SE542954C2

SE542954C2 - Mixer for mixing a gas into pulp comprising a rotor, said rotor comprising a rotor drum

Info

Publication number: SE542954C2
Application number: SE1950312A
Authority: SE
Inventors: Mikael Löfgren; Philip Lundman
Original assignee: Valmet Oy
Priority date: 2019-03-13
Filing date: 2019-03-13
Publication date: 2020-09-22
Also published as: SE1950312A1; WO2020185135A1

Abstract

MIXER FOR MIXING A GAS INTO PULP COMPRISING A ROTOR. SAID ROTOR COMPRISING A ROTOR DRUMTechnical fieldThe 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.BackgroundIn 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.

Description

MIXER FOR MIXING A GAS INTO PULP COMPRISING A ROTOR, SAID ROTORCOMPRISING A ROTOR DRUM Technical fieldThe present disclosure relates to mixers for mixing a lower density medium, suchas 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.

Backgroundln many industrial applications, such as within different chemical industries where various chemicals have to be mixed with different suspensions of raw material orinto combinations of raw material, it is of high importance to homogenize mediumshaving different properties such as different densities to obtain homogenoussuspensions. Such industries include paint-making, pulp- and paper-manufacturingindustries.

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

When mixing treatment media into a fiber suspension, it is of high importance forthe mixing result that an even or homogenous distribution is achieved. When themixed media have very different densities, such as when a gaseous medium ismixed with medium having higher density, such as a pulp suspension, particularconsiderations 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, therotor of the mixer may have a center axis aligned with or arranged perpendicularlyto the general flow direction of the media. Also, the outlet and inlet of the mixer may 2 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 toaccumulate near the rotor center due to the centrifugal force. This undesiredphenomenon 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 mixingmedia or f|uids with different density properties and, in particular, to mixingapparatuses or arrangements for mixing gaseous treatment media into a fiber suspension, such as a lignocellulosic pulp suspension.

Summarv of the invention lt is an object of the present disclosure to a||eviate 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 arrangementthat 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 theskilled 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; 3 said rotor drum having a hollow cylindrical shape with an open end facing said inletfor pulp and gas and a closed end connected to the drive shaft, and having openingsthrough a cylindrical wall thereof; said drive shaft being arranged through a second wall, opposite to said first wall, ofsaid chamber and arranged for rotating said rotor drum around a rotation axiscoinciding with an inflow direction (A) of said pulp and gas through said at least oneinlet for pulp and gas; wherein at least some of the pulp and gas flowing from said inlet for pulp and gas tosaid 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 gasdistribution arrangement comprising a plurality of elongated gas distributionelements extending in a direction generally parallel to the rotation axis, said gasdistribution 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 fedthrough an inlet into the mixer chamber, mixed by being passed through theopenings in the cylindrical wall of the rotating rotor drum, and the pulp-gas mixtureexits the chamber via an outlet. The outlet for the pulp-gas mixture is preferablyarranged 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 themixer to larger dimensions. The mixing energy necessary for accomplishing themixing increases drastically if the dimensions of the mixer are increased in the radialdirection. The peripheral speed increases with increasing machine size if a constantrotating speed is maintained. lncreased peripheral speed in turn leads to increasedcentrifugal 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. 4 ln the mixer according to the present disclosure, a gas distribution arrangementcomprising a plurality of elongated gas distribution elements is used to prevent thegas and pulp from separating. The elongated gas distribution elements, e.g. in theform of rods or bars, extend in a direction generally parallel to the rotation axis ofthe drum at a radia| distance between the rotation axis and the cylindrical wall of therotor drum. The elongated gas distribution elements are attached to and configuredto rotate with the rotor drum. ln some embodiments, the gas distribution elementsare 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 isformed behind the trailing surface of each element. The slipstream has been foundto lead to the formation of a gas rich trail behind the elements. This formationreduces the tendency of the gas to migrate towards the rotor center. lnstead, therotational motion of the elements results in a distribution of the gas along the axiallength of the gas distribution elements, and in turn to the formation of a cylindricalaccumulation of gas through which at least part of the pulp passes. This effectincreases the contact between the pulp and gas compared to the case where a largeportion of the gas accumulates near the rotor center, and as a result improves thedistribution of the gas in the pulp before it reaches the openings in the cylindricalwall of the rotor drum where mixing occurs. The improved gas distribution prior tothe 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 thesolution is easily scalable and can be used for large-scale production withoutexcessive energy requirements or that the machine becomes extremely large, at the same preventing separation and inhomogeneous mixing. ln 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.ln 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. ln 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 gasdistribution elements are preferably arranged symmetrically around the rotation axis. ln some embodiments, the outer rotational diameter of the gas distributionarrangement is smaller than the inner diameter of the inlet for pulp and gas. Thisconfiguration provides for immediate contact between the incoming pulp and gasand the gas distribution elements, before centrifugal separation of the pulp and gashas occurred. ln some embodiments, the gas distribution arrangement and the innerdiameter of the inlet for pulp and gas are concentric. A concentric configuration ofthe gas distribution arrangement and the inlet provides for the most even gasdistribution.

