U.S. patent application number 14/711146 was filed with the patent office on 2015-08-27 for effect of low dose xylanase on pulp in prebleach treatment process.
The applicant listed for this patent is INTERNATIONAL PAPER COMPANY. Invention is credited to GOPAL GOYAL, JACEK KALKA, ALFRED TREPOW, CAIFANG YIN.
Application Number | 20150240423 14/711146 |
Document ID | / |
Family ID | 43973267 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150240423 |
Kind Code |
A1 |
YIN; CAIFANG ; et
al. |
August 27, 2015 |
EFFECT OF LOW DOSE XYLANASE ON PULP IN PREBLEACH TREATMENT
PROCESS
Abstract
A prebleach treatment of pulp with xylanase enzyme prior to
chemical bleaching is disclosed. A process of making pulp comprises
treating a pulp with xylanase in an amount of less than 50 g of
xylanase per ton of pulp. The treating step is carried while
buffering the pulp and xylanase with a buffering agent to a pH of
from about 6.5 to about 7.5 prior to at least one bleaching stage
to form a treated pulp.
Inventors: |
YIN; CAIFANG; (MASON,
OH) ; TREPOW; ALFRED; (BYDGOSZCZ, PL) ; KALKA;
JACEK; (KWIDZYN, PL) ; GOYAL; GOPAL; (MASON,
OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INTERNATIONAL PAPER COMPANY |
Memphis |
TN |
US |
|
|
Family ID: |
43973267 |
Appl. No.: |
14/711146 |
Filed: |
May 13, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12945142 |
Nov 12, 2010 |
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14711146 |
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61323604 |
Apr 13, 2010 |
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61285572 |
Dec 11, 2009 |
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61260133 |
Nov 11, 2009 |
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Current U.S.
Class: |
162/72 |
Current CPC
Class: |
D21C 9/10 20130101; D21C
5/005 20130101; D21H 11/00 20130101; D21C 9/14 20130101; D21H
17/005 20130101 |
International
Class: |
D21H 17/00 20060101
D21H017/00; D21H 11/00 20060101 D21H011/00 |
Claims
1. A process of making pulp comprising: treating a pulp with
xylanase in an amount of less than 50 g of xylanase per ton of
pulp, wherein the treating step is carried while buffering the pulp
and xylanase with a buffering agent to a pH of from about 6.5 to
about 7.5 prior to at least one bleaching stage to form a treated
pulp.
2. The process of claim 1 wherein the pulp is unbleached.
3. The process of claim 1 wherein the xylanase is luminase
enzyme.
4. The process of claim 1 wherein the luminase is
endo-.beta.-1,4-xylanase.
5. The process of claim 1 wherein the pulp is treated with the
xylanase in an amount from about 10 g to about 50 g per ton of
pulp.
6. The process of claim 3 wherein the pulp is treated with the
xylanase in an amount from about 30 g to about 50 g per ton of
pulp.
7. The process of claim 1 wherein the buffering agent is selected
from a group of carbon dioxide (CO.sub.2) and H.sub.2SO.sub.4.
8. The process of claim 1 wherein the pulp is subjected to a first
bleaching stage (D.sub.o) after the treating step.
9. The process of claim 8 wherein the treated pulp causes 10 to 30%
reduction of bleaching chemicals in first bleaching stage (D.sub.o)
without causing an increase in pulp yield loss and bleached
filtrate Chemical Oxygen Demand (COD) loading as compared with a
pulp not being treated in accordance to claim 1.
10. The process of claim 8 wherein the treated pulp after the first
bleaching stage step (D.sub.o) is subjected to an Eop bleaching
stage.
11. The process of claim 10 wherein the treated pulp permits a
reduction of pH by 2 units in the subsequent Eop bleaching stage to
minimize the filtrate Chemical Oxygen Demand (COD) filtration as
compared with a pulp not being treated in accordance to claim
1.
12. The process of claim 11 wherein the pH of treated pulp in Eop
bleaching stage is from about 9 to about 11.
13. The process of claim 10 wherein the treated pulp permits a
reduction of temperature by 15.degree. C. in the Eop bleaching
stage as compared with a pulp not being treated in accordance to
claim 1.
14. The process of claim 13 wherein the temperature in the Eop
bleaching stage is from about 55.degree. C. to about 85.degree.
C.
15. The process of claim 1 wherein the xylanase is added to the
pulp at and/or after the pulp is washed.
16. The process of claim 1 wherein the buffering agent is added to
the pulp simultaneously with, prior to, or after the xylanase is
added to the pulp.
17. The process of claim 1 wherein the retention time for the
treating step of the pulp is from about 15 min. to about 240
min.
18. The process of claim 1 wherein the temperature for the treating
step of the pulp is from about 40.degree. C. to about 70.degree.
C.
19. The process of claim 1 wherein the consistency for the treating
step is from about 2% to about 15%.
20. The process of claim 1 wherein the treated pulp ISO brightness
is at least about 2 to 5% greater than the brightness of the pulp
made by the same or substantially the same bleaching processes
without the pulp being treated in accordance to claim 1.
21. The process of claim 1 wherein treating the pulp with xylanase
in an amount from 10 g to of 50 g of xylanase per ton of pulp and
wherein the treating step is carried while buffering the pulp and
xylanase with a buffering agent to a pH of from about 6.5 to about
7.5 prior to at least one bleaching stage.
22. The process of claim 21 wherein treating the pulp with xylanase
in an amount from 10 g to of 15 g of xylanase per ton of pulp and
wherein the treating step is carried while buffering the pulp and
xylanase with a buffering agent to a pH of from about 6.8 to about
7.2 prior to at least one bleaching stage.
23. The process of claim 1 further comprising a step of recycling
unused enzyme from a prewasher and added to an outlet of a washer
for pH control.
24. In a pulp bleaching system comprising at least one Do stage and
at least one Eop bleaching stage, a prebleach treatment stage which
comprises: pulp; xylanase in an amount of less than 50 g of
xylanase per ton of pulp; and a buffering agent being used
buffering the pulp and xylanase to provide a pH in the prebleach
treatment stage from about 6.5 to about 7.5.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to a pre-bleaching
treatment of pulp. More particularly, the invention relates to the
effect of low dose Xylanase on pulp in pre-bleach treatment
process.
BACKGROUND OF THE INVENTION
[0002] Xylanases are used in the pulp and paper industry to enhance
the bleaching of pulp and to decrease the amount of bleaching
chemicals used in bleaching stages. There have been several
mechanisms proposed for the bleaching action of xylanase. One
mechanism is that lignin is attached to crystalline cellulose
through xylan and xylanase enzymes facilitate bleaching of pulp by
hydrolysing xylan, releasing coloured lignin from the pulp. A
second proposed mechanism is that xylanase removes xylan thereby
improving the alkali extractability of the pulp. Regardless of the
mechanism, xylanase treatment allows subsequent bleaching chemicals
such as chlorine, chlorine dioxide, sodium hydroxide, hydrogen
peroxide, or combinations of these chemicals to bleach pulp more
efficiently than in the absence of xylanase. Pretreatment of pulp
with xylanase prior to chemical bleaching increases the brightness
and quality of the final paper product and reduces the amount of
chlorine-based chemicals which must be used to bleach the pulp.
