DEVICE AND METHOD FOR FRACTIONIZING SUSPENSIONS CONTAINING ELONGATED PARTICLES

Description

[0001] Particle-containing suspension comprising a plurality of substantially tubular fractionating devices and at least one distributor connectable to an inlet opening of each fractionating device, wherein each of the substantially tubular fractionating devices has a tubular flow region surrounded by an annular channel at least over part of its length, and a method for fractionating elongate particle-containing suspensions, in which the suspension is introduced into a substantially tubular fractionating device by means of a distributor, wherein upon flowing through the flow region of the substantially tubular fractionating device, a network of the particles contained in the suspension is formed, which are surrounded by a substantially annular region of the suspension with a reduced particle concentration, wherein at least part of the suspension,preferably the part with reduced particle concentration is at least partially diverted into an annular channel.,

[0002] Fractionation processes and the associated equipment are of great importance for various industrial production processes, e.g., for the manufacture of building materials or in the pulp and paper industry. In the paper industry in particular, used paper has recently been increasingly recycled in order to avoid wasting valuable raw materials and to fractionate native fibers in order to change their properties. For example, waste paper comes from a wide variety of sources, and depending on what the paper was originally used for, the fiber fraction used in the paper also varies. It is therefore necessary to fractionate fibers of different lengths contained in paper or native fibers accordingly in order to be able to use them for the desired purpose.

[0003] Fractionation of the feedstock, which contains particles, especially cellulose fibers of varying lengths, is also essential in the production of packaging materials, as packaging materials typically consist of multiple layers of kraft paper and recycled, printing, or writing paper. The separation of fine materials or fractions containing fibers with lengths shorter than 200 µm is also becoming increasingly important, as these fine materials in particular can negatively impact paper production by complicating or slowing down the dewatering rate in paper production.

[0004] There is therefore a need for a process and apparatus for fractionating suspensions containing particles of different lengths. These fractionation processes must meet several requirements. On the one hand, they should be energy-efficient, while on the other hand, they should have low investment and operating costs, be environmentally friendly, be technically reliable, and offer a high degree of flexibility for adapting to different operating conditions.

[0005] Currently, hydrocyclones are used in the pulp and paper industry not only as cleaning devices but also as fractionation devices, where separation is achieved in the cyclone based on differences in particle settling velocities. In particular, pulp fibers are separated in hydrocyclones based on their cell wall thickness; for example, hydrocyclones are used to separate earlywood from latewood. However, in such processes, the fractionation efficiency is highly dependent on the feed consistency, with a lower feed consistency resulting in higher fractionation efficiency.

[0006] Another currently known method is the use of so-called pressure screens, which screen the material to be fractionated. The size of the screen slots or holes is of utmost importance. Furthermore, in such a process or device, a pressure pulse is generated by a rotor to prevent the screen holes from becoming blocked. The rotor speed in such a process primarily influences energy consumption. Efficient pressure screens for fractionation purposes therefore entail significantly higher energy costs than pressure screens used for sorting or cleaning purposes.

