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Particle separation in a magnetically stabilized fluidized bed
Powder Technology, 64 (1991) 159-164
Particle separation
in a magnetically
159
stabilized fluidized bed
0. Harel, Y, Zimmels Department of Civil Engineering Israel Institute of Technology, Haifa, 32000 (Israel)
and W. Resnick Department of Chemical Engineering, Israel Institute of Technology, Haifa, 32000 (Israel)
Abstract stabilized fluidized beds provide the possibility of performing dry particle separation on the basis of density difference. Such separations are generally performed by wet media methods and are expensive and difficult, if not impossible, to perform for the separation of high density materials. An experimental study of this separation potential is described. Encapsulated magnetite comprised the bed material and bed stabilization was provided by a uniform axial magnetic field. Depth of penetration and settling times were measured for objects of different sizes and densities as a function of bed operating conditions. The separation potential for small particles was also investigated. The experimental results show that dry density separation is possible and that it can be monitored by appropriate manipulation of magnetic field intensity, gas velocity and bed stability which provide different levels of resistance to particle penetration and motion in the bed. Magnetically
Introduction
An unstabilized fluidized bed resembles a fluid in many respects - it can flow, it has a viscosity, it can equalize hydraulic levels - and the bubbles passing through it result in good mixing of the particulate materials which comprise the bed. A bed composed of magnetizable particles and subjected to a magnetic field may experience forces, however, which stabilize against the growth of perturbations in voidage, i.e., against bubble growth in the bed. This stabilized bed will still exhibit some of the wellknown characteristics of the usual fluidized bed, such as fluid-like behavior, in that the solids can move through the bed, but the bed can also be made to be quiescent and free of bubbles and pulsations. Mathematical stability analyses of the state of uniform fluidization have been performed based on equations of motion augmented for stresses due to magnetic polarization [l]. These analyses predict that a fluidized bed may be magnetically stabilized against the growth of bubbles in the bed. As a result of the presence of a magnetic field, the bed structure can change from that of a non-ordered arrangement of particles to a structure of oriented particles [2, 31. The fact that bubble formation can be suppressed in a tluidized bed can be of great interest especially in the case of fluidized bed reactors. In addition, bubble coalescence and slugging as well as particle
carry-over can be controlled or eliminated in the magnetically stabilized bed. The magnetically stabilized fluidized bed (MSFB) can be operated under conditions that range from fluid to frozen states [4, 51. Thus, the MSFB can combine some of the desirable features of fixed bed as well as of fluidized bed contactors, and, in addition, it has features of its own which are not possessed by the fixed nor by the conventional fluidized bed. One of these features is the potential for performing physical separation of particles [6]. The structure and fluidity of the bed, which can be controlled by the magnetic field strength and gas flow, can be exploited for the separation of particles. The separation would take place by virtue of differences in penetration distance or settling velocity through the bed which are experienced by particles of different size and density. In this work, the operational features of an MSFB system were investigated as regards its potential application as a particle separator. Two modes of settling behavior have been observed [S, 71 and were studied here for particle separation: (i) the quasistatic mode in which the particles or objects being studied reach and remain at an equilibrium position because of the resistance to settling offered by the bed yield stress and (ii) the post-yield-stress regime in which particles are able to overcome the yield stress and settle completely through the bed. In the
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in The Netherlands
160
quasi-static mode, particle separation can be effected as a result of different settling levels for particles of different density, whereas separation in the postyield stress regime would be effected as a result of different settling velocities.
