FI98794C - A method and apparatus for mixing a gaseous chemical into a fiber suspension - Google Patents

Description

98794

A method and apparatus for mixing a gaseous chemical into a fiber suspension

The present invention relates to a method suitable for the use described in the preamble of claim 1 and to a device with the basic principles described in the preamble of claim 5. In particular, the invention relates to the mixing of large amounts of gas into a fiber suspension. The aim has been to develop a method and apparatus suitable for mixing ozone supplied with a carrier gas, without, however, excluding other chemicals to be mixed. The method and apparatus according to the invention can be applied particularly well to the mixing of ozone into a medium-density (consistency 8-25%) fiber suspension.

15

If it is desired to mix large amounts of gas into a medium-thick fiber suspension, which is increasingly the case in modern bleaching plants, there is a situation in which the fiber suspension has a consistency of the order of 10-15% and must be able to mix a large volume of gas. That is, during mixing, the medium has about 40 to 80% fiber suspension and about 20 to 60% gas, but most usually about 30 to 50% gas. Evenly supplying such a large amount of gas and achieving a good mixing result is difficult because the gas is separated by local pressure differences into a lower pressure range, if possible. This results in increased chemical loss and uneven bleaching results, as well as deteriorating process runnability.

30 A number of e.g. mixers used to mix ozone, some of which were previously designed to mix liquid chemicals and which could also be used to mix gaseous chemicals the more successfully the amount of gas to be mixed 35. However, these mixers have worked satisfactorily with many of the gaseous chemicals used in bleaching, in which case they have first been tried to mix ozone. However, it has soon been found that, although some agitators have been able to satisfactorily mix a few percent of the gas amounts into the fiber suspension, it has no longer been possible to mix a ten percent increase in the mass of ozone and carrier gas. Attempts have been made to convert the non-aforementioned mixers suitable for mixing large amounts of gas, but the result has invariably been poor, completely unsatisfactory, mixing results.

10

The second group of prior art mixers consists of equipment designed in recent years specifically for mixing large amounts of gas, encouraged and required by ozone bleaching. Many of them have already reached the stage where the prototype is taken to the factory and tested under real practical conditions in their 15 development cycles. Naturally, the results have almost invariably been more positive than with previous modified mixers. However, especially for those who are aware of the potential potential of ozone-20 in bleaching, even current ozone mixers do not perform satisfactorily better under factory conditions. Thus, a stage has been reached in which the pulp mills are relatively satisfied with the bleaching result achieved and its relationship to the investment required to introduce ozone bleaching.

However, product developers on both the equipment and process sides believe that the mixing process can still be significantly improved. Studies have shown that the mixing operation is not efficient enough in most cases or that the resulting mixture of ozone and fiber suspension is not sufficiently homogeneous. This can manifest itself in many different ways. It is possible that the pulp will be unevenly bleached as part of the pulp is damaged due to over-efficient bleaching, with too much ozone being dispensed per unit of pulp and leaving part of the pulp without a sufficient dose of ozone. 98794 3 thus remaining only partially bleached. It is also possible that in the gas separation carried out after the bleaching reaction, ozone is still separated from the pulp, which in practice means that the ozone is not sufficiently mixed with the pulp or that the ozone has not had sufficient time to react with the fibers. It is also possible that ozone consumption is unreasonably high relative to the level of bleaching, again due to poor mixing of ozone with the fiber suspension.

10

Studies have shown that all prior art mixers are characterized in that the inlet pressure of the fiber suspension, or more broadly the pressure effect caused by the inlet, whether positive or negative, can affect the mixing process. It has also been found that the pressure effect of the outlet of the fiber suspension contributes to the mixing operation. It has further been found that the pressure fluctuations caused by the inlet of the fiber suspension can be reflected up to the outlet 20 and the pressure fluctuations of the outlet up to the inlet. As a result, part of the pulp can pass through the mixer very quickly, at worst the mixer can be thought of as having a channel along which part of the pulp flows almost unhindered. Correspondingly, part of the pulp has to stay in the mixer longer. The end result is uneven dosing of the gas to different parts of the fiber suspension, which further results in uneven pulp quality. The reason for the phenomenon described above is that the fluid sounding devices arranged in the mixer alone are not sufficient to prevent pressure fluctuations from being reflected through the device.