[0007] JD Redlinger-Pohn et al, Length-selective separation of cellulose fibers by hydrodynamic fractionation, Chem. Eng. Res. Des. 126 (2017) 54-66 have already described the principle of spontaneous hydrodynamic fractionation. During hydrodynamic fractionation, a fiber network is formed inside a flow channel, in which a wall layer of the flow channel remains essentially free of elongated fibers or particles. This zone contains at most very short particles or fines. By removing part of the suspension, namely that which forms along the wall layer, from the flow channel, for example by arranging a side channel or branch channel, fractionation is achieved into a part of the suspension which contains longer particles and a phase which is predominantly liquid and, for example, only fines or very short particles.This principle is similar to hydrodynamic filtration, which is particularly used in microfluidic devices, for example, to separate non-spherical cells from spherical particles. This inherently stable process, however, has several disadvantages. The liquid flow that forms in the wall region of the flow channel is drawn off through a backward-directed channel. If the geometry between the main flow channel and the backward-directed channel is not optimally selected, the backward-directed channel becomes blocked, necessitating intermittent flushing. Furthermore, it has been found that the interface between the network of elongated particles formed inside the flow channel and the wall layer depends very strongly on the Reynolds number in the flow channel. At low Reynolds numbers up to Reynolds 1,500, a sharp interface is achieved.As the Reynolds number increases, the thickness of the annular stream formed at the edge of the particle-containing suspension becomes smaller. Due to the limited Reynolds number, the process cannot be used economically. Another disadvantage of the known process is that it is essentially intended for microfluidic applications. Scaling up such a device to an industrial scale, in particular multiplying the flow or stream channels as well as increasing the content of elongated particles in the suspension, was not considered possible, although the process offers several advantages over other known processes, in particular extremely low energy consumption, since no rotating parts are required for fractionation.

[0008] The present invention now aims to improve the hydrodynamic fractionation process and a suitable device to such an extent that, on the one hand, it becomes industrially applicable and is also suitable for flows with Reynolds numbers above 10,000 and, moreover, suspensions can be used which have a high content of particles of different lengths.

[0009] To achieve this object, the device according to the invention is essentially characterized in that an end of the flow area facing away from an inlet opening has a substantially conical connecting piece, which conical connecting piece has an outlet opening which is tapered relative to its inlet end and optionally opens into a pipe with an enlarged cross-section, and an outlet which is hinged to the connecting piece and opens into the annular channel and which has a fractionation slot opening in the flow direction at a substantially acute angle between the flow area and the annular channel, wherein the annular channel opens into a collection chamber,that, if appropriate, a fractionation device or another distributor is hinged to an outlet opening of each distributor, and that, if appropriate, at least one further fractionation device is connected to the outlet opening of each fractionation device, if appropriate with the interposition of an intermediate piece provided with a flushing opening. By designing the device such that an end of the flow area facing away from an inlet opening has a substantially conical connecting piece, a fractionation device is created which, in comparison to conventional fractionation devices, has no sharp corners or edges, so that elongated particles or fibers, for example cellulose fibers, cannot get caught or accumulate on the edges, or can only do so with great difficulty, and thus, the inlet area can be prevented from becoming obstructed in the annular channel. By,As is the case with a further development of the present invention, the conical connecting piece further comprises an outlet opening tapered relative to its inlet end, optionally opening into a tube with an enlarged cross-section, resulting in a brief disruption of the fiber network contained in the suspension, whereby any fines and short fibers trapped in the fiber network are drawn into the edge region of the conical connecting piece through a mixing process. By designing the outlet region such that the annular channel is arranged essentially at an acute angle to the flow area of the fractionation device, and by the special construction of the connecting piece, reliable removal of the annular stream, which contains only a small amount of elongated particles or no larger particles at all, is reliably ensured.

[0010] Furthermore, by the annular channel opening into a collection chamber, a reliable removal of the liquid batch, which only contains fines, ie elongated particles or randomly shaped particles with a fiber length of less than 200 µm and / or elongated particles belonging to the short fiber fraction defined above, such as particles with a predefined fiber length of e.g. less than 400 µm, 800 µm or 1000 µm or shorter, which are also referred to as fines, is ensured.

[0011] In this case, elongated particles are understood to mean any solid particle that has a greater longitudinal than transverse extension, such as cellulose fibers, plastic fibers or the like.

[0012] For industrial application of such a device, it is necessary that a plurality of such fractionation devices are arranged in series, in which case the outlet opening of each fractionation device is connected to at least one further fractionation device, optionally with the interposition of an intermediate piece provided with a flushing opening. In such a design, the flushing opening is particularly important in that, despite the special geometry of the device for fractionation, any clumps of elongated particles formed or suspended elongated particles can be quickly flushed out of the device by diluting the suspension and can be fed for further fractionation.In addition, the particle concentration can be adjusted through the flushing opening so that optimal conditions can be guaranteed in all fractionation devices, even those arranged one after the other.

