U.S. patent application number 10/494396 was filed with the patent office on 2004-12-02 for microwave pre-treatment of logs for use in making paper and other wood products.
Invention is credited to Akhtar, Masood, Horn, Eric G, Klungness, John H, Lentz, Michael J, Scott, C. Tim.
Application Number | 20040238134 10/494396 |
Document ID | / |
Family ID | 23365405 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040238134 |
Kind Code |
A1 |
Akhtar, Masood ; et
al. |
December 2, 2004 |
Microwave pre-treatment of logs for use in making paper and other
wood products
Abstract
A method of producing pulp for use in making paper products
using microwave radiation to pretreat the source of pulp prior
further processing. Practising the method of the invention results
in substantial energy savings while decreasing environmental impact
and improving paper qualities.
Inventors: |
Akhtar, Masood; (Madison,
WI) ; Lentz, Michael J; (Madison, WI) ; Horn,
Eric G; (Madison, WI) ; Klungness, John H;
(Madison, WI) ; Scott, C. Tim; (US) |
Correspondence
Address: |
MICHAEL BEST & FRIEDRICH, LLP
ONE SOUTH PINCKNEY STREET
P O BOX 1806
MADISON
WI
53701
|
Family ID: |
23365405 |
Appl. No.: |
10/494396 |
Filed: |
May 3, 2004 |
PCT Filed: |
November 12, 2002 |
PCT NO: |
PCT/US02/36443 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60347818 |
Nov 9, 2001 |
|
|
|
Current U.S.
Class: |
162/20 ; 162/50;
162/72 |
Current CPC
Class: |
D21C 1/00 20130101; D21B
1/02 20130101; D21C 5/005 20130101; D21C 1/10 20130101; D21H 11/08
20130101; D21B 1/021 20130101 |
Class at
Publication: |
162/020 ;
162/050; 162/072 |
International
Class: |
D01B 001/14; D21C
001/00; D21C 003/20 |
Claims
We claim:
1. (Canceled)
2. A method of producing pulp for use in making paper products,
wherein the pulp source comprises wood logs, further comprising the
steps of: treating the wood logs with microwave radiation; chipping
the logs to provide wood chips; and mechanically processing the
wood chips to provide pulp.
3. The method of claim 2 further comprising the step of fiber
loading the mechanically processed pulp.
4. The method of claim 2, wherein the logs comprise a hardwood
species.
5. The method of claim 4 wherein the logs comprise at least one of
an aspen species, a eucalyptus species and an oak species.
6. The method of claim 2, wherein the logs comprise a softwood
species.
7. The method of claim 6, wherein the logs comprise at least one of
a spruce species and a pine species.
8. The method of claim 2, wherein mechanically processing comprises
an RMP process, a TMP process or a CTMP process.
9. The method of claim 2, wherein mechanically processing comprises
a TMP process.
10. A paper produced according to the method of claim 2.
11. The paper of claim 10, wherein the paper demonstrates improved
strength properties over a paper produced according to a method not
including treating logs with microwave radiation.
12. The paper of claim 10 wherein the paper demonstrates at least
about a 10% increase in measurements of tear index, burst index and
tensile index.
13. A method of producing pulp for use in making paper products,
the method comprising the steps of: treating wood logs with
microwave radiation; chipping the logs to provide wood chips;
inoculating the wood chips with a fungus; and mechanically
processing the inoculated wood chips to provide pulp.
14. The method of claim 13, wherein the fungus comprises a white
rot fungus.
15. The method of claim 14, wherein the fungus comprises
Ceriporiopsis subvermispora, Hyphodontia setulos, Phlebia
subserialis, Phlebia brevispora, Phlebia tremellosa or
Phanerochaete chrysosporium.
16. The method of claim 13, wherein the fungus comprises a white or
colorless species of Ophistoma piliferum.
17. A method of producing pulp for use in making paper products,
the method comprising the steps of: treating wood logs with
microwave radiation; chipping the logs to provide wood chips;
treating the wood chips with enzymes; and mechanically processing
the enzyme-treated wood chips to provide pulp.
18. The method of claim 17 wherein the enzymes comprise at least
one of lignin-degrading enzymes, xylanases, pectinases, lipases and
cellulases.
19. A method of reducing energy input requirements during wood
pulping comprising the steps of: treating logs with microwave
radiation; chipping the logs to provide wood chips; and
mechanically pulping the wood chips to provide pulp; wherein the
energy input requirement is reduced at least about 8% over a method
not including the step of treating logs with microwave
radiation.
20. The method of claim 19 wherein the energy input requirement is
reduced at least about 8% to about 15% over a method not including
the step of treating logs with microwave radiation.
21. The method of claim 2 wherein treating wood logs with microwave
radiation comprises microwaving the logs with a 915 MHz microwave
generator for 5 minutes at 50 kW.
22. The method of claim 21 wherein treating the logs with microwave
radiation results in a temperature of 130.degree. C. inside the
logs.
23. The method of claim 12, wherein the burst index is increased at
least about 30%, the tear index is increased at least about 15% and
the tensile index is increased at least about 20%.
