WO2020185135A1

WO2020185135A1 - Mixer for mixing a gas into pulp comprising a rotor, said rotor comprising a rotor drum.

Info

Publication number: WO2020185135A1
Application number: PCT/SE2020/050115
Authority: WO
Inventors: Mikael Löfgren; Philip LUNDMAN
Original assignee: Valmet Ab
Priority date: 2019-03-13
Filing date: 2020-02-06
Publication date: 2020-09-17
Also published as: SE542954C2; SE1950312A1

Abstract

The invention relates to a mixer (1) for mixing a gas into pulp, comprising: a chamber (30) having at least one inlet (32) for pulp and gas and at least one outlet (34) for pulp-gas mixture; said at least one inlet (32) for pulp and gas being arranged through a first wall (36) of said chamber (30); a rotor (10) comprising a rotor drum (20) connected to a drive shaft (12); said rotor drum (20) having a hollow cylindrical shape with an open end (24) facing said inlet (32) for pulp and gas and a closed end (26) connected to the drive shaft (12), and having openings (22) through a cylindrical wall (41) thereof; said drive shaft (12) being arranged through a second wall (37), opposite to said first wall (36), of said chamber (30) and arranged for rotating said rotor drum (20) around a rotation axis (S) coinciding with an inflow direction (A) of said pulp and gas through said at least one inlet (32) for pulp and gas; wherein at least some of the pulp and gas flowing from said inlet (32) for pulp and gas to said outlet (34) for the pulp-gas mixture will pass through said openings (22) in the cylindrical wall (41) of the rotor drum (20); wherein the rotor (10) further comprises a gas distribution arrangement (2), said gas distribution arrangement (2) comprising a plurality of elongated gas distribution elements (4) extending in a direction generally parallel to the rotation axis (S), said gas distribution elements (4) being arranged around the rotation axis (S), between the rotation axis (S) and the cylindrical wall (41) of the rotor drum (20).

Description

Mixer for mixing a gas into pulp comprising a rotor, said rotor comprising a rotor drum.

Technical field

The present disclosure relates to mixers for mixing a lower density medium, such as a gaseous medium, into a higher density medium, such as a liquid medium. More specifically, the disclosure relates to mixers for mixing a gas into a pulp suspension.

Background

In many industrial applications, such as within different chemical industries where various chemicals have to be mixed with different suspensions of raw material or into combinations of raw material, it is of high importance to homogenize mediums having different properties such as different densities to obtain homogenous suspensions. Such industries include paint-making, pulp- and paper-manufacturing industries.

Within the pulp- and paper-making industry, throughout the fiberline, i.e. the different process steps involved when converting wood chips or other fibrous raw material into pulp, there are several positions where mixing apparatuses are used to mix different kinds of media into the pulp suspension. The treatment media added into the fiber suspension may for example be for heating, delignification or bleaching purposes. Often the treatment media are in gaseous or liquid state.

When mixing treatment media into a fiber suspension, it is of high importance for the mixing result that an even or homogenous distribution is achieved. When the mixed media have very different densities, such as when a gaseous medium is mixed with medium having higher density, such as a pulp suspension, particular considerations are necessary to achieve proper mixing with a minimal input of mixing energy. Cylindrical mixers are provided in a wide range of configurations. For example, the rotor of the mixer may have a center axis aligned with or arranged perpendicularly to the general flow direction of the media. Also, the outlet and inlet of the mixer may both be positioned axially with respect to the center axis of the rotor, or one of the inlet or outlet may be positioned perpendicularly to the center axis of the rotor. When a gas is mixed with a liquid in a cylindrical mixer, the gas will typically tend to accumulate near the rotor center due to the centrifugal force. This undesired phenomenon counteracts distribution of the gas in the liquid and increases the mixing energy required to achieve proper mixing.

Thus, there is a need for improved mixing apparatuses or arrangements for mixing media or fluids with different density properties and, in particular, to mixing apparatuses or arrangements for mixing gaseous treatment media into a fiber suspension, such as a lignocellulosic pulp suspension.

Summary of the invention

It is an object of the present disclosure to alleviate at least some of the problems with mixers and methods for mixing a gas into a pulp suspension.

A further object of the present disclosure is to provide a pulp mixing arrangement that can be scaled up without causing separation, inhomogeneous mixing or unreasonable energy consumption. The above-mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure are achieved by the various aspects of the present disclosure.

