SE502666C2 - Reactor with stirrer for ozone bleaching - Google Patents

Abstract

A reactor for bleaching pulp with ozone in a pressurized vessel (1), including a mixer (4) for mixing ozone into the pulp and devices (25) to stir the gas-pulp mixture including agitators (26). According to the invention each agitator stirs the entire quantity of gas-pulp mixture passing through a cross-sectional area of the vessel prior to the agitator, and the distance between the mixer and the first device (25) and between two consecutive devices is less than 8 times the diameter of the vessel. The stirring intensity of each device (25) and the number of devices are so adjusted to each other for a predetermined quantity of ozone supplied that the gas-pulp mixture at the outlet part (3) of said vessel contains less than 10 % of said quantity of ozone. Further, a pressure-reducing means (30) is arranged for removing gas from the treated gas-pulp mixture.

Description

502 666 10 15 20 25 30 35 the gas bubbles merge into larger gas bubbles so that a some of the ozone molecules have too long a path to reach the wall of the gas bubble, adjacent to the liquid phase or possibly directly to the fiber, which may occur especially when the surface of the fiber is relatively dry. reason that the gas bubbles become larger may in turn be due to an excessive pressure drop occurs in the reactor. The object of the present invention is to achieve a mass ozone bleaching reactor that eliminates the above problems. The invention makes it possible to actually that all or practically everything in one step invested ozone will be depleted and thus deposits are created to reduce ozone bleaching to a single step so that economic, constructive and space savings can be made in connection with ozone production and ozone bleaching.

The reactor according to the invention is mainly characterized by that said vessel comprises a device, alternatively several devices mounted one after the other, for stirring the gas-mass mixture, each device comprises at least one rotatable stirring means arranged to stir the whole amount of the gas-mass mixture which passes a cross-sectional area of the vessel immediately before the stirring means, and that the distance between the said the mixer and the only alternative first use order and, where applicable, between two on each other the following devices are smaller than 8xD, preferably less than 4xD, more preferably less than 2xD, most preferred drawn less than lxD, where D is the diameter of the vessel, wherein the agitation intensity of each device and the number of arrangements are thus adapted to each other for an added certain amount of ozone that the gas-mass mixture in the body part of said vessel contains less than 10%, less than 5% and most preferably less than 2% ozone of the said predetermined amount of ozone. Gas-mass- 10 15 20 25 30 35 502 666 the mixture is thus stirred according to the invention at a place or several places in the bleach line, where the mass still contains unreacted ozone, so that throughout the cross section of the bleach line at said stirring point (-n) fibers, liquid and bubbles of ozone-containing gas change their positions in relation to each other, and existing gas bubbles of larger sizes decompose into gas bubbles of smaller sizes.

The invention will be described in more detail in the following with reference to the drawings.

Figure 1 is a side view of a reactor according to the invention with three specific devices for repeated agitation of one gas-mass mixture.

Figure 2 is a longitudinal sectional view of a part of the reactor according to Figure 1 and shows one of the devices with stirring means.

Figure 3 is a cross-sectional view of the reactor taken along line III- III in Figure 2.

Figure 4 is a longitudinal sectional view of the reactor showing the stirring means according to figure 2.

Figure 5 is a longitudinal sectional view of a reactor and shows a device with a stirring means according to another embodiment.

Figure 6 is a longitudinal sectional view of a portion of a reaction and shows a device with stirring means according to a third embodiment.

Figure 7 is a cross-sectional view of the reactor taken along line VII-VII in Figure 6. 582 666 10 15 20 25 30 35 Figure 8 is a longitudinal sectional view of the reactor showing the stirring means according to figure 6.

Figure 9 is a longitudinal sectional view of a portion of a reaction and shows a device with a stirring means according to a fourth embodiment.

Figure 10 is a cross-sectional view of the reactor showing a arrangement with two stirring means according to a fifth embodiment form.

Figure 11 is a cross-sectional view of the reactor and shows a arrangement with two stirring means according to a sixth form of conduct.

Figures 12-14 are diagrams, which contain results from experiments with regard to changes in ozone consumption, cap- patal and viscosity as a function of time in seconds.

