WO1994029513A1

WO1994029513A1 - Reactor for ozone bleaching

Info

Publication number: WO1994029513A1
Application number: PCT/SE1994/000542
Authority: WO
Inventors: Hasse BÖRJESSON; Christen GRÖNVOLD-HANSEN; Axel LÄMÅS; Torbjörn KVARNSTRÖM; Ronny Höglund
Original assignee: Kvaerner Pulping Technologies Ab
Priority date: 1993-06-11
Filing date: 1994-06-06
Publication date: 1994-12-22
Also published as: AU7011794A; SE502666C2; SE9302011D0; SE9302011L

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

Reactor for ozone bleaching

The present invention relates to a reactor for bleaching pulp of cellulosic fibre material with ozone in the manufacture of chemical pulp fed continuously in a bleaching line, the reactor including a pressurized vessel of predetermined length and preferably having constant cross sectional area, an inlet part and an outlet part, said inlet part including or being preceded by a mixer for mixing a predetermined quantity of a gas containing ozone into the pulp flowing through it, said vessel including a device, alternatively several devices arranged one after the other, to stir the gas-pulp mixture, each device including at least one rotatably journalled agitator which is driven at a speed of at least 800 rpm, preferably 1500-3600 rpm, more preferably 1500-3000 rpm, said mixer for initial mixing of the gas containing ozone being provided with a rotor member driven at a speed of 800-3000 rpm, preferably 1200-1500 rpm.

Relatively large quantities of gas containing ozone must be supplied when bleaching pulp with ozone, since the carrier gas can only contain limited quantities of ozone. Although ozone reacts very quickly with the pulp and the gas containing ozone is mixed in extremely efficiently, it has been found that the gas-pulp mixture leaving the following reactor still contains relatively large quantities of ozone out of the predetermined quantity of ozone originally added. Ozone is relatively expensive to produce and all ozone added should therefore be utilized. Since the amount of ozone added is not consumed in the first step the ozone bleaching must be repeated in an additional step with the addition of at least one more dosage of ozone in order to achieve the desired delignification. Several factors probably contribute to the undesired result with unreacted ozone. One factor may be that the gas-pulp mixture is fed through the reactor after the gas containing ozone has been mixed in, in static state, i.e. as a coherent body, without the three phases - fibre material, gas and liquid - noticeably altering their location in relation to each other, so that the transportation of ozone to the liquid phase decreases or ceases altogether. This means that remaining ozone does not come into reactive contact with the fibres, and especially not with those parts of the fibres where oxidation has not yet taken place. Another factor may be that the gas bubbles combine to form larger gas bubbles so that the distance some of the ozone molecules must travel to the wall of a gas bubble adjacent to the liquid phase is too long, or possibly directly to the fibre which may occur particularly if the surface of the fibre is relatively dry. The cause of the gas bubbles becoming larger may in turn be that too great a pressure drop occurs in the reactor.

The object of the present invention is to provide a reactor for bleaching pulp with ozone that eliminates the problems mentioned above. Consequently, the invention enables all or practically all the ozone added in one step to be consumed, thereby creating the conditions for reducing ozone bleaching to a single step and consequently enabling financial, design and space savings to be made in conjunction with ozone production and ozone bleaching.

The reactor according to the invention is substantially characterized in that each agitator is arranged to stir the entire quantity of the gas-pulp mixture passing through a cross-sectional area of the vessel immediately prior to the agitator, that the distance between said mixer and the only, or the first, device and if applicable between two consecutive devices is less than 8xD, preferably less than 4xD, more preferably less than 2xD and most preferably less than lxD, where D is the diameter of the vessel, the stirring intensity of each device and the number of devices being so adjusted to each other for a predetermined quantity of ozone supplied that the gas-pulp mixture at the outlet part of said vessel contains less than 10%, preferably less than 5% and most preferably less than 2% of said predetermined quantity of ozone, and that the outlet part includes or is followed by a pressure-reducing means for removing gas from the treated gas-pulp mixture.

The invention will be described further in the following, with reference to the drawings.

Figure 1 is a lateral view of a reactor according to the invention, with three specific devices for repeated agitation of the gas-pulp mixture.