24. A method of reducing pitch in a wood log comprising the step of
treating a wood log with microwave radiation.
25. The method of claim 24 wherein the wood log comprises a
softwood species.
26. The method of claim 24 wherein the wood log comprises a
hardwood species.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Serial No. 60/347,818, filed Nov. 9, 2001.
BACKGROUND OF THE INVENTION
[0002] In the manufacture of paper from wood, the wood is first
reduced to an intermediate stage in which the wood fibers are
separated from their natural environment and transformed into a
viscous liquid suspension known as a pulp. There are several
classes of techniques which are known, and in general commercial
use, for the production of pulp from various types of wood. The
simplest in concept of these techniques is the so-called refiner
mechanical pulping (RMP) method, in which the input wood is simply
ground or abraded in water through a mechanical milling operation
until the fibers are of a defined or desired state of freeness from
each other. Other pulping methodologies include thermo-mechanical
pulping (TMP), chemical treatment with thermo-mechanical pulping
(CTMP), chemi-mechanical pulping (CMP), and the so-called kraft or
sulfate process for pulping wood. In all of these processes for
creating pulps from wood, the concept is to separate the wood
fibers to a desired level of freeness from the complex matrix in
which they are embedded in the native wood.
[0003] Of the constituents of wood as it exists in its native
state, cellulose polymers are the predominate molecule. Cellulose
is desired for retention in the pulp for paper production. The
second most abundant polymer in the native wood is lignin. Lignin,
the least desirable component in the pulp, is a complex
macromolecule of aromatic units with several different types of
interunit linkages. In the native wood, lignin physically protects
cellulose polysaccharides in complexes known as lignocellulosics
that must be disrupted for there to be accessibility to the
polysaccharides,(e.g., by enzymes) or to separate lignin from the
matrix of the wood fibers.
[0004] Mechanical pulping accounts for about 25% of the wood pulp
production in the world today. This volume is expected to increase
in the future as raw materials become more difficult to obtain.
Mechanical pulping, with its high yield, is viewed as a way to
extend these resources. However, mechanical pulping is electrical
energy-intensive and yields paper with lower strength than chemical
pulps. Kraft pulp is often added to mechanical pulp to impart
strength, but it is much more expensive than mechanical pulp. These
disadvantages limit the use of mechanical pulp in many grades of
paper.
[0005] In the industry, chemical pulps are preferred for a variety
of paper grades, generally for better strength as a result of
superior pulp quality (e.g., higher freeness, higher fiber length,
and lower lignin content). However, chemical pulps are expensive to
produce and fiber yields are generally very low (about 50%). On the
other hand, mechanical pulps have fiber yields in excess of 90%,
but pulp quality is degraded because fiberization is sometimes not
complete and fibers can be severely damaged. Each process has its
own inherent advantages and disadvantages, and papermakers must
weigh these factors when developing a furnish for a particular
paper grade. However, faced with the reality of more restrictive
environmental regulations, increased energy costs, competitive
pricing, and a more diverse raw wood resource, papermakers are
being forced to be more creative in selecting furnish components.
Therefore, efforts must be made to develop new technologies that
improve the quality of mechanical pulps, making them more
attractive as a component in higher quality paper grades.
[0006] In the RMP process, wood chips are refined atmospherically
to make paper. This process requires approximately 100-135 Horse
Power Days (HPD) (about 1800-2400 kWh) energy /ton of wood and
produces pulp with lower strength.
[0007] In thermo-mechanical processes (e.g., TMP and CTMP), high
temperatures are used to separate the fibers during refining. These
processes generally require the refining to be carried out in one
or more steps. The first step is usually a pressurized step with
refining being performed at temperatures above 100.degree. C. and
immediately below or at the softening temperature of lignin. During
this step, the pulp is typically mechanically processed using the
RMP method. In subsequent steps, the pressure and temperature is
usually modulated to achieve the desired state of freeness between
the fibers.
[0008] In the TMP process, a steam pressure of 30lb or less (gauge
pressure) is applied to the chips for 2-5 minutes prior to
refining. This pressure is critical to separate the cell wall
fibers in such a way that the resulting paper has much longer
fibers (increased tear index) than the straight RMP process. If one
exceeds the pressure above 30 lbs during presteaming, then the
lignin will be melted and deposited on the surface of fibers and
the fiber flexibility will be lost resulting in poor quality fibers
that resemble the fibers produced during medium density fiber board
production. Therefore, it is critical to maintain the right gauge
pressure during refining. The drawback of the TMP process is that
it takes significantly higher amounts of energy compared to the RMP
process. For example, the energy requirement during the TMP process
is in the range of 140-220 HPD (about 2500-4000 kWh)/ton of wood.
The steam pressure also results in the darkening of pulp. Thus,
more bleach chemicals are needed to obtain paper of a desired
brightness.
[0009] Relatively large total electric energy amounts or large
quantities of input wood are required to produce pulps using the
above mentioned pulping techniques. In particular, high energy
inputs are generally required to obtain fiber separation in woods
rich in lignin as such woods typically call for extended refining
periods and high temperatures and/or pressures. Studies have also
suggested that even thermal or chemical softening treatments of
such woods does not guarantee a lower total energy consumption in
the production of pulp. This is because unprocessed fibers that are
only mildly separated by the thermal or chemical treatments are
difficult to fibrillate during the refining mechanical process.
