U.S. patent application number 12/991554 was filed with the patent office on 2011-03-10 for prehydrolysis sulfate cooking process.
This patent application is currently assigned to METSO PAPER, INC.. Invention is credited to Olavi Katajisto, Lari Lammi, Antti Viro.
Application Number | 20110056637 12/991554 |
Document ID | / |
Family ID | 39523080 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110056637 |
Kind Code |
A1 |
Katajisto; Olavi ; et
al. |
March 10, 2011 |
PREHYDROLYSIS SULFATE COOKING PROCESS
Abstract
Processes for the preparation of prehydrolyzed pulp from
lignin-containing cellulosic material are described including
prehydrolyzing the cellulosic material with steam at a first
pressure, terminating the prehydrolyzing by at least one addition
of an alkaline liquid at the first pressure followed by at least
one pressure reduction and delignifying the prehydrolyzed
cellulosic material in a cooking stage. The pressure reduction can
be a single pressure reduction or a gradual pressure reduction
carried out in incremental stages.
Inventors: |
Katajisto; Olavi; (Pori,
FI) ; Lammi; Lari; (Pori, FI) ; Viro;
Antti; (Pori, FI) |
Assignee: |
METSO PAPER, INC.
Helsinki
FI
|
Family ID: |
39523080 |
Appl. No.: |
12/991554 |
Filed: |
May 6, 2009 |
PCT Filed: |
May 6, 2009 |
PCT NO: |
PCT/FI09/50364 |
371 Date: |
November 8, 2010 |
Current U.S.
Class: |
162/68 |
Current CPC
Class: |
D21C 3/02 20130101; D21C
1/02 20130101 |
Class at
Publication: |
162/68 |
International
Class: |
D21C 1/02 20060101
D21C001/02 |
Foreign Application Data
Date |
Code |
Application Number |
May 8, 2008 |
FI |
20085425 |
Claims
1-8. (canceled)
9. A process for the preparation of prehydrolyzed pulp from
lignin-containing cellulosic material comprising prehydrolyzing
said lignin-containing cellulosic material with steam in a first
steam phase at a first pressure to obtain prehydrolyzed cellulosic
material, terminating said prehydrolyzing of said cellulosic
material by at least one addition of an alkaline liquid at said
first pressure followed by at least one pressure reduction, and
delignifying said prehydrolyzed cellulosic material in a cooking
stage.
10. The process of claim 9 comprising a batch process.
11. The process according to claim 9 wherein said alkaline liquid
comprises a white liquor.
12. The process according to claim 9 wherein said at least one
addition of said alkaline liquid comprises a single addition of
said alkaline liquid at said first pressure, and wherein said at
least one pressure reduction comprises a single pressure reduction
to a second pressure lower than said first pressure.
13. The process according to claim 12 wherein said first pressure
comprises a pressure of between about 6 and 9 bar and said second
pressure comprises a pressure of between about 3 and 5 bar.
14. The process according to claim 13 wherein said single pressure
reduction is carried out for a period of from about 1 second to
about 10 minutes.
15. The process according to claim 14 wherein said single pressure
reduction is carried out for a period of from about 30 seconds to
about 5 minutes.
16. The process according to claim 9 wherein said at least one
addition of said alkaline liquid comprises a single addition of
said alkaline liquid, and wherein said at least one pressure
reduction comprises a gradual pressure reduction.
17. The process according to claim 16 wherein said first pressure
comprises a pressure of between about 6 and 9 bar, and wherein said
gradual pressure reduction comprises a gradual pressure reduction
from said first pressure to a second pressure, said second pressure
comprising a pressure of between about 3 and 5 bar, and wherein
said gradual pressure reduction is carried out by reducing said
pressure in increments of from about 0.5 to 2 bar.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a national phase entry under 35
U.S.C. .sctn.371 of International Application No. PCT/FI2009/050364
filed May 6, 2009, published in English, which claims priority from
Finnish Application No. 20085425 filed May 8, 2008, all of which
are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an improved prehydrolysis
sulfate (Kraft) cooking process. An improved method is disclosed
for terminating the hydrolysis stage of the prehydrolysis sulfate
cooking process. More precisely, the invention relates to
termination of the hydrolysis stage operating in the steam phase,
wherein such termination is also performed in the steam phase.
BACKGROUND OF THE INVENTION
[0003] The prehydrolysis-sulfate (Kraft) cooking for the production
of special pulps having a high content of alpha cellulose was
developed in the 1930's, see e.g. Rydholm, S. E., Pulping
Processes, pp. 649 to 672, Interscience Publishers, New York, 1968.
