Gas Back-Mixing in a Two-Dimensional Baffled Turbulent Fluidized

Oct 4, 2008 - The effect of louver baffles on gas back-mixing was investigated in a large-scale two-dimensional fluidized bed of fluid catalytic crack...
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Ind. Eng. Chem. Res. 2008, 47, 8484–8491

Gas Back-Mixing in a Two-Dimensional Baffled Turbulent Fluidized Bed Yongmin Zhang,† Chunxi Lu,*,† John R. Grace,‡ Xiaotao Bi,‡ and Mingxian Shi† State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Beijing, 102249, People’s Republic of China, and Fluidization Research Centre, Department of Chemical and Biological Engineering, UniVersity of British Columbia, 2360 East Mall, VancouVer, Canada V6T 1Z3

The effect of louver baffles on gas back-mixing was investigated in a large-scale two-dimensional fluidized bed of fluid catalytic cracking (FCC) particles with helium as tracer gas. The axial gas dispersion coefficient of the baffle-free fluidized bed first increased with increasing superficial gas velocity and then decreased after reaching a maximum near the onset of the turbulent flow regime. “Gulf streaming” of emulsion flow in the baffle-free bed determined the lateral profiles of tracer gas concentration. Solids back-mixing flux was greatly reduced by the addition of a layer of louver baffles, while solids mixing above the baffle layer was enhanced. The modified baffled fluidized bed with multilayer louver baffles not only provided a high efficiency of gas-solids contacting but also greatly suppressed the back-mixing of both gas and solids. 1. Introduction

2. Experimental Setup

Gas mixing, an important concern in the design of gas-solids fluidized-bed reactors, is generally affected by many factors, such as molecular diffusion, convection, turbulent diffusion, and solids mixing. Among these, solids mixing is often considered to be the controlling factor, especially in fluidized beds of group A particles, where the clustering of the particles and small emulsion gas velocities facilitate the entrainment of gas or even small bubbles by descending particles, resulting in significant gas mixing. Accordingly, studies of gas mixing can also help in understanding the flow pattern of the solids phase. Rapid motion of solids in fluidized beds has two main effects: (1) efficient gas-solids contacting and (2) elevated back-mixing of both gas and solids. Although the former is welcome in most applications, the latter is not always desirable, especially in fluidized-bed reactors where narrow residence time distributions (RTD) of gas and solids are favorable, for example in the catalyst strippers of FCC units and in other catalytic fluidizedbed reactors. In such cases, internal baffles can be used to suppress the back-mixing of both gas and solids. Baffles in fluidized beds can also break up bubbles, thereby further strengthening gas-solids contacting, augmenting reaction conversions, and improving product selectivity. Although there have already been many studies on gas mixing in fluidized beds,1-4 most were carried out in the bubbling flow regime. Few gas mixing studies 5-8 have been reported in the turbulent flow regime despite its predominance in industrial practice.9 Moreover, there are few data3 available on the effects of baffles on gas mixing in fluidized beds. Louver baffles are quite common in fluidized-bed reactors. While there have been several studies on louver-baffled fluidized beds,10-12 most were conducted to investigate the effects of louver baffles on hydrodynamics, with little previous work on gas mixing. The aim of this study was to investigate the effects of louver baffles on gas back-mixing and flow patterns of solids over a broad range of operating conditions, covering both the bubbling and turbulent flow regimes.

A schematic of the experimental setup is shown in Figure 1. The main component was a plexiglass column of horizontal cross-section of 500 mm × 30 mm and height 6 m. This “twodimensional design” gave a direct view of the internal flow of the gas and solids in the bed. The gas distributor was a perforated plate with eleven 5 mm diameter holes and 1.44% open area. Two-stage cyclones captured particles entrained in the discharge gas flow and returned them to the dense bed to maintain a constant particle inventory in the column. Air was introduced into the bed by a Roots blower. Equilibrium FCC particles with a mean diameter of 78 µm and a density of 1500 kg/m3 constituted the solids medium. The static bed height was 1.28 m in all of the experiments. According to the correlation of Cai,13 the onset velocity of turbulent flow regime in this column was 0.56 m/s. The superficial gas velocity ranged from 0.2 to about 1.0 m/s, covering both the bubbling and turbulent flow regimes. A steady-state tracer technique was employed to investigate the gas back-mixing. Compared to the stimulus-response tracer technique, the data from a steady-state tracer technique are more credible and repeatable. In this study, tracer gas was continually injected downward through a point injector at the centerline of

