SEPARATING SOLIDS FROM A MIXTURE IN A GAS-SOLID FLUIDISED BED

Abstract: Particles from a mixture of solid particles of different types or categories having different physical properties are fluidised in a gas-solid fluidised bed within which at least one baffle (6) extends transversely by leading an upward gas flow through the bed at a selected velocity. The velocity of the gas flow is selected such that the separation of the single type or category of particles is obtained in one single axial segregation step in the fluidised bed. Separated particles are discharged via a discharge passage and can be collected without requiring further separating steps such as screening. The baffles can be maintained in a stationary state, and a mechanical stirring structure is not necessary for obtaining effective separation. A system for carrying out the method is also described.

Description: TITLE: Separating Solids from a Mixture in a Gas-Solid Fluidised Bed TECHNICAL FIELD The invention relates to the application of gas-solid fluidised beds for separating specific particles from a mixture of solid particles of different types or categories having different physical properties. The particles to be separated can either be a distinct type of particles or a category of particles having at least one property in a predetermined subrange of a continuous distribution range of that property in the mixture. BACKGROUND ART In a publication entitled 'Continuous Separation of Mixed Solids Using a Rotating-Screen Fluidized Bed' (Beeckmans and Yu, The Canadian Journal of Chemical Engineering, Vol. 70, pages 1048-1054), a method for separating one type or category of particles from a mixture of solid particles of different types or categories having different physical properties is described in which the particles are fluidised in a gas-solid fluidised bed provided with a number of screens extending transversely within the bed by leading an upward gas flow through the bed at a selected velocity. A quantity of particles enriched in one of the types or categories of particles is discharged from the fluidised bed via a discharge opening. This publication also discloses a fluidised bed system for separating one type or category of particles from a mixture of solid particles of different types or categories having different physical properties. The described system is provided with a vertically elongate vessel with a gas distributor at its lower end, a gas exhaust passage spaced from the lower end and, at a level between the gas distributor and the gas exhaust passage, a discharge opening and a baffle extending transversely within the vessel. To generate the gas flow, a structure for generating a gas flow through the vessel is provided. More in particular, this publication discribes the discharge of another quantity of particles enriched in another type or category of particles from the fluidised bed via another discharge opening. Thus, a certain extent of separation between the particles of two types is obtained by discharging a mixture enriched in one type or category of particles via one discharge opening and by discharging a mixture enriched in another type or category of particles via another discharge opening. To fluidise the particles to be separated, the fluidised mixture contains a fluidising medium consisting of material selected to give the bed a density between the ultimate densities of the components of the particle to be separated. It describes the use of a fluidising medium with a particle size of 0.2 - 0.6 mm for fluidising primary particles to be separated in a size range of 1 - 5 mm. Furthermore, the bed is stirred by rotating sets of rods between successive screens about the central vertical axis of the bed or by rotating screens about the same axis while sets of eight rods protruding radially inward into the bed in- between each pair of successive screens are maintained in stationary positions. The fluidising medium is discharged via the discharge openings with the primary particles to be separated and subsequently separated from the primary particles to be separated by screening. The fluidising medium is then recycled with fresh particles to be separated. Due to destruction of bubbles by the rotating screens, wake induced vertical mixing was attenuated to an extent that permitted the device to approximate a liquid phase float/sink separator. In practice, however, the necessity to add a fluidising medium is a major drawback, since the fluidising medium will have to be separated from the primary particles. In some applications contamination of the fluidising medium with material from the primary particles will occur and treatments like coating, drying and catalyst regeneration are impaired if not made practically impossible due to the presence of fluidising medium in the bed. For rotating the screens drive means are required which complicate the structure of the separating system, increase its manufacturing and operating costs and impair its reliability. A drawback of the behaviour of the described gas-solid fluidised bed similar to that of a liquid phase float/sink separator is that the fluidised bed is not suitable for separating particles having identical or nearly identical densities but different shapes, sizes, structures and/or textures. It is known as such to provide horizontal perforated baffles in continuous fluidised