This in turn decreases the chlorinated effluent produced by such
processes.
[0003] A group of xylanase enzymes produced from fungi or bacteria
have been practiced by paper industry for pulp prebleaching. It has
been demonstrated that using xylanase enzymes in pulp prebleaching
reduces total ClO.sub.2 consumption by about 10-15% along with a
proportional decrease in the caustic usage. Many paper mills have
been taking advantage of this non-capital approach to minimize the
impact of rising chemical price on the profit.
[0004] The conventional practice of enzyme however causes a number
of the problems that can deter this practice at the paper mills.
These include: excessive yield loss, a significant increase
(20-50%) in effluent Chemical Oxygen Demand (COD) loading to the
Waste Water Treatment Plant (WWTP), interference with wastewater
treatment biological activity, an increase in final effluent COD
discharge, and potential impact on paper machine "sizing"
chemistry.
[0005] These drawbacks particularly the effluents COD increase have
largely prevented many paper mills from using the enzyme technology
for bleaching cost reduction. For example, the xylanase usage has
caused a 20,000 lb/day Biochemical Oxygen Demand (BOD) increase on
HardWood (HW) pulp, responsible for the Waste Water Treatment Plant
odor complaints in one paper mill. The increase in the effluent COD
and its negative impact on the WWTP operation is also a deterrent
to usage of Luminase enzyme application at another mill.
[0006] These challenges have led to intensive investigation to seek
ways to use xylanase enzyme for effective reduction of bleaching
chemicals without causing a significant increase in COD loading and
interference with WWTP operation.
[0007] Therefore, there is a need in the art for methods or
processes of prebleach treatment of pulp with xylanase enzyme prior
to chemical bleaching.
SUMMARY OF THE INVENTION
[0008] The problems were successfully resolved by simultaneous
deployment of the following key strategies in the present
invention. The low enzyme dosage is the key factor one out of the
three strategies.
[0009] Application of a catalytic enzyme dosage (up to 30 mg/1)
made possible by the following improved strategies in enzyme
treatment:
1. Selecting more efficient luminase xylanase enzyme. 2. Installing
bleach plant prewasher to recycle enzyme. 3. Reducing pulp COD
carryover with CO.sub.2 improved washing. 4. Maintaining good pulp
and enzyme mixing. 5. Improved pH control with CO.sub.2. 6.
Decreased Do and Eop delignification stage pH and temperature made
possible by improved bleachability of enzyme pretreatment. 7.
Adjustment in wastewater treatment operation parameters mainly
increasing dissolved oxygen (DO) and macronutrients nitrogen and
phosphorus (N and P) levels in proportion to any increase in
wastewater organic (BOD) concentration as a result of enzyme
treatment.
[0010] One aspect of this invention relates to prebleach treatment
of pulp with xylanase enzyme prior to chemical bleaching. The
unbleached pulp is treated with luminase xylanase up to an amount
of about 10-50 g of luminase xylanase per ton of unbleached pulp to
enhance subsequent bleaching of the unbleached pulp. The treatment
of unbleached pulp is carried out while buffering the unbleached
pulp and luminase xylanase with carbon dioxide to a pH of from
about 6.5 to about 7.5.
[0011] Another aspect of this invention relates to a pulp bleaching
system having a prebleach treatment of pulp which comprises
unbleached pulp and luminase xylanase in an amount from about 10 g
to about to 13 g per ton of unbleached pulp to enhance subsequent
bleaching of the unbleached pulp and carbon dioxide buffering to
provide a pH in the prebleach treatment stage from about 6.5 to
about 7.5.
[0012] The process of the present invention provides one or more
advantages over prior processes. For example, advantages of some of
the embodiments of the process of this invention include 1)
reduction of bleaching chemicals such as, ClO.sub.2,
H.sub.2O.sub.2, NaOH, H.sub.2SO.sub.4 or any combination of the
foregoing, 2) reducing the bleaching cost, 3) reduce steam (energy)
consumption, 4) reduce wastewater cooling cost, 5) minimize pulp
yield loss, 6) minimize wastewater organic Chemical Oxygen Demand
(COD) loading increase to WWTP, 7) improve WWTP efficiency, 8)
decrease final effluent COD discharge, 9) minimize the potential
impact on paper machine sizing chemistry, 10) high pulp brightness
and brightness stability, and/or 11) a combination of two or more
of the aforementioned advantages. Some embodiments of this
invention may exhibit one of the aforementioned advantages while
other preferred embodiments may exhibit two or more of the
foregoing advantages in any combination.
[0013] In this process of invention, a low dosage of luminase
xylanase is required in the prebleach treatment of pulp as compared
to conventional usage of xylanase in treating pulp that use a
higher dosage followed by improved mixing and pH control using
acids including carbon dioxide as buffering agent to provide a pH
in the prebleach treatment stage from about 6.5 to about 7.5.
DETAILED DESCRIPTION OF THE INVENTION
[0014] While this invention is susceptible of embodiment in many
different forms, there is shown and described in tables and
examples and will herein be described in detail preferred
embodiments of the invention with the understanding that the
present disclosure is to be considered as an exemplification of the
principles of the invention and is not intended to limit the broad
aspect of the invention to the embodiments illustrated.
[0015] It is advantageous to define several terms before describing
the invention. It should be appreciated that the following
definitions are used throughout this application.
DEFINITIONS
[0016] Where the definition of terms departs from the commonly used
meaning of the term, applicant intends to utilize the definitions
provides below, unless specifically indicated.
[0017] For the purpose of the present invention, the term "Chemical
Pulp" it is meant any type of virgin fiber, secondary fiber, woody
or nonwoody fiber, softwood, hardwood or a mixture thereof which
has been treated by chemical pulping such as, but not limited to,
Kraft pulp, soda pulp or sulfite pulp and is subsequently in a form
suitable for bleaching. Preferably, the chemical pulp comprises
virgin fiber. Chemical pulp also includes Kraft pulp, soda pulp or
sulfite pulp which has been exposed to an alkali oxygen
delignification stage prior to practicing the method of the present
invention. Other conditions associated with the production of
chemical pulp, including Kraft and sulfite pulps are described in
Pulp Bleaching: Principles and Practice (edited by Dence and Reeve,
1996; which is herein incorporated by reference).
[0018] For the purpose of the present invention, the term
"Xylanases" has been isolated from a variety of organisms including
bacteria and fungi. Xylanase has proven to be a valuable enzyme for
the pre-bleaching of pulp to enhance delignification of wood pulp
by facilitating the removal of lignin from pulp. Therefore,
xylanase prebleaching results in the use of lower amounts of
bleaching chemicals as compared to nonenzymatic bleaching.
[0019] For the purpose of the present invention, the term
"Bleaching" refers to whitening process carried out on pulps by
selective chemical removal of residual lignin and other colored
materials, and with minimal degradation of the cellulosic
constituents. With respect to secondary fibers, bleaching can also
have a dye removal function.
