Note: Descriptions are shown in the official language in which they were submitted.
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Pulping Process
The present invention relates to a process for the removal of lignin from
lignocellulose containing material comprising treating the lignocellulose
containing
material prior to a chemical pulping process with a delignifying gas, wherein
the gas
comprises chlorine dioxide.
Back4round
In chemical pulping processes the objective is to remove the lignin present in
lignocellulose containing material while minimising the damage and loss of the
cellulose
and hemi-cellulose fibres. In these processes, often also referred to as
cooking or
digestion processes, the lignocellulose containing material reacts with
pulping chemicals
at an elevated temperature over a time period sufficient to effect a specified
degree of
delignification. As such, the digestion process is a complex kinetic balance
relating
delignification to inter alia the cooking chemical(s), time and temperature.
These
variables are balanced to produce a pulp with the highest strength, greatest
yield and the
lowest lignin content.
In chemical pulping processes, pulping chemicals are introduced into the
digester along with the lignocellulose containing material and subsequently
heated to a
specific digestion temperature for a predetermined period of time. As the
digestion
proceeds the chemical concentration, temperature and digestion time all effect
the
removal of the lignin from the lignocellulose containing material.
However, as the chemical concentration increases there is more of a tendency
for chemical attack on the cellulose and hemicellulose fibres instead of the
lignin. The
chemicals react with the carbohydrates and break or cleave the fibre chains
resulting in
shorter polymer lengths and overall lower fibre strength, i.e. impaired
viscosity. This effect
is further increased as the temperature and reaction time increase.
Consequently, more
cellulosic material is present in the pulping liquor. When the spent pulping
liquor is
removed from the pulp in the post-washing step, these materials are lost
resulting in
lower pulp yields. )
Side reactions in the cooking process are very temperature dependent. Lignin
removal proceeds slowly at first but accelerates markedly as the temperature
rises above
160°C. Cellulose removal starts at 120-130°C and levels off when
the maximum
temperature is reached. Hemicellulose is composed of two main components,
glucomannan and xylan. Glucomannan removal is rapid at first and becomes even
greater as the temperature increases above 100°C. Xylan on the other
hand follows the
same pattern as lignin removal starting slowly at first and increasing rapidly
as the
temperature increases.
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The rate of penetration and diffusion of the pulping chemicals into the fibre
source determines the efficiency of the digestion. A too short digestion time
results in
non-uniform cooking and a poor pulp product, while excessive digestion
generates
overcooked pulps with poor viscosity.
From the above it can be seen that changes within the digestion process to
increase lignin removal usually have a negative impact on the product pulp
properties.
Higher chemical concentrations and/or temperature result in lower strength and
yield.
Longer digestion times reduce the throughput through the digester and lower
the pulp
production.
To achieve a given degree of delignification, various combinations of heat-up
time, maximum temperature and time at the maximum temperature can be used. To
simplify comparisons of different cooking conditions, the pulp and paper
industry has
developed the H-factor. This value is the sum of the relative rates of
reaction occurring in
the digestion. For a closer definition of the H-factor we refer to Kirk-
Othmer, Encyclopedia
of Chemical Technology, Vol. 20, 4~' Ed., p. 535 which is hereby incorporated
by
reference. Using the H-factor and the kappa number after the cook, different
treatment
and cooking processes can be compared on an equivalent basis. For example,
reducing
the H-factor for the process would allow for shorter cookingldigesting times
and increase
pulp production in existing equipment or allow the digestion to be
accomplished at a
lower temperature resulting in improved yield and strength properties.
Thus, from the reasons set out above it is preferable to pretreat the
lignocellulose containing material in such a way as to facilitate the removal
of lignin in a
subsequent pulping process andlor break and softer the lignin bonded to the
cellulose
and hemicellulose fibres.
If the pulping process is a chemical pulping process the H-factor can be
significantly reduced when preterating the lignocellulose containing material
according to
the invention.
US 4172006 refers to the pretreatment of wood chips with oxygen prior to
adding
a cooking liquor.
US 4750973 relates to a process for reducing carbohydrate losses in sulphate
pulping of wood using sodium hydroxides and sodium sulfide, wherein the wood
is
pretreated in presence of water with oxygen gas and nitrogen oxides.
GB 567774 discloses a process for the treatment of cellulosic raw material
where wood chips are contacted with a aqueous solution of a wetting agent
prior to
subjecting the chips to a solution containing sodium chlorite thereby using
sufficient acid
to insure the liberation of chlorine dioxide.
