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Ind. Eng. Chem. Res. 2004, 43, 5796-5801
Residence Time Behavior of Gas in a Downer Reactor H. Brust and K.-E. Wirth* Institute of Particle Technology, University of Erlangen-Nuremberg, Cauerstrasse 4, D-91058 Erlangen, Germany
A downer reactor is being expected to provide narrow residence time behavior, because of the concurrent movement of gas and solids in the direction of gravity. In the literature, many studies have shown that the gas/solids distributor on the top of the downer greatly influences the flow conditions. However, the influence of the distribution process on the residence time behavior is not known. Therefore, the influence of mixing behavior of gas/solids flows in the entrance section of a downer reactor on the residence time distribution of the gas phase has been studied experimentally. The investigations show that the solids feeding process and geometry of the entrance region crucially influence the gas residence time distribution and can lead to a broad residence time distribution of the gas. However, for certain geometries and different operating conditions, the entire downer plant can be regarded as similar to a plug-flow system. 1. Introduction The application of heterogeneously catalyzed gasphase reactions in circulating fluidized beds is thus far of subordinate importance. Numerous heterogeneously catalyzed reactions with rapid, reversible catalyst deactivation are suitable in principle for implementation in reactor-regenerator systems, whereby advantages are expected compared with implementation in traditional reactors such as fixed beds or fluidized beds.1,2 With many production procedures in chemical industry, the yield of the desired products could be increased if the reaction were purposefully affected by the flow conditions in the reactor. The concurrent upflow circulating fluidized bed (riser) has been established as a state-of-the-art reactor for industrial applications for certain gas/solids reactions (e.g., combustion). Compared to other gas/solids reactors, the riser offers higher gas/ solids contact efficiency and gas/solids throughput.3 However, the core-annulus flow structure of gas and solids in the riser reactor results in axial back-mixing of the solids and a broad residence time distribution. A more uniform flow structure with a reduction of axial back-mixing can be obtained in a concurrent downflow circulating fluidized-bed (downer) reactor. Most heterogeneously catalyzed reactions are accompanied by consecutive and side reactions. Therefore, a narrow residence time distribution of gas and solids is indispensable for the optimization of these reactions, because a wide residence time distribution leads to a decrease in selectivity of the desired product. For that reason, there are two main objectives in designing a gas/solid distributor: (1) rapid radial mixing of gas und solids and (2) reduction of axial back-mixing (plug flow). Recent studies on downer reactors have shown that conditions close to plug flow can be achieved in the fully developed section.4,5 It has also been demonstrated that the design of the gas/solids distributor at the top of the reactor influences the flow pattern significantly.5,6 However, the influence of the gas/solids distribution on the overall behavior of a downer plant is almost unknown. * To whom correspondence should be addressed. Tel.: +499131-8529403. Fax: +49-9131-8529402. E-mail: k.e.wirth@ lfg.uni-erlangen.de.
2. Experimental Section 2.1. Downer Setup. A sketch of the downer plant is shown in Figure 1. The downer has a height of 8.6 m with an inner diameter of 150 mm and is made of stainless steel tubes. The solids stored in a hopper at the top of the plant are transported with a screw feeder to the gas/solids distributor, where the solids are mixed with air supplied from outside. Then, the gas and solids are fed to the downer (inner diameter of 150 mm) by the distributor. At the bottom of the downer, the gas and solids are separated with a two-stage separator consisting of an inertial-type separator and a cyclone. From the separator through a weighing device and standpipe, the solids are fed to the riser, which has an inner diameter of 200 mm and a height of 14.5 m, where they are entrained vertically upward. At the top of the plant, there is another cyclone to separate solids from gas. Gas leaves the plant, and solid material enters the storage hopper to be fed again to the downer. The downer can be operated at superficial gas velocities, UG, of up to 7 m/s with solids mass flow rates GS between 25 and 120 kg/(m2 s). The solids circulating rate can be adjusted with the screw feeder. A fluid catalytic cracking (FCC) catalyst with a mean diameter of 85 µm has been used for the investigations. A short description of the distributor is provided to permit a better understanding of the results described below. The detailed design of the distributor has been presented by Wirth and Schiewe.7 Gas is sucked through a tube, the primary air tube, which is located in the center of the distributor. The solids, which are fluidized in the distributor, are fed through a small annular gap that is concentric to the primary tube. Additional air can be added to this gap to accelerate the solids. Without additional air, there is only fluidization air in the gap. The operating conditions of the distributor can be characterized by the gas velocity ratio, vs/vp.5 The velocity vs characterizes the velocity of gas in the annular gap, whereas vp represents the gas velocity in the primary air tube. Different velocity ratios result in different solids distribution patterns. A minimum value of vs/vp corresponds to no additional air fed to the gap.
