chips in the digester
610.
The remaining part of the wood chips, as reflected by the upper portion 719 of
chips in the digester 610, will be covered with the additional 130.6 cubic
meters
CCE filtrate 717 that will fill in the voids both in and around the chips, as
shown in
Figure 7C. As before, a small band of condensate 713 of approximately 0.6 ¨
0.7
meters in height forms at the surface of the liquid mixture.
-27-
CA 02707330 2010-06-14
[0071] The white liquor pad added to the digester 610 may have an effective
alkali (EA) concentration of 95 to 125 grams NaOH per liter and, more
preferably,
an effective alkali concentration of between 105 and 115 grams NaOH per liter
and,
most preferably, approximately 110 grams NaOH per liter. The equivalent alkali
charge on the wood in such a case may be approximately 4%. After the addition
of
the 30 cubic meters of white liquor pad and the 82.9 cubic meters of CCE
filtrate
716 but before the remaining CCE filtrate 717, the bound liquor in the cone
607 of
the digester 610 is approximately 8.3 cubic meters and the free liquor in the
cone is
approximately 23 cubic meters. The bound liquor in the cylindrical part 608 of
the
digester 610 is about 21.7 cubic meters.
[0072] The white liquor pad preferably provide at least 10% of the total
effective alkali charge applied in the neutralization phase, more preferably
provides
between 13% and 25% of the total effective alkali charge applied in the
neutralization phase, and most preferably provides between 20% and 25% of the
total effective alkali charge applied in the neutralization phase. In the
above
example, the effective alkali charge on wood provided by the white liquor pad
is
4%, while for the rest of the neutralization liquor the effective alkali
charge on wood
is 13.2% from the CCE filtrate, for a total of 17.2% effective alkali charge.
Thus, in
this example, the white liquor pad provides 23% of the total effective alkali
charge
on wood.
[0073] In one aspect, the use of a white liquor pad as described herein may
avoid or reduce pH shock during the neutralization stage since when the CCE
filtrate liquor rich in hemicelluloses meet the wood chips or other similar
material in
the process illustrated in Figures 7B and 7C, the chips or other material will
be
already neutralized by the white liquor. The white liquor pad 715 generally
- 28 -
CA 02707330 2010-06-14
increases the -pH of the wood chips or other similar organic material when it
gets
absorbed into the chip voids. When the CCE filtrate is added, the remaining
white
liquor that has not been absorbed is displaced, and as it rises in the
digester 610 it
continues to neutralize additional wood chips and organic matter before the
CCE
filtrate can reach those chips or organic matter. Since the CCE filtrate
introduction
follows the white liquor pad 715, the CCE filtrate liquor enriched with
hemicelluloses
first meets those chips or organic materials that are already neutralized,
which
avoids or minimizes pH shock, with the possible exception of the small amount
of
chips or organic matter towards the very top of the digester 610. The
hemicelluloses from the CCE filtrate will stay in the solution rather than
being re-
absorbed or deposited on the wood chips or organic materials. This in turn
increases the purity of the pulp brown stock and ultimately leads to an end
product
of higher purity.
[0074] Figures 8 and 9 illustrate liquor and material mixtures and levels
during
the subsequent steps of hot black filling and final liquor displacement, in
accordance with one embodiment as disclosed herein. As shown in Figure 8,
which illustrates the introduction of cooking liquors and displacement of
existing
liquors, a volume of 210 cubic meters of hot black liquor 815 may be added to
the
digester 610 after completion of the neutralization phase. Then, a volume of
144
cubic meters of CCE filtrate (either enriched CCE filtrate or a concentrated
CCE
alkaline filtrate) 817 may be added to the digester 610 followed by another
volume
of 20 cubic meters of hot black liquor 821, thereby displacing the
neutralization
liquors which have by this point become infiltrated with residues and
impurities and
hence take the form of a black liquor 840. In this example, 351 cubic meters
of
- 29 -
CA 02707330 2010-06-14
black liquor 840 are displaced from the digester 610 and sent to a black
liquor
accumulator tank ("AC2"), such as accumulator tank 540b in Figure 5.