Experimental
system and materials
The bed vessel was a 0.089-m diameter cylinder fitted with a low-pressure-drop wire screen distributor plate. The bed material comprised 60/70 U.S. mesh particles with a density of 2700 kg/m3 produced by the encapsulation of -325 U.S. mesh magnetite in a polyester resin. This ‘magnetic sand’ is similar to that used in some earlier fluidized bed studies [B]. Pressure-drop measurements were recorded on a strip-chart recorder and fluidizing gas flow was controlled manually by a needle valve. The minimum fluidization velocity was 0.08 m/s and a charge of 1.3 kg of encapsulated magnetite resulted in a bed rest height of 0.16 m. Bed stabilization was produced by an essentially uniform magnetic field provided by a stack of six 0.05 m thick, flat doughnut or pancake electromagnetic coils connected in series. The stack was activated by a DC power supply capable of supplying O100 A at O-100 V. All elements of the system within the magnetic field were either of glass or aluminum so as not to distort the field. The measured field strength for a coil current of Z amperes (0
izontal air-bearings to a container whose weight was controllable and adjustable by the addition or removal of powder ballast. The ballast container provided the desired counterweight to the object whose penetration distance or settling velocity was to be measured. The ballast container was attached to a tared balance pan and the strings were maintained taut. The entire tracking system was located on a stage that could be moved vertically relative to the MSFB system, and the lowering of the object into the bed was achieved by lowering the stage. The fractional weight of the object as supported by the balance was determined from the balance output readings, and the level to which the object settled at a given coil current and fluidizing gas velocity was taken as the level at which the full weight of the object was recorded by the balance. A schematic representation of the experimental equipment is presented in Fig. 1. Part of the tracking system was also used for the settling velocity measurements. The ballast weights were removed from the ballast container and the string was attached to a magnetic release system. A level indicator on the string was used to allow tracking of settling distance as a function of time. The settling velocity measurements were cross-checked by photographic measurements. A different technique was used in the study of the settling behavior of smaller particles of fluorite, barite, tantalite and tin of specific gravities 3.2, 4.5, 6.6 and 7.4, respectively, in the size range of 8110 US mesh. After fluidization conditions were established, a weighed quantity of test particles was fed to the top of the bed. Fluidization air was stopped after a given time period and the bed was then ‘sliced’ from top to bottom with a vacuum extraction system. The separate slices were analysed to deter-
Fig. 1. Schematic representation ment.
of experimental
equip-
161
mine the quantity of test particles which had reached each slice of the bed. Two studies were done. The first was a study of bed stability and the second was a study of particle penetration and settling. Bed stability was determined by the extent of pressure-drop-fluctuation suppression but the stability was defined somewhat differently than that proposed by Shuster and Kisliak [9] for the measurement of fluidization quality. A stable bed with 100% stability was defined as one in which the pressure-drop fluctuations were essentially zero, whereas an unstabilized lluidized bed was given a stability rating of 0%. The stability P was calculated as
1-s
P=
(
(1)
x100 01
where APi is an average pressure-drop fluctuation about the mean pressure drop across the stabilized bed operated with a superficial gas velocity of V m/ s, and AP,, is its counterpart at the same gas velocity for the unstabilized bed. Thus, an unstabilized fluidized bed would have the stability rating of 0% and a completely stabilized bed would have a stability of 100%. This study, which is reported in detail elsewhere [lo], showed that the MSFB possessed hysteresis and that the bed behavior was dependent upon the bed history. The results of the penetration and settling velocity study are reported here and some implications with respect to the development of a technique for particle separation by specific gravity are suggested.
Experimental Separation
I, Amp.
results by yield-stress
Fluidization conditions were established at a high field strength and the object was then lowered into the bed by lowering the tracking system stage until the object was completely supported by the stabilized bed. This level was taken as the settling depth. The current through the coils was then decreased so as to reduce the field strength and the tracking system stage was further lowered into the bed until the object was again completely supported by the bed. This level was taken as the next settling depth. Some experimental results are presented in Figs. 2, 3 and 4 as depth of penetration into the bed vs. field strength expressed as coil current I(H=30 I Oe) at constant air velocities for cylindrical bodies of dimensions height equal to diameter equal to 0.014 m. The settling behavior for stainless steel, brass and aluminum bodies for a constant air velocity of 0.380 m/s is depicted in Fig. 2, and in Fig. 3 the behavior at the lower constant air velocity of 0.174 m/s is shown. In Fig. 2, it can be observed that the brass and stainless steel bodies were able to overcome the bed yield stress and settle completely even at
effect
The differential level of settling was measured for cylindrical objects of height equal to diameter as a function of magnetic field strength and fluidizing air velocity. It should be noted that bed stability at a given air velocity increases with an increase in field strength (i.e., coil current) and decreases with increase in air velocity at a given field strength. In addition, the greater the stability, the higher the yield stress of the stabilized bed. To perform these experiments with aluminum, brass or stainless steel objects, the ballast container was neutralized and thus the experimental density of these particular objects was the true object density. For the study of variable density objects, the density of the experimental object was adjusted by addition or removal of ballast powder from the ballast container.
