Demarcation of Large and Small Bubbles in Viscous Slurry Bubble

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Demarcation of Large and Small Bubbles in Viscous Slurry Bubble Columns Hae-Ryong Jin,† Dae Ho Lim,† Ho Lim,† Yong Kang,*,† Heon Jung,‡ and Sang Done Kim§ †

School of Chemical Engineering, Chungnam National University, Daejeon 305-764, Korea(Rep.) Korea Institute of Energy Research, Daejeon, 305-731, Korea(Rep.) § Korea Advanced Institute of Science and Technology, Dae jeon 305-701, Korea(Rep.) ‡

ABSTRACT: Demarcation of large and small bubbles was investigated in viscous slurry bubble columns of which diameter was either 0.051, 0.076, 0.102, or 0.152 and 1.5 m in height. The bubble size demarcating the large and small bubbles was considered with variations of gas velocity, liquid viscosity, solid content in the slurry phase, and column diameter. The size (chord length), rising velocity, and holdup of bubbles were measured by means of the dual-electrical resistivity probe (DRP) method, and the holdups of large and small bubbles were determined by adopting the dynamic gas disengagement (DGD) method. The holdups of large and small bubbles which were determined by means of two different methods were compared to each other to find out the bubble size for the demarcation between the large and small bubbles. In addition, the bubble rising velocities were compared with the interstitial gas velocities based on the holdups of bubbles larger than the indicated bubble size, to look for the demarcating bubble size between the large and the small bubbles. It was found that the bubbles of which size were larger than 5.0
6.0  10
3 m can be termed as large bubbles while others as small bubbles, respectively. The holdups of large and small bubbles are correlated in terms of operating variables, respectively, within these experimental conditions.

1. INTRODUCTION Bubble size and its distribution related to the bubble holdup have been regarded as domination factors for determination of the performance and efficiency of reactors or contactors which are employing the slurry bubble columns in conducting Fischer
Tropsch synthesis, dimethyl ether production, methanol synthesis, heavy oil upgrading and hydrogenation, fermentation and biological treatment, flue gas desulfurization, coal liquefaction, wastewater treatment, etc.1
6 Therefore, lots of investigations on the bubble properties and holdup in slurry bubble columns and multiphase flow systems have been conducted by using several measurement techniques.3,6
10 In addition, Wang et al.7 and Liu et al.9 discussed the relation between the Sauter mean diameter and the chord length of bubbles in multiphase flow systems. One of the great contributions obtained from the various kinds of previous research on the bubble size and holdup can be the demarcating of small and large bubbles in the slurry bubble columns because the flow behavior and thus hydrodynamics of large and small bubbles are quite different from each other in the columns. The dynamic gas disengagement (DGD) method has been one of the effective techniques to demarcate the large and small bubble holdups in slurry bubble columns, since the decrease slope of bubble holdup with elapsed time after stopping the gas flow into the column is quite different between the small and large bubbles. It has been generally understood that, for small bubbles, the shape is almost sphere, the size distribution is narrow and a roughly uniform size, which are suitable to comprise the homogeneous bubble flow regime.1
5,11,12 However, the size range of small bubbles in the slurry bubble columns has not been clear. That is, some of the investigators defined that the size of small bubbles was less than 1.0  10
3 m r 2011 American Chemical Society

to distinguish from large bubbles,12,13 while some of them recognized the small bubbles of which size was less than 4.0 or 7.0  10
3 m,4,14,15 but some investigators classified the small bubbles depending on the operation temperature; the diameter of small bubbles at 200 °C was less than 0.5  10
3 m and that at 265 °C was less than 0.3  10
3 m.16 Based on the concept of two phase theory, Krishna et al.4,14,15 suggested that the total gas holdup was composed of the gas holdups in two phases i.e., dense and dilute phases and the dense phase gas holdup, which was approximately constant for churn-turbulent flow regime, was due to the small bubbles and that of dilute phase was to the large bubbles. Lemoine et al.17 proposed an algorithm for predicting the bubble holdup structure and size from the rearrangement of empirical correlations based on the lots of data in literatures. However, the unified demarcation between the small and large bubbles based on their size has been still obscure, which is one of the important factors to analyze the mechanism of transport phenomena as well as hydrodynamics in slurry bubble columns, since the small bubbles are related closely to the dispersion behaviors while the large bubbles are to the convective behaviors in the slurry bubble columns. In the present study, thus, the demarcation of large and small bubbles was investigated by considering the holdup structure, size, and rising velocity of bubbles in the slurry bubble columns. The analysis was conducted by resorting to Special Issue: Nigam Issue Received: March 31, 2011 Accepted: August 29, 2011 Revised: July 21, 2011 Published: August 29, 2011 2062

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Figure 2. Schemes of large and small bubbles in viscous slurry bubble columns.