[0020] For the purpose of the present invention, the term
"Bleaching Chemical" refers to a variety of chemical used in the
bleaching of wood pulp such as chlorine (Cl.sub.2), sodium
hypochlorite (NaOCl), calcium hypochlorite [Ca(OCl).sub.2],
chlorine dioxide (ClO.sub.2), sodium hydroxide, peroxide
(H.sub.2O.sub.2), sodium chlorite (NaClO.sub.2), Oxygen (O.sub.2),
Ozone (O.sub.3) and others.
[0021] For the purpose of the present invention, the term
"Bleaching Sequence" refers to series of stages, each with specific
objectives (e.g., delignification, solubilization, destruction of
chromophoric groups), that contribute to an overall whitening
effect. Typically the pulp is washed between stages.
[0022] For the purpose of the present invention, the term "Kappa
Number" refers to modified permanganate test value on pulp which
has been corrected to 50% consumption of chemical. Kappa number has
the advantage of a linear relationship with lignin content over a
wide range. For pulp samples under 70% yield, the percent Klason
lignin is approximately equal to the Kappa number times a factor of
0.15.
[0023] For the purpose of the present invention, the term "Buffer"
refers to chemical solution that lower pH when acids are added.
Such acids include, but not limited to, CO.sub.2, and
H.sub.2SO.sub.4.
[0024] For the purpose of the present invention, the term
"Buffering Action" refers to ability to neutralize acids and bases
as they are formed during a chemical reaction and thus resist a
change in pH.
[0025] For the purpose of the present invention, the term "Oxygen
Delignification (O-Stage)" refers to treatment of pulp in alkaline
medium with oxygen to degrade and solubilize lignin, typically
employed as the first stage of a bleaching sequence or as a
bleaching "pre-stage". The process is generally carried out at
"medium consistency". Oxygen is added as a gas and magnesium salts
are usually employed as an additive to "protect" the cellulose from
degradation.
[0026] For the purpose of the present invention, the term "Chlorine
dioxide (D) Stages refers to initial delignifying stage (D.sub.o)
and/or brightening stages (D.sub.1 and D.sub.2) in a bleaching
sequence used to produce high-brightness pulp. Traditionally, the
highest and most stable brightness (especially for the softwood
Kraft pulps) is achieved when at least one chlorine dioxide (D)
brightening stage is used with an alkaline extraction in
between.
[0027] For the purpose of the present invention, the term "Alkaline
Extraction Stage-(E-Stage)" refers to essential stage in any
multistage bleaching sequence; it solubilizes the dark-colored
chlorinated and/or oxidized lignin compounds formed in the initial
acid delignification stage (e.g., chlorination or chlorine dioxide)
and in later stages. When used prior to the final bleaching stage,
an E-stage also serves to "activate" the pulp for more effective
brightening.
[0028] For the purpose of the present invention, the term "Peroxide
Extraction Stage-(Ep-Stage)" refers to alkaline extraction stage
supplemented with a peroxide agent.
[0029] For the purpose of the present invention, the term
"Oxidative Extraction Stage-(Eo-Stage)" refers to alkaline
extraction stage supplemented with an oxidizing agent, most
commonly oxygen. Peroxide or hypochlorite may also be used as
supplemental chemicals to provide a brightening effect and/or to
reduce effluent color.
[0030] For the purpose of the present invention, the term
"Oxidative peroxide Extraction Stage-(Eop-Stage)" refers to
alkaline extraction stage supplemented with peroxide and an
oxidizing agent.
[0031] For the purpose of the present invention, the term
"Brightening" refers to 1) any chemical treatment to pulp that
increases its brightness. 2) Chemical modification of colored
elements in high-yield pulps to render them colorless without
removing them, thus retaining the yield advantage of these
pulps.
[0032] For the purpose of the present invention, the term "Chemical
Oxygen Demand (COD)" refers to rapid chemical test to determine the
oxygen demand of all organic matter present in a sample of
wastewater. The limitation of the test is its inability to
differentiate between biologically oxidizable and biologically
inert organic matter.
[0033] For the purpose of the present invention, the term
"Biochemical Oxygen Demand (BOD)" refers to amount of oxygen
consumed in natural aerobic biological processes.
[0034] For the purpose of the present invention, the term
"Carryover" refers to active or inactive enzyme entrained with the
pulp leaving the enzyme treatment stage.
[0035] For the purpose of the present invention, the term
"Consistency" refers to mass or weight percentage of oven dry fiber
in a pulp solution, e.g., pulp and water, or stock (pulp and
additives) and water. It is expressed as a percentage of this
material in the solution, in terms of bone dry (BD), oven dry (OD),
or air dry (AD) weight. Consistency is often described
qualitatively as low, medium, or high without reference to a
standard nomenclature. The following ranges are given as a general
guide: very low consistency (0-1%), low consistency (1-8%), medium
consistency (8-16%), and high consistency (16-40%).
[0036] For the purpose of the present invention, the term
"retention time" refers to contact period of pulp with a bleaching
chemical; usually measured from the point of chemical addition to
the point where residual chemical is washed out or displaced by
another chemical.
[0037] For the purpose of the present invention, the term
"Medium-Consistency Centrifugal Pump a.k.a (MC) Pump" refers to a
pump which generates localized shear forces high enough to fluidize
pulp suspensions up to 15% consistency so that they behave like
Newtonian fluids. Air separation and evacuation is an important
aspect of pump design, since air would otherwise accumulate in the
eye of the pump.
[0038] For the purpose of the present invention, the term "Decker
or Gravity Thickener" refers to a device for increasing the
consistency of dilute fiber suspensions up to the 2-8 percent
level, consisting of a rotating screen drum which is wholly or
partially submerged in an open vat containing the fiber suspension.
Water flows into the cylinder by virtue of the difference in liquid
level between the vat and cylinder. In one type of design (commonly
called a "slusher" or "slush thickener"), the stock moves from the
inlet side of the vat through the dewatering zone to the other side
of the vat where the stock is discharged. In another design
(sometimes call a "roll-type thickener"), pulp is retained on the
cylinder and is couched off by a rubber roll.
[0039] For the purpose of the present invention, the term "High
Density Storage or Buffer Storage" refers to large-volume,
high-density storage of unbleached or bleached pulp to provide
surge capacity between the pulp and paper mills or between sections
of the pulp mill, and thus allow for interruptions in either supply
or demand. It is also defined as storage of pulp within the 8 to
15% range of consistency.
DESCRIPTION OF THE INVENTION
[0040] One aspect of this invention relates to prebleach treatment
of pulp with xylanase enzyme prior to chemical bleaching. The
unbleached pulp is treated with luminase xylanase in an amount
effective up to an amount of about 10-50 g of luminase xylanase per
ton of unbleached pulp to enhance subsequent bleaching of the
unbleached pulp. The treatment of unbleached pulp is carried out
while buffering the unbleached pulp and xylanase enzyme with carbon
dioxide to a pH of from about 6.5 to about 7.5 and with a good
mixing between pulp and luminase enzyme.