[0013] In practice, such a fractionation device can be operated both stationary and transiently, which means that an inlet flow rate and an inlet fiber concentration can be variable over time, ie the flow rate can change as well as the fiber concentration in the suspension containing elongated particles.

[0014] In particular, in order to direct an inlet stream into the fractionation device and, in particular, to be able to connect as many fractionation devices as possible in parallel, at least one distributor is arranged upstream of each fractionation device. According to a further development of the invention, this distributor is designed as a substantially two-pronged, fork-shaped flow distributor. Such a flow distributor makes it possible to evenly divide the feed stream into two fractionation devices without creating an undesirable accumulation of elongated particles in the distributor, particularly at the fork point of the distributor.

[0015] To ensure smooth functioning of the device according to the invention and the corresponding method, it is necessary that a so-called "strand flake" is formed in the annular channel. According to a publication by Duffy, GG & Abdullah, L. Fibre suspension flow in small diameter pipes. Appita J. 56, 290-295 (2003), a strand flake only forms in an annular channel whose ratio of channel diameter to average fiber length is less than 5; moreover, the formation of the strand flake depends on the inlet geometry of the annular channel, the acceleration at the entrance to the annular channel, and the fiber characteristics. It may be necessary to disperse the fiber before it enters the annular channel; dispersion of the fibers can be achieved, for example, by introducing the suspension containing elongated particles during the sorting stage.Poorly formed strand flakes and thus a poorly formed network can, in particular, lead to the tightly bonded fiber flakes becoming separated and being drawn en masse into the annular opening. If tightly bonded fiber flakes are drawn into the opening in this way, selective fractionation of the suspension containing long fibers cannot be ensured, just as blockage-free operation of the device cannot be guaranteed with an undesirably composed suspension.

[0016] If, as corresponds to a further development of the present invention, the device according to the invention is further developed such that a bottom region of the distributor, which is substantially opposite an inlet pipe of the flow distributor and arranged between outlet pipes of the flow distributor, is rounded, the geometry of the distributor reliably prevents the accumulation of elongated particles, particularly in the bifurcation region, so that losses or undesired blockage of the device are also avoided. Furthermore, with such a design of a distributor, it is possible to break up particle agglomerates contained in the suspension and thus to evenly distribute the particles contained in the suspension into both outlet pipes of the distributor.Finally, by providing a constriction in the area where the outlet pipes connect to the inlet pipe, it is possible to accelerate the suspension in this area and thus achieve an elongational flow, which reliably prevents further agglomeration of the elongated particles contained in the suspension. The particle agglomerates are essentially broken up by introducing turbulence, which can be created with the aid of a diffuser or moving components or various other geometries not discussed in detail here.

[0017] For an industrial implementation of such a device, a cascade of flow distributors can be arranged in series, as is the case with a further development of the invention. With such a configuration, it is of course possible for the cross-section of each individual distributor to differ from the cross-section of the preceding distributor in that the cross-section of the inlet pipe of the first distributor is larger or has a larger diameter than that of the subsequent distributor.

[0018] Such a cascade of flow distributors makes it possible to create several hundred or even thousands of outlet openings, each connected to a fractionation device. Of course, it is possible to combine the cascade of flow distributors provided according to the invention with any distributors known from the prior art. An example of such a known distributor is a central distributor or a cross-flow distributor. Of these, the central distributor, for example, can be used as a pre-distributor, particularly due to its excellent damping effect, and the cross-flow distributor, particularly due to its advantageous geometry, according to the invention.

[0019] In order to achieve the most efficient possible separation of the annular liquid flow formed inside the flow area, the invention is further developed in such a way that a ratio of an inner diameter of the flow area to a characteristic length of the elongated particles is selected between 1 and 5, preferably between 2 and 3, in particular with an inner diameter of the flow area between 3 mm and 12 mm. By selecting the ratio between the inner diameter of the flow area and the average fiber length of the elongated particles, it is ensured that a characteristic pressure loss curve is achieved that is essentially congruent with that of water.

