FI120361B - Batch cooking process for the production of kraft pulp - Google Patents

Abstract

According to the invention, polysulfide is used in the manufacture of kraft pulp in the initial stage of a displacement batch process in order to improve the yield. Polysulfide-containing white liquor is added inter-mixed with the first liquor charged to the digester, or as a separate charge before the impregnation liquor is introduced. Preferably, a cooking catalyst, e.g. antraquinone, is added to the white liquor charge as the cooking stage commences.

Description

Batch cooking method for making KRAFT

FIELD OF THE INVENTION

The present invention relates to a process for making kraft pulp, wherein the lignocellulosic material is treated with recycled lye, polysulphide and freshly recrystallized white liquor to remove lignin therefrom. More particularly, the present invention relates to the use of polysulphide white liquor (or other source of polysulphide) in the early stages of modern displacement-krafter cooking, and the benefit thereof in the form of improved pulp yield.

BACKGROUND OF THE INVENTION

In various kraft cooking methods, lignocellulosic material, most often wood chips, is treated at elevated temperatures with an alkaline cooking liquor containing sodium hydroxide and sodium hydrogen sulfide, called white liquor. In these processes, a considerable amount of cellulosic material, especially hemicellulose, is partially wasted in reactions with white liquor. Under the conditions of making kraft pulp, hemicellulose is subjected to an alkaline cleavage reaction, depolymerization, whereupon the hemicellulose is dissolved in the cooking liquor as organic acids. It is a known fact that hemicellulose is stabilized by polysulphide treatment and is then more resistant to cooking liquor under kraft cooking conditions, which results in a higher yield of lignocellulosic material added to the process.

In kraft cooking methods, fresh cooking liquor is obtained from an inorganic melt of a recovery furnace, which is dissolved in water and causticized to produce white liquor. Polysulfide-containing white liquor is obtained from causticized white liquor by various processes which partially oxidize the caustic soda to polysulphide. These processes typically use oxidation catalysts to polymerize white liquor hydrogen sulfide, polysulfide can also be prepared directly by adding elemental sulfur to white liquor. The white liquor containing polysulfide 30 is called polysulfide white liquor or orange liquor.

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The use of polysulfide white liquor has been quite common in conventional kraft cooking operations. Normal white liquor has been converted to polysulphide white liquor and this liquor has been used in the same way as normal white liquor. After impregnation with wood chips in the digester and with the temperature rising above 100 ° C, the polysulfide has reacted with carbon 5 hydrates to make them less susceptible to alkaline decomposition. Later, during cooking, when the temperature has risen above 140 ° C, the remainder of the polysulfide is rapidly degraded by thermal decomposition. When all conventional cooking systems utilize cooking liquors with initial temperatures well below 100 ° C and slowly or gradually raising the temperature in the digester, the polysulfide has been able to react and contribute to higher mass yields.

However, the development of batch cooking systems has led to a significant deviation from conventional cooking technology: white liquor is fed into the digester as '' hot white liquor '' heated using the heat of the previously produced waste liquor, and steam.

For purposes of this disclosure, "hot" refers to a liquor having a temperature at ambient pressure above its boiling point, i.e., liquor to be stored in a pressurized boiler.

The temperature of hot white liquor in a typical displacement batch soup is 140-180 ° C. Polysulfide has been an impossible chemical for the batch-by-batch process. Polysulfide is unstable at higher temperatures, such as those used in kraft cooking: the chemical begins to decompose at temperatures above 100 ° C and completely decomposes at 150 ° C within minutes. Thus, the polysulphide white liquor is not at all stable at the typical hot white liquor process temperature. This has made it impossible to use polysulfide in 25 modern displacement batch cooking methods developed to achieve better energy efficiency, chemical economy and improved bulk quality.

New energy-saving krafter methods using different lye displacements appeared in the early 1980s. Characteristic of displacement cradle methods is that 30 hot black liquors are recovered from the end of the cooking by displacement and then reused in subsequent batches. This is done in part by using hot waste cooking liquor as it is and partly by heating the white liquor with the heat of the hot waste liquor leaving the digester system or being recycled at lower temperatures. This process arrangement results in higher temperatures in the feed liquor collectors. The low temperature white liquor, which is typically at 80 ° C in the storage tank after re-causticization, can no longer be used and the white liquor no longer has 5 stable polysulfide environments.