Figure 1. Schematic diagram of experimental apparatus: 1. main column, 2. gas/liquid distributor, 3. pressure probe, 4. resistivity probe, 5. pressure sensor, 6. A/D converter, 7. data acquisition system, 8. computer, 9. compressor, 10. valve, 11. liquid flowmeter, 12. gas flowmeter, 13. liquid pump, and 14. liquid reservoir.

Table 1. Physical Properties of Liquid Phase surface apparent viscosity 103 tension 103 density (Pa 3 s) (N/m) (kg/m3) water CMC 1 wt % CMC 2 wt % CMC 3 wt % CMC 4 wt %

0.961 11 24 38 50

72.9 73.2 73.3 73.6 73.9

1000 1001 1002 1003 1004

K (Pa 3 sn)

n

0.001 21.69  10
3 43.82  10
3 71.69  10
3 102  10
3

1 0.882 0.847 0.825 0.802

the dynamic gas disengagement and dual-electrical resistivity probe methods.

2. EXPERIMENTS Experiments were carried out in stainless-steel columns of which inside diameter was either 0.051, 0.076, 0.102, or 0.152 and 1.5 m in height, as can be seen in Figure 1. A gas distributor was installed between the main column section and a 0.2 m high stainless-steel distributor box. Oil-free compressed air was fed to the column through a pressure regulator, filter, and a calibrated rotameter. It was admitted to the column through 3.0  10
3 m ID perforated pipes with 44, 65, 88, or 129 holes of 1.0  10
3 m ID drilled horizontally in the grid, depending on the column diameter. The details of gas distributor can be found eleswhere.18
24 The superficial velocity of gas phase ranged from 0.04
0.12 m/s, the pressure was 0.1 MPa, and the viscosity of liquid phase was in the range of 1.0
50.0  10
3 Pa 3 s, respectively. The physical properties of the liquid phase were listed in Table 1. Glass bead of which diameter was in the range of 0.4
0.7  10
6 m was used as a solid phase to comprise the slurry phase. The concentration of solid particles in the slurry phase was in the range of 0
25 wt %. The bubble size in terms of chord length, rising velocity, and frequency were measured by means of the dual-electrical resistivity probe (DRP) system. The probe applied by 5 V DC detected the

difference in conductivity of gas and liquid phases. The resistivity probe, which was installed at 0.4 m above the distributor, consisted of 7  10
3 m diameter stainless-steel pipe coated with epoxy resin. The vertical distance between the two tips of the probe was either 2.0 or 3.0  10
3 m. The probe was located at the center between the wall and the center of the column. The tips of the probe, which were made of platinum wire, had a diameter of 0.2  10
3 m. The analog signals obtained from each probe circuit were processed to produce the digital data. The preselected sampling rate at the personal computer with the DT 2805 Lab Card was 1000 Hz.21
24 The total sampling time was 15 s. The signals were processed off-line. The bubble size (chord length), frequency, and holdup were determined from the relationship between the reformed and digitized probe signals and the bubble dwell and lag time.21
24 In order to determine the holdups of large and small bubbles, the dynamic gas disengagement (DGD) method3,4 was used in the same experimental condition that of bubble property measurement by means of the dual-electric resistivity probe method. The pressure drop variations were measured by pressure transducers located at the bottom and top of the column with elapsed time after stopping the gas flow into the column, during which large and small bubbles were escaped from the column. In addition, the overall bubble holdups in the column were measured independently by means of the static pressure drop method19
22 to check the holdups of large and small bubbles.

3. RESULTS AND DISCUSSION 3.1. Comparison of Bubble Holdups Measured by Two Different Methods. The schemes of the large and small bubbles

rising in the column can be described in Figure 2, of which holdups were obtained by employing the dynamic gas disengagement (DGD) method. The large bubbles could escape from the column easily due to the buoyancy force. Typical pressure drop variations which were measured with a variation of elapsed time after stopping the gas flow can be seen in Figure 3, which were used to calculate the holdups of large and small bubbles in each operating condition.3,4 The typical output signals from the two tips of electrical resistivity probe can be seen in Figure 4, which were reformed and digitized to obtain the digital data in order to determine the bubble properties such as chord length, frequency, rising velocity, and holdup.18
21 The mean value of chord length as a bubble size was determined from the number density distribution of the individual bubble chord length, which can be seen in Figure 5, in which the mean value of bubble size increases with increasing gas velocity. 2063

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Figure 3. Typical example of εG-fluctuation signals in viscous slurry bubble columns (UG = 0.04 m/s, μL = 0.05 Pa 3 s, CS = 0.15, D = 0.152 m). Figure 5. Probability number density of bubble chord length in slurry bubble columns (μL = 0.001 Pa 3 s, CS = 0.05, D = 0.051 m).