The shape and size of the gas distribution elements is preferably selected to providea certain flow resistance during rotation through the incoming pulp and gas. Thecross-sectional shape of the gas distribution elements may for example bequadrangular, such as rectangular or square, triangular or round, such as circularof oval. The size of the gas distribution elements, particularly the radial thickness,should preferably selected to be large enough to achieve a significant gasdistributing effect. However, too large gas distribution elements will cause anunnecessary increase of the energy consumption. A radial thickness of in the rangeof 1-10 %, preferably in the range of 1-6 %, of an inner diameter of said rotor drumhas been found to provide a suitable compromise. ln some embodiments, the radialthickness of said gas distribution elements is less than 10 % of an inner diameter ofsaid rotor drum, preferably less than 6 % of an inner diameter of said rotor drum. lnsome 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 stressduring mixing, the gas distribution arrangement may further comprise a support 6 structure for preventing deformation of the gas distribution elements during rotation.The support structure may for example comprise transverse reinforcement bars ora 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 therotor drum. The length of the gas distribution elements may therefore preferably beselected so as to overlap with the axial extension of the rotor drum. ln someembodiments, the length of the gas distribution elements will correspond to the inneraxial length of the rotor drum, whereas in some embodiments the gas distributionelements will be longer or shorter.

The plurality of gas distribution elements may be of the same length or of differentlengths. ln some embodiments, the gas distribution elements are curved or angledin relation to the rotation axis. ln some embodiments, the gas distribution elementshave a wave or helix shape. ln some embodiments, the cross-sectional area of saidgas distribution elements varies over the longitudinal extension of the gas distribution elements. ln some embodiments, the mixer further comprises an inlet duct connected to the atleast one inlet for pulp and gas of the mixer, said inlet duct having at least one gasinjection nozzle.

The inventors have found that the effect of the gas distribution arrangement can besignificantly improved if the gas distribution elements are positioned close to thepoint where the gas is injected into the pulp. For example, where gas is injected intothe pulp in the duct just before it reaches the mixer, the gas distribution elementsmay advantageously extend through said at least one inlet for pulp and gas and intothe duct, such that they will be positioned closer to the point where the gas is injectedinto the pulp. Thus, in some embodiments, the elongated gas distribution elements extend through said at least one inlet for pulp and gas. 7 Alternatively, in some embodiments the at least one gas injection nozzle is arrangedat 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 closerto 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. ln someembodiments, 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, andoptionally with each other, to form the gas distribution arrangement. ln someembodiments, the plurality of gas distribution elements instead constitutes portionsof a unitary structure, e.g. in the form of a slitted drum or cage like structure havingclosed elongated portions (forming the gas distribution elements) separated by openelongated portions.