According to a first aspect illustrated herein, there is provided a mixer for mixing a gas into pulp, comprising: a chamber having at least one inlet for pulp and gas and at least one outlet for pulp- gas mixture; said at least one inlet for pulp and gas being arranged through a first wall of said chamber; a rotor comprising a rotor drum connected to a drive shaft; said rotor drum having a hollow cylindrical shape with an open end facing said inlet for pulp and gas and a closed end connected to the drive shaft, and having openings through a cylindrical wall thereof; said drive shaft being arranged through a second wall, opposite to said first wall, of said chamber and arranged for rotating said rotor drum around a rotation axis coinciding with an inflow direction (A) of said pulp and gas through said at least one inlet for pulp and gas; wherein at least some of the pulp and gas flowing from said inlet for pulp and gas to said outlet for the pulp-gas mixture will pass through said openings in the cylindrical wall of the rotor drum; wherein the rotor further comprises a gas distribution arrangement, said gas distribution arrangement comprising a plurality of elongated gas distribution elements extending in a direction generally parallel to the rotation axis, said gas distribution elements being arranged around the rotation axis, between the rotation axis and the cylindrical wall of the rotor drum. During operation of a mixer according to the present disclosure, pulp and gas is fed through an inlet into the mixer chamber, mixed by being passed through the openings in the cylindrical wall of the rotating rotor drum, and the pulp-gas mixture exits the chamber via an outlet. The outlet for the pulp-gas mixture is preferably arranged in a direction transverse to the inflow direction of the pulp and gas.

A general problem with mixing through radial slits is that it is difficult to scale up the mixer to larger dimensions. The mixing energy necessary for accomplishing the mixing increases drastically if the dimensions of the mixer are increased in the radial direction. The peripheral speed increases with increasing machine size if a constant rotating speed is maintained. Increased peripheral speed in turn leads to increased centrifugal forces, which cause the gas and pulp to separate. Due to the centrifugal force, the gas will typically tend to accumulate near the rotor center. In the mixer according to the present disclosure, a gas distribution arrangement comprising a plurality of elongated gas distribution elements is used to prevent the gas and pulp from separating. The elongated gas distribution elements, e.g. in the form of rods or bars, extend in a direction generally parallel to the rotation axis of the drum at a radial distance between the rotation axis and the cylindrical wall of the rotor drum. The elongated gas distribution elements are attached to and configured to rotate with the rotor drum. In some embodiments, the gas distribution elements are attached to the closed end of said rotor drum and/or to the cylindrical wall of said rotor drum.

When the gas distribution elements rotate in the pulp-gas mixture a slipstream is formed behind the trailing surface of each element. The slipstream has been found to lead to the formation of a gas rich trail behind the elements. This formation reduces the tendency of the gas to migrate towards the rotor center. Instead, the rotational motion of the elements results in a distribution of the gas along the axial length of the gas distribution elements, and in turn to the formation of a cylindrical accumulation of gas through which at least part of the pulp passes. This effect increases the contact between the pulp and gas compared to the case where a large portion of the gas accumulates near the rotor center, and as a result improves the distribution of the gas in the pulp before it reaches the openings in the cylindrical wall of the rotor drum where mixing occurs. The improved gas distribution prior to the mixing step can improve the mixing result and/or reduce the energy requirement for achieving a desired degree of mixing.

Advantages with the proposed technology include, but are not limited to, that the solution is easily scalable and can be used for large-scale production without excessive energy requirements or that the machine becomes extremely large, at the same preventing separation and inhomogeneous mixing.

In some cases the gas distribution arrangement will have the additional benefit of premixing and fluidizing the pulp-gas mixture, which may reduce the pressure drop.

The gas distribution arrangement comprises at least two gas distribution elements. In some embodiments, the gas distribution arrangement comprises in the range of 2-12 gas distribution elements, preferably in the range of 4-10 gas distribution elements, more preferably in the range of 4-8 gas distribution elements.

In some embodiments, the gas distribution elements are in the form of bars or rods. To ensure even distribution and mixing and to prevent imbalance, the gas distribution elements are preferably arranged symmetrically around the rotation axis.

In some embodiments, the outer rotational diameter of the gas distribution arrangement is smaller than the inner diameter of the inlet for pulp and gas. This configuration provides for immediate contact between the incoming pulp and gas and the gas distribution elements, before centrifugal separation of the pulp and gas has occurred. In some embodiments, the gas distribution arrangement and the inner diameter of the inlet for pulp and gas are concentric. A concentric configuration of the gas distribution arrangement and the inlet provides for the most even gas distribution.

The shape and size of the gas distribution elements is preferably selected to provide a certain flow resistance during rotation through the incoming pulp and gas. The cross-sectional shape of the gas distribution elements may for example be quadrangular, such as rectangular or square, triangular or round, such as circular of oval. The size of the gas distribution elements, particularly the radial thickness, should preferably selected to be large enough to achieve a significant gas distributing effect. However, too large gas distribution elements will cause an unnecessary increase of the energy consumption. A radial thickness of in the range of 1 -10 %, preferably in the range of 1 -6 %, of an inner diameter of said rotor drum has been found to provide a suitable compromise. In some embodiments, the radial thickness of said gas distribution elements is less than 10 % of an inner diameter of said rotor drum, preferably less than 6 % of an inner diameter of said rotor drum. In some embodiments, the radial thickness of said gas distribution elements is larger than 1 % of an inner diameter of said rotor drum.

