Radial Bubble Distribution in a Fluidized Bed with Vertical Tubes

Sep 19, 2012 - Therefore, the radial distribution of bubble characteristics is investigated in a fluidized bed with vertical tubes by means of optical...
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Radial Bubble Distribution in a Fluidized Bed with Vertical Tubes Martin Rüdisüli,† Tilman J. Schildhauer,*,† Serge M. A. Biollaz,† and J. Ruud van Ommen‡ †

Paul Scherrer Institut (PSI), General Energy Research Department, CH-5232 Villigen PSI, Switzerland Delft University of Technology, Product and Process Engineering, Julianalaan 136, 2628 BL Delft, The Netherlands

‡

ABSTRACT: The radial distribution of bubble characteristics such as the bubble frequency, the bubble size, and the bubble rise velocity, is a key to better understanding the flow patterns of gas (and solids) in bubbling fluidized beds. Therefore, the radial distribution of bubble characteristics is investigated in a fluidized bed with vertical tubes by means of optical probes. Measurements are taken for Geldart B particles at two bed heights, three gas velocities, and three vertical tube bank configurations. While the bubble frequency shows a strong dependence on the radial position in the bed and the orientation of the vertical tubes, the bubble size and the bubble rise velocity, in particular in the lower portions of the bed, are more constant. Typical bubble pathways are shown to move from the column wall to the center axis of the bed with increased bed height. Therefore, in order to have a representative mean bubble size and mean bubble rise velocity, which accounts for the whole crosssection of the bed, an area-number weighted mean is introduced.

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complicated gas−solids flow patterns with multiple vortex rings and vigorous circulation of solids may establish.8 Since vertical tubes have an influence on the lateral movement of bubbles and solids,9,10 gas−solids flow patterns with vertical tubes seem to be even more complex than those without internals. Also the determination of a representative bubble size with vertical tubes depends on the radial distribution and the flow pattern of bubbles in a cross-section of the fluidized bed. Due to the shown implications of a heterogeneous bubble growth, the objective of this paper is to measure the radial distribution of rising bubbles in a fluidized bed of Geldart B type particles with different vertical tube banks by means of optical probes. Along with the radial bubble size distribution, the radial distribution of the bubble rise velocity (BRV) and the number of bubbles (= bubble frequency) are investigated. From the obtained spatial distribution of bubbles at several gas velocities and bed heights, the gas (and solids) flow patterns in a fluidized bed with vertical internals is studied and conclusions on bubble coalescence, preferential bubble pathways, and channeling are drawn. Eventually, the radial distribution of the bubble size is used to calculate a representative area-number weighted mean bubble size.

INTRODUCTION The influence of vertical tubes on the axial bubble growth in a fluidized bed has recently been investigated by means of pressure fluctuation measurements (PFM) and optical probing (OP) by Rüdisüli et al.1 It has been shown that in fluidized beds with vertical tubes, the bubble size is significantly reduced compared to beds without internals. The extent of this bubble size reduction, however, depends on the configuration of the tube bank. While the tube arrangement (square or triangular) does not have an influence on the bubble growth, an increased bubble growth reduction can be achieved if the tube-to-tube spacing is narrow and if the tube diameter is small. By means of OP, not only the axial bubble growth, but also the radial components of the bubble growth in a fluidized bed with vertical tubes can be measured. A similar investigation, yet without internals, to detect preferential bubble pathways and to obtain a representative bubble size for an inhomogeneous spatial bubble distribution has been conducted by Werther and Molerus2 and Werther3 by means of a needle-type capacitance probe.4 Their results showed that there is a characteristic radial flow profile of bubbles such that in the lower portions of the bed, bubbles preferably rise in an annulus near the wall, while they move toward the center axis of the bed with increased distance from the distributor plate. This finding of Werther and Molerus2 is supported in a recent experimental study of Lim et al.5 with a pseudo-2D-bed and time-averaged imaging to show that even for a uniform introduction of bubbles, the characteristic gas flow pattern postulated by Werther and Molerus2 is established. The spatial distribution of bubbles, moreover, has a fundamental influence on the gas−solids circulation in a bubbling fluidized bed: Since solids are dragged up in the wake of the bubble,6 due to conservation of mass, it immediately follows that in regions with less bubble activity, solids must move downward again.2 This pattern of upward and downward gas−solids circulation depends on the bed aspect ratio D/H, the superficial gas velocity, and the gas distributor design.7,8 For deep beds with high fluidization velocities even more © 2012 American Chemical Society