Fibrillation is the separation of larger fibers into small,
thread-like structures called fibrils. Fibrillation is necessary to
increase the flexibility of the fibers and to bring about the fine
material characteristics of quality processed pulp. It has been
suggested that a decrease in energy consumption from an established
level in various TMN and CTNP processes has been associated with
the deterioration of certain pulp properties, including a reduction
in the long fiber content of the pulp, a lower tear strength and
tensile strength, and a higher shive content. As a result, high
energy consumption in TMP and CTMP processes has been generally
necessary in current pulping practices.
[0010] Biopulping techniques have been developed to supplement
traditional pulping methods and have been shown to reduce energy
requirements and improve paper properties. Biopulping is defined as
the treatment of wood chips with a "natural" wood decay fungus
prior to mechanical pulping. In this technology, wood chips are
steamed, cooled, inoculated with a fungus, and incubated for two
weeks under forced aeration to remove metabolic heat generated by
the fungus. The process saves a substantial amount of electrical
energy (about 30%), improves paper quality, reduces the
environmental impact of pulping, and enhances economic
competitiveness. However, the economics of the process is highly
dependent on the treatment time and processing costs such as those
associated with ventilation of the pile for two weeks to remove
metabolic heat generated by the fungus.
[0011] The direct application of enzymes has been proposed as a
means to reduce costs associated with energy expenditures and
processing required in traditional pulping methodologies. For
example, lignin-degrading fungi such as Ceriporiopsis
subvermispora, Hyphodontia setulos, Phlebia subserialis, Phlebia
brevispora, Phlebia treniellosa and Phanerochaete chrysosporium.
used in biopulping techniques secrete enzymes inside the wood cell
walls which are responsible for breakdown or modification of
lignin. However, it is known that direct application of isolated
enzymes on wood chips does not yield results similar to those
obtained with fungal pretreatment because these enzymes cannot
penetrate the wood due to their larger size compared to the pore
size in the wood. Live fungus is required to penetrate the wood and
transport enzymes inside the wood cell walls. Because of the low
accessibility of wood chips for enzymatic modification,
incorporation of an enzymatic treatment step into a mechanical
pulping. process can be expected to be successful only after the
primary stage of refining, during subsequent process steps. In
pilot-scale experiments, an energy savings of 10-15% with CBH I
(modified cellulase) has been reported with some improvement in
tensile index, a strength property. However, technical difficulties
have been reported in applying enzymes after primary stage refining
because pulp after primary stage refining enters into secondary
stage refining within seconds. Based on this difficulty, efforts
directed to enzyme treatment are now mainly on reject pulp samples.
Because most of the energy during TMP refining is consumed during
primary and secondary stage refining and not during reject
refining, direct enzyme application is useful only as a downstream
process. Thus, to date, energy savings due to enzymes have been
insignificant.
[0012] Similarly, in certain markets, lumbers are treated with
chemicals to protect them from the environment and provide
stability to the product. Currently, it is not possible for high
molecular weight compounds to penetrate the logs or the lumber for
certain applications, such as wood hardening. A technique that
enhances permeability of wood to larger molecules would therefore
be widely applicable, even outside the pulp and paper industry.
[0013] Another promising technology used in the pulp and paper
industry for improving paper brightness, opacity, and bonding
strength, as well as reducing energy consumption during drying, is
a technique termed "fiber loading." In fiber loading, calcium
carbonate is deposited as a filler within, on the surface of, and
outside the fibers. The process consists of at least two steps.
First, calcium hydroxide is mixed into a pulp fiber slurry. Next,
the pulp and calcium hydroxide mixture is reacted using a high
consistency pressurized reactor (refiner or disk disperser) under
carbon dioxide pressure to precipitate calcium carbonate. Calcium
carbonate formed is termed fiber-loaded precipitated calcium
carbonate (FLPCC). However, most applications of fiber loading have
focused on fiber loading chemical pulps.
[0014] In addition to the above-described efforts to increase pulp
yield, decrease energy consumption and enhance paper quality,
another issue concerning the pulp and paper industry is pitch
content. Pitch is a mixture of hydrophobic resinous materials and
constitutes about 2-8% of the total wood weight depending upon the
species and the time of the year. It causes a number of problems in
wood processing, including at least deposits on tile and metal
surfaces, plugging of drains, discoloration of felt, tears and
other defects in paper and downtime for cleaning. Traditional
methods of controlling pitch include natural seasoning of wood
before pulping and/or adsorption and dispersion of the pitch
particles with chemicals. During the pulping and papermaking
processes, pitch reduction methods can also include adding fine
talc, dispersants and other kinds of chemicals.