10.1021/ie049725s CCC: $27.50 © 2004 American Chemical Society Published on Web 07/22/2004
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Figure 2. Two-point measuring technique.
To obtain the residence time distribution E, a deconvolution operation can be performed. A fast Fourier transformation (FFT) is needed for this operation FFT
X(t), Y(t) 98 X(s), Y(s)
Figure 1. Setup of the downer.
The ratio vs/vp is a characteristic parameter for describing the gas/solid mixing behavior in the entrance section. 2.2. Residence Time Distribution. The overall residence time distribution (RTD) of gas was investigated with a tracer technique. Argon was used as the tracer, and a mass spectrometer was used as the detector. The measurements were carried out in the downer at two axial positions separated by a distance of 8 m. Sampling position 1 was located at the outlet of the gas/solids distributor (Figure 2). The tracer injection can be realized as Dirac delta function or step input. The relation between tracer signal X (sampling point 1) and tracer signal Y (sampling point 2) can be expressed by the mathematical operation of convolution
Y ) EX
(1)
FFT -1 Y(s) X(s) ) E(s) 98 E(t) (2)
Deconvolution is a numerically unstable process, and oscillations can appear. Because of these oscillations, the mathematical solution produces negative values, which are not useful physically. In Figure 3, the setup of the tracer experiment in the downer is illustrated. Figure 3 shows some example signals at the two sampling points. This kind of investigation was also carried in the riser section of the plant to enable a general comparison of the downer and riser. At the right, there are RTDs, which are stable results of deconvolution calculations and are obtained in the riser and downer sections of the experimental setup. The measurements were carried out at almost the same operating conditions. The main differences can be seen clearly: a sharp peak in the case of the downer due to minor back-mixing and uniform flow behavior. In the case of the riser, there is an early peak due to fast flow in the core and a broad tailing of the distribution due to back-mixing of the gas and solids at the wall. This results in a wide residence time distribution in the riser section of the plant. The results presented correctly reflect the known differences in flow conditions. Thus, the possibility exists of regarding the total behavior to be able to meet general statements with this investigation method. 2.3. Axial Dispersion Model. To characterize the residence time distributions in the downer, the onedimensional axial dispersion model was used. It can be described by the equation
∂2C ∂C ∂C ) Dax 2 - u ∂t ∂z ∂z
(3)
In the case of minimal back-mixing, eq 2 can be approximated in dimensionless form8 as
E(Θ) )
[
]
(1 - Θ)2Bo Bo exp π 4
x
1 2
(4)
with Bodenstein number Bo ) uL/Dax and dimensionless time Θ ) t/τ. Because of the minimal back-mixing in a downer system, one is able to describe the residence time behavior therein with the above one-dimensional dispersion model. A good agreement of experimental and model-based data is shown in Figure 4. This applied
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Figure 3. (Left) Setup of tracer experiment in the downer. (Right) Comparison of RTDs: downer vs riser [UG ) 3.5 m/s, GS ) 25 kg/(m2 s)].
Figure 4. Comparison: experimental data vs model data.
model is a suitable model for characterizing the RTDs of downer reactors. Taking into account the two known sections of different flow conditions in the downer (entrance section, fully developed flow section), the calculated axial dispersion coefficient is denoted as Dax,eff, to indicate the overall character of this parameter, because it is a superposition of the two local residence time distributions. 3. Results and Discussion Previous investigations of the flow conditions in downer reactors have shown that there is a strong influence of the gas/solids distributor on the flow pattern in the entrance region. This section has a certain length that is mainly determined by the operating mode of the gas/solids distributor and the velocity ratio vs/vp. Therefore, the distributor should have an influence on the residence time distribution. In Figure 5, the dependence of the residence time behavior on the velocity ratio and the superficial gas velocity is represented. The RTD of the entire plant clearly depends on the mode of operation of the plant (UG) and the operating conditions of the distributor (vs/vp). Considering the dependence of axial back-mixing on the superficial gas velocity, it was found that the Bodenstein number increases with increasing UG (Figure 5). The largest values for Bo are obtained at a minimum velocity ratio vs/vp. A minimum velocity ratio means that the fluidized bed in the gas solid distributor is operated at minimum fluidization. These values of
Figure 5. Dependence of the (a) Bodenstein number Bo and (b) axial dispersion coefficient Dax on the superficial gas velocity UG [GS ) 25 kg/(m2 s)].