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

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

Brief description of the drawinqsThe invention, together with further objects and advantages thereof, will now bedescribed more in detail with reference to the following drawings, in which the same reference numbers are used for similar or corresponding elements: 8 FIG. 1a schematically illustrates a rotor for use in a mixer for mixing gas into pulp;FIG. 1b illustrates a cross-sectional view of a mixer for mixing gas into pulp; FIG. 1c illustrates an embodiment of the inventive mixer in an e|evation 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 rotordrum; FIGS. 6-7 illustrates a part of a cross-sectional view of embodiments of a rotor drumperpendicular 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 e|evated 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 positionedradially 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 providedwith inner protruding portions; FIG. 13 illustrates another embodiment of a mixer with protruding parts inside therotor drum; FIG. 14 illustrates yet another embodiment of a mixer, where the rotor is providedwith outer protruding portions; and FIG. 15 illustrates a cross-sectional view of yet an embodiment of a mixer for mixinggas into pulp; FIG. 16a and 16b are cross-sectional views ofan embodiment of the inventive mixercomprising 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. ln an axial mixing, the pulp and gas flow between a rotor and housing. lf the agitationof the rotor causes an efficient mixing, the length of the mixing zone can be kept 9 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 mixerradius has to be increased. An increase in pulp flow velocity is energy demandingand the conditions in the mixing zone may also deteriorate, which may require anincrease of the mixing zone length as well. lf the radius of the mixer is increased,the energy requirements to reach the same rotational speed varies approximatelyas the square of the radius, which means that the required energy increases fasterthan the radius.

Mixing pulp in an axial mixing at different radial distances can give rise to unevenmixing, since the velocity of any agitating parts varies with the radial distance. lnlarge machines, having a large diameter, there may also be a centrifugal separationbehavior, as mentioned in the background, tending to separate pulp and gas in theradial direction. This may also cause an uneven mixing. Such difficulties may in partbe prevented by limiting the radial extension of the mixing zone. However, mixing atone specific radius reduces the benefit of diameter increase. The increaseddiameter will in such cases only give rise to approximately linear scalar scaling-up of the cross-sectional area of the mixing zone. ln 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 mixingphase. ln 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-gasmixture exits the mixer in a radial or axial direction. Mixing is caused by the rotationalmovement 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 increasingthe axial length. lncreasing the axial length of the mixing zone will increase thethroughput linearly. The scaling in axial length also increases the required energyapproximately 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 approximatelya cubic energy increase, but at the same time a higher turbulence is achieved in themixing zone, which probably improves the mixing. The increase in throughputbecomes approximately linear. The mixing energy necessary for accomplishing themixing increases drastically if the dimensions of the mixer are increased in the radialdirection. The peripheral speed increases with increasing machine size if a constantrotating speed is maintained. lncreased peripheral speed in turn leads to increasedcentrifugal 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 andpulp due to the centrifugal force. ln one embodiment, a mixer for mixing a gas into pulp comprises a chamber and arotor. The chamber has an inlet for pulp and gas and an outlet for mixed pulp. Theinlet for pulp and gas is arranged through a first wall of the chamber. The rotor hasa rotor drum having a hollow cylindrical shape with an open end facing the inlet forpulp and gas and a closed end connected to a drive shaft. The rotor drum gasopenings, i.e. perforations through a cylindrical wall thereof. The rotor is arrangedthrough a second wall, opposite to the first wall, of the chamber. The rotor isarranged for rotating the rotor drum around a rotation axis coinciding with an inflowdirection of the pulp and gas through the inlet for pulp and gas. During operation ofthe mixers, at least some of the pulp and gas flowing from the inlet for pulp and gasto the outlet for the pulp-gas mixture will pass through said openings in the cylindrical wall of the rotor drum.