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

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

Figure 4 is a longitudinal sectional view through the reactor, showing the agitator according to Figure 2.

Figure 5 is a longitudinal sectional view through a reactor, showing a device with an agitator according to another embodiment.

Figure 6 is a longitudinal sectional view through a part of a reactor, showing a device with agitators according to a third embodiment. Figure 7 is a cross sectional view through the reactor along the line VII-VII in Figure 6.

Figure 8 is a longitudinal sectional view through the reactor, showing the agitator according to Figure 6.

Figure 9 is a longitudinal sectional view of a part of a reactor, showing a device with an agitator according to a fourth embodiment.

Figure 10 is a cross sectional view through the reactor, showing a device with two agitators according to a fifth embodiment.

Figure 11 is a cross sectional view through the reactor, showing a device with two agitators according to a sixth embodiment.

Figures 12-14 are diagrams showing results from experiments relating to changes in ozone consumption,

Kappa number and viscosity, respectively, as functions of the time in seconds.

Figure 1 shows a pressurized reactor arranged in a bleaching line in a bleach plant. The reactor is vertical but it may be arranged horizontally. In the embodiment shown in Figure 1 the reactor comprises an oblong tubular unbroken (unitary) vessel 1, an 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 reverse is also possible if it is desired to feed the pulp downwards. The inlet part 2 includes a mixer 4 with an inlet 5 for pulp, connected to a pipe having a pump (not shown) which pumps the pulp into the reactor. A suitable pressure on the output side of the pump is between 3 and 20 bar. The mixer 4 also has an inlet 6 connected to a pipe for the supply of a predetermined quantity of gas containing ozone consisting of a predetermined portion of ozone, the remainder being a carrier gas such as air or oxygen, for instance. The amount of gas containing ozone in relation to the quantity of pulp supplied per time unit is such that the volume ratio between gas and liquid in the gas-pulp mixture leaving the reactor is from 0.1:1 to 0.8:1, preferably 0.2:1-0.6:1. The mixer 4 is of the type that ensures violent homogenous mixing of gas into a pulp suspension, e.g. an MC-mixer, which due to high shearing forces, effects fluidization of the pulp suspension which has a consistency of 5-25%. Such a mixer has a rotor driven at a speed of between 800 and 3000 rpm, preferably 1200-1500 rpm. Although it is preferred to arrange the mixer 4 in the inlet part of the reactor, it may be placed at a suitable point earlier in the bleaching line, i.e. upstream of the reactor. The vessel 1 has circular cross section and may even be designated a pipe, e.g. having the same dimension as a number of other pipes in the bleaching line. The diameter of the vessel is suitably 0.15 m-1 m, preferably between 0.3 m-0.8 m, and most preferably 0.4 m-0.6 m.

The outlet part 3 of the reactor contains equipment 30 for reducing the pressure so that the gas is removed. In the embodiment shown a dP-mixer is used, with the inherent property that stirring is obtained, which is favourable according to the invention if ozone still remains in the gas-pulp mixture. According to an alternative embodiment the outlet part 3 lacks such pressure-reducing means and another similar reactor may be arranged after the first reactor to receive the gas- pulp mixture and perform a second ozone bleaching step. In this case a pressure-increasing or pressure- stabilizing pump may instead be arranged in the pipe between the two reactors. If such a second or more bleaching step is to be performed, however, the carrier gas is preferably removed by means of pressure reducing before the pressure is again increased and more ozone added.