Figure 4. Typical output signals from the dual-electrical resistivity probe (DRP) (UG = 0.12 m/s, μL = 0.038 Pa 3 s, CS = 0.05, D = 0.102 m).

To look for the demarcating bubble size between the large and small bubbles, the holdups based on the bubble size which were determined by means of the DRP method were compared with those measured by the DGD method. The comparison of large and small bubble holdups with a variation of gas velocity can be seen in Figure 6. In Figure 6A, the values of small bubble holdups were determined by cutting the bubble size smaller than the indicated bubble size. That is, the values of εBS were determined as holdups composed of smaller bubbles than the indicated bubble size as a demarcation of DBS. Note that the values of small bubble holdups which were composed of bubbles smaller than 5.0  10
3 m are well fitted with those obtained by means of the dynamic gas disengagement method, with a deviation less than 8%. In addition, the large bubble holdup composed of bubbles of which size are larger than 5  10
3 m is generally coincided with the holdup of large bubbles which was determined by means of the DGD

Figure 6. Comparison of large and small bubble holdups measured by the DGD method with those by the DRP method with a variation of UG (A: μL = 0.038 Pa 3 s, CS = 0.05, D = 0.076 m/B: μL = 0.011 Pa 3 s, CS = 0.05, D = 0.076 m).

method less than 5% deviation, as can be seen in Figure 6B. These mean that the bubbles smaller than 5  10
3 m are comprising the small bubble holdup, in other words, the demarcation between the large and small bubbles can be 5.0  10
3 m in this condition. 2064

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Figure 7. Comparison of large and small bubble holdups measured by the DGD method with those by the DRP method with a variation of μL (A: UG = 0.1 m/s, CS = 0.05, D = 0.102 m/B: UG = 0.04 m/s, CS = 0.05, D = 0.152 m).

The comparison of large and small bubble holdups with a variation of liquid viscosity can be seen in Figure 7, where the holdups decrease with increasing liquid viscosity owing to the increase of bubble size and thus rising velocity with increasing liquid viscosity. In Figure 7A, the values of εBS are composed of bubbles smaller than 5.0  10
3 m and are well fitted with those of εBS obtained by the DGD method. In addition, the large bubble holdup is generally well fitted to the bubble holdups of εBL which are composed of bubbles larger than 5.0
6.0  10
3 m within 8
10% deviation (Figure 7B). These indicate that the demarcation size of bubbles between the large and small bubble can be 5.0
6.0  10
3 m. The comparison of large and small bubble holdups with a variation of solid content in the slurry phase (Cs) can be seen in Figure 8. The holdups of large and small bubbles decrease with increasing solid content. This can be due to that the values of the bubble holdups are including the small bubbles which can be coalesced and integrated easily with increasing solid content in the slurry phase. It has been reported that the bubble coalescence can increase with an increase in the solid content in the slurry phase, thus the bubble size increases, which consequently results in the significant decrease in the holdup of small bubbles.4,15,18 In Figure 8A, the holdups of bubbles smaller than 5.0  10
3 m are well fitted with the small bubble holdups measured by the DGD method, and the εBL values composed of larger bubbles than 5.0  10
3 m are well with the large bubble holdups obtained by

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Figure 8. Comparison of large and small bubble holdups measured by the DGD method with those by the DRP method with a variation of CS (A: UG = 0.04 m/s, μL = 0.011 Pa 3 s, D = 0.076 m/B: UG = 0.04 m/s, μL = 0.011 Pa 3 s, D = 0.152 m).