[0015] Biotechnological and enzymatic methods have also been
developed and used industrially to reduce pitch. It has been
reported that lipases reduce pitch by hydrolyzing triglycerides to
glycerol and free fatty acids in mechanical pulps. A commercial
lipase enzyme product, RESINASE.TM., has been developed (Novo
Nordisk Biochem of North America, Franklinton, N.C. to reduce pitch
deposits from groundwood pine pulp. Another commercial product,
CARTAPIP.TM. (Agra Sol Inc., Raleigh, N.C., U.S.A.) is a fungal
inoculum of the ascomycete Ophiostoma piliferum. A water slurry of
the fungal spores is sprayed onto wood chips as they are piled
prior to pulping. The fungus invades the wood cells, degrading the
pitch.
[0016] However, both traditional and biotechnological methods of
pitch control fail to remove all traces of pitch from most wood
species and thus only alleviate, but do not eliminate the problems
associated with pitch in wood processing. A method that would
provide enhanced pitch reduction would therefore be desirable.
[0017] Yet another dilemma faced by the pulp and paper industry is
blue staining of wood. This problem occurs when freshly cut logs
are stored for a long period of time in wood yards prior to
debarking and chipping. These logs are normally colonized by the
blue stain fungi present in the wood yard. The colonization results
in wood staining and consequently, pulps with lower brightness.
More bleach chemicals are therefore needed to overcome the loss of
brightness which, in turn, results in increased costs for effluent
treatment. This is a serious problem in the southern parts of the
U.S. where the logs are exposed to high temperature and humidity,
which tend to exacerbate the problem. Biotechnological methods of
reducing blue staining include the use of CARTAPIP.TM.(Agra Sol
Inc., Raleigh, N.C., U.S.A.), which, as discussed, is also used in
reducing, pitch. It has been shown that treatment with CARTAPIP.TM.
also controls unwanted colored blue stain microorganisms that lead
to increased costs in the purchase of bleach chemicals. However, as
with pitch reduction methods, efforts to reduce blue staining have
not been completely successful.
[0018] What is needed is an alternative method for producing pulp
in an energy efficient manner that also improves paper strength
properties while decreasing pollution. Also desirable is a method
that enhances permeability and porosity of the internal structure
of wood, thereby providing increased access for fungi, enzymes and
other large molecules and chemicals.
SUMMARY OF THE INVENTION
[0019] Described is a method of pulping wood, including the step of
treating, pretreating or exposing a source of pulp to microwave
radiation to reduce substantially the power requirements, chemical
requirements, or process time to convert the source of pulp to
pulp.
[0020] Also described is a method of producing pulp for use in
making paper products. In a preferred practice, the method includes
steps of treating wood logs with or exposing logs to microwave
radiation, chipping the logs and pulping the wood chips with a
mechanical pulping process. Suitable mechanical pulping processes
include RMP, TMP and CTMP. Optionally, the method can include fiber
loading the pulp.
[0021] Chips obtained from microwaved logs could also be treated
with microorganisms and enzymes to save energy, improve paper
strength, reduce pitch content, and increase chemical penetration
to benefit the pulp and paper and lumber processing industries.
[0022] The method can be used with hard- or softwood species as
pulp sources. Suitable hardwood species include aspen, eucalyptus
and oak. Suitable softwood species include spruce and pine.
[0023] The invention also encompasses a paper produced according to
the methods described. Suitably, the paper demonstrates improved
strength characteristics over methods not including a microwave
step. Most suitably, the paper demonstrates at least a 10% increase
in measurements of tensile index, tear and burst.
[0024] Another facet of the invention is a method of producing wood
pulp that includes steps of treating the wood source, e.g., logs,
with microwave radiation, chipping the logs to provide wood chips,
inoculating the wood chips with a fungus and mechanically
processing the inoculated wood chips to provide pulp. Suitable
fungal species include the "white rot" species commonly used in
biopulping. Included among the suitable species are Ceriporiopsis
subvermispora, Hyphodontia setulos, Phlebia subserialis, Phlebia
brevispora, Phlebia tremellosa or Phanerochaete chrysosporium. An
additional species which can be used in a method of the invention
is a white or colorless species of Ophistoma piliferum, which can
be used to reduce pitch and/or blue staining.
[0025] The invention is also directed to a method of producing pulp
that includes the steps of microwaving wood, chipping the wood,
applying enzymes to the wood chips and mechanically processing the
enzyme-treated wood chips to provide pulp. Suitable enzymes include
lignin-degrading enzymes, xylanases, pectinases, lipases and
cellulases.
[0026] The invention provides for energy savings during wood
pulping and includes a method of reducing energy input
requirements. The method includes steps of treating wood with
microwave radiation, chipping the wood and mechanically pulping the
wood chips, wherein the energy input requirement is reduced at
least about 8% over a method not including the step of treating
logs with microwave radiation. Suitably, the energy requirement is
reduced at least about 8% to about 15%.
[0027] A method of reducing pitch is described wherein a pulp
source is treated with microwave radiation prior to subsequent
process steps.
BRIEF DESCRIPTION OF THE FIGURES
[0028] FIG. 1 is a photograph of a waveguide and chamber for a
60-kW industrial microwave oven that can be used in the methods of
the invention.