According to the invention the reactor includes at least one device 25 which has at least one agitator 26 arranged to effect agitation of the gas-pulp mixture and also disintegration of large gas bubbles. In the embodiment shown three devices 25 are used, spaced a predetermined distance from each other and having their agitators 26 located transverse to the feed direction of the gas-pulp mixture and alternately displaced 90° in relation to each other around the circumference of the vessel 1. Each device 25 includes a motor 7 driving the agitator 26 which, in the embodiment shown in Figures 1-4 consists of a rotor with three rotor blades 8 to stir the gas-pulp mixture. The device 25 comprises an inner wall means 9 which closes off a main portion of the cross-sectional area of the reactor at the stirring point and defines a passage 10 through which the gas-pulp mixture is forced by the pressure prevailing in the bleaching line. The wall means 9 consists of two diametrically opposite plates 11, 12 and a diametrically arranged cylindrical rotor housing 13 to which the plates 11, 12 extend. Said passage 10 is formed by the rotor housing 13, this being provided with two diametrically located identical inlet and outlet openings 14, 15 which communicate with each other via the inner space 16 of the rotor housing. The length of each opening 14, 15, seen in the cross section of the reactor, is from about 60% to 100% of the diameter of the reactor. 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 internally with 4 axial ribs 31 with which the rotor blades 8 cooperate so that the gas-pulp mixture is subjected to fluidizing shearing forces and turbulence occurs in the gas-pulp mixture, thus further increasing the stirring effect and giving improved disintegration of large gas bubbles. The repeated agitation of the gas-pulp mixture ensures that the gas bubbles, liquid phase and fibres change location in relation to each other and that the ozone remaining in the gas bubbles is forced to dissolve in the liquid phase and has the opportunity of reacting with a fibre at a point where oxidation has not yet, or has insufficiently taken place. When the gas-pulp mixture leaves the reactor and the carrier gas is removed from the pulp by reducing the pressure, the carrier gas contains less than 10%, preferably less than 5% and most preferably less than 2% of the amount of ozone added in the mixer 4.

Figure 5 shows a second embodiment of a device 25 for mixing the gas-pulp mixture where each plate 11, 12 consist of an outer portion 17 that can be set in different positions in the longitudinal direction of the reactor, and an inner portion 18 carried by the outer plate portion 17 and adjustably fitted thereto so that it fits tightly against the wall of the rotor housing 13 depending on the axial position of the outer plate portion 17. The inlet opening 14 is located centrally in the rotor housing 13 whereas the outlet opening 15 is directed straight towards one side of the reactor so that the openings 14, 15 form an angle of 90° to each other.

Figures 6-8 show a third embodiment of a device 25 for agitating the gas-pulp mixture, in which the wall means 9 consists of two diametrically arranged plates 11, 12 each comprising an outer portion 17 which is adjustable in various positions in the longitudinal direction of the reactor, and an inner portion 18 carried by the outer plate portion 17 and adjustably fitted thereto leaving a small space between it and the rotor 26. This device has no rotor housing and the passage 10 through the rotor 26 is formed by the opening between the two plates 11, 12. The rotor 26 is of the open type, i.e. it lacks a central body, and includes axial rods 19 secured at each end to circular discs 20, one of which is secured to the driving shaft 21 of the rotor. A central circular disc 22 gives further stability to the rods 19. The rods are distributed in two groups located at different distances from the central shaft of the rotor.

Figure 9 shows a fourth embodiment of a device 25 for stirring the gas-pulp mixture, in which the wall means 9 consists of two transverse plates 11, 12, these plates being slightly longer than the radius of the reactor and being axially displaced in the reactor to form a space for a rotor 26, while at the same time the plates 11, 12 form inlet and outlet openings 14, 15 between their free ends and the reactor wall. The rotor 26 lacks a rotor housing and in this case the passage 10 for gas-pulp mixture is formed by the space 23 between the two plates 11, 12 and the inlet and outlet openings 14, 15. The rotor is of the same type as that shown in Figures 6-8. In the embodiment according to Figure 9 the gas-pulp mixture is subjected to the influence of the rods 19 of the rotor 26 as it flows transversely in the reactor.

Figure 10 shows a fifth embodiment of a device 25 for stirring the gas-pulp mixture, consisting of two rotating agitators 26 fitted in the vessel 1 from opposite directions and having their rotation shafts 24 parallel to each other and located within the same cross section of the vessel 1. The rotor blades 27 have curved end portions connecting with the cylindrical wall of the vessel 1 so that the rotors 26 cover a large portion of the cross-sectional area. The remainder of the cross section is closed by a wall means 9 consisting of two diametrically placed plates 28 secured to the vessel 1.

