Pergamon Chemcal Engineering Science, Vol. 49. No. 15, 2413-2421. 1994 pp. Copyright 8 1994 Ekncr Sheace Ltd

Printed in Great Britain. All ri&ts reserved am-2So9/94 57.00 l 0.00

0009-2509(94)EOO71-W

THREE-DIMENSIONAL FLOW VISUALIZATION OF DILUTELY DISPERSED SOLIDS IN BUBBLING AND

CIRCULATING FLUIDIZED BEDS

MASAYUKI HORIO+ and HIROAKI KUROKI* Department of Chemical Engineering, Tokyo University of Agriculture and Technology, Koganei,

Tokyo 184, Japan

(First received 24 April, 1993; accepted in revised form, 15 March 1994)

Abstract-Three-dimensional flow structures of dilute suspensions in the freeboard of a bubbling Ruidized bed as well as in a circulating Auidized bed of 200 mm internal diameter, roughly a (l/25)-scale model of a commercial boiler, were observed by the laser sheet technique. With this technique the suspension behavior in an arbitrary cross section can be observed. Furthermore, by applying mdtiple laser sheets the three- dimensional structure of the suspension can be investigated. In the dilute suspension above a bubbling fluidized bed, the so called “ghost bubbles” were clearly visualized. In the dilute phase transportation regime the existence of clusters was confirmed. The average shape of a cluster was a paraboloid heading downward and enclosing a gas wake. in the upper side. From a cluster, particles were shed to the dilute phase continuously and these were again absorbed by other clusters. Cluster sizes and their velocity distributions were determined from the video Image analysis. The present data would serve for the future construction of mathematical models on the flow structure of circulating fluidized bed reactors.

1. INTRODUCTION

Circulating fluidized bed technologies have achieved many commercial successes in the fields of industrial boilers and catalytic crackers (Engstrom and Lee, 199 1; King, 1992). Nevertheless, the characteristics of the gas-solid suspension tIow in circulating fluid&d beds are not yet understood well due to their complic- ated nature. Some characteristics so far made clear include the large gas-solid slip velocity (Yerushalmi et al., 1976), the sigmoidal axial distribution of solid holdup (Li and Kwauk, 1980), the formation of the core-annulus flow structure (Bolton and Davidson, 1988; Hartge et al., 1988; Horio et a[., 1988; Rodes, 1990) and the formation of clusters, in other words, denser islands of suspended solids (Horio et al., 1988). As demonstrated by Ishii et al. (19X9), it is easy to apply Nakamura-Capes model to confirm that in the ordinary circulating fluidization condition of fine par- ticles they do not form a core-annulus flow structure, as long as they are suspended homogeneously. Ishii et al. (1989) further developed the clustering annular flow model (CA model), the extended version of Nakamura-Capes model for clustering suspension, and showed that from the experimentally determined cluster sizes and densities the model can predict the core radius quite successfully. From their arguments it is clear that the presence of clusters is essential for the large gas-solid slip velocity, for the large solid holdup compared with the dilute pneumatic transport and for the core-annulus flow development. Since all of these

‘Author to whom correspondence should be addressed. rPresent address: Coal Research Lab., fdemitsu Kosan

Co. Ltd., 3-1 Nakasode Sodegaura, Chiba 299-02. Japan.

are quite important in the design of gas-solid reactors and/or bed-to-surface heat transfer rate control, more information is needed to describe the phenomena completely.

To obtain more detailed information on the cluster behavior, Horio et a!. (1992a) measured radial and axial distributions of cluster sizes and local gas velo- cities in circulating fluidized beds. However, it was based on point measurements with probes, and ac- cordingly, it has been difficult to detect cluster shapes and their dynamic changes. In the course of hydro- dynamic research on suspension behavior in circu- lating fluidized beds, the serious lack of information on three-dimensional suspension behavior has been recognized more and more widely. Arena et al. (1989) and Jin (1992) reported a video analysis of solid suspension behavior in two-dimensional circulating fluidized beds. However, three-dimensional flow visu- alization in more realistic beds has not been at- tempted, due to the dense annular flow which pro- hibits the light penetration. Only very local images (less than 10mm in diameter) taken with an optical fiber image scope have been reported (Takeuchi and Hirama, 1991; Hatano et al., 1992).