Good examples of the development of displacement batch processes are disclosed, for example, in Fagerlund, U.S. Patent 4,578,149 and Östman, U.S. Patent 4,764,251. Another feature of the modern displacement batch process is to combine energy efficiency with efficient use of residual and fresh cooking chemicals to achieve good pulp strength and good lignin removal, in addition to energy efficiency, hydroxyl and hydrogen sulfide concentration and temperature profiling, -B 520 452, Dart, U.S. Patent 5,183,535.

15 The goal of higher mass yield is easily understood. It is always more profitable to produce more pulp from a given amount of wood. The pulp industry has faced increasing pressure on the environment to radically reduce polluting emissions. Reducing organic waste from pulp bleaching requires kraft cooking to continue until the concentration of residual lignin in the pulp has been much lower than today. Prolonged cooking means greater carbohydrate loss. One popular option for the industry has been to use modern displacement batch technology to enable extended cooking while improving energy efficiency and pulp quality. Despite all the advantages of this technique, it has not been possible to exploit the pulp yield using polysulfide white liquor. The present invention avoids these problems and enables the efficient use of polysulfide by a novel displacement batch cooking method.

The use of anthraquinone (AQ) as a cooking catalyst is well known. It works by reducing carbohydrate end groups, stabilizing them against alkaline cleavage, and producing 30 anthraquinone, which is alkali-soluble. Anthrahydroquinone reduces lignin by making it more reactive. In this method, anthraquinone is reconstituted and can react with carbohydrates.

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During the operation of the displacement batch cooking process, it has been found that the properties of the black liquor from the early stages of cooking are different from those of the traditional liquor from the traditional soup. Recycled black liquor 5 from the early stages of the cooking process may complicate evaporation of the black liquor. A particular problem is the formation of deposits on the surfaces of the heat exchanger in the evaporator, which reduces the heat transfer. These evaporator contamination problems are typically related to calcium. In the initial stages of alkaline cooking, the calcium-containing material dissolves in the black liquor from the lignocellulosic material. In traditional soup, the soup proceeds by heating, the temperature rises, and there is no fluid exchange between the 10 women. Then, most of the dissolved calcium-containing material is decomposed in the cooking liquor, calcium carbonate is formed, and as a result, most of the calcium is re-absorbed into the lignocellulosic material in the digester. When such a cooking process is continued, evaporation of the black liquor can normally be carried out without the problems of calcium precipitation. In contrast, the problems of evaporation of black liquors from the early stages of cooking are typically related to the calcium-containing material dissolved in the early stages of cooking. The dissolved calcium-containing material is not degraded and the amount of calcium bound to the dissolved material in the black liquor is high. As described in EP 1 242 674, this problem can be solved by re-feeding the digester with a high calcium precipitation potential at the end of the cooking as a displacement liquor. At this point, the temperature of the digester is high enough to cause calcium release. The calcium is then removed along with the pulp and does not end up in the evaporation or recovery cycle.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process for making pulp using polysulfide white liquor. The method comprises a krafter furnace process which comprises adding polysulphide white liquor during impregnation and pre-treatment at the very beginning of the cooking process, and a final liquefaction step with white liquor and adding any cooking catalyst which is facilitated by the remainder of the initial process. A certain volume of polysulphide white liquor is added first as the liquor to be fed to the digester, or mixed with the impregnation liquor before being pumped into the digester. For purposes of this disclosure, "impregnation liquor" refers to a liquor stored at a temperature below its atmospheric boiling point and used for the first immersion treatment of a lignocellulosic feedstock in a batch cooking process.

Thus, in the present invention, the lignocellulosic material entering the displacement batch process has been pretreated with polysulfide-containing process liquors under conditions favorable to pretreatment and polysulfide stability. The following describes a series of displacement batch process arrangements capable of utilizing polysulfide white liquor. The required dosage amount of fresh polysulphide white liquor is led to the initial stage of the process. The impr-10 impregnation step provides both impregnation and polysulfide pretreatment reactions. The impregnation liquor cycle collects the displaced impregnation liquor, which is reused to fill and pressurize the digester after the first fresh liquor filling. As a result, polysulfide concentration and dosing remain high, and only an excess of lye equilibrium remains to be transferred for chemical recovery or later used in the cooking sequence. The displacement cooking process sequence is then continued with hot black liquor and white liquor fillings, cooking and final displacement.