* To whom correspondence should be addressed. Tel.: +86-1089733803. Fax: +86-10- 89733803. E-mail: [email protected]. † China University of Petroleum. ‡ University of British Columbia.

Figure 1. Schematic of experimental setup.

10.1021/ie800906n CCC: $40.75  2008 American Chemical Society Published on Web 10/04/2008

Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8485

Figure 2. Steady-state gas tracer experimental setup and sampling tap locations.

the two-dimensional column. Previous work by Gilliland and Mason14 has indicated that this method gives lateral tracer gas concentration profiles very similar to those for multipoint injector in the upstream region. The 4 mm i.d. injector was located in the upper part of the bed, whereas gas was sampled upstream, i.e., below the tracer injector. Helium was the nonadsorbing tracer gas, and a chromatograph with a TCD detector determined the helium concentrations in the samples. The flow rate of helium was maintained at 1% of the flow rate of fluidizing air for all superficial gas velocities, resulting in helium injecting velocities of 2-13 m/s. According to the correlation of Merry15 for the jet length of a horizontal injector

[

Lhor Fg,oruor2 ) 5.25 dor Fp(1 - εmf)gdp

]()( ) 0.4

Fg Fp

0.2

dp dor

0.2

(1)

and Karri’s estimation16 on the jet lengths of different types of injectors Lup ≈ 2Lhor ≈ 3Ldown

(2)

the downward helium jet penetration length in this study was estimated to be only 2-7 mm, short enough to have negligible effect on the upstream lateral profiles of tracer gas concentration. In experiments, the measured tracer gas concentration at the nearest upstream sampling tap was always a minimum at that row, further demonstrating the above-mentioned viewpoint. Because of the high sensitivity of TCD detectors to helium, the 1% flow rate of helium was high enough to provide accurate measured tracer gas concentrations while being small enough to avoid interference with the bed hydrodynamics. The sampling tubes were 1 mm i.d. plastic tubes. Wire gauze was inserted into the tips of the sampling tubes to prevent blockage by particles. In the experiment, sampled gas flowed into 2 L sampling bags due to the positive pressure in the bed. The sampling interval for every run was 15-20 min., depending on operating conditions. Figure 2 shows the setup for the steady-state tracer experiments in the two-dimensional column. The helium injector was mounted at the centerline of the column, 1.1 m above the gas distributor. Twenty sampling taps in four rows were mounted below the helium injector, labeled A1-A5, B1-B5, C1-C5, and D1-D5, as indicated in Figure 2. In addition to these sampling taps, another sampling tap, labeled F0 (not marked), was mounted at the gas outlet of the experimental unit to measure the tracer gas concentration at the exit of the freeboard.

Figure 3. Two baffled fluidized bed in this study.

Figure 4. Structure of louver baffle.

