JP3064616B2 - Method for controlling kappa number of pulp in vertical continuous digester with modified cooking method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing pulp by introducing wood chips and an alkaline cooking liquor into a vertical digester having a lower (or modified) cooking (MCC) zone. A method for controlling the value. More specifically, the present invention relates to the kappa number of pulp.
The quickly stabilizes to methods for very small variation thereof.

[0002]

2. Description of the Related Art The mainstream of a method for producing chemical pulp from a lignocellulosic material such as wood which has been conventionally industrially performed is an alkaline cooking method, in particular, a kraft using an alkaline cooking liquor containing sodium sulfide. The method (sulfate method) has been implemented on a large scale because of advantages such as pulp strength, pulp yield, and ease of chemical recovery.

In the alkaline cooking method, a permeation zone is provided in another column, and a two-vessel vertical continuous digester having a digestion zone in another column, or a digestion zone in one vertical column or a percolation zone and It is preferably carried out in a one-vessel vertical continuous digester having a digestion zone.

[0004] In general, as typical pulp obtained from the alkaline cooking method, unbleached pulp is obtained from coniferous wood chips by the Kraft method, and the pulp is delignified and bleached in the bleaching step to obtain whiteness. Kappa number after discharge from digester to obtain high pulp
The pulp having a low degree of cooking in the range of 33 and the unbleached, for example, the kappa number provided to the raw material of the unbleached kraft paper are 40 to 40.
High kappa pulp in the range of 55 and unbleached pulp in the range of 12 to 23 copper for bleaching from hardwood wood chips.

[0005] In any pulp production, the vertical digestion is performed from the viewpoint of stabilizing the operation in the oxygen bleaching process as the subsequent process and the conventional multi-stage bleaching process, stabilizing the unbleached and bleached pulp quality and reducing the production cost. The target kappa value of the pulp discharged from the kettle and the range of its variation are determined, the operation is maintained so as to match the target value, and these target values are based on the type of paper product and the wood chips used as the end use It is usually determined while taking into account the digestibility of the product.

[0006] In a vertical digester having a plurality of digestion zones, when the kappa number of the pulp discharged from the kettle is changed, or even when the kappa number is not changed, the operation is carried out in order to stabilize the kappa number. The operation is carried out by adjusting the factors of the flow rate of the so-called white liquor conventionally supplied to the digestion zone and the effective alkali concentration, temperature and flow rate of the circulating fluid flowing through the circulation system formed in this zone and other various zones. Is being done.

However, even if the effective alkali concentration or temperature of the circulating fluid in each zone is changed, the effective alkali concentration or temperature in each zone does not easily reach the target level, and therefore, the pulp copper discharged from the kettle is removed. It takes a considerable amount of time for the price to reach the target value,
The variation in copper value also increases.

In particular, in Japan, where forest resources are tight, it is almost impossible to supply wood chips of a single grade to a digester stably for a long period of time. Are supplied to the digestion. The cooking rate depends on the alkali addition rate, cooking time and temperature (usually indicated by the H factor) required to obtain the optimum pulp kappa number. In the cooking zone and other zones, efforts are made to change the above-mentioned factors by trial and error to bring the kappa number of the pulp or its variation into the target range.

[0009] Thus, fluctuations in the kappa number of the pulp make the operations in the washing step, the selection step, the oxygen bleaching step (if any), the multi-stage bleaching step, etc. unstable over a long period of time and the quality is high. There was a problem that unsatisfactory pulp was produced in large quantities.

[0010] The circulating liquid of the circulation system formed in each of the plurality of zones of the vertical digester is balanced with white liquor added at a relatively upper portion of the digester and washing liquid introduced from the bottom of the digester. In addition, each time a factor of one digestion operation changes, the temperature of the circulating liquid, the effective alkali and the flow rate of the circulating liquid are rapidly changed in response to the fluctuation, and the temperature and the temperature in the respective zones are further concomitantly changed. If the effective alkali can also be changed quickly, the deviation or variation of the obtained kappa number from the target value can be extremely small even if there are slight fluctuation factors such as a change in the grade. To date, no such cooking operation technique has been established.

