Ind. Eng. Chem. Res. 1994,33, 1889-1896
1889
Characterization of the Flow Transition between Bubbling and Turbulent Fluidization Ahmed Chehbouni, Jamal Chaouki,’ Christophe Guy, and Danilo Klvana Department of Chemical Engineering, Ecole Polytechnique, P.O. Box 6079, S t . “centre ville”, Montrbal, Qubbec, Canada H3C 3A7
A lot of controversy exists in the literature on the actual boundaries of the turbulent regime in gas-solid fluidization. This work shows that this disagreement results from the type of experimental techniques used. It is found that the onset of turbulent fluidization is a t V , and that the velocity uk,which has often been reported in the literature, is an artifact due to the use of differential pressure transducers. Experiments conducted for sand and fluid cracking catalyst (FCC) particles with differential and absolute pressure transducers and with a capacitance probe show that V , is the velocity a t which bubbles and slugs reach their maximum size. At gas superficial velocities higher than U,,the breakup of bubbles increases while the formation and coalescence of these bubbles decrease. The turbulent regime ends a t the transport velocity. Table 1. Properti- of Powders Used in the Experiments
1. Introduction
Although many studies have aimed at characterizing precisely the boundaries of the turbulent regime in a gassolid fluidized bed, a lot of controversy still exists as to whether this regime starts immediately after the bubbling regime or at a higher gas velocity,after a transition regime. For some researchers, the turbulent regime starts at the critical superficialgas velocity, U,, where bubbles or slugs reach their maximum diameter. The hydrodynamics of the gas-solid bed changes then froma regime where bubble formation and bubble coalescence are predominant to a regime where breaking and gradual disappearance of the large bubbles occur. These large bubbles are transformed into smaller bubbles and interstitial gas (Jin et al., 1986; Lee and Kim, 1988; Sun and Chen, 1989; Cai et al., 1990; Brereton and Grace, 1992). For other researchers, U,is only the startup of the transition toward the turbulent regime which actually begins at the gas superficialvelocity U b at which the large bubbles have completely disappeared and the gas-solid bed is almost homogeneous (Yerushalmi and Cankurt, 1979;Satija and Fan, 1985; Mori et al., 1988; Son et al., 1988; Lancia et al., 1988). This paper aims at settling this dispute. First, the change in the bed hydrodynamics is experimentally characterized as a function of the gas velocity from the beginning of the bubbling regime to the transport regime. The boundaries and the main characteristics of the turbulent regime are then evaluated. Second, data relating to the type of fluidization regime and obtained by means of three different experimental techniques are discussed. The analysisof pressure fluctuationsfrom both differential and absolute pressure transducers and the analysis of the local bed density as obtained from a capacitance probe are compared for FCC and sand particles. The characteristics of these particles which are respectively from group A and group B according to Geldart (1973) are given in Table 1. Standard deviation evaluation and spectral analysis of the recorded signals are used as qualitative means to characterize the various fluidization regimes as well as their correspondinghydrodynamic structure. Last, the discrepancies observed in the literature with respect to the definition of the turbulent regime are shown to be related to the type of experimental technique used.
* To whom correspondence should be addressed.
type descriptor particle size distribution (wm) 0-38 38-63 63-90 90-126 90-150 125-180 150-180 180-212 180-250 212-250 250-300 mean particle diameter h m ) particle density (kg/ma) Archimedes number
FCC (mass %)
sand (mass%)
2.20 22.70 29.90 30.20
0.38 4.03 10.18 24.02
12.80 16.47 15.10 1.50 29.82 0.69 78 1450 25
130 2650 208
2. Experimental Section
Two sets of experimental apparatus were used, an 82and a 200-mm4.d. stainless-steelcolumn. In both columns, the fluidizing gas was fed through a bubble cap distributor designed according to the recommendations of Wen et al. (1980). In addition, ports were located at various axial positions of these columna to allow insertion of pressure and capacitance probes. All experiments were performed at ambient temperature and atmospheric pressure and of 450 mm. with a static bed height (Ho) The pressure probes are in stainlesssteel with an internal diameter of 2 mm and an external diameter of 4 mm. This allows for rigidity and for a small dead zone volume (Clark et al., 1988). Ceramic microporous filters (20 pm) are positioned at the head of the probes in order to prevent particles from entering. The end tip of the probes are connected to four absolute transducers and one differential transducer giving an outlet electric current intensity (420 mA) proportional to the measured pressure: 2.90 mA/ kPa for the absolute transducers and 1.625 mA/kPa for the differential transducer. Data acquisitionis performed at 90 readings per second, and the number of readings is equal to 2048. The pressure fluctuation signals are analyzed by means of the average amplitude, the standard deviation, and the dominant frequency obtained by fast Fourier transform (Bendat and Piersol, 1971). The capacitance probe is a cylindrical condenser. A l-mm-diameter stainless- steel rod acts as one pole, and the other pole is a Cmm-diameter cylinder. The length of the larger cylinder is 220 mm. The probe is always
0888-5885/94/2633-1889$04.50/00 1994 American Chemical Society
1890 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 0.26
a
0.14
0.2
FCC
e.... ... .... 1.t
z=46Omm
.