In addition to the baffle-free fluidized bed abbreviated FFB shown in Figure 2, two types of baffled fluidized bed were investigated in this study, as shown in Figure 3: a fluidized bed containing one layer of louver baffles (labeled BFB-1) and the other with three layers of louver baffles (labeled BFB-3). Positions of the 21 sampling taps were the same for all three bed configurations. For BFB-1, the single layer of louvers was mounted between rows A and B of the sampling taps, as shown in Figure 3a. For BFB-3, the configuration of the three layers of louvers can be seen in Figure 3b. Two different types of louvers, baffles I and II, were studied. Figure 4 provides a section view of part of the louver baffles. Three structural parameters, hb, pitch of inclined vanes, dv, and inclination angle of vanes, θv, are most important in the design of louver baffles. Here, the two kinds of louver baffles differed only in vane distances, baffle I having dv ) 40 mm, whereas dv ) 60 mm for baffle II. The other two parameters, a baffle height of 70 mm and a vane inclination angle of 55°, were the same in both cases. BFB-3, a modified louver baffle configuration, contained three layers of baffle II louver baffles, arranged as shown in Figure 3b. The space between each adjacent pair layers of louver baffles was 70 mm, equal to the height of the louvers. The vane inclination directions of each adjacent pair layers alternated, making the gas and solids zigzag, providing interlaced contacting in the space between adjacent layers of baffles. This was shown to be helpful in strengthening gas-solids contacting in a previous paper,17 where pressure fluctuations in BFB-3 were found to be much lower than in FFB, indicating smaller average bubble diameters and more efficient gas-solids contacting. 3. Results and Discussion 3.1. Analysis of Gas Dispersion. When tracer gas enters the upstream region of a fluidized bed from a downward-flowing tracer gas injector, there are several steps. Step 1: Most tracer

8486 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008

Figure 5. Profiles of tracer gas concentration of baffle-free fluidized bed at different superficial gas velocities.

Figure 6. ln(C/CF) vs (z - hj) for baffle-free fluidized bed.

gas enters as bubbles. Step 2: Tracer gas enters the emulsion phase in the downstream region by interphase mass transfer as bubbles rise to the bed surface. Step 3: Tracer gas in the

Figure 7. Axial gas dispersion coefficient and Peclet number for bafflefree fluidized bed.

emulsion phase is dragged downward to the upstream region due to solids descending to replace those carried upward by

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Figure 8. Effects of single-layer louver baffles on helium concentration profiles.

Figure 11. Back-mixing suppression index as a function of superficial gas velocity for two baffle geometries.

Figure 9. Illustration of the internal circulation flow pattern of emulsion above the louver baffle.

Figure 10. Comparison of baffle stripping efficiencies of baffle-free (FFB) and single-layer baffled (BFB-1) fluidized beds.

wakes. Step 4: In the upstream region, tracer gas is transferred back into bubbles from the emulsion and then rises again and may leave the bed. In other words, tracer gas in the upstream emulsion is continuously stripped by the rising bubbles. As a result, the upstream tracer gas concentration decreases as the distance from the tracer gas injector increases.

Two main factors, emulsion downward flux and interphase mass-transfer efficiency, determine the tracer gas concentration in the upstream samples. The larger the solids back-mixing flux, the greater the amount of tracer gas entrained downward and the higher the tracer gas concentration in the upstream samples. On the other hand, the more efficient the interphase mass transfer, the more easily the downward entrained gas in the emulsion is stripped into bubbles and the lower the tracer gas concentrations in the upstream samples. The lateral tracer gas concentration profile for specific operating conditions depends on the solids flow pattern, i.e., the distribution of downward solids back-mixing flux. Where there is a high solids backmixing flux, there will be a relatively high upstream tracer gas concentration. Hence, any internal solids circulation pattern, e.g., “gulf streaming”, will have a strong effect on the lateral tracer gas concentration profiles. 3.2. Gas Back-Mixing in FFB. Figure 5 shows the profiles of tracer gas concentration of FFB at different superficial gas velocities. Here, CF is the tracer gas concentration sampled at sampling tap F0 at the outlet of the column, representing the average tracer gas concentration in the freeboard. All five figure panels have the same ordinate scale to facilitate comparison of tracer gas concentrations. As seen in Figure 5, a significant amount of tracer gas penetrated upstream, indicating strong gas back-mixing in the FFB. Strong gas back-mixing broadens the gas residence time distribution, which is likely to be unfavorable for fluidized bed reactors, e.g., reducing both conversion and selectivity to intermediate products. At all superficial gas velocities tested, the upstream tracer concentrations in the core region were lower than in the wall region, especially at low superficial gas velocities. This is because bubbles tend to rise in the core region carrying solids

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Figure 12. Profiles of tracer gas concentration of BFB-3 at different superficial gas velocities.