[0011]

SUMMARY OF THE INVENTION The inventors of the present invention have conducted intensive studies in view of the present situation, and have found that in a vertical digester having various circulation systems including a cooking circulation system, the cooking conditions of a plurality of cooking circulation systems are controlled. Instead of MCC (Modified Co
By changing only the cooking conditions in the (ntinuous cooking, modified cooking) zone, the fluctuation of the kappa number of the pulp discharged from the digester with respect to the target value, that is, the variation of the kappa number is extremely small, and the kappa number is stabilized. The inventors have found that they can do so and have completed the present invention.

Accordingly, an object of the present invention is to provide a vertical digester having various circulation systems including a digestion circulation system, which minimizes the change factor in the digestion operation and minimizes the kappa number of the produced pulp very quickly. It is an object of the present invention to provide a cooking control method for producing pulp of excellent quality by approaching the pulp value and reducing the variation significantly, and stabilizing the operation in the post-process.

[0013]

A first aspect of the present invention is that a vertical infiltration vessel is installed in front of a vertical digester having a digestion zone. Inside the vertical digester, from the top to the bottom, An upper cooking trim zone, an upper cooking zone, an MCC zone and a washing zone are provided. Co-current cooking is performed in the upper cooking trim zone and the upper cooking zone, countercurrent cooking is performed in the MCC zone, and high heat countercurrent in the washing zone. When wood chips and an alkaline cooking liquor are introduced into a 2-vessel vertical digester in which washing is performed and wood chips are digested, active alkali per 15 weight of absolutely dry wood chips is reduced.
Of the alkaline cooking liquors added in the range of ２０20% by weight (as Na ₂ O), 50-90% is in the permeation vessel, 0-45% is in the upper cooking trim zone, 5-45%
Was added to the MCC zone, and the temperature of each digestion zone was changed in the range of 140 to 175 ° C., and TAPPI test method T-236 for pulp discharged from the bottom of the vertical digester was used.
hm-85, wherein the temperature and alkali concentration in the infiltration vessel, upper cooking trim zone and upper cooking zone are maintained at a constant level, while the cooking temperature of the MCC zone is maintained. Alternatively, a method of controlling the kappa number of pulp by changing the alkali addition rate or both.

A second aspect of the present invention is that a vertical digester is provided with an infiltration zone, a digestion zone, an MCC zone and a washing zone from the top to the bottom. Contact in parallel, M
In the CC zone, countercurrent digestion is performed, and in the washing zone, wood chips and alkaline digestion are performed in the same 1-vessel vertical digester as described above except that it does not have an infiltration zone as well as a 1-vessel vertical digester in which high heat countercurrent washing is performed. In introducing the liquid and digesting the wood chips, the active alkali is 15 to 20% by weight (as Na ₂ O) based on the weight of the absolutely dry wood chips.
Of the alkaline cooking liquor added in the range of 55 to 9
5% is added in the wood chip feed line or infiltration zone and digestion zone, 5-45% is added in the MCC zone, and the temperature of each zone is changed in the range of 140-175 ° C, the bottom of the vertical digester a method of controlling the kappa number of the pulp to be more discharged within the 12 to 55, while the temperature and the alkali concentration in the permeate zone and / or the cooking zone and / or the cooking zone maintained at a certain level, M C C zone Is a method for controlling the kappa number of pulp by changing either the cooking temperature or the alkali addition rate or both the cooking temperature and the alkali addition rate.

The process of the present invention is suitably used in a two-vessel vapor-phase and liquid-phase vertical continuous digester and a one-vessel vapor-phase and liquid-phase vertical continuous digester. FIG. 1 shows wood chips, white liquor, and two-vessel liquid phase vertical continuous digester.
FIG. 2 is a schematic flow chart showing flows of various circulating liquids and the like, and the present invention will be described with reference to the drawings.

In FIG. 1, wood chips (hereinafter referred to as chips) a are supplied to an open-to-atmosphere heating type chip bin 1,
A certain amount of chips is conveyed by a rotary volume meter 2 to a steaming vessel 3, where the chips are heated and degassed and then mixed with an extract supplied from a conduit 7 by a high-pressure feeder 4 to be mixed with an osmotic vessel. Conduit 5 to the top of 6
Introduced via

On the other hand, the main stream of the white liquor is supplied to the top of the permeation vessel 6 via conduits 17 and 18 and is diluted with the circulating liquid into digestion liquor, and most of the liquor is supplied to the strainer provided at the top. And circulated through conduit 7 as an extract. At the top of the infiltration vessel 6, the chips descend in the infiltration vessel 6 with the remaining digested liquor, during which time the liquor is fully penetrated into the chips. A strainer 39 is provided at an intermediate portion of the infiltration vessel 6, and the extract is circulated through conduits 40 and 41 so that the chips can be easily lowered and the cooking liquor can uniformly penetrate the chips.