C
mnd
z=360mm
E Y
L
c atto
Uc=Q76ds
**.a 6.
0.
z=6Omm
-**
0
!
o 0.20.4a10.8 i 1.21.41.81.8 2 2 2 2 4 2 t 2 8 3 3.2 as a4 as a8 at 0.8 a* i 1.1 1.2 superficlal gas velocity (ds) superficial gas velocity (ds) Figure 1. Standard deviation of pressure fluctuations from absolute transducers. Bed diameter D = 82 mm, (a, b) FCC particles, (c, d) sand o ai
a2
particles.
oriented horizontally in the bed, and it can be moved radially across the bed width at different axial locations. Electric insulation is ensured by a 2.5-mm-diameter quartz tube with a relative permittivity equal to 3.8. The varying probe capacitance is made part of an oscillating ac circuit: the change in oscillating frequency is then demodulated to yield a voltage. Three identical probes, except for their tip length which was respectively 2, 4, and 6 mm, were built in order to evaluate the effect of the tip length on the bed local density measurement and to examinate the flow disturbance. The electric capacity of the medium surrounding the probe tip is approximately proportional to its porosity or solid holdup as was already shown by Brereton (1987). The value of the porosity ( E ) was calculated from the read voltage by linear interpolation between the voltage of the gas alone (6 = 1)and the voltage of the fixed bed (t = 0.4 for sand). 3. Results and Discussion
3.1. Absolute Pressure Transducers. Figure 1 presents the variation of the standard deviation of the pressure fluctuations as a function of the superficial gas velocity and the axial position of the pressure probe for, respectively, FCC and sand particles. For each type of solid particles, the critical velocity U,and the values of the standard deviation are not affected by the actual position of the pressure probe providing it is located in the dense bed for the whole experiment. The absolute pressure transducer responds to every pressure fluctuation which occurs within the gas-solid bed (Davidson, 1991).
Figure 1shows that the hydrodynamicsof the fluidized bed are characterized successively, as the gas superficial velocity is increased, by an increase in the standard deviation of pressure fluctuations due to the increase in bubble formation and bubble coalescence up to the critical velocity U,, and a decrease in the standard deviation of pressure fluctuation as larger bubbles begin to break up at velocities higher than U,. The competition between the large bubble formation process and the breakup process continues gradually till the complete disappearance of bubbles and the startup of circulating fluidization. U,is found to be 0.75 rnh for sand particles and 0.26 m/s for FCC (Figure 1). The transport velocity is 2.4 and 1.1 m/s respectively for these particles in the 82-mmdiameter column. The transport values for FCC and sand particles are found to be in agreement with the correlation of Perales et al. (1990). The experimental data obtained with absolute pressure transducers clearly indicate that there is only one transition velocity before the onset of circulating fluidization. This is in line with findings of researchers who have used absolute transducers (Jin et al., 1986; Lee et Kim, 1988; Cai et al., 1990; Sun and Chen, 1989) and who have suggested that turbulent fluidization starts at U,and ends at the transport velocity, UQ(Table 2). Moreover, those researchers who, although they used absolute transducers, believe that there is another critical velocity between U, and U, (Satija and Fan, 1985; Son et al., 1988) had to use a different technique to evaluate the onset velocity of the turbulent regime. They suggested an alternative proce-
Ind. Eng. Chem. Res., Vol. 33, No. 8,1994 1891 Table 2. Transducer T Y DUsed ~ and Velocities Detected ~~~
transducers reference Y e r u b i and cankurt (im) Satija and Fan (1986) Jin et al. (1986) Mori et al. (1988) Lee and Kim (1988) Son et al. (1988) Schnitzlein et al. (1988) Sun and Chen (1989) Cai et al. (1989) Perales et al. (1890) Grace and Sun (1991) Mei et al. (1991) Horio et al. (1992)
velocities detected
used not specified absolute absolute not specified absolute absolute differential absolute absolute differential differential differential differential
a
-
u,,9,plateau uc uc U,,Uk,plateau uc uc
e..
