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Ind. Eng. Chem. Res. 1998, 37, 2548-2552
GENERAL RESEARCH Hydrodynamic Aspects of a Circulating Fluidized Bed with Internals N. Balasubramanian Department of Chemical Engineering, Indian Institute of Technology, Madras 600 036, India
C. Srinivasa kannan* Chemical Engineering Division, Central Leather Research Institute, Madras 600 020, India
An attempt is made to examine the influence of internals (baffles) in the riser of the circulating fluidized bed. Experiments are conducted in a circulating fluidized bed, having perforated plates with different free areas. It is noticed from the present work that a circulating fluidized bed having 45% free area gives uniform solids concentration and pressure drop along the length of the riser. In addition to the uniformity, the circulating fluidized bed with internals gives higher pressure drop (solids concentration) compared to a conventional circulating fluidized bed. For internals having 67.6% free area the pressure drop is higher at the lower portion of the riser compared to the upper portion, similar to a conventional circulating fluidized bed. For 30% free area plates the solids concentration varies axially within the stage and remains uniform from stage to stage Introduction A circulating fluidized bed offers a number of advantages for contacting gas and solids in chemical industries. They include effective and bubble-free contact between gas and solids, high gas throughput per unit cross section, and easy handling of even cohesive materials. Circulating fluidized beds find application in calcination of aluminum hydroxide, coal combustion, and catalytic processes such as fluid catalytic cracking and Fischer-Tropsch synthesis. A circulating fluidized bed (CFB) system consists of a riser to provide contact between gas and solids and a gas-solid separator to facilitate separation and recycle of solids into the riser. The operation of CFB often presents a region of high solids concentration at the bottom of the riser and a region of low solids concentration near the exit of the riser. Even within these regions solids concentration varies longitudinally, presenting difficulties in scale-up and reliable estimation of its kinetic performance. The present study aims at the use of internals in the riser of the CFB for improving the contact between the phases and for obtaining uniform solids concentration along the length of the riser. A number of publications on the advantages and disadvantages of CFB, on its industrial applications, and especially on the characteristics of the CFB riser have appeared in the literature. Significant among them are due to Yerushalmi and Cankurt (1979), Li and Kwauk (1980), Yerushalmi and Avidan (1985), Weinstein et al. (1986), Hartge et al. (1986), and Rhodes et al. (1988). The studies on the use of internals in the riser of CFB reported in the literature are due to Zheng et * Corresponding author.
al. (1990), Van der Ham et al. (1993, 1994a,b), and Jiang et al. (1991). Zheng et al. (1990) noticed solids being lifted in the dilute phase in the center and moving downward in the dense phase near the wall. The improvement of radial solids distribution was attempted using different types of internals such as ring, inverse cone tube, and several perforated plates. The perforated plate internal was found to improve the radial distribution of solids over other types of internals. The ring type internals intensified the nonuniformity in radial distribution of particles. The use of inverse cone tube internals could not establish better radial distribution as the gas tending to flow to the wall. The two perforated plate internals with 200-mm baffle spacing was found to be optimum spacing for better radial mixing of solids. When four perforated plates with 200-mm spacing were used, a denser bed holdup was obtained. Jiang et al. (1991) provided four horizontal ring type baffles with an open area of 56% kept approximately 1200 mm apart in a riser of 102-mm i.d. and 6320-mm height. The authors reported nearly linear pressure variation in the axial distance for low solids rate and a significant axial pressure variation with an increase in solids rate, with breaks in pressure profiles occurring around the baffle area. The authors on comparing solids holdup in the riser with baffles to that without baffles observed that the riser with baffles gave higher solids holdup especially at high gas rates. Van der Ham et al. (1993, 1994a,b) dealt with the hydrodynamics of a regularly packed CFB. It is noted that industrial application of baffles involves various innovative designs beyond ring, inverted cone, and perforated type baffles which are
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Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2549
Figure 1. Schematic diagram of the experimental setup: (a) conventional circulating fluidized bed, (b) circulating fluidized bed with internals; (1) test section, (2) solids hopper, (3) gas-solid separator, (4) bag filter, (5) air blower, (6) surge tank, (7) contral valve, (8) flowmeter, (9) butterfly valve, (10) pressure taps, (11) plate stack, (12) air heater.