Figure 1a illustrates schematically a rotor 10 for use in a mixer for mixing gas intopulp. The rotor 10 comprises a shaft 12 and a rotor drum 20. The rotor drum 20 hasa number of openings 22, in this embodiment in the shape of slits 23. ln other words,the rotor drum 20 defines openings 22. The slits 23 are elongated in an axialdirection A of the rotor 10. Pulp and gas are intended to be introduced into a firstopen end 24 of the rotor drum 20 with a flow direction parallel to the axial directionA. A second end 26, opposite to the first end 24, of the rotor drum 20 is closed and 11 attached to the shaft 12. This forces the pulp and gas to change their flow directioninto a mainly radial flow direction, indicated by the reference r. The pulp and gascome into contact with the rotor drum 20 when it tries to escape through theopenings 22. Since the rotor drum is intended to rotate in a rotation direction R, thismotion 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 theopenings 22, i.e. in the present embodiment the slits 23, in a radial direction. Therotor drum 20 has in this embodiment a front-end surface 28. ln one embodiment, the rotor drum 20 defines slits 23. The slits 23 have their mainextension direction directed non-perpendicular with respect to the rotation axis ofthe rotor 10. ln one embodiment, the slits are straight slits. However, as is described further below, also other shapes are feasible in alternative embodiments. ln the embodiment of Figure 1a, 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, whichin turn to some degree determines the width of the mixing zone. A long mixing zonehaving changing radii may lead to differing mixing conditions at the beginning andend, respectively, of the mixing zone. On the other hand, a too short mixing zonemay instead lead to an incomplete mixing. lt has been found that a thickness of therotor drum that is less than 10 % of an inner diameter of the rotor drum gives rise toacceptable small mixing differences. However, preferably, a thickness of the rotordrum is less than 6 % of an inner diameter of the rotor drum. Moreover, it is alsopreferred 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 1b illustrates a cross-sectional view of an embodiment of a mixer 1 for mixinggas into pulp having a similar rotor 10. The mixer 1 comprises a chamber 30. Thechamber 30 has an inlet 32 for pulp and gas and an outlet 34 for mixed pulp. Theinlet 32 for pulp and gas is arranged through a first wall 36 of the chamber 30. The 12 rotor 10 has a rotor drum 20 that is perforated, and the rotor drum has a generalcylindrical shape. The rotor 10 is arranged through a second wall 37, opposite to thefirst wall 36, of the chamber 30. Pulp and gas entering the chamber 30 through thein|et 32 in the axial direction A will flow into the interior of the rotor drum 20 throughthe first open end 24.

Preferabiy, an inner radius of the rotor drum at the end facing the in|et for pulp andgas is equal to or larger than a radius of the in|et for pulp and gas. This ensures asmooth entrance into the rotor drum. Due to the closed second wall 37, the pulp andchemical is, when entered into the rotor drum, changing flow direction into a radiallydirected 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 in|et 32 forpulp and gas. The rotor 10 is arranged with a small gap against the front-end surface28 of the rotor drum 20. The small gap ensured that most of the material travellingfrom the in|et 32 for pulp and gas to the outlet 34 for mixed pulp will pass throughthe openings 22 in the rotor drum 20. A mixing of the pulp occurs in a radial directionr when it passes the openings 22, and the mixed pulp exits the chamber 30 throughthe outlet 34, in this embodiment in the radial direction r. ln the present embodiment, the rotor drum 20 has a constant inner radius. Thisensures that the mixing conditions are as homogeneous as possible for all pulp passing the mixer 1. ln the present embodiment, the outlet 34 for mixed pulp is arranged in a directiontransverse to the inﬂow direction of the pulp and gas. However, in alternativeembodiments, the output from the chamber 30 may also be provided parallel to the inflow direction.

Figure 1c illustrates the embodiment of a mixer 1 similar to the one of Figure 1b in an elevation view. 13 The mixer thus comprises a rotor body in shape of a rotor drum that mixes in radialdirection. ln the embodiment above, the rotor drum has slits where the pulp canpass through the rotor drum that rotates with a relatively high velocity. The rotationvelocity 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 receivethe 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 entirerotor drum. Since the pulp and the gas or liquid have to be transported through therotor drum openings, the pulp suspension will be subjected to mixing. Since themixer mixes radially, there will be an increase in pressure due to the addition ofenergy that will rotate the pulp suspension. This rotation will naturally cause a static DFGSSUFG lnCfeaSe.