[0041] In the preferred embodiment of this invention, the prebleach
treatment of pulp with lower levels (e.g., 10-20 g/t) of luminase
(a bacterial-produced endo-.beta.-1,4-xylanase) enzymes (e.g.,
Verenium's Luminase PB-100 and PB-200) to enhance subsequent
bleaching of pulp by chlorine dioxide, thus reducing usage of
chlorine dioxide. The optimum pH range for prebleaching treatment
of pulp with luminase is about 6.8-7.2 (i.e., slightly acidic to
slightly neutral) by using acid and preferably carbon dioxide as
the pH buffering agent to provide this pH range. A medium
consistency (MC) centrifugal pump is required to provide a good
mixing of pulp and luminase enzyme. Use of lower levels of luminase
enzyme decreases the filtrate chemical oxygen demand (COD) on the
pulp bleaching system. In the preferred embodiment of this
invention, the luminase enzyme can be added to brownstock or post
02 washer repulper or standpipe prior to medium consistency pump
(aka, MC pump). Luminase enzyme is most typically added at the
suction side of MC pump following the addition of acid or D
filtrate for pH control. Alternatively, the Luminase enzyme can be
added at the thich stock pump to improve pulp consistency from 10%
to 7% consistency. The enzyme treatment is completed in a
brownstock High Density (HD) tower or storage tank.
[0042] one further aspect of the present invention is directed to a
process that ensures all enzymes are effectively consumed in the
enzymatic treatment of the pulp and/or there is no or substantially
no active enzymes present in the pulp composition prior to
bleaching of the enzyme-treated pulp. The enzyme-treated pulp
composition prior to bleaching may contain from 0 to 50 g of active
xylanase, preferably from 0 to 40 g, more preferably 0 to 30 wt %,
and most preferably 0 to 20 g of active xylanase per ton of enzyme
treated pulp. This range may include less than 0.25, 0.5, 1, 2, 3,
4, 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41,
42, 43, 44, 45, 46, 47, 48, 49, and 50 g of active xylanase per ton
of pulp transferred to the bleaching step, including any and all
ranges and subranges contained therein.
[0043] The inventors identified several factors affecting bleach
filtrate COD increase in enzyme treatment. The key factors are
listed and discussed below:
[0044] 1--Type of Enzyme--While many xylanase products are
available on the market, all xylanase enzymes do not have the same
effectiveness in terms of bleaching chemical reduction and the
extent of the side effects such as filtrate COD increase. The
inventors discovered, Verenium's Luminase xylanase enzyme
outperforms other market xylanase products because of the two key
differences between Luminase and market xylanase products: 1)
Luminase is produced from bacteria rather than fungi for other
market xylanase, and 2) Luminase seems to have better penetration
and reactivity than other xylanase products (determined
collectively by the enzyme's protein size, shape, and charge). The
selection of luminase enzyme has made it possible to reduce enzyme
dosage that not only improves the net bleaching cost reduction, but
provides a mechanism to minimize filtrate COD increase commonly
observed in enzyme trials. Traditionally, paper mills used 75-300
g/t enzyme dosage based largely on recommendations from enzyme
suppliers. Dropping the enzyme dosage to 10-50 g/t will
substantially reduces the net bleaching cost as well as pulp yield
loss and bleach filtrate COD increase
[0045] 2--Enzyme dosage--There exist an optimum enzyme dosage which
above that level, filtrate BOD/COD will increase without improving
bleaching cost reduction (actually decreasing net cost savings
because of higher enzyme cost). The filtrate COD increase with
increasing enzyme dosage. That optimum enzyme dosage was found to
be as low as 10 g/t of pulp. Some paper mills applied as high as
300 g/t enzyme dosage in the past. This is believed to be the key
reason for the elevated filtrate COD increase associated with
xylanase enzyme pulp treatment.
[0046] 3--Enzyme Conditions--It was noted that pH is perhaps the
most important operating parameter in enzyme treatment of pulp. The
natural xylanase optimum pH is about 6.5-7.5. If paper mills
operate outside of the enzyme optimum pH range, it will reduce the
enzyme effectiveness while still increasing filtrate COD.
Generally, paper mills add H.sub.2SO.sub.4 to adjust unbleached
pulp pH to achieve the optimum enzyme treatment pH in the
brownstock HD tower. The inventors discovered that compared with
H.sub.2SO.sub.4, CO.sub.2 provides a buffer at a neutral to
slightly acidic pH range and improves pH uniformity in the
brownstock HD tower. Both effects can translate to more enzyme
effectiveness and a lower enzyme dosage. Other key enzyme treatment
conditions affecting the effectiveness and COD increase are
temperature, retention time, and unbleached pulp carryover.
[0047] The temperatures employed in the enzyme treatment stage may
vary widely and any temperatures employed in conventional enzyme
treatment may be used. For example, useful temperatures can be as
low as about 40.degree. C. or lower and as high as about 70.degree.
C. or higher. In the enzyme treatment of this invention, the
temperature is usually from about 40.degree. C. to about 70.degree.
C., preferably from about 45.degree. C. to about 65.degree. C., and
most preferably from about 50.degree. C. to about 60.degree. C.
[0048] The retention time employed in the enzyme treatment stage
may vary widely and any retention time employed in conventional
enzyme treatment may be used. Usually, retention time will be at
least about 30 minutes. Retention times are preferably from about
15 min. to about 240 min., and are more preferably from about 30
minutes to about 240 min. and most preferably from about 40 min. to
about 240 min.
[0049] The consistency (CSC) of the pulp may vary widely and any
consistency that provides the desired increase in pulp brightness
may be used. The pulp may be treated under low or medium
consistency conditions (i.e. from about 2% to about 15% based on
the total weight of the mixture of pulp and bleaching chemicals).
The consistency is preferably from about 5% to 15%, more preferably
from about 8% to 15%, and most preferably from about 10% to about
12%.
[0050] Optionally, a washer is disposed after the enzyme treatment
step and prior to bleaching to wash the pulp. The washed pulp is
then sent to bleaching and the remaining composition may be
utilized elsewhere in the process of the invention. For example and
preferably in this embodiment, the remaining composition may
contain enzyme, preferably active enzyme. Thus, washing the pulp
after the enzyme treatment step removes a substantial portion of
the enzyme from the pulp composition after the enzyme treatment
step and prior to bleaching. When the remaining composition
contains enzyme after washing, the composition may in part or in
whole be recycled for use at any one or more of the aforementioned
upstream enzyme addition points described above in the process of
the invention including but not limited to the high shower side or
the repulper side of the brown stock decker. Alternatively, the
remaining composition containing enzyme after washing may be used
as a substitute to dilution water that may be mixed with fresh
Luminase before entered on the repulper side of the brown stock
decker. A person of ordinary skill in the art would appreciate that
the remaining composition containing enzyme after washing can be
added anywhere between the repulper side of the brown stock decker
and/or prior to the enzyme treatment step including but not limited
to those enzyme addition points described hereinabove in the
process of the invention.
[0051] 4--ClO.sub.2 Usage in D.sub.o stage--After enzyme treatment,
the pulp is much easier to delignify. It is important to reduce the
ClO.sub.2 charge in the D.sub.o stage. If the paper mill operates
the D.sub.o stage at above a critical ClO.sub.2 charge, there is no
net improvement in D.sub.oEop delignification efficiency, while
proportionally increasing D.sub.o stage filtrate COD.