Fig. 2. Settling behavior of cylindrical body as a function of coil current at constant air velocity (V=O.380 (m/s); cylinder dimensions: L = D =0.014 m).
Fig. 3. Settling behavior of cylindrical body as a function of coil current at constant air velocity (V=O.174 (m/s); cylinder dimensions: L =D = 0.014 m).
Fig. 4. Settling behavior of variable-density cylindrical body at constant airvelocity (V= 0.321 (m/s); cylinder dimensions: L =D=O.O14 m).
a field strength corresponding to 26 A at the fluidizing velocity of 0.38 m/s. The lower density aluminium object, however, did not begin to overcome the yield stress until the current had been reduced to 18 A and did not settle completely until the current had been reduced to 14 A. At the lower fluidizing gas velocity of 0.174 m/s, the results in Fig. 3 show that even the higher density objects did not settle completely until the field strength had been reduced substantially below that corresponding to 26 A, and even the lower density aluminum object did not completely overcome the yield stress until the coil current had been reduced to 2 A. The settling results for the variable density studies are shown in Fig. 4 at the constant air velocity of 0.321 m/s. The higher specific gravity objects were able to overcome the bed yield stress at higher field strengths than were the lower specific gravity bodies. These results are not unexpected in the light of the settling behavior described for the aluminum, brass and stainless steel objects. The results can be related to the bed stability as follows: at any given bed stability, i.e., combination of gas velocity and magnetic field strength, the higher specific gravity bodies will settle to a greater depth than will the bodies of lower specific gravity and, the greater the stability, the greater will be the bed yield stress which must be overcome. Settling behavior of a cylindrical body (dia. 0.1 m; sp.gr. = 8.6) is shown as a function of coil current in Fig. 5 for a number of constant air velocities. As would be expected, the object settles to greater depths in the bed as the coil current is decreased at a given air velocity. In other words, the yield stress at any given air velocity is a function of the coil current, i.e., the field strength. In addition, the bed stability is a direct function of the field strength at a given gas velocity [lo].
Fig. 5. Settling behavior of constant specific gravity cylindrical body as a function of coil current for several constant air velocities (specific gravity= 8.6; cylinder dimensions: L=D=O.Ol m).
Inspection of the settling curves shows that the curves fan out, thereby indicating that a potential exists for separating objects according to density. Different settling depths are already observed at a field strength corresponding to 26 A. A stepwise decrease in Z could facilitate the stepwise settling of less dense objects to the bottom. Thus, if a bed were to be operated in crossflow mode [ll] with the field strength decreasing in the direction of bed motion, objects should be separable according to density along this direction. Differential settling velocities - post-yield-stress regime In this part of the program, the fluidization conditions were established and then the experimental object was placed so that its lower surface just made contact with the top of the bed. The magnetic latch was then operated to release the experimental object. The settling behavior, settling times and velocities were followed and measured by observing and photographing a marker on the tracking stage cable. A plot is presented in Fig. 6 of the settling time through the bed as a function of specific gravity for a cylindrical object with height equal to diameter equal to 0.01 m at different levels of bed stability at a fluidizing gas velocity of 0.292 m/s. The fact that the heavier particles settle more rapidly than do the lighter particles would indicate that separation of particles according to density difference could be effected by virtue of these differences in settling velocities. It can also be observed that the settling velocity is a sensitive function of specific gravity and bed stability, particularly in the low density range. For control purposes, this would mean that the residence time of less dense objects could be readily monitored by control of bed stability.