One advantage with the proposed technology is that the mixing zones will maintaina symmetric mixing energy effort. The solution is easily scalable and can be usedfor large productions without demanding enormous energy efforts or that themachine becomes extremely large. By extending the rotor body, the time in themixing zones is inﬂuenced. The pressure drop through the mixer is reduced since apart 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. Bymixing in radially increasing direction, a natural separation cannot occur since thepulp and gas are forced to pass the mixing zone for mixing. lf a difference in innerradius of the drum and outer radius of the drum is small, the difference in speedbecomes small. At a small difference in speed, about the same mixing intensitieswill be present around the entire drum. lf 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 2illustrates schematically a part of a rotor drum 20 having openings 22 in the shapeof curved slits. Note that, in order to facilitate the understanding of the figures, thedrawing is made in the plane of the rotor drum surface, i.e. the depicted planeillustration is in reality a part of a cylindrical surface. The rotor drum 20 is rotated inthe direction of the arrow R. The curved shape will tend to move the pulp somewhat 14 towards the middle, which may be advantageous if the pulp tend to get stuck at the ends of the rotor drum 20. ln different embodiments at least a part of the slits is directed in a direction that isnon-parallel to the rotation axis of the rotor drum. This is the condition in Figure 2.Another embodiment of such slits is i||ustrated schematically in Figure 3. Also herethe drawing is made in the plane of the rotor drum surface. The slits 23 are heredirected in an angle with respect to the rotor drum 20 rotation axis. The slits are alsoof a non-constant width. ln this embodiment, the width is increased in the inner partof the rotor drum 20, closest to the second end 26. This design may take care oftendencies to build up congestions of pulp in the inner part. However, in alternativeembodiments, the width may instead be decreased in the inner part of the rotor drum20. ln Figure 4, two types of slits 23 are provided. Also here the drawing is made in theplane of the rotor drum surface. A first type of slits covers essentially the full lengthof the rotor drum 20, whereas shorter slits are provided there between. Such adesign 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 arotor 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 numberof rods 25 extending along the cylindrical shape of rotor drum 20. The openings 22in the shape of slits 23 are defined by the rods 25. This also leads to that theopenings 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 byother means. Referring back to Fig. 1a, it can for instance be noticed that the slits23 in that embodiment are slightly cone-shaped. However, 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 drumperpendicular to the rotational axis. ln this embodiment, the increasing cross-sectionin the direction of increasing radial distance is enhanced. The additional tilting of thesides of the slits 23 also results in that the surfaces 19 defining the openings 22 ofthe rotor drum 20 are inclined in relation to the radial direction r. Such changingcross-section and/or inclined opening surfaces 19 may influence the pressure dropover the openings 22.

Figure 7 illustrates a part of a cross-sectional view of an embodiment of a rotor drumperpendicular to the rotational axis. ln this embodiment, the cross-section in thedirection of increasing radial distance is constant. However, the tilting of the surfaces19 of the slits 23 results in that the surfaces 19 defining the openings 22 of the rotordrum 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 8illustrates a part of a rotor drum, where the openings 22 are provided in the shapeof holes 17.

Also the rotor drum shape can be varied. ln the embodiments shown above, theradius of the rotor drum has been constant along the entire axial extension of therotor drum. However, 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.However, in order to ensure a high shear action on the pulp suspension, it might incertain applications be advantageous to provide static portions of the mixer in closeproximity to the rotor drum. Figure 9A illustrates such an embodiment in an elevatedcross-sectional view. Besides the rotor drum 20, a stator drum 40 is arrangedconcentrically with the rotor drum 20. The stator drum 40 is also perforated. ln thepresent embodiment, the stator drum 40 is positioned radially outside the rotor drum20. The openings in the stator drum 40 can be of any kind. They can be of the sametype as in the rotor drum 20 or different therefrom. 16 Figure 9B is another cross-sectional view of the embodiment of Figure 9A. Here, itcan be seen that the stator drum 40 and rotor drum 20 are concentric. ln thisparticular embodiment, both the stator drum 40 and the rotor drum 20 presentstraight slits parallel to the rotationa| axis S of the rotor. ln this particularembodiment, the stator drum 40 has more slits than the rotor drum 20, and whichstator drum slits are somewhat broader than the rotor drum slits. However, in otherembodiments, other relations can be employed.

