EP3380667B1

EP3380667B1 - Method and system for oxygen delignification of cellulose pulp

Info

Publication number: EP3380667B1
Application number: EP16868982.6A
Authority: EP
Inventors: Håkan DAHLLÖF
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2015-11-27
Filing date: 2016-11-16
Publication date: 2020-01-08
Anticipated expiration: 2036-11-16
Also published as: BR112018005525B1; EP3380667A1; EP3380667A4; SE1551539A1; WO2017091129A1; SE540043C2; BR112018005525A2

Description

Background of the Invention

The present invention relates to a method and system for oxygen delignification.
A number of different processes for oxygen delignification have been disclosed, most with a focus on achieving a larger delignification at less order of viscosity losses. Some have been based upon findings in laboratory environment where conditions differ quite much compared to mill systems. In most laboratory tests is used a small experimental reactor charged with a small amount of pulp. Said laboratory reactor equipped with mixing capabilities, such as a quantum mixer, that induce a thorough mixing effect in the entire pulp sample with a weight of a couple of hundred grams. This kind of mixing effect may never be obtained in commercial mill systems, producing thousands of tons of bleached pulp per day, as the residence time for the pulp in mill mixers is quite short and often less than one second. While high order of delignification may be obtained in laboratory trials, the same results may be very difficult to reach in a commercial mill system.
The wood costs are a substantial part of a mills variable cost and measures to increase the pulping yield is normally very cost efficient. Cooking to higher Kappa no and maximizing delignification degree in oxygen delignification improves overall yield and reduce wood costs. However, this shift in higher kappa number in the pulp from the digester, increase the demand on delignification capabilities of the oxygen stage in a mill environment.
Basic principles in oxygen delignification with remixing and advantages therefrom in mill system was discovered as a result from production increase rebuilds, where existing single stage oxygen delignification obtained increased delignification degree by adding a second reactor ahead of the existing reactor with remixing in-between. Despite all kind of modifications of the old single stage system, the kappa number could not be further reduced with preserved pulp quality. However, after rebuilding to two phases in delignification with remixing in between kappa number could be further reduced without pulp quality deterioration. This experience from mill systems supported the fact that remixing in delignification or bleaching systems using gaseous agents needed remixing in order to expose the entire pulp suspension evenly to the delignification agents.
In a presentation entitled "Two-stage MC-oxygen delignification process and operating experience", which was given by Shinichiro Kondo, from the Technical Div. Technical Dept. OJI PAPER Co. Ltd., at the 1992 Pan-Pacific Pulp & Paper Technology Conference ('99 PAN-PAC PPTC), Sept. 8-10, Sheraton Grande Tokyo Bay Hotel & Towers, the positive effects of remixing between stages was disclosed, but then with highest pressure at start and least pressure in second reactor.
Advantages with heating in between phases in oxygen stage delignification in mill system was discovered when trying to optimize single stage oxygen delignification, using higher temperature and larger charges of alkali, a certain kappa number could be reached without too much penalties in pulp strength losses. This end point in delignification was often found at kappa numbers in the range 6-8.
US 5217575 presented an improved system for the oxygen bleaching of pulp of medium consistency where this temperature profiling surprisingly showed that a lower temperature in a first phase may result in better delignification. In US 5217575 was shown that if an optimized single stage operated at 105°C was preceded by a phase operated at 20-40°C lower temperature, a better delignification could be obtained at higher selectivity.
US6221206 disclosed an alternative for temperature profiling where the difference in temperature should be kept between 0°C and 15°C. The aim was to obtain an improved yield and an improved viscosity, while retaining the same dwell time, in connection with industrial implementation.
In order to improve oxygen delignification further has also been proposed to alter consistency during the oxygen delignification stage, and in US 4259150 was proposed to operate a first phase at lower consistency followed by thickening ahead of a following phase. This system was argued to function on high kappa pulp and by using repeated phases able to reduce kappa number from 70 down to 15.
SE-C 505 147 presents an alternative with instead a high pulp concentration, in the range of 25-40%, in a first phase and a lower concentration of 8-16% in a second phase, at the same time as the temperature in the second phase is to be higher than, or the same as, the temperature in the first phase. The advantages of the solution in accordance with SE-C 505 147 are stated to be the possibility of admixing more oxygen in the first high-consistency phase without the risk of channeling. Here were unused quantities of oxygen bled off after the first phase for further admixture in a second mixer prior to the second phase.
A high-pressure alternative has been disclosed in SE526843 , where the initial pressure in first reactor is established above 15 bar, and where the pressure was maintained at high level throughout all phases of the oxygen delignification stage. The idea is to increase the amount of oxygen dissolved in the liquid phase per liter liquid, and thus being able to react with the pulp. However, as been realized during the development of the present invention the amount of inert gases produced increases throughout the phases, the net amount of oxygen dissolved may in fact decrease as the total pressure in the system is only the sum of the partial pressures from each gas present (CO, CO₂, O₂, etc.).