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EXPERIMENTAL SECTION All experiments in this paper were conducted in a glass column fluidized bed (“Glas15”) with an internal diameter of D = 0.143 m (cf. Figure 1). The gas inlet was a porous plate distributor of 10 pore size. The total height of the Glas15 was 0.9 m and the settled bed height was 0.5 m by default. As bed material, porous aluminum oxide (γ−Al2O3) Puralox NWa-155 (= NWA) by Sasol Germany GmbH was used. NWA has a mean particle size of 289 μm, a density of 1350 kg/m3, and a minimum Received: Revised: Accepted: Published: 13815

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Figure 1. Schematic drawing of the experimental unit “Glas15”. Abbreviations are explained in the Nomenclature section.

fluidization velocity umf of approximately 0.041 m/s. Therefore, NWA can be classified as Geldart B.11 For more information on the Glas15 and NWA, refer to Rüdisüli et al.12 and Rüdisüli et al.,13 respectively. Bubble characteristics are measured by means of optical probing (OP) mounted at the wall of Glas15 at 0.243 and 0.450 m above the distributor plate (cf. Figure 2). The optical probes are in-house-made and composed of two stems with an outer diameter of 5.2 mm. To evaluate the bubble size, bubble frequency, and bubble rise velocity (BRV) a bubble linking algorithm is used. This bubble linking algorithm is based on least-squares regression techniques and evaluates the vertical bubble chord length by measuring the response of an ellipsoidal bubble pierced by two reflective optical fiber probes spaced at 1 cm. As shown by Rüdisüli et al.,14 the locally measured vertical chord length of the bubble can be taken as an adequate approximation of the overall mean bubble size in the bed. For more information on optical probing and the bubble linking algorithm, refer to Rüdisüli et al.12 To investigate the radial distribution of bubble characteristics, the optical probe is radially moved from the center of the bed to the wall of the glass column by increments of 1 cm. Optical probe measurements are recorded for each radial position during 20 min at a sampling rate of 400 Hz. Measurements are taken under ambient conditions (i.e., pressure and temperature) and for (relative) gas velocities u0/umf at 2.3, 4.5, and 6.8. The recorded time series are evaluated afterward by means of MATLAB. All vertical tube banks in this study are made from hollow steel rods. Due to geometrical constraints with triangular tube arrangements, OP can only be applied to square tube arrangements, viz. Sq/15/9, Sq/15/18, and Sq/20/9. This abbreviated nomenclature of the tube banks means, for

Figure 2. Photo of the optical probe setup at the wall of “Glas15”.

instance, squared tube arrangement (= Sq), 15-mm tube diameter, and 9-mm tube-to-tube spacing. Thus, the tube bank configurations vary in their tube diameter and the tube-to-tube spacing. For more information on physical properties of the tube bank configurations, refer to Rüdisüli et al.1 An important feature of the tube banks is that adjacent tubes are interconnected at their bottom by plastic “U”-shaped connectors, such as would be used in industrial vertical heat exchanger tubes. Depending on the orientation of these U's, the relative position of the probe tip to the tubes and the U is altered. Figure 3 displays Sq/15/9, Sq/15/18, and Sq/20/9 along with the relative positions of the probe tip and the U at the bottom. Measurements with Sq/20/9 are conducted for both relative positions 0° and 90°, since with Sq/20/9 the largest difference in the orientation of the U and the tubes relative to the probe position is found. Sq/15/9 and Sq/15/18 are more radial-symmetric with respect to the optical probe positioning.