As the gas distribution elements may be subjected to significant mechanical stress during mixing, the gas distribution arrangement may further comprise a support structure for preventing deformation of the gas distribution elements during rotation. The support structure may for example comprise transverse reinforcement bars or a ring structure fixating the gas distribution elements to each other or to the inside walls of the rotor drum.

The gas distribution elements help to distribute the gas in the axial direction of the rotor drum. The length of the gas distribution elements may therefore preferably be selected so as to overlap with the axial extension of the rotor drum. In some embodiments, the length of the gas distribution elements will correspond to the inner axial length of the rotor drum, whereas in some embodiments the gas distribution elements will be longer or shorter.

The plurality of gas distribution elements may be of the same length or of different lengths. In some embodiments, the gas distribution elements are curved or angled in relation to the rotation axis. In some embodiments, the gas distribution elements have a wave or helix shape. In some embodiments, the cross-sectional area of said gas distribution elements varies over the longitudinal extension of the gas distribution elements. In some embodiments, the mixer further comprises an inlet duct connected to the at least one inlet for pulp and gas of the mixer, said inlet duct having at least one gas injection nozzle.

The inventors have found that the effect of the gas distribution arrangement can be significantly improved if the gas distribution elements are positioned close to the point where the gas is injected into the pulp. For example, where gas is injected into the pulp in the duct just before it reaches the mixer, the gas distribution elements may advantageously extend through said at least one inlet for pulp and gas and into the duct, such that they will be positioned closer to the point where the gas is injected into the pulp. Thus, in some embodiments, the elongated gas distribution elements extend through said at least one inlet for pulp and gas. Alternatively, in some embodiments the at least one gas injection nozzle is arranged at the entrance of the mixer chamber, or even such that it extends into the mixer chamber. Alternatively, the point where the gas is injected into the pulp can be moved closer to the gas distribution elements, e.g. by arranging the gas injection nozzle at the entrance of the mixer chamber, or even inside the mixer chamber.

The gas distribution arrangement can be obtained in various ways. In some embodiments, the plurality of are provided in the form of individual elements, e.g. bars or rods. The individual elements are then assembled with the rotor drum, and optionally with each other, to form the gas distribution arrangement. In some embodiments, the plurality of gas distribution elements instead constitutes portions of a unitary structure, e.g. in the form of a slitted drum or cage like structure having closed elongated portions (forming the gas distribution elements) separated by open elongated portions.

The distribution of the gas may be further improved by introducing a second set of gas distribution elements at a different radius from the first set of gas distribution elements. In some embodiments, the gas distribution arrangement comprises at least two sets of gas distribution elements, a first set of gas distribution elements being arranged at a first radial distance and a second set of gas distribution elements being arranged at a second radial distance between the rotation axis and the cylindrical wall of the rotor drum.

Further embodiments and advantages of the mixer will be appreciated from the following detailed description.

Brief description of the drawings

The invention, together with further objects and advantages thereof, will now be described more in detail with reference to the following drawings, in which the same reference numbers are used for similar or corresponding elements: FIG. 1 a schematically illustrates a rotor for use in a mixer for mixing gas into pulp; FIG. 1 b illustrates a cross-sectional view of a mixer for mixing gas into pulp;

FIG. 1 c illustrates an embodiment of the inventive mixer in an elevation view;

FIG. 2-4 illustrate schematically embodiments of a part of a rotor drum;

FIG. 5 illustrates a cross-sectional view of one embodiment of a rotor having a rotor drum;

FIGS. 6-7 illustrates a part of a cross-sectional view of embodiments of a rotor drum perpendicular to the rotational axis;

FIG. 8 illustrates a part of an embodiment of a rotor drum;

FIG. 9a illustrates an embodiment of a mixer in an elevated cross-sectional view; FIG. 9b illustrates another cross-sectional view of the embodiment of FIG. 9a;

FIG. 10 illustrates another embodiment of a mixer with the stator drum positioned radially inside the rotor drum;

FIG. 11 illustrates yet another embodiment of a mixer with two stator drums;

FIG. 12 illustrates yet another embodiment of a mixer, where the rotor is provided with inner protruding portions;

FIG. 13 illustrates another embodiment of a mixer with protruding parts inside the rotor drum;

FIG. 14 illustrates yet another embodiment of a mixer, where the rotor is provided with outer protruding portions; and

FIG. 15 illustrates a cross-sectional view of yet an embodiment of a mixer for mixing gas into pulp;

FIG. 16a and 16b are cross-sectional views of an embodiment of the inventive mixer comprising a gas distribution arrangement; and

FIG. 17a and 17b are cross-sectional views of alternative embodiments of the inventive mixer comprising a gas distribution arrangement.

Detailed description of preferred embodiments

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of mixing conditions in a pulp mixer.