Very few of the prior art solutions identify the problem with residual inert gases in the pulp suspension, and only US6391152 identifies this problem, but here is degassing proposed in a pressurized pump; "Where appropriate, it should also be possible to degas exhaust gases (residual gases) in immediate conjunction with the second pump, preferably by means of the pump being provided with internal degassing, preferably a pump termed a "degassing pump". Such type of degassing, performed in a centrifugal part of a pump separating some gases in center, may not extract all gases and especially not those gases dissolved in the liquid phase at the prevailing pressure.
In US6221207 is also degassing done in upper part of a down flow tower in the last phase. This specific system was developed for temperature profiling using low pressure steam between a first up-flow reactor and a subsequent down-flow reactor, where degassing from top of second reactor only kept the oxygen dissolved and oxygen remaining after venting. This system has never been used, and the risk is very high that the major part of all oxygen charged may be vented away from the second reactor, i.e. obtaining no reaction effect from the vented part of charge.
In US6514380 a method is disclosed, which comprises the steps of: cooking comminuted cellulosic fibrous material to produce brown stock having shives, washing the brown stock to produce chemical pulp at a consistency of between about 6-18 %, oxygen delignifying the chemical pulp and separating gas from the pulp during the oxygen delignification step, screening the oxygen delignified pulp to produce an accept fraction and a shive-containing reject fraction (where all the steps above are practiced in a main fiber line), further treating the accept fraction, and directly transporting the shive-containing reject fraction back to the main fiber line before the oxygen delignification step. The disclosed method is, however, not directed to improvement of the oxygen delignification step itself, and is, for example, silent about the pressures that prevail in the oxygen delignification step.
EP1061173 relates to a method for treating lignocellulosic pulp, which method comprises the steps of: mixing the lignocellulosic pulp with alkali in a pulp soaking stage to create a mixture, mixing said mixture with both oxygen and steam to form a combined steam, and feeding said combined steam to a pressurized oxygen delignification stage to enable a reaction between the oxygen and the lignin in the lignocellulosic pulp, to remove colored materials from the lignocellulosic pulp. The disclosed method is directed to reduction of capital equipment by obviating one stage of a two-stage oxygen delignification plant, which is achieved by the extra addition of alkali before the oxygen delignification step. Apart from the investments costs, the general advantages of a two-stage process are, however, acknowledged also in this document.
CA2190573 relates to a method for delignifying chemical pulp, comprising the steps of: feeding cooked pulp having a temperature above 70 °C from the brown stock washing to the delignification stage, adding alkali and oxygen and mixing the alkali and oxygen into the pulp, where at least part of the alkali is unoxidized white or green liquor. The claimed process is said to preferably be carried out in at least two delignification steps and appears in practice to require an extra gas separating step between two delignification steps. In reality, on a pulp mill scale, there is, however, still a question how much the use of unoxidized white liquor impact the kappa reduction in the oxygen delignification stage.
The greater part of the prior art has consequently been aimed at keeping the pulp suspension pressurized throughout all oxygen delignification stages, most often pressurized to highest pressure in inlet to first stage and using this pressure to be the driving force for pulp flow throughout the system, which makes sense as the installed pumping effect may be kept to a minimum.
Besides the optimization of the delignification conditions in the stages is also filtrate handling in the oxygen delignification having an impact on kappa number reduction and pulp strength. Usage of dirty filtrate with high COD content in the oxygen delignification calls for high amount of oxygen charge, as a large part of the oxygen charge is consumed in reaction with the COD in the filtrate.
Also, if high kappa pulp, with kappa numbers after cook in the range 40-120, where a kappa number above 20 may be considered high for hardwood, is to be delignified, large amounts of oxygen is needed for delignification that may result in channeling in the stages. With high kappa number in pulp is also a large part of the oxygen charge consumed during delignification of the high kappa pulp.
So, the combination of high kappa pulp and filtrates with high level of COD impose immense problems for charging and mixing the necessary amount of oxygen to the pulp suspension
In following parts is the normal terminology for oxygen delignification stages used. A stage, i.e. an oxygen delignification stage in this case, is the treatment done between two wash positions. What is done in said stage may also be divided into phases, with some changes effectively brought into next phase in aspects of chemical charges, heating and/or venting. Also, each phase may in turn be divided into zones where a first and a second zone of one phase may differ as to changes of chemical charges, heating and/or venting.
In summary, an oxygen delignification stage between 2 wash positions may be divided into phases, and each phase may in turn be divided into zones. This nomenclature is used throughout the description even if some prior art documents describe 2-phase delignification systems as 2 stage systems.
The wash positions may be performed in a number of ways, for example;