beds to subdivide the bed into stages which narrows the residence time distribution of both gas and solids. Examples of such fluidised beds are described in WO 92/02458 and in a publication entitled 'A Model of Solid Baσkmixing between Stages in a Gas-Fluidized Bed with Perforated Baffles' (Jun et al., Powder Technology, vol. 73, pages 37-41) . However, narrowing the residence time distribution - i.e. more closely approaching the desired situation of plug flow - would be counterproductive to demixing and segregation which is desired for separating types or categories of solid particles. Moreover, although approaching plug flow is helpful for narrowing the distribution of the extent to which the particles are treated, if the particles are treated during passage through the fluidised bed, an even more evenly distributed treatment would be obtained if the particles were separated on the basis of the extent to which the individual particles have been treated. SUMMARY OF THE INVENTION In view of the above considerations, it is an object of the present invention to provide a method and a system in which types or categories of particles having physical properties within specific ranges can be effectively separated from particles having at least one different physical property in a simpler manner, without admixing a fluidising medium, without mechanically stirring and even if the particles have identical or about identical densities. In accordance with the present invention, this object is achieved by selecting the velocity of the gas flow in a method of the type described above such that the separation of the single type or category of particles is obtained in one single axial segregation step in the fluidised bed. The invention is based on the insight that by selecting the flow velocity of the gas adequately, the separation of the type of particles to be separated can be achieved without previously admixing a fluidising medium and without mechanically stirring the fluidised bed, since the gas flow causes sufficient axial disturbance to promote the desired demixing of the fluidised bed, while not disturbing the fluidised bed to an extent that material axially moving in wakes of rising bubbles is remixed, which is counterproductive with respect to the goal of obtaining segregation in the fluidised bed. Particles axially moving in wakes of rising " bubbles are at least to a large extent refrained from further following the bubbles at the baffle or baffles, while passing bubbles are not substantially disturbed at the baffle or baffles, so the stirring effect thereof is substantially maintained. The invention can also be embodied in a fluidised bed system comprising at least two discharge openings at different levels, which is specifically adapted for carrying out the invention by providing the system with at least one collecting station for collecting separated particles, the collecting station being in direct and open communication with one of the discharge openings. Thus, the separated particles, which do not have to be separated from any fluidising medium by screening, can be discharged directly to the collecting station. The invention can also be embodied in a fluidised bed system as described hereinabove which is adapted for treating 5 particles within the vessel, the treatment including a change of at least one physical property such that treated particles form another type or category of particles than untreated particles, but which may also be provided with only a single discharge opening. In such a fluidised bed system untreated 10 particles are to remain in the vessel until they have been treated. Therefore, a single discharge opening for discharging treated particles separated from untreated or partially untreated particles is in principle sufficient for enabling discharging the separated particles in such a system. 15 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of a first example of a fluidised bed system according to the invention, 20 Fig. 2 is a schematic representation of a second example of a fluidised bed system according to the invention, Fig. 3 is a schematic representation of a section of a rod assembly for mounting baffles, Fig. 4 is a schematic top plan view of a baffle 25. supported by three rods, Figs. 5 and 6 are top plan views of alternative baffles, Fig. 7 is a graphics showing jetsam concentration curves when different types of baffles are used, and Fig. 8 is a graphics showing jetsam concentration curves 30 at different gas flow velocities. MODES FOR CARRYING OUT THE INVENTION In the drawings identical reference numerals are used to 35 refer to corresponding parts of different embodiments,. The invention is first explained with reference to a presently most preferred embodiment schematically shown in Figs. 1, 3 and 4 and with reference to a most pref rred mode of operation. The fluidised bed system as shown in Fig. 1 is intended for separating one type or category of particles from a mixture of solid particles of different types or categories having different physical properties. The core of the fluidised bed system is formed by a vertically elongate vessel 1 provided with a gas distributor 2 at its lower end, a gas exhaust and recycling passage 3 spaced from the lower end and two discharge openings 4, 5 at different levels between the gas distributor 2 and the gas exhaust passage 3. Between