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

Figure 11 illustrates yet another embodiment of a mixer. ln this embodiment, thereare two stator drums 40. The stator drums 40 are arranged concentrically with therotor drum 20. The stator drums 40 are as before perforated. One of the two statordrums 40 is positioned radially outside the rotor drum 20 and the other one of thetwo stator drums 40 is positioned radially inside the rotor drum 20.

The rotor may further be provided with inner protruding portions, protruding into avolume inside the rotor drum. Figure 12 illustrates one such embodiment, where theinner protruding portions 42 protrude inwards from an inner surface 41 of the rotordrum 20. The shape, direction and position in circumferential and axial directions ofthe 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, angulardistribution of pulp flow and/or pre-mixing of gas into the pulp.

Figure 13 illustrates another embodiment with protruding parts inside the rotor drum20. Here, the inner protruding portions 42 protrude outwards towards an innersurface 41 of the rotor drum 20.

Supporting protruding parts may also be provided outside the rotor drum. ln Figure14, the rotor 10 further comprises outer protruding portions 44, protruding into avolume 46 outside the rotor drum. ln this embodiment, the outer protruding portions44 are attached to an outer surface 45 of the rotor drum 20. By these outer 17 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 mixer1 for mixing gas into pulp. ln this embodiment, the inner protruding portions 42comprise a rotationally symmetric flow directing structure 29 provided at therotational axis S. ln such a mixer 1, the incoming pulp and chemical substances,travelling substantially in the axial direction A will be deviated by the flow directingstructure 29 to obtain at least a velocity component in the radial direction r. ln theillustrated embodiment, the flow directing structure 29 is illustrated as a cone.However, in alternative embodiments also other designs with rotationally symmetricbodies having a successively increasing diameter along the axial direction can be used.

The rotor of the mixer further comprises a gas distribution arrangement, comprisinga 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 embodimentof a mixer 1 for mixing gas into pulp. The mixer 1 is generally described withreference to Figure 1a-1c above, but further includes inlet duct 31 connected to theinlet for pulp and gas 32 of the mixer, said inlet duct 31 having a number of gasinjection nozzles 33 for injecting the gas to be mixed with the pulp. The mixer 1further includes a gas distribution arrangement 2. The gas distribution arrangement2 is attached to the rotor drum 20 opposite to the inlet for pulp and gas 32. The gasdistribution arrangement 2 includes 5 elongated gas distribution elements 4 (not allshown) arranged around the rotation axis S, between the rotation axis and thecylindrical wall 41 of the rotor drum. The gas distribution elements 4 are arrangedsymmetrically around the rotation axis S. The gas distribution elements 4 are formedof 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 stressduring mixing, the gas distribution arrangement 2 may further comprise a support 18 structure 6, in the form of a steel ring fixating the gas distribution elements to eachother near a distal end 8 of the gas distribution elements 4, for preventingdeformation of the gas distribution elements during rotation.

The inventors have found that the effect of the gas distribution arrangement can besignificantly improved if the gas distribution elements are positioned close to thepoint where the gas is injected into the pulp. ln the embodiment of Figures 16a and16b, the gas distribution arrangement 2 and the inner diameter of the inlet for pulpand gas 32 are concentric and the outer rotational diameter of the gas distributionarrangement 2 is slightly smaller than the inner diameter of the inlet for pulp and gas32. The length of the gas distribution elements 4 exceeds the length the mixerchamber 30 and the gas distribution elements extend partially through the inlet forpulp and gas 32 and into the inlet duct 31 _ Accordingly, the gas distribution elements4 will be positioned closer to the injection nozzles 33 where the gas is injected intothe pulp. This configuration provides for immediate contact between the incomingpulp and gas and the gas distribution elements, before centrifugal separation of thepulp and gas has occurred.

