Study on the FCC Process in a Novel RiserDownerCoupling Reactor (I

This paper presents an experimental study on the hydrodynamics and mixing behaviors in a riser-downer-coupling reactor proposed for the fluid catalyti...
0 downloads 0 Views 199KB Size
Download PDF
Ind. Eng. Chem. Res. 2005, 44, 733-741

733

Study on the FCC Process in a Novel Riser-Downer-Coupling Reactor (I): Hydrodynamics and Mixing Behaviors Huie Liu, Rensheng Deng, Lei Gao, Fei Wei,* and Yong Jin Department of Chemical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

This paper presents an experimental study on the hydrodynamics and mixing behaviors in a riser-downer-coupling reactor proposed for the fluid catalytic cracking (FCC) process. Special attention is placed on two parts: the annular riser and the riser-downer junction. In comparison to the conventional riser, the annular riser has a more uniform radial flow structure, which can be observed from the radial distribution of particle velocity and solid concentration; however, the axial particle mixing seems to be similar. The carefully designed junction can serve as an inlet structure of the downer part to realize the uniform distribution of solids, while it does not increase the back-mixing of particles of the whole coupling reactor. The particle dispersion mechanism has also been investigated, and the differences in the microstructures can explain the characteristic particle mixing behaviors observed in the coupling reactor. 1. Introduction The circulating fluidized bed (CFB) is widely used in various industries: for example, a riser type is the most common reactor adopted in the fluid catalytic cracking (FCC) process. The riser has the advantages of high gas-solid contact efficiency, high gas/solid flux, and high operating flexibility. However, serious nonuniformity of local hydrodynamic properties and significant axial gas and solid back-mixing are observed in the riser reactor,1,2 which reduces the yields of desired intermediate products such as gasoline. As a promising future substitute for the riser reactor, the downer type provides much more uniform radial profiles of gas velocity, particle velocity, and solid fraction3 as well as a “close to plug flow” pattern.4 Unfortunately, the performance of downer reactors is strongly dependent on the entrance structure,5-7 showing the lack of flexibility in their operations. And, the solid fraction in a downer reactor is much lower than that in a riser type with the same solid flux, which may be unfavorable for certain gassolid catalytic reactions.8,9 New designs of bed configuration are needed to overcome the above problems before downer reactors can be widely applied to commercial production. A novel riser-downer-coupling circulating fluidized bed reactor (abbreviated as “coupling reactor” in the following) has been proposed for the FCC process,10 as shown in Figure 1. The original idea is to take the advantages of both riser and downer reactors to obtain the highest yields of desired products. It has two main parts: the riser part and the downer part, which are connected by a carefully designed junction. To ensure a good transition between the riser and the downer, the reactor adopts the configuration of a dual cylinder, with the annulus being a riser and the inner cylinder being a downer. The regenerated catalyst is introduced from the lower part of the annular riser, where it mixes with the dispersed oil droplets sprayed through nozzles. The * To whom correspondence should be addressed at the Fluidization Laboratory of Tsinghua University (FLOTU). Tel: 86-10-62785464. Fax: 86-10-62772051. E-mail: wf-dce@ mail.tsinghua.edu.cn.

Figure 1. Schematic diagram of the FCC process adopting a riser-downer-coupling reactor.

cracking reaction occurs on the hot catalyst particles in the annual riser and the downer in succession; finally the gas products and solids are separated by a fast separator. The products are discharged and sent away for separation, while the deactivated catalyst is led to the regenerator, where the coke is burned in the air and the reactivated catalyst can be reused for cracking reactions. This coupling reactor shows many promising characteristics. First, the solid fraction is high with extensive gas-solid mixing in the annular riser, which can ensure a high conversion of the reactants in the initial stage. The subsequent downer reactor possesses a uniform radial flow structure and a low axial back-mixing that can efficiently suppress the occurrence of the secondary reaction, which may be significant, especially at the high feed conversion in consecutive reactions. These properties are beneficial to the maximum yield of desired products such as gasoline in the FCC process. Second, a uniform gas-solid suspension is formed in the riser

10.1021/ie040174f CCC: $30.25 © 2005 American Chemical Society Published on Web 01/20/2005

734

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

Figure 3. Comparison of radial profiles of solid fraction between annular riser and conventional riser. Gs and j denote the solid flux and the averaged voidage over the cross-section, respectively.