the DGD method (Figure 8B). The comparison of holdups of large and small bubbles with a variation of column diameter can be seen in Figure 9, where the large and small bubble holdups decrease with increasing column diameter in all the cases studied. Since the population of bubbles increases with increasing column diameter in a given gas velocity, the probability of bubble coalescence can increase, which leads to the increase in the bubble size and thus consequent increase in the rising velocity of bubbles. Therefore, the holdups of large and small bubbles decrease with an increase in the column diameter. In addition, it has been pointed out that the decrease of large bubble holdup with column diameter can be due to the decrease of wall effect with increasing column diameter.4 In Figure 9A and B, the decrease trend and values of bubble holdups, which are composed of bubbles larger or smaller than 5.0  10
3 m, are generally well fitted to those determined by means of the dynamic gas disengagement method, respectively, within 10% deviation. From Figures 6
9, it is reasonable to state that the demarcation between the large and small bubbles can be 5.0
6.0  10
3 m; it can be termed large bubble when the bubble chord length is larger than 5.0
6.0  10
3 m (DB g 5.0
6.0  10
3 m) and termed small bubble if DB is smaller than 5.0
6.0  10
3 m (DB < 5.0
6.0  10
3 m) within these experimental conditions. It has been reported that the flow behaviors of small bubbles have been strangely affected by the inertia of liquid flowing around the bubble, and thus the small bubbles have been observed to travel down at the side of the column.12,13 Zhang et al.12 pointed out that the descending bubbles are approximately 2065

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Figure 10. The rising and interstitial velocities of large bubbles with a variation of UG (μL = 0.011 Pa 3 s, CS = 0.05, D = 0.102 m).

Figure 9. Comparison of large and small bubble holdups measured by the DGD method with those by the DRP method with a variation of D (A: UG = 0.1 m/s, μL = 0.050 Pa 3 s, CS = 0.05 / B: UG = 0.04 m/s, μL = 0.038 Pa 3 s, CS = 0.05).

smaller than 4.0  10
3 m and their distributions are narrow. Krishna and Sie4 found that the bubble size distribution was narrow and roughly uniform, and its size was smaller than 7.0  10
3 m when the bubble column was operated in the homogeneous bubbly flow regime, in which the large bubbles could not appear. From the experimental results in the airparaffin oil system, Krishna et al.15 pointed out that the small bubble size was in the range of 1.0
4.0  10
3 m. Recently, Lemoine et al.17 suggested the correlation to predict the Sauter mean bubble diameter. The correlation can be written as eq 1, without the effects of gas sparger type 1:22 0:02 1:65 μ0:08 L σ G FG T UG0:14 1:52 0:12 FG μSL  0:30 D ð
2:29XW þ 2:ε1CV þ 2:77Fp dp Þ  ð1
εG Þ1:56 εG D þ 1

DB ¼ 37:19

From the calculation of Sauter mean diameters of bubbles based on the correlations of Lemoine et al.,17 the demarcation size between the small and large bubbles were in the range of 4.0
8.0  10
3 m depending on the experimented conditions of this study. Therefore, it is reasonable to state that the demarcation size to classify the large and small bubbles in slurry bubble columns can be in the range of 5.0
6.0  10
3 m based on the experimental results of this study. 3.2. Comparison of Bubble Rising Velocity and Interstitial Gas Velocity. The bubble rising velocity and interstitial gas velocity were also considered to examine the demarcation size of large and small bubbles in slurry bubble columns. That is, the rising velocities of bubbles which were measured by means of the DRP method were compared with the interstitial gas velocities estimated with the holdup values of bubbles larger than the indicated bubble size. In Figure 10, the interstitial gas velocities, UG/εBL, were plotted with respect to the superficial gas velocity. The interstitial gas velocities were estimated with the holdups of bubbles larger than 5.0, 6.0, 7.0, or 8.0  10
3 m, respectively. In this figure, the interstitial gas velocities increase almost linearly with increasing the superficial gas velocity in all the cases studied. Based on the drift flux theory,25
27 the interstitial gas velocity can be expressed as a function of superficial gas velocity as eq 4 UG ¼ C1 UG þ U0 εBL

ð1Þ And the Sauter mean bubble diameter of large bubbles can be written as eq 2, and thus that of small bubbles can be obtained from eq 3
5 0:22 0:33 8:60 0:042 2:37 2:74 εG εGL Þ DBL ¼ D0:96 B ð1
10 FL μL σ L UG

ð2Þ

εBS εG εBL ¼ ¼ DBS DB DBL

ð3Þ

ð4Þ

In eq 4, C1 and U0 are constant, and U0 can be a rising velocity of a single bubble in the given continuous liquid medium. Actually, the rising velocity of a single bubble can be estimated by eq 5. The value of single bubble rising velocity determined from the intercept   σLG 0:25 ð5Þ U0 ¼ 1:53 μL 2066

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Figure 11. The rising and interstitial velocities of large bubbles with a variation of μL (UG = 0.04 m/s, CS = 0.05, D = 0.051 m).

