U.S. patent application number 14/789359 was filed with the patent office on 2016-01-07 for methods for the oxygen-based delignification of pulp.
The applicant listed for this patent is Linde Aktiengesellschaft. Invention is credited to Naresh J. Suchak.
Application Number | 20160002858 14/789359 |
Document ID | / |
Family ID | 55016619 |
Filed Date | 2016-01-07 |
United States Patent
Application |
20160002858 |
Kind Code |
A1 |
Suchak; Naresh J. |
January 7, 2016 |
METHODS FOR THE OXYGEN-BASED DELIGNIFICATION OF PULP
Abstract
Pulp is delignified by forming a mixture of pulp and caustic,
adding oxygen and feeding this oxygen-containing mixture to a first
step reactor. The lignin in the oxygen-containing mixture will be
partially delignified and will be fed to a second step reactor
where the remainder of the lignin in the pulp will be delignified.
In this manner, fast reacting lignin can be treated in the first
step reactor and slow reacting lignin can be treated in the second
reactor. The delignified pulp is recovered from the apparatus and
after washing can be forwarded to a bleaching unit.
Inventors: |
Suchak; Naresh J.;
(US) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Linde Aktiengesellschaft |
Munich |
|
DE |
|
|
Family ID: |
55016619 |
Appl. No.: |
14/789359 |
Filed: |
July 1, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62020446 |
Jul 3, 2014 |
|
|
|
Current U.S.
Class: |
162/15 ;
162/243 |
Current CPC
Class: |
D21C 7/00 20130101; D21C
9/147 20130101; D21C 9/02 20130101; D21H 17/675 20130101; D21H
21/32 20130101; D21C 11/0007 20130101; D21C 9/10 20130101 |
International
Class: |
D21H 21/32 20060101
D21H021/32; D21H 17/67 20060101 D21H017/67; D21C 7/00 20060101
D21C007/00; D21C 11/00 20060101 D21C011/00 |
Claims
1. A method for delignifying pulp comprising the steps of: (a)
Forming a mixture of the pulp and caustic; (b) Feeding oxygen to
the mixture and feeding the oxygen-containing mixture to a first
step reactor wherein lignin will delignify; (c) Feeding the
oxygen-containing mixture to a second step reactor wherein lignin
will delignify; and (d) Recovering the delignified pulp.
2. The method as claimed in claim 1 wherein the lignin in step b)
is fast reacting lignin.
3. The method as claimed in claim 1 wherein the lignin in step c)
is slow reacting lignin.
4. The method as claimed in claim 1 wherein the pulp is selected
from the group consisting of medium and high consistency pulp.
5. The method as claimed in claim wherein the caustic is sodium
hydroxide
6. The method as claimed in claim 1 wherein the oxygen is greater
than 80% purity.
7. The method as claimed in claim 1 wherein steam is added to the
mixture of pulp and caustic before being fed into the first step
reactor.
8. The method as claimed in claim 1 wherein the oxygen-containing
mixture comprises a gas phase and a liquid phase.
9. The method as claimed in claim 8 wherein the first step reactor
is a reactor where the gas phase is the continuous phase and the
liquid phase is the dispersed phase.
10. The method as claimed in claim 1 wherein supplemental oxygen
may be added to the mixture of pulp and caustic after step (b).
11. The method as claimed in claim 1 wherein the oxygen-containing
mixture is present in the first step reactor for about 0.25 to 5
minutes.
12. The method as claimed in claim 1 wherein the second step
reactor is a reactor column.
13. The method as claimed in claim 1 wherein the oxygen-containing
mixture is present in the second step reactor for about 2 to 40
minutes.
14. The method as claimed in claim 1 wherein the delignified pulp
is fed to a washing unit.
15. The method as claimed in claim 13 wherein water is added to the
washing unit.
16. The method as claimed in 13 wherein the delignified pulp is fed
to a pulp bleaching operation.
17. An apparatus for delignifying pulp comprising a mixer, a first
step reactor and a second step reactor.
18. The apparatus as claimed in claim 17 wherein the first step
reactor is a reactor where the gas phase is the continuous phase
and the liquid phase is the dispersed phase.
