- Email: [email protected]
Inter-particle forces that suppress bubbling in gas fluidised beds
Shorter Communications Pergamon Press.
Chemical Engineering Science, 197 1, Vol. 26, pp. 1293-1294.
Printed in Great Britain.
Inter-particle forces that suppress bubbling in gas fluid&d
beds
(Received 14 December 1970) IT IS WELL known that, when fine particles are fluidised with a gas, fluidisation occurs at a velocity markedly less than that at which bubbles first appear[ 1,2,3]. With commonly used materials fluidised with air, the phenomenon is confined to particles in the approximate range 50-80 pm average diameter [4] for below this it is usually impossible to fluidise such finely divided powders. The commonly accepted reason for this behaviour is that it results from the dominance of surface forces as the ratio of surface to body forces increases with diminishing particle size. It is usually attributed to electrostatic forces as a charge is readily generated when dry gas is used and the particle size at which the effect becomes observable depends on the kind of solid material. Suppression of bubble formation occurs strongly in low density resin powders as large as 125 pm particle diameter whilst some inorganic salts ground to around 10 pm dia. can still be fluidised quite normally. This phenomenon has now been observed with comparatively coarse steel shot when electrostatic forces are presumably absent. The alternative force responsible for bubble suppression is magnetic. Figure 1 shows the pressure drop/flow rate curves for three grades of steel shot obtained by sieving from the same
sbperflcial
air
coarsely graded batch. The sieve fractions were -120 +lSO, -72 +85 and -52 +60 BSS corresponding to mean particle diameters of 125, 194 and 280 pm and all observations were made in a 14 cm dia. bed with a uniformly porous distributor. Two of the materials had become magnetised (accidently, as a result of recovering the material with a magnet after particle mixing experiments). All show the same characteristic curve although in the case of the two magnetised materials there was a distinct difference between the minimum fluidisation velocity and the miniium bubbling point. This difference was barely observable with the unmagnetised material. For the purpose of this note, the minimum fluidisation velocity is defined by the extrapolation of the packed bed pressure drop line to the pressure W/,4 where W is the bed weight and A its cross-sectional area. This is shown in Fig. 1. Figure 2 shows the change in bed height with increasing flow rate for the same three beds. Velocity and bed height are expressed in dimensionless form for ease ofcompatison. With steady bubbling, bed height is notoriously difficult to measure and the upper part of the curves show characteristic scatter. With the two magnetised materials, however, there is appreciable and uniform bed expansion before bubble formation occurs. For the 125 pm particles this was as great as
valocity* u. cm/sac
Fig. 1.
1293
@
STt?
Shorter Communications
first observed
/I’
1.12
I.06 I-&-74Ocm bca
L_
.
4:
iwbble , 1.c
fwmothl
I 2cI
IFI Gas vdoclty.
I
u&,.
Fig. 2. 12 per cent. The rate of expansion is very similar to tbat found for liquid fluidised beds [5]. There was no observable expansion of the unmagnetised material before bubbles appeared and a different definition of U,.f. would make this coincident with the minimum bubbling point, an impossible adjustment witb the other materials. The particle density of the shot varied from 7500 for the coarsest grade to 7180 Kg/m3 for the finest. The angle of repose of tmmagnetised material was 24” but this increased with magnetisation to 27.5” for the 280 pm and 3 1.5” for the 125 Km material. Attempts to measure the degree of magnetisation with a conventional magnetometer have not been successful presumably because of the random orientation of the polenormal sample of shot. There is no reason to think that the two size grades were equally magnetised and the different changes in angle of repose reflect this. Figure 3(a) is a microphotograph of unmagnetised shot and 3(b) the same grade of material after magnetisation when particles are plainly adhering one to another. This chance discovery of the effect of magnetism makes
possible a quantitative study of bubble suppression by interparticle forces. The electrostatic surface charge on fluidised particles is more or less uncontrollable whilst the degree of magnetism can be determined with precision although the method of measurement requires some development. Further work is continuing along these lines. Department of Chemical Engineering University College, London England
4 d, H Ho U U,,,. W Ap
J. A. AGBIM A. W. NIENOW P. N. ROWE
NOTATION cross sectional area of bed, cm* average particle diameter, pm bed height, cm initial bed height, cm supertlcial air velocity, cm/set minimum fluidisation velocity (superllcial), cm/set total mass of the bed, Kg pressure drop across the bed, Kg/cm2
REFERENCES 111 RICHARDSON J. F. and DAVIES L., Nature, Land. 1963 199898. [2] RICHARDSON J. F., Chapter in Fhidisation (Edited by DAVIDSON and HARRISON), Academic Press, London (in press 1971). [3] ROWE P. N., Chem. Engng Sci. 1969 24 415. [4] ROWE P. N., PARTRIDGE B. A., CHENEY A. G., HENWOOD G. A. and LYALL E., Trans. Insf. them. Engrs 43T271. [5] RICHARDSON J. F. and ZAKI W. N., Trans. Inst. them. Engrs. 1954 32 35.