Figure 1 shows a pressurized reactor, which is arranged in a bleaching line in a bleaching plant. The reactor is arranged standing, but alternatively it can be arranged horizontally. At the embodiment shown in Figure 1 comprises the reactor an elongated tubular unbroken (uniform) vessel 1, a inlet part 2 and an outlet part 3. The inlet part 2 is arranged at the bottom and the outlet part 3 at the top of the reactor, but the inverse relationship can be used, if one in instead want to feed mass down. Inlet part 2 inside- comprises a mixing apparatus 4 with an inlet 5 for pulp, connected to a line by a pump (not shown), which pumps the mass into the reactor, a suitable one pressure at the outlet side of the pump is between 3 and 20 bar.

Furthermore, the mixing apparatus 4 has an inlet 6, which is the end of a line for the supply of a predetermined amount of ozone-containing gas consisting of a predetermined 10 15 20 25 30 35 502 666 proportion of ozone and the remainder of a carrier gas, such as e.g. air or acid. The amount of ozone-containing gas in relation to it amount of mass added per unit time is such that the volume ratio of gas to liquid in the outgoing the gas-mass mixture amounts to from 0.1: 1 to 0.8: 1, preferably 0.2: 1-2, 6: 1. The mixer 4 is off the type that ensures intense homogeneous mixing of gas in a pulp suspension, e.g. and s.k. MC mixer, as by high shear forces causes a fluidization of the pulp suspension, which has a consistency of 5-25%. One such a mixer has a rotor which is driven at a speed of 800-3000 rpm, preferably 1200-1500 rpm. Even if it it is preferred to arrange the mixing apparatus 4 in the inlet part of the reactor, it can be placed on a suitable place earlier in the bleach line, ie. upstream of the reactor.

The vessel 1 has a circular cross-section and can also be designated as a wire, e.g. of the same dimension as some others wires in the bleach line. The diameter of the vessel is suitable 0.15-1 m, preferably 0.3-0.8 m and most preferably 0.4-0.6 m.

The outlet part 3 of the reactor comprises an equipment 30 for to lower the pressure so that the gas is removed. At the shown embodiment, a so-called dP mixer, as in itself in addition, it has the property of stirring, which according to the invention is favorable if ozone is continued residues remain in the gas-mass mixture. According to a alternative embodiment, the outlet part 3 lacks one pressure reducing means, and a further similar reaction tor can then be arranged after the first reactor to receive the gas-mass mixture and perform a second ozone bleaching step, wherein a pressure raising or pressure stabilizing pump can instead be arranged in the line between the two reactors. If such a second or however, several ozone bleaching steps should be performed, it is preferred that the carrier gas is removed by a pressure reduction before the pressure is raised again and ozone is added.

LH CD 10 15 20 25 30 35 O\ (JN ON According to the present invention, the reactor comprises at least a device 25 having at least one rotary agitator means 26, which is arranged to effect a stirring of the gas-mass mixture and also a decomposition of larger ones gas bubbles. In the embodiment shown, three were used at a predetermined distance from each other 25, which have their stirring means 26 transverse the feed direction of the gas-mass mixture and the alternative offset 90 ° in the circumferential direction of the vessel 1 in relation to eachother. Each device 25 comprises a motor 7, which drives the stirring means 26, which in the case shown in FIG. The embodiment shown in Figures 1-4 consists of a rotor with three rotor blades 8 for stirring the gas-mass mixture. An- the arrangement 25 comprises an inner wall member 9, which closes the main part of the cross-sectional area of the reactor at the point of contact and delimits a passage 10, through which the gas-mass mixture is pressed by the prevailing pressure in the bleach line. The wall member 9 consists of two diametrically opposed standing plates 11, 12 and a diametrically arranged cylinder up to which the plates ll, 12 extends. Said passage 10 is formed by the rotor housing 13 risk rotor housing 13, in that this is provided with two diametrically located uniform inlet and outlet openings 14, 15, which communicate communicates with each other via the internal space 16 of the rotor housing.

Each opening 14, 15 has a length seen in the cross section of the reactor. average from about 60% to 100% of the reactor diameter meter. The width of the openings 14, 15 is about 15-40% of the diameter of the reactor and 50-80% of the diameter of the rotor housing 13.