In an axial mixing, the pulp and gas flow between a rotor and housing. If the agitation of the rotor causes an efficient mixing, the length of the mixing zone can be kept relatively short, and there are no advantages in increasing the mixing zone length. For larger mixers, either the flow velocity of the pulp has to increase or the mixer radius has to be increased. An increase in pulp flow velocity is energy demanding and the conditions in the mixing zone may also deteriorate, which may require an increase of the mixing zone length as well. If the radius of the mixer is increased, the energy requirements to reach the same rotational speed varies approximately as the square of the radius, which means that the required energy increases faster than the radius. Mixing pulp in an axial mixing at different radial distances can give rise to uneven mixing, since the velocity of any agitating parts varies with the radial distance. In large machines, having a large diameter, there may also be a centrifugal separation behavior, as mentioned in the background, tending to separate pulp and gas in the radial direction. This may also cause an uneven mixing. Such difficulties may in part be prevented by limiting the radial extension of the mixing zone. However, mixing at one specific radius reduces the benefit of diameter increase. The increased diameter will in such cases only give rise to approximately linear scalar scaling-up of the cross-sectional area of the mixing zone. In this disclosure the term“radial mixing” is used to denote mixing processes, wherein the pulp flows in a radial direction with respect to a rotor during the mixing phase. In radial mixing the pulp and gas flow axially in to a cylindrical rotor drum, passes radially through openings in the cylindrical rotor drum and the pulp-gas mixture exits the mixer in a radial or axial direction. Mixing is caused by the rotational movement of the rotor drum. When mixing occurs at essentially one specific radius, the pulp is exposed to homogeneous speed conditions in the entire mixing zone and an improved homogeneous mixing is achieved.

Radial mixing is scalable, not only by increasing the diameter, but also by increasing the axial length. Increasing the axial length of the mixing zone will increase the throughput linearly. The scaling in axial length also increases the required energy approximately linearly. Scaling up the diameter, while keeping the peripheral speed, results in a decrease in rotational speed. The increase in energy in such circumstances scales approximately linearly to the increase in diameter. Scaling up the diameter, while keeping the rotational speed, results in approximately a cubic energy increase, but at the same time a higher turbulence is achieved in the mixing zone, which probably improves the mixing. The increase in throughput becomes approximately linear. The mixing energy necessary for accomplishing the mixing increases drastically if the dimensions of the mixer are increased in the radial direction. The peripheral speed increases with increasing machine size if a constant rotating speed is maintained. Increased peripheral speed in turn leads to increased centrifugal forces, which cause the gas and pulp to separate. Due to the centrifugal force, the gas will typically tend to accumulate near the rotor center.

Thus, both axial and radial mixing suffer from the problem of separation of gas and pulp due to the centrifugal force. In one embodiment, a mixer for mixing a gas into pulp comprises a chamber and a rotor. The chamber has an inlet for pulp and gas and an outlet for mixed pulp. The inlet for pulp and gas is arranged through a first wall of the chamber. The rotor has a rotor drum having a hollow cylindrical shape with an open end facing the inlet for pulp and gas and a closed end connected to a drive shaft. The rotor drum gas openings, i.e. perforations through a cylindrical wall thereof. The rotor is arranged through a second wall, opposite to the first wall, of the chamber. The rotor is arranged for rotating the rotor drum around a rotation axis coinciding with an inflow direction of the pulp and gas through the inlet for pulp and gas. During operation of the mixers, at least some of the pulp and gas flowing from the inlet for pulp and gas to the outlet for the pulp-gas mixture will pass through said openings in the cylindrical wall of the rotor drum.

Figure 1 a illustrates schematically a rotor 10 for use in a mixer for mixing gas into pulp. The rotor 10 comprises a shaft 12 and a rotor drum 20. The rotor drum 20 has a number of openings 22, in this embodiment in the shape of slits 23. In other words, the rotor drum 20 defines openings 22. The slits 23 are elongated in an axial direction A of the rotor 10. Pulp and gas are intended to be introduced into a first open end 24 of the rotor drum 20 with a flow direction parallel to the axial direction A. A second end 26, opposite to the first end 24, of the rotor drum 20 is closed and attached to the shaft 12. This forces the pulp and gas to change their flow direction into a mainly radial flow direction, indicated by the reference r. The pulp and gas come into contact with the rotor drum 20 when it tries to escape through the openings 22. Since the rotor drum is intended to rotate in a rotation direction R, this motion will then shear the pulp so that the properties of the pulp become fluid, becomes turbulent and is mixed with the gas. The mixed pulp passes through the openings 22, i.e. in the present embodiment the slits 23, in a radial direction. The rotor drum 20 has in this embodiment a front-end surface 28.

In one embodiment, the rotor drum 20 defines slits 23. The slits 23 have their main extension direction directed non-perpendicular with respect to the rotation axis of the rotor 10.

In one embodiment, the slits are straight slits. However, as is described further below, also other shapes are feasible in alternative embodiments.

In the embodiment of Figure 1 a, the slits are directed parallel to the rotation axis S. Also here, there are alternative embodiments presenting other slit directions.