Figure 2. Experimental apparatus used in the study of hydrodynamics and mixing behaviors.

due to the sufficient mixing there, which then enters the downer smoothly through the junction. Thus, difficulties in designing a special entrance structure for the downer can be avoided. Finally, feedstocks from various sources vary widely in components and concentration, and requirements for products also change from time to time according to market demands. In the coupling reactor, the length ratio of the riser to the downer can be adjusted by adopting nozzles at different positions; as a result, the reaction time and further the feed conversion can be changed in a wide range. This provides a reactor with the flexibility to treat different stocks and modify product schemes. In this paper, we present our experimental study on the hydrodynamics and mixing behaviors in the novel coupling reactor, which was mainly carried out on a laboratory-scale cold model. This work can serve as a foundation for its future commercial application. 2. Hydrodynamics Study on a Cold Model Many investigations on hydrodynamics in a riser or downerhavebeencarriedoutinthelasttwodecades.3,4,8,11-15 However, the flow structure in the annular riser and the riser-downer junction is rarely reported in the literature, even though it is very important to the development of a novel coupling reactor for commercial applications. Figure 2 shows the experimental apparatus used in the hydrodynamics study. The whole setup is made of Plexiglas with a total height of 11.2 m, and the reactor is composed of a main riser, an annular riser, and a downer with lengths of 7.6, 4.5, and 5.5 m, respectively. The inner diameter of both the main riser and the downer is 192 mm, and the cross-sectional area of the annular riser is the same as that of the main riser and the downer. The gas-solid suspension rises through the

main riser, passes through the annular riser, and finally enters into the downer. The gas and solids are separated in the fast separator at the exit of the downer and a subsequent two-stage cyclone, and then the particles enter the slow bed for the next cycle. The fluxes of the gas and solid are monitored by a rotator meter and a solid flux meter, respectively. The particles used in the experiments are those of FCC catalyst with an average diameter of 54 µm and a density of 1710 kg/m3. 2.1. Flow Behaviors of Gas and Solids in the Annular Riser. A conventional riser usually has the shape of a long cylinder in which the solid fraction exhibits a profile with a dilute core and a dense annular near the wall. For both gas and solid, high velocities appear in the central zone, while there are low or even negative velocities in the wall regime. Corresponding to the nonuniform flow structure, severe axial backmixing of both gas and solids is also observed. To improve such a pattern far away from plug flow, various methods were explored, such as the modification of the bed configuration or material properties,16-22 introduction of swirling air flow,23 and adoption of particles with special size distribution.24-28 These methods are successful to some extent; however, the researchers have usually focused on the hydrodynamics without considering the mixing behaviors of gas and solid, although the latter is very important to reactions in the FCC process, where the desired products are intermediates. On the basis of this understanding, both the hydrodynamics and mixing behaviors in the annular riser are examined in the present study. In the annular riser, the inner cylinder can be viewed as a vertical internal along the whole bed; thus, supposedly it can function like some other internals used in riser reactors. For example, the aggregation of particles near both the inner and outer walls may reduce the nonuniformity in the radial distribution of solid fraction and gas/particle velocity. This statement is to be tested in the following experiments. Additionally, the average solid concentration here is limited by the apparatus structure, and the hydrodynamics and mixing behaviors in a high-density annular riser will be studied in the future. 2.1.1. Radial Distribution of the Solid Fraction. The radial distribution of the solid fraction in the annular riser is measured using a two-path optical fiber density probe system as adopted by Lin et al.29 Figure 3 shows the comparison of radial solid fraction (1 - ) profiles between the conventional riser and the annular riser under the same superficial gas velocity (Ug) and

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 735

averaged cross-sectional solid fraction. The abscissa is the relative radial position, denoted by Φ: for the conventional riser, Φ ) r/R, and for the annular riser, Φ ) (r - R1)/(R2 - R1), where r denotes the radial distance from the bed center and R1 and R2 denote the radii of the inner and outer cylinders, respectively. As shown in Figure 3, particles in the annular riser tend to aggregate near both the inner and outer walls, resulting in a higher local solid fraction than in other radial locations. In comparison to the conventional riser, the solid fraction near the outer cylinder is slightly lower. It can also be seen from Figure 3 that the distribution of the solid fraction is not symmetric around the radial location of (R1 + R2)/2: the minimum appears at Φ ) 0.3-0.4, and the solid fraction at the outer wall is slightly higher than that at the inner wall. This may be due to the differences in the curvatures between the two walls, which influence the flow behaviors of the gas-particle suspension to different extents. The results agree qualitatively with the data obtained by Wang et al.17 It can also be seen that the gap between the maximum and minimum of solid fractions in the annular riser is lower than in the conventional riser. By adoption of the nonuniformity index defined by Bai et al.12