19. The apparatus as claimed in claim 17 wherein the second step
reactor is a reactor column.
20. The apparatus as claimed in claim 17 wherein the mixer is in
fluid communication with the first step reactor.
21. The apparatus as claimed in claim 17 wherein the first step
reactor is in fluid communication with the second step reactor.
22. The apparatus as claimed in claim 17 further comprising a
washing unit.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from U.S.
provisional application 62/020,446 filed Jul. 3, 2014.
BACKGROUND OF THE INVENTION
[0002] Oxygen delignification is the step between digesting wood
chips in pulp making and pulp bleaching operations. Oxygen
delignification is designed to dislodge and disintegrate residual
lignin left in the pulp after the digestion step using oxygen and
alkali. It is the direct extension to delignification that occurs
during digestion. Contacting pulp in an aqueous alkaline medium
with oxygen causes oxidation of lignin which not only breaks
molecules but also makes it water soluble. Oxidation of color
imparting groups reduces the Kappa Number lowering the consumption
of bleach chemicals in the bleach plant. Delignification with
oxygen is a more gentle way of reducing the Kappa Number than by
extended digesting and bleaching. In recent decades, new pulp mills
have been increasingly adopting oxygen delignification systems as
an advantageous step in reducing environmental impact and achieve a
better return on economic investment.
[0003] The most commonly practiced oxygen delignification consists
of the following steps. The first step involves adding washed pulp
into a mixer, adding caustic, adding oxygen and steam to bring the
temperature to a range of 70.degree. to 95.degree. C. and
introducing this pulp mixture into the bottom of a vertical tall
reactor in a continuous fashion. The pulp will flow upwards while
lignin in the reactor gets oxidized in the alkaline medium thereby
dissolving and disintegrating the lignin and dislodging it from the
pulp fibers. The reactor is maintained at 5 to 10 bar pressure to
improve on oxygen update. The residence time for pulp flowing
through a commercially practiced reactor is in the range of between
20 and 100 minutes.
[0004] Oxygen delignification can be performed with both medium as
well as high consistency pulp. Due to limited effectiveness,
difficulty in mixing of oxygen and other operating problems with
high consistency pulp, oxygen delignification has not achieved
widespread success when compared with medium consistency pulp.
[0005] Oxygen delignification works with pulps from both types of
woods, hardwood and softwood, reducing the Kappa Number up to 35%
and 50% respectively. In the case of hardwood, a two stage approach
is needed where two reactors are placed in series. The first stage
is maintained at a higher pressure and lower temperature with less
residence time while the second stage is usually maintained at
lower pressure but at higher temperatures and greater residence
times.
[0006] There has been limited success using a single stage high
efficiency reactor towards achieving short term delignification at
smaller and experimental scales. Consequently, there has not been
any commercial apparati embodying this concept.
SUMMARY OF THE INVENTION
[0007] In a first embodiment of the invention, there is disclosed a
method for delignifying pulp comprising the steps of: [0008] (a)
Forming a mixture of the pulp and caustic; [0009] (b) Feeding
oxygen to the mixture and feeding the oxygen-containing mixture to
a first step reactor wherein lignin will delignify; [0010] (c)
Feeding the oxygen-containing mixture to a second step reactor
wherein lignin will delignify; and [0011] (d) Recovering the
delignified pulp.
[0012] In step b) the fast reacting lignin or that lignin which
contains the more fast reacting components will delignify first
when mixed with the oxygen. This delignified lignin as well as that
lignin that was not dellgnified will be passed through with the
remainder of the pulp to the second step reactor in step c) where
the lignin containing the slow reacting components will delignify.
Some fast reacting lignin may of course carry over from the first
step reactor to the second step reactor where it will delignify
along with the slow reacting lignin.
[0013] The pulp that is treated is selected from the group
consisting of medium and high consistency pulp.
[0014] The caustic employed is typical of that used in pulp and
paper operations and typically is sodium hydroxide.