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RESULTS Number of Analyzed Bubbles. The number of bubbles analyzed by the optical probe (i.e., the bubble frequency) for each radial position and for each tube bank configuration is shown in Figure 4 at u0/umf = 2.3. For each radial position, experimental runs of 20 min sampled at a frequency of 400 Hz are taken. To have a more natural interpretation of the radial distribution, the number of analyzed bubbles at each radial 13816

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interconnected tubes. Auxiliary tubes such as explained in Rüdisüli et al.1 are shown in a lighter and thinner gray. Investigated tube bank configurations encompass Sq/15/18 (= wider tube-to-tube spacing), Sq/20/9 (= larger tube diameter), and Sq/15/9 (= reference) along with no internals in Figure 4d. In Figure 4, the radial distribution of the number of analyzed bubbles varies for all tube bank configurations, for all radial probe positions, and for both probe heights (z = 0.243 m and z = 0.45 m). Basically, the maximum number of bubbles is larger between the tubes. This gives a characteristic wriggly distribution. Typically, at the lower probe height of z = 0.243 m, bubbles are predominantly found at about D/4, and not in the center of the bed (= D/2). At 0.450 m probe height, in turn, bubbles rather move toward the center of the bed. A similar conclusion can also be drawn for the case without internals (cf. Figure 4d), which is in agreement with the findings of Werther and Molerus.2 If the total number of analyzed bubbles is compared between the two probe heights 0.243 and 0.450 m, the influence of bubble coalescence can be seen. In the bed without internals, the total number of bubbles decreases by −35% from the lower to the upper probe height. This is a clear indication that bubbles grow by coalescence. Contrarily, for all internals the

Figure 3. Employed vertical tube banks (a) Sq/15/18 (wide tube-totube spacing), (b) Sq/20/9 (large tube diameter), and (c) Sq/15/9 (reference). Principal tubes are represented with dark blue dots. Auxiliary tubes are displayed as light blue dots. “U”-shaped connectors at the bottom of the tubes are shown as green bars and horizontal support spacers are given as black lines. The measurement positions of the optical probe are marked with a red line and cross ( × ) at 0° and with a blue line and cross ( × ) at 90° for (b) Sq/20/9.

position is mirrored at the center axis of the bed. Moreover, the vertical tubes are displayed as gray shaded columns. The relative orientation of the probe position to the tube bottom is indicated at the bottom of the tube with a black rectangle, if the probe is located between two interconnected tubes, or with an extension of the gray shaped area, if the probe is parallel to two

Figure 4. Number of analyzed bubbles (= bubble frequency) as a function of the radial optical probe position for (a) Sq/15/18, (b) Sq/20/9, (c) Sq/15/9, and (d) no internals at a relative gas velocity u0/umf = 2.3. The total sampling time is 20 min and the sampling frequency is 400 Hz. Black rectangles at the bottom of the gray shaded areas (= tubes) indicate that the probe is located between two tubes interconnected by a “U” at their bottom. If the gray shaded area at the bottom of the tubes is extended, the probe is parallel to two tube interconnected by a “U”. Note that dashed lines are mirrored at the center axis and only meant to guide the eye. 13817

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Figure 5. Number of analyzed bubbles as a function of the radial probe position for Sq/15/9 with (a) u0/umf = 4.5 and (b) u0/umf = 6.8. For further information on the exact interpretation of the figures, refer to the caption of Figure 4.