the two discharge passages 4, 5 five baffles 6 are disposed, each baffle being horizontally oriented. The vessel is further provided with a feeding passageway 10 with a debouchment 11 also at a level between the two discharge passages 4, 5. If the gas leaving the fluidised bed does not have any specific desirable or undesirable properties in comparison with ambient air, it is generally more advantageous to pass the gas through the fluidised bed on a once-through basis, i.e. without recycling the gas. The system further includes a circulating device 7 for generating a gas flow through the vessel 1. Such devices are well-known in the art, so the circulating device 7 is only depicted very schematically. The system may also be provided with a cyclone (not shown) for removing particles and dust from gas leaving the vessel 1. Downstream of each discharging passage 4, 5 a collecting station 8 respectively 9 for collecting separated particles is provided. The collecting stations 8, 9 are each in direct open communication with one of the discharge openings 4, 5. In operation, a mixture of solid particles of different types or categories having different physical properties is fed into the vessel 1 via the feeding passageway 10. The particles are fluidised in the vessel 1 by an upward gas flow generated by the circulating device 7 so that a gas-solid fluidised bed is formed within which the baffles extend transversely. The velocity of the gas flow is selected such that the separation of the single type or category of particles is obtained in one single axial segregation step in the fluidised bed as depicted by segregated layers 16, 17 in the vessel 1. The separated particles 16, 17 can be discharged directly via the discharge passages 4, 5 into the collecting stations 8, 9 respectively. By selecting the flow velocity of the gas adequately, the separation of the types of particles to be separated can be achieved without previously admixing a fluidising medium and without mechanically stirring the fluidised bed, since the gas flow causes sufficient axial disturbance to promote the desired demixing of the fluidised bed, while not disturbing the fluidised bed to an extent that material axially moving in wakes of rising bubbles is remixed, which is counterproductive with respect to the goal of obtaining segregation in the fluidised bed. More in particular, when a bubble passes a baffle, material travelling in the wake of that bubble is knocked out of the wake of that bubble, so axial remixing is limite . Between successive baffles unobstructed free spaces are provided so that bubbles formed in the fluidised bed are free to rise upward through one or more of the baffles. A certain " extent of bubble formation has a positive effect on the separating effectivity in particular if the bubbles are formed in the lowest partially segregated layer, probably because the bubbles disturb and stir the fluidised bed to a certain extent which allows particles to move either upward or downward, and thus to de ix, as the bed settles. The free unobstructed spaces between successive baffles allow free passage of bubbles from baffle to baffle, so that a well distributed stirring effect is achieved. If the fluidised bed has a height of 16 cm, preferably at least three to five baffles are used. In larger riuldisβd bed the number of baffles is preferably about proportionally larger. To obtain a simple, reliable construction, the baffles are mounted so as to be stationary. More in particular, the baffles are positioned by three assemblies, each assembly including a threaded tie-rod 12 carrying a plurality of spacers 13. The number of spacers 13 between successive baffles 6 determines the spacing between successive baffles 6. To provide sufficient supporting surface for the baffles 6 and to guide the rods 12 relative to the walls of the vessel 1, support rings 14 are provided adjacent each baffle 6. In Fig. 2 an alternative fluidised bed system according to the invention is shown. This fluidised bed system is to a large extent similar to the fluidised bed system shown in Fig. T However, the fluidised bed system according to Fig. 2 is provided with an injection unit for injecting a coating material or a binder for granulation into the vessel 1. This unit is schematically depicted and denoted by reference numeral 15. The feeding passageway 10 has its debouchment 11 in an upper part of the vessel 1 so that freshly fed particles are admixed to the top of the fluidised bed. The vessel 1 has a single discharge passage 5 communicating with a single collecting station 9. The fluidised bed system according to this embodiment of " the invention can for example be used for separating coated particles from uncoated and partially coated particles. Even if the coating material has a density similar or identical to that of the particles, particles will tend to move downward more as the amount of coating applied to that particle increases. Compared with fluidised beds in which a plug flow state is approached, the fluidised bed according to the invention provides the advantage that the extent to which each particle tends to move downward is dependent on the extent to which the treatment of that individual particle is completed. Therefore, the method according to this embodiment of the invention results in a more even distribution of the intensity of the treatment applied to