Process and apparatus for white liquor oxidation

Abstract

In a process for oxidation of white liquor, in particular of white liquor used in a process for production of paper or cellulose, the oxygen required for the oxidation is supplied to the reactor or the reactors in which the oxidation is carried out at least partially in the form of oxygen-containing nanobubbles. Due to the relatively long lifetime of the nanobubbles this very efficiently provides oxygen also for oxidation reactions in the white liquor proceeding at different rates.

Description

[0001] The invention relates to a process for the oxidation of white liquor, in which white liquor is brought into contact with oxygen in a reactor, thereby oxidizing sulfur compounds in the white liquor. The invention further relates to a corresponding device.

[0002] White liquor is the digestion medium used in sulfate pulping. It is essentially an aqueous solution of NaOH and Na2S. It is used in the kraft pulping process as cooking liquor for wood pulping. The cooking liquor consumed during pulping, known as black liquor, is then concentrated and incinerated. The molten inorganic chemicals remaining as a residue from combustion are dissolved to form green liquor, which consists essentially of sodium carbonate and sodium sulfide. The sodium carbonate is then converted into sodium hydroxide by causticization, thus producing white liquor again.

[0003] A portion of the white liquor is often also used in other pulp production processes. In particular, white liquor can be used to adjust the pH of alkaline processes such as alkaline oxygen delignification, alkaline extraction, or peroxide bleaching. This is particularly advantageous because these bleaching steps are often integrated into the liquor recovery process. If pure caustic soda were used instead of white liquor, the Na / S ratio in the white liquor would change due to the constant addition of Na to the cycle.

[0004] However, the sulfide in white liquor causes undesirable side reactions during alkaline delignification and bleaching steps. It disrupts the oxygen delignification process, reduces the effectiveness of the bleaching agents, and increases cellulose degradation during bleaching. Therefore, if white liquor is to be used in these process steps, this sulfide must be oxidized: (a) For oxygen delignification at least to thiosulfate ("partially oxidized white liquor") (b) For peroxide bleaching to sulfate ("totally oxidized white liquor").

[0005] The first reaction step (a) to form thiosulfate proceeds very quickly, while the second reaction step (b) to form sulfate takes significantly longer. These process steps are often carried out in two separate reactors.

[0006] Air, oxygen-enriched air, or pure oxygen can be used as the oxidant for white liquor oxidation. A uniform gas introduction and rapid dissolution of the gas are crucial for this process.

[0007] WO 00 / 44978 A1 describes a process in which white liquor, which mainly contains sodium sulfide, sodium hydroxide, and water, is first contacted with an oxygen-containing gas to oxidize sodium sulfide to sodium thiosulfate. The white liquor is then contacted with hydrogen peroxide to oxidize sodium thiosulfate to sodium sulfate.

[0008] US Pat. No. 5,500,085 B1 A describes a two-stage process for white liquor oxidation in a kraft process. In a first step, sulfide is removed from the white liquor using oxygen, and in a second step, a significant portion of the sulfur compounds still present in the white liquor are converted to sulfates. The resulting white liquor is used as an alkali source for various processes in the downstream pulp production process.

[0009] Various prior art processes and devices for white liquor oxidation are also described in WO 2013 / 78885 A1. WO 2013 / 78885 A1 itself proposes a process for white liquor oxidation in which a partial stream of white liquor is taken from a stream of white liquor passing through a line, intensively mixed with oxygen in a mixer, and then fed back into the main white liquor stream. This is intended to achieve intensive mixing of white liquor and oxygen and to cause rapid oxidation of the sulfides. This intensive mixing causes the oxygen to be present in the form of small bubbles, thus creating a high surface-to-volume ratio, which favors the reaction of the sulfur compounds in the white liquor. However, the oxygen bubbles tend to coagulate and, due to their buoyancy, quickly rise to the surface, significantly reducing the efficiency of the process.This applies in particular to the rather slow sulfate-forming reactions.

