NOTE

Local Bubble Behavior in Three-Phase Fluidized Beds Z. CHEM, C. ZHENG, I! FENG and H. HOFMANM

Department of Chemical Engineering, Beijing University of Chemical Technology, 100029 Beijing, China

4 Institut fur Technische Chemie I, Der Universitcit Erlangen-Numberg, 8520 Erlangen, Germany

Bubble behavior, including bubble Sauter diameter, bubble rise velocity, bubble frequency and local gas holdup in different radial and axial positions, was measured using a dual electro-conductivity probe in air-water-glass beads fluid- ization systems. It has been found that the bubble characteristics differ significantly in various flow regimes, depending on the operating conditions; the radial distribution of bubble parameters also changes from one flow regime to another. Thus, it is necessary to employ local bubble behavior in the modeling of three-phase fluidized beds.

Le comportement des bulles, et notamment le diametre des bulles de Sauter, la vitesse d’ascension des bulles et la reten- tion de gaz locale dans des positions radiales et axiales differentes, a ete mesure a I’aide d’une sonde d’electroconductivite duale dans des systemes de fluidisation a billes air-eau-verre. On a trouve que les caracteristiques des bulles variaient de faqon significative dans divers regimes d’ecoulement selon les conditions de fonctionnement; la distribution radiale des parametres des bulles change avec le regime d’ecoulement. C’est pourquoi le comportement des bulles local est a inclure dans la modelisation des lits fluidises triphasiques.

Keywords: bubble behavior, fluidization, three-phase fluidized bed, flow regime.

n the modeling of three-phase fluidized-bed reactors, bub- I ble characteristics such as bubble size, bubble rise velocity and local gas holdup, are of fundamental importance. On the other hand, bubbles play a major role on the flow structure and phase holdups in the beds. Investigation of bubble behavior would provide a basic knowledge for the analysis of hydrodynamic phenomena of the fluidized beds. Therefore, many researchers have recently paid attention to this aspect (Fukuma et al., 1987; Han and Kim, 1990), though the effect of flow regimes has not been discussed in their work.

Moreover, it has been found that flow regimes in a three- phase fluidized bed depend mainly upon operating condi- tions (Muroyama et al., 1985; Fan et al., 1986; Chen et al., 1995a), and the bubble behavior is significantly different in different flow regimes. However, surprisingly little work has been found in the literature concerning the effect of flow regimes on bubble behavior.

The present work is aimed at an investigation into local bubble behavior in different flow regimes under various operating conditions by means of a dual electro-conductivity probe. The effect of flow regimes on the bubble behavior is discussed.

Experimental

Experiments were carried out in a Plexiglas column with a 0.285 m inside diameter and a 4.1 m height in air-tap water-glass beads systems. The distributor is shown in Figure 1. It was found that the orifice size of gas passage- way on the distributor has a remarkable effect on the flow pattern: the large diameter of the orifices would result in an early onset of the turbulent bubble regime (TBR) without undergoing the homogeneous bubble regime (HBR) or the

*Author to whom correspondence should be addressed. Present address: Department of Chemical and Petroleum Engineering,

University of Wyoming, Laramie, WY 82071. E-mail address: chen@esig2.uwyo.edu

liquid gas 200x 0.8mm - m i - I 1 l l

p = = \ d

u Figure 1 -Distributor structure (not to scale).

TABLE 1 Measurement Positions

gas

liquid

Axial positions (x), m 0.653 1.055 1.458 2.005 2.543 Radial positions (r), m 0.0 0.0355 0.071 5 0.1065 0.1295

transition regime (TR), even if the gas velocity was very low. The diameter of 0.8 mm used in this study was found to ensure that the systems can operate in HBR at low gas velocities. 200 orifices for the gas and 61 tubes for the liq- uid to pass through were evenly spaced on the distributor. A 120-mesh wire screen was mounted on the distributor sur- face; this screen is necessary to prevent the particles from blocking the orifices. Spherical glass beads, tap water and compressed air were used as solid, liquid and gas phases, respectively. Glass beads were weighed before being loaded into the beds. Gas and liquid flow rates were controlled by valves and measured by rotameters.