the particles. The treatment applied in the vessel 1 may be in the form of many other types of treatment, such as granulation by adding a binder, drying, baking. For some applications, the discharge passage will have to be located adjacent an upper section of the fluidised bed and the feeding passage will have to debouch in a lower part of the fluidised bed. If the treatment consists for example of the continuous roasting of popcorn, popped popcorn particles will, due to their reduced density, concentrate at the top of the fluidised bed from where they can be discharged. Since in the method according to the invention the fluidised bed consists of the particles to be separated and does not include a fluidising medium, the particles to be separated as such are fluidised by the gas flow and not by interaction with a fluidising medium. It is therefore possible to separate the particles having the same density on the basis of the dimensions, the shape and the texture of the particles. If a fluidising medium is required this would not be possible, because a fluidising medium having a density between the densities of the components to be separated cannot be provided if both components have the same density. Generally, an improved separating effect is achieved if the superficial velocity of the gas flow is at least above a minimum fluidising velocity of the type or category of " particles in the mixture having the second highest minimum fluidising velocity. In most cases the optimum lies at a superficial velocity of the gas flow close to or slightly below the minimum fluidising velocity of the type or category of particles in the mixture having the highest minimum fluidising velocity. Preferably, the gas flow velocity should be about 75-105 % and more preferably 85-100 % of the minimum fluidising velocity of the type of particles having the highest minimum fluidising velocity. A particularly effective separation is achieved if the superficial velocity of the gas flow is at or above a minimum velocity at which bubbles rise through the fluidised bed, espeσially when a .vessel 1 having obstruction-free interspaces between successive baffles is used. On the other hand, if the superficial velocity of the gas flow is too high (substantially above the minimum fluidising velocity of the type or category of particles in the mixture having the highest minimum fluidising velocity), the fluidised bed is generally mixed too much by the bubbles rising through the bed, which has an adverse effect on the segregating effectivity. The method according to the invention can also be used to separate three or more types of particles in a single fluidised bed. For this purpose, the velocity of the gas flow in an upper section of the fluidised bed should preferably be lower than in a lower section of the bed. This may for example be achieved by using a vessel having a larger internal cross- section in an upper portion than in a lower portion. Preferably the particles should then be fed and discharged batchwise in alternating fashion to avoid inadvertent discharging of particles passing the level of discharge passage while moving to a preferred area adjacent another discharge opening. Now an example of a method carried out according to the invention is described in more detail. Use was made of a 8.7 cm ID glass vessel 1 with a porous glass distributor 2. ithin " the vesεel 1 five baffles 6 were positioned so as to be stationary, horizontally and in vertically spaced succession by three threaded tie-rods 12 extending vertically within the vessel 1 at angular intervals of 120°. The interspace between successive baffles 6 was maintained at 3 cm by spacers 13 guided over the rods 13. The mixture to be separated consisted of 750 g glass ballotini having a surface mean diameter of 559 μm and 750 g glass ballotini having a surface mean diameter of 268 μm. The particle density was 2500 kg/m 3 for both types of particles. The minimum fluidising velocities of the two types of particles were 0.23 m/s and 0.072 m/s respectively. Segregation of the fluidised bed was determined by starting from a well-mixed condition until the bed reached a steady state. Then the gas flow was stopped and the bed was sectioned in layers with a vacuum technique. When uncovered the baffles could be removed individually from the bed without disturbing it by loosening screws at the top of the vessel 1. The concentration of particles of the two types in each layer of the bed was determined by screening. Fig. 7 shows the results obtained at a gas flow velocity of 0.208 m/s with no baffles and with baffles according to Fig. 4, Fig. 5 and Fig. 6. The jetsam concentration is the concentration of particles tending to concentrate in the bottom part of the bed. The results show that the best results are achieved with the baffles according to Fig. 4 which were made of wire mesh. Fig. 8 shows the results obtained at different gas flow velocities and using slightly different (cleaned) particles, while using baffles as shown in Fig. 4. The results show that already at 0.178 m/s a useful degree of segregation is obtained, in particular when the purpose is to discharge jetsam particles at the lower end of the vessel 1. Best results were obtained at a gas velocity of 0.208 m/s whereas at the gas flow velocity of 0.249 m/s many jetsam particles were apparently mixed to upper portions of the vessel 1.