[0010] Another process for white liquor oxidation is described in JP2006-342434.

[0011] The invention is therefore based on the object of specifying a method and a device for white liquor oxidation in which the efficiency of the reaction between the supplied oxygen and the sulphur compounds contained in the white liquor is improved compared to prior art methods and which ensures an efficient oxygen supply, in particular for the comparatively slow formation of sulphates.

[0012] This object is achieved by a method having the features of patent claim 1. Advantageous embodiments of the invention are specified in the subclaims.

[0013] According to the invention, the oxygen required for the oxidation of the white liquor is introduced at least partially in the form of nanobubbles with a diameter between 20 nm and 1 µm. The nanobubbles are generated either directly in a reactor where the oxidation of the white liquor takes place, or indirectly, by introducing oxygen into a line that conveys water or an aqueous fluid directly or indirectly into such a reactor. Therefore, at least within the reactor, the oxygen is present in the white liquor at least partially in the form of nanobubbles.

[0014] "Nanobubbles" or "nanovesicles" are understood here to mean gas bubbles with a diameter between 20 nm and 1 µm. The term "nanobubble" is used specifically to distinguish them from larger bubbles with a diameter between 1 µm and 100 µm, which in the context of the present invention are referred to as "microbubbles." Various studies have shown that nanobubbles with a diameter of over 20 nm can remain stable in water for a long period of several weeks or even longer. Unlike microbubbles, they do not rise to the water surface because the rising movement caused by the - comparatively low - buoyancy force is disturbed and almost completely canceled out by Brownian motion. At the same time, the zeta potential at the surface of the nanobubbles is large enough to compensate for the surface tension and thus prevent the nanobubble from dissolving.Only at a diameter significantly below 20 nm does surface tension take over, causing the nanobubbles to collapse and disappear within fractions of a second. Furthermore, due to the repulsive interactions between their surfaces, nanobubbles are not prone to coagulation. The size of nanobubbles ranges from an average diameter between 20 nm and less than 1 µm, preferably between 20 nm and 500 nm, and particularly preferably between 20 nm and 200 nm.

[0015] Methods and devices for generating nanobubbles in aqueous systems are described, for example, in US 2012 / 0175791 A1, US 2019 / 0083945 A1, US 6,382,601 B1, US 10,293,312 B2, or WO 2017 / 217402 A1, without, however, limiting the method of introducing the nanobubbles according to the present invention to these previously known systems. Essential to the present invention is that the device is designed such that a substantial portion of the oxygen supplied to an aqueous fluid is generated in the fluid in the form of nanobubbles. This is achieved, for example, by introducing the oxygen through a nozzle or a bubbling device with a section made of a porous material, such as sintered ceramic, whose pore diameters are sufficiently large to form stable nanobubbles of the desired size in the fluid.For example, the diameters of the pores of the porous material are also in the nano range, i.e. less than 1 µm.

[0016] Nanobubbles are capable of mass transfer with their environment. A nanobubble loaded with a specific gas can release gas molecules into or absorb them from the surrounding solution, depending on the saturation of this gas in the solution. In the context of white liquor oxidation, the nanobubbles are filled with oxygen or an oxygen-containing gas, such as air or oxygen-enriched air, and thus represent a stable reservoir of oxygen. The oxygen introduced in the form of nanobubbles has only a very low tendency to coagulate into larger gas bubbles and / or rise to the surface.

[0017] Parameters such as pH and salinity have a particular influence on the minimum size of the nanobubbles at which they can be stable in the white liquor. To ensure that the largest possible proportion of the oxygen in the white liquor can be present in the form of stable nanobubbles, it is therefore advisable to select the type of injection system that takes into account the average size of the bubbles generated during injection and their stability under the conditions prevailing in the white liquor. This can be achieved empirically, for example, by testing various injection systems before permanent operation and determining their suitability for the respective chemical system.