The thickness of the rotor drum 20 will define the length of the openings 22, which in turn to some degree determines the width of the mixing zone. A long mixing zone having changing radii may lead to differing mixing conditions at the beginning and end, respectively, of the mixing zone. On the other hand, a too short mixing zone may instead lead to an incomplete mixing. It has been found that a thickness of the rotor drum that is less than 10 % of an inner diameter of the rotor drum gives rise to acceptable small mixing differences. However, preferably, a thickness of the rotor drum is less than 6 % of an inner diameter of the rotor drum. Moreover, it is also preferred if the thickness of the rotor drum is larger than 1 % of an inner diameter of said rotor drum, to ensure a complete mixing.

Figure 1 b illustrates a cross-sectional view of an embodiment of a mixer 1 for mixing gas into pulp having a similar rotor 10. The mixer 1 comprises a chamber 30. The chamber 30 has an inlet 32 for pulp and gas and an outlet 34 for mixed pulp. The inlet 32 for pulp and gas is arranged through a first wall 36 of the chamber 30. The rotor 10 has a rotor drum 20 that is perforated, and the rotor drum has a general cylindrical shape. The rotor 10 is arranged through a second wall 37, opposite to the first wall 36, of the chamber 30. Pulp and gas entering the chamber 30 through the inlet 32 in the axial direction A will flow into the interior of the rotor drum 20 through the first open end 24.

Preferably, an inner radius of the rotor drum at the end facing the inlet for pulp and gas is equal to or larger than a radius of the inlet for pulp and gas. This ensures a smooth entrance into the rotor drum. Due to the closed second wall 37, the pulp and chemical is, when entered into the rotor drum, changing flow direction into a radially directed flow.

The rotor 10 is arranged for rotating the rotor drum 20 around the rotation axis S, which coinciding with an inflow direction of the pulp and gas through the inlet 32 for pulp and gas. The rotor 10 is arranged with a small gap against the front-end surface 28 of the rotor drum 20. The small gap ensured that most of the material travelling from the inlet 32 for pulp and gas to the outlet 34 for mixed pulp will pass through the openings 22 in the rotor drum 20. A mixing of the pulp occurs in a radial direction r when it passes the openings 22, and the mixed pulp exits the chamber 30 through the outlet 34, in this embodiment in the radial direction r.

In the present embodiment, the rotor drum 20 has a constant inner radius. This ensures that the mixing conditions are as homogeneous as possible for all pulp passing the mixer 1.

In the present embodiment, the outlet 34 for mixed pulp is arranged in a direction transverse to the inflow direction of the pulp and gas. However, in alternative embodiments, the output from the chamber 30 may also be provided parallel to the inflow direction.

Figure 1 c illustrates the embodiment of a mixer 1 similar to the one of Figure 1 b in an elevation view. The mixer thus comprises a rotor body in shape of a rotor drum that mixes in radial direction. In the embodiment above, the rotor drum has slits where the pulp can pass through the rotor drum that rotates with a relatively high velocity. The rotation velocity will then shear the pulp so that the properties of the pulp becomes as water, becomes turbulent and is mixed with the gas. The rotor drum is hollow to receive the pulp axially and arranged to change the direction of the pulp to be radially mixed. Since the rotor drum is symmetric, the mixing will be performed around the entire rotor drum. Since the pulp and the gas or liquid have to be transported through the rotor drum openings, the pulp suspension will be subjected to mixing. Since the mixer mixes radially, there will be an increase in pressure due to the addition of energy that will rotate the pulp suspension. This rotation will naturally cause a static pressure increase.

One advantage with the proposed technology is that the mixing zones will maintain a symmetric mixing energy effort. The solution is easily scalable and can be used for large productions without demanding enormous energy efforts or that the machine becomes extremely large. By extending the rotor body, the time in the mixing zones is influenced. The pressure drop through the mixer is reduced since a part of the energy is used for creating an increase of potential by rotation.

Since the drum is hollow, the pulp comes from the inside and passes outwards. By mixing in radially increasing direction, a natural separation cannot occur since the pulp and gas are forced to pass the mixing zone for mixing. If a difference in inner radius of the drum and outer radius of the drum is small, the difference in speed becomes small. At a small difference in speed, about the same mixing intensities will be present around the entire drum. If the mixing intensity can be kept on an even level over the fluidizing point, the mixer will use low amounts of energy.

The openings in the rotor drum can be designed in many different ways. Figure 2 illustrates schematically a part of a rotor drum 20 having openings 22 in the shape of curved slits. Note that, in order to facilitate the understanding of the figures, the drawing is made in the plane of the rotor drum surface, i.e. the depicted plane illustration is in reality a part of a cylindrical surface. The rotor drum 20 is rotated in the direction of the arrow R. The curved shape will tend to move the pulp somewhat towards the middle, which may be advantageous if the pulp tend to get stuck at the ends of the rotor drum 20.

