U.S. patent application number 14/241141 was filed with the patent office on 2014-10-09 for kraft cooking method using polysulfide cooking liquor.
The applicant listed for this patent is Mikael Lindstrom, Fredrik Wilgotson. Invention is credited to Mikael Lindstrom, Fredrik Wilgotson.
Application Number | 20140299283 14/241141 |
Document ID | / |
Family ID | 47756632 |
Filed Date | 2014-10-09 |
United States Patent
Application |
20140299283 |
Kind Code |
A1 |
Lindstrom; Mikael ; et
al. |
October 9, 2014 |
KRAFT COOKING METHOD USING POLYSULFIDE COOKING LIQUOR
Abstract
The method is for the preparation of kraft pulp with increased
pulping yield from lignin-containing cellulosic material using
polysulfide cooking liquor. In order to increase carbohydrate
stabilization and hence the yield from a kraft cooking process a
first impregnation stage is using polysulfide cooking liquor
implemented at high alkali and polysulfide concentration and at a
low temperature.
Inventors: |
Lindstrom; Mikael; (Lidingo,
SE) ; Wilgotson; Fredrik; (Sundsvall, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lindstrom; Mikael
Wilgotson; Fredrik |
Lidingo
Sundsvall |
|
SE
SE |
|
|
Family ID: |
47756632 |
Appl. No.: |
14/241141 |
Filed: |
August 30, 2011 |
PCT Filed: |
August 30, 2011 |
PCT NO: |
PCT/SE2011/051038 |
371 Date: |
May 6, 2014 |
Current U.S.
Class: |
162/19 |
Current CPC
Class: |
D21C 1/06 20130101; D21C
3/02 20130101 |
Class at
Publication: |
162/19 |
International
Class: |
D21C 1/06 20060101
D21C001/06 |
Claims
1. A method for the preparation of kraft pulp with increased
pulping yield from lignin-containing cellulosic material using
polysulfide cooking liquor, wherein the lignin-containing
cellulosic material is heated to a temperature in the range
50-100.degree. C. followed by adding polysulfide cooking liquor to
a first impregnation stage which in turn is followed by cooking
stages resulting in a kraft pulp with a kappa number below 40
characterized in that the impregnation stage is conducted at high
alkali concentration, low temperature and high polysulfide
concentration using polysulfide cooking liquor at a liquor-to-wood
ratio in the range 2.0 to 3.2, and that the temperature is between
80-120.degree. C. during a retention time resulting in a H-factor
in the range 2-20 and preferably 2-10 of the impregnation
stage.
2. The method according to claim 1 wherein the effective alkali
concentration during the impregnation stage is above 60 g/l when
adding the polysulfide cooking liquor.
3. The method according to claim 2 wherein the polysulfide
concentration during the impregnation stage is above 3 g/l, or
above 0.09 mol/l, when adding the polysulfide cooking liquor.
4. The method according to claim 3 wherein more than 90% of the
total charge of cooking liquor needed for completion of the cooking
stages to the intended kappa number below 40 (andra I text) is
charged to the first impregnation stage, and that at least 175 kg
of effective alkali (EA as NaOH) for softwood and 160 kg of
effective alkali for hardwood per ton of chips is charged.
5. The method according to claim 4 wherein the alkali concentration
is reduced by at least 8 g/l by adding additional cooking liquids
having lower alkali concentration than the alkali concentration
prevailing at end of the first impregnation stage when increasing
the temperature to cooking temperature, said cooking liquids in at
least part thereof includes black liquor.
6. The method according to claim 5 wherein no black liquor is added
to the first impregnation stage.
7. The method according to claim 6 wherein the white liquor added
to the first impregnation stage has an alkali concentration above
100 g/l and a polysulfide concentration above 4 g/l .
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for the
preparation of kraft pulp with increased pulping yield from
lignin-containing cellulosic material using polysulfide cooking
liquor.
BACKGROUND OF THE INVENTION
[0002] In conventional kraft cooking implemented in the
1960-1970-ies in continuous digesters was the total charge of white
liquor added to the top of the digester. It soon emerged that the
high alkali concentrations established at high cooking temperatures
was detrimental for pulp viscosity.
[0003] Cooking methods was therefore developed in order to reduce
the detrimental high alkali peak concentrations at start of the
cook, and thus was split charges of alkali during the cook
implemented in cooking methods such as MCC, EMCC, ITC and Lo-Solids
cooking.