The following is a further discussion of the most important properties of ozone specifically for mixing.

35 Ozone is the most reactive of the bleaching chemicals used. In addition, ozone is not selective in the slightest, but reacts with all the 4 98794 reactants that fall in its path, including those that should not be affected. It can even be argued that ozone cannot be compared to any other bleaching chemical for the reasons mentioned above. Due to the above-mentioned properties of ozone 5, ozone must be introduced almost to the fiber level in the fluidized mixture to deal with each fiber. Diffusion, as with other bleaching chemicals, cannot be relied upon, in which case it is sufficient for the chemical to be introduced in the vicinity of a reasonably sized fiber floc, from which it 10 finds its own path to the fibers.

Ozone can only be produced industrially in relatively dilute mixtures. In other words, only about 5 to 7% of the ozone feed gas is present, the remainder being the so-called carrier gas, which is usually oxygen or nitrogen, although other inert gases, or at least gases inert compared to ozone, can also be used. Thus, even if relatively small amounts of ozone are sufficient for bleaching, the carrier gas must be fed and mixed in an amount of about 20 times the amount of ozone.

The object of the present invention is to obviate the above-mentioned disadvantages inherent in the devices and methods according to the prior art by the method and device according to the invention, the characteristic features of which appear from the appended claims.

The method and apparatus of the invention will now be described in more detail with reference to the accompanying Figures, Figure 1 shows a preferred embodiment of the present invention, Figure 2 shows another preferred embodiment of the present invention, Figure 3 shows a third preferred embodiment of the present invention.

Fig. 4 98794 shows a preferred fourth embodiment of the present invention, Fig. 5 shows the change in power consumption 5 of both the mixer according to the invention and the mixer according to the prior art as a function of the mixer speed and the gas content of the fiber suspension to be mixed.

Figure 1 shows a mixer according to a preferred embodiment of the invention, consisting of an elongate substantially 10 cylindrical mixer housing 10, two ends 12 and 14, a common incoming fiber suspension 16, an outgoing fiber suspension 18 and a gas / gas mixture 20 to be mixed, and a rotatable second end an internally arranged rotor 15 22. The rotor 22 consists of vanes and agitating members suitably mounted on the shaft 24. The shaft 24 of the rotor 22 is connected to conventional actuators (not shown).

In the embodiment of Figure 1, the fiber suspension to be treated is fed either radially or tangentially through a joint 16 arranged in the mixer housing 10 and an opening 26 in the wall of the housing to the first mixing chamber 28, the so-called Fig. 25 is in the embodiment from the connection 20 at the end 12 of the mixing housing 10. However, it is also possible to arrange this gas supply connection in the housing wall 30 (shown, inter alia, in Figure 2), in the fiber suspension supply connection 16 or even somewhat further from the mixer. to the pulp supply pipe 30 (not shown). It is only necessary to ensure that the gas is not fed into the pulp so early that a substantial part of it is consumed in the reactions before it is effectively mixed with the pulp, which would naturally run the risk of part of the fiber suspension being treated too efficiently, i.e. the fibers themselves.