In different embodiments at least a part of the slits is directed in a direction that is non-parallel to the rotation axis of the rotor drum. This is the condition in Figure 2. Another embodiment of such slits is illustrated schematically in Figure 3. Also here the drawing is made in the plane of the rotor drum surface. The slits 23 are here directed in an angle with respect to the rotor drum 20 rotation axis. The slits are also of a non-constant width. In this embodiment, the width is increased in the inner part of the rotor drum 20, closest to the second end 26. This design may take care of tendencies to build up congestions of pulp in the inner part. Flowever, in alternative embodiments, the width may instead be decreased in the inner part of the rotor drum 20. In Figure 4, two types of slits 23 are provided. Also here the drawing is made in the plane of the rotor drum surface. A first type of slits covers essentially the full length of the rotor drum 20, whereas shorter slits are provided there between. Such a design increases the dynamic action of the rotor drum 20, thereby avoiding static flow paths through the mixer.

Figure 5 illustrates a cross-sectional view of one embodiment of a rotor 10 having a rotor drum 20 comprising an inner disc as the second end 26 and an annular part (not shown) as the first end. The first and second end 26 are connected by a number of rods 25 extending along the cylindrical shape of rotor drum 20. The openings 22 in the shape of slits 23 are defined by the rods 25. This also leads to that the openings 22 of the rotor drum 20 have different cross-sections at different radial distances.

Similarly, differing cross-sections at different radial distances can be achieved by other means. Referring back to Fig. 1 a, it can for instance be noticed that the slits 23 in that embodiment are slightly cone-shaped. Flowever, in alternative embodiments, the slits may be designed to be straight. Figure 6 illustrates a part of a cross-sectional view of an embodiment of a rotor drum perpendicular to the rotational axis. In this embodiment, the increasing cross-section in the direction of increasing radial distance is enhanced. The additional tilting of the sides of the slits 23 also results in that the surfaces 19 defining the openings 22 of the rotor drum 20 are inclined in relation to the radial direction r. Such changing cross-section and/or inclined opening surfaces 19 may influence the pressure drop over the openings 22.

Figure 7 illustrates a part of a cross-sectional view of an embodiment of a rotor drum perpendicular to the rotational axis. In this embodiment, the cross-section in the direction of increasing radial distance is constant. Flowever, the tilting of the surfaces 19 of the slits 23 results in that the surfaces 19 defining the openings 22 of the rotor drum 20 are inclined in relation to the radial direction r. The openings of the rotor drum may also be designed in many other ways. Figure 8 illustrates a part of a rotor drum, where the openings 22 are provided in the shape of holes 17.

Also the rotor drum shape can be varied. In the embodiments shown above, the radius of the rotor drum has been constant along the entire axial extension of the rotor drum. Flowever, in alternative embodiments, rotor drums with varying radius may also be used, e.g. rotor drums in the shape of a frustum of a cone.

The motion causing the pulp to become water-like is provided by the rotor drum. Flowever, in order to ensure a high shear action on the pulp suspension, it might in certain applications be advantageous to provide static portions of the mixer in close proximity to the rotor drum. Figure 9A illustrates such an embodiment in an elevated cross-sectional view. Besides the rotor drum 20, a stator drum 40 is arranged concentrically with the rotor drum 20. The stator drum 40 is also perforated. In the present embodiment, the stator drum 40 is positioned radially outside the rotor drum 20. The openings in the stator drum 40 can be of any kind. They can be of the same type as in the rotor drum 20 or different therefrom. Figure 9B is another cross-sectional view of the embodiment of Figure 9A. Flere, it can be seen that the stator drum 40 and rotor drum 20 are concentric. In this particular embodiment, both the stator drum 40 and the rotor drum 20 present straight slits parallel to the rotational axis S of the rotor. In this particular embodiment, the stator drum 40 has more slits than the rotor drum 20, and which stator drum slits are somewhat broader than the rotor drum slits. Flowever, in other embodiments, other relations can be employed.

Figure 10 illustrates another embodiment of a mixer 1 . In this embodiment, the stator drum 40 is positioned radially inside the rotor drum 20.

Figure 1 1 illustrates yet another embodiment of a mixer. In this embodiment, there are two stator drums 40. The stator drums 40 are arranged concentrically with the rotor drum 20. The stator drums 40 are as before perforated. One of the two stator drums 40 is positioned radially outside the rotor drum 20 and the other one of the two stator drums 40 is positioned radially inside the rotor drum 20.

The rotor may further be provided with inner protruding portions, protruding into a volume inside the rotor drum. Figure 12 illustrates one such embodiment, where the inner protruding portions 42 protrude inwards from an inner surface 41 of the rotor drum 20. The shape, direction and position in circumferential and axial directions of the inner protruding portions 42 may be adapted according to different applications. The provision of the inner protruding portions 42 may improve e.g. pulp flow, angular distribution of pulp flow and/or pre-mixing of gas into the pulp.

Figure 13 illustrates another embodiment with protruding parts inside the rotor drum 20. Flere, the inner protruding portions 42 protrude outwards towards an inner surface 41 of the rotor drum 20.