The basic idea was to remove as much hemicellulose as possible from
cellulose fibers in connection with delignification, so as to
obtain a high content of alpha cellulose. This is essential because
the various end uses of such pulps, dissolving pulp for instance,
do not tolerate short-chained hemicellulose molecules with a
randomly grafted molecular structure.
[0004] A separate prehydrolysis step permits the desired adjustment
of the hydrolysis of hemicelluloses by varying the hydrolysis
conditions. In the prehydrolysis-kraft cooking process the
necessary delignification is not carried out until a separate
second cooking step. The prehydrolysis is carried out either as a
water or steam phase prehydrolysis, or in the presence of a
catalyst. In the former processes, organic acids liberated from
wood during the process perform a major part of the hydrolysis,
whereas in the latter process, small amounts of mineral acid or
sulfur dioxide are added to "assist" the prehydrolysis. In the
prehydrolysis stage carried out in steam phase direct steam is
introduced to the chip column in the digester. Due to the material
and energy balance, very little, if any, liquid hydrolysate phase
is generated, as all condensate is trapped in the porosity of the
wood material.
[0005] Traditionally after prehydrolyzing the cellulosic material
in a reactor, the hydrolysate and the prehydrolyzed cellulosic
material are neutralized in the reactor with alkaline neutralizing
liquor so as to produce neutralized hydrolysate and neutralized
prehydrolyzed cellulosic material. There is hydrolysate both in the
free liquid outside the chips and also trapped and immobilized
inside the chips. If desired, part of the hydrolysate can be
recovered before the neutralization step. In the neutralization
step, the contents of the digester are prepared for later
delignification, to be carried out by alkaline kraft cooking.
[0006] The prehydrolyzed material is conventionally neutralized
with alkali-containing liquid, either by displacement or
circulation. Neutralization is achieved by selecting an appropriate
neutralizing alkali charge, which results in a clear alkaline
neutralization end point. Suitable neutralizing agents to be used
contain caustic soda, and the preferred agent is alkaline Kraft
cooking liquor, i.e., white liquor. The displacement process can be
performed, for example, by filling up the digester with white
liquor so that the leading edge of the white liquor neutralizes the
hydrolysate. In the circulation process alkali-containing liquid is
pumped into the digester so as to displace the hydrolysate from
outside the chips. Neutralization is completed by circulating the
liquid in the digester and thus mixing the contents therewith.
[0007] In addition to the primary neutralization function, the
neutralization step also serves as an alkaline
hemicellulose-dissolving step. The strong alkali and the high
temperature directly dissolve and, on the other hand, degrade
hemicellulose by the so-called end-wise peeling reaction. The pulp
is thus further purified, which leads to higher pulp viscosity and
higher alpha cellulose content. In other words, the neutralization
step also becomes, in part, an alkaline extraction stage prior to
the cooking step. Therefore, the liquor-to-wood ratio in this step
is preferably relatively low, such as between about 2.5 and
3.5.
[0008] The neutralized prehydrolyzed cellulosic material is
delignified with alkaline cooking liquor (containing sodium sulfide
and sodium hydroxide). The delignification step has been a
conventional Kraft cooking method, where white liquor has been
added to the digester and the cooking has been carried out as a
single step.
[0009] As is disclosed above the acidic hydrolysis stage of the
prehydrolysis sulfate (Kraft) cooking of the prior art is
terminated (i.e. neutralized) e.g. by filling up the digester with
white liquor or with some other alkali-containing liquid to effect
the pH change and to be able to transfer from the acidic
hemicellulose-splitting condition/state to an alkaline
lignin-dissolving sulfate cooking state. The neutralization, i.e.
termination of the hydrolysis stage, is performed in the same way
independently of whether or not the prehydrolysis is operated in a
liquid or in a steam phase. Both of the above-mentioned
neutralization methods i.e. circulation and displacement are
time-consuming, which makes the uniformity of the neutralization
poorer.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, many of these
deficiencies in the prior art have now been overcome by the
invention of a process for the preparation of prehydrolyzed pulp
from lignin-containing cellulosic material comprising
prehydrolyzing the lignin-containing cellulosic material with steam
in a first steam phase at a first pressure to obtain prehydrolyzed
cellulosic material, terminating the prehydrolyzing of the
cellulosic material by at least one addition of an alkaline liquid
at the first pressure followed by at least one pressure reduction,
and delignifying the prehydrolyzed cellulosic material in a cooking
stage. This process preferably comprises a batch process.
[0011] In accordance with one embodiment of the process of the
present invention, the alkaline liquid comprises a white
liquor.