[0052] 5--pH and Temperature of Eop stage--Lower Eop stage pH is
the key to leverage the bleach filtrate COD. Backing off Eop pH is
made possible by effective enzyme treatment particularly with using
CO.sub.2 for pH adjustment (better enzyme penetration and
efficiency due to more buffered pH with CO.sub.2). The reduced Eop
pH and temperature also contributes to an improved bleaching cost
reduction and a minimum filtrate COD increase. The reduction in the
Eop conditions (pH or NaOH charge, temperature and sometime H2O2
charge) after enzyme treatment is made possible by the
functionality of the luminase enzyme to break down the lignin-xylan
bonds, making residual lignin more accessible and reactive for
removal in the first two delignification (D.sub.oEop) stages of the
bleach plant. The reduced temperature in the Eop filtrate can
improve COD removal efficiency in mill wastewater treatment plant
(WWTP).
[0053] Subsequently, all the enzyme application principles were
applied at one paper mill and have achieved surprisingly excellent
results that were not known previously such as:
1) Enzyme pretreatment has allowed the mill to cut all the steam
usage off in all of its bleach stages, leading to lower wastewater
temperature and improved WWTP operation efficiency. 2) The COD
increase is minimized to less than 10% with the help of prebleach
washer and by manipulating the enzyme dosage and enzyme application
(e.g., CO.sub.2) and optimizing D.sub.oEop delignification
operation conditions (e.g., D.sub.o ClO.sub.2 charge and Eop pH and
temperature). 3) The actual final effluent COD discharge to one
paper mill receiving water after Wastewater Treatment Plant (WWTP)
is actually 4% lower with enzyme compared with the period without
enzyme because of a) the positive impact of lower wastewater
temperature on WWTP operation and b) the readily biodegradable COD
(sugars) added from enzyme treatment. 4) Specifically, enzyme
pretreatment at one paper mill enables the reduction of Eop pH by
0.29 and 0.23 units for SoftWood (SW) and HardWood (HW)
respectively. The total temperature reduction in D.sub.o, Eop, and
D.sub.1 stages for SW and HW are 8.degree. C. and 6.degree. C.,
respectively. There is no impact on paper machine (PM) sizing
chemistry.
[0054] One paper mill achieved better reduction in bleach chemical
and steam consumption and lower additional load of COD from bleach
plant compared with other paper mills, attributable to many factors
including: [0055] The use of better xylanase enzyme (Luminase)
[0056] The availability of a pre bleach plant washer to recycle
[0057] The CO.sub.2 use in post O.sub.2 washer [0058] Availability
of a MC centrifugal pump for efficient mixing [0059] Tight control
of enzyme treatment conditions on particularly pH (6.7-7.0) [0060]
Lower enzyme dosage (11-13 g/t versus as high as 500 g/t for other
mills) [0061] Reduction of D.sub.o kappa factor or ClO.sub.2 charge
[0062] Reduction of D.sub.o and Eop stage temperature [0063]
Reduction of Eop stage pH.
[0064] Since a majority of COD is generated during enzyme treatment
prior to ECF bleaching, the prebleach plant washer allows the paper
mill to send some part of COD to the recovery boiler for burning
rather than sewering. This has contributed to the observed low COD
increase in mill wastewater and is one of the key recommendations
for practicing enzyme treatment with minimum COD increase.
[0065] For some mills, the aeration power (to increase the
dissolved oxygen (DO) level) and macronutrients nitrogen and
phosphorus (N and P) concentrations in the wastewater treatment
plant needs to be increased to successfully treat a higher COD
enzyme treated effluent and for a higher treatment efficiency.
[0066] However, one of the advantages of a preferred embodiment of
this invention is the reduction of bleaching chemicals such as
ClO.sub.2, H.sub.2O.sub.2, NaOH, or combination thereof in the Eop
and/or Ep stages as compared to the same or substantially the same
bleaching processes which do not include the enzyme treatment
stage. For example, the reduction in the amount of H.sub.2O.sub.2
is typically at least about 5%. In the most preferred embodiments
of the invention when hydrogen peroxide is used as a bleaching
agent in the Eop or Ep extraction stage. The amount of hydrogen
peroxide in the bleaching liquor is preferably from about 5 to
about 100 pounds per ton of pulp on a dry basis. The hydrogen
peroxide is conventionally obtained from suppliers as a mixture of
50% water and 50% hydrogen peroxide on a weight basis, but other
proportions of water and hydrogen peroxide can be used, provided
they are equivalent to from 5 to 100 pounds of H.sub.2O.sub.2
chemical. These amounts of hydrogen peroxide can be applied to the
methods of brightening mechanical as well as chemical pulps
according to the present invention. The amount of extraction agent
oxidant used (e.g. hydrogen peroxide and hypochrite) used in the
practice of the process of this invention can vary widely and any
amount sufficient to provide the desired degree of brightness can
be used. The amount of bleaching agent used is usually at least
about 0.2% based on the dry weight of the pulp. Preferably the
amount of bleaching agent is from about 0.5% to about 1%, more
preferably from about 0.5% to about 0.8% and most preferably about
0.5-0.8% on the aforementioned basis.
[0067] In addition, the Eop or Ep pulp brightness and viscosity
were higher than those treatments without enzyme treatment, which
indicates the positive impact of enzyme treatment on the peroxide
efficiency and selectivity in the Ep stage. For example, the
viscosity is typically at least about 5%, preferably at least about
10%, more preferably from about 5% to about 15% and most preferably
from about 5% to about 10% greater than the viscosity of the pulp
made by the same or substantially the same bleaching processes
which do not include the enzyme treatment stage. For example, the
brightness is typically at least about 2%, preferably at least
about 10%, more preferably from about 5% to about 15% and most
preferably from about 7% to about 10% greater than the brightness
of the pulp made by the same or substantially the same bleaching
processes which do not include the enzyme treatment stage.
[0068] Conventional process parameters employed in these oxidative
extraction stage can be described in, for example "Pulp Bleaching
Principles and Practice of Pulp Bleaching" Carlton W. Dence and
Douglas W. Reeve, TAPPI Press, 1996 and references cited therein.
Accordingly, they will not be described in greater detail.
[0069] In the preferred embodiment of this invention in which a
prior D.sub.o extraction stage is used, the pH in the D.sub.o stage
of the present invention is higher than the pH of the conventional
D.sub.o bleaching stage. The advantages of higher pH are higher
brightness, less AOX formation, elimination of H.sub.2SO.sub.4
addition and associated BaSO.sub.4 scale formation or a combination
of two or more thereof. The pH of the D.sub.o stage is preferably
in the range from greater than 3 to about 6. Any pH within this
range can be used. For example, the pH can be as high as about 4,
5, or 6 and as low as about 2.5 to about 3. In the preferred
embodiments of the invention, the pH is from about 3 to about 6. In
the more preferred embodiments of the invention, the pH is from
about 4 to about 6 and in the most preferred embodiments of the
invention, the pH is from about 4.5 to about 5.5.
[0070] However, one of the advantages of a preferred embodiment of
this invention is the reduction of bleaching chemicals such as
ClO.sub.2 in the D.sub.0 stage as compared to the same or
substantially the same bleaching processes which do not include the
enzyme treatment stage. For example, the reduction in the amount of
ClO.sub.2 is typically at least about 5%, preferably at least about
10%, more preferably from about 5% to about 20% and most preferably
from about 10% to about 20% less than the amount of ClO.sub.2 used
in the same or substantially the same bleaching processes which do
not include the enzyme treatment stage to obtain the same or
substantially level of pulp brightness in the Eop and or Ep stages.