..
Z= (I 60-260)mm
*.
; *.
U,,plateau
uc uc
U,,uk, plateau Uc, plateau U,,uk,plateau U,,uk,plateau dure where absolute pressure fluctuations are monitored at a point above the minimum height achieved in slugging and one looks for a sharp change in the probability of the local pressure reaching the outlet pressure. This corresponds to the complete disappearance of large bubbles. The experimental data presented here and observations by Brereton and Grace (1992)show that the above behavior occurs at the onset of circulating fluidization. 3.2. Differential PressureTransducer. In contrast to the absolute pressure transducer, the response of the differential transducer is affected by the distance between its probes (Davidson, 1991) and by its axial position. Pressure fluctuations are functions of the bed density and the bubble diameter. Experimentaldataof the normalized standard deviation of differential pressure fluctuations are presented in Figure 2for sand particles and for different distances between the two probes. As recommended by Brereton and Grace (19921,normalization has beencarried out by dividing by the time mean differential pressure. The curves are similar, except for Figure 2c, to the ones obtained by other researchers who used differential pressure transducers (Schnitzlein et al., 1988;Mori et al., 1988;Mei et al., 1989;Perales et al., 1990;Grace and Sun, 1991)(Table 2). They show not only the critical velocity, U,,but also another critical velocity, ut, at which the pressure fluctuations level off (beginning of a plateau). Yerushalmi and Cankurt (1979)have defined this velocity Uk as the onset of turbulent fluidization which is characterized by a stable suspension of solid aggregates with a low entrainment. Is there, then, an actual change in the hydrodynamic structure of the bed between U,and the transport velocity, and especially at u k ? As discussed above, breakup of large bubbles starts at U,. Although formation and breakup of large bubbles their still coexist as the gas velocity is increased above U,, residence time in the bed and their number decrease gradually until the onset of pneumatic transport (Brereton and Grace, 1992). However, in a limited volume of the bed such as the one bounded by the two probes of a differentialtransducer, the decrease in the bed density is dampened as well as the variability of the bubble size. This leads to a slower decrease of the standard deviation of the pressure fluctuation in the small volume than in the overall bed. Figure 2 shows that, under identical operating conditions, the plateau is well defined and uk is about 1 m/s when the two probes are 0.1 m apart while the plateau is not so well defined and u k is higher (1.2 m/s) when the two probes are 0.2 m apart. When the two probes are 0.4 m apart, no more plateau and therefore no more Uk are found. In this case, the experimental curve is similar to the ones obtained with an absolute pressure transducer. Exactly the same behavior was obtained with the 200mm4.d. fluidized bed.
r
1 I
1
.
o
a2
a4
01
ae
i
1.2
1.4
1.8
1.a
2
ft
C
a4
1
c
e
z=(lM)lbCCO)mm
Figure 2. Normalized standard deviation of pressure fluctuations from differentialtransducer. Bed diameter D = 82 mm, sand particlee.
The differential transducer responds to local pressure fluctuations generated by bubbles or slugs, passing near the end of the two probes (Clark et al., 1991;Davidson, 1991). Therefore, as the pressure taps get closer together, there will be an increasing correlation because the strongest signals are from events in close proximity to each probe.
1892 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 1'
[
1,-
E
6 400 200 0o
az a4
a6
aa
1
1.2 i , 4 1.8 1.8
z
2 2 2 4 21
za
Supsrflclrl a06 Veloclty (m/a)
Figure 3. Normalized standard deviation of pressure fluctuations and bulk density from differentialtransducer. Bed diameter D = 82 mm, sand particles.