[0020] The characteristic length of the elongated particles is understood to be the length-weighted mean length, without taking into account the fine fraction possibly contained in the suspension, which is typically determined by a particle length of less than 200 µm.

[0021] For example, as is the case with a further development of the invention, the collection chamber has a volume multiple of the annular channel. In particular, the device is designed such that the collection chamber, for example, is hinged to the annular channel as an annular collection chamber. Preferably, the collection chamber has a cross-sectional area that is enlarged relative to the total cross-section of the flow area. It goes without saying that, according to the present invention, a plurality of parallel fractionation geometries can also be directly connected in one stage to a single, correspondingly large-volume collection chamber.

[0022] As is the case with a further development of the invention, the fractionation slot is designed as an uninterrupted annular gap to reliably prevent elongated particles from becoming trapped on parts protruding into the interior of the fractionation device. Preferably, the fractionation slot is dimensioned such that its opening is between 0.05 and 1.5 times the average particle length of the particles flowing through the annular channel. This dimensioning of the fractionation slot ensures that the entire circumference of the flow area can be utilized for fractionation and that it is not disadvantageously constricted.In particular, small slot widths corresponding to 0.05 times the average particle length of the particles flowing through the annular channel or larger have proven to be particularly preferred with regard to the separation behavior, since they make it possible to separate elongated particles, for example fibers from softwood pulp with, for example, an average particle length of 2 to 3 mm, in particular 2.2 mm, from the suspension without any blockage or obstruction of the annular channel.

[0023] In order to keep the energy expenditure for fractionation low, in particular, the device is designed such that the ratio of the internal diameter of the flow area to a characteristic length of the elongated particles is between 1 and 5, preferably 3. In a conventional pulp suspension, this corresponds to an internal diameter of a flow area of the essentially tubular fractionation device of between 3 mm and 12 mm, preferably between 4 mm and 10 mm and in particular between 6 mm and 8 mm. Such an internal diameter is selected in accordance with the liquid flow, whereby a pressure drop of the suspension containing elongated particles corresponds to a pressure drop of water under the same process conditions.

[0024] With such a geometry of the device, it has now surprisingly been possible to avoid misplacing an essentially conical connecting piece.

[0025] A further reduction in the tendency for the substantially conical connecting piece to become blocked by trapped elongated particles could be achieved by further developing the device such that the substantially conical connecting piece has a conical surface with a radius. By providing the substantially conical connecting piece with a curvature or radius, in particular an inward curvature of the conical surface, a design is created that has no sharp corners or edges, so that the blocking and, in particular, the snagging of elongated particles, for example, cellulose fibers, inside the device becomes virtually impossible, thus enabling stable and energy-saving operation of the device.

[0026] According to a further development of the invention, entrained elongated particles can slide reliably off the walls of the device in that the connections between the essentially conical connecting piece and the adjacent elements of the fractionation device, in particular the flow area, the annular channel, and the intermediate piece, are rounded. The rounded design of all connecting parts and also the acute angle of the articulation of the annular channel to the conical connecting piece ensures that the elongated particles slide along the individual parts of the device and makes it virtually impossible for the device to become blocked by trapped particles. Of course, the selected geometric design of the device also reliably prevents the deposition of other particles that may be present in the suspension containing elongated particles.

[0027] Surprisingly, it was shown that by selecting the process conditions, such as the high Reynolds number present in the flow area, i.e. the high flow velocity present in the flow area and in particular the geometry of the device, such as the arrangement of the backward-facing annular channel and the geometry of the fractionation slot, a blockage of the conical connecting piece due to the formation of a very high velocity gradient on the wall of the flow area, through which the elongated particles are safely entrained in the flow direction, can be reliably prevented.