Supporting protruding parts may also be provided outside the rotor drum. In Figure 14, the rotor 10 further comprises outer protruding portions 44, protruding into a volume 46 outside the rotor drum. In this embodiment, the outer protruding portions 44 are attached to an outer surface 45 of the rotor drum 20. By these outer protruding portions 44, flow properties of the pulp outside the rotor drum can be influenced.

Figure 15 illustrates a schematic cross-sectional view of an embodiment of a mixer 1 for mixing gas into pulp. In this embodiment, the inner protruding portions 42 comprise a rotationally symmetric flow directing structure 29 provided at the rotational axis S. In such a mixer 1 , the incoming pulp and chemical substances, travelling substantially in the axial direction A will be deviated by the flow directing structure 29 to obtain at least a velocity component in the radial direction r. In the illustrated embodiment, the flow directing structure 29 is illustrated as a cone. However, in alternative embodiments also other designs with rotationally symmetric bodies having a successively increasing diameter along the axial direction can be used. The rotor of the mixer further comprises a gas distribution arrangement, comprising a plurality of elongated gas distribution elements extending in a direction generally parallel to the rotation axis.

Figures 16a and 16b illustrates a schematic cross-sectional view of an embodiment of a mixer 1 for mixing gas into pulp. The mixer 1 is generally described with reference to Figure 1 a-1 c above, but further includes inlet duct 31 connected to the inlet for pulp and gas 32 of the mixer, said inlet duct 31 having a number of gas injection nozzles 33 for injecting the gas to be mixed with the pulp. The mixer 1 further includes a gas distribution arrangement 2. The gas distribution arrangement 2 is attached to the rotor drum 20 opposite to the inlet for pulp and gas 32. The gas distribution arrangement 2 includes 5 elongated gas distribution elements 4 (not all shown) arranged around the rotation axis S, between the rotation axis and the cylindrical wall 41 of the rotor drum. The gas distribution elements 4 are arranged symmetrically around the rotation axis S. The gas distribution elements 4 are formed of square profile steel bars having a thickness of about 4% of the inner diameter of the rotor drum 20.

As the gas distribution elements 4 may be subjected to significant mechanical stress during mixing, the gas distribution arrangement 2 may further comprise a support structure 6, in the form of a steel ring fixating the gas distribution elements to each other near a distal end 8 of the gas distribution elements 4, for preventing deformation of the gas distribution elements during rotation. The inventors have found that the effect of the gas distribution arrangement can be significantly improved if the gas distribution elements are positioned close to the point where the gas is injected into the pulp. In the embodiment of Figures 16a and 16b, the gas distribution arrangement 2 and the inner diameter of the inlet for pulp and gas 32 are concentric and the outer rotational diameter of the gas distribution arrangement 2 is slightly smaller than the inner diameter of the inlet for pulp and gas 32. The length of the gas distribution elements 4 exceeds the length the mixer chamber 30 and the gas distribution elements extend partially through the inlet for pulp and gas 32 and into the inlet duct 31. Accordingly, the gas distribution elements 4 will be positioned closer to the injection nozzles 33 where the gas is injected into the pulp. This configuration provides for immediate contact between the incoming pulp and gas and the gas distribution elements, before centrifugal separation of the pulp and gas has occurred.

An alternative embodiment is shown in Figure 17a. In this embodiment, the gas distribution elements 4’ are shorter, and overlap only partially with the axial length of the mixer chamber 30. Instead, the point where the gas is injected into the pulp is placed closer to the gas distribution elements 4’ by arranging the gas injection nozzles 33’ inside the mixer chamber. Another embodiment is shown in Figure 17b. In this embodiment, the gas distribution elements 4” are attached to the open end 24, rather than to the closed end 26, of the rotor drum 20. The gas distribution elements 4” overlap only partially with the axial length of the mixer chamber 30. Instead, the gas distribution elements 4” extend partially through the inlet 32 for pulp and gas and into the inlet duct 31. Accordingly, the gas distribution elements 4” will be positioned closer to the injection nozzles 33 where the gas is injected into the pulp. This configuration provides for immediate contact between the incoming pulp and gas and the gas distribution elements, before centrifugal separation of the pulp and gas has occurred. The gas distribution arrangement can be obtained in various ways. In some embodiments, the plurality of are provided in the form of individual elements, e.g. bars or rods. The individual elements are then assembled with the rotor drum, and optionally with each other, to form the gas distribution arrangement. In some embodiments, the plurality of gas distribution elements instead constitute portions of a unitary structure, e.g. in the form of a slitted drum or cage like structure having closed elongated portions (forming the gas distribution elements) separated by open elongated portions. The distribution of the gas may be further improved by introducing a second set of gas distribution elements (not shown) at a different radius from the first set of gas distribution elements. Thus, in some embodiments the gas distribution arrangement may include at least two sets of gas distribution elements, a first set of gas distribution elements being arranged at a first radial distance and a second set of gas distribution elements being arranged at a second radial distance between the rotation axis and the cylindrical wall of the rotor drum.

