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The use of slow speed stirring to initiate particulate fluidisation
Shorter Communications [8] [9]
FABULA A. G., Ph. D. thesis, Department of Aeronautical Engineering, Pennsylvania 1966. PARNAS A. L., NASA T-T F-432 1967.
Chemical Engineering Science, 1969, Vol. 24, pp. 194-l 95.
Pergamon Press.
State University, Appendix G.
Printed in Great Britain.
The use of slow speed stirring to initiate particulate fluidisation (Received 4 July 1968) INTRODUCTION generally found that beds composed of particles of small size or low density give rise to particulately fluidised systems over a wide range of gas velocities. However, because of the large specific surface of such solids and the relatively small gravitational force per particle, adhesive forces are frequently sufficiently great to give rise to serious channeling. The present note gives some results obtained by using a slow speed stirrer to break down channels within the bed. IT
IS
EXPERIMENTAL WORK Fluidisation was carried out in a perspex tube 4in. dia. and 18 in. deep, fitted with a porous bronze distributor, details of which are given ekewhere[l]. A stirrer was mounted above the tube in such a way that the vertical position of the blade could be adjusted with the stirrer in motion. The leading edge of the blade of the stirrer (see Fig. 1) was so shaped as to minimise interference with the flow of fluidising air. The stirrer speed was controlled using a variable transformer to control the power supplied to the motor. Experiments were carried out with two powders which could not normally be fluidised without serious channeling. Their properties are listed in Table 1. Preliminary trials
Fig. 1.Stirrer blade details. showed that the bed pressure drop and voidage were not sensitive to changes in stirrer speed over the range 30-120 revlmin and therefore a standard speed of 60revlmin was adopted. The effect of stirring on a channeling bed was tested by lowering the revolving stirrer into a bed in which the air
Table 1 Phenolic resin Bed weight g Particle diameter p Particle density g/cm3 Free falling velocity (calculated) u0 cm/set Experimentally determined value ui cmlsec Minimum fluidising velocity (u& cmlsec Minimum gas velocity for bubbling u,~~ 9 i$xperimental) n (calculated)
180 121 0.24
Aggregated silica
10
16.3 19.6
18.6 7.6
0.18 1.11
194
10.3 0.16
2%
2.92 18.4
6.2 8.4 4.4
30 0.05 3.0
6.74
7.50 3.7
flow was maintained at a velocity of about twice uw NO effect was observed until the blade was within about 1 cm of the distributor when the channels abruptly disappeared and the surface of the bed rose as particulate fluidisation took place. Once this condition had been reached, the stirrer could be removed without any noticeable effect on the quality of fluidisation provided that the bed voidage was above a certain minimum value (064 for phenolic resin) and no bubbles were present. Measurements of bed pressure drop and bed height were made for velocities up to the point at which spontaneous bubbling occurred. This limiting velocity Us was sensitive to the presence of the stirrer, lower values being obtained when the stirrer was present as the stirrer blade acted as a bubble source. EXPERIMENTAL RESULTS Experimental results were expressed by plotting the logarithm of the voidage against the logarithm of the gas velocity for the range of conditions over which particulate expansion was obtained (see Fig. 2). The two powders studied gave rise to particulate fluidisation over a very wide 1,3I.2 I.1 I.0
-
0.9
-
0.6
-
” 0.7 Y
-
.~0.6
-
i
Hisftdic msin &d wsighll80 q
0 Stirsr used
x !%rrerrsm~sd
8 3
0.5
-
0.4
-
In
11
062
1
0.64
*
1III
(II
0.66 voidW6
066
0.70
0.72
(6)
Fig. 2. Relation between gas velocity and voidage for a stirred bed of phenolic resin. range of flowrates, and the behaviour was preprsented normal equation for particulate fluidisation: UC -=
by the