of a straight line in Figure 10 was 0.285 m/s, while the estimated single bubble rising velocity by eq 5 was 0.246 m/s. These two values of U0 were well agreed with each other within 13.7% deviation. Note in Figure 10 that the mean values of bubble rising velocity (UB) which were measured by means of the DRP method are generally well fitted with the values of interstitial gas velocities (UG/εBL) which were estimated with the holdups of bubbles larger than 5.0  10
3 m. The bubble rising velocity, UB, and the interstitial gas velocity, UG/εBL, are also plotted with a variation of liquid viscosity in Figure 11. In Figure 11, the mean values of bubble rising velocity are well fitted with the values of interstitial gas velocities estimated with the holdups of bubbles larger than 5.0 or 6.0  10
3 m. It can be noted in Figure 11 that the rising velocity of bubbles increases with an increase in the liquid viscosity. This can be due to the increase of bubble size owing to the increase of bubble coalescence in the column with an increase in the liquid viscosity.1
3,18 In addition, the values of UB and UG/εBL are plotted as a function of solid content in the slurry phase, CS, and column diameter, D, in Figures 12 and 13, respectively. The values of bubble rising velocity are generally well fitted with the interstitial gas velocity based on the holdups of bubbles larger than 5.0  10
3 m (Figure 12), while the values of UB are located between the two kinds of UG/εBL values based on the holdups of bubbles larger than 6.0  10
3 m as well as 5.0  10
3 m (Figure 13). From Figures 10
13, it has been noted that the bubble size to demarcate the large and small bubbles in the slurry bubble columns can be 5.0
6.0  10
3 m; if the bubbles of which size are larger than 5.0
6.0  10
3 m, then they can be treated as large bubbles and in the case of DB < 5.0
6.0  10
3 m, then the bubbles can be treated as small bubbles. These results can support that the demarcating bubble size between the large and small bubbles is in the range of 5.0
6.0  10
3 m from the comparison of holdups of large and small bubbles, which were measured by means of two kinds of different techniques, as discussed in the earlier section.

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Figure 12. The rising and interstitial velocities of large bubbles with a variation of CS (UG = 0.1 m/s, μL = 0.050 Pa 3 s, D = 0.152 m).

Figure 13. The rising and interstitial velocities of large bubbles with a variation of D (UG = 0.04 m/s, μL = 0.001 Pa 3 s, Cs = 0.05).

The holdups of large and small bubbles in slurry bubble columns were well correlated in terms of operating variables within this operating conditions as eqs 6 and 7, with a correlation coefficient of 0.90 and 0.84, respectively

2067


0:17
0:16
0:30 CS D εBL ¼ 0:139U0:63 G μL

ð6Þ

μ
0:340 C
0:069 D
0:041 εBS ¼ 0:018U0:689 G L S

ð7Þ

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4. CONCLUSION The demarcating bubble size between the large and the small bubbles in viscous slurry bubble columns could be successfully investigated by comparing the holdups of large and small bubbles obtained by the DGD method with those determined by means of the DRP method. In addition, the information from the comparison between the bubble rising velocity and the interstitial gas velocity could support the results on the demarcating bubble size to distinguish the large bubbles from the small bubbles. The demarcating bubble size was found to be in the range of 5.0
6.0  10
3 m depending on the experimental conditions such as gas velocity, liquid viscosity, solid content in the slurry phase, and column diameter. The holdups of large and small bubbles were well correlated in terms of operating variables. ’ AUTHOR INFORMATION Corresponding Author

*Phone: 82-42-821-5683. Fax: 82-42-822-8995. E-mail: kangyong@ cnu.ac.kr.

’ ACKNOWLEDGMENT Financial support from Korea Institute of Energy Research (A7-2802) is greatly appreciated. )

Dedication

This paper was prepared for dedication to Professor K. D. P. Nigam on the occasion of his retirement from IIT.

’ NOMENCLATURE CS particle fraction in the slurry phase [-] solid content in the slurry phase [vol%] CV Sauter mean diameter of bubbles [m] DB D column diameter [m] K Fluid consistency index [Pa 3 s] L column height [m] bubble chord length [m] LV n flow behavior index [-] molecular weight [kg] MW P pressure [MPa] t time [s] T temperature [K] gas velocity [m/s] UG rising velocity of large bubbles [m/s] UB concentration of purest component in the liquid mixXW ture [wt%] Greek Letters

εG εBL εBS εL μ F σ

total gas phase holdup [-] large bubble holdup [-] small bubble holdup [-] liquid phase holdup [-] viscosity [kg/m 3 s] density [kg/m3] surface tension [N/m]

Subscripts

B BS BL G

bubble phase small bubble large bubble gas phase

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GL L P SL W

large gas liquid phase particle slurry phase weight percent

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