To obtain three-dimensional images of solid sus- pension flows it would be possible to apply the laser sheet technique, which has recently become popular in the field of single-phase flow and gas-liquid flow analysis (Shioji et al., 1991). With this technique the solid suspension behavior in three-dimensional circu- lating fluidized beds could also be observed as long as the downflow is not too dense. In the case of very thick downtlow some lighting and imaging techniques would circumvent this difficulty. As a preliminary challenge to the three-dimensional flow visualization

2413

CES 49:15-B

2414 MASAYUKI HORIO and HIROAKI KUROKI

for circulating Ruidized beds, the visualization of rather lean but not too dilute suspensions in a labor- atory scale circulating fluidized bed was conducted.

2. EXPERIMENTAL

The test rig used in the present work (Fig. l), same as the one in our previous work (Horio et al., 1989), was made of PMMA resin tube. The riser was 200 mm in internal diameter and 1600 mm high, roughly a (l/25)-scale model of a commercial scale boiler. The inside of the riser tube was covered by cellophane tape to avoid the accumulation of static electricity. Eight pressure taps were placed along the riser. Static pres- sure distribution was measured by both semicon- ductor pressure transducers (COPAL ELEC- TRONICS P-A300) and water manometers. Solid holdup I ~ E was calculated assuming Idp/dzl = (1 - &)p, 9. The circulation rate of particles was meas- ured by a semicontinuous particle flow meter (5) (Horio er al., 1992b), and controlled by the aeration to the downcomer as well as by a butterfly valve (6). Table 1 shows the properties of particles used and the experimental conditions. The superficial gas velocity I+, was less than 0.6 m/s, and solid circulation flux G, was less than 0.6 kg/m2 s.

materials particles : FCC fluid : air

particle properties 4 61.3 pm 4 1,780 kg/m3 Ut 0.198 m/s

operating conditions uo : 0.15-0.60 m/s

GS : 0.016 -0.60 kg/m%

It is interesting here to know how realistic the operating conditions of the present experiments are. According to the scaling law of circulating fluidized beds (Horio rt al., 1989) the present conditions corres- pond to u0 < 3 m/s, G, < 3 kg/m” s in the corres- ponding commercial scale boiler (0, = 5 m, atmo- spheric, T = 1123 K). Since operating conditions of

commercial scale boilers range over u0 = 4- 10 m/s and G, = 20-50 kg/m’ s, the present experimental conditions are a little more moderate than the com- mercial conditions. In the present experiment there was no significant dense bed formation at the bottom of the column and, accordingly, the flow was in the dilute transport mode, except for the case of u0 = 0.15 m/s and G, = 0.016 kg/m’ s where dense bed formation (bed height: 400mm above the dis- tributor) and bubbling were observed. Since in com- mercial scale circulating iluidized bed boilers the major part of furnace is occupied by the “lean but not too dilute suspension”, it is clear that the present experimental conditions are not too unrealistic.

@ Riser

@ Cyclone

@ Downcomer

@ Pressure tap

- @ Particle flow meter

@ 8utterfly valve

@ Distributor

@ Windbox

@ To filter

To illuminate the suspension flow, one or three He-Ne laser light sources (output = 10 mW, ,J = 632.8 nm, beam diameter = 0.8 mm) were applied. As shown in Fig. 2(a), each laser sheet, formed with a glass rod lens of 4 mm in diameter, was applied through the transparent riser in the axial and radial directions. Flow patterns both in vertical and hori- zontal cross sections were visualized by gdjusting the angle of the laser sheet. For the visualization of vertical planes a laser sheet was applied through the center of the riser. The area of visualization was adjusted as roughly 200 mm x 200 mm. In the pre- sent work, the laser sheets were applied between 750 and 1350mm above the distributor. To see flow patterns of vertical and horizontal planes simultan- eously a set of three laser sheets intersecting at right angles were applied [Fig. 2(b)]. The visualized flow data were recorded by a video system in 60 frames per second. The shutter speed of the video camera was set at l/100 s. The still pictures printed out from the video system were analyzed to determine the cluster behavior.