in diameter. If the packed bed voidage were about 0.42, the density of the aggregates would be about 0.038 g/cm*. Experimentally determined values of uI are tabulated in Table 1 and are compared with the free falling velocity u0 of the sphere with the same surface mean diameter as the phenolic resin powder and the aggregates of silica. Values of the index n are compared with values available in the literature [2] for uniform spherical particles in a liquid. The experimental values of n found here are somewhat larger than those predicted for liquid fluidisation. This is in agreement with the only other experimental results available for gas fluidised systems [ 1, 31. The minimum gas velocity for spontaneous bubbling (Us) and the corresponding value of voidage (eti) are also given in Table 1. It will be noted that particulate fluidisation is obtained over a wide range of velocities with both powders, u,,,&+ being 4.1 for the phenolic resin and 18.4 for the silica. Stirrer power consumption was very low; the presence of a fluidised bed within the tube did not increase the power required by more than 0.3 W. CONCLUSIONS P. slow agitator is capable of breaking down channels within a packed bed and enabling good quality particulate fluidisation to be obtained in beds of solids which otherwise will not fluidise. The stirrer blade is effective only if it is situated less than about 1 cm from the distributor. It appears that the action of the stirrer is to move solids into the channels which are thereby destroyed. At a higher point in the bed the channels are so well developed that the gas is able to sweep aside any particles which are introduced. Once particulate fluidisation has been established, it will continue in the absence of the stirrer, and the relation between bed voidage and gas velocity is of the same form as that normally obtained for particulate fluidisation. Acknowledgment-The authors wish to thank Mr. W. L. Linton, of the Research and Development Department, Imperial Smelting Corporation, for a sample of finely divided silica, and for information*conceming the behaviour of fluidised beds of this material. Department of Chemical Engineering K. GODARD University College J. F. RICHARDSON Swansea
e”
NOTATION
4
voidage expansion coefficient in Eq. (I) gas velocity relative to the container constant in Eq. (1) minimum gas velocity at which spontaneous bubbling occurs minimum fluidising velocity
where e is the voidage obtained at a superficial velocity of u, and u, and n are constants for the particular system. u, is the extrapolated value of velocity corresponding to a voidage of unity and should be comparable with the free falling velocity of the particles. The finely divided silica gave rise to stable loose aggregates approximately 500~
REFERENCES [l] GODARD K. E. and RICHARDSON J. F., Tripartite Chemical Engineering Conference, [2] RICHARDSON J. F. and ZAKI W. N., Trans. Instn Chem. Engrs 1954 32 35. [3] DAVIES L. and RICHARDSON J. F., Trans. lnstn Chem. Engrs 1966 44 293.
195
Montreal 1968.
FABULA A. G., Ph. D. thesis, Department of Aeronautical Engineering, Pennsylvania 1966. PARNAS A. L., NASA T-T F-432 1967.
Chemical Engineering Science, 1969, Vol. 24, pp. 194-l 95.
Pergamon Press.
State University, Appendix G.
Printed in Great Britain.
The use of slow speed stirring to initiate particulate fluidisation (Received 4 July 1968) INTRODUCTION generally found that beds composed of particles of small size or low density give rise to particulately fluidised systems over a wide range of gas velocities. However, because of the large specific surface of such solids and the relatively small gravitational force per particle, adhesive forces are frequently sufficiently great to give rise to serious channeling. The present note gives some results obtained by using a slow speed stirrer to break down channels within the bed. IT
IS
EXPERIMENTAL WORK Fluidisation was carried out in a perspex tube 4in. dia. and 18 in. deep, fitted with a porous bronze distributor, details of which are given ekewhere[l]. A stirrer was mounted above the tube in such a way that the vertical position of the blade could be adjusted with the stirrer in motion. The leading edge of the blade of the stirrer (see Fig. 1) was so shaped as to minimise interference with the flow of fluidising air. The stirrer speed was controlled using a variable transformer to control the power supplied to the motor. Experiments were carried out with two powders which could not normally be fluidised without serious channeling. Their properties are listed in Table 1. Preliminary trials
Fig. 1.Stirrer blade details. showed that the bed pressure drop and voidage were not sensitive to changes in stirrer speed over the range 30-120 revlmin and therefore a standard speed of 60revlmin was adopted. The effect of stirring on a channeling bed was tested by lowering the revolving stirrer into a bed in which the air
Table 1 Phenolic resin Bed weight g Particle diameter p Particle density g/cm3 Free falling velocity (calculated) u0 cm/set Experimentally determined value ui cmlsec Minimum fluidising velocity (u& cmlsec Minimum gas velocity for bubbling u,~~ 9 i$xperimental) n (calculated)
180 121 0.24
Aggregated silica
10
16.3 19.6
18.6 7.6
0.18 1.11