Using wash presses, where a displacement washing is performed of the pulp followed by a dewatering to high consistency;
Using wash filters, but washing effect is obtained by diluting down the pulp to low consistency and a subsequent dewatering on the drum;
Using drum displacement wash machines, and a displacement wash of pulp is performed on the drum and optionally finished by some thickening process using vacuum draining or press rolls;
Using simple dilute and dewater machines, i.e. dilute the pulp to low consistency and subsequently dewatering the low consistency pulp to medium or high consistency pulp using a dewatering screw.

In this description are low, medium and high consistency of pulp used to define the amount of liquid in the pulp suspension. Low consistency (LC) in pulp is typically in the range 2-8%, medium consistency (MC) in the range 8-18%, and high consistency (HC) typically above 30%. LC pulp is so diluted that it behaves like water and the pulp suspension may be readily pumped by conventional liquid pumps. MC is an intermediate range that is pumpable but requires special MC-pumps that fluidize the suspension, while high consistency pulp is not pumpable by conventional pumps and instead require transport screws etc. for feeding and transport.
In this description is also COD content of the filtrate used, COD standing for "Chemical Oxygen Demand" and is a parameter describing the total content of oxidizable material in the filtrate.

SUMMARY OF THE INVENTION

One common aim of the invention is to improve oxygen delignification further and optimizing the order of oxygen dissolved in the pulp suspension such that the oxygen may react with the cellulose to a far greater extent than previously possible and make the oxygen delignification better suitable to high kappa cooking.
While the shift to high kappa cooking and increased delignification order in oxygen stages has been known for many years to be beneficial for pulp strength from laboratory trials, mill implementations has been less frequent due to difficulties adding the necessary amounts of oxygen in a reactive phase after high kappa cooking.
It has been found during laboratory trials that the inert gases, especially CO and CO₂, that are formed during the oxygen stage acts parasitically on the solubility of oxygen in the liquid phase, and more or less substitutes the amount of dissolved oxygen. For the efficiency of the delignification effect on the cellulose pulp it is of outmost importance that the oxygen to the greatest extent possible is dissolved in the liquid phase such that the liquid may penetrate the cellulose fiber network and get in contact with the oxidizable material, i.e. lignin, in the cellulose fiber matrix.
In laboratory trials with multi-phase delignification is often the small test reactor filled with an excess amount of oxygen, i.e. the pulp sample is not in a hydraulically filed volume instead only occupies 5-10 of the entire test reactor, so these testing's may never come close to real environment in reactors being hydraulically filled with pulp suspension and where oxygen is not in excess.
Another aim is enable almost a total withdrawal of all inert gases that are formed and dissolved in the pulp suspension after a first phase of the oxygen delignification stage, where the oxidation process is fastest and produces most of the inert gases. By dropping the pressure of the entire pulp suspension to a pressure close to atmospheric pressure, could both the content of non-dissolved gases be removed, but also a large part of gases being dissolved in the liquid at higher pressure. A flashing effect is thus obtained. By venting away the inert gases before charging a fresh charge of oxygen to the pulp suspension could also the amount of oxygen being dissolved in the liquid phase be increased considerably as competing gas content has been reduced to a large extent.
The method according the invention is related to oxygen delignification of a medium consistency cellulose pulp suspension having a kappa number exceeding 18, and said pulp suspension passing through a first and a second oxygen delignification phases located between 2 wash positions (W₁, W₂) for the pulp where said phases are separated by heating the pulp at least 5°C using steam after the first phase and ahead of the second phase, and both phases includes charge of alkali and oxygen to each of said first and second phase. The inventive features of the method are that both the first and second phase is pressurized to an initial pressure exceeding 5 bar, and after the first phase is the pulp depressurized to an excess pressure below 0.5 bar, preferably depressurized to atmospheric pressure, i.e. within a pressure interval between 0 to 0.5 bar, and residual gases released from the pulp during depressurization are vented away before the start of the second phase, and wherein the charge of oxygen to the second phase is charged to the pulp after the residual gases has been vented away, and wherein the charge of oxygen to the second phase is distributed into the pulp using a mixer, thus establishing increased partial pressure of the oxygen in the pulp suspension. This conceptual principle enables better delignification at higher viscosity, i.e. improved selectivity of the oxygen delignification stage.