• 98794 6

The tip 32 of the mixer rotor 22 preferably extends some distance into the premix space 28, where the vanes 34 arranged in the tip 32 of the rotor 22 cause intense fluidization of the fiber suspension, breaking the large fiber flocs and distributing the feed gas evenly throughout the premix space. Preferably, ribs 36 are provided on the wall of the premixing chamber 28 to prevent the fiber suspension from rotating excessively with the vanes 34 of the rotor 22. Most preferably, the ribs extend 10 along the entire length of the device, possibly only by changing their height in different zones of the mixer. It is also possible to add fixed mixing members 38 to the end 12 of the housing 10, the sole purpose of which is to increase the turbulence of the pulp in the premixing space 28 and to prevent the pulp 15 from rotating excessively with the rotor 22. Preferably, the agitating members 38 at the end 12 of the housing 10 are located radially inside the rotor blades 34 at a distance therefrom. Both the rotor blades 34 and the ribs 36 on the housing wall are preferably substantially axial, although fluidization means in the other direction 20 may also be considered. If necessary, the blades 34 of the rotor 22 can be made to feed some of the fiber suspension to the next zone. Much more important to the direction of the vanes 34 and fins 36 is the distance between the vanes 34 and fins 36 and the other dimensions thereof 25, by means of which the fluidization level of the premixing space is adjusted to suit the mixing. Issues affecting the required level of fluidization include e.g. the amount of fiber suspension to be treated (t / h), the consistency of the fiber suspension, the amount of gas to be mixed, the origin of the fibers, etc. Since the above 30 factors cause the widest variety of combinations, no universally valid dimensions or sizing principles can be given.

The tip portion 32 of the rotor 22 extending into the premixing space 28 is, for example, conical, so that when the surface of the rotor 22 turns towards the direction of the axis, the so-called fluidization zone is moved. homogenization zone 7 to zone 40. In said zone 40, the mixture of fiber suspension and gas is fluidized so efficiently that virtually all of the fiber flocs in the suspension are broken into microflocks of not more than a few fibers that the gas can be evenly distributed / dispensed throughout the mixture. In this zone of very intense turbulence 40 it is possible to mix the gas so precisely on the surface of the microflocks that the consumption of gas can be minimized and at the same time it can be ensured that all 10 microflocks and the fibers therein are treated uniformly.

Very strong fluidization of the homogenization space 40 is achieved by means of preferably radial pins 42 on the surface of the rotor 22 and teeth 44 arranged in the wall 30 of the housing 10. With regard to the so-called in the form of pins 42, they may be round and radial, but rectangular or pyramidal members may well be considered. In fact, both the pins and the so-called

The teeth 20 may be very similar in shape.

Figure 1 shows two rows of pins 42 on the surface of the rotor 22 in the circumferential direction and one toothed ring 44 between them on the wall 30 of the housing 10. Of course, both the pin frames 42 and the tooth frames 44 may be different in number than shown. Preferably, the pins 44 of adjacent rings are positioned so that they are not sequentially viewed from the end of the rotor 22 but interleaved. The same applies to the teeth 44 if there are more than one toothed ring described. Preferably, each ring gear is formed by an intact ring 46 arranged in the wall of the housing 30 and teeth 44 extending inwardly thereof towards the shaft. Thus, the flow is clearly restricted at the ring gear 44.

The number of both teeth 44 and pins 42 in each circumference varies between 2-15 depending on the size of the device. Another way to restrict the flow in the homogenization zone is, of course, to arrange the rotor pin circumference starting from an annular flange extending radially towards the wall of the agitator housing arranged on the surface of the root.

By restricting the flow just described above, it is possible to prevent the pressure fluctuations of the inlet and the outlet from being reflected to one another. Forcing the flow of the fiber suspension through a sufficiently small flow channel ensures that the mixing operation is optimal in the homogenization zone, whereby the gas is absolutely evenly distributed throughout the fiber suspension. The operation of the throttle illustrated in Figure 1 is as follows. In the direction of flow of the rotor, the first pin circumference "throws" against the wall and counter-wall of the fiber suspension housing, from which the flow must constrict to move in the axial direction to avoid a throttle to move towards the shaft, from which the fiber suspension is again thrown against the housing wall.

The homogenization zone 40 is followed by a zone of weaker turbulence 20, the so-called maintenance zone 48, also referred to as reaction zone or exit zone. In the embodiment of the figure, the diameter of the rotor 22 is clearly smaller than in the homogenization zone 40, and the rotor 22 is provided with vanes 50 corresponding to the wings 34 of the premixing zone 28.