[0075] The alkali charge added with the CCE filtrate in digester 610 in the
process shown in figure 8 is between 7 and 12% expressed as effective alkali
over
dry wood and more preferably around 8.9%, expressed in terms of NaOH over dry
wood. The total alkali charge needed for the cooking phase is complemented
with
the alkali added together with the enriched hot black liquor. After the
addition of the
combination of black liquors 815, 821 and CCE filtrate 716, the total liquid
volume
inside the digester 610 is approximately 312 cubic meters, the total liquid
mass
inside the digester 610 is about 353 tons, and the density of the liquor
inside the
digester 610 is approximately 1.13.
[0076] Figure 9 illustrates the introduction of displacement liquor at the
end of
the cooking process resulting in the displacement of the spent cooking
liquors. As
shown in Figure 9, a volume of 475 cubic meters of displacement liquor 930 may
be
added to the digester 610 at the end of the cooking phase. The cooking
liquors,
which have by this point become infiltrated with pulp residues and impurities,
may
be discharged as a first volume of 220 cubic meters of a relatively strong and
hot
black liquor 942 which is stored in a first black liquor accumulator tank
("AC1", e.g.,
tank 540a in Figure 5) for holding a black liquor of this type, and a second
volume
of 255 cubic meters of relatively weaker black liquor 941 which is stored in a
second black liquor accumulator tank ("AC2", e.g., tank 540b in Figure 5) for
holding a black liquor of weaker type. Some amount of cooking liquor remains
bound to the cooked wood chips or other pulp-bearing organic materials. The
process yields approximately 31.1 bone dry tons of cooked pulp, with roughly
41.1
tons of solids having been dissolved in the cooking and related processes.
- 30 -
CA 02707330 2010-06-14
[0077] Figure 10 is a process flow diagram of a cooking process 1000 as may
be used in a cold caustic extraction pulp manufacturing process, in accordance
with
one or more embodiments as disclosed herein. The process 1000 in Figure 10
begins with a wood chip feeding step 1005 in which wood chips or other pulp-
containing organic materials along with steam are fed into a digester capable
of
withstanding high pressure. As previously noted, the particular choice of wood
type
or other plant or organic materials may depend upon the desired end products.
The steam is introduced to improve the packing of the chips inside the
digester.
The digester may then be heated in one or more steps; in this example, the
digester is heated to a pre-determined temperature (for example, be between
110
to 130 C and, more specifically, may be approximately 120 C) by steam or
otherwise in an initial heating step 1018, followed by heating to a pre-
hydrolysis
temperature (to around 165 C for example) in a subsequent step 1020, although
these two steps may, in some embodiments, potentially be combined. The heating
time may depend to some degree upon the particulars of the equipment, the
volume of the digester, the volume of wood chips, and the nature of the
organic
materials being heated.
[0078] Once the pre-hydrolysis temperature is attained, the digester is
held at
that temperature for a suitable period of time, e.g., 35 to 45 minutes, or any
other
time sufficient to complete a pre-hydrolysis stage 1025. Next, a
neutralization step
1030 is carried out. In a preferred embodiment, a neutralization solution
comprising
a white liquor 1015 is first added to the digester, followed by introduction
of a CCE
filtrate liquor 1016. The white liquor 1015 may take the form of, e.g., a
mixture of
sodium hydroxide and sodium sulfide, with an effective alkali content in
accordance
with any of the embodiments described elsewhere herein. For instance the white
- 31 -
CA 02707330 2010-06-14
liquor 1015 may have between 80 to 150 grams per liter effective alkali as
sodium
hydroxide (NaOH), and preferably between 100 to 110 grams per liter of
effective
alkali as sodium hydroxide. The sulfidity of the white liquor 1015 is
preferably
between 20 and 30% but may, in some embodiments, vary. The CCE filtrate 1016
may comprise recycled CCE alkaline filtrate that is obtained from a downstream
CCE washing process and optionally concentrated by evaporation or other means.
The concentration of effective NaOH in the CCE filtrate 1016 may be between,
e.g.,
50 to 75 grams per liter, although it may vary according to the particular
process.
[0079] Preferably, the white liquor 1015 is introduced first as a "pad" in
accordance with the process described for Figures 7A - 7C, followed by the CCE
filtrate liquor 1016. Then, in step 1035, a second portion of hot black liquor
1017 is
introduced into the digester, as described in connection with Figure 8.