The amount of extraction agent used (e.g. sodium hydroxide,
magnesium hydroxide, potassium hydroxide, etc.) used in the
practice of the process of this invention can vary widely and any
amount sufficient to provide the desired lignin extraction
efficiency and the desired degree of brightness can be used. The
amount of caustic used is usually at least about 0.5% based on the
dry weight of the pulp. Preferably the amount of bleaching agent is
from about 1% to about 8%, more preferably from about 1.5% to about
3% and most preferably about 1-2% on the aforementioned basis.
[0071] In the most preferred embodiments of the invention, the
amount of chlorine dioxide in the Do extraction stage liquor is
preferably from about 10 to about 100 pounds per ton of pulp on a
dry basis. The chlorine dioxide is conventionally obtained from
suppliers as a mixture of 90% water and 10% chlorine dioxide on a
weight basis, but other proportions of water and chlorine dioxide
can be used, provided they are equivalent to from 10 to 100 pounds
of ClO.sub.2 chemical. These amounts of chlorine dioxide can be
applied to the methods of brightening mechanical as well as
chemical pulps according to the present invention.
[0072] The plant source of pulp for use in this invention is not
critical and may be any fibrous plant which can be subjected to
chemical pulp bleaching. Examples of such fibrous plants are trees,
including hardwood fibrous trees such as aspen, eucalyptus, maple,
birch, walnut, acacia and softwood fibrous trees such as spruce,
pine, cedar, including mixtures thereof. In certain embodiments, at
least a portion of the pulp fibers may be provided from non-woody
herbaceous plants including, but not limited to, kenaf, hemp, jute,
flax, sisal, or abaca although legal restrictions and other
considerations may make the utilization of hemp and other fiber
sources impractical or impossible. The source of pulp for use in
the practice of this invention is preferably hardwood and softwood
fibrous trees, more preferably Eucalyptus, Spruce and Aspen and is
most preferably Aspen and Spruce.
[0073] The pulp used in the process of this invention can be
obtained by subjecting the fibrous plant to any chemical pulping
process. Following the wood digestion process, pulp is separated
from the spent pulping liquor. The spent pulping liquor is then
recovered and regenerated for recycling. The pulp is then bleached
and purified in a bleach plant operation.
[0074] The pulp of this invention can also be used in the
manufacture of paper and packaging products such as printing,
writing, publication and cover papers and paperboard products.
Illustrative of these products and processes for their manufacture
are those described in U.S. Pat. Nos. 5,902,454 and 6,464,832.
[0075] For example, in the paper or paperboard making process, the
bleached pulp of this invention or pulp mixtures comprising the
bleached pulp of this invention is formulated into an aqueous paper
making stock furnish which also comprises one of more additives
which impart or enhance specific sheet properties or which control
other process parameters. Illustrative of such additives is alum
which is used to control pH, fix additives onto pulp fibers and
improve retention of the pulp fibers on the paper making machine.
Other aluminum based chemicals which may be added to furnish are
sodium aluminate, poly aluminum silicate sulfate and poly aluminum
chloride. Other wet end chemicals which may be included in the
paper making stock furnish for conventional purposes are acid and
bases, sizing agents, dry-strength resins, wet strength resins,
fillers, coloring materials, retention aids, fiber flocculants,
defoamers, drainage aids, optical brighteners, pitch control
chemicals, slimicides, biocides, specialty chemicals such as
corrosion inhibitors, flame proofing and anti-tarnish chemicals,
and the like.
[0076] The aqueous paper making stock furnish comprising the
bleached pulp and the aluminum based compounds is deposited onto
the forming wire of a conventional paper making machine to form a
wet deposited web of paper or paperboard and the wet deposited web
of paper or paperboard is dried to form a dried web of paper or
paperboard. Paper making machines and the use of same to make paper
are well known in the art and will not be described in any great
detail. See for example, Pulp and Paper Chemistry and Handbook for
Pulp & Paper Technologies, supra. By way of example, the
aqueous paper making stock furnish containing pulp, aluminum based
and other optional additives and usually having a consistency of
from about 0.3% to about 1% is deposited from the head box of a
suitable paper making machine as for example a twin or single wire
Fourdrinier machine. The deposited paper making stock furnish is
dewatered by vacuum in the forming section. The dewatered furnish
is conveyed from the forming section to the press section on
specially-constructed felts through a series of roll press nips
which removes water and consolidates the wet web of paper and
thereafter to the dryer section where the wet web of paper is dried
to form the dried web of paper of this invention. After drying, the
dried web of paper may be optionally subjected to several dry end
operations such as and various surface treatments such as coating,
and sizing and calendering.
[0077] The paper manufactured in accordance with this invention can
be used for conventional purposes. For example, the paper is useful
as printing paper, publication paper, newsprint and the like.
[0078] The present invention is described in more detail by
referring to the following examples and comparative examples which
are intended to more practically illustrate the invention and not
to be a limitation thereon.
Example 1
[0079] The lab investigation on Hardwood (HW) pulp shows an
excellent Luminase enzyme response in that application of Luminase
enzyme at a dosage as low as 10 g/t reduces 20% of ClO.sub.2 and
NaOH with 1-2% higher extracted pulp brightness as shown in Table
1.
[0080] The filtrate COD in the D.sub.o stage is strictly dependent
upon the enzyme application dosage, decreasing with decreasing
enzyme dosage. Therefore, one way to manipulate the filtrate COD
increase for luminase enzyme treatment is to control the enzyme
dosage rate. Luminase enzyme treatment at 10 g/t for birch pulp
bleaching actually leads to a 5% decrease in filtrate COD versus
about 10% COD increase at the 15 g/t enzyme dosage while achieving
the same 20% of ClO.sub.2 and NaOH reduction at both enzyme
dosages. The process results and conditions are set forth in the
following Table 1.
TABLE-US-00001 TABLE 1 Birch Pulp Luminase Lab Study PB 100 enzyme,
g/t 0 (Control) 10 15 30 100 Luminase Treatment Conditions: 8% CSC,
pH 5.6 (w/H.sub.2SO.sub.4), 60.degree. C., 40 min Brightness, %
47.5 46.5 47.2 48.4 No washing between Luminase and D.sub.o stage
Do: 45 min, 10% CSC, vat pH 4, 60 C. ClO.sub.2 charge, % 1.15 0.9
0.9 0.9 0.9 Brightness, % 73.7 68.8 70.5 70.9 70.5 Filtrate COD,
mg/l 1504 1286 1748 2016 2372 Ep: 45 min, 10% CSC, 70 C., 0.45%
H.sub.2O.sub.2 NaOH charge, % 1.0 0.8 0.8 0.8 0.8 Brightness, %
76.6 77.8 78.3 78.1 78.8 P no 1.8 2.6 2.5 2.4 2.4 Filtrate COD,
mg/l 1060 1142 1066 1084 1046
Example 2
[0081] The lab results on HW pulps are summarized in Table 2. The
luminase enzyme dosage study on HW pulp shows that a luminase
dosage at 20 g/t works as well as a high enzyme dosage at 100 g/t.