The method is further distinguished in that more than 90% of the inert gases, such as carbon monoxide and carbon dioxide, formed during the oxidation process in the first phase, are vented away by the depressurization, and wherein the pulp suspension for the depressurization effect is fed to an upper part of a standpipe and exposed to pressure in the range 0-0.5 bar in the upper part of the standpipe having a height exceeding 3 meter. This venting principle applied here reduce the pressure of the pulp suspension to such a level that the solubility all gases decreases, i.e. follows Henry's law, and enable efficient degasification in the stand pipe volume. It is specifically the inert gases formed during the first phase of delignification that needs to be removed. Even if some residual oxygen may be lost in this venting is the parasitic effect on oxygen solubility by the competing inert gases more dominant, and by proper dosing of the oxygen charge to the first phase could most of the oxygen charged by consumed in the first phase.
In a preferred embodiment of the method is the first phase divided into at least 2 zones, and both zones include charge of oxygen to each of said first and second zones of the first phase. This division of the first phases into multiple zones, i.e. at least 2 zones, enable a full utilization of the reaction kinetics of oxygen delignification, as most of the reactions in an oxygen delignification stage occurs during the first half of the total retention time of the oxygen delignification stage, and the bulk volume of inert gases are formed in this first half.
In yet a preferred embodiment of the inventive method is the retention time for the pulp suspension in the first phase shorter than the retention time in the second phase, and that the retention time of the first zone in the first phase is shorter than the retention time of the second zone in the first phase. This sizing of reactors may enable an equal amount of oxygen charged to each phase, near the charge volume possible to mix into the pulp suspension with the mixers used, said charge being further optimized such that more than 90% is consumed in each phase and is not wasted in the following degassing step.
In another embodiment of the inventive method is the medium consistency cellulose pulp suspension fed to the first phase obtained from a preceding thickening process producing a high consistency cellulose pulp at a consistency above 30%, and that the medium consistency cellulose pulp suspension is produced by mixing the high consistency cellulose pulp from the thickening process with oxidized filtrate. Using oxidized filtrate for forming the medium consistency pulp avoids adding oxidizable matter into the pulp suspension, which oxidizable matter produce hydroxyl radicals during the oxygen stage and in presence of cellulose pulp reduce the viscosity of the pulp.
In a final embodiment of the inventive method is the pulp suspension after the first zone in the first phase depressurized to an excess pressure below 0.5 bar, preferably depressurized to atmospheric pressure, i.e. within a pressure interval between 0 to 0.5 bar, and residual gases released from the pulp during depressurization are vented away before the start of the second zone of the first phase, and wherein a charge of oxygen to the second zone in the first phase is charged to the pulp after the residual gases has been vented away, and wherein the charge of oxygen to the second zone of the first phase is distributed into the pulp using a mixer, thus establishing increased partial pressure of the oxygen in the pulp. This repeated venting of inert gases formed initially in the oxygen stage is especially suitable for oxygen delignification of high kappa pulps and when the filtrate added contains high concentration of COD, which in total introduce huge amounts of oxidizable material to the oxygen stage, and thus generates extreme amounts of inert gases during the oxygen delignification.