[0080] In a next step 1040, a second white liquor 1019 is added to the
digester
for cooking purposes. As an alternative, the white liquor 1019 can be replaced
by
recycled CCE alkaline filtrate that is obtained from a downstream CCE washing
process and optionally concentrated by evaporation or other means. During this
phase as indicated by step 1045, the contents of the digester are heated, by
steam
or other means, to an appropriate cooking temperature; this temperature is
maintained in a cooking step 1050 for a period suitable to achieve
delignification of
the wood pulp in the digester. The cooking temperature may be in the range
between 140 and 180 C, and is preferably in the range between 150 to 160 C,
although could be any suitable temperature. The heating can be over a period
of
to 30 minutes or other suitable period. The digester is held at the cooking
temperature for a suitable period for the cooking process, such as between 15
to
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CA 02707330 2010-06-14
120 minutes. The temperature range and the cooking time are generally chosen
for target H factor, as previously described.
[0081] After cooking, as indicated by step 1055, a diluted black liquor
1034 in
general from the brown stock washing step is introduced to the digester and
the
pulp contents are discharged for downstream processing. Then, the digester is
discharged and may be washed and cleaned, as indicated by step 1060, and the
next batch of wood chips may be processed in the same fashion, as indicated by
step 1070.
EXAMPLES
[0082] The processes of embodiments of the present invention are
demonstrated in the following examples. Analytical results described in the
examples are obtained using the general process illustrated in Figure 11,
which
lists a series of steps performed in general accordance with the process flow
1000
of Figure 10, and are described with reference to a bench scale digester of
approximately 20 liters volume to simulate an industrial process. Differences
between the procedure illustrated in Figure 11 and the specific examples are
explained in more detail below.
[0083] As indicated in Figure 11, the process normally begins with digester
pre-steaming for 30 minutes to attain initial temperature and humidity in the
digester, along with the addition of wood chips (in this case eucalyptus) to
the
digester; although in the case of a laboratory no steam packing may be needed.
The digester is then heated further by providing steam to the digester, for a
period
of approximately 60 minutes to bring the temperature to 165 C. A pre-
hydrolysis
step is then carried out for, e.g., approximately 40 minutes at a temperature
of 165
C. Then, in some examples, a CCE allkaline filtrate or a first white liquor
pad is
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CA 02707330 2010-06-14
added as part of a neutralization process. This process takes approximately 15
minutes and is carried out at a temperature of roughly 150 C. Next, a first
hot
black liquor is added to fill the remainder of the digester. The introduction
of the
first hot black liquor takes approximately 15 minutes and is carried out a
temperature of 140 C. Next, a second hot black liquor is added to the
digester
during a displacement step, which is carried out for 23 minutes at a
temperature of
approximately 146 C. These two hot black liquor steps collectively represent
a hot
liquor fill as would be carried out in an industrial operation. Next, a white
liquor or
CCE alkaline filtrate is added to finish the displacement process, starting
the
cooking phase. If necessary, some hot black liquor may also be fed to the
digester.
This mixture of white liquor (or CCE alkaline filtrate) and hot black liquor
may be
carried out for, e.g., 12 minutes at a temperature of approximately 152 C and
a
pressure of 10.0 bar. For the cooking step, the liquor is circulated through
digester
at a rate of approximately 3 liters per minute during 3 minutes at a slightly
reduced
pressure of 9.1 bar. The contents of the digester are then heated back up to,
e.g.,
roughly 160 C over a period of 14 minutes, and then maintained at that
temperature during a suitable cooking period for about, e.g., 23 minutes.
Next, a
diluted liquor is introduced as a displacement liquor and the contents of the
digester
are discharged for downstream processing. The diluted liquor continues to be
introduced at a rate of one liter per minute and is circulated in the digester
for a
sufficient period. The digester is then discharged, and may be washed and
cleaned to ready for a new batch.
[0084] Figures 13A and 13B are tables summarizing various process
conditions and results according to Examples 2 ¨ 9 described below. In
particular,
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CA 02707330 2010-06-14
Figure 13A shows the process conditions and parameters for the various
different
examples, and Figure 13B shows the corresponding results in tabular form.