194
10.3 0.16
2%
2.92 18.4
6.2 8.4 4.4
30 0.05 3.0
6.74
7.50 3.7
flow was maintained at a velocity of about twice uw NO effect was observed until the blade was within about 1 cm of the distributor when the channels abruptly disappeared and the surface of the bed rose as particulate fluidisation took place. Once this condition had been reached, the stirrer could be removed without any noticeable effect on the quality of fluidisation provided that the bed voidage was above a certain minimum value (064 for phenolic resin) and no bubbles were present. Measurements of bed pressure drop and bed height were made for velocities up to the point at which spontaneous bubbling occurred. This limiting velocity Us was sensitive to the presence of the stirrer, lower values being obtained when the stirrer was present as the stirrer blade acted as a bubble source. EXPERIMENTAL RESULTS Experimental results were expressed by plotting the logarithm of the voidage against the logarithm of the gas velocity for the range of conditions over which particulate expansion was obtained (see Fig. 2). The two powders studied gave rise to particulate fluidisation over a very wide 1,3I.2 I.1 I.0
-
0.9
-
0.6
-
” 0.7 Y
-
.~0.6
-
i
Hisftdic msin &d wsighll80 q
0 Stirsr used
x !%rrerrsm~sd
8 3
0.5
-
0.4
-
In
11
062
1
0.64
*
1III
(II
0.66 voidW6
066
0.70
0.72
(6)
Fig. 2. Relation between gas velocity and voidage for a stirred bed of phenolic resin. range of flowrates, and the behaviour was preprsented normal equation for particulate fluidisation: UC -=
by the
in diameter. If the packed bed voidage were about 0.42, the density of the aggregates would be about 0.038 g/cm*. Experimentally determined values of uI are tabulated in Table 1 and are compared with the free falling velocity u0 of the sphere with the same surface mean diameter as the phenolic resin powder and the aggregates of silica. Values of the index n are compared with values available in the literature [2] for uniform spherical particles in a liquid. The experimental values of n found here are somewhat larger than those predicted for liquid fluidisation. This is in agreement with the only other experimental results available for gas fluidised systems [ 1, 31. The minimum gas velocity for spontaneous bubbling (Us) and the corresponding value of voidage (eti) are also given in Table 1. It will be noted that particulate fluidisation is obtained over a wide range of velocities with both powders, u,,,&+ being 4.1 for the phenolic resin and 18.4 for the silica. Stirrer power consumption was very low; the presence of a fluidised bed within the tube did not increase the power required by more than 0.3 W. CONCLUSIONS P. slow agitator is capable of breaking down channels within a packed bed and enabling good quality particulate fluidisation to be obtained in beds of solids which otherwise will not fluidise. The stirrer blade is effective only if it is situated less than about 1 cm from the distributor. It appears that the action of the stirrer is to move solids into the channels which are thereby destroyed. At a higher point in the bed the channels are so well developed that the gas is able to sweep aside any particles which are introduced. Once particulate fluidisation has been established, it will continue in the absence of the stirrer, and the relation between bed voidage and gas velocity is of the same form as that normally obtained for particulate fluidisation. Acknowledgment-The authors wish to thank Mr. W. L. Linton, of the Research and Development Department, Imperial Smelting Corporation, for a sample of finely divided silica, and for information*conceming the behaviour of fluidised beds of this material. Department of Chemical Engineering K. GODARD University College J. F. RICHARDSON Swansea
e”
NOTATION
4
voidage expansion coefficient in Eq. (I) gas velocity relative to the container constant in Eq. (1) minimum gas velocity at which spontaneous bubbling occurs minimum fluidising velocity
where e is the voidage obtained at a superficial velocity of u, and u, and n are constants for the particular system. u, is the extrapolated value of velocity corresponding to a voidage of unity and should be comparable with the free falling velocity of the particles. The finely divided silica gave rise to stable loose aggregates approximately 500~
REFERENCES [l] GODARD K. E. and RICHARDSON J. F., Tripartite Chemical Engineering Conference, [2] RICHARDSON J. F. and ZAKI W. N., Trans. Instn Chem. Engrs 1954 32 35. [3] DAVIES L. and RICHARDSON J. F., Trans. lnstn Chem. Engrs 1966 44 293.
195
Montreal 1968.