∫01( - j)2φ dφ

S2 ) 2

(1)

we can calculate the indices for the conventional riser and annular riser to be 0.000 47 and 0.000 09, respectively. It can be concluded that the uniformity in the radial distribution of the solid fraction is significantly improved in the annular riser. 2.1.2. Radial Distribution of Particle Velocity. The particle velocity (Vp) in the annular riser is measured using laser Doppler velocimetry (LDV), and the results are shown in Figure 4. The ordinate is the relative particle velocity Vp/Ug, and the abscissa is the relative radial position. It can be seen that the particle velocity is very low near the two walls: close to zero. Furthermore, the particle velocity is negative near the outer wall, showing significant backflow of particles here. High velocities can be observed in the central zone with a maximum at Φ ) 0.3-0.4, corresponding to the minimum of the solid fraction. The particle velocity increases more sharply from the inner wall to the central zone than that from the outer wall to the central zone. Such an asymmetric distribution of velocity also appears for the laminar axial flow of a single-phase fluid in the annular zone between two concentric cylinders. A more sharply increasing rate for the fluid velocity near the inner wall was observed, but the asymmetry was not significant.30-32 The location corresponding to the velocity maximum is

Rmax )

x

R22 - R12 R2 2 ln R1

()

(2)

The radial distribution of local gas velocity (Vg) is

Vg/Ug ) 2

R22 - r2 - 2Rmax 2 ln

R2 r

R22 + R12 - 2Rmax2

(3)

Figure 4. Radial distribution of particle velocity in the annular riser: (a) Ug ) 3.8 m/s; (b) Ug ) 5.7 m/s; (c) 1 - j ) 0.022.

For example, for the laminar gas flow between two concentric cylinders with the same dimension as the annular riser, the maximum velocity occurs at Φ ) 0.49 and the gas velocity has a radial distribution as indicated by the dashed line shown in Figure 4c. It can be seen that the addition of particles into the fluid results in a more significant nonuniformity in the radial distribution of velocity. At the same superficial gas velocity, the higher the averaged solid fraction over the cross-section, the larger the gap that exists between the maximum and minimum particle velocity, as shown in Figure 4a,b. At the same averaged solid fraction over the cross-section, the radial nonuniformity decreases with increasing superficial gas velocity, as shown in Figure 4c. In Figure 5 the radial distributions of particle velocity in the annular and conventional risers are compared at similar superficial gas velocities and averaged crosssectional solid fractions. The particle velocity near the wall (Φ ) 1) is about zero for both reactors. However, the maximum velocity in the annular riser is lower than that in the conventional riser; moreover, the gradient of particle velocity between the center and the wall is also lower. In this sense, the double-wall effects in the

736

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

Figure 5. Comparison of the radial profiles of particle velocity between the conventional riser and the annular riser: (a) high solid flux; (b) low solid flux.

Figure 6. Structure of the riser-downer junction.

annular riser can reduce the nonuniformity in radial distribution of particle velocity. However, particle aggregation and particle backflow occur in both walls, and significant differences are observed between the particle velocity at the wall region and in the center zone. The influence of such a radial structure on the product yields has yet to be examined. 2.2. Hydrodynamics in the Riser-Downer Junction. Another subject of interest is the junction connecting the annular riser and the downer, as shown in Figure 6. The gas can easily change from its upward flow in the riser to the downward flow in the downer, while the motion of particles must be examined in order to obtain a uniform distribution at the downer inlet. Here the solid fraction and particle velocity are also measured using the two-path optical fiber probe system and the LDV system mentioned above, respectively.