EXAMPLE 1
Kraft process using a combination of white liquor and hot black liquors in the
neutralization and cooking step
[0085] According to a first example, a 20-liter bench scale digester is pre-
heated with steam to 120 C over a period of 30 minutes. 4700 grams of oven
dried pulp-containing organic material such as eucalyptus or other wood chip
is
added to the digester. The lab sequence operations follows the Figure 11. The
digester is heated to 165 C over a period of 60 minutes and held at 165 C
for a
further 40 minutes to complete the pre-hydrolysis stage. 4.51 liters of a
first white
liquor ("WL1") with an effective alkali of 124.7 g NaOH per liter is added to
the
digester over fifteen minutes at a temperature of 150 C. The H factor
calculation
starts at this point. Then, 10.8 liters of a first hot black liquor ("HBL1")
with an
effective alkali of 25.3 g NaOH per liter is added over 15 minutes at a
temperature
of 140 C to complete the neutralization step. 10.0 liters of a second hot
black
liquor ("HBL2") with an effective alkali of 25.3 g NaOH per liter is then
added to the
digester to displace the spent HLB1 and WL1 over a period of 23 minutes at a
temperature of 146 C, followed by addition of the cooking liquor consisting
of a
mixture of 1 liter of HBL2 and 4.16 liter of a second white liquor ("WL2")
with an
effective alkali concentration of 124.7g NaOH per liter added over a period of
12
minutes at 10 bar and 152 C. One meaningful process parameter during this
series of operations is the Total Effective Alkali charge, which is generally
expressed in terms of alkali percentage on the wood chips weight (dry basis)
that is
calculated considering the entire volume of all added liquors and their
respective
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CA 02707330 2010-06-14
concentrations. For this example, the total equivalent effective alkali charge
on the
wood is 12% EA as NaOH for the neutralization phase, and 11 /o EA as NaOH for
the cooking phase, Samples of the displaced WL1 and HBL1 after the
neutralization step (the "Neutralysate") are collected to measure and follow
the pH
behavior, typically from the beginning of the displacement operation to the
end of
that operation. The displaced liquor is collected for later recovery.
[0086] The cooking liquor, comprising of HBL2 and WL2, is circulated at a
rate
of 3 liter per minute for 3 minutes under a pressure of 9.1 bar. The digester
is then
heated to 160 C over a period of 14 minutes, and held at 160 C for another 23
minutes. An aliquot of the reaction mixture is taken to measure the
concentration of
NaOH at the end of the reaction ("EoC"). The EoC is approximately 23.3 g NaOH
per liter.
[0087] The digester is then cooled, and the reaction mixture is washed
twice
with a diluted caustic solution. Each wash uses 15-liter of an aqueous
solution
containing approximately 0.2 g NaOH per liter of a diluted liquor solution
("DL").
The spent liquor after the first wash contains approximately 21.9g NaOH per
liter,
and may be used to prepare a next batch of hot black liquor. The spent liquor
after
the second wash contains approximately 13.0g NaOH per liter and is combined
with the Neutralysate. The combined liquor has an EA of 6.4g NaOH per liter
(equivalent to 3.88% NaOH). In the mill this mixture may be evaporated to form
a
more concentrated caustic solution for the recovery boiler burning.
[0088] The lab bench digester is cleaned by first circulating DL (diluted
liquor)
through the digester at 1 liter per minute for 10 minutes, and then washed
twice first
with 33 liter of pure water and then with 45 liter of pure water. The spent
washing
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CA 02707330 2010-06-14
liquor from the first wash contains approximately 0.9g NaOH per liter and may
be
used to prepare the next batch of DL.
[0089] The resulting brown stock shows a Kappa Number of 11.9, a viscosity
of 1117 ml/g, a S10 solubility of 3.54% and a S18 solubility of 2.7%. The
reaction
has a 39.8% yield. When screened, the mixture has a 0.4% rejection rate,
resulting
in a screening yield of 39.4%. The H factor for the reaction is 333.
[0090] Figure 12 is a graph charting the pH and effective alkali
concentrations
of the Neutralysate out of various samples, indicating the leveling off of
alkali
content signaling the general completion of the cooking stage.