From a system point of view the invention comprises a system for oxygen delignification of a medium consistency cellulose pulp suspension having a kappa number exceeding 18, and where said system is located between 2 wash positions, said medium consistency pulp suspension first pressurized by a first pump passing the pulp to at least a first oxygen mixer and directly thereafter to at least a first oxygen delignification reactor in a first delignification phase. After the first delignification phase is the pulp suspension heated in a steam mixer ahead of a second phase in a second oxygen delignification reactor such that said phases are separated by heating the pulp at least 5°C using steam after the first phase and ahead of the second phase, and both phases include charge of alkali and oxygen to each of said first and second phase. The system is further distinguished in that both the first and second phase is pressurized by said pumps to an initial pressure exceeding 5 bar in each of the oxygen delignification reactors, and after the first phase is the pulp depressurized to an excess pressure below 0.5 bar over a valve located in an outlet from the first oxygen delignification reactor, preferably depressurized to atmospheric pressure, i.e. within a pressure interval between 0 to 0.5 bar, in a vented standpipe. Residual gases released from the pulp during depressurization are vented away in a degassing line connected to the standpipe before the start of the second phase, and wherein the charge of oxygen to the second phase is charged to a second oxygen mixer to the pulp after the residual gases has been vented away, and wherein the charge of oxygen to the second phase is distributed into the pulp using the second mixer, thus establishing increased partial pressure of the oxygen in the pulp in a second oxygen delignification reactor.
The system is designed in such a way that more than 90% of the inert gases, such as carbon monoxide and carbon dioxide, formed during the oxidation process in the first phase, are vented away by the depressurization over the valve located in the outlet from the first oxygen delignification reactor. The pulp suspension for the depressurization effect is fed from the valve and to an upper part of a standpipe and exposed to pressure in the range 0-0.5 bar in the upper part of the standpipe having a height exceeding 3 meters. A stand pipe connected to atmosphere and with this height enable a sufficient practical volume for establishing a retention time in this stand pipe that may allow inert gases to separate over time. Even though it is not necessary, additional equipment may be added in the standpipe to improve separation of inert gases, such as stirrers or sonication/ultrasonication, but the costs are most often not motivated versus the effect obtained, as most of the separation effect occurs when the pressure is reduced suddenly over the valve and ahead of the standpipe.
Further, the system is designed such that the first phase is divided into at least 2 zones, with reactors in each zone and both zones includes charge of oxygen to each of said first and second zones of the first phase using mixers ahead of each zone. As indicated above, this staging of zones enables maximal charges of oxygen to each phase and zones, without causing losses of oxygen in subsequent degassing and minimum risks for gas channeling in the system. This kind of staging is also preferably designed such that the retention time for the pulp suspension in the first phase is shorter than the retention time in the second phase, and that the retention time of the first zone in the first phase is shorter than the retention time of the second zone in the first phase, said retention times established by increasing storage volume in reactors of each phase or zone.
In the final embodiment of the inventive system is the medium consistency cellulose pulp suspension fed to the first phase obtained from a preceding thickening process producing a high consistency cellulose pulp at a consistency above 30%, and that the medium consistency cellulose pulp suspension is produced by mixing the high consistency cellulose pulp from the thickening process with oxidized filtrate obtained from a filtrate tank in a wash arranged after the last phase and having passed through an oxidizing reactor (RE). This additional reactor is typically only needed when the COD levels in the filtrate is above 100 g/l, which may be the case if high or medium kappa pulp is fed to the oxygen stage.
The invention is described in more detail with reference to the figures in accordance with the following figure list.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a first embodiment of the inventive oxygen delignification system suitable for incoming pulp with low kappa numbers and low concentration of COD in the filtrate obtained in the oxygen wash, and
Fig. 2 shows a second embodiment of the inventive oxygen delignification system, suitable for incoming pulp with medium kappa numbers or medium concentration of COD in the filtrate obtained in the oxygen wash, and
Fig.3 shows a third embodiment of the inventive oxygen delignification system, with similar set-up as in figure 2 but with an additional venting also after the first zone in the first stage, and optional degassing pumps, suitable for incoming pulp with medium kappa numbers and medium concentration of COD in the filtrate obtained in the oxygen wash; and
Fig.4 shows a fourth embodiment of the inventive oxygen delignification system, with similar set-up as in figure 2 but with an additional oxidizing reactor for the filtrate returned to the oxygen stage; suitable for incoming pulp with high kappa numbers and high concentration of COD in the filtrate obtained in the oxygen wash.