Next, the mixture of chips and the cooking liquor from the bottom of the permeation vessel 6 is passed through a conduit 37 provided with a thermometer 45.
The remaining cooking liquid and chips which are introduced into the top of the digester 8 through the strainer and drained by the strainer provided at the top descend in the digester 8. The extract generated by the above-described liquid removal is circulated to the bottom of the permeation vessel 6 through a conduit 36 provided with a sensor 43 for measuring the alkali concentration and a heater 16 to which medium-pressure steam 35 is supplied. Is returned (this line is called the transfer cycle). On the other hand, the sensor 43 provided in the return line of the transfer circulation measures the alkali concentration in the circulation return liquid (extract), and the required amount of alkali in the form of white liquor is transferred via the conduits 17, 19 and 20 to the transfer circulation. It is supplied to the return liquid.

The digester 8 is provided with an upper digestion trim zone A, an upper digestion zone B, an MCC zone C, and a washing zone D from the top to the bottom. Each zone is provided with a strainer 9, 10, 11, and 12. Are largely divided through. As the mixture of chips and cooking liquor descends in the digester 8, first a part of the cooking liquor is extracted from the strainer 9 provided at the lower part of the upper cooking trim zone A, and this extract is passed through the conduit 38. It is circulated together with the circulation return liquid of the transfer circulation line flowing through the conduit 36. Chips that are partially digesting are then considerably digested while passing down the upper cooking zone B while passing down the zone while moving co-currently with the cooking liquor (cocurrent cooking),
From the strainer 10 provided in the lower part of this zone, a digested liquid having a high solids content (referred to as black liquor) is extracted and sent to a recovery step via a conduit 30 (not shown).

The considerably digested chips are further lowered and M
While passing through CC zone C, it flows upward from the bottom, and is replenished with white liquor as described below to be brought into contact with the cooking liquor of which temperature and alkali concentration have been adjusted to make the final degree of delignification. (Countercurrent cooking). That is, MCC
The liquid extracted from the strainer 11 provided at the lower part of the zone is returned to the continuous digester 8 again by a circulation line composed of the conduit 24, the heater 14, and the conduit 27.
The circulation line is provided with a sensor 44 for measuring the alkali concentration and a thermometer 46. The white liquor branched from the conduit 19 to maintain the required alkali concentration while constantly monitoring the alkali concentration and the temperature is provided. The liquid is replenished to the conduit 24 via the conduit 21, and the temperature of the circulating liquid is adjusted in the heater 14 by the medium-pressure steam 33. New conduit 24
A part of the circulating liquor containing the white liquor replenished to the chip is brought into contact with the chips in countercurrent as described above, and after the modified cooking (MCC) is performed, the strainer 10 and the other liquor are combined with the black liquor. And collected outside the system via the conduit 30.

The chips after the digestion are further lowered and moved in the washing zone D, and come into contact with the liquid flowing upward from the bottom in a countercurrent manner, whereby high-heat countercurrent washing is performed. In the lower part of the washing zone D, a strainer 12 and a conduit 2 are provided.
5, a circulating line composed of a heater 15, a thermometer 47 and a conduit 28 is provided.
The temperature is adjusted by the medium-pressure steam 34 supplied to 5.
The washing liquid is introduced into the digester 8 from the conduit 29, is taken into the circulation line as an extract from the strainer 12, and after being adjusted in temperature as described above, is introduced into the washing zone D via the circulation line, and the digested chips Of the strainer 10 through the MCC zone.
Together with other surplus liquid from the system via the conduit 30 to the outside of the system. The digested chips that have been washed are placed in the discharge pipe 31.
And then sent to the next washing step, where the pressure is released and the pulp is defibrated by mechanical action.

In the digestion of chips in the two-vessel continuous digester described above, the pressure of the steaming vessel 3 is:
0.5-2.0 kg / cm ² (gauge pressure) range, temperature is 110-130 ° C., top pressure in the permeation vessel 6 is 12.0-15.0 kg / cm ² range, temperature Is in the range of 115 to 145 ° C., while the temperature of the upper digestion zone A, the temperature of the upper digestion zone B and the temperature of the MCC zone C of the continuous digester 8 are all 1
40 to 175 ° C., preferably 1 for softwood wood
50-170 ° C, 140-155 ° C for hardwood,
The heater outlet temperature of each circulation line is adjusted so that the temperature of the washing zone D can be operated in the range of 120 to 155 ° C.