[0018] The addition of oxygen in the form of nanobubbles can therefore be used in the white liquor oxidation process both for partial oxidation, in which the sulfide contained in the white liquor is oxidized to thiosulfate, and for complete oxidation, in which the sulfur compounds contained in the white liquor are reacted with oxygen to form sulfate. In principle, it is sufficient to add the amount of oxygen required for complete oxidation in the form of nanobubbles at the beginning of the process, for example, before it is fed to a first reactor used for white liquor oxidation. However, if a two-stage oxidation takes place in two separate, sequentially connected reactors and a partial stream of the white liquor that is only partially oxidized in the first reactor is withdrawn, for example as an alkali source for oxygen delignification, it is advantageous to add oxygen in the form of nanobubbles to both reactors.Of course, the supply of oxygen according to the invention in the form of oxygen-containing nanobubbles can also be used if only a single-stage process is carried out with only one reactor in which a partial or complete oxidation of the white liquor is carried out.

[0019] The arrangement and operation of mechanical means such as stirrers, rotors, etc. in connection with the supply of oxygen must be such that the stability of the nanobubbles is not impaired by mechanical influences such as strong shear forces or cavitations.

[0020] The white liquor treated with oxygen according to the invention is particularly advantageous as an alkali source in the bleaching stages of a pulp bleaching process, especially in alkaline oxygen delignification and / or peroxide bleaching. Due to the long lifetime of the nanobubbles, it is also conceivable that a portion of the oxygen supplied during white liquor oxidation may also be present in the bleaching stages in the form of nanobubbles, where it directly supports the respective bleaching reaction.

[0021] The object of the invention is also achieved by a device for the oxidation of white liquor with the features of patent claim 5. A device according to the invention is equipped with a reactor in which white liquor is brought into contact with oxygen and sulfur compounds in the white liquor are thereby oxidized, wherein the reactor itself and / or a feed line for the white liquor or for an aqueous fluid to be fed to the reactor, which feed line is fluidly connected to the reactor, is assigned an introduction device for introducing oxygen in the form of nanobubbles with a diameter between 20 nm and 1 µm.

[0022] The feed device is arranged on the reactor and / or the feed line in such a way that oxygen can be fed in the form of oxygen-containing nanobubbles directly into the fluid in the reactor or feed line. For example, the feed device is equipped with a nozzle or a bubbling system that has a section made of a porous material, such as sintered metal or sintered ceramic, whose pore diameters are large enough to create stable nanobubbles of the desired size in the fluid.

[0023] An embodiment of the invention will be explained in more detail with reference to the drawing. The only drawing ( Fig.1 ) shows a flow diagram for a white liquor oxidation, in which the treated white liquor is subsequently fed into a bleaching process.

[0024] Fig. 1shows the addition of treated white liquor to a process 1 for bleaching pulp, such as is used, for example, during the production of cellulose fibers. During the bleaching process 1, an aqueous pulp suspension 2, which contains lignin in addition to pulp, passes through several successive stages, two of which are shown here: an alkaline oxygen delignification 3 and an oxygen-enhanced peroxide bleach 4. Additional bleaching stages, such as an oxygen-enhanced extraction, may also be present but are not shown here.

[0025] During oxygen delignification (3), the pulp suspension (2) is treated with oxygen in one or more reactors at high temperatures in an alkaline environment. In this process, significant portions of the lignin still present in the suspension are removed by reaction with oxygen. For reasons of clarity, only one process step for oxygen delignification (3) is shown here in an abstract manner; however, oxygen delignification (3) can be carried out either in a single reactor or—as is common in today's bleaching processes—in several stages in series-connected reactors.