In all figures are reactors indicated to be hydraulically filled with the pulp suspension, and in the same manner is the retention volume of the pulp suspension in the stand pipes indicated in the same way.

DETAILED DESCRIPTION

In figure 1 is disclosed a first embodiment of the inventive oxygen delignification stage located between 2 wash positions W₁ and W₂ in a fiberline producing bleached qualities of cellulose pulp. The pulp may be obtained from any kind of continuous or batch chemical pulping process and even mechanical pulp, but most preferably the pulp is obtained from a continuous kraft pulping process.
The oxygen delignification system is typically located directly after a pulp screening stage or immediately preceding said pulp screening stage.
Normally a number of bleaching stages follows said oxygen delignification, including chlorine dioxide delignification and/or bleaching, alkali extraction, peroxide bleaching, ozone bleaching, hot acid treatment, chelating stages, etc.
Pulp is fed to the oxygen delignification stage from a wash position W₁. In this case a wash press where the pulp fed to a standpipe 10 has a high consistency and the wash filtrate sent to a filtrate tank FT₁. The pulp is diluted to medium consistency in the standpipe by adding filtrate from the filtrate tank FT₂ that is obtained from the second wash press W₂, together with the bulk charge of alkali necessary for establishing the alkaline conditions for the oxygen stage.
The alkali charge is typically in the order of 25 kg/ADT pulp and the consistency about 12%. A protector in form of a minor charge of MgSO4 is also charged. The temperature of the pulp suspension in this stage is typically around 85-95°C.
The pulp suspension is initially pressurized by a first pump P₁ to a pressure above 5 bar(e), and preferably about 10 bar, and this pump also bring about a mixing effect of the added liquid charges (alkali, in form of sodium hydroxide oxidized white liquor or white liquor, and filtrate). The conditions are now set for starting the oxygen delignification which starts with passing the medium consistency cellulose pulp through a mixer M₂ dedicated for mixing oxygen into the pulp suspension. Once the oxygen is mixed into the pulp suspension, the pulp suspension is fed to an upflow tower, i.e. an oxygen delignification reactor O2_1B, wherein the oxygen delignification process proceeds. In this initial phase of the oxygen delignification the consumption of alkali and oxygen is extremely high and a lot of inert gases such as CO and CO₂ are formed. Due to the exothermic heat release during the delignification is the temperature increased some 5°C, i.e in the range 1-7°C. However, in some mill systems the heat release is quite large and rapid initially, indicating a high oxidation rate.
At the end of the first phase is the pulp depressurized from a residual pressure of about 6 bar(e), (if the initial pressure was about 10 bar) in top of reactor O2_1B and sent to a standpipe 11, typically held at atmospheric pressure and vented directly to atmosphere. The standpipe is sufficiently high, typically more than 5 meter, and enable also inert gases dissolved in the liquid phase to release over time. In the bottom of the standpipe 11 will most of the inert gases formed in first phase have been released and vented to atmosphere.
Next pump P₂ pressurize the pulp suspension again to a pressure above 5 bar(e), and preferably about 10 bar, and this pump also bring about a mixing effect of the added liquid charges (alkali). The conditions are now set for starting a second phase of the oxygen delignification which starts with passing the medium consistency cellulose pulp through a second mixer M₃ dedicated for mixing a second fresh charge oxygen into the pulp suspension.
This second mixer M₃ is preferably also used to mix in steam. The amount of steam added is typically in the order that an increase of about 5°C is obtained in the pulp suspension, reaching a temperature of about 100-105°C. The order of temperature increase is dependent on the amount of exothermic heating in previous stages. Once the second charge of oxygen is mixed into the pulp suspension, the pulp suspension is fed to a second or final upflow tower, i.e. an oxygen delignification reactor O2₂, wherein the oxygen delignification process proceeds in a second phase. In this second or final phase of the oxygen delignification stage the consumption of alkali and oxygen is moderate and the amount of inert gases such as CO and CO₂ formed are produced at a far lower rate and at less volume.
At the end of the second phase is the pulp depressurized from a residual pressure of about 6 bar(e), (if the initial pressure was about 10 bar) in top of reactor O2₂ and sent to a standpipe 12, typically held at atmospheric pressure and vented directly to atmosphere. The standpipe is sufficiently high, typically more than 5 meter, and enable also inert gases dissolved in the liquid phase to release over time. In the bottom of the standpipe 12 will most of the inert gases formed in first phase have been released and vented to atmosphere.
After finishing of the oxygen delignification, the vented pulp suspension is fed to a final wash press W₂, conventionally called the oxygen wash position, that ends the oxygen delignification stage. The wash filtrate obtained from this final wash press W₂ is collected in a second filtrate tank FT₂, and used as dilution liquid in the first standpipe 10. This wash filtrate is also normally used as washing/displacement liquid in the first wash press W₁.