Each probe will "see" the same event (the diameter and the velocity of the bubbles are almost the same at each probe, Figure 2a). When the pressure taps get further apart, they cease to be the same events (the bubbles will have time to split, for example), and then the correlation will decrease (Figure 2c, no plateau). Figure 3 shows that, although the distance between the two probes is identical to the experiment in Figure 2b, no more plateau is observed if the axial position of the probes is changed. From Figure 2, the critical velocity U, is found to be a function of the distance (h)between the probes: for h = 0.1 m, U, = 0.57 m/s, for h = 0.2 m, U, = 0.6 m/s, and for h = 0.4 m, U, = 0.7 m/s. This dependence is consistent with the fact that, with the absolute transducer for which h could be considered as the overall height of the bed, U, is equal to 0.75 m/s. Figures 2b and 3 show that U, is a function of the position of the probes even for the same distance between the two. When they are located at 50 and 250 mm from the distributor, U, is equal to 0.75 m/s, while at 150 and 350 mm, U, is equal to 0.60 m/s. The local variation of the stable bubble diameter with the height above the distributor can account for variations of U, (Figures 2 and 3). This is in agreement with previous work (Werther (1984) and others), where the mean bubble diameter, dmb, increases with height H, until a critical value H* where dmb remains constant. It is expected that the dynamic equilibrium between coalescence and splitting of bubbles is reached earlier at the top of the bed than at the bottom where bubble coalescence is predominant. Therefore, U, decreases slightly with height (Figure 2b and 3). This is in agreement with recent data reported by Sun and Grace (1991) for FCC particles. The effect of column diameter and the other parameters on the value of U,is reported elsewhere (Chehbouni et al., 1993). The typical pressure-time curves as obtained by the differential pressure transducer and their related power spectral densities are shown respectively in parts a and b of Figure 4. Two trends can clearly be seen in the hydrodynamics of the fluidized bed, as a function of U: 1. From U = 0.1 m/s to U,, the amplitude of the pressure fluctuations and the energy level of the spectral densities increase with the gas superficial velocity. Fluctuations become more and more regular; this translates into a smaller range of frequencies (dominant frequency). This is due to the formation and coalescence of bubbles and slugs. 2. Above U,, in the turbulent regime, as the superficial velocity is increased, fluctuations become more and more irregular; their amplitude and their spectral density decrease. Spectral densities spread over higher frequen-
cies, and no dominant frequency still exists. Flow irregularity is found to increase up to the transport velocity where all fluctuations vanish. The presented experimental data show that uk,which is only detected by a differential pressure transducer, is a function of the distance between the two probes and of the vertical position of the probes with respect to the fluidized bed. uk is related not to any change in the hydrodynamic structure of the bed, but to the decrease in the variability of fluctuations due to the smaller volume of bed which is actually investigated. 3.3. Capacitance Probe. The capacitance probe gives information on the local hydrodynamic structure surrounding its head while pressure transducers are sensitive to a much larger volume of the bed, if not the whole bed itself. Only a few papers have dealt with the transition to the turbulent regime as measured by capacitance probes (Lanneau, 1960; Kehoe and Davidson, 1970; Massimilla, 1973;Carotenuto et al., 1974;Crescitelli et al., 1978;Lancia et al., 1988). All of these researchers have found, although they have used different types of probes, that, as the gas superficial velocity is increased, bubble coalescence occurs. After bubbles reach their maximum size, the hydrodynamic structure of the bed changes: large bubbles break up into smaller bubbles. Beyond this first change which happens at U,, some disagreement appears with respect to the description of the hydrodynamics of the fluidized bed up to the transport velocity. Lanneau (1960) observed some small bubbles and a high solid holdup and, for higher gas velocity, uniformity in the bed which coincides with the rapid entrainment of the solid. For Kehoe and Davidson (19701, the slugs still exist beyond U, and the velocity at which these cannot be detected anymore is set arbitrarily. Massimila (1973) and Crescitelli et al. (1978) found that the bed structure changes periodically from slug to small bubbles. Carotenuto et al. (1976) indicate that the turbulent bed shows irregular small bubbles with small solid aggregates but found that the transition between bubbling and turbulent regimes is hard to characterize. Nevertheless, all these researchers mentioned only one transition velocity which is a t the beginning of the breakup of large bubbles. Only Lancia et al. (1988) found that, beyond U,, the slug regime deteriorates progressivelyuntil the complete disappearance of the slugs at Uk. Then the turbulent regime starts and is characterized by a better homogeneity and a lower solid holdup due to the large bed expansion. In this study, the local gas holdup, e, and the normalized standard deviation of the local density were measured as a function of the Superficial gas velocity for sand and FCC particles. Figure 5 presents experimental data for the sand particles in a82-mm4.d. fluidized bed as obtained by three capacitance probes which are different, one from the other, by their head size, L. The particular geometries of the miniaturized probes were designed to minimize flow disturbance as suggested by Werther and Molerus (1973). The volumes associated with such miniaturized probes are respectively 1.57, 3.14, and 4.71 mm3, which ensures that even small bubbles can be detected. It is obvious that the gas holdup experimental value does not depend on the probe size (Figure 5). Moreover, the presented data are similar to the ones obtained by Lancia et al. (1988). However, the proposed interpretation is quite different. The gas holdup increases with the gas superficial velocity until U, (around 0.8 m/s). Above U,,the gas holdup remains practically constant until the transport velocity (around 2.3 m/s, Figure 5). Two opposite phenomena take place. On the one hand, the gradual disappearance of large bubbles and slugs decreases gas holdup, and on the other hand, bubbles are no more the main vehicle for gas
Ind. Eng. Chem. Res., Vol. 33, No. 8,1994 1893
b
+am-
U=0.12 m/a
$
&a=-
:
1
a'\.* 0
z
0
1
2
3
4
6
6
6
26
6
2-
$!
