The Fluidized Bed TRANSITION STATE IN THE VERTICAL PNEUMATIC TRANSPORT OF PARTICLES
development I
RICHARD H. WILHELM AND STEPHEN VALENTINE' PRINCETON UNIVERSITY, PRINCETON, N. J. c
Dynamic masses of suspended particles may form dense-phase fluidized beds, such as have become important industrially. Under certain conditions, however, a stable bed is not formed but the particles are transported by the suspending stream. It is desirable to know when fluidized beds are formed and when particles are carried away pneumatically. The present paper contributes toward the solution of this problem. Experiments were performed in a 4-inch vertical, unobstructed tube with claj spheres in three sizes between 0.02 and 0.03 foot in diameter. It was found that fluidized beds were formed as the direction of motion of the particles changed from cocurrent to countercurrent, relative to the rising air stream. The fluidized beds formed in these open-tube experiments were presumed to be similar to those normally formed in screen-supported beds because the velocity-expansion relations in both cases were alilce. The complex manner in which the formation of a fluidized bed in an open, vertical tube depends upon particle properties and upon flow rates of supporting fluid and particle streams is illustrated by means of experimental ciirves relating these parameters.
To Particle Separafion
Par f icle Mechanism
air^^^
/ /
HIS work presents an experimental study of the behavior of a stream of solid par-
T
apparatus, when ticles gasthat velocities has is introduced received are high previous intoand a rising experimental particles stream areofattention, transported gas. Theparticularly upper upward limit and t of o obtain out operation, of opthe
Blower
erating conditions for conveying of specific materials. Recent fundamental papers (3-4,10) have dealt with pressure drop-velocity-particle concentration relations. In past studies the object has been to study the system after acceleration has been substantially completed and the velocity of particles in the pipes may be taken as Present address, Sooony-Vacuum Oil Co , Inc., New York, N. Y.
1
I
*
I
I
I
I l l [
I
I
Equipmenf
Particle
Figure 1. Elements of Apparatus
1
I
,I
I I l l /
I
I
I
1
I l I l 1
--
WHlTLU I
-
-
\
-
-
II
3
--n
Re= dpPf(uq-us) I
I
I
I
I
l
l
1
IO3
I
I
I
I
I
1 I
1'O4
I
r
I
1
1
1
1
1
INDUSTRIAL AND ENGINEERING CHEMISTRY
1200
I;
Vol. 43, No. 5
GAS VELOCITY, u 8 , tps O :
67
’p
75
80 8 , .
I
8;5
O
,Y
Figure 4. Weight of Particles Suspended us. Gas 1-elocity at Constant Values of Horsepower
,
I
60
QAS VELOCITY, ug
I
6S
I
70
I
76
I
88
80
U
90
Figure 3. Solid Feed Rate v s . Gas Velocity at Constant \'slues of Horsepower
constant. The experiments here reported are precisely in the zone of high acceleration in which particles enter the tube from a side inlet and are accelerated from rest upward or downmud, depending upon the gas velocity imposed. The apparatus arrangement permitted either cocurrent or countercurrent flow of particles, with the added feature that the particles could escape at either end of the apparatus. Zenz (la)reported a similar study which apparently was performed a t about the same time as the present one. He used separate pieces of equipment for cocurrent and countercurrent measurements, solids being fed a t the appropriate ends. Limiting conditions were reached as particle concentration mounted and an ultimate instability and slugging action ensued in each unit. It was postulated that fluidized bed formation is approached under these limiting con_ditions. The present experiments provide a direct link betxeen cocurrent and c o u n t e r c u r r e n t operation, because the experimental arrangement permitted a stable phase formation which has the visual and quantitative characteristics of a fluidized bed.
I
tions 1 or 2, for the computation of the fraction void. Section 1 Tyas 3.8 feet long and section 2 was 1.1 feet long. *\ir was supplied by an exhaust blower. The motor was cradled as a Sprague dynamometer and its brake horsepower was determined. The total pressure drop through the apparatus also was measured by means of appropriate manometers. Rate of air flow was measured by means of a standard nozzle as recommended by the National Bureau of Standards (8). The nozzle was preceded by a calming section and straightening vanes in accordance IT ith the code of the American Society of Heating and Ventilating Engineers (1). Screen and hopper to separate overhead particles were provided a t the entry of the calming section. Three sizes of clay spheres were used in the primary measurements: Material A was 0.0297 foot in diameter, material B, 0.0247 foot in diameter, and material C, 0.0201 foot in diameter. The density of the particles was 195 pounds per square foot. Because of strong end conditions it seemed desirable to “calibrate” the equipment by means of single-particle suspension tests. The result of these measurements in terms of a drag coefficient-Reynolds number plot is given in Figure 2. Particles of different density and size were used. The characteristic curve is noted t o be parallel t o that of Lapple and Shepherd ( 5 ) and higher by a factor of about 1.4. The three spheres that
I
I
I
w
+
EXPERIMEXTAL
The apparatus is illustrated diagrammatically in Figure 1.