Bo (Bo > 100) indicate a flow condition that is quite similar to “plug flow”. At this operating condition, the axial dispersion coefficient Dax decreases slightly with increasing UG. Here, some still existing influence of the distributor is probably declining. Considering other conditions, it was found that Dax first increases with increasing UG and then decreases at high UG. At a high velocity ratio, the solids are decelerated by the main gas flow. Because the gas velocity in the gap, vs, is proportional to UG at constant velocity ratio, when UG increases, the entrance velocity of the solid also increases. An increasing solid velocity leads to an enlargement of the entrance section because
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Figure 6. Pressure profile of the downer.
Figure 7. Dependence of Dax on the solids flux GS [vs/vp ) 0.07 (minimum)].
of the deceleration of the solids, which can be recognized in a pressure profile (Figure 6). This extended entrance section leads to a longer backmixing section, and this effect extends the overall backmixing behavior. The subsequent decrease of Dax at high superficial gas velocities is the result of a maximum entrance velocity of solids, which cannot be exceeded. Then, the superficial gas velocity is the controlling parameter, so that the gas back-mixing can be reduced again through a reduction of the entrance section. At a minimum solids mass flow rate, the solids flux GS has a recognizable influence on the gas residence time behavior. With an increasing solids flux, the back-mixing increases, and therefore, the Bodenstein number decreases. These circumstances are represented in Figure 7. At rising superficial gas velocities in the representation of Dax, however, it can be seen that the influence is reduced to a minimum. At minimum fluidization in the distributor, the velocity ratio becomes negligible at rising superficial gas velocities. Under these conditions, the gas/solids flow is determined strongly by the superficial gas velocity. The flow in the total plant is forced more and more in the direction of gravity with only slight back-mixing. At the superficial gas velocity UG ) 4 m/s, the axial dispersion coefficient is of the order of magnitude of the appropriate coefficient of a singlephase turbulent pipe flow. With the help of the empirical equation9
1 3 × 107 1.35 ) + 0.125 2.1 Pe Re Re
(4)
the axial dispersion coefficient Dax and the Peclet number, Pe, of a turbulent gas flow can be calculated. The equation is valid for Re > 2000. At higher superficial gas velocities, the axial dispersion coefficient of gas in the gas/solids flow is smaller than that in the
Figure 8. Dependence of Dax on the solids flux GS.
single-phase flow. In that case, turbulence is of reduced by the presence of particles, which, in turn, leads to a reduction axial back-mixing. The results at constant values of vs/vp show different tendencies (Figure 8). With vs/vp ) 0.5, the dispersion coefficient is almost constant. However, at a high value of vs/vp ) 2, one recognizes a partially clear increase of the axial back-mixing, which itself reflects an increasing Dax. In both cases, the influence of the solid mass flow rate on the gas dispersion can be seen very clearly. For a fluidized bed operated within the range of pneumatic transport, Lee and Wu10 determined dispersion coefficients between 0.2 and 0.3 m2/s at solids circulating rates of GS < 25 kg/(m2 s), which is in the range of magnitude for the results obtained in this work at operating conditions with vs/vp < 0.5. The experimental procedure and data evaluation were accomplished in a similar way. However, the entrance section in the proximity of the gas/solids distributor was not included within the investigations of Lee and Wu. To understand the residence time behavior in the investigated downer, additional measurements were carried out to characterize the local flow pattern. At this point, a short description is provided. For detailed information, see Brust and Wirth.11 The flow pattern can be characterized with different types of measurements to study the local behavior of the gas/solids flow. In this case, the local solids velocity has been measured at a distance from the distributor of z ) 1.2 m with a capacitance probe system, which provides a high time resolution.12 At a distance of z ) 6.1 m, a laser doppler anemometer (LDA) system was applied. The additional air (secondary air) has a positive influence on the radial solids distribution. In the case of the dependence of the flow condition on the solids
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Figure 10. Dependence of the residence time behavior on the superficial gas velocity UG and the velocity ratio vs/vp.
4. Conclusion
Figure 9. Solids velocity related to superficial gas velocity.
velocity, a significant influence can also be recognized by the mode of operation of the distributor (Figure 9a). Without use of the secondary air (vs/vp ) minimum), an even distribution of velocity is reached. The average value of the solids velocity is within the range of the superficial gas velocity plus the single-particle fall velocity. With an increase of the volumetric flow rate of the additional air, the gas/solid flow is being split into two different parts, with high flow rates in the wall region and low values, up to negative, in the core of the flow. Thus, a clear back flow is obtained. At a distance of z ) 6.1 m, the dependence on the ratio no longer exists. The velocity profile is almost evenly distributed (Figure 9b). It was shown that different flow conditions exist in the entrance section of the downer depending on the operating conditions. The presented local axial back flow of solids and gas for certain operating conditions leads to broad RTDs for the gas. The local flow conditions can have a considerable influence on the overall behavior of the downer plant. An aim of a high velocity ratio is the improvement of the gas/solids mixing process. However, an increasing vs/vp ratio negatively affects the axial back-mixing. Figure 10 shows the dependence of the residence time behavior of the gas in the downer on the two crucial operating parameters. A limit curve could be determined, with whose help back-mixing can be adjusted during operation of the downer. Depending on the reaction technical requirements, a certain measure of flexibility can be ensured in the downer plant.