Figure 6. Radial bubble size distribution for (a) Sq/15/18, (b) Sq/20/9, (c) Sq/15/9, and (d) no internals at a relative gas velocity u0/umf = 2.3. Also included are the area-number weighted means calculated with eq 1. For further information on the exact interpretation of the figures, refer to the caption of Figure 4.

frequent bubble splitting. Since coalescence is the principle mechanism of bubble growth,15 this is a potential explanation why vertical tubes effectively delay the growth of the bubble size. The influence of the gas velocity on the number of analyzed bubbles can be found in Figure 5 for the reference tube bank

number of analyzed bubbles between the lower and the upper probe height remains either unchanged at 100% (Sq/15/18) or even increases by +10% (Sq/20/9 and Sq/15/9). However, an increase of +10% in this context is certainly not overly large. Nonetheless, this indicates that by means of vertical tubes, bubble coalescence is either inhibited or compensated by more 13818

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0

1.8 3.9 1.5 3.6 1.6 3.5 2.2 3.7 ± ± ± ± ± ± ± ± 4.0 7.9 3.3 6.9 3.1 6.4 4.5 8.0

1

2.0 (1016) 3.6 (1230) 1.4 (323) 3.9 (979) 1.6 (534) 4 (1029) 2.3 (1137) 3.6 (884) ± ± ± ± ± ± ± ± 4.2 7.5 3.2 7.0 3.2 6.9 4.5 7.9 (1140) (962) (602) (743) (609) (1166) (1290) (936)

2

2.0 3.6 1.4 4.0 1.6 3.6 2.2 3.5 ± ± ± ± ± ± ± ± 4.0 6.9 3.0 6.6 3.1 5.9 4.4 7.4 (1516) (906) (768) (772) (810) (1137) (1326) (789)

3

1.8 3.8 1.4 3.5 1.7 3.6 2.1 3.6 ± ± ± ± ± ± ± ± 3.8 7.1 3.2 5.6 3.4 5.7 4.3 7.6

4

1.9 (1608) 3.6 (778) 1.6 (698) 3.1 (693) 1.7 (872) 3.7 (626) 2 (1052) 3.6 (591) ± ± ± ± ± ± ± ± 4.0 6.3 3.2 5.5 3.3 5.5 4.1 7.8 (578) (280) (487) (374) (773) (305) (638) (345)

5

1.9 3.6 1.3 3.3 1.8 2.6 1.8 3.7 ± ± ± ± ± ± ± ± 3.8 6.3 2.8 4.8 3.4 4.0 3.8 7.1 (301) (74) (532) (218) (589) (137) (277) (112)

6

1.7 2.8 1.5 2.9 1.5 3.2 1.9 3.4 ± ± ± ± ± ± ± ± 3.1 5.3 2.8 4.2 3.0 4.1 3.7 7.7 (86) (13) (385) (87) (206) (56) (109) (52)

7

1.7 2.3 1.2 2.9 1.8 3.0 2.1 4.1 ± ± ± ± ± ± ± ± 4.1 6.2 2.7 4.3 3.4 4.4 4.2 8.1 mm mm mm mm mm mm mm mm no internals

Sq/15/9

Sq/20/9

Equations 1 through 3 can also be used to calculate a number-area weighted mean bubble rise velocity (BRV). Bubble Size Distribution. The radial bubble size distribution of Sq/15/18, Sq/15/9, and Sq/20/9 as well as no internals at u0/umf = 2.3 can be found in Figure 6. To see the spread of the bubble sizes and the number of bubbles to calculate the mean bubble size plus its standard deviation at each radial position, a summary of all measurements is shown in Table 1. Apparently, at 0.243 m the bubble size is fairly uniformly distributed across the bed cross-section, while at 0.450 m probe height, bubbles are larger toward the center of the bed. In particular at 0.450 m probe height, the measured bubble size depends on the presence of tubes. This is indicated by the wriggly shape of the radial bubble size distribution. However, there is no clear trend whether there is an increase or a decrease of the bubble size close to the tubes. With Sq/15/18,

243 450 243 450 243 450 243 450

(3)