[0029] FIG. 2 is a graph showing radial temperature as a function
of microwave power level for 20- and 50-kW treated logs.
[0030] FIG. 3 is a photograph showing steam jet issuing from end of
a log after microwave treatment at 50 kW for 5 minutes.
[0031] FIG. 4 is a photograph showing extensive radial checking
after microwave treatment at 50 kW for 5 minutes.
[0032] FIG. 5 is a scanning electron micrograph of a tangential
fracture surface after microwave treatment at 50 kW for 5
minutes.
[0033] FIG. 6 is a scanning electron micrograph of a tangential
fracture surface after microwave treatment at 50 kW for 5
minutes.
[0034] FIG. 7 is a graph showing freeness as function of refiner
energy consumption for several microwave pretreatments.
[0035] FIG. 8 is a graph showing refiner energy savings as a
function of microwave power level for several microwave
pretreatments.
[0036] FIG. 9 is a graph showing tensile index as a function of
microwave power level for microwave-pretreated black spruce
TMP.
[0037] FIG. 10 is a graph showing estimated annual pulp cost
savings for 800 ton/day for a mill based on substitution of
microwave-pretreated TMP for kraft pulp.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] It has now been discovered that a pulping process that
includes a pre-treatment or exposure of the pulp source to
microwave radiation allows for increased porosity and permeability
of the pulp source. Generally speaking, this improved pulping
process is most applicable to wood, generally in the form of logs.
The increase in permeability after microwaving pretreatment is due,
in part, to breakage of pit membranes and vessel cell ends caused
by steam pressure generated inside the wood. Breakage of pit
membranes and vessel cell walls by microwave exposure substantially
increases access of process chemicals to wood. During the
microwaving process, some of the water in the wood is converted to
steam. Major advantages of microwave over other conventional
methods are increased pulp yield, high speed, low or no chemical
use, low wood inventories, low waste production, and low process
cost during papermaking.
[0039] Not to be bound by theory, it is believed that the microwave
process leads to steam pressure build-up inside the logs. This
separates cell walls, increasing porosity and permeability so that
less energy is required during subsequent refining and also results
in a stronger paper product.
[0040] As used herein, "mechanical processing" and "mechanically
processing" refer to processing methods in which mechanical,
electrical or thermal energy is used to break down intact wood into
constituent fibers to produce wood pulp with a desired level of
freeeness. Suitable methods include TMP, RMP and CTMP. TMP is a
preferred method.
[0041] As used herein, "biopulping" refers to a method used in the
production of pulp that includes the use of a biological system to
perform, or to assist in performing, the pulping of wood.
Preferably, biopulping is carried out by inoculating steamed wood
chips with a species of fungi known to degrade or modify lignin.
Preferred fungal species include the so-called "white rot" fungi.
Preferred among the white rot species are species of Ceriporiopsis
subvermispora, Hyphodontia setulos, Phlebia subserialis, Phlebia
brevispora, Phlebia tremellosa or Phanerochaete chrysosporium.
[0042] As used herein, the terms "reduced energy input
requirements," "improved strength properties," and "enhanced
permeability" are relative terms that indicate a reduction,
improvement or enhancement, respectively, over a pulping method
that does not include a microwave treatment (including
modifications of a method to accommodate a microwave step), but
otherwise including the same steps as the described methods.
Suitably, the method of the invention reduces the energy input
requirement at least about 8%. Most suitably, the method of the
invention reduces the energy input requirement at least about 8% to
about 15%. Paper produced according to the method of the invention
suitably demonstrates at least about a 10% increase in strength
properties. The permeability of wood to chemicals also is enhanced
by exposure of the wood to microwave radiation according to one
aspect of a method of this invention.
[0043] The benefits of microwave pre-treatment can be realized in
many aspects of paper manufacturing. Microwave pretreatment of wood
can reduce electrical power requirements, improve paper quality,
reduce pitch and reject contents, improve paper machine operation
and save energy during drying of pulp, etc. The technology also has
potential for improving existing biopulping processes, by
preventing blue staining of wood, enhancing the penetration of
enzymes and other large molecules into wood, improving fiber
loading processes, and improving chemical penetration during lumber
processing.
[0044] In a method of the invention, the steps of treating logs
with microwave radiation, chipping the logs and pulping the wood
chips with a mechanical pulping process are carried out.
[0045] Microdry, Inc. (Crestwood, Ky. is a manufacturer of custom
industrial microwave ovens suitable for use in the present
invention Individual logs can be manually placed in the microwave
chamber until appropriate treatment time and frequency is
determined. Treatment parameters are dependent upon a number of
factors, including type of wood, diameter of the log and moisture
content. After optimization of treatment parameters, however, a
continuous belt transport system capable of accommodating logs can
be used. Microwaving can be done prior to or after debarking.
[0046] Chipping of logs is within those of skill in the art and be
can be accomplished with any known suitable techniques. One
suitable technique is to use a Sprout-Waldron Model D2202 single
rotating 300 mm diameter disk refiner. After chipping, a mechanical
pulping process is carried out. Mechanical pulping processes
include RMP, TMP and CTMP. In thermomechanical pulping, high power
refiners are used to mechanically reduce wood chips to fiber. To
aid in this process, elevated temperatures are used to soften the
wood. Several refining "passes" are generally required to obtain a
target freeness. The first pass is usually defibration at
temperatures above 100.degree. C. and immediately below or at the
glass transition temperature of lignin (T.sub.g<124.degree. C.).