[0004] Other cooking methods was implemented using black liquor
impregnation ahead of cooking stages where residual alkali in the
black liquor was used to neutralize the wood acidity and to
impregnate the chips with sulfide. One such cooking method sold by
Metso is Compact Cooking where black liquor with relatively high
residual alkali level is withdrawn from earlier phases of the cook
and charged to a preceding impregnation stage.
[0005] One aspect of alkali consumption during the cooking process,
i.e. including impregnation, is that a large part of the alkali
consumption is due to the initial neutralization of the wood
acidity, and as much as 50-75% of the total alkali consumption is
occurring during the neutralization process. Hence, a lot of alkali
is needed to be charged to the initial neutralization. This
establish a cumbersome problem as high alkali concentrations had
been found to be detrimental for pulp viscosity when charged to top
of digesters in conventional cooking. One solution to meet the high
alkali consumption and necessity to reduce alkali concentration in
top of digester was to charge large volumes of alkali treatment
liquors, preferably black liquor having a residual alkali content,
but having low alkali concentration, which resulted in presence of
relatively large amount of total alkali per kg of wood material but
still at low alkali concentration.
[0006] IN U.S. Pat. No. 7,270,725 (=EP1458927) Metso disclosed a
pretreatment stage using polysulfide cooking liquor ahead of black
liquor treatment. In this process was the polysulfide treatment
liquor drained after the pretreatment stage and before starting the
black liquor treatment. The polysulfide treatment stage was also
preferably kept short with treatment time in the range 2-10
minutes.
[0007] In a recent granted US patent, U.S. Pat. No. 7,828,930, is
shown an example of a kraft cooking process where 100% of the
cooking liquor, in form of polysulfide liquor also named as orange
liquor, is charged to top of digester and start of an impregnation
stage. Here is also the temperature raised from 60.degree. C. to
120.degree. C. at start of the polysulfide treatment stage.
However, as shown in example 1 is a liquor to wood ratio of about
3.5 established in the top of the digester by adding a proper
amount of water. This order of liquor/wood ratio is often perceived
as a standard liquor/wood ratio in continuous cooking necessary for
a steady process. According to this proposal is a part of the
residual polysulfide treatment liquor at relative high alkali
concentration withdrawn and replaced with cooking liquor at
relative low alkali concentration at start of the cooking stage,
and the withdrawn residual polysulfide treatment liquor is added at
later stages of the cook.
[0008] There has thus been an ongoing development of cooking
methods where both alkali concentrations at start of cook is
reduced, and increased yield from the cooking process is sought for
using among others addition of polysulfide cooking liquor that
stabilize the carbohydrates.
SUMMARY OF THE INVENTION
[0009] The invention is based upon the surprising finding that
concentration of polysulfide should be kept high in a low
temperature pretreatment stage at relatively long retention time
before cooking, using liquor to wood ratios well below that as
commonly used. The stabilization effect of carbohydrates, the major
objective for polysulfide addition, has shown to be improved
dramatically if using a liquor to wood ratio of about 2,9 instead
of the conventional liquor to wood ratio of about 3,5, and all
other conditions equal. This non proportional effect of low liquor
to wood ratio has not been disclosed or realized before despite the
numerous proposals for improving cooking yield using polysulfide
cooking liquor.
[0010] One object of the present invention is to provide an
improved method for the preparation of kraft pulp with increased
pulping yield from lignin-containing cellulosic material using
polysulfide cooking liquor, wherein the lignin-containing
cellulosic material is heated to a temperature in the range
50-100.degree. C. followed by adding polysulfide cooking liquor to
a first impregnation stage which in turn is followed by cooking
stages resulting in a kraft pulp with a kappa number below 40, and
wherein the impregnation stage is conducted at high alkali
concentration, low temperature and high polysulfide concentration
using polysulfide cooking liquor at a liquor-to-wood ratio in the
range 2.0 to 3.2, and that the temperature is between
80-120.degree. C. during a retention time resulting in a h-factor
in the range 2-20 and preferably 2-10 of the impregnation stage.
This low h-factor is indicative for that no cooking or
delignification effect is obtained in the first impregnation stage,
and hence is no reduction in pulp viscosity seen as could be the
case if high alkali concentrations are at hand in cooking stages at
higher temperatures.
[0011] According to one preferred embodiment of the method is the
effective alkali concentration during the impregnation stage above
60 g/l when adding the polysulfide cooking liquor.
[0012] According to another preferred embodiment of the method is
the polysulfide concentration during the impregnation stage above 3
g/l, or above 0.09 mol/l, when adding the polysulfide cooking
liquor.