EXAMPLE 2
Kraft process using white liquor in the neutralization and cooking step
[0091] According to a second example, the same pulping process as described
in Example 1 is repeated, using white liquor in both neutralization and
cooking
phases. The Neutralysate has a pH of 10.2, and the final cooking liquor has an
EoC of 26.7 g NaOH per liter. The P factor for the pre-hydrolysis is 310 and
the H
factor for the cooking reaction is 394. For this example the total equivalent
effective
alkali charge on the wood are respectively: 12% EA as NaOH for the
neutralization
phase and 11% EA as NaOH for the cooking phase.
[0092] The resulting brown stock shows a Kappa Number of 10.3, a viscosity
of 988 ml/g, an S10 solubility of 3.6% and an S18 solubility of 2.7%. The
reaction
has a 39.3% yield. When screened, the mixture has a 0.13% rejection rate,
resulting in a screening yield of 39.1%.
EXAMPLE 3
Kraft process using CCE 54 in the neutralization and white liquor in the
cooking
step respectively
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CA 02707330 2010-06-14
[0093] According to a third example, the same pulping process as described
in
Example 1 is repeated, except that white liquor for the neutralization is
replaced
with a filtrate from the CCE step having an EA of 54 g NaOH per liter
("CCE54").
The Neutralysate has a pH of 8.6, and the cooking mixture has an EoC of 23.5 g
NaOH per liter. The P factor for the pre-hydrolysis is 300, and the H factor
for the
cooking reaction is 364. For this example the total equivalent effective
alkali charge
on the wood are respectively: 12% EA as NaOH for the neutralization phase and
11% EA as NaOH for the cooking phase.
[0094] The resulting brown stock shows a Kappa Number of 11.0, a viscosity
of 1059 ml/g, an S10 solubility of 4.0% and an S18 solubility of 3.1%. The
reaction
has a 40.3% yield. When screened, the mixture has a 0.16% rejection rate,
resulting in a screening yield of 40.2%.
EXAMPLE 4
Kraft processing using CCE54 in the neutralization and cooking step
[0095] According to a fourth example, the same pulping process as described
in Example 1 is repeated, except that CCE54 replaces the white liquor in both
the
neutralization and cooking step. The Neutralysate has a pH of 11.0, and the
cooking mixture has an EoC of 18.5 g NaOH per liter. The P factor for the pre-
hydrolysis is 297 and the H factor for the cooking reaction is 419. For this
example
the total equivalent effective alkali charge on the wood are respectively: 12%
EA as
NaOH for the Neutralization phase and 11% EA as NaOH for the Cooking phase.
[0096] The resulting brown stock shows a Kappa Number of 10.8, a viscosity
of 1118 ml/g, an S10 solubility of 4.5% and an S18 solubility of 3.6%. The
reaction
has a 40.4% yield. When screened, the mixture has a 0.09% rejection rate,
resulting in a screening yield of 40.3%.
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CA 02707330 2010-06-14
EXAMPLE 5
Kraft processing using 'Weak" white liquor in the neutralization and cooking
step
[0097] According to a fifth example, the same pulping process as described
in
Example 1 is repeated, except that a white liquor having an EA of 54 g NaOH
per
liter ("WL54") is used in both the neutralization and cooking step. The
Neutralysate
has a pH of 11.3, and the cooking mixture has an EoC of 18.8 g NaOH per liter.
The P factor for the pre-hydrolysis is 300, and the H factor for the cooking
reaction
is 429. For this example the total equivalent effective alkali charge on the
wood are
respectively: 12% EA as NaOH for the neutralization phase and 11% EA as NaOH
for the cooking phase.
[0098] The resulting brown stock shows a Kappa Number of 11.2, a viscosity
of 1158 ml/g, an S10 solubility of 3.7% and an S18 solubility of 3.1%. The
reaction
has a 40.2% yield. When screened, the mixture has a 0.12% rejection rate,
resulting in a screening yield of 40.0%.