An alternative embodiment is shown in Figure 17a. ln this embodiment, the gasdistribution elements 4' are shorter, and overlap only partially with the axial lengthof the mixer chamber 30. lnstead, the point where the gas is injected into the pulpis placed closer to the gas distribution elements 4' by arranging the gas injectionnozzles 33' inside the mixer chamber.

Another embodiment is shown in Figure 17b. ln this embodiment, the gasdistribution elements 4” are attached to the open end 24, rather than to the closedend 26, of the rotor drum 20. The gas distribution elements 4” overlap only partiallywith the axial length of the mixer chamber 30. lnstead, the gas distribution elements4” 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 injectionnozzles 33 where the gas is injected into the pulp. This configuration provides forimmediate contact between the incoming pulp and gas and the gas distributionelements, before centrifugal separation of the pulp and gas has occurred. 19 The gas distribution arrangement can be obtained in various ways. ln someembodiments, 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, andoptionally with each other, to form the gas distribution arrangement. ln someembodiments, the plurality of gas distribution elements instead constitute portionsof a unitary structure, e.g. in the form of a slitted drum or cage like structure havingclosed elongated portions (forming the gas distribution elements) separated by openelongated portions.

The distribution of the gas may be further improved by introducing a second set ofgas distribution elements (not shown) at a different radius from the first set of gasdistribution elements. Thus, in some embodiments the gas distribution arrangementmay include at least two sets of gas distribution elements, a first set of gasdistribution elements being arranged at a first radial distance and a second set ofgas distribution elements being arranged at a second radial distance between therotation axis and the cylindrical wall of the rotor drum.

The gas intended to be mixed with the pulp can be of essentially any kind. ln oneembodiment, the gas comprises bleaching agents.

While the invention has been described with reference to various exemplaryembodiments, it will be understood by those skilled in the art that various changesmay be made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention. ln addition, many modifications may bemade to adapt a particular situation or material to the teachings of the inventionwithout departing from the essential scope thereof. Therefore, it is intended that theinvention not be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the invention will include allembodiments 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 arrangedthrough 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) facingsaid 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 saidfirst 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 gasthrough 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 andgas 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 distributionarrangement (2), said gas distribution arrangement (2) comprising a plurality ofelongated gas distribution elements (4) extending in a direction generally parallel tothe rotation axis (S), said gas distribution elements (4) being arranged around therotation axis (S), between the rotation axis (S) and the cylindrical wall (41) of therotor drum (20).

2. The mixer (1) according to claim 1, characterized in that said gas distributionelements (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 inthat said gas distribution arrangement (2) comprises in the range of 2-12 gasdistribution 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). 21

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

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

7. The mixer (1) according to any one of the preceding claims, characterized inthat a radia| thickness of said gas distribution elements (4) is larger than 1 % of aninner diameter of said rotor drum (20).

8. The mixer (1) according to any one of the preceding claims, characterized inthat 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 inthat 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 saidplurality of gas distribution elements (4) are of different lengths.

11. The mixer (1) according to any one of the preceding claims, characterized inthat said mixer (1 ) further comprises an inlet duct (31) connected to the at least oneinlet (32) for pulp and gas of the mixer, said inlet duct (31) having at least one gasinjection nozzle (33; 33').

12. The mixer (1) according to any one of the preceding claims, characterized inthat said elongated gas distribution elements (4) extend through said at least oneinlet (32) for pulp and gas. 22

13. The mixer (1) according to any one of the preceding claims, characterized inthat 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 inthat a cross sectiona| 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 inthat 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 inthat said plurality of gas distribution elements (4) constitute portions of a unitarystructure.

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

19. The mixer (1) according to any one of the preceding claims, characterized inthat 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 inthat 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. 23

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