= = = = = = = =

and wi as the percental number of bubbles n wi = 8 i ∑ j = 1 nj

(2)

bed height

⎧ 2 ⎪ π((pi + 0.5) ) if i = 1 ⎪ A ⎪ ⎪ π (p + 0.5)2 − (p − 0.5)2 ) i ai = ⎨ ( i if 2 ≤ i ≤ 7 ⎪ A ⎪ ⎪ π((D/2)2 − (p − 0.5)2 ) i ⎪ if i = 8 ⎩ A

z z z z z z z z

with ai the annular area of the total cross-sectional area A represented by the eight radial probe positions pi

tube bank

(1)

Sq/15/18

8

dB̅ =

radial position [cm]

⎛ ⎞ wi ·ai ⎟ ⎜ d ∑⎜ 8 ⎟ B,i i = 1 ⎝ ∑ j = 1 wj · aj ⎠

Table 1. Mean Bubble Size (± Standard Deviation) and Number of Analyzed Bubbles (In Parentheses) Displayed in Figures 6 and 4, Respectively, for All Tube Banks Investigated and a (Relative) Gas Velocity of u0/umf = 2.3

(Sq/15/9) at u0/umf = 4.5 and 6.8. Compared to u0/umf = 2.3, at higher gas velocities, the number of analyzed bubbles almost doubles for the lower probe height, yet decreases by about 20% for the upper one. Also, the shape of the radial distribution of the number of analyzed bubbles changes between u0/umf = 2.3 and the two higher gas velocities. For higher gas velocities, bubbles can more frequently be found toward the center of the bed, also at the lower probe height. At the higher probe height of z = 0.45 m, the wriggly shape of the curve in Figure 4c can no longer be found for u0/umf = 4.5 (cf. Figure 5a). At u0/umf = 4.5, a more bell-shaped distribution of the number of analyzed bubbles is found. Moreover, the total number of analyzed bubbles between the lower and the upper probe height decreases by more than 50%. This is in stark contrast to the same tube bank at u0/umf = 2.3 (cf. Figure 4c), where no decrease in the total number of analyzed bubbles is seen. Thus, at higher gas velocities, bubble coalescence is not suppressed as efficiently as with low gas velocities. Area-Number Weighted Mean. In the previous section, it has been shown that the number of analyzed bubbles depends on the radial position of the optical probe. Moreover, since the annular area covered by the probe is a function of the radial probe position, a representative mean bubble size must take into account both the number of analyzed bubbles and the annular area covered by the probe. Therefore, an area-number weighted mean bubble size is calculated as

(925) (1385) (315) (990) (444) (1249) (1069) (898)

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Figure 7. Radial bubble size distribution for Sq/15/9 with (a) u0/umf = 4.5 and (b) u0/umf = 6.8. For further information on the exact interpretation of the figures, refer to the caption of Figure 4.

Figure 8. Radial bubble rise velocity distribution for (a) Sq/15/18, (b) Sq/20/9, (c) Sq/15/9, and (d) no internals at a relative gas velocity u0/umf = 2.3. Also included are the area-number weighted means calculated with eq 1. For further information on the exact interpretation of the figures, refer to the caption of Figure 4.

bubble growth will be discussed more thoroughly in the section on Tube Orientation. As a comparison, the radial bubble size distribution in the bed without internals can be found in Figure 6d. As opposed to the cases with internals, the case without internals also shows a uniform bubble size distribution at 0.450 m. This is noteworthy,

it seems that between the larger distance tubes, bubbles grow larger. On the other hand, with Sq/15/9 and Sq/20/9 rather the opposite trend is observed. It must be noted that for all tube banks, the relative orientation of the probe and the tubes with their U-interconnection at the bottom are different. This orientation of the tubes and the influence of the U on the 13820

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Figure 9. Radial bubble rise velocity (BRV) distribution for Sq/15/9 with (a) u0/umf = 4.5 and (b) u0/umf = 6.8. For further information on the exact interpretation of the figures, refer to the caption of Figure 4.