The gas intended to be mixed with the pulp can be of essentially any kind. In one embodiment, the gas comprises bleaching agents.

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1 . A mixer (1 ) for mixing a gas into pulp, comprising:

a chamber (30) having at least one inlet (32) for pulp and gas and at least one outlet (34) for pulp-gas mixture; said at least one inlet (32) for pulp and gas being arranged through a first wall (36) of said chamber (30);

a rotor (10) comprising a rotor drum (20) connected to a drive shaft (12);

said rotor drum (20) having a hollow cylindrical shape with an open end (24) facing said inlet (32) for pulp and gas and a closed end (26) connected to the drive shaft (12), and having openings (22) through a cylindrical wall (41 ) thereof;

said drive shaft (12) being arranged through a second wall (37), opposite to said first wall (36), of said chamber (30) and arranged for rotating said rotor drum (20) around a rotation axis (S) coinciding with an inflow direction (A) of said pulp and gas through said at least one inlet (32) for pulp and gas;

wherein at least some of the pulp and gas flowing from said inlet (32) for pulp and gas to said outlet (34) for the pulp-gas mixture will pass through said openings (22) in the cylindrical wall (41 ) of the rotor drum (20);

characterized in that the rotor (10) further comprises a gas distribution arrangement (2), said gas distribution arrangement (2) comprising a plurality of elongated gas distribution elements (4) extending in a direction generally parallel to the rotation axis (S), said gas distribution elements (4) being arranged around the rotation axis (S), between the rotation axis (S) and the cylindrical wall (41 ) of the rotor drum (20).

2. The mixer (1 ) according to claim 1 , characterized in that said gas distribution elements (4) are attached to the closed end (26) of said rotor drum (20) and/or to the cylindrical wall (41 ) of said rotor drum (20).

3. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution arrangement (2) comprises in the range of 2-12 gas distribution elements (4), preferably in the range of 4-10 gas distribution elements (4), more preferably in the range of 4-8 gas distribution elements (4).

4. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution elements (4) are arranged symmetrically around the rotation axis (S).

5. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution elements (4) are in the form of bars or rods.

6. The mixer (1 ) according to any one of the preceding claims, characterized in that a radial thickness of said gas distribution elements (4) is less than 10 % of an inner diameter of said rotor drum (20), preferably less than 6 % of an inner diameter of said rotor drum (20).

7. The mixer (1 ) according to any one of the preceding claims, characterized in that a radial thickness of said gas distribution elements (4) is larger than 1 % of an inner diameter of said rotor drum (20).

8. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution arrangement (2) further comprises a support structure (6) for preventing deformation of the gas distribution elements (4) during rotation.

9. The mixer (1 ) according to any one of the preceding claims, characterized in that said plurality of gas distribution elements (4) are of the same length.

10. The mixer (1 ) according to any one of claims 1 to 8, characterized in that said plurality of gas distribution elements (4) are of different lengths.

1 1 . The mixer (1 ) according to any one of the preceding claims, characterized in that said mixer (1 ) further comprises an inlet duct (31 ) connected to the at least one inlet (32) for pulp and gas of the mixer, said inlet duct (31 ) having at least one gas injection nozzle (33; 33’).

12. The mixer (1 ) according to any one of the preceding claims, characterized in that said elongated gas distribution elements (4) extend through said at least one inlet (32) for pulp and gas.

13. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution elements (4) are curved or angled in relation to the rotation axis (S).

14. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution elements (4) have a wave or helix shape.

15. The mixer (1 ) according to any one of the preceding claims, characterized in that a cross sectional area of said gas distribution elements (4) varies over the longitudinal extension of the gas distribution elements (4).

16. The mixer (1 ) according to any one of the preceding claims, characterized in that said plurality of gas distribution elements (4) are provided in the form of individual elements.

17. The mixer (1 ) according to any one of the preceding claims, characterized in that said plurality of gas distribution elements (4) constitute portions of a unitary structure.

18. The mixer (1 ) according to any one of the preceding claims, characterized in that said gas distribution arrangement (2) comprises at least two sets of gas distribution elements (4), a first set of gas distribution elements (4) being arranged at a first radial distance and a second set of gas distribution elements (4) being arranged at a second radial distance between the rotation axis (S) and the cylindrical wall (41 ) of the rotor drum (20).

19. The mixer (1 ) according to any one of the preceding claims, characterized in that said outlet (34) for pulp-gas mixture is arranged in a direction transverse to said inflow direction (A) of said pulp and gas.

20. The mixer (1 ) according to any one of the preceding claims, characterized in that an inner radius of said rotor drum (20) at the end (24) facing said inlet (32) for pulp and gas is equal to or larger than a radius of said inlet (32) for pulp and gas.

21. The mixer (1 ) according to any one of the preceding claims, characterized in that said rotor drum (20) has a constant inner radius.