[0028] With such a fractionation device, it is now surprisingly possible to achieve a precise fractionation of a liquid stream containing elongated particles, with which even suspensions having a higher concentration of elongated particles can be reliably separated.

[0029] In addition, a higher Reynolds number results in a higher flow velocity, which makes it possible to increase the throughput of the introduced suspension containing elongated particles, thus making it possible to provide a device and a method that operate extremely energy-efficiently with compact dimensions.

[0030] The invention further aims to provide a method for processing suspensions in such a way that the elongated particles contained therein can be separated even from streams with a Reynolds number greater than 10,000. The Reynolds numbers cited in this context refer to the viscosity of water at 25 °C ± 2 °C.

[0031] A method for fractionating a suspension containing elongated particles is defined in claim 11. In this case, the essentially tubular fractionating device is operated with a Reynolds number of over 10,000, wherein a ratio of an inner diameter of the flow area to a characteristic length of the elongated particles is selected between 1 and 5, preferably between 2 and 3, and that a stream of the suspension enriched with elongated particles is either discharged through an outlet opening or optionally introduced into at least one further fractionating device, optionally with the interposition of an intermediate piece provided with a flushing opening.

[0032] This type of process minimizes the tendency for blockage inside the device, ensuring a high concentration of elongated particles only within a single flow zone, further reducing the tendency for the device to block. At the same time, a ring flow is generated inside the flow zone. Even at high Reynolds numbers of over 10,000, a thin liquid film close to the wall or a thinned-out edge zone is maintained, ensuring reliable separation of fine material from the suspension without simultaneously generating excessive separation of elongated particles from the flow zone. This process draws fine material out of the network of long particles located inside the ring-shaped channel, further improving the desired fractionation.Furthermore, if the essentially conical connecting piece has only a low reduction rate, i.e., a small taper, the tendency for it to become clogged is further reduced. The formation of the thin fluid film near the wall further ensures that wall friction in the annular channel is kept low, thus essentially preventing channel blockage.

[0033] Thus, a liquid film is formed along the wall of the annular channel, which contains almost no elongated particles. By forming this liquid film close to the wall, the wall friction in the channel is kept low and thus blockage of the channel is prevented.

[0034] In this case, a process control in which flow conditions are created in the flow device with which Reynolds numbers of over 10,000 can be maintained during operation is extremely surprising, since the state of the art with similarly designed devices could only handle Reynolds numbers in the maximum range of 4,000. At higher Reynolds numbers, the annular flow in the prior art was destroyed to such an extent that the separation of liquid containing fine material was no longer possible. Only by modifying the device, in which only rounded connections are present, and by carefully selecting the diameter to the mean particle length, is it surprisingly possible to also process suspensions with higher solids contents and thus to safely and reliably process flows with Reynolds numbers of over 10,000 within the device.

[0035] If the Reynolds number of the suspension in the fractionation device inside the device is selected to be between greater than 10,000 and 100,000, preferably greater than 15,000, as is the case in a further development of the invention, it is not only possible to fractionate suspensions with a higher particle concentration in the process according to the invention, but in particular to increase the throughput through the device to such an extent that the device and the process can be used on an industrial scale.

[0036] According to a further development of the method according to the invention, the method is selected such that the suspension containing elongated particles is introduced into the flow region with a crowding number of the elongated particles in the liquid between 60 and 360, in particular approximately 200. With such a process, in which the diameter, in particular the inner diameter, of the flow region is selected as described above, it is possible to drastically reduce the area of the open surface compared to conventional devices, in particular by a factor of approximately 300, for example. At the same time, with such a process on an industrial scale, a significantly smaller number of fractionation devices can be used than was described in the prior art.This is achieved in particular by selecting a suitable tube diameter in relation to the fiber length, as well as by increasing the concentration of the suspension separated using the method and device according to the invention to crowding numbers of even more than 200, and by increasing the Reynolds number in the flow maintained in the device to values of over 10,000, if not even over 100,000. The Reynolds number is calculated using the formula Re = vL / u, where v is the flow velocity, L is the characteristic length of the system, i.e., the flow area of the tubular fractionation device, and u is the kinematic viscosity of the flowing liquid. This type of process makes it possible to keep the flow regime stable regardless of the Reynolds number.