A 4-inch glass tube was fitted with side ports to permit introduction of a stream of particles and of air. Particles left the system either through an overhead bend in the pipe or by moving downward, accumulating in the collection leg. A slide valve was provided to permit collection and measurement of particle population in sec-
QAS VELOCITY, rps. 55
60
bp
79
75
90
Figure 5. Pressure Drop us. Gas Velocity at Constant Values of Horsepower
w
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
0
A
I
1201
10000 I1000 12000 13000 REYNOLDS NUhBER, DPPf ug/lr --.FLU~D-I ZONE T~ANSPORT
9000
'
-
6
/SUSPENDING
VELOCITY! 70.6 fpS.
GAS VELOCITY, Ug, fps. A t
'55
I
60
I
65
I 70
I 75
i
80
Figure 6. Fraction Void us. Gas Velocity with Solid Feed Rate as Parameter
I
85
INDUSTRIAL AND ENGINEERING CHEMISTRY
1202
Vol. 43, No. 5
D1 SCUSSION
Gas Velocity uc
\"
. 5 -3
B
& ? 3
? Q .
Gas Velocity u,
Figure 7.
Typical Curies
Fraction void and pressure gradient us. gas velocity at constant solid feed rate
By means of croqs plots and interpolation of the experimcnral data of Figures 3, 4, and 5 a convenient graph is obtained of iraction void against gas velocity with solid feed rate as the painmcter. Figure 6 gives such a graph for material B. The solid lines are in the region covered by experimental data. Fraction void is defined as (V-V,)/V,where T i is the total volume in n-hich the action is taking place, and V , is the volume of the solid particles. The sequential behavior as gas velocity changes can bcst be described with the aid of Figuies 7 and 8. Figure 7 shows a fraction void-gas velocity curve for a single solid feed rate as ne11 as the corresponding pressure drop curve. Figure 8 is a repremitation of what is observed a s gas velocity is taken from a very high value to a low one. Consider a high gas velocity, .4. Under these conditions all particles are transported upxyard and out of the equipment. The particle population in the t,ransport, section 1 of Figure 1 is sptarse and does not vary greatly with fluid velocity change. Pree,sure drop through the section is primarily that due t o fluid friction in the empty pipe. The flow is cocurrent with respect to gas and particles. As air velocity is decreased to condition B, an increase in particle population takes place in section 1, solid and air coiitinuing through the sect'ion. At one point in the velocity railye the particles are observed visibly to collect and fall abruptly as an accumulated group from upper section 1 of the apparatus to lower section 2. Thereafter, a.t lower velocities than this critical value, the movement of particles is downward in countercurrent f l o to ~ the air. A relatively high particle density is imniediat,elv formed in section 2 and a t conditions C as illustrated in Figures 7 and 8. Bubble formation and an upper interface are visible in the resuking bed of particles. The critical zone is so labeled liecause in changing from cocurrent to countercurrent flow, events occur more rapidly than can be measured readily. The act,ions take place under conditions of rapid acceleration. A minimum in bed voidage is measured a t D,a t a velocity less than the suspending velocity for individual particles. As velocity is decreased further t.oward E, the bed, through which the descending stream of particles is now passing, decreases markedly in particle content, as shown by the rising fraction void curve. Ultimately a t zero air velocity the fraction void is that measured by simply pouring particles a t different rates through an open tube of the length of the apparatus. The entire sequence of events is rerei'sible. As may be anticipated, the pressure drop curve is an iuverse of the fraction void curve.
'
n-ere used in the transport experiments are indicated as TPC, -4, B, and C. The measured variables in the transport experinients 11 ere air flow rate, horsepower, solid feed rate, pressure drop, and particle holdup. It was convenient to perform experiments a t constant The position of maxinium densities or minimum fraction voide horsepower input. The solid feed rate of material into the test Eection was varied between 90 and 1200 pounds per hour. C n A B E At a constant horsepower a t the motor, increments of solid feed rate caused a decrease of air velocity, which was measured. Holdup was measured by simultaneously inserting a paddle into the holdup slot and cutting the air supply by means of the motor s~vitc h, Figures 3, 4,and 5 present experimental data for material B. Figure 3 shows the solid feed rate versus gas velocity a t constant horsepower. Figure 4 gives the corresponding weight of material suspended versus gas I velocity, and Figure 5 gives Figure 8. Pictorial Representation of Transition from Cocurrent to Countercurrent the pressure drop versus gas Flow of Particles in Rising Gas Stream velocity. The data for the A . Pure upward transport. Low particle population in upper column only B . Pure upward transport. Increased particle population in upper column only other particle sizes as well as C. Slugging region. Heavy concentration in lower column. Negligible population i n upper column D . Fluidized bed. Maximum particlc concentration i n lower column only further details regarding the E. Fluidized bed. Decreased particle eoncentration in lower column 0. Upward motion apparatus and experimental e. Downward motion procedure are available ( 9 ) . Gas velocity decreases left t o right
.'. .
l
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1951
m] E-
Typcai Re M 6 cwve for the fiuidiizdion of sdld norticies
Marial B
-3 :p
-
Material C
.b!
16
Figure 9.