[0099] Comparison of the S18 solubility in Examples 2 and 5 suggests that
higher alkali concentration may help suppress hemicellulose redeposition in
the
cooking step. Comparison of the results in Examples 3, 4 and 5 suggest that
the
use of CCE filtrate has a negative impact on the hemicellulose content in the
final
product. To further reduce hemicellulose content while maximizing the
utilization of
CCE filtrates, the following experiments are performed.
EXAMPLE 6
Kraft process using CCE60 in the neutralization and cooking step
[00100] According to a sixth example, the same pulping process as described in
Example 1 is repeated, except that a CCE filtrate having an EA of 60 g NaOH
per
liter ("CCE60") replaces white liquor in both the neutralization and cooking
step.
- 39 -
CA 02707330 2010-06-14
The cooking temperature due to the higher alkali charge in the cooking phase
is
lowered from 160 to 155 C, but the cooking time is correspondingly increased.
The
Neutralysate has a pH of 11.2, and the cooking mixture has an EoC of 24.5 g
NaOH per liter. The P factor for the pre-hydrolysis is 272, and the H factor
for the
cooking reaction is 389. For this example the total equivalent effective
alkali charge
on the wood are respectively: 12% EA as NaOH for the neutralization phase and
12.5% EA as NaOH for the cooking phase.
[00101] The resulting brown stock shows a Kappa Number of 11.4, a viscosity
of 1155 ml/g, an S10 solubility of 4.6% and an S18 solubility of 3.6%. The
reaction
has a 40.7% yield. When screened, the mixture has a 0.07% rejection rate,
resulting in a screening yield of 40.6%. While CCE60 allows the cooking
temperature be reduced by 5 C, the cooking time and alkali charge for cooking
are
lengthened and the hemicellulose content in the brown stock is not reduced as
compared to when CCE54 is used.
EXAMPLE 7
Kraft process using CCE60 in the neutralization and a combination of CCE60 and
HBL40 in the cooking step respectively
[00102] According to a seventh example, the same pulping process as
described in Example 6 is repeated, except that a more highly concentrated hot
black liquor having an EA of 40.0 g per liter ("HBL40") is used in the cooking
step.
In this example the total alkali charge in the cooking step increased to 13.0%
because of the use of more highly concentrated black liquor (HBL40) as a
portion
of the cooking liquor.
[00103] In addition, while the cooking temperature is also at 155 C as in
Example 6, the cooking time is shorter and comparable to the cooking time in
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CA 02707330 2010-06-14
Examples 2 ¨ 5 where the cooking is performed at 160 C. As consequence the H
factor for the cooking reaction is lower at 377. The Neutralysate has an EA of
3.1 g
NaOH per liter, and the cooking mixture has an EoC of 29.5 g NaOH per liter.
The
P factor for the pre-hydrolysis is 301. For this example the total equivalent
effective
alkali charge on the wood are respectively: 12% EA as NaOH for the
neutralization
phase and 13% EA as NaOH for the cooking phase
[00104] The resulting brown stock shows a Kappa Number of 10.3, a viscosity
of 1107 ml/g, an S10 solubility of 4.1% and an S18 solubility of 3.1%. The
reaction
has a 40.1% yield. When screened, the mixture has a 0.09% rejection rate,
resulting in a screening yield of 40.0%.. Compared to Example 6, a lower
hemicellulose content as evidenced by S18 solubility is observed. Thus, the
use of
a higher alkali concentration and a combination of alkaline fluids in the
cooking step
appears to result in reduced hemicellulose content.
EXAMPLE 8
Kraft process using a CCE70 in the neutralization and cooking step
[00105] According to an eighth example, the same pulping process as described
in Example 7 is repeated, except that a CCE filtrate having an EA of 70 g NaOH
per
liter ("CCE70") is used in the neutralization step and a combination of CCE70
and
HBL40 is used in the cooking step. In addition the effective alkali charge in
the
cooking phase is 15%.
[00106] The Neutralysate has a pH of 11.6, and the cooking mixture has an EoC
of 36.1 g NaOH per liter. The P factor for the pre-hydrolysis is 304 and the H
factor
for the cooking reaction is 301.