Overall, the HW pulp has excellent response to luminase enzyme,
resulting in 6 lb/t (20%) ClO.sub.2 reduction and 3 lb/t (25%) NaOH
reduction with 1-2% higher final brightness and viscosity. The
process results and conditions are set forth in the following Table
2:
TABLE-US-00002 TABLE 2 HW Pulp Luminase Lab Study Luminase, g/t 0
29 30 100 Luminase: 65 min, 10% CSC, 60 C. Brightness, % 50.9 51.7
52.2 D0: 20 min, 5% CSC, 63 C., 0.5% H2SO4 ClO.sub.2, % 1.0 0.8 0.8
0.8 Brightness, % 72.7 75.5 75.9 76.7 Filtrate COD, mg/l 516 685
770 955 Ep: 90 min, 10% CSC, 0.1% H.sub.2O.sub.2 Temperature, C. 72
72 72 72 NaOH, % 0.6 0.45 0.45 0.45 Brightness, % 74.9 78.4 79.1
79.6 P# 2.8 2.7 2.5 2.3 Filtrate COD, mg/l 776 774 746 714 D1: 180
min, 10% CSC, 0.6% ClO2 Temperature, C. 72 72 60 60 Brightness, %
87.5 88.6 89.5 89.2 Reverted Brightness, % 85.5 87.0 88.0 87.2
Viscosity, cps 17.2 18.7 19.5 19.5 Tappi Dirt, ppm 0 0 0 0 Filtrate
COD, mg/l 332 308 267 284
Examples 3 & 4
[0082] As shown in Tables 3 and 4 below, the lab investigation
shows excellent HW pulp response to Luminase enzyme treatment.
Application of Luminase enzyme at a dosage as low as 30 g/t reduces
about 20% of ClO.sub.2 and NaOH with higher extracted pulp
brightness without increasing bleach filtrate COD. The chemical
cost savings is substantial. Furthermore, the bleaching cost
reduction is the same for the enzyme dosage as low as 30 g/t and as
high as 125 g/t. The process results and conditions are set forth
in the following Table 3.
TABLE-US-00003 TABLE 3 Hardwood Batch Low Mid High Enzyme Enzyme
Enzyme Stage Control Dose Dose Dose Enzyme 0 0.063 0.126 0.252
#/adt Do % ClO2/ 0.73/13.14 0.6/10.8 0.6/10.8 0.6/10.8 #/adt COD
mg/L 362 434 444 426 Brightness, 68.4 73.0 72.9 71.9 ISO EOP NaOH
0.9/16.2 0.7/12.6 0.7/12.6 0.7/12.6 %/#/adt Temperature 80/176
68.3/155 60/140 60/140 .degree. C./.degree. F. COD mg/L 1174 1050
784 680 Brightness, 81.4 82.3 79.9 81.5 ISO D1 % ClO2/ 0.5/9 0.5/9
0.5/9 0.5/9 #/adt COD mg/L 542 503 525 484 Brightness, 86.8 88.2
88.0 87.9 ISO High Mid Low Temp E- Temp E- Temp E- Stage Control
Low Enzyme Low Enzyme Low Enzyme Enzyme #/adt 0 .063 .063 .063 Do %
ClO2/ 0.73/13.14 0.6/10.8 0.6/10.8 0.6/10.8 #/adt COD mg/L 344 258
258 258 Brightness, ISO 71.6 73.8 73.8 73.8 EOP NaOH 0.9/16.2
0.7/12.6 0.7/12.6 0.7/12.6 %/#/adt Temperature 80/176 70/158 65/149
60/140 .degree. C./.degree. F. COD mg/L 1082 935 788 818
Brightness, ISO 82.6 83.6 83.3 82.3 D1 % ClO2/ 0.5/9 0.5/9 0.5/9
0.5/9 #/adt COD mg/L 620 568 572 580 Brightness, ISO 89.2 89.1 89.6
89.1
[0083] For EO HW pulp, Luminase enzyme treatment did not result in
a significant change in filtrate COD. A decrease in Eop temperature
however leads to a reduced Eop filtrate COD without adversely
affecting the Eop efficiency.
Example 5
[0084] Similarly, the luminase enzyme dosage study on pine pulp
shows that there is no performance difference between a luminase
dosage at 30 g/t and 100 g/t as shown in Table 5 The luminase
enzyme of pine pulp results in 4 lb/t (15%) ClO.sub.2 reduction and
4 lb/t (25%) NaOH reduction with a significantly higher final
brightness and viscosity. The process results and conditions are
set forth in the following Table 5.
TABLE-US-00004 TABLE 5 SW Pulp Luminase Lab Study Luminase, g/t 0
30 100 Luminase: 65 min, 10% CSC, 60 C. Brightness, % 43.4 43.1 D0:
60 min, 3.8% CSC, 65 C., 0.8% H.sub.2SO.sub.4 ClO.sub.2, % 1.3 1.1
1.1 Brightness, % 68.0 66.2 66.9 Filtrate COD, mg/l 555 655 785 Ep:
110 min, 10% CSC, 73 C., 0.3% H.sub.2O.sub.2 NaOH, % 0.9 0.7 0.7
Brightness, % 78.2 78.7 79.3 P# 1.0 1.2 1.1 Filtrate COD, mg/l 1036
996 996 D1: 210 min, 10% CSC, 0.41% ClO.sub.2 Temperature, C. 74 65
65 ClO.sub.2, % 0.41 0.41 0.41 Brightness, % 88.3 88.7 89.1
Reverted Brightness, % 86.3 86.5 87.0 Viscosity, cps 18.0 20.0 18.7
Tappi Dirt, ppm 0 0 0 Filtrate COD, mg/l 330 252 246
[0085] The observed filtrate COD increase for both HW pulp and pine
pulp is decreased by reducing luminase enzyme dosage and can be
further minimized by dropping Eop temperature.
[0086] Excellent luminase enzyme responses were achieved in lab and
mill trial/practice for all the pulps investigated, demonstrating
the viability of luminase as a non-capital cost reduction approach
for mills provided that the effective luminase enzyme treatment
conditions in the brown stock HD can be achieved. The filtrate COD
can be minimized by carefully controlling the enzyme dosage,
mixing, and pH in the luminase application as well as Eop stage pH
and temperature.
Xylanase Application Logistics
[0087] Xylanase Addition and Reaction Vessel--
[0088] As discussed hereinabove, the luminase xylanase can be added
to brownstock or post O.sub.2 washer repulper or standpipe prior to
MC pump. Luminase xylanase is most typically added at the suction
side of MC pump following the addition of acid or D filtrate for pH
control. The xylanase treatment is completed in a brownstock high
density (HD) tower.