& . a
1,6-
C
s 3 p
U=0.7 m/a
N
is 4 5. c
-
8
1
1-
Ha i
1
a
n 0
0
1
2
3
4
b
6
N
Taw-
U=1.7 m/e
8
P -. U=2.1 m/a s 6
6
e
iaw
U=2.1 mla
4-
F b r e 4. (a) Pressure fluctuations from differential transducer. Bed diameter D = 82 mm; sand particles, z = 60-260 mm. (b)Power spectral densities related to pressure fluctuations. Bed diameter D = 82 mm; sand particles, z = W 2 6 0 mm.
transport as gas percolates more and more through the gas-solid emulsion and, by doing so, increases the gas holdup. This increase is due to the large bed expansion and to the decrease of the size of the solid aggregates. Above 2 m/s, bed expansion is dominant and transport of particles takes place. This is confirmed in Figure 6, where the time necessary for the bed to get empty, in the absence of solid recycle, is presented as a function of the gas superficial velocity. The bed is found empty when the capacitance probe does not detect any particle. The transport velocity is evaluated as the superficial velocity above which its increase results only in slight decrease of
the emptying time. The plot of this time vs gas velocity allowsfor a graphical interpolation to obtain U, as defined by Perales et al. (1990). Figure 7a shows that, for FCC particles in the same fluidized bed, the gas holdup presents one obvious change in slope at U,. Above U,, the plateau is not as apparent as for sand since the bed expansion is very large for these type A particles and the gas holdup continues increasing. In the 200-mm4.d. fluidized bed, the same behavior is observed, except that both transition velocities U,and U, increase with bed diameter (Chehbouni et al., 1993) and the local gas holdup, e, slightly decreases (Figure 7b).
1894 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 1
FCC
A
a (D=82mm)
H
'ti
E
B
..I,, a6
o
I
E ar E
0
'.u^ ^
BP
P
; e
Utr=2.3 m/a
Uc=O.8mls
,
ae
,
,$,
a 2 a4 a6 a8
1
,
I
,
I
1.2 1.4
,
,
,
I
1,s 1.8
,
I
2
--0 nr
,
22
2 4 2s 2 8
ai as a8 1 aup.rflclal gae velocity (mla)
a2
ai
1.1
1.4
c
3
SuperflclelGam Velocity (We)
Figure 5. Normalizedstandarddeviation of local densityand average local void fraction from capacitance probes located at 250 mm above the gas distributor (r = 0 mm, sand particles). Bed diameter D = 82 mm.
ne
1 ,
b (D=POOmm) A
ao
600
400
a !l
300
as-
m -5
A
ai
= 200 as
'
o
100
0
"
'
1
'
1
'
"
"
1
'
'
'
Study of the capacitance probe signal is much more meaningful since it allegedly gives a true measure of the bed microstructure. Figures 5 and 7 also present a graph of standard deviation of local density inferred from a capacitance probe signal normalized with respect to the local density for sand and FCC in both columns. Only two different trends can be sorted out: 1. From U = 0.1 m/s to U,, the normalized standard deviation of local density increases with the gas superficial velocity. At U,, bed heterogeneity is maximum. 2. Above U,, as the superficial velocity is increased, normalized standard deviation of local density decreases. This is representative of the turbulent regime where small structures (bubbles and aggregates) are present. Exactly the same behavior was obtained with the measured void fraction fluctuations and their related power spectral densities (Figure 8) for the same four gas superficial velocities as in Figure 4. As the velocity increases, bubbles coalesce and slugs appear. A characteristic low dominant frequency at U = 0.7 m/s (Figure 8b) with a relatively high spectral energy is representative of the slug regime. Beyond 0.7 m/s, the periodicity disappears (no dominant frequency); the spectral density decreases gradually while frequencies reach higher values. This is related to the gradual decrease in the solid aggregate size and bubble size as well as the chaotic particle movement. The transition velocities Ucand U, obtained by means of the three different experimental techniques are slightly different. The two major causes include (a)the sensitivity
1
a2
'
1
a4
'
1
'
1
"
"
'
1
'
l
as a8 i 1.2 1.4 auperflclel gaa velocity (mb)
'
I
1.0
~
1.8
' o 2
c
Figure 7. (a) Normalized standard deviation of local density and average local void fraction from capacitance probes located at 250 and 350 mm above the gas distributor in the bed medium. FCC D = 82 mm. (b) Normalized standard deviation of local density and average local void fraction from capacitance probe located at 150 mm above the gas distributor in the bed medium (r = 0 mm). Bed diameter D = 200 mm, FCC particles.