The wall 30 of the housing 10 in the maintenance zone 48 is preferably provided with ribs 52 which, however, are lower than the corresponding ribs 36 of the premixing zone 28. As can be seen from the name of the zone, the zone 48 is intended to maintain a sufficiently strong turbulence 30 or fluidization level in the fiber suspension. to stand out, but could continue the reaction made possible by the even gas distribution of the homogenization zone 40 to almost the fiber level. The maintenance zone 48 is also intended to accelerate the rotational speed of the mixture of fiber-35 pension and gas so that the mixture can be removed from the device, preferably via a tangential joint 18. However, the rotational speed must be kept at a level that does not allow the gas to separate around the rotor 22. Such a gas separation tendency can be further complicated by arranging at the end 14 of the housing 10 fixed vanes 54 between the blades 50 of the rotor 22 and the surface 5 of the rotor 22, preferably extending from the axial maintenance zone 48. When the fiber suspension has a suitable velocity correctly, the fiber suspension-gas mixture can be removed from the mixer 10 so that the gas cannot separate, but the bleaching chemical in the residual gas can continue the reaction without hindrance in the exhaust pipe and / or in the subsequent bleaching reactor, if deemed necessary. However, this separate bleaching reactor is not required if the residence time of the fiber suspension-gas mixture in the maintenance or reaction zone is sufficient for ozone bleaching. In most cases, however, this will require a considerable lengthening of the reaction zone, resulting in additional energy consumption if a sufficient level of turbulence is to be maintained at all times to eliminate gas evolution.

Figure 2 shows a mixer according to another preferred embodiment of the invention. In the illustrated solution 25, the diameter of the agitator rotor 122 is not reduced after the homogenization zone 40, but is increased by a conical spacer 156 so as to reach a relatively large diameter rotor 122 at the outlet 18 having fins 158 on the surface to maintain a sufficiently high turbulence level. through the entire suspension. The embodiment of the figure also shows a second toothed ring 144 located at the conical spacer 156 and thus not having to extend its teeth so deep into the homogenization space 40. Further in the premixing zone 28, the blades 134 at the tip 132 of the rotor 122 are slightly different from the embodiment of Figure 1. - 10 98794 keavat. That is, wing extensions extending from the attachment point of the wings 134 toward the homogenization zone 40 are omitted. Of course, the various variations shown in Figure 2 can be applied individually without any need to use all of them exactly as shown in the figure. Figure 2 further shows that the gas inlet 120 may be in the agitator housing wall 130 and that the pulp inlet may also be at an angular position with respect to the outlet 18 (90 ° angle shown).

10

Fig. 3 shows a mixer according to a preferred third embodiment of the invention, which may have a different construction according to either Fig. 1 (shown in Fig.) Or Fig. 2 or some combination of variations shown therein, except that the fiber suspension inlet 226 is axial and gas (shown in the figure) input connection 220 radial. That is, the fiber suspension inlet 226 is preferably located at the end 12 of the housing and the gas inlet 220 is preferably located in the wall 20 30 of the housing 10.

Figure 4 shows another alternative to the rotor tip shown in the previous figures, which according to the figure may be provided with agitator blades 360 extending almost or even up to the axis 25 in addition to the axial but spaced apart blades 334 of the previous embodiments. The figure also shows the possibility that the diameter of the rotor 22 may be practically constant over the entire length of the rotor 22, i.e. at least in the homogenization zone 40 and the maintenance zone 48.

Example

The experiments performed compared a modified version of a known chemical mixer for mixing large amounts of gas with a mixer according to our invention. The easiest way to compare said mixers was to monitor the change in energy required for mixing as a function of the amount of gas in the gas-fiber suspension mixture. Indeed, in our experiments, it has been found that in optimal mixing, the need for mixing power should be reduced in proportion to the amount of gas added to the suspension. In other words, a 20% gas addition should reduce the mixing power by only about 20%.