During this pass, chips are typically fiberized under pressure
using an aggressive plate pattern to produce a high freeness pulp.
This pulp is then further reduced in multiple passes through an
atmospheric refiner until the desired pulp freeness is obtained.
The inventors have surprisingly found that microwave treatments
alter the structure of wood such that fiberization occurs more
easily during mechanical pulping, thereby reducing refiner energy
requirements and improving the pulp.
[0047] Optionally, the method can include fiber loading the pulp.
Fiber loading is described in U.S. Pat. No. 5,223,090, issued Jun.
29, 1993, and is incorporated herein by reference.
[0048] Further methods of the invention include producing pulp by
treating logs with microwave radiation, chipping the logs to
provide wood chips, inoculating the wood chips with a fungus and
mechanically processing the inoculated wood chips. Microwave
treatment, chipping and mechanical processing is carried out as
described above. Included among the suitable species for
inoculation of the wood chips are Ceriporiopsis subvermispora,
Hyphodontia setulos, Phlebia subserialis, Phlebia brevispora,
Phlebia tremellosa or Phanerochaete chrysosporium. When microwaved
logs are debarked, chipped and inoculated with biopulping fungus,
the treatment time is substantially reduced as compared to
conventional biopulping without the use of microwave pretreatment.
As discussed, it is believed that the enhanced porosity of the
microwaved chips provides faster colonization of these chips by the
fungus. Further, microwaved logs or chips from these logs can be
inoculated with CARTAPIP.TM. or other fungal species to remove blue
stain microorganisms or pitch. As described above, the enhanced
porosity facilitates colonization, thereby reducing treatment and
incubation times.
[0049] A method of the invention for reducing pitch and/or blue
staining can be carried out using a colorless species of Ophistoma
pilifetm, which can be used to reduce pitch and/or blue staining.
One species of Ophistoma piliferum is sold under the trade mark
CARTAPIP.TM. by Agra Sol Inc. of Raleigh, N.C., U.S.A. In the
method of the invention, this fungus is suitably applied to wood
chips subsequent to microwaving as described. U.S. Pat. No.
5,607,855, issued March 4, 1997, describes a suitable method of
reducing pitch with fungi and is incorporated herein by reference.
Even without the use of CARTAPIP.TM., microwaving of logs can be
used to reduce or remove resinous material. Not to be bound by
theory, it is believed that some of the components of this resinous
material that are sticky, such as triglycerides, are converted into
a less sticky material after microwaving.
[0050] The invention is also directed to a method of producing pulp
that includes the steps of microwaving wood, chipping the wood and
applying enzymes to the wood chips. Suitable enzymes include
lignin-degrading enzymes, xylanases, pectinases, lipases and
cellulases.
[0051] The invention provides for energy savings during wood
pulping and includes a method of reducing energy input
requirements. The method includes steps of treating wood with
microwave radiation, chipping the wood and mechanically pulping the
wood chips, wherein the energy input requirement is reduced at
least about 8% over a method not including the step of treating
logs with microwave radiation. Suitably, the energy requirement is
reduced at least about 8% to about 15%. The inventors have
discovered that higher energy savings correlate with higher power
levels used during the microwave pretreatment step. Energy savings
are also observed during debarking and chipping compared to logs
that were not microwaved.
EXAMPLES
[0052] Details of the invention will become more apparent by
reference to the following non-limiting examples, which, in some
cases, illustrate laboratory-scale embodiments and results achieved
thereby.
Example 1
Microwaving Logs and Structural Effects
[0053] Microdry, Inc. (Crestwood, Ky. is a manufacturer of custom
industrial microwave ovens suitable for use in the present
invention. A high capacity microwave oven was used for initial
tests (FIG. 1). This oven is connected to a variable-power (up to
60 kW) 915-MHz frequency generator. Individual logs can be manually
placed in the microwave chamber until appropriate treatment time
and frequency is determined. Treatment parameters are dependent
upon a number of factors, including type of wood, diameter of the
log and moisture content. After optimization of treatment
parameters, however, a continuous belt transport system capable of
accommodating logs can be used.
[0054] Microwaved logs or chips obtained from these logs
demonstrate increased porosity as has been observed in treated
logs. In general, as shown in FIG. 2, it has been determined that
higher power levels result in higher log temperatures, with steeper
temperature gradients from bark to pith. Of particular interest are
results obtained using spruce logs microwaved for 5 min at 50 kW.
Within a couple of minutes, splitting became intense and steam jets
shot out the ends of the logs (FIG. 3) In just 5 minutes, the logs
had lost about 25% of their weight or nearly all of their moisture.
A visual examination of the ends of the logs revealed extensive
radial checking (FIG. 4). Several fracture surfaces from logs
treated at 5 min/50 kW were sampled to identify possible
morphological changes in the fiber structure. A scanning electron
microscope was used to obtain images of both tangential and radial
surfaces (FIGS. 5 and 6).