163
Fig. 6. Settling time behavior of cylindrical body as a function of specific gravity for several levels of bed stability (V= 0.292 (m/s); cylinder dimensions: L = D = 0.01 m).
termined by analysing sections of the bed which were obtained by extracting ‘slices’ of the bed. The ,experimental results are presented in Figs. 8 and 9 in which the distribution of the particles at different settling times is presented for the top and bottom sections of the bed for the four materials studied. The settling experiments were performed at a fluidizing air velocity of 0.38 m/s and magnetic field strength which produced a bed stability equal to 90%. Results of the analysis for the top section or ‘slice’ are shown in Fig. 8. As can be observed, most of the densest material, tin, had already been settled out of the top section after 5 s and the tin was completely settled out after 12 s. The least dense material, fluorite, however, required more than 40 s to settle and be cleared out of the top bed section. The same general pattern is observed in Fig. 9, which showed, for example, that essentially all of the tin had already reached the bottom section of the bed after 16 s, whereas only part of the lower-density fluorite had reached the bottom slice even after 70 s. These results provide obvious and unambiguous evidence of the potential of the MSFB for performing dry particle separations by specific gravity.
Fig. 7. Settling velocity of a cylindrical body as a function of specific gravity at 90% bed stability (V=O.292 (m/s); cylinder dimensions: L = D = 0.006 m).
A plot of settling velocity vs. object density is presented in Fig. 7 at a gas velocity of 0.292 m/s and 90% bed stability. This plot is for a cylindrical object with height equal to diameter equal to 0.006 m. It is again observed that the settling velocity is a sensitive function of object specific gravity. An object with a specific gravity of 7 settles at a velocity which is almost twice as large as the settling velocity of an object with a specific gravity of 6. The rate of change of settling velocity increases with density, a characteristic which would be of especial importance for the separation of particles of high but different densities.
Fig. 8. Distribution of settling particles with time in bed top section (V=O.38 (m/s); bed stability =90%).
Small-particle separation The settling characteristics of small particles of fluorite, barite, tantalite and tin were studied. Particle size was 8/10 US mesh. The fluidization conditions were first established and then a layer of the selected material was introduced at the top of the bed. Fluidization was continued for a given period of time, fluidization gas flow was then stopped and the distribution of the experimental particles was de-
L-.------7-.-.--T * 1”
i’
,,*’
?
I i
i’
2”
30 TIME
/J40
50
fir3
70
(Seconris)
Fig. 9. Distribution of settling particles with time in bed. bottom section (V=O.38 (m/s); bed stability=90%).
164 Conclusions It has been shown that magnetically stabilized fluidized beds can be used for the dry separation of particles according to density. The separation can be effected as a result of the fact that objects and particles of different specific gravities possess different settling characteristics in an MSFB. The experimental results reported here show that the separation operation can be monitored and controlled by appropriate manipulation of magnetic field intensity, gas velocity and bed stability which provide different levels of resistance to particle penetration and particle motion in the bed. Two modes of operation can be used to obtain particle separation: the yield-stress mode in which particles, which are larger than the bed material, can be separated as a result of different settling levels achieved by particles of different density; and the post-yield-stress-flow mode in which differential settling velocity effects the separation. In both modes, the system’s sensitivity to density of the objects being separated increases in the higher density range. This feature is unique to MSFB systems and reflects the effect of bed structure on the separation mechanism. Presently available wet separation methods do not provide satisfactory solutions for density separation of heavy metals and, in addition, the cost of wet separation processes may be prohibitive. The potential application of the dry MSFB density separation appears promising, but further investigation of its operational as well as its economic features is necessary.
Experimental equipment of the type used in this work could be used to supply the data necessary for the design and evaluation of MSFB solids separation devices.
Acknowledgements The authors acknowledge the help and assistance of Gena Perlov for his advice on the experimental system and the design of the air-bearing tracking system.
References 1 R. E. Rosensweig, Ind. Eng. Chem., Fundam., 18 (1979) 260.