An optional feature of the present invention is a novel way of using a cooking catalyst such as angtrakone (AQ). Because the formation of polysulphide white liquor reduces the concentration of hydrogen sulphide ions, the lignin removal rate is slower, which reduces cooking capacity. The cooking catalyst can be used to maintain a normal kraft cooking speed and production level. In the present invention, the catalyst can be introduced into a digester together with hot white liquor. In this way, it cannot be lost through displacements and, on the other hand, it enters the hot cooking stage where it is most needed. This allows the technical and economic benefits of 25 displacement batch soups to be preserved while fully exploiting the benefits of polysulphide and cooking catalyst.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a block diagram of a crafts displacement batch cooking system according to the invention.

The top of the figure shows the required containers, outgoing or incoming flows, indicated by letters. Below this is the chronological sequence of the cooking phase starting from the top and the vertically labeled streams showing the flows through the digester during the various phases. The letters representing the various input and output flows correspond to their source or destination in the container diagram.

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Figure 2 illustrates another embodiment of a craftsmanship batch cooking system according to the invention.

DESCRIPTION OF THE INVENTION

In the following, the method according to the invention will be explained in detail with reference to the accompanying Figure 1. It is clear that the liquor is removed from the bottom of the digester and the displaced gases 10 and the liquor are discharged from above.

Krafter rotation is started by feeding wood chips or other lignocellulosic material into the digester. After feeding the lignocellulosic material, the machine is filled in a stepwise sequence using a specific volume of fresh polysulphide white liquor X1 from polysulphide white liquor reservoir 6, and impregnation liquor A1 from reservoir 4 so that the digester is completely filled. The volume of polysulphide white liquor may be added as a separate first portion forming a lye raising pad when the second lye is pumped from below. Alternatively, the polysulfide white liquor is mixed with the impregnation liquor.

The overflow A2, which is led to the impregnation liquor container 4, is preferred for removing the diluted first front portion of the air and lye. After the flow A2 is stopped, the kettle is pressurized and the impregnation is completed within a suitable period of time. Thereafter, the cooking process is continued by pumping in a certain volume of hot black liquor B1 from the hot black liquor collector 1. The cooler impregnation liquor A3, which is displaced by the hot black liquor 25, is led to the impregnation liquor reservoir 4 for reuse in subsequent batches. The cooking sequence then continues by pumping into the digester a volume of hot white liquor C from tank 3. A certain amount of cooking catalyst can be added to the digester along with white liquor C. The liquor D1, which is displaced by the hot liquor approximately above the atmospheric boiling point, is led to size 2 of the hot black liquor.

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At the end of the sequence described above, the kettle is near the final cooking temperature. Final heating is performed using direct or indirect steam heating and digester recycling. After the desired cooking time, when the lignin removal has progressed to the desired final reaction stage, the waste liquor is ready to be displaced by washing filtrate E1 from vessel 5. The volume A4 from impregnation liquor vessel 4 can be used as first part of displacement liquor to deactivate 24

In the final displacement, the first displaced volume B2, displaced by the wash filtrate 10, is led to the black liquor collector 1. The second portion D2 of the displaced black liquor, diluted with the washing filtrate but still above the atmospheric boiling point, manufacturing process. The cooking sequence described above can then be repeated.

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The hot black liquor from the collector 2 is fed to an evaporator which is first used to heat the white liquor to the tank 3 and / or to heat the water by means of a heat exchanger.

The reservoir 6 is arranged to act as a reservoir of polysulfide white liquor from a polysulfide production plant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made to the figure explained in the preceding section.

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In the process of the present invention, a volume of polysulfide white liquor corresponding to 25 to 90% of the total active alkali dose, resulting in a dose of 0.5% to 5% polysulfide based on absolute dry wood, is added to the digester before the impregnation charge is completed. Impregnation is carried out using a temperature of 70 ° C to 120 ° C, preferably 85 ° C to 95 ° C, and a time period of about 20 minutes to about 120 minutes, preferably about 30 minutes to about 60 minutes, more preferably about 25 minutes to about 40 minutes.

8

In a preferred embodiment, the polysulfide white liquor is added as a separate portion before the addition of the next lye.

In one preferred embodiment, the volume of the polysulfide white liquor corresponds to 50-75% of the total active dose of alkali in the batch.

According to a particular embodiment of the process according to the present invention, a cooking catalyst is used. Preferred catalysts are anthraquinone, anthraquinone sulfonate and hydro anthraquinone. Any other suitable redox catalyst used by those skilled in the art may be used.