[0055] Based on the results of exploratory mechanical pulping
trials, it was evident that microwave pretreatment can
substantially lower refiner energy requirements while improving
pulp quality. To verify this, a more extensive evaluation was
undertaken using the logs that were microwave pretreated at several
different power levels. The logs were debarked and chipped, then
refined by the established TMP protocol. FIG. 7 shows pulp freeness
as a function of total refining energy for the last three
atmospheric refining passes, indicating total energy savings for
all microwave pretreatments. Of particular interest is the
relationship of increased energy savings to increased microwave
power levels, as can be seen in FIG. 8. Handsheets made from these
pulps also exhibited an increase in mechanical properties, with
only moderate reductions in brightness. As with total energy
reduction, an increase in mechanical properties seems to correlate
with an increase in microwave power level, as can be seen in FIG.
9. Because pulp quality is improved, kraft components can be
reduced, with a resultant savings in total pulp cost, as
demonstrated in FIG. 10. An estimate of capital costs for 20-kW and
50-kW systems could range from $7.5 to $12.5 million.
Example 2
Microwave Pretreatment of Spruce Logs
[0056] Spruce logs were divided into two lots. One lot was frozen
and used as a control. The other lot was treated for 5 minutes with
a high power microwave generator (50kW at 915 MHz). During
microwaving, significant moisture loss was observed and a
temperature of 130.degree. C. inside the log was recorded. Prior to
refining atmospherically, both the control and the microwaved logs
were completely submerged in water overnight to maintain the same
moisture content in both the logs. Logs were then debarked,
chipped, and refined through the RMP process. Following results
were obtained (Table 1):
1TABLE 1 Energy requirements and paper strength properties during
RMP process Parameters Control Treatment (Microwaved) Energy during
refining (Wh/kg) 2411 2051 Burst index (kN/g) 0.98 1.33 Tear index
(mNm.sup.2/g) 3.31 3.91 Tensile index (Nm/g) 23.6 28.6 Breaking
length (m) 2408 2912
[0057] The data in Table 1 indicates that the microwave treatment
improved all major strength properties significantly with reduced
energy input requirements. The observed enhancement of strength
properties was surprising because microwaving resulted in a drying
of logs which is typically associated with a decrease in paper
strength properties.
[0058] Other highly unexpected results were obtained during
bleaching. Although the initial pulp brightness of the treated
samples was approximately 4 points lower than the control, as
reported in Table 2, the microwave-treated samples demonstrated
increased susceptibility to bleaching chemicals. As can be seen
from the data, control samples required 2% hydrogen peroxide to
reach the target brightness of 73% ISO, whereas treated samples
required only 1.5% hydrogen peroxide to reach to the same level of
brightness.
[0059] Thus, an additional advantage of the invention is a
reduction in amounts of bleaching chemicals required during
bleaching. This, in turn, increases the opacity of the resulting
paper and reduces the effluent treatment costs associated with
paper production.
2TABLE 2 Brightness response Brightness Treatments (% ISO)
Control.sup.1 Initial Brightness 59.6 1.5% Hydrogen Peroxide + 1.5%
Sodium Hydroxide 71.4 2% Hydrogen Peroxide + 2% Sodium Hydroxide
73.6 Treatment.sup.2 Initial Brightness 55.5 1.5% Hydrogen Peroxide
+ 1.5% Sodium Hydroxide 73.4 .sup.1Control produced using the
conventional TMP process. Fifty five percent of this pulp was mixed
with 45% of the Ground Wood Pulp (GWP.sup.a). The initial
brightness of this mixed pulp before bleaching was 59.6% ISO. Fifty
percent of this mixture was bleached with 1.5% and 2% of hydrogen
peroxide. .sup.2Treatment produced from microwaved logs using the
conventional RMP process. Fifty five percent of this pulp was mixed
with 45% of GWP. The initial brightness of this mixed pulp before
bleaching was 55.5% ISO. Fifty percent of this mixture was bleached
with only 1.5% hydrogen peroxide. .sup.aGWP was obtained from a
mill producing lightweight coated (magazine) paper.
Example 3
Microwave Pretreatment of Pine Logs
[0060] Objective: To achieve electrical energy savings and
improvements in paper strength by microwaving pine logs prior to
mechanical pulping.
[0061] Materials: Pine logs were received from a mill specializing
in the production of light weight coated paper. Logs were
microwaved at Microdry in Louisville, Ky. Logs were debarked and
chipped to a nominal size of 6-14 mm. Chips were placed in plastic
freezer bags and frozen to prevent the growth of contaminating
microorganisms. Log discs were cut before debarking and chipping
that was approximately 3 centimeters thick. Moisture content varies
from approximately 50%-56% depending on the microwave treatment
time.
[0062] Microwave Treatments: Logs were subject to three microwaving
conditions. Logs were microwaved at 50 kW for 5 minutes (50/5), 20
kW for 6 minutes (20/6), and 20 kW for 8 minutes (20/8).