Fig. 1. Schematic of-experimental apparatus.

With the present visualization system it was im- possible to visualize the suspension flow when G, was higher than 0.6 kg/m2 s. This was because the pen- etration of the laser light from the outside of the riser wall was prevented by the particle downflow over the wall which became denser with increasing G,. For the visualization of the suspension flow in higher G, conditions, it is necessary to avoid the thick solid

Dilutely dispersed solids in bubbling and circulating Ruidized beds

Riser 1 ,600

r-7-k

He-Ne laser

Video camera

400

Video system

(a)

Riser

A set of three faser sheets

Fig. 2. Schematics of flow visualization system: (a) flow visualization in vertical cross section; (b) three-dimensional

visualization.

downflow both for the TV camera and the laser

illumination. In our another series of experiments (Kuroki and Horio, 1994) a small TV camera was inserted into the core region of the riser. A special

(a) Vertical cross section light guide was also attached on the inner wall of the riser so that the laser sheet can penetrate into the core region. We have been calling such a system the “in- ternal picturing” in contrast with the “external picturing” system used in the present work.

3. RESULTS AND DlSCUSSIONS

3.1. Particle behavior in the freeboard of a bubbling juidieed bed

An example of axial voidage distributions of a bubbling fluidized bed obtained from pressure mea- surements is shown in Fig. 3. Figure 4 shows the photographs of vertical and horizontal cross sections of the freeboard under the same condition as that of Fig. 3. A denser phase (the bright part in pictures) and a leaner phase (the dark part in pictures) can be distinguished clearly. The double white spots on ver-

(b) Horizontal cross section tical cross section in Fig. 4 are the reflections of the Fig. 4. The visualized photographs ofcross sections in dilute

laser light from the external surface of the column. phase above a bubbling fluidized bed (IQ, = 0.15 m/s, G, =

These photographs indicate that, even in the dilute 0.016 kg/m* s, z = 750 mm).

region in the freeboard, the gas flow does not suspend particles homogeneously allowing the presence of gas velocities of the gas pockets at the view height were pockets. From the video images it was confirmed that obtained for 50 samples randomly taken from the the gas pockets drifted upward changing their shapes video images. As shown in Fig. 5, the average rise and sizes. The average sizes as well as the average rise

. _ velocities of gas pockets were faster than uO, and the

E (4 Fig. 3. Voidage distribution under the operating condition

for Fig. 4.

1 ,200

-z r 800 G= 0.018 kg/m%

N

2415

2416

0.3

g.2

J

MA&

uQ= 0.15 m/s A

\

G= 0.016 kg/m%

A

/ I

800 1,000 1,200 1,

4Y UKI HORIO and HIROAKI KUROKI

(a) Vertical cross section 2 (mm)

Fig. 5. Average rise velocities and sizes of gas pockets in axial direction.

1,600, I

N I ‘r FCC I

I- & (-)

Fig. 6. Solid holdup distribution along the riser axis corres- ponding to the flow of Fig. 7.

pocket sizes decreased with increase in height. From the motion of particles around gas pockets, it was confirmed that a gas pocket has vortices both in the vertical and the horizontal planes.

These gas pockets are similar to the ghost bubbles of Pemberton and Davidson (1984) which are sup- posed to be formed at the bed surface as a result of vortex ejection from erupting bubbles. The average size of gas pockets represents the scale of turbulence of the dilute suspension. Horio et al. (1980) pointed out that the radial particle transport and, accordingly, the formation axial distribution of particle concentration in the freeboard are determined by the decaying turbulence generated at the bed surface by erupting bubbles. The above observation seems to support their model visually.

3.2. Particle behavior in a dilute transport condition Figure 6 shows the axial solid holdup distribution.

Figure 7 shows the solid distributions under the condition corresponding to Fig. 6. As observed in

(b) Horizontal cross section Fig. 7. The visualized photographs in the dilute trans- portation regime (14~ = 0.58 m/s, G, = 0.22 kg/n? s, z =

750 mm).