It was shown that the residence time distribution of gas in a downer reactor is crucially effected by the operating mode of the gas/solids distributor. The application of additional air improves the gas/solids mixing process at the outlet of the distributor, but operating the plant at high values of the velocity ratio vs/vp generates a strong back-mixing of the solid material in the entrance region of the downer. This fact leads to a wide residence time distribution of the gas. Nevertheless, it is possible to operate the downer in plug-flowlike conditions. To achieve this behavior, high superficial gas velocities and low solids entrance velocities are essential. The investigated gas/solids distributor provides a wide range of adjustable residence time behaviors. Finally, the presented investigation method makes a simple approach available for estimating the efficiency of a reactor. Acknowledgment Financial support of these experimental investigations by the Deutsche Forschungsgemeinschaft (DFG) and the Max-Bucher-Stiftung is gratefully acknowledged. Notation Bo ) Bodenstein number C ) tracer concentration D ) downer diameter, m Dax ) axial dispersion coefficient, m2/s E(t) ) residence time distribution, 1/s GS ) solids circulating rate, kg/(m2 s) L ) distance between sampling positions, m Pe ) Peclet number r ) radial position, mm Re ) Reynolds number t ) time, s UG ) superficial gas velocity, m/s vs ) gas velocity of secondary air, m/s vp ) gas velocity of primary air, m/s vs/vp ) velocity ratio in the gas/solids distributor X(t) ) tracer signal at position 1, 1/s Y(t) ) tracer signal at position 2, 1/s z ) length coordinate, m Greek Symbols ) voidage υ ) viscosity, m2/s Θ ) dimensionless time τ ) hydrodynamic residence time, s
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5801 Dimensionless Groups Bo ) (UGL)/Dax Pe ) (UGD)/Dax Re ) (UGD)/ν Θ ) t/τ
Literature Cited (1) Kwauk, M. Fast Fluidization. In Advances in Chemical Engineering; Wei, J., Ed.; Academic Press: San Diego, 1994. (2) Berruti, F.; Chauki, J.; Godfroy, L.; Pugsley, T. S.; Patience, G. S. Hydrodynamics of CFB Risers: A Review. Can. J. Chem. Eng. 1995, 73, 579-602. (3) Grace, J. R.; Avidan, A. A.; Knowlton, T. M. Circulating Fluidized Beds; Blackie Academic & Professional: London, 1997. (4) Zhang, H.; Zhu, J.-X.; Bergougnou, M. A. Flow Development in a Gas-Solids Downer Fluidized Bed. Can. J. Chem. Eng. 1999, 77, 194-198. (5) Lehner, P. Stro¨mungszustand und Wa¨rmeu¨bergang in vertikal-abwa¨rts gerichteten Gas-Feststoff-Stro¨mungen. Ph.D. Thesis, University of Erlangen-Nuremberg, Erlangen, Germany, 2000. (6) Johnston, P. M.; de Lasa, H. I.; Zhu, J.-X. Axial flow structure in the entrance region of a downer fluidized bed: Effects of the distributor design. Chem. Eng. Sci. 1999 54, 2161-2173.
(7) Wirth, K.-E.; Schiewe, T. Flow structures in a downer reactor. In Fluidization IX; Fan, L. S., Knowlton, T. M., Eds.; Engineering Foundation: New York, 1998. (8) Baerns, M.; Hofmann, H.; Renken, A. Chemische Reaktionstechnik; Georg Thieme Verlag: Stuttgart, Germany, 1987; Band 1. (9) Wen, C. Y.; Fan, L. T. Models for Flow Systems and Chemical Reactors. In Chemical Processing and Engineering; Albright, L. F., Maddox, R. N., Mcketta, J. J., Eds.; Marcel Dekker: New York, 1975; Vol. III. (10) Li, Y.; Wu, P. A study on axial gas mixing in a fast fluidized bed. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; 1990; pp 581-586. (11) Brust, H.; Wirth, K. E. Local flow pattern in the entrance section of a downer, manuscript to be published. (12) Richtberg, M. Charakterisierung der lokalen Stro¨mungsverha¨ltnisse in einer druckaufgeladenen zirkulierenden Wirbelschicht. Ph.D.Thesis, University of Erlangen-Nuremberg, Erlangen, Germany, 2001.
Received for review April 7, 2004 Revised manuscript received July 1, 2004 Accepted July 2, 2004 IE049725S