[00107] The resulting brown stock shows a Kappa Number of 11.0, a viscosity
of 1119 ml/g, an S10 solubility of 4.0% and an S18 solubility of 2.9%. The
reaction
- 41 -
CA 02707330 2010-06-14
has a 40.0% yield. When screened, the mixture has a 0.13% rejection rate,
resulting in a screening yield of 39.9%. Compared to Examples 6 and 7, a lower
hemicellulose content as evidenced by S18 solubility is also observed. This
reinforces that the use of a higher alkali concentration and a combination of
alkaline
fluids in the cooking step appears to result in reduced hemicellulose content.
EXAMPLE 9
Kraft Process Using White Liquor Pad
[00108]
According to a ninth example, the same pulping process as described in
Example 7 is repeated, except that for neutralization step the CCE60 is
replaced
with first a volume of white liquor with having an EA about 125 g NaOH per
liter in
the form of a white liquor pad as previously described, being followed by the
CCE
filtrate (CCE60).. The effective alkali charge in the neutralization step is
increased
from 12% to 16% (4% due to the while liquor pad). As a consequence the
effective
alkali charge in the cooking phase is reduced from 13% to 11%.
[00109] The Neutralysate has an EA of 4.5 g NaOH per liter, and the cooking
mixture has an EoC of 31.7 g NaOH per liter. The P factor for the pre-
hydrolysis is
303 and the H factor for the cooking reaction is 367.
[00110] The resulting brown stock shows a Kappa Number of 9.7, a viscosity of
1103 ml/g, an S10 solubility of 4.0% and an S18 solubility of 3.0%. The
reaction
has a 39.9% yield. When screened, the mixture has a 0.03% rejection rate,
resulting in a screening yield of 39.9%. A lower hemicellulose content as
evidenced by S18 solubility is also observed. The delignification degree
measured
as Kappa Number (KN) is lower for the same level of viscosity (about 1100
ml/g)
which indicates a better process selectivity, as reflected by the ratio
between
viscosity and Kappa Number.
- 42 -
CA 02707330 2010-06-14
[00111] Comparison of the results in Examples 2 to 9 (as summarized in the
tables shown in Figures 13A-13B) suggest that hemicelluloses redeposition may
be reduced through the use of a white liquor pad in the neutralization step
and the
use of a combination of CCE filtrate and higher concentrated black liquor in
the
cooking step. In addition, the use of higher concentrated hot black liquor
results in
higher effective alkali charge, which is desirable as this often, leads to a
better
delignification selectivity (lower Kappa number for same viscosity level). The
use of
a white liquor pad and a combination of CCE filtrate and more concentrated hot
black liquor also may result in reduced cooking temperature with no adverse
effect
on cooking time or pulp quality. Further experiments on industrial scales are
performed to confirm the benefits of the invention.
EXAMPLE 10
Industrial Scale Kraft Process With and Without White Liquor Pad
[00112] A kraft cooking process is performed as generally described in
relation
to Figures 4 and 5. A conventional neutralization step is performed as
illustrated in
Figures 6A and 6B, and an improved process using a white liquor pad is
performed
as illustrated in Figures 7A - 7C. In the improved process, 40 cubic meters of
white
liquor having an effective alkali (EA) level of 110 g NaOH per liter ("WL110")
is
pumped first into the digester at a rate of 180 m3/hour at the beginning of
the
neutralization step (a filling period of 13 minutes), followed by 72.9 cubic
meters of
CCE filtrate with an effective alkali (EA) level of approximately 60 grams
NaOH per
liter. The concentration of the CCE filtrate from the CCE washing process
(e.g.,
process 424 in Figure 4) was in this case adjusted from 53 ¨ 55 grams NaOH per
liter to 60 grams NaOH per liter by adding concentrated white liquor, a
process that
may be referred to as enrichment with white liquor. After the neutralization
step,
- 43 -
CA 02707330 2010-06-14
and following the industrial digester operation sequence described in
reference to
Figure 10, first a volume of hot black liquor with an effective alkali (EA)
level of
approximately 45 grams NaOH per liter ("HBL45") and then a volume of CCE
alkaline filtrate with an effective alkali (EA) level of 60 grams NaOH per
liter are
added to displace the spent neutralization liquor. The wood chips are then
cooked
at a temperature of approximately 150 - 153 C to achieve the target H factor.
Small adjustments of cooking conditions were made to achieve a target
viscosity.