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WO 8908165 refers to a method for the pretreatment of wood chips with sulphur
dioxide gas prior to alkaline deligification operations.
DE 1049220 discloses a method comprising subjecting wood chips to carbon
acid before sulfite cooking.
JP 49020241 refers to a pulping process comprising the steps of reacting
chlorine dioxide or a mixture consisting of chlorine dioxide and chlorine with
wood chips
in the presence of water soluble cellulose derivatives and thereafter removing
inter alia
the oxidised lignin by extraction.
US 5474654 refers to a delignification process where chlorine dioxide gas is
used on pulp obtained from pulping processes such as chemical kraft, sulfide
or
mechanical processes.
US 3591451 and US 3919041 disclose the use of gaseous chlorine dioxide
subsequent a pretreatment step which may be either mechanical, chemical or a
combination thereof.
Summary of the present invention
In accordance with the present invention it has surprisingly been found that
the
lignin content of lignocellulose containing material can be reduced prior to a
chemical
pulping processes by providing a process according to the claims. More
specifically, the
invention relates to a process for the removal of lignin from lignocellulose
containing
material, where the material prior to a chemical pulping process is treated
with a
delignifying gas comprising chlorine dioxide.
An advantage by treating lignocellulose containing material prior to a pulping
process in accordance with the present invention is that the pulp yield and
pulp properties
such as pulp viscosity subsequent a pulping process are significantly improved
at given
(corresponding) H-factors.
Thus, by implementing the present invention in an existing pulping process,
improved pulp yield and pulp viscosity is obtained at corresponding Kappa
numbers.
Another advantage is that effluent streams originating from the pulping
process
are reduced since in-specific properties such as pulp yield and viscosity can
be obtained
at decreased pulping chemical dosages (lower H-factors of the cooking
operation can be
used).
A still further advantage of the present invention is that by pretreating
cellulose
containing material prior to a chemical pulping process with chlorine dioxide
or any gas
comprising chlorine dioxide the lignin content of the pulp (expressed by the
kappa
number) dramatically decreases after chemical pulping processes having similar
H
factors while not significantly. impairing the viscosity over pretreatments
where oxidising
gases other than chorine dioxide have been used.
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Further advantages of the present invention are apparent from the
specification.
Detailed description of the invention
Suitable lignocellulose containing material used in the present invention can
be
any lignocellulose containing material derived from natural sources such as
softwood,
hardwood, gum, straw, bagasse and/or bamboo. The physical state of the
lignocellulose
containing material is not critical, however, a physical state providing a
large surface area
is preferred that maximises penetration of the delignifying gas and optionally
processing
chemicals. Suitably, the lignocellulose containing material is in the form of
chips with a
size which is governed by the process equipment and process parameters.
The lignocellulose containing material is suitably treated according to any
method known to the skilled artisan which renders the diffusion of the
delignifying gas
within the fibre source to the lignin more effective such as steaming and/or
evacuation.
According to the present invention the lignocellulosic material is treated
with a
gas comprising chlorine dioxide such as gaseous chlorine dioxide. Suitably,
the
delignifying gas mixture comprising chlorine dioxide is a non-liquid
containing gas. The
chlorine dioxide containing gas may contain other gases such as nitrogen,
oxygen, air or
steam or mixtures thereof. The chlorine dioxide containing gas may also
contain small
amounts of chlorine, however, the gas is suitably substantially free from
chlorine,
preferably having less than 10% by volume, more preferably less than 1 % by
volume of
chlorine. The concentration of chlorine dioxide in the gas is not critical for
the invention.
Thus, the lignocellulose containing material may be treated with substantially
pure
chlorine dioxide gas. The upper limit of the amount of chorine dioxide
comprised in the
gas mixture is purely set by safety considerations. Suitable concentrations of
chlorine
dioxide comprised in the gas mixture are from about 0.05 up to about 100% by
volume,
more preferably from about 0.05 up to about 50 % by volume and most preferably
from
about 1 up to about 20 % by volume.
The delignifying gas containing chlorine dioxide is believed to selectively
attack
the lignin leaving the majority of the cellulose and hemicellulose fibres
intact. The pre
removal of the lignin allows conditions to be optimised in the cooking process
in terms of
pulp yield, strength and production rate.