[0037] The crowding number is defined by RJ Kerekes and CU Schell, "Characterisation of Fiber Flocculation by a Crowding Factor." J. Pulp Paper Sc. 18, 1 (1992), 32-38, which represents a factor that defines the average number of fibers present in a spherical control volume whose sphere diameter corresponds to the average fiber length in the suspension.

[0038] Furthermore, such a process control makes it possible to maintain a substantially laminar flow of the suspension containing elongated particles inside the system, whereby the tendency of the device to become blocked with an accumulation of elongated particles is further reduced and, in particular, the energy requirement for the passage can be further reduced.

[0039] According to a further development of the invention, the process is designed such that the suspension is introduced through a plurality of distributors, in particular a cascade of distributors, into a plurality of fractionation devices corresponding to the number of discharge openings of the distributors. With such a process, the large-scale separation of suspensions containing elongated particles is achieved quickly and easily, and in particular with the lowest possible energy consumption.

[0040] The invention will be explained in more detail below with reference to exemplary embodiments shown in the drawings. Fig. 1 a section through a schematic representation of a substantially tubular fractionation device according to the invention, Fig. 2 a section through a multi-channel module with distributors around four substantially tubular fractionation devices according to the invention.

[0041] In Fig. 1 is a section through a substantially tubular fractionation device 1 for suspensions containing elongated particles, in which suspension a suspension is introduced into a flow area 4 at 2 with the flow direction according to the arrow 3. A diameter or inner diameter of the flow area 4 of the substantially tubular fractionation device 1 preferably has a ratio of the inner diameter of the flow area to the average particle length of between 1 and 5, preferably 3. In a conventional pulp suspension, such a ratio corresponds to an inner diameter of the flow area 4 of between 3 mm and 12 mm, preferably between 4 mm and 10 mm and in particular between 6 mm and 8 mm. At the outlet end of the flow area 4, a substantially conical connecting piece 5 is articulated, from which conical connecting piece 5 an annular channel 6 branches off.The annular channel 6 forms an acute angle with the flow area 4, wherein the conical connecting piece 5 is designed such that the inner walls thereof are slightly curved in order to prevent elongated particles contained in the suspension from getting stuck.

[0042] At the mouth of the annular channel 6, a fractionation slot 7 is formed, the slot width of which is selected such that it is essentially not blocked by the elongated particles contained in the suspension, but is sufficiently small to reliably prevent excessive entrainment of elongated particles. Preferably, a slot width of between 0.05 times and greater than the average particle length of the particles flowing through the annular channel is selected, with slot widths of approximately 0.25 times the average particle length being preferred. A discharge opening 8 of the flow region 4 opens into a collection chamber 9, which collection chamber 9 can, for example, be designed as a trough. Instead of a collection chamber 9, however, the device can also be designed such that the collection chamber 9 is designed as an intermediate piece, to which intermediate piece a further flow region 4 is hinged.In order to reliably prevent essentially elongated particles from getting stuck in this flow area of the essentially tubular fractionation device 1 or to dilute the suspension to the optimal concentration for fractionation, as shown in . Fig. 1 As shown, a flushing opening or a flushing valve 10 may be provided.

[0043] The annular channel 6, with which the annular flow formed in the flow area 4 consisting of liquid containing essentially no particles belonging to the previously defined long fiber fraction is discharged, in turn opens into a collecting chamber 11, which collecting chamber 11 has a cross section which is enlarged in relation to a total cross section of the flow area 4, whereby a laminar or stress-free inflow of the withdrawn liquid into the collecting chamber 11 is ensured and a uniform withdrawal of the liquid can be ensured over the entire circumference of the annular channel 6.