2 W. Resnick, D. Boadi and Y. Zimmels, IEEE Trans. Magn., 24 (1988) 757. 3 J. Arnaldos, J. Casal and L. Puigjaner, Powder TechnoL, 36 (1983) 33. 4 J. H. Siegell, Powder Technol., 55 (1988) 127. 5 W. K. Lee, AZChE Symp. Ser., 79 (122) (1983) 87. 6 R. E. Rosensweig, W. K. Lee and J. H. Siegell, Sep. Sci Technol., 22 (1987) 25. 7 0. Harel, W. Resnick and Y. Zimmels, J. Msg. Msg. Man., 83 (1990) 498. 8 D. Wolf and W. Resnick, Znd. Eng. Chem., Fundam., 4 (1965) 77. 9 W. W. Shuster and P. Kisliak, Chem. Eng. Progr., 48 (1952) 455. 10 Y. Zimmels, W. Resnick and 0. Harel, Powder TechnoL, 64 (1991) 49. 11 J. H. Siegel1 and C. A. Coulaloglou, AZChE Symp. Ser., 80 (241) (1984) 129.
Particle separation
in a magnetically
159
stabilized fluidized bed
0. Harel, Y, Zimmels Department of Civil Engineering Israel Institute of Technology, Haifa, 32000 (Israel)
and W. Resnick Department of Chemical Engineering, Israel Institute of Technology, Haifa, 32000 (Israel)
Abstract stabilized fluidized beds provide the possibility of performing dry particle separation on the basis of density difference. Such separations are generally performed by wet media methods and are expensive and difficult, if not impossible, to perform for the separation of high density materials. An experimental study of this separation potential is described. Encapsulated magnetite comprised the bed material and bed stabilization was provided by a uniform axial magnetic field. Depth of penetration and settling times were measured for objects of different sizes and densities as a function of bed operating conditions. The separation potential for small particles was also investigated. The experimental results show that dry density separation is possible and that it can be monitored by appropriate manipulation of magnetic field intensity, gas velocity and bed stability which provide different levels of resistance to particle penetration and motion in the bed. Magnetically
Introduction
An unstabilized fluidized bed resembles a fluid in many respects - it can flow, it has a viscosity, it can equalize hydraulic levels - and the bubbles passing through it result in good mixing of the particulate materials which comprise the bed. A bed composed of magnetizable particles and subjected to a magnetic field may experience forces, however, which stabilize against the growth of perturbations in voidage, i.e., against bubble growth in the bed. This stabilized bed will still exhibit some of the wellknown characteristics of the usual fluidized bed, such as fluid-like behavior, in that the solids can move through the bed, but the bed can also be made to be quiescent and free of bubbles and pulsations. Mathematical stability analyses of the state of uniform fluidization have been performed based on equations of motion augmented for stresses due to magnetic polarization [l]. These analyses predict that a fluidized bed may be magnetically stabilized against the growth of bubbles in the bed. As a result of the presence of a magnetic field, the bed structure can change from that of a non-ordered arrangement of particles to a structure of oriented particles [2, 31. The fact that bubble formation can be suppressed in a tluidized bed can be of great interest especially in the case of fluidized bed reactors. In addition, bubble coalescence and slugging as well as particle
carry-over can be controlled or eliminated in the magnetically stabilized bed. The magnetically stabilized fluidized bed (MSFB) can be operated under conditions that range from fluid to frozen states [4, 51. Thus, the MSFB can combine some of the desirable features of fixed bed as well as of fluidized bed contactors, and, in addition, it has features of its own which are not possessed by the fixed nor by the conventional fluidized bed. One of these features is the potential for performing physical separation of particles [6]. The structure and fluidity of the bed, which can be controlled by the magnetic field strength and gas flow, can be exploited for the separation of particles. The separation would take place by virtue of differences in penetration distance or settling velocity through the bed which are experienced by particles of different size and density. In this work, the operational features of an MSFB system were investigated as regards its potential application as a particle separator. Two modes of settling behavior have been observed [S, 71 and were studied here for particle separation: (i) the quasistatic mode in which the particles or objects being studied reach and remain at an equilibrium position because of the resistance to settling offered by the bed yield stress and (ii) the post-yield-stress regime in which particles are able to overcome the yield stress and settle completely through the bed. In the
Q Elsevier Sequoia/Printed
in The Netherlands
160
quasi-static mode, particle separation can be effected as a result of different settling levels for particles of different density, whereas separation in the postyield stress regime would be effected as a result of different settling velocities.