The rotor housing 13 is provided on its inside with 4 axial ribs. boron 31, with which the rotor blades 8 cooperate so that the gas the pulp mixture is subjected to fluidizing shear forces and turbulences occur in the gas-mass mixture, which increases the stirring effect further and provides an improved decomposition of larger gas bubbles. Through the repeated the agitation of the gas-mass mixture is ensured that the the bubbles, the liquid phase and the fibers get new positions in 10 15 20 25 30 35 502 666 relationship to each other and that the remaining ozone in the gas bubbles are forced to go into solution in the liquid phase and gets the opportunity to react with a fiber in a place there oxidation has not yet taken place or insufficient oxidation tion took place. Then the gas-mass mixture leaves the reactor and the carrier gas is removed from the pulp by depressurizing the inside holds the carrier gas less than 10%, preferably less than 5% and most preferably less than 2% ozone of that in the mixture apparatus 4 charged the amount of ozone.

Figure 5 shows another embodiment of a device 25 for mixing the gas-mass mixture, where each plate ll, 12 consists of an outer part 17, which is adjustable in different positions in the longitudinal direction of the reactor, and an inner part 18, which is supported by the outer sheet metal part 17 and is adjustable mounted next to it to tightly connect to the rotor the wall of the housing 13 depending on the outer sheet metal part 17 axial position. The inlet opening 14 of the rotor housing 13 is centrally located, while the outlet opening 15 is directed straight to one side of the reactor so that the openings 14, 15 forms 90 ° with each other.

Figures 6-8 show a third embodiment of a device arrangement 25 for stirring the gas-mass mixture, there the wall member 9 only consists of two diametrically arranged plates 11, 12, each of which consists of an outer part 17, which is adjustable in different positions in the longitudinal direction of the reactor, and an inner part 18, which is supported by the outer plate part 17 and is adjustably mounted thereto to provide one small distance to the rotor 26. This device lacks rotor housing, and the passage 10 through the rotor 26 is formed by the connection between the two plates ll, 12. The rotor 26 is of it open type, ie. it lacks a central body, and contains takes axial rods 19 which are at both ends fixed to round discs 20, one of which is fixed to the rotor drive shaft 21. A central round disc 22 provides additional stability to the rods 19.

(II CÜ 10 15 20 25 30 35 The rods are divided into two groups at different distances to the center axis of the rotor.

Figure 9 shows a fourth embodiment of a device For agitating the gas-mass mixture, where the wall means 9 only consists of two transverse plates 11, 12, a length slightly greater than the radius of the reactor and is that has axially displaced in the reactor so that they form between them a space for receiving a rotor 26 at the same time as the plates 11, 12 form the inlet and outlet openings, respectively. 14, 15 between its free ends and the reactor wall.

The rotor 26 lacks a rotor housing and in this case is thus formed the passage 10 for the gas-mass mixture of the space 23 between the plates 11, 12 and the inlet and outlet openings 14, 15. The rotor is of the same design as it according to Figures 6-8. In the embodiment according to Figure 9 the gas mass mixture is exposed to the action of the rotor 26 closes 19 as it flows in a transverse direction in the reactor.

Figure 10 shows a fifth embodiment of a device For agitating the gas-mass mixture consisting of two rotary stirring means 26, which are mounted in the vessel 1 from opposite directions and have their axes of rotation 24 parallel to each other and located within the same cross section of the vessel 1, the rotor blades 27 having curved end portions which connects to the 1 cylindrical wall of the vessel so that the rotors 26 cover a larger part of the cross-sectional area. The the remaining part of the cross-section is closed by a wall means 9, which consists of two diametrically placed plates 28, rotor housing 29 which is limited to the outside of the vessel 1 for which are attached to the vessel 1. Each rotor 26 has one to enclose the rear of the engine 7. Passage 10 for the gas-mass mixture is thus formed by the two openings 32, 33 between the plates 28 and the vessel wall, in which openings 32, 33 the two rotors 26 extend. The rotors 26, which is of the open type as previously described, 10 15 20 25 30 35 502 666 rotates in different directions and each in such a direction that the rotor blades 27 follow the feed of the gas-mass mixture direction of movement in the vessel as they pass through its center.