Figure 7 shows the radial profiles of the solid fraction and particle velocity at 0.11, 0.2, 0.5, 0.8, and 3.84 m below the downer inlet, with a superficial gas velocity of 5.0 m/s and a solid flux (Gs) of 70.1 kg/(m2 s). At a position of 0.11 m below the inlet, a zone with a high solid fraction can be found at a relative radius of 0.90.95, and a low solid fraction is found in the center. This can be explained by the fact that particles tend to aggregate near the wall region after transiting from the riser to the downer. Then, particles disperse quickly during their downflowing process and they are quite uniformly distributed along the radial dimension after traveling 0.2 m in the downer. Later, the radial uniformity of the solid fraction profile increases gradually along the flow direction. At a position of 0.8 m away from the inlet, the solid fraction shows a pattern with a flat central zone and a dense annular near the wall, which is close to the typical density profile in the fully developed zone of the downer. Figure 7 also showed that the radial distribution of particle velocity becomes uniform very soon after entering the downer, for example, at 0.8 m below the entrance it has already evolved into a regular, fully developed pattern. The analysis on the time series of particle velocity in the inlet region can provide us with the particle fluctuating velocity, which represents the intensity of the fluctuation of particle velocity. The evolution of particle fluctuating velocity in the downer is shown in Figure 8. The radial distribution of particle fluctuating velocity is not uniform at the axial position of 0.11 m, and the magnitude of the fluctuating velocity is relatively high. This may result from the strong particle-particle and particle-wall collisions caused by the significant change in flow direction. With the particles moving downward, the fluctuation of particle velocity decreases and finally reaches a regular distribution in the fully developed downer. Such a strong fluctuation of particle velocity leads to excellent particle mixing as well as rapid momentum exchange in the inlet zone; as a result, a uniform distribution of the solid fraction and particle velocity can be obtained in a very short time. Thus, the riser-downer junction exhibits a new development in the design of entrance structure for downer reactors. However, particle clustering near the wall is still observed in the very early stage of particle motion in the downer, which is undesired for the gas-solid contact. Further study is necessary to optimize the junction structure in order to obtain a better transition. 3. Mixing Behaviors A further question is about the mixing behaviors of particles in the coupling reactor, which are related to the heat and mass transfer in chemical processes. For example, the FCC catalyst is the location where the cracking reaction occurs, and catalyst particles with different residence time lengths can result in varied reaction rates, due to the differences in deactivation extents. The quality of particle mixing directly determines the residence time distribution (RTD) and, further, the product yields. On the other hand, particles are the major heat carriers, due to their high heat capacity compared to gas; thus, the heat transfer coefficient in an endothermic reaction such as FCC is strongly dependent on the particle mixing behaviors. In the following the axial back-mixing of particles in the

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 737

Figure 7. Evolution of radial profiles of solid fraction and particle velocity in the downer. Ug and Gs are 5.0 m/s and 70.1 kg/(m2 s), respectively, and L is the distance between the measuring point and the downer inlet.

annular riser and also the whole reactor is studied using the phosphor tracer technique developed by Wei et al. in 1994. Due to the special structure of the annular riser, the light source should be specially arranged in order to inject the tracer particle pulse. Two blocks of black cloth are used to cover the outer cylinder with a gap of 5 mm width left between them. Six flashtubes are uniformly allocated around the gap; thus, the light emitted from the flashtubes can be transmitted through transparent Plexiglas walls and lighten all the particles in the crosssection corresponding to the gap. This method makes it possible to inject a pulse of the surface light source. The optical-electrical sensor is placed at a position of 1.98 m downstream to measure the light intensity. The particles adopted in the study of mixing behaviors are a kind of phosphor particle with physical properties similar to those of the FCC catalyst: e.g., they have the same particle size and density. 3.1 Data Processing Method. A one-dimensional two-component dispersion model33 is used to describe the particle mixing behaviors in the riser. According to this model, the axial flow of the particle phase is composed of two components: dispersed particles and clusters. Suppose that the dispersion of each component can be described by a one-dimensional axial dispersion model and no interchange between them is taken into consideration; then the model can be described as

Dad

∂2Cd ∂x2

- Upd

∂Cd ∂Cd ) ∂x ∂t

with the boundary conditions

x ) 0: Cc ) Cc0‚δ(t) Cd ) Cd0‚δ(t)

(7)

x ) - ∞: Cc ) 0.0 Cd ) 0.0

(8)

The analytical solution of the equations is

E(θd) )

E(θc) )

xPed

x4πθd3 xPec

x4πθc3

Ped )

(6)

exp -

(1 - θc)2Pec 4θc

(10)

θc )