Figure 13. Comparison of average tracer gas concentrations for baffle-free (FFB) and three-layer baffled (BFB-3) fluidized beds at row C.

Figure 14. Comparison of axial gas dispersion coefficients for baffle-free (FFB) and three-layer baffled (BFB-3) fluidized beds.

in their wakes and due to drift, while solids in the wall region descend to replace the upward-moving solids. In the core region, the bubble fraction is large and downward solids flux is small,

resulting in less entrained tracer gas and lower tracer gas concentrations. In the wall region, the bubble fraction is low and most solids descend, resulting in higher tracer gas concen-

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trations. This solids circulation pattern, often called gulf streaming, is common in fluidized beds of height/diameter ratio greater than 1. The tracer gas concentration profiles are consistent with this type of circulation pattern. As the superficial gas velocity increased, two changes in tracer gas concentration profiles of FFB can be observed in Figure 5. First, the upstream tracer gas concentration decreased with increasing superficial gas velocity due to the increasing bubble fraction in the bed, resulting in increased interphase masstransfer rates and enhanced stripping.18-20 Second, the lateral tracer gas concentration gradient decreased as u0 increased, particularly when the bed reached the turbulent flow regime, due to the more random, smaller scale, rapidly forming and disappearing voids in the turbulent flow regime. The simple one-dimensional axial dispersion model is commonly employed as a simple one-parameter means of comparing the magnitude of vertical mixing in fluidized beds. This method is used here as a simple means of comparing the baffled and baffle-free cases, even though the authors recognize the limitation of the axial dispersion model in view of the two-phase nature and gulf-streaming macromixing inherent in the gas fluidized beds studied. The steady form of the one-dimensional axial dispersion model is u0 ∂C ∂2C ) Da,g 2 ε ∂z ∂z

(3)

with the boundary conditions z ) hj, C ) C0; z ) -∞, C ) 0 where C0 is the injection volume concentration of tracer gas; i.e., C0 )

QHe QHe + u0 At

(4)

The analytical solution of eq 3, with u0 and ε as constants, is

( )

ln

u0 1 C ) (z - hj) C0 ε Da,g

(5)

where C is estimated from the average of the five lateral tracer concentrations at each row of sampling taps, and ε is the averaged bed voidage obtained experimentally, as referred to in Zhang et al.17 The axial gas dispersion coefficient is derived from the slope of ln(C/C0) vs (z - hj). In view of the difficulty in measuring the injection concentration, C0, the helium concentration measured at the freeboard exit, CF, was used here. Despite that CF may not equal C0, it introduces no error in calculating Da,g. Figure 6 shows the linear fitting of ln(C/CF) vs (z - hj) to derive the slopes at different superficial gas velocities in FFB. The fitted axial gas dispersion coefficients and Peclet numbers are plotted in Figure 7. Da,g for the FFB first increased with increasing superficial gas velocity and then began to decrease beyond a superficial gas velocity of ∼0.55 m/s, corresponding reasonably well with the onset of the turbulent flow regime. The Peclet number increased monotonously with increasing superficial gas velocity, in agreement with a number of previous studies.5-8 3.3. Effect of Louver Baffles on Gas Back-Mixing. The one-layer louver baffle configuration, i.e., BFB-1 shown in Figure 3a, was employed to investigate the effect of louver baffles on gas back-mixing, with the bottom surface of the baffle layer 0.8 m above the gas distributor, located between the row A and row B sampling taps. Figure 8 compares the lateral tracer gas concentration profiles above and below the baffle layer with