10

Figure 5 shows the decrease in power consumption of a prior art modified chemical mixer as a function of gas concentration and rotor speed. The figure compares the power required to mix the 20% gaseous mass with the power required to mix the gaseous mass alone. In other words, the 100% line indicates the relative amount of power required to mix the pulp alone, and the lower curves indicate the relative amount of power required to mix the mass containing 20% gas. It can be seen that, for example, in the speed range used in the experiments, the power consumption of the prior art mixer at gaseous mass varied between about 40% and 23% of the power required to mix non-gaseous mass. The mixer according to the invention had a power consumption of between 78 and 82% over the entire speed range. In other words, the mixer according to the invention reduced the power consumption by only 18 to 22%, while the mixer according to the prior art reduced the power consumption by 60 to 77%.

30 The explanation for the power but reduction of the power consumption of the mixer in the prior art is that a large part of the mixing elements of the mixer rotates in a gas bubble, whereby the power requirement is naturally reduced to almost non-existent. Correspondingly, the small reduction in power consumption of the mixer according to our invention means that the power requirement is reduced only by the amount by which the addition of gas reduces the density of the pulp. In other words, the prior art mixer 12 98794 has not been able to mix the gas at all, but the gas has been allowed to separate around the mixer members.

As can be seen from the above, it has been possible to develop a chemical mixer that is much more reliable than previous equipment used in the process, capable of mixing large amounts of gas into a medium thick mass without fear of gas separation either during the mixing operation or when removing the suspension from the mixer.

Although different structures are shown in each of the above figures, all structures are interchangeable and combinable, so it is clear that the structures shown in the different figures can be freely combined.

As is well apparent from the claims, the invention also includes an embodiment in which both the premixing zone and the maintenance zone are removed with respect to the solution shown in the figures. In other words, the homogenization zone can handle the entire mixing operation. The only objection to its exclusive use is the high power it requires to minimize the upstream and downstream zones.

Claims (27)