The use of a delignifying gas comprising chlorine dioxide overcomes several
unsolved problems. Treatment of the lignocellulosic containing material with
solutions
containing for example chlorine dioxide is limited by the rate of chlorine
dioxide diffusion
through the solution to the fibre source followed by the diffusion of the
chlorine dioxide
within the fibre source to the lignin. The result is a slow delignification
process that works
primarily on the fibre source surface and an aqueous effluent stream
containing chlorine
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dioxide, dissolved fibre components and chlorides, which is difficult to treat
in an
environmentally correct manner.
It has been found that a gas comprising chlorine dioxide does not have the
diffusion barriers that limit the process when a solution is used. The gas
passes rapidly
and uniformly into the fibre source resulting in even delignification
throughout the
material. Furthermore, there are no aqueous effluent streams. The degraded
lignin and
lignin by-products are carried with the lignocellulose containing material
into the digestion
process where additional delignification occurs. The total dissolved lignin is
then removed
in the normal washing step following the cooking process.
Preferably, the gas comprising chlorine dioxide is applied on lignocellulose
containing material free from any surrounding aqueous solution. Preferably,
the moisture
content of the lignocellulose containing material is from about 30 weight % up
to about 60
weight % based on oven dry material, more preferably from about 40 up to about
50
weight %.
The chlorine dioxide containing gas employed in the present invention is
suitably
produced using a chlorine dioxide generation process as described in the US
patens US
4770868, US 5091166, US 5091197 and US 5380517, which all are incorporated by
reference.
The gas comprising chlorine dioxide is generally applied in amounts which are
suitable for removal of lignin to a desired degree. Usually, increased applied
amount of
chlorine dioxide comprised in the gas (mixture) increases the degree of
delignification.
The charge of chlorine dioxide is from about 0.5 kg/tonne up to about 300 kg
active CI2
per tonne of oven dry material, more preferably from about 2 kg up to about
100 kg active
CIZ per tonne oven dry material and most preferably from about 30 kg/tonne up
to about
50 kg active CIz per tonne oven dry material.
The present invention may be performed at any location prior to chemical
pulping processes. The lignocellulose containing material may be treated with
the gas
comprising chlorine dioxide in any type of equipment. For practical reasons
the
equipment should be gas tight. The treatment with the gas comprising chlorine
dioxide
may also be carried out in the same equipment (vessel) as is used for the
subsequent
pulping process.
Suitably, the treatment is carried out in an equipment such as a vessel which
is
essentially free from an aqueous solution, i.e. the treatment is carried out
in the absence
of aqueous solutions. By essentially free from an aqueous solution is meant
that a minor
amount aqueous solution can be present during the treatment with a gas
comprising
chlorine dioxide as long as the removal efficiency of lignin is not
significantly impaired, i.e.
as long as the overall diffusion (diffusion of the gas in respect of the
totality of material
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treated) of the gas containing chlorine dioxide to the lignocellulose
containing material is
not significantly impaired.
The delignifying gas is suitably admixed with the lignocellulose containing
material in an equipment which is at any suitable pressure including
atmospheric,
subatmospheric or superatmospheric pressures. Suitably, the treatment with the
delignifying gas is carried out at a pressure ranging from about 10 kPa up to
about 500
kPa, preferably from about 50 kPa up to about 250 kPa. The most preferred
pressure
ranges from about 80 kPa up to about 120 kPa.
The temperature during the treatment according to the present invention is not
critical and can be carried out at surprisingly low temperatures including
ambient
temperatures. The upper temperature level in the treatment is set by
economical and
safety considerations. Temperatures may range from about 10 °C up to
about 400 °C,
suitably from about 15 °C up to about 200 °C, more preferably
from about 20 °C up to
about 95 °C and most preferably from about 25 °C up to about 90
°C. Suitable
temperature ranges are also those obtained by combining any of the lower
temperature
level of above ranges with any of the higher temperature levels.
According to one preferred embodiment of the present invention the
lignocellulose containing material is pretreated prior to being subjected to
the gas
comprising chlorine dioxide. The pretreatment may be accomplished in the same
equipment used for the gas comprising chlorine dioxide, yet, the pretreatment
can also be
performed in any suitable equipment located upstream the treatment with the
delignifying
gas. Suitably, the pretreatment includes various steaming and/or evacuation
processes.
The pretreatment is believed to open up the lattice structure of the
lignocellulose
containing material thereby improving the diffusion of the delignifying gas
into the
material.