[0015] The oxygen is generally pure oxygen but oxygen
concentrations greater than 80% purity will be effective for
creating the oxygen-containing mixture. Supplemental oxygen may be
added to the mixture of pulp and caustic after step (b) and before
the oxygen-containing mixture is fed to the second step
reactor.
[0016] Steam may be added to the mixture of pulp and caustic before
being fed into the first step reactor in order to increase its
temperature.
[0017] The first step reactor is a reactor where the gas phase is
the continuous phase and the liquid phase is the dispersed
phase.
[0018] The oxygen-containing mixture is present in the first step
reactor for about 0.25 to 5 minutes.
[0019] The second step reactor is preferably a reactor column.
[0020] The oxygen-containing mixture is present in the second step
reactor for about 2 to 40 minutes.
[0021] After the delignified pulp is recovered in step (d), it may
be fed to a washing unit where water may be added to the washing
unit.
[0022] The delignified pulp may also be fed to a pulp bleaching
operation.
[0023] In another embodiment of the invention, there is disclosed
an apparatus for delignifying pulp comprising a mixer, a first step
reactor and a second step reactor.
[0024] In the apparatus the first step reactor is a reactor where
the gas phase is the continuous phase and the liquid phase is the
dispersed phase.
[0025] The second step reactor is a reactor column.
[0026] The mixer is in fluid communication with the first step
reactor while the first step reactor is in fluid communication with
the second step reactor.
[0027] This apparatus can further comprise a washing unit
[0028] Studies have demonstrated that the addition of oxygen
utilized in delignification will also cause oxygen to react with
cellulose and hemicelluloses and no-lignin components in the pulp
system.
[0029] The residual in the pulp after digestion is often
characterized as containing "fast reacting" and "slow reacting"
lignin. Many non-lignin based oxidizable structures present in the
pulp that consumes oxygen have been described by pulp and paper
scientists.
[0030] The present invention divides the oxygen consuming reaction
in oxygen delignification as "fast" and "slow" oxidation without
specifying them to be either specific lignin or non-lignin
reactions. Thus fast reacting lignin will be that part of the
lignin in the pulp that contains more of the faster reacting
components when the lignin is delignified. The slow reacting lignin
will be that part of the lignin in the pulp that contains more of
the slower reacting components when the lignin is delignified.
[0031] The methods of the present invention achieve high levels of
oxygen delignification by using two steps. In the first step, a
reactor is employed that offers high efficiency with respect to
mass transfer. The reactor is capable of transporting oxygen from
the gas phase to the liquid phase at extremely high rates. The pulp
that is introduced into the first reactor has the highest
concentration of both the fast and the slow reacting components.
Oxygen is a sparingly soluble gas and by utilizing a high
efficiency mass transfer reactor and elevated pressure, the aqueous
medium surrounding the pulp can be maintained to saturation levels
which maximize and maintain rapid reaction rates between the fast
reacting components and oxygen.
[0032] The residence time of the pulp in the first step reactor or
the feed rate of the pulp to the high efficiency reactor is
adjusted accordingly to ensure depletion of almost all the fast
reacting components or fast reacting lignins of the pulp before it
exits the reactor.
[0033] The typical high efficiency reactor may offer 10 to 1000
times greater mass transport per unit volume compared to
conventional gas-liquid reactors such as continuously stirred tank
reactors or mechanically agitated contactors, bubble columns,
etc.
[0034] The reactor employed for the first step of the invention is
one where the gas phase is the continuous phase and the liquid
phase is the dispersed phase. The power required for the gas liquid
contacting is provided to the gas phase. In such a reactor, the gas
phase has extremely high turbulence for transporting gas to the
aqueous phase and only a partial amount of the energy from this
turbulence is conveyed to the liquid phase thereby avoiding
undesirably loss of fiber strength of the pulp.
[0035] Some of the manufacturers of such high efficiency reactor's
attribute unusually high mass transport to ultrasonic phenomena.
One such reactor available is made by Quantum Technologies of
Akron, Ohio. However, for purposes of the invention, the method
described herein is not limited solely to this one reactor.