Upct t (11) ) L tc

A kind of regression technique is used to evaluate the coefficients Ped, Pec, Upd, Upc, and γ. If the overall axial particle dispersion in the reactor also satisfies the onedimensional model, we have

σt2

2

C ) Cd + Cc Cd0 ) γC0 Cc0 ) (1 - γ)C0

(9)

UpdL UpcL Updt t Pec ) θd ) ) Dad Dac L tc

ht 2 (5)

] ]

(1 - θd)2Ped 4θd

where

(4)

∂ Cc ∂Cc ∂Cc Dac 2 - Upc ) ∂x ∂t ∂x

[ [

exp -

)

2 8 + Pe Pe2

(12)

where

ht ) td‚γ + tc‚(1 - γ)

(13)

738

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

Figure 10. RTD curves of particles detected at various positions in the coupling reactor. Figure 8. Radial distribution of particle flucuating velocity (Ustd) in the downer.

Figure 11. Axial particle Peclet numbers of the coupling reactor, riser and downer.

Figure 9. Comparison of (a) particle RTD curve and (b) axial particle Peclet number (Pea) between the annular riser and the conventional riser, where x is the distance between the tracer pulse injection position and the detecting position.

Thus, the overall Peclet number Pe can be obtained and used to evaluate the extent of particle dispersion. 3.2. Particle Mixing Behaviors in the Annular Riser. Figure 9a shows the RTD curves of particles in both the conventional riser and the annular riser under the same operating conditions. It can be seen that they have similar shapes with a long tail. By simulation of the results with the one-dimensional axial dispersion model, the axial Peclet number of particles can be obtained as shown in Figure 9b. It can be seen that the two types of risers have axial Peclet numbers of particles in the same order of magnitude (ranging from 3 to 8), denoting no significant differences in the axial mixing behaviors between them. Thus, it can be concluded that although the structure of the annular riser can improve the uniformity of the radial distribution of solid fraction and particle velocity, it has little influence on the axial mixing behaviors and cannot change the severe backmixing nature of the riser reactor. The explanation of such a somewhat strange observation is related to the mechanism governing the mixing behaviors in the riser, which will be discussed later in this paper.

3.3. Overall Particle Mixing Behaviors in the Coupling Reactor. To investigate the variation of particle mixing behaviors through the reactor, the phosphor tracer particles were injected at a certain position in the annular riser (6.43 m upstream from the downer exit) and detected at different positions: the outlet of the annular riser, the inlet of the downer, and the outlet of the downer. Figure 10 shows the RTD curves obtained at a superficial gas velocity of 5.5 m/s and a solid flux of 41.6 kg/m2s. Also, the Peclet numbers can then be calculated using the one-dimensional twocomponent dispersion model mentioned above. In the annular riser, the Peclet number is 3.98, a typical value observed in the conventional riser. After passing through the junction, the particle RTD curve changes slightly in shape at the inlet of the downer, and the corresponding Peclet number increases to 6.02, which means that the junction does not increase the back-mixing of the particles. At the outlet of thedowner, the particle RTD curve is similar to that at the inlet of the downer, only with a certain shift in time. This obviously results from the close to plug flow pattern in the downer. The Peclet number here is 14.1, twice as high as in the riser, indicating that the large back-mixing in the riser can be offset to some degree by the successive downer. The comparison of typical Peclet number among a coupling reactor, a single riser, and a single downer are shown in Figure 11. The results are obtained under the following operating conditions: a superficial gas velocity of 5.5-8.66 m/s and a solid flux of 20-90 kg/(m2 s). The Peclet number in the coupling reactor is around twice as large as in a single riser, indicating a notable decrease in solid axial mixing, which is beneficial to the gasoline yield in the FCC process. In comparison to the downer, the coupling reactor has a lower Peclet number; however, it possesses a higher solid fraction that can raise the conversion of feedstock and more extensive mixing, which ensures the desired gas-solid contact. As stated above, one feature of the coupling reactor is the flexibility to treat various feeds and meet special product requirements, which can be realized by changing the position of the nozzles. To study the particle mixing in the above cases, the tracers are injected at different positions of the coupling reactor, i.e., the main

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 739

Figure 12. RTD curves for different tracer injection positions, where TIP means tracer injection position and ART denotes average residence time. The injection positions a-d are located in the main riser, the bottom of the annular riser, the outlet of the annular riser, and the inlet of the downer, respectively, which have distances of 9.26, 6.43, 4.47, and 3.73 m away from the detection position, respectively.