Each rotor 26 is provided with a rotor housing 29 limited to the outside of the vessel 1 to surround the rear portion of the motor 7. The passage 10 for the gas-pulp mixture is thus formed by the two openings 32, 33 between the plates 28 and vessel wall, in which openings 32, 33 the two rotors 26 extend. The rotors 26, which are of the open type described earlier, rotate in different directions and each in such a direction that the rotor blades 27 follow the feed direction of the gas-pulp mixture in the vessel when they pass through its mid portion.

Figure 11 shows a sixth embodiment of a device 25 for stirring the gas-pulp mixture, consisting of two rotating agitators 26 fitted in the vessel from opposite directions and having their rotation shafts 24 parallel to each other and located within the same cross section of the vessel. The rotor blades 27 are straight, their length being equal or substantially equal to the diameter of the vessel so that the two rotors 26 cover a cross- sectional area corresponding to the size of the circular cross section of the vessel. No inner wall means to provide permanent throttling is therefore necessary in this embodiment. The rotors 26 are enclosed in a common, rectangular rotor housing 34 with two opposite parallel end walls 35, 36 and two opposite parallel side walls 37, 38, these walls extending parallel to the central axis of the vessel and substantially touching the inner circumference of the vessel. The rotor housing 34 is provided with two circular flanges 39 for assembly in two opposing cylinder parts of the vessel. The passage 10 for gas-pulp mixture is thus formed by the rotor housing 34 inserted as a square intermediate part, the cross- sectional area of which encloses the cross-sectional area of the cylindrical part of the vessel so that no fixed throttling exists.

The length of the reactor may be between 0.3 m and 20 m, preferably between 0.5 m and 6 m. The rotors 26 are driven at a speed of at least 800 rpm, preferably 1500-3600 rpm, more preferably 1500-3000 rpm in order to achieve sufficiently effective agitation. The more vigorous the agitation, the greater will be the consumption of the remaining ozone and the fewer devices 25, i.e. stirring points, will be required to achieve the object of the invention. According to a preferred embodiment three devices 25 are used, their rotors being driven at a speed of 3000 rmp. The devices 25 may be arranged relatively close together (for instance with flange connection) so that the time during which the gas- pulp mixture assumes static state is extremely brief, viz. from almost 0 to about 10 seconds, preferably about 0.1-5 seconds. The devices 25 may also be of the same type of equipment as used for the initial mixing of the gas containing ozone and the pulp, i.e. several identical MC mixers arranged consecutively where the rotors are thus arranged in flow direction instead of transverse thereto as shown in the drawings. The reactor is suitably arranged to treat a pulp to which at least 3 kg, preferably at least 4 kg, most preferably at least 5 kg per ADMT, is supplied. By way of example it may be mentioned that for a normal 24-hour production of ozone- bleached pulp a reactor vessel with a diameter of 600 mm can be used, the time interval between the initial mixing apparatus and the first agitator and between two agitators being 10 seconds.

The following describes a text performed on laboratory scale to establish the consumption of ozone both during a number of stirring phases and also during intermediate static phases after vigorous mixing of the ozone with a pulp suspension in an initial mixing phase, whereby the initial mixing phase and the first static phase can be considered to simulate operating conditions prevailing in known bleaching with ozone using a mixer and then a reactor, in which the gas-pulp mixture is pressed forward in a static state. The pulp used had undergone an acid treatment step with H2SO4 and had the following properties: Consistency 10%

Kappa number 12.1

Viscosity 920 dm³/kg

In the experiment 4 kg ozone/ADMT was supplied to a treating vessel equipped with a rotor which was connected at regular intervals to be run at a speed of 3000 rpm. The rotor had an acceleration time of 3 seconds to reach a speed of 3000 rpm and a retardation time of 2 seconds to 0 rpm. The pressure in the treating vessel was 4 bar. The initial mixing, when all ozone was added, was performed for a period of 1 second and agitation was performed three times after that, each time lasting 1 second. The static time for the rotor before the first stirring and between two stirrings was in each case 4 seconds, including said retardation time (2 seconds). Samples were extracted after each mixing/stirring phase and after each static phase to measure ozone content, Kappa number and viscosity. The results can be seen from the accompanying diagrams according to Figures 12, 13 and 14 where ozone consumption, Kappa number and viscosity are shown as a function of the time. The acceleration time (3 seconds) has not been plotted on the time axes. The areas marked with wavy lines indicate mixing and stirring phases, while the areas between represent static phases. The graphs show clearly that delignification