A method for admixing large quantities of gas into a media suspension of media consistency (8-25%) by means of a rotary rotor arranged in a mixer housing, which process consists mainly of the following steps: a) said gas and fiber suspension is introduced into the mixer; ) the gas involved in the fiber suspension is in a fluidized state and c) the mixture obtained in this way is measured out of the mixer, characterized in that step b) is at least divided into two parts: b2) homogenizing part, in which the fiber suspension is fluidized all the way to the fiber and micro flock level and the gas are brought into contact with each fiber / micro flock, during which part the flow of the gas-fiber suspension mixture is suppressed, so that the influence of the pressure variations caused by the inlet flow and / or outlet flow on the mixing process is eliminated, and is eliminated. - / reaction portion in which the gas-fiber suspension mixture 20 f the level of fluidization is maintained so high that it is sufficient to prevent the formation of gas bubbles and the gas separation. 2. A method according to claim 1, characterized in that in step b) before sub-stage b2) there is also included a sub-stage b) premix, in which the fiber suspension is fluidized all the way to the flock level and the gas is evenly distributed throughout the suspension. 30 Method according to claim 1 or 2, characterized. hence, the rotational movement of the fiber suspension caused by the rotor is braked without interruption during step b2). 35 4. A method according to claim 1, characterized in that the rotational movement of the fiber suspension caused by the rotor is slowed during each step a), b) and c). 98794 Apparatus for admixing large gas volumes in a medium consistency fiber suspension (8 - 25%), which consists mainly of a mixer housing (30), its two ends (12, 14), at least two arranged in said mixing housing 5 (30). conduits (16, 18), i.e. lines for the fiber suspension to be treated and for the gas-fiber suspension mixture to be wet out of the device, and by a rotating rotor (22) arranged within the housing (30) and its shaft (24), characterized in that the housing (30) in the device is distributed in the axial direction at least in two of the following zones: a pre-mixing zone (28), homogenization zone (40) and a living zone. reaction zone (48), and within said homogenization zone there are devices (42, 44, 46, 144) bed for throttling the flow and homogenizing the gas-fiber suspension mixture. 6. Apparatus according to claim 5, characterized in that said devices comprise eating at least one eating at least one of said rotor (22) and mixer housing (30) and ring mixer (46) and of mixing means (42) working together with the same. 7. Device according to claim 5, characterized in that the housing (30) of the device is distributed in the axial direction in three zones: a pre-mixing zone (28), homogenization zone (40) and a housing, ie. reaction zone (48). 8. Apparatus according to claim 5 or 7, characterized in that the pre-mixing zone (28) is provided with devices (34, 36, 38, 134, 334, 360) for fluidizing said fiber suspension to the flock level and for distributing said gas evenly. within the entire premix zone (28). 35 9. Device according to claim 5 or 7, characterized in that the homogenization zone (40) is provided with 98794 devices (42, 44, 46, 144) for fluidising the gas-fiber suspension mixture mixed within the premix zone (28). or ro-floc level and for conducting said gas into contact with each fiber / micro-floc. 10. Device according to claim 5 or 7, characterized in that the reaction zone (48) is provided with means (50, 52, 54, 158) to maintain the level of turbulence in the homogeneous gas-fiber suspension mixture formed within the homogenization zone (40) sufficient to prevent the formation of gas bubbles and to cure the mixture is homogeneous. Device according to Claim 8, 9 or 10, characterized in that said device comprises a rotor (22), which or part thereof extends from one end (14) of the device through the entire device in the vicinity of the opposite device. the second end (12). 12. Device according to claim 8, 9 or 10, characterized in that said means include mixing means (50, 158) arranged on the surface of the rotor (22). 13. Apparatus according to claim 12, characterized in that, in said devices, mixing means (52) arranged on the inner surface of the mixer housing (30) which cooperate with the mixing means (50, 158) of the rotor (22). 30 Device according to claim 12, characterized in that said devices include at least one end (14) of the mixing means (54) arranged by the mixer housing (30) which cooperates with the mixing means (50) of the rotor (22). Device according to claim 13 or 14, characterized in that said mixing means (52, 54) are located at a radial distance from the mixing means (50, 158) of the rotor (22). Device according to claim 8, 9 or 10, characterized in that said devices include moldings (36, 52) on the wall of the mixer housing (30) and the blades (34, 134, 334, 50, 158) which lie in the radial direction at a distance from said moldings (36, 52). 10 Device according to Claim 8, 9 or 10, characterized in that in said devices there are provided pin-shaped means (42) arranged on the surface of the rotor (22) and with these co-operating mixing means (44, 46, 144) extending in mesh from the mixer housing. (30) wall. 18. Device according to claim 17, characterized in that said pin-shaped means (42) are arranged to extend outwardly from the annular piece arranged on the surface of the rotor 20 (22). 19. Apparatus according to claim 13, characterized in that said mixing means (44, 144) are arranged to extend leaking from the annular piece (46) attached to the inside of the wall of the mixer housing (30). 20. Device according to claim 19, characterized in that said annular pieces (46) constitute said devices for throttling the flow. 30 Apparatus according to claim 19, characterized in that said mixing means (44, 144) are teeth in the inner edge of said ring (46). 22. Apparatus according to claims 13 and 17, characterized in that both the pin-shaped means (42) and the mixing means (44, 144) extending leaking from the wall of the housing (30) are reinforced in rows which are parallel to the circuit. 23. Apparatus according to claim 22, characterized in that the pin-shaped means (42) and the mixing means (44, 144) extending outwardly from the wall of the housing (30) in the axial direction at a distance from each other. Device according to claim 5 or 7, characterized in that the cross-sectional area of the flow between the mixer housing (30) and the rotor (22) is at least within the homogenization zone (40). Device according to claim 17, characterized in that the number of the tapping mixers (42) per one tapping circuit is 2-15. Device according to claim 5, characterized in that the outlet line for the gas-fiber suspension mixture 20 is located at one end of the mixer housing. Device according to claim 16, characterized in that an opening is provided between the blade (50) of the rotor (22) and the rotor (22). that the blade (50) is located at a distance from the rotor (22).