[0063] Chip fiberization, pulp refining and handsheet production:
Microwaved wood chips were fiberized in a Sprout-Waldron Model
D2202 single rotating 300 mm diameter disk refiner. Energy
consumption was measured using an Ohio Semitronic Model WH 30-11195
integrating Wattmeter attached to the power supply side of the 44.8
kW electric motor. Feed rate through the refiner was between 10 kW
and 15 kW. Energy reported in WH/kg. Refiner plate settings were
0.025 inch, 0.014 inch, 0.010 inch, and 0.008 inch. Pulp was
collected at each pass as hot water slurry. Between the passes the
pulp slurry was dewatered to approximately 25% solids in a porous
bag by vacuum. Dilution water at 85 degrees Celsius was then added
each time as the pulp was fed into the refiner. Samples of the pulp
were taken and tested for the Canadian Standard Freeness (CSF).
Samples refined to 100 CSF. Handsheets were prepared and tested
using TAPPI standard testing methods.
[0064] Results: See Table 3.
3TABLE 3 Pine Treatments Sample Identification Burst Tear Energy
Including Log Size (kN/g) (mN-m{circumflex over ( )}2/g) Savings
(%) Control 0.47 1.99 -- 50/5 0.56 2.43 11.7 20/6 0.53 2.20 6.9
20/8 0.52 2.14 9.1
Example 4
Microwave Pretreatment of Aspen Logs
[0065] Objective: To achieve electrical energy savings and
improvements in paper strength by microwaving aspen logs prior to
mechanical pulping.
[0066] Materials: Aspen logs were received from a mill specializing
in the production of light weight coated paper. Logs were
microwaved at Microdry in Louisville, Ky. Logs were debarked and
chipped at FPL to a nominal size of 6-14 mm. Chips were placed in
plastic freezer bags and frozen to prevent the growth of
contaminating microorganisms. Log discs were cut before debarking
and chipping that was approximately 3 centimeters thick. Moisture
content varies from approximately 50%-56% depending on the
microwave treatment time.
[0067] Microwave Treatments: Logs were subject to three microwaving
conditions. Logs were microwaved at 50 kW for 5 minutes (50/5), 20
kW for 6 minutes (20/6), and 20 kW for 8 minutes (20/8).
[0068] Chip fiberization, pulp refining and handsheet production:
Microwaved wood chips were fiberized in a Sprout-Waldron Model
D2202 single rotating 300 mm diameter disk refiner. Energy
consumption was measured using an Ohio Semitronic Model WH 30-11195
integrating Wattmeter attached to the power supply side of the 44.8
kW electric motor. Feed rate through the refiner was between 10 kW
and 15 kW. Energy reported in WH/kg. Refiner plate settings were
0.025 inch, 0.014 inch, 0.010 inch, and 0.008 inch. Pulp was
collected at each pass as hot water slurry. Between the passes the
pulp slurry was dewatered to approximately 25% solids in a porous
bag by vacuum. Dilution water at 85 degrees Celsius was then added
each time as the pulp was fed into the refiner. Samples of the pulp
were taken and tested for the Canadian Standard Freeness (CSF).
Samples refined to 100 CSF. Handsheets were prepared and tested
using TAPPI standard testing methods. Table 4 describes the
results.
4TABLE 4 Aspen Treatments Sample Identification Burst Tear Energy
Including Log Size (kN/g) (mN-m{circumflex over ( )}2/g) Savings
(%) Control 0.46 1.85 -- 50/5 0.47 1.85 3.4 20/6 0.49 1.86 1.9 20/8
0.50 1.98 1.8
Example 5
Pitch Reduction
[0069] Logs were microwaved as described in Example 1. The control
consisted of logs that did not undergo microwave treatment. All
logs were then chipped and the chips were extracted with
dichloromethane (DCM). A significant decrease in pitch was observed
in the microwave pre-treated samples. Results are shown in Table
5.
5TABLE 5 Dichloromethane extraction Treated Treated Microwaved
Microwaved Pitch/Resin Acids Control 20 kw/6 min 50 kw/5 min DCM
Extractives 1.81 1.70 1.55 (% dry weight basis) % reduction over
control -- 6 14 Resin Acids b Pimaric acid 98.5 77.1 83.0
Sandaracopimaric acid 134 95.9 93.0 Isopimaric acid 359 278 341
Levopimaric acid 246 145 258 Dehydroabietic acid 1310 839 892
Abietic acid 147 151 191 Neoabietic acid <25 <25 <25
Chlorodehydraoabie-tic acid <25 <25 <25
Dichlorodehydrobioetic acid <25 <25 <25 Total identified
resin acids 2290 1590 1860 (.mu.g/dry weight basis) % reduction
over control -- 31 19
Example 6
Enzyme Application
[0070] Logs are microwaved as described in Example 1. Logs are then
chipped and sprayed with compositions containing a mixture of
lipases, xylanases, pectinases, cellulases and lignin-degrading
enzymes. Upon mechanical processing to provide pulp, a decrease in
energy input requirements and an increase in paper strength and
desirable optical characteristics are noted.
* * * * *