According to an alternative embodiment of the process of the present invention, the following portions are displaced from the digester: 15 - the hot digestion liquor fraction B2 and portions D1 and D2 of lower temperature and lower chemical concentration are collected in a collector for use in the hot black liquor pretreatment step. In this embodiment, the hot black liquor collectors 1 and 2 of Figure 1 are combined. The black liquor stream to the evaporator is taken from a single hot black 20 collector which is used to heat the white liquor and / or water to the white liquor tank 3 by means of a heat exchanger as shown in Fig. 1 for the tank 2.

If high dosage amounts of polysulfide white liquor are used in the impregnation step of the process, longer impregnation times than in the prior art and / or dual-tank system and / or transferring the impregnation liquor to the hot black liquor can be used to avoid excessive residual alkali concentration in the evaporator. Thus, according to another embodiment of the process of the present invention, shown in Figure 2, a certain volume of impregnation liquor from the impregnation vessel 4 is led to a tank of hot black liquor 1 for use in treatment with hot black liquor, B1. The current 9 from the impregnation vessel 4 to the hot black liquor tank 1 can be heated by means of a heat exchanger through a flow from the hot black liquor collector 2 to the evaporator.

In addition to the white liquor feed prior to the cooking step, one or more white liquor feeds may be performed during the cooking.

The main advantage of the method according to the present invention is that it provides a '' upstream end '' of the craft batch batch cooking system from a chemical point of view and in terms of process organization. The polysulfide feed used reflects an improvement in pulp yield from cooked lignocellulosic material.

The following examples illustrate the invention. The following abbreviations are used: WBL warm impregnation black liquor 15 DWBL displaced WBL (from digester) HBL hot black liquor RHBL displaced HBL (from digester) DCBL displaced digestate liquor WF wash filtrate 20 PSWL polysulfide white liquor

Preparation of Normal "Comparison" Pine Kraft Pulp Using Displacement Batch Technique: 4.0 kg of pine (Pinus sylvestris) chips (oven-dry) were measured in a chipping basket placed in a 20 liter jacketed displacement batch digester. The kettle lid was closed. The impregnating black liquor consisting of the washing filtrate and the white liquor (WBL, 80-90 ° C, 25gEA / L) was pumped in for about 10 minutes, followed by an impregnation at 90 ° C under an excess pressure of 2.5 bar. The impregnation step took 40 minutes (total WBL filling and impregnation combined). After impregnation, a hot black liquor pretreatment step was initiated by passing hot black liquor (HBL 1 at 150 ° C, about 20 gEA / L) to the bottom of the digester, displacing the waste impregnation liquor (DWBL) from the top of the digester. The pretreatment step with hot black liquor took 30 minutes. After 10 pre-treatment steps with hot black liquor, hot white liquor (121 gEA (NaOH) / l, 37% sulfide) was fed to the bottom of the digester, displacing a corresponding volume of hot waste black liquor (RHBL) from the digester. A heating step of 45 minutes with cooking-liquor circulation raised the temperature from 150 ° C to a cooking temperature of 167 ° C. With an H-factor of 250, the white liquor feed was fed to the bottom of the digester by displacing a corresponding volume of hot waste black liquor (DCBL 1) from the digester. After reaching the desired H-factor at cooking time, the final displacement was started by passing cool DWBL to the bottom of the digester and wash filtrate (WF) sequentially by displacing the hot waste black liquor from the top of the digester (DCBL 2). The proportion of the first displaced hot black liquor, together with the previously displaced hot black liquor (DCBL 2 + DCBL 1) earlier in the cooking step, covered the volume required for the next boiling hot black liquor (HBL 1). The final displacement step took approximately 40 minutes. After the final displacement, the pulp was disrupted, washed with deionized water, screened and analyzed. The cooking procedure was repeated to give equilibrium to the cooking process, and the soups began to repeat in line with the industrial batch cooking system.

An experimental arrangement for making pulp according to the invention is described below.