most intriguing difference between the radial distribution of the BRV and the bubble size is that at the wall, the BRV is almost equal for all tube banks, regardless of the probe height. Therefore, wall effects are strong enough to substantially decelerate the BRV independently of the tube bank configuration. In contrast, the area-number weighted mean BRV does again coincide well with the default mean BRV at p = 2 for all tube bank configuration and for both probe heights. Eventually, the radial BRV distribution is displayed for higher gas velocities (u0/umf 4.5 and 6.8) in Figure 9. Compared to u0/ umf = 2.3, the radial BRV distribution is particularly different for the lower probe height. As with the largest bubble sizes in Figure 7, the fastest BRV can be found in the center of the bed, while at u0/umf = 2.3 the radial BRV distribution is quite uniformly distributed across the cross-section. At the upper probe height, the distribution of the BRV does not show a clear increase toward the center of the bed compared to u0/umf = 2.3, in particular at u0/umf = 6.8. By tendency, the fastest BRV can be found between the tubes. This is in contradiction to Gallucci et al.16 who claim that the fastest bubble rise velocity can be found along the tube walls. Tube Orientation. The influence of the tube bank orientation of Sq/20/9 relative to the probe position is displayed at 0° and 90° in Figure 10 for u0/umf = 2.3. For a top view of Sq/20/9 and the shifted probe positions, refer to Figure 3b. With the default case (0°), the optical probe is always between two interconnected tubes and above a U. At 90°, the probe is only above a U for the outermost tubes, while the two more central tubes are in parallel to the probe. The relative difference between these two tube orientations is the most distinct compared to the other tube banks in Figure 3. Figure 10a and b show the number of analyzed bubbles for the default orientation (0°) and for the tube bank shifted by 90°, respectively. Apparently, the tube orientation has a significant influence on the number of analyzed bubbles. While Figure 10a features the typical gas flow pattern of Werther and Molerus,2 Figure 10b shows a rather formless radial distribution of the number of analyzed bubbles. The radial bubble size distribution in Figure 10c and d and the radial bubble rise velocity (BRV) distribution in Figure 10e and f are almost independent of the azimuthal tube bank shift. Moreover, their area-number weighted means are also nearly identical. Therefore, the findings from the previous sections on

since the number of analyzed bubbles in Figure 4 shows the typical flow pattern of Werther and Molerus2 with bubbles moving from the wall to the center of the bed with increasing bed height. For the bubble size this tendency does not apply. Without internals, larger bubbles are not predominantly found toward the center. Thus, for the particles and gas velocities tested, not the bubble size, but the bubble frequency is heterogeneous in space. Apparently, for the lower probe position (z = 0.243 m), the area-number weighted mean from eq 1 is very similar to the default mean at a probe position of p = D/3 = 2 cm used in the study of Rüdisüli et al.1 Therefore, in this specific case, no adaptation of the mean is needed for the lower probe position. Moreover, the bubble size at D/3 for the higher probe position (z = 0.450 m) also coincides fairly well with the area-number weighted mean in Figure 4. However, due to the wiggly shape of the radial bubble size distribution, this coincidence should not be taken as a generic rule. It is always advisible to use the area-number weighted mean as the representative bubble size. Figure 7 shows the radial bubble size distribution with the reference tube bank (= Sq/15/9) for higher gas velocities u0/ umf 4.5 and 6.8. As with the number of analyzed bubbles in Figure 5, a distinctively different radial bubble size distribution is found for higher gas velocities. While at 0.243 m probe height, the bubble size distribution is fairly homogeneous for u0/umf = 2.3, it is more concentrated toward the center of the bed for higher gas velocities. At 0.45 m, the radial bubble size distribution becomes more and more irregular at higher gas velocities. At u0/umf = 4.5, the largest bubble can be found close to the tubes, while at u0/umf = 6.8, the largest bubbles can rather be found between the tubes. Both possibilities are plausible and documented in literature.16 Again, for the lower probe height, the bubble size at D/3 (p = 2 cm) agrees fairly well with the area-number weighted mean. In turn, at 0.45 m, no representative mean bubble size can be found by just putting the optical probe at D/3. A representative mean can only be obtained by measuring the whole radial bubble size distribution and using eq 1. Bubble Rise Velocity Distribution. The radial distribution of the bubble rise velocity (BRV) including the areanumber weighted mean BRV is displayed in Figure 8. Since the bubble size and the bubble rise velocity are related,17 it is obvious that Figure 8 and Figure 6 display similar trends. The 13821