1294
Fig. 3.
(Facing page 1294)
Chemical Engineering Science, 197 1, Vol. 26, pp. 1293-1294.
Printed in Great Britain.
Inter-particle forces that suppress bubbling in gas fluid&d
beds
(Received 14 December 1970) IT IS WELL known that, when fine particles are fluidised with a gas, fluidisation occurs at a velocity markedly less than that at which bubbles first appear[ 1,2,3]. With commonly used materials fluidised with air, the phenomenon is confined to particles in the approximate range 50-80 pm average diameter [4] for below this it is usually impossible to fluidise such finely divided powders. The commonly accepted reason for this behaviour is that it results from the dominance of surface forces as the ratio of surface to body forces increases with diminishing particle size. It is usually attributed to electrostatic forces as a charge is readily generated when dry gas is used and the particle size at which the effect becomes observable depends on the kind of solid material. Suppression of bubble formation occurs strongly in low density resin powders as large as 125 pm particle diameter whilst some inorganic salts ground to around 10 pm dia. can still be fluidised quite normally. This phenomenon has now been observed with comparatively coarse steel shot when electrostatic forces are presumably absent. The alternative force responsible for bubble suppression is magnetic. Figure 1 shows the pressure drop/flow rate curves for three grades of steel shot obtained by sieving from the same
sbperflcial
air
coarsely graded batch. The sieve fractions were -120 +lSO, -72 +85 and -52 +60 BSS corresponding to mean particle diameters of 125, 194 and 280 pm and all observations were made in a 14 cm dia. bed with a uniformly porous distributor. Two of the materials had become magnetised (accidently, as a result of recovering the material with a magnet after particle mixing experiments). All show the same characteristic curve although in the case of the two magnetised materials there was a distinct difference between the minimum fluidisation velocity and the miniium bubbling point. This difference was barely observable with the unmagnetised material. For the purpose of this note, the minimum fluidisation velocity is defined by the extrapolation of the packed bed pressure drop line to the pressure W/,4 where W is the bed weight and A its cross-sectional area. This is shown in Fig. 1. Figure 2 shows the change in bed height with increasing flow rate for the same three beds. Velocity and bed height are expressed in dimensionless form for ease ofcompatison. With steady bubbling, bed height is notoriously difficult to measure and the upper part of the curves show characteristic scatter. With the two magnetised materials, however, there is appreciable and uniform bed expansion before bubble formation occurs. For the 125 pm particles this was as great as
valocity* u. cm/sac
Fig. 1.
1293
@
STt?
Shorter Communications
first observed
/I’
1.12
I.06 I-&-74Ocm bca
L_
.
4:
iwbble , 1.c
fwmothl
I 2cI
IFI Gas vdoclty.
I
u&,.
Fig. 2. 12 per cent. The rate of expansion is very similar to tbat found for liquid fluidised beds [5]. There was no observable expansion of the unmagnetised material before bubbles appeared and a different definition of U,.f. would make this coincident with the minimum bubbling point, an impossible adjustment witb the other materials. The particle density of the shot varied from 7500 for the coarsest grade to 7180 Kg/m3 for the finest. The angle of repose of tmmagnetised material was 24” but this increased with magnetisation to 27.5” for the 280 pm and 3 1.5” for the 125 Km material. Attempts to measure the degree of magnetisation with a conventional magnetometer have not been successful presumably because of the random orientation of the polenormal sample of shot. There is no reason to think that the two size grades were equally magnetised and the different changes in angle of repose reflect this. Figure 3(a) is a microphotograph of unmagnetised shot and 3(b) the same grade of material after magnetisation when particles are plainly adhering one to another. This chance discovery of the effect of magnetism makes
possible a quantitative study of bubble suppression by interparticle forces. The electrostatic surface charge on fluidised particles is more or less uncontrollable whilst the degree of magnetism can be determined with precision although the method of measurement requires some development. Further work is continuing along these lines. Department of Chemical Engineering University College, London England
4 d, H Ho U U,,,. W Ap
J. A. AGBIM A. W. NIENOW P. N. ROWE
NOTATION cross sectional area of bed, cm* average particle diameter, pm bed height, cm initial bed height, cm supertlcial air velocity, cm/set minimum fluidisation velocity (superllcial), cm/set total mass of the bed, Kg pressure drop across the bed, Kg/cm2
REFERENCES 111 RICHARDSON J. F. and DAVIES L., Nature, Land. 1963 199898. [2] RICHARDSON J. F., Chapter in Fhidisation (Edited by DAVIDSON and HARRISON), Academic Press, London (in press 1971). [3] ROWE P. N., Chem. Engng Sci. 1969 24 415. [4] ROWE P. N., PARTRIDGE B. A., CHENEY A. G., HENWOOD G. A. and LYALL E., Trans. Insf. them. Engrs 43T271. [5] RICHARDSON J. F. and ZAKI W. N., Trans. Inst. them. Engrs. 1954 32 35.
1294
Fig. 3.
(Facing page 1294)