[0013] According to a further embodiment of the method is more than
90% of the total charge of cooking liquor needed for completion of
the cooking stages to the intended kappa number below 40 charged to
the first impregnation stage, and that at least 175 kg of alkali
(EA as NaOH) per ton of chips is charged for softwood and at least
160 kg of alkali per ton of chips for hardwood.
[0014] According to yet another embodiment of the method is the
alkali concentration reduced by at least 8 g/l by adding additional
cooking liquids having less alkali concentration than the alkali
concentration prevailing at end of the first impregnation stage
when increasing the temperature to cooking temperature, said
cooking liquids in at least part thereof includes black liquor.
[0015] In a most preferred embodiment of the method is no black
liquor added to the first impregnation stage.
[0016] When using the inventive method has also preferably the
white liquor added to the first impregnation stage an alkali
concentration above 100 g/l and a polysulfide concentration above 4
g/l.
[0017] The lignin-containing cellulosic materials to be used in the
present process are suitably softwood, hardwood, or annual
plants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic cooking system capable of implementing
the inventive method;
[0019] FIG. 2 demonstrate an example of the alkali profile
established with the inventive method;
[0020] FIG. 3 show the dramatic impact on increased yield when
increasing the polysulfide concentration above 0.15 mol/L
[0021] FIG. 4 show the relative stabilization of carbohydrates as a
function of liquid to wood ratio during the impregnation stage.
DETAILED DESCRIPTION OF THE INVENTION
[0022] In FIG. 1 is shown a 2-vessel kraft cooking system, having a
first hydraulic impregnation vessel B and a second steam/liquid
phase digester C, wherein the inventive method could be
implemented. In this type of system is first the lignin containing
cellulosic material Ch fed to a bin A wherein the cellulosic
material is heated to a temperature in the range 50-100.degree. C.
by using addition of steam St. The lignin containing cellulosic
material could preferably be wood chips. From the lower part of the
bin A is then the heated chips suspended in treatment liquor in a
chute C located above the high pressure sluice feeder SF. The
treatment liquor here is preferably only polysulfide cooking
liquor, WL, and preferably is the entire charge of cooking liquor
needed for the cooking process charged here.
[0023] The chips suspended in the treatment liquor are fed to the
sluice feeder and displaced liquid is fed out from the bottom
outlet of the sluice feeder and returned to the chute in a low
pressure circulation. The chips in the sluice feeder is pressurized
by the return flow from the vessel B and fed out to the top
separator TS in top of the vessel B.
[0024] Thus, the first impregnation stage is implemented in the
vessel B and preferably only with the polysulfide cooking liquor
and as small amount as possible of additional liquids such as wood
moisture, steam condensates, and especially no black liquor nor
additional water or filtrates. The resulting liquor-to-wood ratio
established should be in the range 2.0 to 3.2 and the temperature
should be in the range 80-120.degree. C.
[0025] After the sufficient retention time in vessel B, which
should have a retention time resulting in an H-factor in the range
2-20 of the impregnation stage, the impregnated chips will be fed
to the steam/liquid phase digester C together with the residual
treatment liquor. Here is shown a conventional transfer system with
dilution in bottom of the vessel B using withdrawn treatment liquor
from the top separator TS in the top of vessel C. At this point is
the chip suspension heated to full cooking temperature, in the
range 140-170.degree. C. depending upon type of cellulosic
material, and additional liquid is added in order to reduce the
alkali concentration at this point. In this embodiment is shown
addition of black liquor obtained from a screen section withdrawing
black liquor and sending a part of this black liquor to recovery
REC. Hence, no detrimental effects upon pulp viscosity would occur
by this dilution with black liquor. In this embodiment is shown a
digester C with 2 concurrent cooking zones, one cooking zone above
the first screen section and a second cooking zone above the final
screen section in bottom of digester. In a conventional manner is a
final counter current wash zone implemented in bottom of digester
by addition of wash water/Wash. The final pulp with a kappa number
below 40 is fed out from bottom in flow Pu.
[0026] In FIG. 2 is disclosed the alkali concentration profile that
could be established in a system like that disclosed in FIG. 1,
with alkali consumption of about 110 kg/BDT in the impregnation
vessel, 45 kg/BDT in the first cooking zone in vessel C and 15
kg/BDT in last cooking zone in vessel C. In the top of the first
impregnation vessel B is an alkali concentration of about 67 g/l
established and this alkali level drops down to about 32 g/l in the
bottom of vessel B, where a dilution is made by return flows added
to bottom. Combined with the dilution with black liquor in top of
digester vessel C the cooking in top of digester starts at an
alkali concentration of about 22 g/l. Due to the dilution to a
liquor to wood ratio of about 6,5 is however sufficient total
amount of alkali present. During the cook the alkali concentration
drops evenly, first to a level of about 16 g/l at first withdrawal
screen, and finally down to about 8 g/l in final withdrawal screen.