of these experimental techniques is different and (b) the bubble diameter is a function of height H,above the distributor (Werther, 1978, 1984). Table 3 summarizes details of transducers and capacitance data used to determine the lower (U,) and upper (Utr)boundaries of the turbulent regime obtained in the 82-mm-i.d. column. An absolute pressure transducer gives a weighted average of pressure fluctuations throughout the bed; it is affected by the bubbles occurring in the whole bed, so ita response is independent of probe location within the bed (Roy and Davidson, 1989). The experimental value for Ucis then an average value for the whole bed. For a differential pressure transducer which responds to the local bubbles passing between its probes and canceling out any events occurring outside the volume in between, the two probes are affected by their location and by the distance from each other. The experimental value for Uc is then an average value for the fraction of bed between the two probes. As discussed before, this value depends on the locations of the probes. Nevertheless, the two pressure transducer techniques give almost similar experimental values, when the distance between the two probes is large. The capacitance probe gives more local information on the solid and gas holdups. It is not surprising then that the reported value for U, is different from the one obtained by the other techniques. The higher value may be due to the fact that the capacitance probe is more sensitive to the presence of small bubbles which delay the onset of the turbulent regime. This is consistent with the fact that U,
Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 1898 b 0.00%
4
1
u=0.12m/s
a003
0.9
J= 0.8
3
9
0.7
8 0.6
(D
$
0.5
UI
0.4 0.30 1 1
r
f
. "
'
1
u=0.12m/s ' " " 2 3 4 5 I
.
'
c
2
6
I
1
2 0.0025 -
%
* 0.8
6
8
L
0.003
0.9
4
U=0.7
m/s
9
5!
0.7
QI
F 0.6 0.5 UI
0.4 0.3,!,"
"
1
2
'
I
3
.
I
4
..' I
ii
'
'
5
.
J
6 1
1 C
f
0.9
*P 0.8 $!
p
0.7
0.8
0
8 0.5k UI
0.4-
'
I U=1.7 m/s
0.003
0.41
u=2.1 m/s
t
1=2.1 m/s
I
I 8
(l/s) Figure 8. (a) Void fraction fluctuations from capacitance probe. Bed diameter D = 82 mm,sand Particles, L = 4 mm, z = 250 mm, r = 1 mm. (b) Power spectral densities related to void fraction fluctuations. Bed diameter D = 82 mm,sand particles, L = 4 mm, z = 250 mm, r = 0 mm.
Table 3. Summary of the Test Methods and Their Results ( D = 82 mm) solids measurement technique U,(m/s) UQ(m/s) 0.26 1.1-1.2 FCC absolute transducer 0.28 1.2 FCC differential transducer 0.4 1-1.1 FCC capacitance probe 0.75 2.4 sand absolute transducer 0.57-0.75 2.4 sand differential transducer 0.7-0.8 2-2.3 sand capacitance probe ~~
appears later at the bottom of the bed where small bubbles are predominant. The pressure transducer techniques and the capacitance probes technique give similar results for the transport velocity (Table 3). The emptying time method is not easy
to use due to the difficulty to define a precise criterion. The nonexistence of ut,corroborates some studies which have been published recently. Schnitzlein and Weinstein (1988) have suggested characterizing the hydrodynamics of the fluidized bed by two regimes: a low velocity regime where t increases with the gas velocity (up to U,)and a high velocity regime where t changes only a little. Perales et al. (1990) found that uk and U, are similar for sand and FCC particles, while Bi and Fan (1992) showed that u k = U, for sand particles. Brereton and Grace (1992) did not observe any structural change between U,and U,. In sufficiently large columns, Svensson et al. (1993) showed that no transition velocity was found from 2 to 5 m/s for 320-pm sand particles.