Further, in the Kraft method, the white liquor having an alkali concentration of 105 to 115 g / liter (as active alkali, Na ₂ O) introduced from the conduit 17 is 15 to 20% by weight, preferably 15 to 20% by weight, based on the weight of the absolutely dry wood. Coniferous wood 1
A liquid volume corresponding to 7 to 19.5% by weight, in the case of hardwood timber, 15 to 19% by weight, ranges from 50 to 90%, preferably 60 to 85%, from the conduit 18 to the top of the infiltration vessel, conduits 19 and 20 0 to 45%, preferably 0 to 20%, from the transfer circulation line to the upper digestion trim zone A.
%, From conduits 19 and 21 via the circulation line
It is appropriately distributed to the C zone C in the range of 5 to 45%, preferably 10 to 40% as needed.

On the other hand, the alkali concentration at the zone inlet of the circulating liquid in the cooking zone is in the range of 5 to 35 g / l, preferably 8 to 30 g / l (effective alkali indication as Na ₂ O) in the upper cooking trim zone circulating liquid. MCC
The zone circulating liquid is 5-35 g / liter, preferably 8-
30 g / liter (effective alkali indication as Na ₂ O)
Range.

In the present invention, while maintaining the temperature and alkali concentration in the infiltration vessel, the upper cooking trim zone and the upper cooking zone at a constant level, the cooking temperature or the alkali addition rate of the MCC zone or the cooking temperature and the alkali addition rate are controlled. By selecting and changing either of them, the kappa number of the pulp is controlled, and the variation in kappa number is suppressed to σ _n-1 of 1 or less.

[0026] Such conclusions were obtained by calculating operating factors affecting the kappa number of the pulp discharged from the digester and its contribution rate, calculating a prediction formula for the target kappa number of the pulp, and conducting a number of experiments. It was obtained by performing regression analysis.

The operating factors include the temperature of each zone of the vertical digester, the inlet (not shown) and the outlet temperature and residence time of the heater provided in the circulation line of each zone,
H factor is calculated from these ([0031] ginseng
Irradiation), an alkali activity alkali alkali addition rate [oven dry wood chips per weight for each zone (NaOH + Na
₂ O ) ] , the alkali concentration of the circulating fluid in each zone
[The effective alkali (NaOH + 1 / 2Na) is indicated by an alkali concentration meter sensor provided in the circulation line.
₂ S ) concentration is displayed as Na ₂ O ] , and a formula for estimating a target kappa value predicted based on these factors is shown in Formula 1.

[0028]

[Formula 1] Prediction formula of target kappa value = A + a × (target kappa value−actual kappa value) + b × (alkali concentration in transfer circulating fluid, g /
Liter) + c × (H of upper cooking trim zone and upper cooking zone
Factor) + d × (H factor of MCC digestion zone) + e × (Activated alkali addition rate of MCC digestion zone,%) where A, a, b, c, d and e are constants.

The kappa number prediction formula shown in the formula 1 is incorporated into a high-order computer of the process computer,
The alkali concentration, H
Variables such as a factor and an alkali addition rate are determined at intervals of 0.5 to 60 minutes, preferably at intervals of 2 to 10 minutes, and an equation is set such that a predicted kappa number obtained based on this equation matches the target kappa number. The cooking variables inside are changed automatically. This formula also incorporates an automatic correction term for disturbances such as variations in wood chip mix, and the pulp kappa value measured manually, periodically or irregularly, is manually entered into a computer, Alternatively, an automatic sampling device is installed in the blow line, the kappa value of the pulp is automatically measured, the results are automatically input periodically, and a correction proportional to the difference between the predicted value and the actual value of the kappa value is automatically calculated. To always maintain the accuracy of automatic control of copper value.

The temperature in the MCC zone at a constant production rate is changed at a constant alkali addition rate, and when only the temperature is changed, 0.5 to 7.0 ° C., preferably 1.0 to 7.0 ° C.
~ 6.0 ° C. If the change in temperature is less than 0.5 ° C, the manifestation of the effect becomes extremely slow, and if it exceeds 7.0 ° C, not only the adjustment becomes difficult but also the pulp quality deteriorates, which is not suitable. Using this temperature, the H factor is calculated by Equation 2 and introduced into Equation 1.

