TECHNICAL FIELD

The present invention relates to a mixing apparatus for mixing media or fluids with different density properties and, in particular to a mixing apparatus for mixing treatment media into a fiber suspension, e.g. lignocellulosic pulp

suspension.

BACKGROUND OF THE INVENTION

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 media having different properties such as different densities to obtain homogenous suspensions. Such industries include paint manufacturing and pulp and papermanufacturing.

Within the pulp and paper 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 kind of media into the pulp suspension. The treatment media added to 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.

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. In many cylindrical mixer configurations, particularly wherein the outlet is arranged perpendicularly to the inlet and a center axis of the outlet is perpendicular to a center axis of the rotor, or in a tangential direction in relation to the rotor, there is a remaining rotation of the media flowing out from the mixer, e.g. the pulp suspension. This remaining rotation of the pulp suspension may cause a separation of media or fluids when the mixed media have different densities due to the centrifugal force, which is an undesired phenomenon particularly frequent when gaseous media are mixed with media with higher density such as pulp.

Thus, there is a need for improved mixing apparatuses for mixing media or fluids with different density properties and, in particular, for mixing apparatuses for mixing treatment media into a fiber suspension, e.g. lignocellulosic pulp

suspension.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved mixing apparatus for mixing media or fluids with different density properties.

Another object of the present invention, is to provide an improved mixing apparatus for mixing treatment media into a fiber suspension, e.g. lignocellulosic pulp suspension

These and other objects are achieved by the present invention by means of a mixing apparatus for mixing media or fluids according to the claims.

According to an aspect of the present invention, there is provided a mixing apparatus for mixing fluids, for example, a gaseous medium into a pulp

suspension. The mixing apparatus comprises a housing having a mixing chamber, at least one inlet opening communicating with the mixing chamber. The inlet opening enables fluids to be fed into the mixing chamber. A drive shaft is connectable to a drive device for rotation of the drive shaft in operation and a rotor is arranged in the mixing chamber and is adapted to be connected to the drive shaft to rotate with the drive shaft. At least one outlet is provided in the housing and is communicating with the mixing chamber. The at least one outlet is communicating with an outlet duct for enabling a discharge of mixed fluids. The rotor has a rotor axis extending through the mixing chamber and being concentric with the drive shaft, wherein the rotor includes mixing elements arranged around the rotor and along its extension. The rotor includes a discharge end arranged with discharging elements for discharging the fluids radially. A stator is arranged in the mixing chamber including stator openings allowing mixed fluids discharged by the discharging elements of the rotor to flow radially through the stator openings from the mixing chamber and further toward the at least one outlet, whereby a tangential movement of the discharged fluids is prevented.

The present invention is based on the insight that by, during discharge of the fluids, stopping or preventing the remaining rotation of the fluids, e.g. the pulp suspension, and thus the tangential movement of the fluids by means of the stator, the separation of fluids with different density properties caused by the centrifugal force can be minimized. This effect is also improved by the uniform radial discharge of fluids from the mixing zone within the mixing chamber.

According to an embodiment of the present invention, a center axis of the outlet duct is perpendicularly, e.g. radially, displaced in a direction towards the at least one outlet in relation to a center axis the rotor.

In embodiments of the present invention, an interior volume of the outlet duct increases in proportion to a volume of the discharged fluid in a flow direction.

According to embodiments of the invention, the stator is shaped as tubular element.

In embodiments of the invention, the stator openings is arranged radially adjacent to and outside the discharging elements of the rotor in a flow direction.

According to embodiments of the invention, the stator openings are distributed along the stators entire circumference to provide a symmetric discharge flow of fluid.

In embodiments of the invention, the mixing elements of the rotor are protrusions with a curved or angled trailing surface in a direction of rotation of the rotor.

According to embodiments of the invention, the mixing elements of said rotor are distributed along substantially entire longitudinal extension of the rotor.

In embodiments of the present invention, the discharging elements of the rotor are protrusions curved or angled in a flow direction.

According to embodiments of the present invention, the stator is

encircled in a radial direction by a discharge space delimited by an interior wall of the housing, the discharge end of the rotor and the stator. The discharge space communicates with the outlet duct via the at least one outlet, wherein, during operation, fluids are forced to flow through the stator openings and in a semi- circular flow in the discharge space towards the at least one outlet and outlet duct. Further advantageous embodiments of the device according to the present invention and further advantages of the present invention emerge from the de- tailed description of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, for exemplary purposes, in more detail by way of embodiments and with reference to the enclosed drawings, in which:

Fig. 1 illustrates a schematical axial cross-section of a first preferred embodiment of the present invention;

Fig. 2 illustrates a schematical partly cross-sectional view of a first preferred embodiment of the present invention;

Fig. 3 illustrates a schematical view of the mixing apparatus and the outlet opening in a direction perpendicular to a center axis of the rotor and shaft;

Fig. 4 illustrates a schematical cross-section view of the mixing apparatus in a direction along the extension of the center axis of the rotor and shaft;

Fig. 5 illustrates schematically the outside of the housing of the mixing apparatus according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the drawings, similar or corresponding elements are denoted by the same reference numbers. Generally, in the drawings, arrows indicate the direction of the flow of fluid in the mixing apparatus, into and out of the mixing apparatus.

For the purpose of this disclosure, the term longitudinal refers to the direction along which a body, part or element has its greatest extension. Further, when the term longitudinal is used in connection with the axes of shafts or similar, the longitudinal axis corresponds to the rotational axis of the shaft or similar.