[0089] The effective low dosage luminase xylanase practice to
achieve improved bleaching cost reduction without a pulp yield loss
and filtrate COD increase, it requires the implementation of part
or all of the following key application strategies where
practical:
a) Improved pH control to 6.5-7.5, preferably 6.8-7.2 is desirable
in brownstock high density (HD) tower b) Low unbleached pulp COD or
saltcake carryover at <10 kg/ton of pulp, preferably <5 kg/t
desirable after the washer c) Optional use of a pre-bleach plant
washer (pre-washer) to reduce carryover and recycle xylanase d)
Reduction in the extraction (E, Eo, or Eop) stage pH by up to 2
units (to as low as 9) and temperature by up to 20.degree. C. (to
as low as 60.degree. C.) as compared with the case without luminase
xylanase treatment
[0090] In the preferred embodiment of this invention, the luminase
xylanase can be added to last brownstock or post O2 washer repulper
or standpipe prior to a thick stock pump (aka, TS pump) or a medium
consistency pump (aka, MC pump). For the paper mills with a MC
pump, luminase xylanase is typically added at the suction side of
MC pump. For the paper mills with a TS pump, luminase xylanase must
be added at the last brownstock or post O2 washer repulper to get
initial xylanase and pulp mixing followed by adding mechanical
action in the TS pump. The pulp consistency can be diluted from
typically 10-12% to 7-8% to further help xylanase distribution and
mixing.
[0091] Luminase xylanase may be added to the pulp line following
the addition of acid or D filtrate for pH control before being
pumped to a brownstock high density (HD) tower or storage tank
where the xylanase treatment is completed.
[0092] The acids used for pH control include, but not limited to,
CO.sub.2, H.sub.2SO.sub.4, HCl. The buffering agent CO.sub.2 can be
added at the inlet (vat) of the washer to reduce the pH of the pulp
while simultaneously reducing pulp carryover. Alternatively,
CO.sub.2 can be added at the outlet (repulper) of the washer for pH
control.
[0093] The addition of a pre-washer will help to practice
low-dosage luminase xylanase in three ways: 1) to reduce the
unbleached pulp carryover for improved xylanase efficiency (as
xylanase preferentially reacts with COD or saltcake carryover in
filtrate before reacting with pulp), 2) to recycle residual
xylanase, and 3) to allow the pulp mill to send some part of COD to
the recovery boiler for burning rather than dumping in a sewer.
[0094] Optionally, a washer is disposed after the enzyme treatment
step and prior to bleaching to wash the pulp. The washed pulp is
then sent to bleaching and the remaining composition may be
utilized elsewhere in the process of the invention. For example and
preferably in this embodiment, the remaining composition may
contain enzyme, preferably active enzyme. Thus, washing the pulp
after the enzyme treatment step removes a substantial portion of
the enzyme from the pulp composition after the enzyme treatment
step and prior to bleaching. When the remaining composition
contains enzyme after washing, the composition may in part or in
whole be recycled for use at any one or more of the aforementioned
upstream enzyme addition points described above in the process of
the invention including but not limited to the high shower side or
the repulper side of the brown stock decker. Alternatively, the
remaining composition containing enzyme after washing may be used
as a substitute to dilution water that may be mixed with fresh
Luminase before entered on the repulper side of the brown stock
decker. A person of ordinary skill in the art would appreciate that
the remaining composition containing enzyme after washing can be
added anywhere between the repulper side of the brown stock decker
and/or prior to the enzyme treatment step including but not limited
to those enzyme addition points described hereinabove in the
process of the invention.
[0095] As noted above, one aspect of the present invention is
directed to a process that ensures all enzymes are effectively
consumed in the enzymatic treatment of the pulp and/or there are no
or substantially no active enzymes present in the pulp composition
prior to bleaching of the enzyme-treated pulp. The enzyme-treated
pulp composition prior to bleaching may contain from 0 to 50 g of
active xylanase, preferably from 0 to 40 g, more preferably 0 to 30
wt %, and most preferably 0 to 20 g of active xylanase per ton of
enzyme treated pulp. This range may include less than 0.25, 0.5, 1,
2, 3, 4, 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23,
24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
41, 42, 43, 44, 45, 46, 47, 48, 49, and 50 g of active xylanase per
ton of pulp transferred to the bleaching step, including any and
all ranges and subranges contained therein.
[0096] For some mills, because of unusual pulp response to xylanase
treatment and/or practical challenges to implement all the
application aspects for COD minimization, there still will be some
COD increase in bleach filtrate or wastewater to treatment plant
during the low-dosage luminase xylanase practice. The mill may
increase the aeration power to raise the dissolved oxygen (DO)
level to >1 mg/1, preferably to >2 mg/1, throughout the
aeration basin and to increase macronutrients nitrogen and
phosphorus (N and P) addition to the wastewater to successfully
treat a higher COD xylanase treated effluent and for a higher
treatment efficiency in wastewater treatment plant. For example,
for 100 parts of BOD (COD), about 5 parts nitrogen and 1 part
phosphorus is required.
[0097] Xylanase Application System--
[0098] The process of making pulp in a pulp bleaching system
comprises at least one Do stage and at least one Eop bleaching
stage. A prebleach treatment stage comprises pulp and xylanase in
an amount of less than 50 g of xylanase per ton of pulp. A
buffering agent is used for buffering the pulp and xylanase in
order to provide a pH in the prebleach treatment stage from about
6.5 to about 7.5.
Brightness
[0099] Approximately 5 grams of pulp is rolled or pressed on a disc
and is permitted to completely dry. The brightness is measured on
both sides of the brightness pad, at least four readings per side
and then the average is calculated. These readings are performed on
a GE brightness meter which reads a directional brightness or on an
ISO brightness meter which reads a diffused brightness. Both
instruments are made by Technidyne Corp.
Viscosity
[0100] The viscosity is a measurement used to compare a relative
strength property of the pulp. This property is used to determine
the percentage of hardwood/softwood for making different grades of
paper. A Cannon-Fenske (200) viscometer tube, calibrated for 25 C,
is used for testing bleached pulps. The sample size is 0.2000
grams, using 20 ml, 1.0 molar CED and 20 ml DI water mixed
thoroughly to break down the pulp fiber.
Permanganate Number
[0101] The Permanganate Number indicates the amount of lignin that
is in the pulp. (The Kappa number is generally used only on the
brownstock, while the value for the Permanganate Number is
comparative to the bleached pulp.) The procedure for determining
the Permanganate Number is: [0102] 1. Weigh exactly 1.00 gram
sample. [0103] 2. Put the sample in a blender with 700 ml DI water
and blend about 45 seconds, pour the sample into a battery jar on a
stir plate. [0104] 3. Add exactly 25 ml of 0.1 N Potassium
Permanganate and 25 ml 4N H.sub.2SO.sub.4, starting a timer set for
5 min. [0105] 4. When the timer stops, add 6 ml 1 Molar KI and
allow it to mix thoroughly to kill the reaction. [0106] 5. Titrate
to a starch end point with 0.1N Sodium Thiosulfate. Record mls
titrated. [0107] 6. In 700 ml DI water without the pulp sample, use
the same reagents and titrate to use as a blank. Using an
accurately prepared Potassium Permanganate, the blank should be
25.0 [0108] 7. Subtract the mls titrated with the sample from the
mls titrated for the blank and the result will be the P Number.
Dirt
[0109] Pulp dirt count is done by a visual count of all the dirt
spots on the brightness pad and is the size weighted sum of the
total dirt spots according to a Tappi temperature rate.
[0110] Various modifications and variations may be devised given
the above-described embodiments of the invention. It is intended
that all embodiments and modifications and variations thereof be
included within the scope of the invention as it is defined in the
following claims.
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