[0031]

[Equation 2] H factor = (retention time, Hr) × e ^x And the activation energy of lignin contained in the wood chips was set to 32 Kcal / Mol.

The change of the alkali addition rate in the MCC zone is preferably 0.02 to 9.0% (expressed as active alkali Na ₂ O) per weight of the absolutely dry wood chips when the temperature is constant and only the alkali addition rate is changed. Is 0.05 to 7.5
It is changed in the range of%. The fluctuation of the alkali addition rate is 0.0
If it is less than 2%, the change in kappa number becomes extremely slow,
If it exceeds 9.0%, the kappa number rapidly changes, which is not desirable.

On the other hand, when both the temperature and the alkali addition rate are changed, the change of the temperature and the alkali addition rate is appropriately selected and combined within the above-mentioned range.

The same digestion control as described above is applied to the one-vessel vertical continuous digester, and the pulp obtained in this manner has a kappa number variation of 1 or less. Stable operation in the process (if any), multi-stage bleaching process, etc. is achieved, and in the production of bleached pulp, stable operation of the bleaching process and production of pulp of high quality become possible, and raw materials for unbleached paper In the production of unbleached pulp to be supplied to pulp, it is possible to produce pulp having stable brightness and paper strength.

[0035]

EXAMPLES The present invention will be specifically described below with reference to examples, but of course, the present invention is not limited thereto.

Examples 1 to 3 A two-vessel vertical continuous digester (NUKP8) shown in FIG.
(00 ADT / day), kraft digestion of a softwood wood chip consisting of 15% Douglas fir and 85% domestic pine was managed by computer under the standard digestion conditions shown in Table 1 to produce pulp having a kappa number of 25. The kappa number target value of the pulp was then changed to 27, (1) the cooking temperature in the MCC zone and the cooking temperature in the other zones were left as they were (the same as the standard one shown in Table 1), and 3.7% alkali addition rate per absolutely dry wood chip weight
From Example 1 to 2.9% and the total alkali addition rate was reduced from 18.2% to 17.4% as Example 1,
(2) Example 2 was an example in which the alkali addition rate was kept as it is (the same as the standard one shown in Table 1), and only the heater outlet temperature of the circulating liquid in the MCC zone was reduced from 164 ° C to 161 ° C to perform cooking. And (3) reducing the alkali addition rate to the MCC zone from 3.7% to 3.3%, and reducing the total alkali addition rate from 18.2% to 17.8%.
C zone circulating fluid heater outlet temperature from 164 ° C to 16
The example which reduced to 3 degreeC and digested was set to Example 3.

The cooking conditions used are shown in Table 1 together with the standard cooking conditions. In each case, pulp discharged from the digester was collected every 15 minutes by a blow line, and TAPP was collected.
The kappa number of the pulp was measured by the method described in Test Method T-236hm-85, and is shown in Table 2 together with the variation.

Examples 4-6 Pulps having a kappa number of 25 were produced by continuing cooking under the same standard cooking conditions as in Examples 1-3, and then the pulp kappa number target value was changed to 23. , (1) leave the cooking temperature in the MCC zone and the temperature in the other zones unchanged (same as the standard one shown in Table 3),
Increase the alkali addition rate in the zone from 3.7% to 4.5%
The total alkali addition rate from 18.2% to 19.0%
Example 4 was an example in which cooking was performed by increasing the temperature of the circulating fluid in the MCC zone from 164 ° C. to 167 ° C. (2) The alkali addition rate was kept as it was (the same as the standard one shown in Table 3). Example 5 where the temperature was increased to ℃ and the digestion was performed.
And (3) the alkali addition rate to the MCC cooking zone from 3.7% to 4.1%, and the total alkali addition rate to 1
Example 6 was an example in which the digestion was performed by increasing the temperature from 8.2% to 18.6% and further increasing the heater outlet temperature of the circulating liquid in the MCC zone from 164 ° C to 165 ° C.

The cooking conditions used are shown in Table 3 together with the standard cooking conditions. Table 4 shows the obtained pulp copper values and variations.