Bubble behavior in 5 different axial positions and 5 differ- ent radial positions (see Table 1) of the column was measured over wide ranges of gas velocity, liquid velocity, particle diameter and particle loading. A dual electro-conductivity

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 76, APRIL, 1998 315

A n 40.0 ,

TR

1 .o 3.0 5.0 7.0 9.0 I, mm

Figure 2 - Bukble chord length distribution dp = 0.67 mm, E, = 1.62%, x = 1.458 m;r = 0, Ul = 6.53 c d s ,

ug = 5.33 c d s

I I

0.0 0.2 0.4 0.6 0.8 I .o I .2 v, m l s

Figure 3 - Bubbje rise velocity distribution dp = 0.755 mm, E, = 4.37%,x = 2.005 m, r = 0, U,= 6.53 c d s ,

Ug = 2.78 c d s

probe was used, and the conductivity signals were logged into a microcomputer and processed to give bubble parame- ters. The structure and measurement principle of the electro- conductivity probe were described previously (Chen et al., 1994). The probe consisted of two tungsten filaments of 80 ptn in diameter. The vertical distance between two tips of the probe was 0.5 mm, which allowed the minimum bubble size detectable by the probe to be approximately 1 mm (Buchholz et al., 1981).

The sampling time was 100 - 600 s, depending on the operating conditions. The number of bubbles measured available for estimates of bubble size and bubble rise veloc- ity was over 100 per sample, the largest one being about 600. Results and discussion

Generally, the bubble Sauter diameter was obtained from the measured bubble chord length by an empirical correla- tion which resulted from image analysis and chord length measurements by the same probe in a two-dimensional bed (Ueyama and Miyauchi, 1976). However, the bubble shapes in three-dimensional beds are certainly different from those

0.0 1 ' 1 ' 1 I 0.0 0.2 0.4 0.6 0.8

rlR I

Figure 4 - Radial distribution of bubble Sauter diameter in dif- ferent flow regimes:-U, = 6.53 cm/s, x = 2.005 m

d = 0.755 mm, E, = 4.37%, Ug = 11.6 cm/s + B = 1.8 mm, E, _= 2.40%, Ug = 3.0 c d s A 4 = 0.755 mm, E, = 1.94%, Ug = 3.0 c d s

.'- I

0.4 I / I ' I I

rlR

Figure 5 - Radial distribution of bubble rise velocity in different flow regimes: Ul= 6,53 c d s , x = 2.005 m

d = 0.755 mm, E, = 4.37%, Ug = 11.6 cm/s + c f = 1.8 mm, E, _= 2.40%, Ug = 3.0 c d s A 4 = 0.755 mm, E, = 1.94%, Ug = 3.0 c d s

in two-dimensional beds. With this consideration in mind, in the present study, the calibrating correlation for the bubble chord length was directly obtained in the test bed by taking pictures and measuring the chord length of bubbles near the column wall simultaneously. The resulting correlation is d, = 1.871, + 0.97 x lW3; it indicates that bubbles with a small chord length are more spherical in shape, while bubbles with a large chord length are more spheroidal.

It has been found that the probability distribution of bubble size follows a log-normal distribution under various operat- ing conditions, while that of bubble rise velocity basically obeys a normal distribution. It should be noted that the oper- ating variables, especially gas and liquid velocities, affect undoubtedly the distribution shapes. As examples, Figures 2 and 3 show the distributions of bubble size and bubble rise velocity in TR and HBR, respectively, according to the cri- teria proposed in a previous work (Chen et al., 1995a) in which it was found that the flow regime was mainly depen- dent on the gas and liquid velocities which predominantly influenced the bubble behavior. Thus, it is clear that the bubble behavior is indeed different in different flow regimes.

316 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 76, APRIL, 1998

L 70.0 4 I

0.0 I I I ' I I i 0.0 0.2 0.4 0.6 0.8 I .o

rIR

Figure 6 - Radial distribution of bubble frequency in different flow regimes: dp = 0.53 mm, E, = 6.67%, x = 0.653 m, U, = 3.48 c d s + Ug = 5.3 cm/s; W Ug = 12.0 cm/s; A Ug = 1.8 c d s

Figure 4 represents the distribution of the bubble Sauter diameter, along with the radial direction of the column. It can be clearly seen that the bubble size decreases with an increase in the distance from the central axis of the column; this is in agreement with the findings reported by Han and Kim (1990), who found that large bubbles tend to move towards the center of a fluidized bed and small bubbles rise in the vicinity of the wall. It is also discernible that in the homogeneous bubble regime, bubble sizes distribute uni- formly along the radius of the bed; the distribution becomes progressively steep from the HBR to the transition regime, and to the turbulent bubble regime. The similar phenomena can be observed in the distribution of bubble rise velocity, as shown in Figure 5 . It is worth noting that in the same flow regime in the range of this study, these distributions are essentially similar; in the case of the transition of flow regimes resulting fkom the changes of operating conditions, the distribution of bubble behavior changes significantly.