[00113] The various experimental conditions and resulting pulp quality are
summarized below in Table 1 below.
=
-44 -
CA 02707330 2010-06-14
Cooking Conditions Pulp Quality
Entry WL CCE HBL H Cooking Kappa Viscosity S18
Pad filtrate conc. (g Factor Temp. No. After
Used? conc. (g NaOH/1) ( C) Cooking
NaOH/1)
1 No 62.6 45 200 153 10.7 1013 3.6
2 No 63.4 45 200 151 10.2 1028 3.7
3 Yes 67.7 45 200 151 8.5 921 2.9
4 Yes 63.5 45 175 151 8.5 942 3.1
Yes 62.2 45 150 151 8.8 1025 3.1
6 Yes 63.7 45 125 151 11.2 1074 3.2
7 Yes 63.4 45 150 152 9.9 953 3.0
8 Yes 61.0 45 140 152 10.5 1031 2.7
9 Yes 61.8 45 125 152 10.4 1033 3.0
Yes 58.6 45 125 150 10.6 1031 3.0
Table 1
As illustrate by the results above, the use of a white liquor pad before the
addition of CCE filtrate in the neutralization step results in reduced
hemicellulose
redeposition on the fibers, as evidenced by the lower S18 solubility in the
resulting
pulp (by comparison of entries 1-2 with entries 3-10 in Table 1). Without the
white
liquor pad, the S18 solubility remains at 3.6% or above. The use of a white
liquor
pad, optionally with CCE filtrate and black liquor, as well as white liquor
enrichment
during neutralization and cooking, enables achieving simultaneously an S18
solubility of approximately 3.0% or less with a Kappa number of roughly
between
10 and 11 (although more broadly the kappa number value may range between
about 8 and 12 depending upon process parameters), and in general provides a
higher quality pulp product as compared to conventional techniques.
-45 -
CA 02707330 2010-06-14
[00114] Figure 14 is a graph of S18 versus Kappa number fora process
according to one embodiment as disclosed herein, based on quantities used for
an
industrial run (similar to the quantities described with respect to the
cooking
processes explained in connection with Figures 6 ¨ 9). As shown in Figure 14,
the
318 value (in percent) and Kappa number for a conventional cooking process is
illustrated by the line 1405, while the S18 and kappa number values for a
process
using the white liquor pad as detailed herein is shown by line 1410. The
values
when using the white liquor pad are superior. In particular, the process based
on
embodiments disclosed herein may yield an S18 value in the range of 3.0,
indicating a low residual hemicellulose content.
[00115] The kappa number values and solubility values provided above
represent post-cooking characteristics of the brown stock, prior to downstream
cold
caustic extraction and bleaching. After conventional cold caustic extraction
is
performed, the kappa number would generally be reduced to approximately 7 to
9,
and the S18 solubility may be below 1.7% and may reach the range of 1.5%.
These values represent a highly purified pulp with an alpha cellulose content
of
approximately 97.5% before bleaching, and having favorable viscosity
characteristics, achieved in a manner that is efficient and lower cost than
conventional methods for performing high quality pulp processing.
[00116] In addition, use of a white liquor pad as described herein may
avoid or
reduce pH shock during the neutralization stage since when the CCE filtrate
liquor
rich in hemicelluloses meet the chips, they will be already neutralized by the
white
liquor. The white liquor pad generally increases the -pH of the wood chips or
other
similar organic material when it gets absorbed into the chip voids. The white
liquor
pad elevates the pH of the chips or other similar material after the
prehydrolysis
-46 -
CA 02707330 2015-06-09
stage but before the CCE filtrate liquor enriched with hemicelluloses first
meet the
chips. By this effect, pH shock is avoided or minimized, and the
hemicelluloses from
the recycled CCE filtrate will stay in the solution rather than being re-
absorbed or
deposited on the pulp. This in turn increases the purity of the pulp brown
stock and
ultimately leads to an end product of higher purity.
[00117]
While preferred embodiments of the invention have been described herein,
many variations are possible which remain within the concept and scope of the
invention. Such variations would become clear to one of ordinary skill in the
art after
inspection of the specification and the drawings. The invention therefore is
not to be
restricted except within the scope of any appended claims.
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