those for FFB at two representative operating conditions: one in the bubbling flow regime and the other in the turbulent flow regime. It can be seen that the upstream tracer gas concentrations were greatly reduced after inserting a layer of louver baffles, due to a reduction of solids back-mixing flux across the baffle, demonstrating its effective suppression of back-mixing of both gas and solids. At relatively low superficial gas velocities, the suppression of solids back-mixing by baffles is likely due to the removal of the solids material in the wakes of bubbles by baffles when bubbles pass through baffles, as reported by van Dijk et al.21 on the basis of an X-ray technique and sieve baffles. With further increases in u0, a layer of gas may appear below baffles, further suppressing solids back-mixing.17 The tracer gas concentrations of row A in BFB-1 were larger than for FFB, demonstrating that louver baffles can also strengthen the mixing of gas and solids above the baffle. This enhanced gas and solids mixing may also be related to the solids circulation behavior observed in experiments. When u0 reached ∼0.4 m/s, an internal solids circulation could be clearly observed above the louver baffles. As shown in Figure 9, due to the inclination of the baffle vanes, bubbles were directed to one side as they passed through the louver baffle, resulting in a region of higher voidage on that side and a region of lower voidage on the other side. The difference in voidage between the two regions caused solids circulation above the louver baffle, with solids descending in the low-voidage region and rising in the high-voidage region. These two regions are labeled “bubbleladen” and “emulsion-downflow” regions, respectively. There was a transition region between these two regions where solids rose or descended randomly. The bubble-laden region occupied two-thirds to three-fourths of the column width. As u0 increased, the area occupied by the emulsion-downflow region decreased, while the particle downward velocities increased, often resulting in bridging or defluidization in the corner above the louver baffle. This region of internal solids circulation was usually 0.3-0.4 m in height, beyond which the bed recovered to a more normal state. As shown in Figure 3a, most row A sampling taps were located in the emulsiondownflow region, accounting for the high tracer gas concentrations of row A in BFB-1. On the basis of the average tracer gas concentrations of rows A and B, a stripping efficiency is defined as

(

ηstr ) 1 -

)

Cav,B × 100% Cav, A

(6)

to evaluate the stripping effect of the baffles. For strippers, such as those in FCC units, stripping efficiency depends on axial solids mixing and on interphase contacting. To achieve high stripping efficiency, reduced axial solids back-mixing and high interphase mass-transfer efficiency are preferred. It can be seen in Figure 10 that the beds with louver baffles had much higher stripping efficiencies than FFB, no doubt due both to the suppression of solids back-mixing and enhancement of gas-solids contacting. As superficial gas velocity increased, the stripping efficiencies increased slightly for all three fluidized beds. Louver baffles with smaller vane pitch tended to have higher stripping efficiencies. Assuming the downward drag on gas by solids is the cause of gas back-mixing and that the tracer gas concentration is proportional to the solids back-mixing flux, we introduce a backmixing suppression index, defined as IBMS ) 1 -

Cav,B,BFB-1 Cav,B,FFB ⁄ Cav,A,BFB-1 Cav,A,FFB

(7)

8490 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008

to estimate the reduction of solids back-mixing flux due to the louver baffles. As shown in Figure 11, IBMS mostly increased with increasing superficial gas velocity but appeared to reach a maximum within the turbulent flow regime. IBMS for baffle I was 0.55-0.75, whereas it was 0.45-0.65 for baffle II, demonstrating a very large reduction in solids back-mixing for both configurations. Louver baffles with smaller vane pitches were more effective in suppressing solids back-mixing. 3.4. Gas Back-Mixing in BFB-3. Figure 12 shows tracer gas concentration profiles for BFB-3 at different superficial gas velocities. Compared with Figure 5, the tracer gas concentrations of BFB-3 at each row were much lower than for FFB. In addition, as the distance from the helium injector increased, helium concentrations decreased much faster in BFB-3 than in FFB. The lateral helium concentration profiles in BFB-3 were similar to those in FFB, with the strongest gas back-mixing near the wall. However, the lateral tracer gas concentration gradients were smaller than in FFB. In Figure 13, the average tracer gas concentrations of row C in FFB and BFB-3 are compared, revealing the global effect of BFB-3 on gas back-mixing. Another back-mixing suppression index, IBMS, BFB-3 ) 1 - Cav,C,BFB-3 ⁄ Cav,C,FFB