Figures 6 and 7 report the distributions of bubble fre- quency and normalized local gas holdup. Bubble frequency represents the number of bubbles passing through the mea- surement point per second. It can be found from these figures that both bubble frequency and local gas holdup decrease along the radial direction towards the column wall; in differ- ent flow regimes, they exhibit different distributions in the radii of fluidized beds. It has been found that although bubble behavior follows different radial distributions in different flow regimes, it is possible to correlate these distributions in a similar model, but with different model parameters (Chen et al., 1995b). Bubble size and bubble rise velocity may be

2 2 correlated as = 1 - C, (i) and -% = 1 - C2 (i) ;

dEO h 0 whle bubble frequency and local gas holdup may be correlated

In the above equations, subscript 0 indicates bubble parame- ters at the axis of the bed, which change with the axial posi- tion; Ci (i=l, 2, 3 and 4) are the equation parameters which are a function of the operating conditions and should be determined from the experimental measurements. Detailed

0.8 -

0.4 - f t -

0.0 0.2 0.4 0.6 0.R I .o r/R

Figure 7 - Radial distrib3tion of local gas holdup in different flow regimes: dp = 0.53 mm, E = 6.67%, x = 0.653 m, U, = 3.48 cm/s

W Ug = 12.0 cds ; 6 Ug = 5.3 cm/s; A Ug = 1.8 cm/s

functionality of Ci is given elsewhere (Chen et al., 1995b). From either the experiments or the above correlations, it is clear that bubble behavior is a strong function of radial and axial positions of the bed. Therefore, it appears essential to use local bubble behavior in the modeling of three-phase fluidized beds.

Conclusion

The experimental data obtained in this work reveal that the operating variables and local positions of three-phase fluidized beds have significant effects on bubble behavior. Bubble properties change with operating conditions and dif- ferent regularities in different flow regimes. This suggests that it is essential to use the local data instead of mean-value parameters over the entire column to describe the flow hydrodynamics of each phase and that the effect of the flow regimes needs to be taken into account in the modeling of three-phase fluidized-bed reactors.

Acknowledgement

help in the preparation of the manuscript.

Nomenclature

dB = bubble Sauter diameter, m 5 = bubble Sauter diameter at the axis of the bed, m = particle diameter, mm 4 = bubble frequency, l/s

fBo = bubble frequency at the axis of the bed, I/s HBR = homogeneous bubble regime le = bubble chord length, m P = probability density function of bubble parameters r = radial coordinate of the bed, m R = column radius, m TR = transition regime TBR = turbulent bubble regime U = superficial gas velocity, m/s 4 = superficial liquid velocity, m/s VB = bubble rise velocity, m/s VBo = bubble rise velocity at the axis of the bed, m/s x = axial coordinate of the bed, m

Greek letters E~ = local gas holdup E@ E~ = average solid holdup

The authors would like to thank Ms. Kirse Kelly for her kind

= gas holdup at the axis of the bed

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References

Buchholz, R., W. Zakrzewski and K. Schugerl, “Techniques for Determining the Properties of Bubbles in Bubble Columns”, Int. Chem. Eng. 21, 180-187(1981).

Chen, Z., C. Zheng and Y. Feng, “An Electro-conductivity Micro- probe Used to Determine Bubble Characteristics in Multiphase Flow Systems”, Petrochem. Technol. (China) 23,415-50( 1994).

Chen, Z., C. Zheng, Y. Feng and H. Hofmann, “Distributions of Flow Regimes and Phase Holdups in Three-phase Fluidized Beds”, Chem. Eng. Sci. 50, 2153-2159(1995a).

Chen, Z., C. Zheng, Y. Feng and H. Hofmann, “Modeling of Three-phase Fluidized Beds Based on Local Bubble Characteristics Measurement”, Chem. Eng. Sci. 50,23 1-236 1995b).

Fan, L. S., S. Satija and K. Wisecarver, “Pressure Fluctuation Measurements and Flow Regime Transitions in GasLiquiMolid Fluidized Beds”, AIChE J. 32, 338-340 (1986).

Fukuma, M., K. Muroyama and A. Yasunishi, “Properties of Bubble Swarm in a Slurry Bubble Column”, J. Chem. Eng. Japan 20,28-33( 1987).

Han, J. H. and S. D. Kim, “Radial Dispersion and Bubble Characteristics in Three-phase Fluidized Beds”, Chem. Eng. Commu. 94,9-26 (1 990).

Muroyama, K. and L. S. Fan, “Fundamentals of Gas-Liquid-Solid Fluidization”, AIChE J. 31, 1-34 (1985).

Ueyama, K. and T. Miyauchi, “Determination of Mean Bubble Diameters Based on Observation by a Two-point Electric Probe”, Kagaku Kogaku Ronbunshu 2,430-43 1 ( 1 976).

Manuscript received May 13, 1997; revised manuscript received March 17, 1998; accepted for publication March 18, 1998.

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