[0012] In accordance with another embodiment of the process of the
present invention, the at least one addition of the alkaline liquid
comprises a single addition of the alkaline liquid at the first
pressure, and the at least one pressure reduction comprises a
single pressure reduction to a second pressure lower than the first
pressure. In a preferred embodiment, the first pressure comprises a
pressure of between about 6 and 9 bar and the second pressure
comprises a pressure of between about 3 and 5 bar. Preferably, the
single pressure reduction is carried out for a period of about 1
second to about 10 minutes, preferably from about 30 seconds to
about 5 minutes.
[0013] In accordance with another embodiment of the process of the
present invention, the at least one addition of the alkaline liquid
comprises a single addition of the alkaline liquid, and the at
least one pressure reduction comprises a gradual pressure
reduction. Preferably, the first pressure comprises a pressure of
between about 6 and 9 bar, and the gradual pressure reduction
comprises a gradual pressure reduction from the first pressure to a
second pressure, the second pressure comprising a pressure of
between about 3 and 5 bar, and the gradual pressure reduction is
carried out by reducing the pressure in increments of from about
0.5 to 2 bar.
[0014] One object of the present invention is to provide an
improved process for the preparation of pulp from lignin-containing
cellulosic material, in which the prehydrolysis is carried out in a
steam phase and the termination (i.e. neutralization of the
contents of the digester) of the prehydrolysis stage is also
carried out in a steam phase in order to overcome many of the
problems associated with the prior art. According to the present
invention steam phase termination is carried out by adding alkaline
liquid in a prevailing pressure into the digester and then
temporarily reducing the pressure of the digester. As a result of
the pressure reduction the added alkaline liquid evaporates to the
gas space of the digester, thus neutralizing the prehydrolyzed
material in the digester. The termination stage according to the
present invention and its modifications will be described in more
detail below.
[0015] The use of a momentary decrease of pressure when
impregnating wood by overpressure is known from Canadian Patent No.
597,147. The purpose of this method is to get air contained in the
wood cells to escape therefrom with great force. There may be one
or several decreases of pressure, and heating of the impregnating
liquor may be used in connection therewith.
[0016] The present invention is directed to a process for the
preparation of prehydrolyzed pulp in a digester from
lignin-containing cellulosic material comprising [0017] a)
prehydrolyzing the cellulosic material in a first steam phase at a
first pressure to obtain prehydrolyzed cellulosic material [0018]
b) terminating the prehydrolyzing operation in a termination stage
[0019] c) delignifying the prehydrolyzed cellulosic material in a
cooking stage, wherein the termination stage comprises at least one
addition of an alkaline liquid at the first pressure followed by at
least one pressure reduction.
DETAILED DESCRIPTION
[0020] According to the present invention the prehydrolysis stage
is carried out in a steam phase, by introducing direct steam to the
chip column in the digester at a temperature of from about 150 to
180.degree. C., preferably at a temperature of 160 to 170.degree.
C. The steaming is normally initiated with a low-pressure steam
(LP; appr. 3.5 bar, 150.degree. C.) and then continued with a
medium pressure steam (MP; appr. 10.5 bar, 200.degree. C.) until
the desired temperature in the venting line of the digester is
obtained, i.e. the temperature of the digester contents is about
150 to 180.degree. C. The temperature inside the digester is
measured from the temperature of the venting gas in the venting
line. During the prehydrolysis stage the pressure inside the
digester can vary between about 4.8 bar and 10.0 bar, preferably it
is in the range of from about 6 to about 9 bar.
[0021] A suitable prehydrolyzing time is from about 10 to 200
minutes, preferably from about 20 to 120 minutes.
[0022] According to the present invention after completing the
prehydrolysis stage, the contents of the digester are neutralized,
i.e. the prehydrolysis stage is terminated by a termination stage,
which stage comprises at least one addition of an alkaline liquid
at a prevailing pressure (i.e. at a first pressure) followed by at
least one pressure reduction to a second pressure. Thus, according
to the present invention the first pressure is always higher than
the second pressure. The pressure difference between the first
pressure and the second pressure, i.e. the magnitude of the
pressure drop, depends on several process variables, e.g. strength
of the alkali, degree of packing and pressure of the digester. The
pressure of the digester is reduced so that the alkali
charged/introduced evaporates and thus neutralizes the
prehydrolyzed material in the digester. This termination stage can
also be called a neutralization stage, as the acidic prehydrolyzed
material is neutralized during this step. The amount of alkali thus
charged is determined so that all of the liquid (free liquid
outside the chips and liquid trapped and immobilized inside the
chips) in the digester is neutralized, and alkaline conditions are
obtained that are at a pH of 7 or greater.