The embodiment shown in figure 1 is preferably used for oxygen delignification of low kappa pulp and with filtrate having low content of COD. By venting the suspension down to atmospheric conditions after the first phase could almost all of the inert gases formed be vented away from the pulp suspension, and in the subsequent second charge of oxygen could an optimal amount of oxygen be dissolved into the pulp suspension without competing with residual amounts of inert gases.
In figure 2 is disclosed a second embodiment of the inventive oxygen delignification stage located between 2 wash positions W₁ and W₂ in a fiberline producing bleached qualities of cellulose pulp. This second embodiment differs from the first embodiment by an additional pre-reactor O2_1A forming a first zone in the first phase. This additional pre-reactor adds yet an oxygen delignification with an additional mixer M₁ ahead of this additional pre-reactor O2_1A, and may suitably be designed as a vertical pipe loop, or using two standard standpipes together with an upper bend connecting the standpipes. In this embodiment is no venting after the pre-reactor O2_1A used, and the second zone of the first phase may follow directly in reactor O2_1B.
The embodiment shown in figure 2 is preferably used for oxygen delignification of medium kappa pulp or with filtrate having medium concentration of COD, where a slightly larger order of oxidation takes place in first phase.
In figure 3 is disclosed a third embodiment of the inventive oxygen delignification stage located between 2 wash positions W₁ and W₂ in a fiberline producing bleached qualities of cellulose pulp. This third embodiment differs from the first embodiment by an additional pre-reactor O2_1A forming a first zone in the first phase, and a subsequent standpipe 14. This additional standpipe 14 adds yet a position for venting of inert gases from the pulp suspension. Additionally, in this embodiment is also degassing MC-pumps used in all MC pumps P_1A, P_1B, P₂ and P₃, enabling venting of inert gases ahead of all oxygen mixers M₁, M₂ and M₃, as well as ahead of the final wash W₂.
The embodiment shown in figure 3 is preferably used for oxygen delignification of medium kappa pulp and with filtrate having medium concentration of COD, where a slightly larger order of oxidation takes place in first phase.
Finally, in figure 4 is disclosed a fourth embodiment of the inventive oxygen delignification stage located between 2 wash positions W₁ and W₂ in a fiberline producing bleached qualities of cellulose pulp. This fourth embodiment differs from the third embodiment by an additional oxidation reactor RE for the filtrate pumped by pump P₄ from the filtrate tank FT₂ and to standpipe 10.
Both oxygen and alkali may be added to the oxidation reactor RE, as well as additional oxidation chemicals such as peroxide (H₂O₂). In this ultimate version of the invention may most of the oxidant added to the pulp suspension be used for delignification of the lignin content of the pulp, and almost no oxidation of dissolved lignin or other organic content in the liquid phase of the pulp suspension takes place in presence of pulp, avoiding viscosity losses in the pulp due to formation of hydroxyl radicals when oxidizing COD content in the liquid phase.
The embodiment shown in figure 4 is preferably used for oxygen delignification of high kappa pulp and with filtrate having high concentration of COD, where the largest order of oxidation takes place in first phase.
The residence time in reactors O2_1A, O2_1B and O2₂ should be continuously increasing. Typically, the residence time in a pre-reactor like O2_1A may be in the range 1-10 minutes, the residence time in a second reactor like O2_1B in the second zone of the first phase may be in the range 20-60 minutes, and the residence time in a reactor like O2₂ in the second or final phase may be in the range 40-180 minutes.
If only two reactors are used, i.e. O2_1B and O2_2, are used, the residence time of the first phase may be shorter than 20 minutes, and preferably in the range 5-60 minutes in total, followed by a residence time in the range 40-180 minutes for the second phase.
In all embodiments shown are the inert gases formed during a first phase of the oxygen delignification stage vented away ahead of the final phase in the oxygen delignification stage, such that the oxygen charge added to the final phase may obtain highest possible level of oxygen solved in the liquid phase of the pulp suspension. In some applications where the oxidation produces more inert gases, with higher kappa number and/or higher COD content in filtrate, may additional venting be implemented also after a first zone in the first phase.
What embodiment of the invention is to be chosen depends on the rate and amount of oxidation that occurs in first phase, which order of oxidation may be controlled by measuring the temperature profile of the pulp suspension.
If the temperature increase due to the exothermic reactions are modest and slow it may be fully sufficient with a 2-reactor system, with venting in between, and further costs for a third reactor and additional mixer could likely not be motivated by the smaller improvement in selectivity.
If the temperature increase is very rapid in the first phase it is fair to assume that the filtrate contains high COD content, as this COD content is much easier to oxidize than the organic matter bound to the cellulose, and hence the motivation to invest in a filtrate oxidation reactor RE may be fully motivated by the improvement in selectivity.
Accordingly, it is to be understood that the embodiments disclosed are potential embodiments of the present invention and has been described by way of illustration and not limitation. Features from the fourth embodiment, such as the oxidation reactor for the filtrate, may for instance also be implemented in a 2-reactor system, and degassing MC pumps may also be used as a complement in such 2 reactor system.