Fig. 8. The three-dimensional visualization (q, = 0.58 m/s, G, = 0.22 kg/m’ s, z = 750 mm).

Fig. 4, a gas pocket above a bubbling fluidized bed had a clear boundary probably because of the pres- ence of a rather stable swirl of gas in it. In the case of dilute transport there are no more gas pockets of such clear boundaries. In contrast, the presence of “denser islands of particles”, or “clusters”, can be seen clearly.

Dilutely dispersed solids in bubbling and circulating fluid&red beds 2417

cross section

cross section

Fig. 9. The sequential pictures showing a paraboloidal cluster (uO = 0.50 m/s, G, = 0.18 kg/m2 s, z = 850 mm).

2418 MASAYUKI HORIO and HIROAKI KUROKI

The average shape of a cluster found in the vertical cross section is a parabola or a horseshoe shape heading downward and having thin tails upward. In the horizontal cross section the clusters are roughly circular but their boundaries are interconnected to form a network. The existence of particle downflow near the riser wall was also confirmed.

With respect to the cluster shape no consensus has yet been established. Some assumed that clusters are spherical (Yerushalmi et al., 1978; Ishii et al., 1989) and other investigators assumed string like patterns in suspensions (Li and Kwauk, 1980). To investigate the three-dimensional shape of a cluster a set of three laser sheets intersecting at right angles was applied as shown in Fig. 8. With this setup we were able to observe the vertical and the horizontal cross sections simultaneously. As can be seen more clearly in Fig. 9, the closeup pictures of the intersection line of the vertical and horizontal laser sheets, it was found that a typical cluster was a paraboloid facing downward having a gas wake in its inside, which agrees well with the early prediction by Ishii et al. (1989). Particles were emitted from the periphery of the paraboloid forming a tail or a film like dilute particle flow which again merged into other clusters. The shape of a cluster always changed randomly. Of course a para- boloid was soft and its cross section was not always in

(a) Original photo

(b) Cluster size reading Fig. IO. Definition nf cluntrr si7e

a perfect circle. However, the important thing is that under a given operating condition the sizes of clusters were rather uniform. This implies that the gas-solid slip velocity, or the gas drag force acting on the suspension, is determined by the nose part, not by the tail part, of a cluster.

Let us define a cluster size as the distance between the outer surface of the tails as shown in Fig. 10. Clusters were sampled from the core region (within 70mm from the column axis) of video images and their sizes and velocities were determined from frame- by-frame analysis of video images. Figure 11 shows

the average cluster size as a function of particle holdup. The average cluster sizes were calculated from 100 clusters sampled randomly. For the same superfi- cial gas velocity z+, it was shown that cluster showed sizes having a tendency to decrease with the increase in solid circulation rate. Here, it would be worth to argue whether such relationships between cluster size and gas velocity and between cluster size and solid circulation rate are consistent with the postulate that clusters are suspended in the stream as large bodies (Horio et al., 1992a). Since the increase in gas velocity increases the .gas drag force acting on clusters, the maximum suspendable cluster size should be in- creased. This is cdnsistent with the present data. However, although the increase in solid circulation velocity likely increases the drag force, it reduced the cluster size in our experiment. If particles shed from a cluster start ascending accelerated by the gas flow and then if they are absorbed by another cluster, the role of shed particles is only a mediator of momentum transfer from gas to clusters. Instead, the increase in particle concentration may increase the particle shed- ding rate and, as a result, may decrease the cluster size. It seems, however, that it is still too early to start any further discussion about it.

In Fig. 12, the rise velocity of clusters is plotted against their sizes. The wide scatter of cluster velocit- ies over the range from - 0.05 to 0.7 m/s indicates

FCC

0.02 0.04 0.06 0.08 f

1 -E (-)

Fig. 11. The average cluster sizes as a function of solid holdup (clusters were sampled at core region; within 70 mm

from column axis).