Figure 11 shows a sixth embodiment of a device For agitating the gas-mass mixture consisting of two rotary agitators 26 mounted in the vessel from opposite directions and have their axes of rotation 24 parallel to each other and located within the same cross section of the vessel, the rotor blades 27 being straight and having a length which is equal to or substantially equal to the diameter of the vessel so the two rotors 26 cover a cross-sectional area which is equal to the circular cross-sectional area of the vessel. At this embodiment, no internal wall means is thus needed, which form fixed throttles. The rotors 26 are enclosed in a common rectangular rotor housing 34 having two counter- standing parallel end walls 35, 36 and two opposite parallel side walls 37, 38, which walls extend parallel to the center axis of the vessel and tangential to the the inner circumference of the vessel. The rotor housing 34 is provided with two circular flanges 39 for their mounting in two standing cylindrical parts of the vessel. Passage 10 for gas the pulp mixture is thus formed by it as a square intermediate part mounted the rotor housing 34, whose cross-sectional area encloses the cross-sectional area of the cylindrical part of the vessel so that there are no fixed throttles.

The length of the reactor can be between 0.3 m and 20 m, preferably typically between 0.5 m and 6 m.

The rotors 26 are driven at a speed which is convenient at least 800 rpm, preferably 1500-3600 rpm, more preferably drawn 1500-3000 rpm, to provide a sufficient efficient stirring. The higher the intensity of the stirring the greater the turnover of residual ozone and yes fewer devices 25, i.e. stirring points, needed for to achieve the object of the invention. According to a preferred '5ø2 ess 10 15 20 25 30 35 10 embodiment, three devices 25 are used, the rotors of which operates at 3000 rpm. The devices 25 can be arranged rela- close to each other (for example with flange joints) so that the time during which the gas-mass mixture takes up static condition is very short, namely from near O to about 10 seconds, preferably about 0.1-5 seconds.

The devices 25 can also be of the same type of equipment which was used for the initial mixture of ozone-containing gas and pulp, e.g. a plurality of successive lenta MC mixers, where the rotors are thus arranged in the flow direction instead of across it as shown on the drawings. The reactor is suitably arranged to be trade a mass to which at least 3 kg has been charged, preferably at least 4 kg, most preferably at least 5 kg of ozone per ADMT. As an example it can be mentioned that for a normal daily production of ozone-bleached pulp, a reactor vessel can be used with a diameter of 600 mm, the time interval between the initial mixer and the first the stirring means and between two stirring means is 10 seconds.

In the following, an experiment, which was carried out in the laboratory, is reported. ration scale to determine ozone consumption partly during a number of stirring phases, partly during stagnant phases after ozone has been mixed intensively with a pulp suspension in an initial mixing phase, wherein this initial mixing phase and the first the standstill phase can be considered to simulate the operating condition prevailing in known bleaching with ozone using a mixer and a subsequent reactor, in which gas the pulp mixture is pressed forward in a static state.

The pulp used had undergone an acid treatment. steps with HZSO4 and had the following properties: Consistency 10% Chapter 12.1 Viscosity 920 dm3 / kg 10 15 20 25 30 502 666 ll In the experiment, 4 kg of ozone / ADMT were charged in a treatment vessel, which was equipped with a rotor that was connected at regular intervals to run at a speed of 3000 rpm. The rotor had a turn-up time of 3 seconds to 3000 rpm and a deceleration time of 2 seconds more 0 rpm. The pressure in the treatment vessel was 4 bar. The the initial mixing, when all the ozone was charged, was carried out for a period of 1 second and three subsequent stirring each took place over a period of 1 second.

The standstill time of the rotor before the first stirring and between two agitations was in all cases 4 seconds, which included said deceleration time (2 seconds).

Samples were taken after each mixing / stirring phase and after each standstill phase to measure ozone content, kappatal and viscosity. The results are shown by subsequent diagrams according to Figures 12, 13 and 14, where ozone consumption, kappa number and viscosity set aside as function of the time, whereby the warm-up time (3 seconds) has not been plotted on the time axes. The zig-zag dashed the areas constitute the mixing and stirring phases, while they the intermediate areas represent the standstill phases. The curves clearly show that delignification (kappa number reduction) and ozone consumption occurs when the gas-mass mixture is stirred. During each stirring phase, a large amount of ozone is converted by that amount of ozone which remains when the stirring phase begins, while practical no ozone is converted during the standstill phases.