Experimental
system and materials
The bed vessel was a 0.089-m diameter cylinder fitted with a low-pressure-drop wire screen distributor plate. The bed material comprised 60/70 U.S. mesh particles with a density of 2700 kg/m3 produced by the encapsulation of -325 U.S. mesh magnetite in a polyester resin. This ‘magnetic sand’ is similar to that used in some earlier fluidized bed studies [B]. Pressure-drop measurements were recorded on a strip-chart recorder and fluidizing gas flow was controlled manually by a needle valve. The minimum fluidization velocity was 0.08 m/s and a charge of 1.3 kg of encapsulated magnetite resulted in a bed rest height of 0.16 m. Bed stabilization was produced by an essentially uniform magnetic field provided by a stack of six 0.05 m thick, flat doughnut or pancake electromagnetic coils connected in series. The stack was activated by a DC power supply capable of supplying O100 A at O-100 V. All elements of the system within the magnetic field were either of glass or aluminum so as not to distort the field. The measured field strength for a coil current of Z amperes (0
izontal air-bearings to a container whose weight was controllable and adjustable by the addition or removal of powder ballast. The ballast container provided the desired counterweight to the object whose penetration distance or settling velocity was to be measured. The ballast container was attached to a tared balance pan and the strings were maintained taut. The entire tracking system was located on a stage that could be moved vertically relative to the MSFB system, and the lowering of the object into the bed was achieved by lowering the stage. The fractional weight of the object as supported by the balance was determined from the balance output readings, and the level to which the object settled at a given coil current and fluidizing gas velocity was taken as the level at which the full weight of the object was recorded by the balance. A schematic representation of the experimental equipment is presented in Fig. 1. Part of the tracking system was also used for the settling velocity measurements. The ballast weights were removed from the ballast container and the string was attached to a magnetic release system. A level indicator on the string was used to allow tracking of settling distance as a function of time. The settling velocity measurements were cross-checked by photographic measurements. A different technique was used in the study of the settling behavior of smaller particles of fluorite, barite, tantalite and tin of specific gravities 3.2, 4.5, 6.6 and 7.4, respectively, in the size range of 8110 US mesh. After fluidization conditions were established, a weighed quantity of test particles was fed to the top of the bed. Fluidization air was stopped after a given time period and the bed was then ‘sliced’ from top to bottom with a vacuum extraction system. The separate slices were analysed to deter-
Fig. 1. Schematic representation ment.
of experimental
equip-
161
mine the quantity of test particles which had reached each slice of the bed. Two studies were done. The first was a study of bed stability and the second was a study of particle penetration and settling. Bed stability was determined by the extent of pressure-drop-fluctuation suppression but the stability was defined somewhat differently than that proposed by Shuster and Kisliak [9] for the measurement of fluidization quality. A stable bed with 100% stability was defined as one in which the pressure-drop fluctuations were essentially zero, whereas an unstabilized lluidized bed was given a stability rating of 0%. The stability P was calculated as
1-s
P=
(
(1)
x100 01
where APi is an average pressure-drop fluctuation about the mean pressure drop across the stabilized bed operated with a superficial gas velocity of V m/ s, and AP,, is its counterpart at the same gas velocity for the unstabilized bed. Thus, an unstabilized fluidized bed would have the stability rating of 0% and a completely stabilized bed would have a stability of 100%. This study, which is reported in detail elsewhere [lo], showed that the MSFB possessed hysteresis and that the bed behavior was dependent upon the bed history. The results of the penetration and settling velocity study are reported here and some implications with respect to the development of a technique for particle separation by specific gravity are suggested.