The removal of lignin from lignocellulose containing material is conducted
prior to
a chemical pulping process. Any chemical pulping process known to the skilled
artisan
can be employed within the scope of the present invention exemplified by the
sulphite,
bisulphite, kraft (sulphate), soda, soda anthraquinone (soda AQ) or organosolv
process
or modifications and/or combinations thereof. Suitable chemical pulping
processes are
further disclosed in Rydholm, Pulping Processes, Interscience Publisher and
Ullman's
Encyclopedia of Industrial Chemistry , 5'" Edition, VoLA18, 1991, pages 568
and 569,
which documents all are incorporated by reference.
According to a preferred embodiment of the present invention the treatment of
the lignocellulose containing material with a delignifying gas comprising
chlorine dioxide
is carried out prior to a soda anthraquinone (soda AQ) pulping process. The
cooking
liquor in a soda AQ,chemical pulping process is essentially free from sulphur
containing
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compounds which are the predominant compounds causing malodorous compounds,
i.e.
sulphurous compounds. Accordingly, one further advantage with the present
invention
according to this embodiment is the minimisation of malodorous compounds.
The process may be operated in either batch or continuous mode.
Subsequent the process according to the present invention the obtained pulp
may be delignified and bleached using any available technique such as totally
chlorine
free bleaching (TCF), elementary chlorine free bleaching (ECF) or bleaching
sequences
containing chlorine, although not preferred. The pulp can also be subjected to
oxygen
delignification subsequent the pulping process.
To further illustrate the invention the following examples are provided. All
parts
and percentages are by weight unless otherwise specified. Temperatures are in
degrees
Celsius.
Example 1
Southern pine softwood chips obtained from a commercial pulp supplier were
used. There was no special handling, separation or classification of the chips
prior to the
experiment.
A portion of the wood chips were cooked under Kraft pulping conditions with a
fixed H factor to establish the baseline for the process. All the Kraft cooks
were performed
at 170°C with a 22% effective alkali with a liquor to wood ratio of
4.55:1. The Kappa
numbers of the untreated wood chips after cooking at various H factors are
summarised
in Table 1
Table 1: Untreated Wood Chips
H Factor 600 800 1200
Kappa 72 42 28.2
No.
Samples of the wood chips were treated in a batch mode using chlorine dioxide
containing gas at 2.7 volume % at a temperature of about 50°C and a
pressure of about
100 kPa at different time periods. Each batch was then cooked under Kraft
pulping
conditions at an H factor of 600. The resulting Kappa Numbers after cooking
are shown in
Table 2.
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Table 2 - Kappa Number Results
Batch Treat Kappa % Kappa
ment After Reduction
time digestion
[min]
reference0 72
1 15 57 27.8
2 30 46 36.1
3 60 38 47.2
Example 2
The procedure of example 1 was repeated with the difference that each batch
was then cooked under Kraft pulping conditions at an H factor of 1200. The
resulting
Kappa Numbers after cooking are shown in Table 3.
Table 3 - Kappa Number Results
Batch Treat Kappa % Kappa
went After Reduction
time Cook
[min]
reference0 28.2
1 15 27.9 1.1
2 30 25.4 9.9
3 60 24.4 13.5
Example 3
A sample of the wood chips according to example 1 was treated in batch mode
using chlorine dioxide gas at 5.5 volume % at a temperature of about
50°C and a
pressure of about 100 kPa for 30 minutes. The treated chips were then cooked
under
Kraft pulping conditions at an H factor of 600. The resulting Kappa Number
after cooking
was 38.
Example 4
A sample of the wood chips was treated in batch mode using chlorine dioxide
gas at 2.5 volume % at a temperature of about 50°C and a pressure of
about 100 kPa for
60 minutes. The treated chips were then cooked under Soda-Antraquinone
conditions to
an H factor of 1600. The resulting Kappa Number after cooking was equal to 37.
Untreated chips cooked at an H-factor of 1600 under the same Soda-AQ
conditions had a
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final Kappa Number of 64. Thus, a Kappa No. reduction of 42 % of the pulp was
obtained.
From the above examples it is evident that the kappa number after cooking was
substantially lower at a constant H factor when the lignocellulose containing
materials
were pre-treated with chlorine dioxide containing gas prior to the cooking
process.
This means also that a given kappa number after cooking can be achieved at a
lower H factor (less dosage of inter alia cooking chemicals) when using
lignocellulose
containing materials that have been pretreated with chlorine dioxide according
to the
invention as compared to untreated chips.