Figure 13. Axial particle Peclet numbers (Pe) corresponding to tracer injections at different positions. Positions a-d are the same as those shown in Figure 12.

riser, the bottom and outlet of the annular riser, and the inlet of the downer, and detection is conducted at the outlet of the coupling reactor. The results are shown in Figure 12, where all RTD curves are more symmetrical than that for a single riser. When the tracer injection position comes closer to the outlet of the reactor, the RTD curve tends to be narrower. This indicates that variation of the injection position has effects on both the average residence time and the extent of axial particle mixing. For example, when the feedstock is sprayed at the inlet of the downer, not only the reaction time is shorter compared to the case with feed injected into the annular riser but also the cracking process encounters a particle flow pattern much closer to plug flow. Figure 13 shows the Peclet numbers corresponding to various injection positions. For those injections in the riser, e.g., positions a-c, the change in Peclet number is insignificant. All of them are of the same order of magnitude, indicating that they have similar axial mixing behaviors here. However, the sharp increase of Peclet number from 30 at the outlet of the annular riser

to 200 at the inlet of the downer shows that, although the axial particle back-mixing in the junction is less than at the riser as stated above, it is still severe in comparison to that in the downer. Thus, the design of the junction structure is important for controlling the RTD of the particles in the coupling reactor. It can also be seen from Figure 13 that the Peclet number does not change significantly with operating conditions such as the superficial gas velocity, which can be explained by the mechanism of particle dispersion discussed in the following. 4. Analysis of Particle Dispersion Mechanism The RTD curve of particles in the riser has a long tail or is bimodal, while it has a symmetrical single peak in the downer, suggesting that significant differences exist between the particle dispersion mechanisms in the riser and downer. Wei et al.33 suggested that two modes of particle dispersion exist in the riser; one is due to the dispersion of dispersed particles, and the other is due to the clusters. On the other hand, there is only one dispersion mode in the downer, similar to that of the dispersed particles in the riser. The probability density distribution (PDD) of local solid fraction exhibits bimodal profiles at different radial positions in the riser,29 as shown in Figure 14. The corresponding PDD curves in the downer are obtained under similar operating conditions,34 which have the shape of a single peak in Figure 14. On the basis of these studies, it can be suggested that a stable two-phase microstructure exists in the riser: a dilute void phase and a dense cluster phase. Such a structure exists throughout the cross-section of the riser, with the intensity of the two phases changing at different radial positions: the void phase dominates in the center, while the cluster phase becomes significantly important near the wall. Oppositely, only the void phase exists in the downer, with no cluster phase appearing at any radial position. Both the void phase and the cluster phase are stable, which cannot be destroyed by changing operating conditions such as superficial gas velocity and solids flux. Note that the “cluster phase” here is different from the macro-scale “ribbons” and “U-shape” clusters observed in the riser and the microscopic particle aggregation directly resulting from strong particle-particle forces. The two-phase structure is expected to be responsible for the bimodal profile of particle RTD in the riser. The cluster phase has a higher density and a lower velocity than the void phase; thus, the particles inside form

Figure 14. PDD profiles at different radial positions in the riser and downer.