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Figure 10. Radial distribution of (a,b) the number of analyzed bubbles, (c,d) the local mean bubble size, and (e,f) the local mean bubble rise velocity (BRV) at two bed heights (0.243 and 0.45 m) in a fluidized bed with Sq/20/9 internals fluidized at u0/umf = 2.3. In (a), (c), and (e) the optical probe is inserted at an azimuthal angle of 0°, while in (b), (d), and (f) the azimuthal probe angle is 90° (cf. Figure 3b). For further information on the exact interpretation of the figures, refer to the caption of Figure 4.

the bubble size and the BRV are valid irrespective of the tube orientation. Since the Us at the bottom of the tubes can be regarded as horizontal tubes, their influence on the bubble growth may be clarified by the aid of literature findings: Glass and Harrison18 investigated the interaction of bubbles with horizontal tubes and found that below the tube a “cushion” of gas with high voidage and variable thickness is established. Above the tube, in the lee of the gas stream, a zone of only little particle movement and low voidage is witnessed (cf. Figure 11). This region of reduced particle motion at the upstream side of the horizontal

tube may even result in local and temporal defluidization which is in particular detrimental if the tubes are also used for heat exchange.7 Gogolek and Grace7 claim that bubble splitting occurs only if the bubble strikes the tube near its nose and if they are considerably larger than the tube diameter. Since, in this setup, the Us at the bottom are positioned 10 mm above the distributor plate, bubbles are still small and most likely swerve around the tube. Even if the bubble is effectively split and two daughter bubbles of roughly equal size are generated,19 at an optical probe height of z = 0.243 m above the distributor, 13822

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Figure 11. Flow field and bubble movement around a horizontal tube such as the U-shaped connector at the bottom of the vertical tube. Below the tube there is a high voidage gas cushion and above the tube there is a defluidized zone inaccessible to bubbles.18 However, if only one row of horizontal tubes is used, split bubbles will recombine above the tube again.

bubbles would have recombined again.7 This is also a reason that a single row of horizontal tubes, such as the Us at the bottom of the vertical tube banks, does not have a significant influence on the bubble size higher up in the bed. Solids Motion. Because bubble growth is immediately linked to solids circulation in a fluidized bed, all findings from this study on the bubble growth can also be used to indirectly describe the solids motion in the fluidized bed. It is shown that bubbles within vertical tube banks rise predominantly in a pattern similar to that postulated by Werther and Molerus.2 That is, in the lower portions of the bed, they are more concentrated toward the wall regions of the bed, while higher up in the bed they gradually move toward the center axis of the bed (cf. Figure 12). This does not mean that in the lower portions the center axis of the bed is completely devoid of bubbles and that close to the eruption zone no bubbles can be found near the walls anymore, but there is a inhomogeneous distribution of the bubbles also in fluidized beds with vertical tubes. With respect to the solids circulation, the upward movement of solids is in the wake and drift of bubbles,6 so the same motion patterns of rising bubbles apply for rising solids. On the other hand, due to the conservation of mass, solids descent where bubbles are less active. That is, along the walls in the upper portions and toward the center axis in the lower portions of the bed. Again this is not a strict rule, but rather a tendency. It must be noted that this solids motion pattern is specifically valid for this test rig with Geldart B particles and rather low gas velocities. To obtain a more general and more comprehensive understanding of the gas−solids interactions and motion in a fluidized bed with vertical internals, additional experiments with other particles and at other gas velocities are needed. Moreover, by means of tracer studies (e.g., PEPT), the exact pathways of selected particles could be made visible.