[0026] Oxygen delignification 3 requires an alkaline environment with a pH value of approximately 11 at a temperature between 80°C and 105°C. The alkaline environment is created by adding a caustic solution to the reactor(s), as explained in more detail below. The suspension has a medium consistency of, for example, 10% to 14% consistency. Oxygen or an oxygen-containing gas is introduced into the reactor(s). In the currently rather uncommon case of a single-stage oxygen delignification, the treatment takes place at a pressure of, for example, 7 to 8 bar in the inlet and 4.5 to 5.5 bar in the outlet of the (single) reactor. The treatment time (retention time) is, for example, 50 to 60 minutes. In the case of a two-stage oxygen delignification, the pressure and reaction time in the two reactors are usually different.For example, in the first stage a pressure of 7 to 10 bar and a retention time of 10 to 15 minutes is usual, and in the second stage a pressure of 3 to 5 bar with a retention time of approximately 1 hour.

[0027] In the peroxide bleaching stage 4, a peroxide, in particular hydrogen peroxide (H2O2), is added to the suspension as an additional bleaching agent. The efficiency of this process step can be significantly improved by adding oxygen ("PO", oxygen-enhanced peroxide bleaching). The treatment takes place in a reactor, for example at atmospheric pressure and a temperature between 85°C and 90°C, or under elevated pressure at temperatures between 100°C and 110°C. The peroxide bleaching stage 4 also takes place in an alkaline environment, which is created by adding a lye, as also explained in more detail below. The suspension 5 of bleached pulp produced in the bleaching stages 3 and 4 is subsequently fed to further process steps not of interest here.

[0028] In the example shown here, white liquor is used to create the alkaline environment in bleaching stages 3 and 4. The white liquor, consisting predominantly of sodium sulfide and sodium hydroxide, is used in the Kraft process to break down cell walls and can subsequently be recycled. In the example shown in Fig. 1 For example, recovered white liquor 6 is fed to the bleaching stages 3, 4, but a partial stream of the white liquor intended for digestion can also be diverted and used in the manner described here.

[0029] To be usable in bleaching stages 3 and 4, the sodium sulfide contained in the white liquor, which would interfere with the bleaching process, must be removed. The white liquor 6 is then fed to a white liquor oxidation process 7. In the white liquor oxidation process 7, the sulfide is converted into thiosulfate ("partially oxidized white liquor") and / or sulfate ("fully oxidized white liquor") by adding oxygen in the form of air, an oxygen-rich gas, or pure oxygen (with a purity of 95 vol.% or more). Partially oxidized white liquor is suitable for the bleaching process in oxygen delignification 3, while fully oxidized white liquor is also suitable for peroxide bleaching 4.

[0030] In the embodiment shown here, the white liquor 6 is first fed to a first reactor 8, in which partial oxidation of the white liquor 6 takes place. A partial stream of the resulting partially oxidized white liquor is fed to the oxygen delignification unit 3 via a feed line 9. The remaining partial stream of partially oxidized white liquor is fed to a second reactor 10, in which complete oxidation of the white liquor takes place. The fully oxidized white liquor is fed to the peroxide bleaching unit 4 via a feed line 11.

[0031] The oxygen required for the oxidation of the white liquor can be supplied directly or indirectly to reactors 8, 10. According to the invention, at least part of the oxygen is introduced in the form of nanobubbles, i.e., small bubbles with an average diameter between 20 nm and 1000 nm. The exemplary embodiment shown here illustrates various possible locations where oxygen can be introduced in the form of nanobubbles.

[0032] For example, for the partial oxidation of the white liquor, oxygen in the form of nanobubbles can be introduced directly into the reactor 8 via an oxygen feed line 12 or by feeding oxygen in the form of nanobubbles via an oxygen feed line 13, which opens into a feed line 14 for white liquor leading to the reactor 8.