[0044] The collection chamber 11 has a volume which exceeds that of the annular channel 6 by several times.

[0045] When carrying out the method according to the invention, a suspension is introduced into the flow area 4 at 2. In its interior, an annular stream forms, the liquid flowing along the pipe wall of the flow area 4 contains almost no fibers or elongated particles, or at most fine particles of extremely short length, and in whose center a stream essentially enriched with particles is formed, wherein the concentration of particles in the interior or in the central region of the flow area 4 is significantly higher than in the region of its pipe wall. In this case, a liquid film forms along the wall of the annular channel, which contains almost no elongated particles. By forming this liquid film close to the wall, the wall friction in the channel is kept low, thus preventing the channel from blocking.This low-concentration suspension stream is introduced at the discharge end from the flow area 4 into the conical connecting piece 5, in which, due to the different diameters of the individual channels and the pressure and suction conditions prevailing in the device, the annular liquid stream is withdrawn or discharged almost quantitatively and without backmixing into the annular channel 6 and subsequently introduced into the collection chamber 11. At the same time, the stream, which has an increased concentration of longer particles, is introduced at the outlet opening 8 either into a collector 17 or fed to another fractionation device.

[0046] In order to make this small-sized device suitable for industrial applications, according to the present invention, each fractionating device 1 is connected to a substantially at least two-pronged fork-shaped distributor 12, in particular a current distributor 12, as shown in Fig. 2 is shown. In such a flow distributor 12, a bottom region 15 of the distributor 12, which is opposite an inlet pipe 13 and arranged between the outlet pipes 14, is rounded. Such a rounded design of the bottom region 15 of the distributor 12 also ensures that the distributor 12 does not become blocked by stuck elongated particles or particle clumps and, in particular, ensures a uniform inflow and uniform distribution of the suspension into both outlet pipes 14. As further shown in Fig. 2 As can be seen, for use on an industrial scale, a cascade of distributors 12 is arranged one after the other in order to achieve a multiplication of the number of fractionation devices 1, which are fed by one and the same inlet. The inlet is in Fig. 2 schematically shown at 16.

[0047] When displaying Fig. 2 Fractionation facilities 1 are essentially as in Fig. 1 shown, although further discussion of these fractionation devices 1 does not appear necessary here. In the illustration in Fig. 2The four fractionation devices 1 are connected to a common collection basin 17, into which the suspension enriched with elongated particles is discharged. If further homogenization of the particle size of the particles contained in the suspension is desired, a further row of essentially tubular fractionation devices can be hinged to the first row of fractionation devices 1, thus discharging further batches of small particles or fines from the suspension.

[0048] In industrial use, such a device can have more than 1,000 individual fractionation devices 1 or can be enlarged as desired.

[0049] The fractionation achieved by the process is not exclusively limited to the range of lower concentrations, but can also extend into the range of higher concentrations, whereby fractionation takes place regardless of the concentration.

[0050] With such a device and such a method, in particular the separation of a batch of, for example, cellulose fibers that is homogeneous in terms of particle size from a suspension containing cellulose fibers of different lengths, for example from waste paper, is possible industrially without the use of excessive energy, as is the case with pressure sorters, for example, and with almost no loss of material.

Claims

1. An apparatus (1) for fractionating a suspension containing elongated particles, comprising a plurality of substantially tubular fractionation devices (1) as well as at least one distributor (12) which can be connected to an inflow opening (2) of each fractionation device (1), wherein each of the substantially tubular fractionation devices (1) has a tubular through-flow region (4) which is surrounded, at least over a portion of its length, by an annular channel (6), wherein one end of the through-flow region (4) facing away from an inflow opening has a substantially conical connecting piece (5), which conical connecting piece (5) having an outlet opening (8) which is tapered with respect to its intake end and which optionally opens into a tube with an enlarged cross-section, as well as an outlet coupled to the connecting piece (5) opening into the annular channel (6), the outlet having, in the direction of flow, a fractionation gap (7) opening at a substantially acute angle between the through-flow region (4) and the annular channel (6), wherein the annular channel (6) opens into a collecting chamber (9), wherein optionally, a fractionation device (1) or a further distributor (12) is coupled to an outlet opening of each distributor (12) and wherein optionally, at least one further fractionation device (1) is connected to the outlet opening from each fractionation device (1), optionally with the interposition of an intermediate piece provided with a rinsing opening (10).