Figure 12. Flow pattern of bubbles and solids in a fluidized bed with vertical tubes and Geldart B particles fluidized at (relative) gas velocities u0/umf between 2.3 and 6.8. Broad light blue arrows show the preferential downward pathways of solids, while thin black arrows show the upward lift of solids in the wake of bubbles.

particles are Geldart B and investigated gas velocities range from u0/umf 2.3 to 6.8. It is shown that in particular the bubble frequency, i.e., the number of bubbles analyzed per time unit, is dependent on the radial position and the orientation of tubes in the bed. Generally, bubbles rise more frequently between the vertical tubes instead of around them. In the upper portions of the bed, most bubbles are found toward the center axis of the bed, while lower in the bed, more bubbles are found toward the walls of the bed. This flow pattern has already been postulated in literature by Werther and Molerus2 and Lim et al.5 for beds without internals. Thus, the bubble and consequently also the solids motion patterns in fluidized beds with vertical tubes are qualitatively similar to those in beds without internals. Although the general flow pattern of bubbles between the different vertical tube bank configurations is similar, there are still significant differences in the exact pathways of rising bubbles depending on the tube-to-tube spacing and the tube diameter. A direct relation of preferential bubble pathways on the tube bank configuration, however, is not clear. Without internals, there is a significant reduction of the total number of analyzed bubbles between the lower and the upper measurement position, which can be attributed to bubble coalescence. Contrarily, with vertical tube banks at low gas velocities, the number of bubbles remains almost constant at both probe heights. Thus, bubble coalescence in beds with vertical tubes is either inhibited or compensated by more frequent bubble splitting. For higher gas velocities (u0/umf 4.5 and 6.8), however, a considerable reduction of the total number of analyzed bubbles can also be found with internals.

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CONCLUSION The radial distribution of the bubble frequency, the bubble size, and the bubble rise velocity has been investigated in an ambient pressure bubbling fluidized bed with different vertical tube bank configurations by means of optical probing (OP). Used 13823

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Opposed to the bubble frequency, the bubble size and the bubble rise velocity (BRV) show less variation in their radial distribution, respectively. In particular for the bubble size in the lower portions of the bed, a highly uniform radial distribution is found at u0/umf = 2.3. Consequently, although bubbles are more frequently found at certain radial positions of the bed, their size remains almost unaffected. On the other hand, higher up in the bed, large bubbles are more likely found toward the center axis of the bed. For the BRV similar trends can be measured, however, the tendency to have a higher BRV in the center axis of the bed is already found in lower portions of the bed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge “Verband Schweizerische Gasindustrie” (VSG/ASIG) for their funding of this project. NOMENCLATURE A = Cross sectional area [m2] a = Annular area [%] D = Column diameter [m] H = Column height [m] i = Incremental number [−] j = Incremental number [−] n = Number of bubbles analyzed [−] P = Wetted perimeter [m] p = Probe position [m] w = Number of bubbles analyzed [%] z = Bed/probe height [m] dB = Bubble size/diameter [m] umf = Minimum fluidization velocity [ms−1] u0 = Superficial gas velocity [ms−1]

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SUBSCRIPTS AND SUPERSCRIPTS hyd = Hydraulic tube = Vertical tube

ABBREVIATIONS BFB = Bubbling fluidized bed BRV = Bubble rise velocity NWA = Puralox NWa-155 particles OP = Optical probing PEPT = Positron emission particle tracking PFM = Pressure fluctuation measurement

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REFERENCES

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dx.doi.org/10.1021/ie3004418 | Ind. Eng. Chem. Res. 2012, 51, 13815−13824