1896 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994
4. Conclusion The analysis of experimental data obtained b y various techniques indicates that the hydrodynamics of fluidized beds shows only one structural change between the minimum bubbling velocity and the transport velocity. As the gas superficial velocity is increased f r o m the minimum bubbling velocity, bubble coalescence increases. At U,, large bubbles start to break up into small bubbles and interstitial gas. This phenomenon deteriorates gradually until bubbles completely disappear and transport occurs. The turbulent regime starts then at U, and continues up to the transport velocity. The velocity uk, which has been observed b y some researchers, has no physical existence. It is an experimental artifact due to the use of a differential pressure transducer. Nomenclature D = bed diameter, mm d,b = mean bubble diameter, m m f = frequency , 5-1 Ho= static bed height, mm H = height above the distributor, mm H* = height above the distributor at which stagnation of bubble growth begins, m m
h = vertical spacing of differential transducer probes, mm L = t i p length of capacitance probe, m m r = radial position, m m t = time, s U = superficial gas velocity, mas-1 U,= critical gas velocity at which predominant flow of bubbles varies from the coalesced to breakup conditions (onset of turbulent regime), m w l Uk = superficial gas velocity corresponding to leveling out of pressure fluctuation amplitude as superficial gas velocity is increased when a differential transducer is used, m d Ut, = transport velocity , m-s-1 z = vertical position of probe, measured from distributor, mm
Literature Cited Bendat, J. S.; Piersol, A. G. Random Data-Analysis and Measurements Procedures; John & Sons Interscience-Wiley: New York, 1971. Bi, H.; Fan, L. S. Existence of Turbulent Regime in Gas-Solid Fluidization. AIChE J. 1992,38,297. Brereton, C. M. H. Fluid Mechanics of Highvelocity Fluidized Beds. Ph.D. Dissertation, The University of British Columbia, Canada, 1987. Brereton, C. M. H.; Grace, J. R. The Transition to Turbulent Fluidization. Trans. Znst. Chem. Eng. 1992,70,Part A, 246-251. Cai, P.; Jin, Y.; Yu, Z. Q.; Wang, Z. W. Mechanism of Flow Regime Transition from Bubbling to Turbulent Fluidization. AZChE J. 1990,36,955-956. Carotenuto, L.; Crescitelli, S.; Donsi, G. High Velocity Behaviour of Fluidized Beds: Characterization of Flow Regimes. Quad. Zng. Chim. Ztal. 1974,10, 185-193. Chehbouni, A.; Chaouki, J.; Guy, C.; Klvana, D. Effete de Diffbrenta Parametres sur lee Vitesses de Transition de la Fluidisation en Regime Turbulent. (Effecta of Different Variables on the Transition Velocities of Turbulent Fluidization Regime.) Submitted for publication to Can. J . Chem. Eng. 1993. Clark, N. N.; Atkinson, C. M. Amplitude Reduction and Phase Lag in Fluidized-Beds Pressure Measurements. Chem.Eng. Sci. 1988, 43,1547-1557. Clark, N. N.; McKenzie, E. A.; Gautam, M. Differential Pressure Measurementa in SluggingFluidized Bed. Powder Technol. 1991, 67,187-199. Crescitelli, S.;Donsi, G.; Russo, G.; Clift, R. High VelocityBehaviour of Fluidized Beds: Slugs and Turbulent Flow. Chisa Conf. 1978, 1-20. Davidson, J. F. The Two-Phase Theory of Fluidization: Successes and Opportunities. AZChE Symp. Ser. 1991,281 (87),1.