Claims

A method for oxygen delignification of a medium consistency cellulose pulp suspension having a kappa number exceeding 18,
said pulp suspension passing through a first and a second oxygen delignification phases located between 2 wash positions (W₁, W₂) for the pulp where said phases are separated by heating the pulp at least 5°C using steam after the first phase and ahead of the second phase, and both phases includes charge of oxygen to each of said first and second phase; characterized in that

both the first and second phase is pressurized to an initial pressure exceeding 5 bar,

and after the first phase is the pulp depressurized to an excess pressure below 0.5 bar, preferably depressurized to atmospheric pressure, i.e. within a pressure interval between 0 to 0.5 bar

and residual gases released from the pulp during depressurization are vented away before the start of the second phase,

and wherein the charge of oxygen to the second phase is charged to the pulp after the residual gases has been vented away, and wherein the charge of oxygen to the second phase is distributed into the pulp using a mixer, thus establishing increased partial pressure of the oxygen in the pulp suspension.
A method according to claim 1 characterized in that more than 90% of the inert gases, such as carbon monoxide and carbon dioxide, formed during the oxidation process in the first phase, are vented away by the depressurization, and wherein the pulp suspension for the depressurization effect is fed to an upper part of a standpipe and exposed to pressure in the range 0-0.5 bar in the upper part of the standpipe having a height exceeding 3 meter.
A method according to claim 1 characterized in that the first phase is divided into at least 2 zones, and both zones includes charge of oxygen to each of said first and second zones of the first phase
A method according to claim 3 characterized in that the retention time for the pulp suspension in the first phase is shorter than the retention time in the second phase, and that the retention time of the first zone in the first phase is shorter than the retention time of the second zone in the first phase.
A method according to claim 1 characterized in that the medium consistency cellulose pulp suspension fed to the first phase is obtained from a preceding thickening process producing a high consistency cellulose pulp at a consistency above 30%, and that the medium consistency cellulose pulp suspension is produced by mixing the high consistency cellulose pulp from the thickening process with oxidized filtrate.
A method according to claim 3 characterized in that and after the first zone in the first phase is the pulp depressurized to an excess pressure below 0.5 bar, preferably depressurized to atmospheric pressure, i.e. within a pressure interval between 0 to 0.5 bar
and residual gases released from the pulp during depressurization are vented away before the start of the second zone of the first phase,

and wherein a charge of oxygen to the second zone in the first phase is charged to the pulp after the residual gases has been vented away, and wherein the charge of oxygen to the second zone of the first phase is distributed into the pulp using a mixer, thus establishing increased partial pressure of the oxygen in the pulp.
A system for oxygen delignification of a medium consistency cellulose pulp suspension having a kappa number exceeding 18, and where said system is located between 2 wash positions (W₁, W₂)
said medium consistency pulp suspension first pressurized by a first pump (P_1A) passing the pulp to at least a first oxygen mixer (M₂) and directly thereafter to at least a first oxygen delignification reactor (02_1B) in a first delignification phase, and after the first delignification phase is the pulp suspension heated in a steam mixer (M₃) ahead of a second phase in a second oxygen delignification reactor (02₂) such that said phases are separated by heating the pulp at least 5°C using steam after the first phase and ahead of the second phase, and both phases includes charge oxygen to each of said first and second phase; and charge of alkali to at least first phase characterized in that

both the first and second phase is pressurized by said pumps to an initial pressure exceeding 5 bar in each of the oxygen delignification reactors,

and after the first phase is the pulp depressurized to an excess pressure below 0.5 bar over a valve (V₂) located in an outlet from the first oxygen delignification reactor, preferably depressurized to atmospheric pressure, i.e. within a pressure interval between 0 to 0.5 bar, in a vented standpipe (11),

residual gases released from the pulp during depressurization are vented away in a degassing line (DG₁) connected to the standpipe before the start of the second phase,

and wherein the charge of oxygen to the second phase is charged to a second oxygen mixer (M₃) to the pulp after the residual gases has been vented away, and wherein the charge of oxygen to the second phase is distributed into the pulp using the second mixer, thus establishing increased partial pressure of the oxygen in the pulp in a second oxygen delignification reactor (02₂).
A system according to claim 7 characterized in that more than 90% of the inert gases, such as carbon monoxide and carbon dioxide, formed during the oxidation process in the first phase, are vented away by the depressurization over the valve (V₂) located in the outlet from the first oxygen delignification reactor (O2_1B) and wherein the pulp suspension for the depressurization effect is fed from the valve and to an upper part of a standpipe and exposed to pressure in the range 0-0.5 bar in the upper part of the standpipe having a height exceeding 3 meter.
A system according to claim 7 characterized in that the first phase is divided into at least 2 zones, with reactors in each zone and both zones includes charge of oxygen to each of said first and second zones of the first phase using mixers ahead of each zone.
A system according to claim 9 characterized in that the retention time for the pulp suspension in the first phase is shorter than the retention time in the second phase, and that the retention time of the first zone in the first phase is shorter than the retention time of the second zone in the first phase, said retention times established by increasing storage volume in reactors of each phase or zone.
A system according to claim 7 characterized in that the medium consistency cellulose pulp suspension fed to the first phase is obtained from a preceding thickening process (W₁) producing a high consistency cellulose pulp at a consistency above 30%, and that the medium consistency cellulose pulp suspension is produced by mixing the high consistency cellulose pulp from the thickening process with oxidized filtrate obtained from a filtrate tank in a wash arranged after the last phase and having passed through an oxidizing reactor (RE).