Because no reaction occurs unless gas and mass stirred vigorously, one might assume that it is the mass transport of ozone to the liquid phase, which is the most limiting factor and that the chemical reaction between ozone in the liquid phase and the fibers occur momentarily.

Claims (9)

0 "! (I) l \ J 10 15 20 25 30 35 V a device (25), 666 12 P A T E N T K R A V Reactor for bleaching pulp of cellulosic fibrous material with ozone in the production of chemical pulp, which is fed continuously in a bleaching line, which reactor comprises a pressurized vessel (1) of predetermined length and with preferably constant cross-sectional area, part (2) and an outlet part (3), comprises or is preceded by a mixing apparatus (4) for an inlet, which inlet part (2) mixes in a predetermined amount of an ozone-containing gas in the flowing mass, which vessel (1) alternatively comprises several devices mounted one after the other (25), for agitating the gas-mass mixture, each device (25) comprising at least one rotatably mounted agitator (26), which is operated at a speed of at least 800 rpm, preferably 1500-3600 rpm, the mixing apparatus (4) for the initial admixture of more preferably 1500-3000 rpm, the mixed ozone-containing gas having a rotor means driven at a speed of 800-3000 rpm, preferably 1200-1500 rpm, characterized in that each The agitating means (26) is arranged to agitate the whole amount of the gas-mass mixture passing a cross-sectional area of the vessel immediately before the agitating means (26), that the distance between said mixing apparatus (4) and the only, alternatively first device (25) and, where appropriate, between two consecutive devices (25) is less than 8xD, than 2xD, most preferably less than 1xD, where D is preferably less than 4xD of the vessel, more preferably smaller diameter, the agitation intensity of each device (25 ) and the number of devices (25) are adapted to each other for a supplied predetermined amount of ozone that the gas-mass mixture at the outlet part (3) of said vessel (1) contains less than 10%, preferably less than 5% and most preferably less than 2 % ozone of said predetermined amount of ozone, and that the outlet portion (3) comprises or is followed by a pressure reducing means (30) for removing gas from the treated gas mass. breathing. Reactor according to Claim 1, characterized in that it comprises 2-5, preferably 3 devices (25) for stirring the gas-mass mixture. Reactor according to claim 1 or 2, characterized in that each device (25) has a wall member (9) arranged in the vessel (1), which delimits a passage (10) with a flow area which is reduced in relation to the cross-sectional area of the vessel (1) before and after the passage (10), which passage area (10) flow area is preferably located in a plane across the vessel (1), wherein at least one stirring means (26) is arranged in that of the wall means ( 9) delimited the passage (10) and aligned across the vessel (1). Reactor according to Claim 3, characterized in that the stirring means (26) of two successive devices (25) are displaced in the circumferential direction of the vessel (1) by approximately 90 ° relative to one another. Reactor according to one of Claims 3 and 4, characterized in that it has a length of between 0.3 m and 20 m, preferably between 0.5 m and 6 m. Reactor according to one of Claims 3 to 5, characterized in that the vessel is cylindrical and has a diameter between 0.15 m and 1 m, preferably between 0.3 m and 0.8 m, most preferably between 0 , 4 m and 0.6 m. Reactor according to any one of claims 1-6, characterized in that it is arranged to treat a mass to which at least 3 kg has been charged, preferably at least 4 kg, most preferably at least 5 kg of ozone per ADMT. Û L! 10 15 20 25 30 35 2 666 14 8. A rotatably mounted stirring means (26), a reactor according to any one of claims 1-7, characterized in that each device (25) comprises two having their axes of rotation (24) parallel to each other and located in each its half of the vessel's cross section and across the vessel. Reactor according to claim 8, characterized in that each device comprises a rectangular rotor housing (34), with the feed direction of the gas-mass mixture, which rotor having walls (35, 36, 37, 38) which are parallel housings (34) form an intermediate part of the vessel (1) and has a cross-sectional area which in projection encloses the cross-sectional area of the vessel and that the two agitating means (26) fill the entire cross-sectional area of the rotor housing (34).