-1 FRACTION VOID, C I I I 0.90 0.94 0.98 Fraction Void us. Reynolds Number
1.00
For screen-eupported fluidization and for conditions of maximum particle population in transport axperimente
*
a
in Figure 6 is of particular interest. The system then has all the appearances of a fluidized bed with an upper interface and bubbles rising through it. Operation is relatively stable. At velocities higher than the minimum, the bed becomes increasingly turbulent with strong slugging action until the unstable region is entered and the particles are thrown into the section of the column above the feed point. I t is interesting t o note that the positions of the minima transfer t o increasing gas velocities as the particle feed rate is decreased, approaching as a limit a t zero particle rate the velocity of suspension for a single particle. The formation of a fluidized bed by means of particle aggregation may be due to interparticle Bernouilli forces. As a basis for comparison, the fraction void-velocity relationships without particle interaction were computed for the given apparatus by standard procedures of particle dynamics (6). Symmetrical curves were obtained on either side of the single particle suspension velocity and these reached lower values of fraction void as the line representing this velocity was reached asymptotically. In the experiments, because of interparticle attraction, particles no longer act as individuals but as groups of particles and they tend to fall more rapidly. The net effect is that the system develops a minimum at a velocity less than the free fall velocity a t which this minimum would otherwise occur. On the supposition that the minima represent actual fluidization conditions, fraction void-velocity ’ relationships for these minimum conditions were plotted for experiments with the three sizes of particles. The results are shown in Figure 9, Reynolds number being proportional t o velocity. Straight lines are obtained on the semilogarithmic plot over the narrow range of fractions void terminating in the measured free fall velocities of the individual particles. Also shown in the figure is a line from the fluidization of 5-mm. glass beads in water as measured by Wilhelm
1203
and Kwauk (11) in screen-supported experiments. These data are the closest in Reynolds number range reported in the literature to those of the present work. The slopes of the curves from the transport experiments are closely similar to those obtained in independent fluidization experiments. This similarity in characteristics gives strong support to the supposition that we are dealing with the formation of a true fluidized bed in the region of transition from cocurrent t o countercurrent flow of particles. The ability t o measure the minima in the fraction void-velocity curves results from the experimental arrangement in which the particles are introduced through a side port into the vertical duct and have freedom of motion in either vertical direction. When the stream of solid particles is introduced into an end of the tube, as was done in the cocurrent and the countercurrent experiments of Zenz ( I I ) ,in the cocurrent continuous fluidization experiments of Lewis, Gilliland, and Bauer (67,or in any normal countercurrent or cocurrent contacting equipment, then the conditions of the experiment exist on either side of the zone of uncertainty in, Figure 6. It is not possible in such equipment to go past t h e minimum from one lobe of the curve t o another. As the minimum fraction void is approached from either side in such equipment, a dense bed begins to form and an increasing volume of it accumulates because of the continuous supply of solids into the end of the tube. Ultimately a choking up or flooding of the equipment takes place. Regardless of the type of equipment used, quality of fluidization that occurs or tends to occur near the condition of the minimum will depend upon the physical character of the system as outlined by Morse (7). Smooth or particulate fluidization is favored in shallow beds with small particle sizes and densities, by small differences in density between particle and fluid, and by high fluid viscosities. The reverse conditions, which are characteristic of the present experiments and those of Zenz, favor aggregative fluidization with a high degree of nonuniformity or bubble formation in the bed.
ACKNOWLEDGMENT
The authors wish to express their appPeciation t o the SoconyVacuum Oil Go., Inc., for the support of a fellowship which made this investigation possible. Thanks are also due to R. C. Reichhart and the Century Fan and Ventilator Co., for a gift of a blower and allied equipment, and to P. B. Gordon of Wolff and Munier, Inc., for valuable design suggestions.
LITERATURE CITED
(1) Am. Soo. Heating Ventilating Engrs., New York, N. Y . , “1947 Guide,” p. 95. (2) Belden, D. H., and Kassel, L. S.,IND. ENG.CHEM.,41, 1174 (1949). (3) Farber, L., Ibid., 41, 1184 (1949). (4) Hariu, 0. H., and Molstad, M. C., Ibid., 41, 1148 (1949). (5) Lapple, C. E., and Shepherd, C. B., Ibid., 32, 605 (1940). (6) Lewis, W. K., Gilliland, E. R., and Bauer, W. C., I W . , 41, 1104 (1949). (7) Morse, R. D., Ibid., 41, 1117 (1949). (8) National Bureau of Standards, Bur. Standards J . Research, 2 , 561 (1929). (9) Valentine, S., 111, “Co-Current Flow, Counter-Current Flow a n d
Bed Formation in a Fluidized Solid-Gas System,” M.S.E. thesis, School of Engineering, Princeton University, Princeton, N. J., May 1950. (10) Vogt, E. G., and White, R. R., IND.ENQ. CHEM.,30, 1731 (1948). (11) Wilhelm, R. H., and Kwauk, M., Chem, Eng. Progress, 44, 201 (1948), (12) Zens, F. A., IND.ENG.CHEM.,41, 2801 (1949).
RECEIVED Auguat 31, 1950.