740

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

relatively stagnant regions and lead to the long tail (or the second peak) in the RTD profile of the riser. In contrast, the downer has a uniform phase composed of voids, shown as a single peak in its particle RTD curve. When the gas-solid mixture is introduced into the riser, a two-phase structure with both voids and clusters forms, regardless of the length and configuration of the riser. Thus, although the uniformity in the radial distribution of solid fraction and particle velocity is improved after introducing the annular riser in the coupling reactor, the basic element, cluster, remains unchanged and still results in a wide RTD. Coupling a downer to a riser cannot destroy the stable two-phase structure in the riser, although a one-phase structure can be obtained during the downflowing of the gas-solid mixture in the downer. Thus, the overall Peclet number of the coupling reactor increases with decreasing length of the riser part and increasing length of the downer part. The significant difference in flow structure between the riser and the downer is caused by the flow direction of the gas and solids. They flow along the gravitational and antigravitational directions in the riser and downer, respectively, which results in different particle aggregation mechanisms. Actually, stable two-phase structures exist in any gas-upward-moving fluidized bed such as a bubbling bed, turbulent bed, and fast bed.29 Further studies on the relationship between the flow direction and particle aggregation mechanisms have yet to be conducted. 5. Conclusions As two special parts adopted in the novel riserdowner-coupling reactor, the hydrodynamics and particle mixing behaviors in the annular riser and the riser-downer junction have been studied in detail. In comparison to the conventional riser, the annular riser possesses a more uniform radial flow structure, while the axial particle back-mixing is of the same order of magnitude. The carefully designed junction does not show significant axial particle back-mixing; however, it can realize the uniform distribution of a gas-solid mixture at the inlet of the downer. All these features make the coupling reactor a promising candidate for the FCC process to achieve the highest yields of desired products. Acknowledgment This work has been supported by the National Science Foundation of China under Contract Nos. 20176024 and 20236020. Literature Cited (1) Helmrich, H.; Schurgerl, K.; Janssen, K. Decomposition of NaHCO3 in Laboratory and Bench Scale Circulating Fluidized Bed Reactors. In Circulating Fluidized Bed Technology; Basu, P., Ed.; Pergamon Press: Oxford, U.K., 1986; pp 161-166. (2) Bai, D.; Jin, Y.; Yu, Z.-Q. Circulating Fluidization. Chem. React. Eng. Technol. (China) 1991, 7, 303-317. (3) Zhang, H.; Zhu, J.-X.; Bergounou, M. A. Hydrodynamics in Downflow Fluidized Beds (1): Solids Concentration Profiles and Pressure Gradient Distributions. Chem. Eng. Sci. 1999, 54, 54615470. (4) Wei, F.; Wang, Z.; Jin, Y.; Yu, Z.-Q.; Chen, W. Dispersion of Lateral and Axial Solids in a Cocurrent Downflow Circulating Fluidized Bed. Powder Technol. 1994, 81, 25-30.