[0036] Once the fast reacting components are oxidized, the fast
mass transfer of oxygen of gas to the aqueous phase is not
critical. The slow reacting components can be oxidized by holding
the pulp exiting the first step in a chamber or a column type
reactor with a predefined residence time as the second step of the
invention. As discussed previously, the pulp leaving the first step
will always be saturated with oxygen if operated as described for
the first step. Since in the second step, the oxidation reaction is
very slow, supplemental oxygen is limited and also very slowly
consumed. Providing supplemental oxygen in the second step can be
accomplished by allowing the complete output from the first step
containing both phases. The gas phase consisting of excess oxygen
and the aqueous phase consisting of pulp can be introduced at the
bottom of the chamber or column utilized in the second step. Both
the aqueous and gas phase in the chamber or column of the second
step will flow co-currently upward. As the gas phase is lighter, it
flows upward in the form of bubbles faster and rises through the
aqueous pulp in the chamber/column supplementing the depleted
oxygen and agitating the pulp.
[0037] The pulp exiting the second step reactor is then washed and
sent to further processing steps such as bleaching.
[0038] The combination of the first step and the second step
provide high efficiency oxygen delignification at lower costs than
conventional techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] The figure is a schematic of a pulp delignification process
according to the methods of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Turning to the figure, a schematic of a pulp delignification
system according to the methods of the invention is shown. A mixer
A receives pulp through line 1 and caustic through line 2. The
combination of pulp and caustic is mixed in the mixer A and fed
through line 3 and a pump (not shown) to the first step reactor B.
The pump will raise the pressure of the pulp and caustic
mixture.
[0041] Oxygen is fed through line 4 to join with the pulp caustic
mixture in line 3 before being fed into the first step reactor B.
Steam may also be introduced upstream of the first step reactor B
to raise the pulp and caustic mixture temperature to between
50.degree. C. and 100.degree. C.
[0042] As discussed, there is an improved rapid reaction between
the fast reacting lignin components or fast reacting lignin and the
oxygen thereby resulting in the depletion of all the fast reacting
components of the pulp before it exits the first step reactor
B.
[0043] The pulp which now contains mostly the slow reacting lignin
components will be fed from the first step reactor B through line 5
into the bottom section of the second step reactor chamber or
column C. The second step reactor chamber or column C is designed
to allow for additional retention time for the pulp to contact the
oxygen thereby delignifying the slow reacting components.
[0044] The excess oxygen from the first step reactor B will be
adjusted so as to provide the required oxygen feed rate in the
second step reactor chamber or column C.
[0045] The gas and aqueous phase streams will rise through the
reactor chamber or column C and the gas will be exhausted from the
top through line 10. The mix of water and delignified pulp will be
fed from the second step reactor chamber or column C through line 6
and will be washed in washer D which is assisted in this by the
addition of fresh water through line 7. The pulp will be separated
from the water and fed through line 9 to a pulp bleaching operation
(not shown). The remaining water from the washing step can be
discharged from the washing unit D through line 8 where it can be
reused or cleaned up prior to discharge into the environment.
[0046] The advantages of the present invention are manifest in a
number of areas. A smaller reactor column can be employed thereby
reducing the capital required for equipment. Due to residency time
of between 0.25 and 5 minutes for the first step reactor and 2 to
40 minutes for the second step reactor, the amount of feedstock in
progress in the vessel is lessened.
[0047] The equipment is one twelfth to one half the size of
conventional oxygen delignification equipment making for an easier
retrofit into existing mills that do not have oxygen
delignification.
[0048] The methods of the present invention will provide for a
better Kappa Number reduction that that achieved by traditional
oxygen delignification systems.
[0049] The pulp produced by this invention has better pulp strength
as it exposes the cellulose fiber in the pulp to the caustic and
oxygen for shorter durations of time.
[0050] In depth analysis of feed stock and its complex chemistry is
not necessary to implement the two step method of the
invention.
[0051] While this invention has been described with respect to
particular embodiments thereof, it is apparent that numerous other
forms and modifications of the invention will be obvious to those
skilled in the art. The appended claims in this invention generally
should be construed to cover all such obvious forms and
modifications which are within the true spirit and scope of the
invention.
* * * * *