Dilutely dispersed solids in bubbling and circulating Auidized beds 2419

0.9

0.8

0.7

z 0.6

E 0.5

y 0.4

= 0.3

0.2

0.1

0

-0.1

t

FCC up0.6mIs dp=61 .3pm G,=0.266kg/m4 pp=1760kg/m3 z =660mm

. .

. l -0 .

l m

0 5 10 15 20 30

&I (mm)

Fig. 12. The relation between sizes and rise velocities of clusters sampled at core region (within 70 mm from column

axis).

not only the turbulent nature o,f cluster motion but also the large slip velocities between clusters and the gas surrounding them.

The transient motion and shape change of clusters are shown in Fig. 13 . The time interval between the video frames was l/60 s. Clusters were mostly moving upward in the riser. However, some of them stayed at one height for seconds (cluster A in Fig. 13) and others even moved downward, which again shows that clusters are suspended in the gas stream as large bodies.

Even in the dilute transport regime lateral trans- port of clusters took place as shown in Fig. 14 (see cluster B). The lateral transport then induced the solid downflow over the wall as shown in Fig. 15.

4. CONCLUSIONS

With the laser sheet illumination the behavior of lean but not too dilute suspension of FCC particles in a laboratory-scale circulating fluidized bed was suc- cessfully visualized. The present findings can be sum- marized as follows:

(1) In the dilute suspension above a bubbling fluid- ized bed the eddies ejected from the bed surface as a result of bubble eruption do not disappear immedi- ately but drift upwards in the form of gas pockets or the so called ghost bubbles.

Fig. 13. The motion of clusters in vertical plane with time (u,, = 0.58 m/s, G, = 0.22 kg/m2 s, z = 1350 mm).

indicating that clusters were suspended as large bodies.

(2) Even in the dilute transportation regime the suspensions tend to form clusters. A typical cluster was in a shape of soft paraboloid with its round nose facing downward enclosing a gas wake in it and the film-like flow of shed particles on its upper side.

(3) The shape of clusters changed frequently but under a given operating condition the sizes of clusters were rather uniform. Some clusters stayed at one height for seconds and others even moved downward in solid holdup and with the decrease in gas velocity.

(4) Lateral transport of clusters and formation of solid downflow over the column wall were clearly visualized.

(5) The size of clusters decreased with the increase

0 s

l/60 s

2160 s

3160 s

4160 s

2420

200 mm MASAYUKI HCIRIO and HIROAKI KUROKI

Fig. L4. Movement of clusters in the radial direction (u, = 0.58 m/s, G, = 0.22 kg/m2 s, z = 1150 mm).

200 mm

lJ60 s

2J60 s

3160 s

(6) The velocity of clusters of the same size scat- % average rise velocity of gas pockets, m/s tered widely and indicated a large gas-solid slip % terminal velocity, m/s

velocity. z height above distributor, mm

NOTATION

median particle diameter, pm cluster size, mm average cluster size, mm average diameter of gas pockets, mm column diameter, m gravity acceleration, m/s2 average solid mass flux, kg/m2 s static pressure, pa temperature, K superficial gas velocity, m/s cluster rise velocity, m/s

Fig. 15. The solid downflow near the riser wall (u. = 0.58 m/s, G, = 0.22 kg/m’s, I = 750 mm).

Greek letters E voidage, dimensionless d wave length, nm

PP particle density, kg/m3

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Dilutely dispersed solids in bubbling and circulating fluidized beds 2421

Engstrom, F. and Lee, Y. Y., 1991, Future challenges of circulatine fluid&d bed combustion technology. in Circu- lating Flzdized Bed Technology, Vol. III (Ediied by P. Basu, M. Horio and M. Hasatani) pp. 15-26. Pergamon, Oxfoid.

. _

Hartge, E.-U., Rensner, D. and Werther, J., 1988, Solid concentration and velocity patterns in circulating fluidjzed beds, in Circulating Fluidized Bed Technology, Vol. II (Edited by P. Basu and J. F. Large) pp. 165-180. Per- gamon, Oxford.

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_ ~.

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