Geldart, D. Types of Gas Fluidization. Powder Technol. 1973, 7, 285-292. Grace, J. R.; Sun, G. Influence of Particle Size Distribution on Performance of Fluidized Bed Reactors. Can. J. Chem. Eng. 1991, 69,1126-1134. Horio, M.; Ishii, H.; Nishimuro, M. On the Nature of Turbulent and Fast Fluidized Beds. Powder Technol. 1992,70, 229-236. Jin, Y.; Yu, Z.; Wang, Z.; Cai, P. A Criterion for Transition from Bubbling to Turbulent Fluidization. In Fluidization V; Ostergaarol,K., Sorenson, A., Eds.; EngineeringFoundation: New York, 1986;pp 289-296. Kehoe, P. W. K.; Davidson, J. F. Continuously Slugging Fluidized Beds. Chemeca 70;Institution of Chemical Engineers Symposium Series 33;Butterworthe: Australia, 1970; pp 97-116. Lancia, A.; Nigro, R.; Volpicelli, G. Transition from Slugging to Turbulent Flow Regimes in Fluidized Beds Detected by Means of Capacitance Probes. Powder Technol. 1988,56,49-56. Lanneau, K. P. Gas-Solid Contactingin Fluidized Beds. Trans.Znst. Chem. Eng. 1960,38,125-143. Lee, G. S.;Kim, S. D. Pressure Fluctuations in Turbulent Fluidized Beds. J. Chem. Eng. Jpn. 1988,21,515-521. Massimilla, L. Behaviour of Catalytic Beds of Fine Particles at High Gas Velocities. AIChE Symp. Ser. 1973,69 (128),11-13. Mei, J. S.;Rockey, J. M.; Lawson, W. F. Flow Regime Transition in Fluidized Beds of Coarse Particles. Proceedings of the 1991 International Conference on Fluidized Bed Combustion; ASME: Montrhl, 1991;pp 1225-1232. Mori, 5.;Hashimoto, 0.;Haruta, T.; Mochizuki, K.; Matautani, W.; Hiraoka, S.;Yamada, I.; Kojima, T.;Tuji, K. Turbulent Fluidization Phenomena. In Circulating Fluidized Bed Technology ZI; Basu, P., Large, J. F., as. Pergamon ; Press: Oxford, 1988;pp 105-111. P e d e s , J. F.; Coll, T.; Llop, M. F.; Puigjaner, L.; Arnaldos, J.; Cassal, J. On the Transition from Bubbling to Fast Fluidization Regimes. In Circulating Fluidized Bed Technology ZZI; Basu, P., Horio, M., Hasatani, as. Pergamon ; Press: Oxford, 1990; pp 73-78. Roy, R.; Davidson, J. F. Similarity Between Gas-Fluidized Beds at Elevated Temperature and Pressure. In Proceedings of the Sirth Engineering Foundation Conference on Fluidization; Engineering Foundation: New York, 1989; pp 293-299. Satija, S.;Fan, L. S. Characteristics of SluggingRegime and Transition to Turbulent Regime for Fluidized Beds of Large Coarse Particles. AZChE J. 1985,31,1554-1562. Schnitzlein,M.; Weinstein,H. Flow Characterization in High-Velocity Fluidized Beds Using Pressure Fluctuations. Chem. Eng. Sci. 1988, 43,2605-2614. Son, J.E.; Choi,J. H.; Lee, C. K. Hydrodynamicsin a Large Circulating Fluidized Bed. In Circulating Fluidized Bed Technology Zl; Basu, Pergamon ; Press: Oxford, 1988;pp 113-120. P., Large, J. F., as. Sun, G.; Chen, G. Transition to Turbulent Fluidization and ita Prediction. In Fluidization Vl; Grace, J. R., Shemilt, L. W., Bergougnou, M. A., Eds.; Engineering Foundation: New York, 1989. Svenason, A.; Johnason, F.; Leckner, B. Fluid-Dynamics of The Bottom Bed of Circulating Fluidized Bed Boilers. Proceedings of the 1993Znternational Conferenceon Fluidized Bed Combustion; ASME 1991;p 887. Wen, C. Y.; Krishman, R.; Kalyanaraman, R. Particle Mixing Near the Grid Region of Fluidized Beds. In Fluidization; Grace, J. R., Masten, J. M., Eds.; Plenum, New York, 1980. Werther, J. Scale-up of Fluidized Bed Reactors. Ger. Chem. Eng. 1978,1,243-251. Werther, J. Hydrodynamics and Maae Transfer Between the Bubble and Emulsion Phases in Fluidized Beds of Sand in Cracking Catalyst. In Fluidization VI; Kunii, D., Toei, R., Eds.; Engineering Foundation: New York, 1984,pp 93-101. Werther, J.; Molerus, 0. The Local Structure of Gas Fluidized BedsI. A Statistically based Measuring System. Znt. J . Multiphase Flow 1973,1, 103-122. Yerushalmi, J.; Cankurt, N. T. Further Studies of the Regimes of Fluidization. Powder Technol. 1979,24,187-205. Received for review August 9, 1993 Revised manuscript received January 25, 1994 Accepted May 9,1994O ~~~
~~
~~~~
Abstract published in Advance ACS Abstracts, July 1,1994.