(reduction in Kappa number) and ozone consumption occur when the gas-pulp mixture is agitated. During each stirring phase a large quantity of ozone is consumed out of the quantity of ozone remaining when the stirring phase commenced, while practically no ozone is consumed during the static phases. Since to reaction occurs unless gas and pulp are stirred vigorously it could be assumed that it is the mass transportation of ozone to the liquid phase which is the most limiting factor and that the chemical reaction between ozone in liquid phase and the fibres occurs instantaneously.

Claims

C L A I S

1. A reactor for bleaching pulp of cellulosic fibre material with ozone in the manufacture of chemical pulp fed continuously in a bleaching line, the reactor including a pressurized vessel ( 1 ) of predetermined length and preferably having constant cross-sectional area, an inlet part (2) and an outlet part (3), said inlet part (2) including or being preceded by a mixer (4) for mixing a predetermined quantity of a gas containing ozone into the pulp flowing through it, said vessel (1) including a device (25), alternatively several devices (25) arranged one after the other, to stir the gas-pulp mixture, each device (25) including at least one rotatably journalled agitator (26) which is driven at a speed of at least 800 rpm, preferably 1500-3600 rpm, more preferably 1500-3000 rpm, said mixer (4) for initial mixing of the gas containing ozone being provided with a rotor member driven at a speed of 800-3000 rpm, preferably 1200-1500 rpm, characterized in that each agitator (26) is arranged to stir the entire quantity of the gas-pulp mixture passing through a cross-sectional area of the vessel immediately prior to the agitator (26), that the distance between said mixer (4) and the only, or the first, device (25) and if applicable between two consecutive devices (25) is less than 8xD, preferably less than 4xD, more preferably less than 2xD and most preferably less than lxD, where D is the diameter of the vessel, the stirring intensity of each device (25) and the number of devices (25) being 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%, preferably less than 5% and most preferably less than 2% of said predetermined quantity of ozone, and that the outlet part (3) includes or is followed by a pressure-reducing means (30) for removing gas from the treated gas-pulp mixture.

2. A reactor as claimed in claim 1, characterized in that it includes 2-5, preferably 3 devices (25) for stirring the gas-pulp mixture.

3. A reactor as claimed in claim 1 or 2, characterized in that each device (25) has a wall means (9) in the vessel (1) defining a passage (10) with a through-flow area which is reduced in relation to the cross-sectional area of the vessel (1 ) before and after the passage ( 10), the through-flow area of said passage (10) preferably being located in a plane transverse to the vessel ( 1 ) , at least one agitator (26) being arranged in the passage ( 10) defined by the wall means (9 ) and directed transversely to the vessel (1).

4. A reactor as claimed in claim 3, characterized in that the agitators (26) in two consecutive devices (25) are displaced about 90° in relation to each other around the circumference of the vessel (1 ) .

5. A reactor as claimed in any of claims 3 and 4, characterized in that its length is between 0.3 m and 20 m, preferably between 0.5 m and 6 m.

6. A reactor as claimed in any of claims 3-5, characterized in that the vessel is cylindrical and has a diameter of 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.

7. A reactor as claimed in any of claims 1-6, characterized in that it is arranged to treat a pulp to which at least 3 kg, preferably at least 4 kg, most preferably at least 5 kg per ADMT, is supplied.

8. A reactor as claimed in any of claims 1-7, characterized in that each device (25) includes two rotatably journalled agitators (26), the rotation axis (24) of which being parallel to each other, one agitator being positioned in each half of the cross-section of the vessel and transverse thereto.

9. A reactor as claimed in claim 8, characterized in that each device includes a rectangular rotor housing (34) having walls (35, 36, 37, 38) which are parallel to the feeding direction of the gas-pulp mixture, said rotor housing (34) forming an intermediate part of the vessel (1) and having a cross-sectional area that in projection encloses the cross-sectional area of the vessel, and that the two agitators (26) cover the entire cross-sectional area of the rotor housing (34) .