Experimental Separation
I, Amp.
results by yield-stress
Fluidization conditions were established at a high field strength and the object was then lowered into the bed by lowering the tracking system stage until the object was completely supported by the stabilized bed. This level was taken as the settling depth. The current through the coils was then decreased so as to reduce the field strength and the tracking system stage was further lowered into the bed until the object was again completely supported by the bed. This level was taken as the next settling depth. Some experimental results are presented in Figs. 2, 3 and 4 as depth of penetration into the bed vs. field strength expressed as coil current I(H=30 I Oe) at constant air velocities for cylindrical bodies of dimensions height equal to diameter equal to 0.014 m. The settling behavior for stainless steel, brass and aluminum bodies for a constant air velocity of 0.380 m/s is depicted in Fig. 2, and in Fig. 3 the behavior at the lower constant air velocity of 0.174 m/s is shown. In Fig. 2, it can be observed that the brass and stainless steel bodies were able to overcome the bed yield stress and settle completely even at
effect
The differential level of settling was measured for cylindrical objects of height equal to diameter as a function of magnetic field strength and fluidizing air velocity. It should be noted that bed stability at a given air velocity increases with an increase in field strength (i.e., coil current) and decreases with increase in air velocity at a given field strength. In addition, the greater the stability, the higher the yield stress of the stabilized bed. To perform these experiments with aluminum, brass or stainless steel objects, the ballast container was neutralized and thus the experimental density of these particular objects was the true object density. For the study of variable density objects, the density of the experimental object was adjusted by addition or removal of ballast powder from the ballast container.
Fig. 2. Settling behavior of cylindrical body as a function of coil current at constant air velocity (V=O.380 (m/s); cylinder dimensions: L = D =0.014 m).
Fig. 3. Settling behavior of cylindrical body as a function of coil current at constant air velocity (V=O.174 (m/s); cylinder dimensions: L =D = 0.014 m).
Fig. 4. Settling behavior of variable-density cylindrical body at constant airvelocity (V= 0.321 (m/s); cylinder dimensions: L =D=O.O14 m).
a field strength corresponding to 26 A at the fluidizing velocity of 0.38 m/s. The lower density aluminium object, however, did not begin to overcome the yield stress until the current had been reduced to 18 A and did not settle completely until the current had been reduced to 14 A. At the lower fluidizing gas velocity of 0.174 m/s, the results in Fig. 3 show that even the higher density objects did not settle completely until the field strength had been reduced substantially below that corresponding to 26 A, and even the lower density aluminum object did not completely overcome the yield stress until the coil current had been reduced to 2 A. The settling results for the variable density studies are shown in Fig. 4 at the constant air velocity of 0.321 m/s. The higher specific gravity objects were able to overcome the bed yield stress at higher field strengths than were the lower specific gravity bodies. These results are not unexpected in the light of the settling behavior described for the aluminum, brass and stainless steel objects. The results can be related to the bed stability as follows: at any given bed stability, i.e., combination of gas velocity and magnetic field strength, the higher specific gravity bodies will settle to a greater depth than will the bodies of lower specific gravity and, the greater the stability, the greater will be the bed yield stress which must be overcome. Settling behavior of a cylindrical body (dia. 0.1 m; sp.gr. = 8.6) is shown as a function of coil current in Fig. 5 for a number of constant air velocities. As would be expected, the object settles to greater depths in the bed as the coil current is decreased at a given air velocity. In other words, the yield stress at any given air velocity is a function of the coil current, i.e., the field strength. In addition, the bed stability is a direct function of the field strength at a given gas velocity [lo].
Fig. 5. Settling behavior of constant specific gravity cylindrical body as a function of coil current for several constant air velocities (specific gravity= 8.6; cylinder dimensions: L=D=O.Ol m).
Inspection of the settling curves shows that the curves fan out, thereby indicating that a potential exists for separating objects according to density. Different settling depths are already observed at a field strength corresponding to 26 A. A stepwise decrease in Z could facilitate the stepwise settling of less dense objects to the bottom. Thus, if a bed were to be operated in crossflow mode [ll] with the field strength decreasing in the direction of bed motion, objects should be separable according to density along this direction. Differential settling velocities - post-yield-stress regime In this part of the program, the fluidization conditions were established and then the experimental object was placed so that its lower surface just made contact with the top of the bed. The magnetic latch was then operated to release the experimental object. The settling behavior, settling times and velocities were followed and measured by observing and photographing a marker on the tracking stage cable. A plot is presented in Fig. 6 of the settling time through the bed as a function of specific gravity for a cylindrical object with height equal to diameter equal to 0.01 m at different levels of bed stability at a fluidizing gas velocity of 0.292 m/s. The fact that the heavier particles settle more rapidly than do the lighter particles would indicate that separation of particles according to density difference could be effected by virtue of these differences in settling velocities. It can also be observed that the settling velocity is a sensitive function of specific gravity and bed stability, particularly in the low density range. For control purposes, this would mean that the residence time of less dense objects could be readily monitored by control of bed stability.