(8)

is used here to estimate the extent of solids back-mixing flux suppressed by BFB-3. As shown in Figure 14, its value was within the range of 0.75-0.95, indicating that back-mixing of both gas and solids was greatly weakened by the three layers of louver baffles. Axial gas dispersion coefficients for FFB and BFB-3 are compared in Figure 14. It can be seen that the axial gas dispersion coefficients for BFB-3 were far lower than for FFB, again indicating strong suppression of gas back-mixing by the louver baffles. In BFB-3, the flow patterns of both gas and solids were much closer to a plug flow. Generally, the back-mixing of both gas and solids is weaker in small-scale fluidized beds, strengthening with increasing bed scale.22 For larger commercial fluidized vessels, axial gas dispersion coefficients can be 1-2 m2/s, 22 several times larger than in the baffle-free column of this study. On the other hand, baffles, e.g., vertical tubes and plates, are found capable of minimizing scale-up effect in fluidized beds.23 Therefore, louver baffles of similar geometry are expected to have relatively small scale-up effect when employed in large fluidized beds, i.e., exerting stronger suppression on gas/solids back-mixing. 4. Conclusions Gas back-mixing was studied in a two-dimensional fluidizedbed column operating in both bubbling and turbulent flow regimes by steady-state gas tracer technique. Experiments in the baffle-free fluidized bed showed that the gulf streaming of solids determined the lateral profiles of tracer gas concentration. The axial gas dispersion coefficient of the baffle-free fluidized bed first increased with increasing superficial gas velocity and then began to decrease after reaching a maximum at a superficial gas velocity of ∼0.55 m/s, near the onset of the turbulent flow regime. Experiments with louver baffles showed that both gas and solids back-mixing could be greatly suppressed by louver baffles, whereas the mixing of gas and solids above the baffles could be strengthened due to the promotion of internal emulsion circulation. The louver baffle with smaller vane distance was superior in suppressing gas and solids back-mixing. The modified baffled fluidized bed with multilayer louver baffles,

labeled BFB-3, not only provided a high efficiency of gas-solids contacting but also greatly suppressed the back-mixing of both gas and solids. Acknowledgment The authors are grateful to the National Natural Science Foundation of China and to the Chinese Scholarship Council for their financial assistance of this study. Nomenclature At ) area of column cross-section (m2) C ) tracer gas concentration (%) C0 ) initial tracer gas concentration at the injector (%) CF ) tracer concentration in freeboard (%) Cav, A, Cav, B, and Cav, C ) average tracer gas concentrations at rows A-C (%) dv ) vane separation distance (see Figure 4) (m) dor ) diameter of distributor orifices (m) dp ) mean particle diameter (m) Da,g ) axial gas dispersion coefficient (m2/s) g ) gravitational acceleration (m/s2) hb ) baffle height (see Figure 4) (m) hj ) helium injector height above the distributor (m) Hb ) height of the bottom of the louver baffles above the distributor (m) H0 ) static bed height (m) IBMS ) back-mixing suppression index IBMS,BFB-3 ) back-mixing suppression index for BFB-3 Ldown, Lhor, Lup ) jet lengths of downward, horizontal, and upward injectors (m) QHe ) flow rate of tracer helium (m3/s) u0 ) superficial gas velocity (m/s) uor ) injecting velocity (m/s) uc ) onset gas velocity of turbulent flow regime (m/s) z ) height (m) zL ) lateral distance from the column centerline (m) Greek Letters ∆hj ) vertical distance of sampling tap below the tracer gas injector (m) ∆hA,D ) vertical distance between row A and row D sampling taps (m) ε ) voidage εmf ) voidage at minimum fluidization θv ) inclination angle of vane (see Figure 4) (deg) Fg ) gas density at bed operating conditions (kg/m3) Fg,or ) gas density of gas entering the grid orifices (kg/m3) Fp ) particle density (kg/m3) ηstr ) baffle stripping efficiency (%)

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ReceiVed for reView June 9, 2008 ReVised manuscript receiVed August 26, 2008 Accepted August 28, 2008 IE800906N