[0033] Due to the comparatively long service life of the nanobubbles, the oxygen input via the oxygen supply lines 12, 13 is also sufficient for the subsequent complete oxidation of the white liquor in the reactor 10. Alternatively, for complete oxidation, an additional input of oxygen in the form of nanobubbles takes place, either via an oxygen supply line 15 directly into the reactor 10 or via an oxygen supply line 16, which flows into a supply line 17 for partially oxidized white liquor leading to the reactor 10. Furthermore, the oxygen in the form of nanobubbles can also be introduced into a supply line for an aqueous medium, such as fresh water, which flows into the supply line 14, 17, although this is not shown here.

[0034] The nanobubbles are generated at the junction of the oxygen supply lines 12, 13, 15, 16 with the respective fluid-carrying line 14, 17 or the respective reactor 8, 10 at suitable feed devices 18, 19, 20, 21. All that is required is that, during operation, the feed devices 18, 19, 20, 21 are surrounded by water or an aqueous fluid, at least by a device that generates the nanobubbles, for example, a nozzle or a bubbling system or a section thereof, so that the nanobubbles can form in the aqueous phase. The nanobubbles are then carried along by the flow of the respective fluid and thus reach the respective reactor 8, 10 of the reaction.

[0035] Furthermore, within the scope of the invention, it is by no means necessary for the oxygen to be introduced exclusively in the form of nanobubbles. Rather, it is also possible for the oxygen to be introduced in the form of nanobubbles in addition to other methods of introducing oxygen, such as those known from the prior art.

[0036] With the process and device according to the invention, it is possible to utilize the oxygen introduced into the white liquor during the various oxidation reactions with significantly greater efficiency than is the case with prior art processes. The small size of the nanobubbles enables a uniform distribution of oxygen in the white liquor and provides a sustainably available oxygen reservoir for the comparatively slow oxidation of the sulfur compounds in the white liquor to sulfate. List of reference symbols:

[0037] 1. Bleaching process 2. Pulp suspension 3. Alkaline oxygen delignification 4. Peroxide bleaching 5. Bleached pulp suspension 6. White liquor 7. White liquor oxidation 8. Reactor 9. Feed line (for partially oxidized white liquor) 10. Reactor 11. Feed line (for fully oxidized white liquor) 12. Oxygen feed line 13. Oxygen feed line 14. Feed line (for white liquor) 15. Oxygen feed line 16. Oxygen feed line 17. Feed line (for partially oxidized white liquor) 18. Feed device 19. Feed device 20. Feed device 21. Feed device

Claims

1. Process for oxidizing white liquor, in which white liquor is contacted with oxygen in a reactor (8, 10) and hence sulfur compounds in the white liquor are oxidized, characterized in that the oxygen required for the oxidation is supplied to the reactor (8, 10) at least partly in the form of oxygen-containing nanobubbles having a diameter between 20 nm and 1 µm.

2. Process according to Claim 1, characterized in that the oxygen-containing nanobubbles are supplied at least partly by generating nanobubbles in a feed (14, 17) for the white liquor or for a fluid to be fed into the white liquor, for example for fresh water, which is fluidically connected to the reactor (8, 10).

3. Process according to Claim 1 or 2, in that the oxidation of the white liquor proceeds in multiple stages each conducted in a separate reactor (8, 10) and the oxygen required is fed to one of the reactors (8, 10) or multiple reactors (8, 10) at least partly in the form of oxygen-containing nanobubbles.

4. Process according to any of the preceding claims, characterized in that the white liquor that has been treated with the oxygen is used as alkali source in bleaching stages (3, 4) of a chemical pulp bleaching operation (1).

5. Apparatus for oxidizing white liquor, comprising a reactor (8, 10) in which white liquor is contacted with oxygen and hence sulfur compounds in the white liquor are oxidized, characterized in that the reactor (8, 10) and / or a feed (14, 17) for the white liquor or for an aqueous fluid to be supplied to the reactor that has flow connection to the reactor has an assigned introduction device (18, 19, 20, 21) for introduction of oxygen in the form of nanobubbles having a diameter between 20 nm and 1 µm.

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