It is to be noted that a part of the withdrawn black liquor at
concentration of about 16 g/l is recirculated back to top of vessel
C. With this alkali profile is an improved usage of the polysulfide
obtained as it is used in the first impregnation stage at high
alkali concentration, low temperature and high polysulfide
concentration.
[0027] In FIG. 3 is disclosed the improved carbohydrate yield as a
function of the polysulfide concentration, when about 1% lignin is
still present in the pulp. Here is shown the dramatic increase in
yield when increasing the polysulfide concentration above 0.15
mol/L. There is basically a linearly increasing yield when the
concentration increases between 0 to 0.15 mol/l. In this initial
range is the yield increased from about 45% up to about 46.2%.
However, when the concentration reach 0.2 mol/L is the yield
increased to about 48.3%.
EXAMPLES
[0028] A series of tests has been made simulating a system as that
shown in FIG. 1 using a white liquor having an alkali concentration
of about 117 g/L and a polysulfide concentration of about 6 g/l.
The charges of flows to the first impregnation stage are in tests
#1-7 using part flows a-e. This results in a liquor to wood ratio
shown in row L/W. The respective concentrations established are
shown in rows f to j. S.sub.nS.sup.2- Despite the presence of a
number of different polysulfide ions, each polysulfide ion can be
considered to consist of one atom "sulfide sulfur", i.e. sulfur in
the formal oxidation state S(-II), and n atoms of polysulfide
"excess sulfur", i.e. sulfur in the formal oxidation state
S(0).
[S-II)]=[HS-]+.SIGMA.[S.sub.nS.sup.2-]
[S(0)]=.SIGMA.n[S.sub.nS.sup.2-]
[0029] Finally, the Xs factor has been calculated using the
formula:
Xs=[S(0)]/[S(-II)]
[0030] and the carbohydrate stabilization has been calculated using
the formula*:
Log[S(0)]+1.7 log[OH-]-1.6 log(1/Xs-1/4)
(*see Teder, A. (1965): Svensk Papperstidn. 68:23, 825)
TABLE-US-00001 [0031] #1 #2 #3 #4 #5 #6 #7 a WL (m.sup.3/BDT) 1.79
1.79 1.79 1.79 1.79 1.79 1.79 b Moisture (m.sup.3/BDT) 0.82 0.82
0.82 0.82 0.82 0.82 0.82 c Condensate (m.sup.3/BDT) 0 0.3 0.3 0.3
0.3 0.3 0.3 d BL to feed (m.sup.3/BDT) 0.0 0.0 0.0 0.5 1.0 1.5 2.0
e Knots to feed (m.sup.3/BDT) 0 0 0.3 0.3 0.3 0.3 0.3 L/W 2.61 2.91
3.21 3.71 4.21 4.71 5.21 f NaOH (g/L) 80.4 72.1 65.9 59.2 54.1 50.0
46.8 g OH (mol/L) 2.0 1.8 1.6 1.5 1.4 1.3 1.2 h PS (g/L) 4.12 3.70
3.35 2.90 2.56 2.28 2.07 i PS (mol/L) 0.13 0.12 0.10 0.09 0.08 0.07
0.06 j HS (mol/L) 0.07 0.08 0.10 0.11 0.12 0.13 0.14 Xs 1.81 1.37
1.1 0.83 0.67 0.56 0.48 Carbohydrate stab 220 134 100 68 47 31 19
(test #3 is reference)
[0032] In FIG. 4 is disclosed the relative carbohydrate
stabilization from above examples as a function of liquor to wood
ratio during impregnation. The test #3 is used as the reference,
i.e. 100%. The relative carbohydrate stabilization is roughly
increasing linearly when decreasing the liquor to wood ratio during
impregnation from 5,2 down to 3,7. However a dramatic improvement
is obtained if the liquor to wood ratio is reduced to and further
below 3,2. While the relative carbohydrate stabilization increase
from about 19 to about 68 in the liquor to wood ratio from 5,2 down
to 3,7, it is increased to astonishing 100 and further to about 134
and up to 220 at liquor to wood ratio of 3,2 , 2,9 and 2,6
respectively.
* * * * *