2. The apparatus (1) as claimed in claim 1, wherein the distributor (12) is substantially constructed as a fork-shaped flow distributor (12) with at least two prongs.

3. The apparatus (1) as claimed in claim 1 or claim 2, wherein a base region (15) of the distributor disposed substantially opposite to a supply tube (13) of the flow distributor (12) and between outflow tubes (14) of the flow distributor (12) is rounded in construction.

4. The apparatus as claimed in claim 3, wherein a constriction is formed in the region of a link of the outflow tube (14) with the supply tube (13).

5. The apparatus (1) as claimed in one of claims 1 to 4, wherein a cascade of flow distributors (12) disposed one behind the other is provided.

6. The apparatus (1) as claimed in claim 1, wherein a ratio of an internal diameter of the through-flow region (4) to a characteristic length of the elongated particle is selected so as to be between 1 and 5, preferably between 2 and 3, in particular with an internal diameter of the through-flow region of between 3 mm and 12 mm.

7. The apparatus (1) as claimed in one of claims 1 to 6, wherein the fractionation gap (7) is constructed as an uninterrupted annular gap.

8. The apparatus (1) as claimed in one of claims 1 to 7, wherein the collecting chamber (9) is coupled to the annular channel (6) and wherein a volume of the collecting chamber (9) is a multiple of a volume of the annular channel (6).

9. The apparatus (1) as claimed in one of claims 1 to 8, wherein the conical envelope of the substantially conical connecting piece (5) has a curvature.

10. The apparatus (1) as claimed in claim 9, wherein the connecting regions of the substantially conical connecting piece (5) with the adjacent elements of the fractionation device (1), in particular the through-flow region (4), the annular channel (6) and the intermediate piece, are rounded in construction.

11. A method for the fractionation of suspensions containing elongated particles, in which the suspension is introduced into a substantially tubular fractionation device (1) by means of a distributor (12), wherein, when a flow passes through the through-flow region (4) of the substantially tubular fractionation device (1), a network of the particles contained in the suspension which is surrounded by a substantially annular region of the suspension with a reduced particle concentration is formed, wherein at least a portion of the suspension, preferably the portion with a reduced particle concentration, is diverted at least in part into an annular channel (6), wherein the substantially tubular fractionation device (1) is operated with a Reynolds number of more than 10000, wherein a ratio of an internal diameter of the through-flow region (4) to a characteristic length of the elongated particles is selected so as to be between 1 and 5, preferably between 2 and 3, and wherein a flow of the suspension enriched with elongated particles through an outlet opening is either discharged or is optionally introduced into at least one further fractionation device (1), optionally with the interposition of an intermediate piece provided with a rinsing opening (10).

12. The method as claimed in claim 11, wherein the suspension containing the elongated particles is introduced into the liquid with a crowding factor for the elongated particles of between 60 and 360, in particular approximately 200.

13. The method as claimed in claim 11 or claim 12, wherein a Reynolds number for the suspension in the fractionation device (1) is selected so as to be between more than 10000 and 100000, preferably more than 15000.

14. The method as claimed in claim 13, wherein a substantially laminar flow is formed inside each fractionation device (1).

15. The method as claimed in one of claims 12 to 14, wherein the suspension is introduced by a plurality, in particular a cascade, of distributors, into a plurality of fractionation devices (1) which corresponds to a number of the discharge openings of the distributor.