4.0 kg of pine (Pinus sylvestris) chips (oven-dried) were measured in a chips basket 20 placed in a 20 liter jacketed displacement batch digester. The kettle lid was closed. The chips were steamed until the digester temperature was 100 ° C for about 5 minutes. The impregnation liquors were sequentially introduced as follows: first, polysulphide white liquor black liquor (PSWL, about 4.5-5 liters feed volume) was introduced; next, recycled waste impregnation liquor (DWBL) was fed to hydraulically fill the digester. The impregnation-filling step lasted for about 11-13 minutes, and then the impregnation was carried out at 90 ° C under an excess pressure of 2.5 bar. The impregnation phase lasted 40 minutes (total filling and impregnation combined). After impregnation, a hot black liquor pretreatment step was performed by passing hot black liquor (HBL 1, 150 ° C, about 20 g / EA / L) to the bottom of the digester, displacing the waste impregnation liquor from the top of the digester (DWBL). The pretreatment step with hot black liquor took 30 minutes. After the pretreatment step with hot black liquor, a supply of hot white liquor (121 gEA (NaOH) / l, 37% sulphide content) was introduced by displacing a corresponding volume of hot waste black liquor (RHBL) from the digester. The cooking catalyst was fed with WL mixed with a small volume of hot black liquor. A heating step of 45 minutes with cooking liquor circulation raised the temperature from 150 ° C to a cooking temperature of 167 ° C. With an H-factor of 250 at the bottom of the digester, another white liquor feed was introduced by displacing a corresponding volume of hot waste black liquor (DCBL 1) from the digester. After reaching the desired H-factor at cooking time, the final displacement was started by successively introducing a certain volume of cool DWBL and wash filtrate (WF) to the bottom of the digester by displacing the hot waste liquor from the top of the digester (DCBL 2). The proportion of the first displaced hot black liquor, together with the previously displaced hot black liquor 10 during the cooking step (DCBL 2 + DCBL 1), covered the volume of 17 liters required for the next soup hot black liquor (HBL 1). After the final displacement, the pulp was disintegrated, washed with deionized water, sieved and analyzed. The cooking process was repeated to balance the cooking process, and the soups began to repeat in line with the industrial batch cooking system.

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EXAMPLE 1. Description of the Improvement in Yield obtained by Addition of Polysulfide White Alcohol

The reference soup follows the same addition and temperature steps as the soups of the invention. All soups were cooked until the same desired kappa number was reached. A clear improvement in yield was observed, which was proportional to the polysulfide addition.

PS Input,% Total Yield,% Rejects,% Cooking Calculated From Wood Kappa Calculated From Wood 25 Comparison - 25.9 44.0 0.26 SB-PSAQ 1.21 25.9 45.8 0.36 SB-PSAQ 1 , 58 25.8 46.8 0.24 30 Example 2. Effect of anthraquinone on maintaining lignin removal rate during cooking

It can be seen that the reference soup requires a significantly higher H-factor to achieve the desired kappa number 12

PSWL

in impregnation, PS,% yield,% reject,% method% wood H-factor EoC, g / 1 kappa # wood wood 5__ comparison - - 1285 16 26.9 43.6 1.01 PS pad, AQ 0% 70 % 1.45 1106 19.1 27.1 44.3 0.15 PS pad, AQ 0.05% 70% 1.45 1030 19.9 26.7 44.5 0.16 10 PS pad, AQ% 0 , 1 70% 1.45 1096 18.1 27.3 45.8 0.1

Claims (11)

A batch cooking process for making kraft pulp in an improved yield from lignin-containing cellulosic material, comprising the steps of - filling and pressurizing a boiler with a pre-batch impregnating liquor to which said impregnating liquor is blended with a specific volume, or a specific volume; impregnating the cellulosic material with the resulting impregnation liquor mixture; 10. replacing the impregnation liquor with hot waste cooking liquor; reacting the impregnated cellulosic material with said hot waste liquor; - displacing the hot waste cooking liquor with a certain volume of hot white liquor and cooking the cellulosic material with said white liquor until the desired degree of delignification is achieved; - displacement of the liquor used for cooking. The method of claim 1, wherein the amount of polysulfide white liquor added corresponds to 25 to 90% of the total active dose of the batch. 20 The method of claim 1, wherein the amount of polysulfide white liquor added corresponds to 50-75% of the total active dose of the batch. The method of claim 1, wherein the dosage amount of the polysulfide corresponds to 0.5 to 5% relative to absolute dry wood. The method of claim 1, wherein the volume of the polysulfide white liquor is added separately to the digester prior to the impregnation liquor. The process according to claim 1, wherein a certain amount of cooking catalyst is added to the hot white liquor. The process according to claim 6, wherein the cooking catalyst is anthraquinone, anthraquinone sulfonate, hydroanthraquinone or a redox catalyst derivative thereof. The method of claim 1, wherein the hot waste liquid displaced from the digester is collected in one hot liquor container. The method of claim 1, wherein after completion of the cooking, a certain volume of impregnation liquor is introduced into the digester as the first displacement liquor volume. 10 The method of claim 1, wherein a certain amount of impregnation liquor is transferred from the impregnation liquor container to the hot waste cooking liquor container. The method of claim 10, wherein the cooking system comprises two tanks of hot waste liquor.