[0023] According to one embodiment of the present invention, in the
termination stage first about 2 to 15% (as NaOH on wood) of a
suitable alkaline liquid, for example alkalized black liquor,
preferably white liquor, is added into the digester at the
prevailing pressure i.e. at the first pressure. The first pressure
is the pressure of the end of the prehydrolysis stage, i.e. between
4.8 bar and 10.0 bar, preferably in the range of from about 6 bar
to about 9 bar. The pH of the digester contents cannot be directly
measured, thus, normally, the alkaline conditions are ensured by
feeding some extra amount of alkaline liquid into the digester.
Secondly, after the addition of the alkaline liquid, the pressure
of the digester is momentarily reduced from the first pressure to a
second pressure, which is from about 2 to 7 bar, preferably from
about 3 to 5 bar. The pressure is reduced for a period of about 1
second to 10 minutes, preferably for a period of about 30 seconds
to 5 minutes. After the momentary pressure reduction the process is
continued by an alkaline delignifying step. In the delignifying
step the digester is filled up with the cooking chemicals, the
temperature is increased to the cooking temperature, which is
maintained until the desired degree of delignification is
obtained.
[0024] According to a further embodiment of the present invention
the pressure reduction of the termination stage is performed
gradually. In that case first about 2 to 15% (as NaOH on wood) of
suitable alkaline liquid, for example alkalized black liquor,
preferably white liquor, is charged into the digester at the
prevailing pressure. After the addition of the alkaline liquid the
pressure is reduced gradually step-by-step, for example, at
increments of about 0.5 to 2 bar at a time, to obtain the end
pressure, which is from about 2 to 7 bar, preferably from about 3
to 5 bar. After completing the termination stage, the digester is
filled up with the cooking chemicals and thus preparation of the
pulp is continued by an alkaline delignifying step.
[0025] According to a still further embodiment of the present
invention, the total amount of the needed (2to 15% as NaOH on wood)
suitable alkaline liquid, for example alkalized black liquor,
preferably white liquor, is gradually added into the digester and
after each addition the pressure is reduced from the previous
pressure. For example, first, half of the needed alkaline liquid is
added into the digester and immediately after that the pressure is
reduced e.g. by an increment of from about 0.5 to 1 bar, preferably
from about 0.5 to 0.7 bar. The pressure is reduced for a suitable
period of time, i.e. for a period of about 1 second to 10 minutes,
preferably for a period of about 30 seconds to 5 minutes. After the
momentary pressure reduction, the rest of the alkaline liquid is
added into the digester followed by a second pressure drop. The
amount of alkaline liquid needed can also be added in more than two
batches. Each addition of alkaline liquid is followed by a pressure
reduction. The pressure can be reduced e.g. gradually so that after
the addition of alkaline liquid the reduction of the pressure is in
increments of from about 0.5 to 1 bar, preferably from about 0.5 to
0.7 bar from the previous pressure.
[0026] Persons skilled in the art can determine the optimal
termination conditions and procedural steps based on the process
variables in question.
[0027] The neutralizing liquid is preferably introduced into the
digester through the bottom of the digester, but other possible
methods are also within the scope of the present invention.
[0028] After the termination stage the prehydrolyzed cellulosic
material is delignified by conventional sulfate cooking. In the
beginning of this conventional Kraft cooking, the cooking
chemicals, e.g. white liquor and spent black liquor, are fed into
the digester. The digester contents are then heated by direct steam
or by means of indirect heating of the recirculating pulping liquor
in a heat exchanger. A suitable cooking temperature is from about
150.degree. C. to 180.degree. C., preferably from about 150.degree.
C. to 165.degree. C. for hardwoods, and from about 155.degree. C.
to 170.degree. C. for softwoods. After the desired degree of
delignification is reached, the digester contents are blown to a
blow tank.
[0029] In the prior art after completing prehydrolysis the contents
of the digester are neutralized, in other words the prehydrolysis
stage was completed by a liquid phase neutralization process, i.e.
circulation or displacement, which are very time-consuming.
Therefore, the process conditions in the upper part of the digester
are totally different compared to the process conditions in the
lower part of the digester at the end of the neutralizing step. In
the process according to the invention, termination or
neutralization of the prehydrolysis stage is carried out very
quickly compared to the prior art processes. This leads to several
advantages, namely to time savings in the cooking cycle, resulting
in increased production per m.sup.3 of the digester, to improved
homogeneity in the hemicelluloses content of the cooked pulp, to
improved uniformity of the lignin content of the cooked pulp, to
increased viscosity of the cooked pulp and also to better
unbleached pulp brightness.
[0030] Although the invention herein has been described with
reference to particular embodiments, it is to be understood that
these embodiments are merely illustrative of the principles and
applications of the present invention. It is therefore to be
understood that numerous modifications may be made to the
illustrative embodiments and that other arrangements may be devised
without departing from the spirit and scope of the present
invention as defined by the appended claims.
* * * * *