(5) Wei, F.; Liu, J.; Jin, Y.; Yu., Z.-Q. Hydrodynamics and Solids Mixing in the Entrance of a Downer. In Circulating Fluidized Bed Technology V; Kwauk, M., Li, J., Eds. Science Press: Beijing, China, 1997; pp 122-127. (6) Lehner, P.; Wirth, K. E. Effects of the Gas/Solids Distributor on the Local and Overall Solids Distribution in a Downer Reactor. Can. J. Chem. Eng. 1999, 77, 199-206. (7) 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, 21612173. (8) Wang, Z.; Bai, D.; Jin, Y. Hydrodynamics of Cocurrent Downflow Circulating Fluidized Bed. Powder Technol. 1992, 70, 271-275. (9) Deng, R.; Wei, F.; Jin, Y.; Zhang, Q.; Jin, Y. Experimental Study of the Deep Catalytic Cracking Process in a Downer Reactor. Ind. Eng. Chem. Res. 2002, 41, 6015-6019. (10) Jin, Y.; Wei, F.; Cheng, Y.; Wang, Z.; Wang, J. A Novel Design of an Integrated Riser-downer Reactor. Chinese Patent Pending 00100823.4, 2000. (11) Hartge, E. U.; Rensner, D.; Werther, J. Solids Concentration and Velocity Patterns in Circulating Fluidized Beds. In Circulating Fluidized Bed Technology II; Basu, P., Large, J. E., Eds.; Pergamon Press: Oxford, U.K., 1988; pp 165-180. (12) Bai, D.; Jin, Y.; Yu, Z.; Gan, N. Radial Profiles of Local Solids Concentration and Velocity in a Cocurrent Downflow Fast Fluidized Bed. In Circulating Fluidized Bed Technology III; Basu, P., Horio, M., Hasatani, M., Eds.; Pergamon Press: Toronto, 1991; pp 157-162. (13) Bai, D.; Jin, Y.; Yu, Z.; Zhu, J. The Axial Distribution of the Cross-Sectionally Averaged Voidage in Fast Fluidized Bed. Powder Technol. 1992, 71, 51-58. (14) Zhu, J.-X.; Jin, Y.; Yu, Z.-Q.; Grace, J. R.; Issangya, A. Cocurrent downflow circulating fluidized bed (downer) reactors-a state of the art review. Can. J. Chem. Eng. 1995, 73, 662-677. (15) Zhang, H.; Zhu, J.-X. Hydrodynamics in Downflow Fluidized Beds(2): Particle Velocity and Solids Flux Profiles. Chem. Eng. Sci. 2000, 55(19), 4367-4377. (16) Gan, N.; Jiang, D.; Bai, D.; Jin, Y.; Yu, Z.-Q. Concentration Profiles in Fast Fluidized Bed with Bluff Body. J. Chem. Eng. Chin. Univ. 1990, 3, 273-277 (in Chinese). (17) Wang, Z.; Yao, J.; Liu, S.; Li, H.; Guo, M. Study on Characteristics of Flow in an Internal Fast Circulating Fluidized Bed. Chem. Eng. 1995, 5, 26-30 (in Chinese). (18) Zheng, C. G.; Tung, Y. G.; Zhang, W. N.; Zhang, J. G. Impact of Internals on Radial Distribution of Solids in a Circulating Fluidized Bed. Eng. Chem. Metallurgy 1990, 11(4), 296-302 (in Chinese). (19) Zheng, C.; Tung, Y.; Li, H.; Kwuak, M. Characteristics of Fast Fluidized Beds with Internals. In Fluidization VII; Potter, O. E., Nicklin, D. J., Eds.; Engineering Foundation: New York, 1992; pp 275-284. (20) Jiang, P.; Bi, H. T.; Jean, R. H.; Fan, L. S. Baffles Effects on Performance of Catalytic Circulating Fluidized Bed Reactor. AIChE J. 1991, 37, 392-1340. (21) Zhu, J.-X.; Salah, M.; Zhou, Y. Radial and Axial Voidage Distributions in Circulating Fluidized Bed with Ring-Type Internals. J. Chem. Eng. Jpn. 1997, 30(5), 928-937. (22) Wei, F.; Yang, Y.; Jin, Y. Effect of Internals on Hydrodynamics in High-Density Riser. J. Chem. Ind. Eng. 2000, 51, 812815 (in Chinese). (23) Ran, X.; Wei F.; Wang Z.; Jin Y. Lateral Solids Dispersion in a High-Density Riser with Swirling Air Flow. Powder Technol. 2001, 121, 123-130. (24) Sun, G.; Grace, J. R. The Effect of Particle Size Distribution on the Performance of a Catalytic Fluidized Bed Reactor. Chem. Eng. Sci. 1990, 45, 2187-2194. (25) Wang, Y.; Wei, F.; Jin, Y.; Yu, Z.-Q. Radial Profiles of Solids Concentration and Velocity in a Fine Particle Riser. Powder Technol. 1998, 96, 262-266. (26) Ma, X.; Kato, K. Effect of Interparticle Adhesion Forces on Elutriation of Fine Powders from a Fluidized Bed of a Binary Particle Mixture. Powder Technol. 1998, 95, 93-101. (27) Wei, F.; Cheng, Y.; Jin, Y.; Yu, Z.-Q. Axial and Lateral Dispersion of Fine Particles in a Binary-Solids Riser. Can. J. Chem. Eng. 1998, 76, 19-26. (28) Du, B.; Wei, F.; Lateral Solids Mixing Behavior of Different Particles in a FCC Riser. In Fluidization X; Basu, P., Large, J. E., Eds.; Engineering Foundation: New York, 2001; pp 197-204.

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 741 (29) Lin, Q.; Wei, F.; Jin, Y. Transient Density Signal Analysis and Two-phase Microstructure Flow in Gas-solids Fluidization. Chem. Eng. Sci. 2001, 56, 1-11. (30) Wang, Z. Fundamental Hydrodynamics; Higher Education Press: Beijing, 1987 (in Chinese). (31) Dai, G.; Chen, M. Chemical Engineering Hydrodynamics; Chemical Industry Press: Beijing, 1988 (in Chinese). (32) Han, Z. The Principle of Transfer Processes; Press of Zhejiang University: Zhejiang Province, 1988 (in Chinese). (33) Wei, F.; Zhu, J. Effect of Flow Direction on Axial Solids Dispersion in Gas-solids Cocurrent Upflow and Downflow System. Chem. Eng. J. 1996, 64, 345-352.

(34) Zhang, M.; Qian, Z.; Yu, H.; Wei, F. The Solids Flow Structure in a Circulating Fluidized Bed Riser/Downer of 0.42m Diameter. Powder Technol. 2003, 129(1-3), 46-52.

Received for review June 10, 2004 Revised manuscript received November 19, 2004 Accepted November 19, 2004 IE040174F