163
Fig. 6. Settling time behavior of cylindrical body as a function of specific gravity for several levels of bed stability (V= 0.292 (m/s); cylinder dimensions: L = D = 0.01 m).
termined by analysing sections of the bed which were obtained by extracting ‘slices’ of the bed. The ,experimental results are presented in Figs. 8 and 9 in which the distribution of the particles at different settling times is presented for the top and bottom sections of the bed for the four materials studied. The settling experiments were performed at a fluidizing air velocity of 0.38 m/s and magnetic field strength which produced a bed stability equal to 90%. Results of the analysis for the top section or ‘slice’ are shown in Fig. 8. As can be observed, most of the densest material, tin, had already been settled out of the top section after 5 s and the tin was completely settled out after 12 s. The least dense material, fluorite, however, required more than 40 s to settle and be cleared out of the top bed section. The same general pattern is observed in Fig. 9, which showed, for example, that essentially all of the tin had already reached the bottom section of the bed after 16 s, whereas only part of the lower-density fluorite had reached the bottom slice even after 70 s. These results provide obvious and unambiguous evidence of the potential of the MSFB for performing dry particle separations by specific gravity.
Fig. 7. Settling velocity of a cylindrical body as a function of specific gravity at 90% bed stability (V=O.292 (m/s); cylinder dimensions: L = D = 0.006 m).
A plot of settling velocity vs. object density is presented in Fig. 7 at a gas velocity of 0.292 m/s and 90% bed stability. This plot is for a cylindrical object with height equal to diameter equal to 0.006 m. It is again observed that the settling velocity is a sensitive function of object specific gravity. An object with a specific gravity of 7 settles at a velocity which is almost twice as large as the settling velocity of an object with a specific gravity of 6. The rate of change of settling velocity increases with density, a characteristic which would be of especial importance for the separation of particles of high but different densities.
Fig. 8. Distribution of settling particles with time in bed top section (V=O.38 (m/s); bed stability =90%).
Small-particle separation The settling characteristics of small particles of fluorite, barite, tantalite and tin were studied. Particle size was 8/10 US mesh. The fluidization conditions were first established and then a layer of the selected material was introduced at the top of the bed. Fluidization was continued for a given period of time, fluidization gas flow was then stopped and the distribution of the experimental particles was de-
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Fig. 9. Distribution of settling particles with time in bed. bottom section (V=O.38 (m/s); bed stability=90%).
164 Conclusions It has been shown that magnetically stabilized fluidized beds can be used for the dry separation of particles according to density. The separation can be effected as a result of the fact that objects and particles of different specific gravities possess different settling characteristics in an MSFB. The experimental results reported here show that the separation operation can be monitored and controlled by appropriate manipulation of magnetic field intensity, gas velocity and bed stability which provide different levels of resistance to particle penetration and particle motion in the bed. Two modes of operation can be used to obtain particle separation: the yield-stress mode in which particles, which are larger than the bed material, can be separated as a result of different settling levels achieved by particles of different density; and the post-yield-stress-flow mode in which differential settling velocity effects the separation. In both modes, the system’s sensitivity to density of the objects being separated increases in the higher density range. This feature is unique to MSFB systems and reflects the effect of bed structure on the separation mechanism. Presently available wet separation methods do not provide satisfactory solutions for density separation of heavy metals and, in addition, the cost of wet separation processes may be prohibitive. The potential application of the dry MSFB density separation appears promising, but further investigation of its operational as well as its economic features is necessary.
Experimental equipment of the type used in this work could be used to supply the data necessary for the design and evaluation of MSFB solids separation devices.
Acknowledgements The authors acknowledge the help and assistance of Gena Perlov for his advice on the experimental system and the design of the air-bearing tracking system.
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