Comparative Examples 1-2 The cooking was continued under the same standard cooking conditions as in Examples 1-3, and then the alkali addition rate and temperature were changed, and (1) the target value of the pulp kappa number was set to 27. The modified example was referred to as Comparative Example 1, and (2) the example in which the target value of the pulp copper value was changed to 23 was referred to as Comparative Example 2. The cooking conditions used are shown in Table 5 along with the standard cooking conditions. The pulp kappa value was measured in the same manner as in Examples 1 to 3, except that the pulp sampling interval was changed every hour, and the results are shown in Table 6 together with variations.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

[Table 4]

[0045]

[Table 5]

[0046]

[Table 6]

As is clear from Tables 2, 4 and 6, the method of the present invention can obtain the target pulp kappa value very quickly and has little variation (see Examples 1 to 6). In the method (Comparative Examples 1 and 2), it takes a long time to obtain the target kappa value, and the dispersion is extremely large.

[0048]

According to the present invention, there is provided a cooking apparatus having a plurality of digestion zones, one of which is provided with an MCC zone capable of countercurrent cooking.
In a method of controlling wood digestion in a vessel and a 2-vessel vertical continuous digester, the target is controlled by operating either the digestion temperature in the MCC zone or the digestion conditions comprising the alkali addition rate or both the temperature and the alkali addition rate. The kappa value level of pulp can be achieved extremely easily and in a short time, and the dispersion can be reduced, so that pulp with stable quality can be manufactured and the operation in the post-process is stabilized. It has the effect of being able to.

[Brief description of the drawings]

FIG. 1 is a schematic flow chart showing the flow of wood chips, white liquor, various circulating liquids, and the like in a two-vessel liquid-phase vertical continuous digester.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Chip bin, 3 ... Steam vessel, 4 ... Pressurized feeder, 6 ... Permeation vessel, 8 ... Digester, A ... Upper digestion trim zone, B ... Upper digestion zone, C ... MCC zone, D ... Washing zone, 14, 15, 16 ... heater, 1
7 ... White liquid introduction pipe

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) D21C 3/02

Claims (2)

(57) [Claims] 1. A vertical infiltration vessel is installed in front of a vertical digester having a digestion zone, and an upper digestion trim zone, an upper digestion zone, and a lower digestion are provided inside the vertical digester from top to bottom. A zone and a washing zone are provided, an upper cooking trim zone and an upper cooking zone are co-currently cooked, and a lower cooking zone is a countercurrent cooking,
In the washing zone, when wood chips and an alkaline cooking liquor are introduced into a 2-vessel vertical digester in which high heat countercurrent washing is performed to digest wood chips, active alkali per weight of the absolutely dry wood chips is converted to (Na ₂ O). ) 50% of the alkaline cooking liquor added in the range of 15 to 20% by weight.
９０90% is added to the permeation vessel, 0-45% by weight is added to the upper cooking trim zone, 5-45% by weight is added to the lower cooking zone, and the temperature of each cooking zone is 140-175.
C., in a method of controlling the kappa number of pulp discharged from the bottom of the vertical digester from the bottom of the vertical digester according to TAPPI test method T-236hm-85 within the range of 12 to 55, using an infiltration vessel and an upper digestion trim. A method of controlling the kappa number of pulp by changing the cooking temperature and / or the alkali addition rate in the lower cooking zone while maintaining the temperature and alkali concentration in the zone and the upper cooking zone at constant levels. 2. A vertical digester is provided with an infiltration zone, a digestion zone, a lower digestion zone, and a washing zone from the top to the bottom, and in the infiltration zone and the digestion zone, the wood chips and the cooking liquor are co-current. In contact with the lower cooking zone, countercurrent cooking is carried out, and in the washing zone, the wood is placed in the same 1-vessel vertical cooking vessel as above except that it does not have a 1-vessel vertical digester in which high heat countercurrent washing is carried out and an infiltration zone. When cooking wood chips by introducing chips and an alkaline cooking liquor, 55 to 50% of alkaline cooking liquors to which active alkali is added in the range of 15 to 20% by weight (as Na ₂ O) per absolute dry wood chip weight. 95% by weight is added in the wood chip feed line and / or infiltration zone and / or cooking zone, 5 to 45% by weight is added in the lower cooking zone, And the temperature of each zone is 140 ~
A method for controlling the kappa number of pulp discharged from the bottom of a vertical digester within a range of 12 to 55, which is changed in the range of 175 ° C., wherein the temperature and alkali concentration in the infiltration zone and the digestion zone or the digestion zone are kept at a certain level. A method for controlling the kappa number of pulp by changing the cooking temperature and / or the alkali addition rate of the lower cooking zone while maintaining the kappa number.