Claims: CLA MS 1. A method for separating one type or category of particles from a mixture of solid particles of different types or categories having different physical properties comprising the steps of fluidising the particles in a gas-solid fluidised bed comprising at least one baffle (6) extending transversely within said bed by leading an upward gas flow through said bed at a selected velocity and discharging a quantity of particles at least enriched in at least one of said types or categories of particles from the fluidised bed via a discharge passage (4, 5), characterised in that the velocity of the gas flow is selected such that said separation of said single type or category of particles is obtained in one single axial segregation step in the fluidised bed. 2. A method according to claim 1, wherein said mixture of particles consists of two types or categories of particles. 3. A method according to claim 1 or 2, wherein the particles of the separated type or category of particles have at least different dimensions, shapes or textures than other particles in the mixture and substantially the same density. 4. A method according to any of the preceding claims, wherein the superficial velocity of the gas flow is above a minimum velocity at which bubbles rise through the fluidised bed. 5. A method according to any of the preceding claims, wherein the superficial velocity of the gas flow is above a minimum fluidising velocity of the type or category of particles in the mixture having the second highest minimum fluidising velocity. 6. A method according to claim 4 or 5, wherein the superficial velocity of the gas flow is around or below a minimum fluidising velocity of the type or category of particles in the mixture having the highest minimum fluidising velocity. 7. A method according to claim 4 or 5, wherein the superficial velocity of the gas flow is 75-105 % of the minimum fluidising velocity of the type or category of particles having the highest minimum fluidising velocity. 8. A method according to claim 4 or 5, wherein the superficial velocity of the gas flow is 85-100 % of the minimum fluidising velocity of the type or category of particles having the highest minimum fluidising velocity. 9. A method according to any one of the preceding claims, wherein the fluidised bed comprises a plurality of baffles (6) arranged in vertically spaced succession and wherein the gas passes through unobstructed free space between successive baffles (6). 10. A method according to any one of the preceding claims, wherein the baffles (6) are maintained in stationary positions. 11. A method according to any one of the preceding claims, wherein the velocity of the gas flow in an upper section of the fluidised bed is lower than in a lower section of said bed. 12. A method according to any one of the preceding claims, wherein a single type or category of particles is fed to the fluidised bed, the particles are treated in said bed, said treatment including a change of at least one physical property such that treated particles form another type or category of particles than untreated particles, and the treated type or category of particles is separated from the untreated type or category of particles and discharged from the bed. 13. A fluidised bed system for separating one type or category of particles from a mixture of solid particles of different types or categories having different physical properties comprising: a vertically elongate vessel (1) provided with a gas distributor (2) at its lower end, a gas exhaust passage (3) spaced from said lower end and, at a level between said gas distributor (2) and said gas exhaust passage (3), at least two discharge passages (4, 5) at different levels and at least one baffle (6) extending transversely within said vessel; and a device (7) for generating a gas flow through said vessel (1); characterised by: a collecting station (8, 9) for collecting separated particles, said collecting station (8, 9) being in direct and open communication with one of said discharge passages (4, 5). 14. A fluidised bed system for separating one type or category of particles from a mixture of solid particles of different types or categories having different physical properties comprising: a vertically elongate vessel (1) provided with a gas distributor (2) at its lower end, a gas exhaust passage (3) spaced from said lower end and, at a level between said gas distributor (2) and said gas exhaust passage (3), at least one discharge passage (5) and at least one baffle (6) extending transversely within said vessel (1); and a device for generating a gas flow through said vessel; characterised by: treatment means (15) for treating particles within said vessel (1), said treatment including a change of at least one physical property such that treated particles (17) form another type or category of particles than untreated particles (16); and a collecting station (9) for collecting separated treated particles, said collecting station (9) being in direct and open communication with said discharge passage (5). 15. A fluidised bed system according to claim 13 or 14, wherein the fluidised bed comprises a plurality of baffles (6) arranged in vertically spaced succession and at least one unobstructed free space between successive baffles (6). 16. A fluidised bed system according to any one of the claims 13-15, wherein the baffles (6) are mounted in a stationary position.

Inventors: HARTHOLT GERTJAN PIETER, HOFFMANN ALEX CHRISTIAN, JANSSEN LEON PETER BERNARD MAR, LA RIVIERE RENE, HARTHOLT, GERTJAN, PIETER, HOFFMANN, ALEX, CHRISTIAN, JANSSEN, LEON, PETER, BERNARD, MARIE, LA RIVIERE, RENE
Applicants: HARTHOLT GERTJAN PIETER, HOFFMANN ALEX CHRISTIAN, JANSSEN LEON PETER BERNARD MAR, RIVIERE RENE, UNIV GRONINGEN, HARTHOLT, GERTJAN, PIETER, HOFFMANN, ALEX, CHRISTIAN, JANSSEN, LEON, PETER, BERNARD, MARIE, LA RIVIERE, RENE, RIJKSUNIVERSITEIT TE GRONINGEN
Application year: 1995
Main classification: B01J8
IPC codes: B01J8