With reference first to Fig. 1 , the mixing apparatus 10 comprises a housing 12, wherein the interior thereof is called a homogenization chamber or mixing chamber 14 with an inlet duct 16 having an inlet 18 into the mixing chamber 14. Further, the mixing apparatus 10 comprises an outlet duct 20 having an outlet 22 from the mixing chamber 14 and a rotor 24 arranged axially with the direction of flow from the inlet opening 18 and transverse to the direction of flow to the outlet 22. The housing 12 is, in this embodiment of the present invention, preferably of a cylindrical, shell or helix shaped design, as shown in Fig. 5. The housing 12 is further provided with an end part or end cap 26. The end cap 26 includes a substantially central opening for a shaft 28 of the rotor 24 with necessary sealing, and possibly bearings for the shaft 28.

The substantially cylindrical wall of the housing 12 is, as explained above, provided with inlet and outlet 18, 22. Preferably, the shape of the inlet opening 18 such that a center axis and an axis of symmetry that is concentric with the center axis RR of the rotor 24 and the shaft 28.

The outlet 22 is in some embodiments of the present invention shaped as a rectangle as shown in Fig. 3. Fig. 3 illustrates the mixing apparatus 10 and the outlet 22 schematically in a direction perpendicular to a center axis RR of the rotor 24 and shaft 28.

The outlet duct 20 is, according to preferred embodiments of the present invention, shaped such that a center axis R _ou of the outlet duct 20 is radially displaced in a direction towards the outlet 22 in relation to the center axis RR of the rotor 24 and such that an interior volume of the outlet duct 20 increases in proportion to a volume of the discharged fluid.

Flence, the rotor 24 is mounted to the shaft 28 in the mixing chamber 14 and rotates with the drive shaft 28. The shaft 28 is driven by a motor, for example, an electrical motor. The rotor 24 has a number of mixing elements 32 arranged around the rotor and along its longitudinal extension.

Further, the rotor 24 includes a discharge end 34 arranged with discharging elements 36 for discharging the fluids in a direction substantially radially, in relation to the center axis RR of the rotor 24 towards to outlet 22 and outlet duct 20.

The purpose of the mixing elements 32 is to cause turbulence in the fluids or media and thereby to obtain a homogenization or mixing of the fluids or media and to avoid a separation of, for example, gas from a medium. Flowever, as the skilled person realizes, the shape and design of the mixing elements 32 as well as their number and exact location on the rotor and in relation to the inner wall of the housing, to the inlet opening, depend on the medium or media to be treated, the consistency of the medium or media, gas content of media, volume flow through the mixing chamber, amount of gas added to medium or media to mention a few influencing parameters. In addition to cause turbulence, the mixing elements causes a flow of media through the mixing chamber towards the end part of the rotor, where for example, the flow velocity of the medium or media can be regulated by means of the inter alia the rotational speed of the rotor.

The mixing elements 32 can, according to certain embodiments, be distributed along the longitudinal extension of the rotor 24 in a gradually dislocated manner such that each line of mixing elements 32 are arranged with an inclination in relation to the center axis RR of the rotor 24. Further, in certain embodiments, the mixing elements 32 are protrusions with a curved or angled trailing surface in a direction of rotation of the rotor 24 and the discharging elements 36 are

protrusions curved or angled in a flow direction.

A stator 38 is arranged at the end of the mixing chamber 14 adjacent to the outlet 22. The stator 38 comprises a number of stator openings 40 arranged radially adjacent to and outside the discharging elements 36 of the rotor 24 in a flow direction (indicated with arrows).

Preferably, the stator 38 is shaped as tubular element provided inside the housing, for example as an insert lining or attached to the housing by means of for instance bolts or screws. An important purpose of the stator 38 and the stator openings 40 is, in essence, to function as an end of the mixing zone within the mixing chamber 14 and to stop or prevent a tangential movement of the

discharged fluids when entering into and flowing through the stator openings 40.

A common problem in prior art mixing devices is a separation of gas from medium during the outflow or discharge caused by remaining rotation in the media since the centrifugal force acts differently on gases compared to liquids or pulp suspensions, for example. This is a pronounced problem when gas is mixed with pulp suspension.

Turning now to Fig. 4 which illustrates a schematical cross-section view of the mixing apparatus in a direction along the extension of the center axis RR of the rotor 24 and shaft 28, i.e. the center axis RR of the rotor 24 points out from the figure. As shown in Fig. 4, the stator 38 is encircled in a radial direction by a discharge space 42 communicating with the mixing chamber 14 via the stator openings 40 and the outlet duct 20 via the outlet 22.

The discharge space 42 is delimited by an interior wall 44 of the housing 12, the discharge end 34 of the rotor 24 and the stator 38. In Fig. 4, the flow of media is indicated with arrows B, C, and D. Arrow B illustrates the flow of media in the mixing chamber 14, where the rotation of the rotor 24 causes turbulence and rotation of media and thereby homogenization and mixing of the media, for example, gaseous medium and pulp suspension. Arrows C illustrate discharge of media through the stator openings 40 and arrows D illustrate the semi-circular flow of media following the interior wall 44 of the housing 12 towards the outlet 22 and then further to the outlet duct 20. As can be seen in Fig. 4, the volume of and the width (seen as a distance between the stator 38 and the interior wall 44 of the housing 12) of the discharge space 42 increases in a flow direction.

The invention shall not be considered limited to the embodiments illus- trated, but can be modified and altered in many ways by one skilled in the art, without departing from the scope of the appended claims.