focused toward minimizing erosion and pressure drop. Such options are considered in industry for circulating fluidized-bed reactor operation. Special attention should be given with respect to erosion of baffles on scale-up. Experimental Section Figure 1 shows the schematic diagram of the experimental setup. The riser (1), made of Plexiglas, has an internal diameter of 52 mm and a height of 2200 mm. A gas solid separator (3), at the top of the riser, facilitated the separation of solids from the exiting gas. The separated solids flow into a solids hopper (2), from which they are recycled back into the riser through a control valve (7). A butterfly valve (9) provided at the bottom of the riser facilitated the measurement of solids holdup. The solids holdup was measured by arresting the flow of solids and air into the riser and collecting the solids within the riser. Pressure taps (10) were provided at different locations along the length of the riser at close intervals, for the measurement of pressure drop. Perforated plates of known plate geometry were provided in the riser using a central rod for support. Air was allowed at a fixed flow rate in the riser. The solids were introduced by opening the solids control valve provided at the bottom of the hopper. The solids circulation rate was measured by collecting the solids in the riser outlet. After attaining the steady state, the pressure drop and solids holdup were measured. The system was considered to operate at steady state once the pressure drop was constant with respect to time. Experiments were repeated to check the reproducibility of the data. Experiments were conducted in the conventional CFB system (i.e., without internals) and with the modified CFB system (i.e., with perforated plates in the riser) for various operating conditions. Horizontal perforated plates with 30%, 40%, 45%, and 67.6% were used in the present study. These perforated plates were placed in the riser at 600, 1000, 1400, and 1800 mm from the solids feed point. Table 1 gives the details of the
Table 1. Details of Material and Plate Geometry Used in the Present Studya material
dp (µm)
Fs
Ug
Gs(max)
sand sand resin resin silica gel FCC
412 177 530 385 384 81
2650 2650 1480 1480 676 900
3.5-5.0 3.0-4.0 3.5-4.5 3.0-4.0 2.7-3.3 2.7-3.3
42 36 26 30 20 35
a Type of baffle: perforated plate. Baffle perforation: 3 mm. Baffle free area: 30%, 40%, and 45%.
materials, the plate geometry, and range of variables covered in the present study. The suspension density, (1 - )Fs, estimated from the measured solids holdup and the measured pressure drop, ∆P/L, is found to satisfy the following relationship:
∆P/L ) W/AL ) (1 - )Fs + Fg
(1)
The frictional pressure drop and the accelerational effects are neglected. Results and Discussion Figure 2 shows typical variation of the total pressure drop in the conventional CFB without baffles with solids and gas flow rates. It can be ascertained from the figure that the variation in the pressure drop with an increase in solids rate is a sigmoidal curve. At low solids rate, the pressure drop increases slowly, corresponding to pneumatic transport, and at high solids rate, it approaches an asymptotic value, corresponding to conventional fluidization. Figure 3 shows a typical variation of the pressure drop along the length of the riser in the conventional CFB without baffles. The initial portion, corresponding to the lower portion of the riser, is the higher holdup region with high solids concentration while the upper portion of the riser with low solids concentration is the lower holdup region. It is noticed from the figure that, depending on solids flow rate, the pressure drop (and
2550 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 2. Variation of the pressure drop with solid and gas flow rates in the conventional CFB.
Figure 5. Variation of total pressure drop with solids flow rate at different gas flow rates in the modified CFB.
Figure 3. Variation of pressure drop along the length of the riser in the conventional CFB.
Figure 4. Variation of the pressure drop along the length of the riser for conventional and modified CFBs at different solids flow rates.
hence the solids concentration) can be very different in the two regions. The extent of higher holdup region visa`-vis the lower holdup region, as well as the solids concentration in each of the regions, depends on the flow rates of the phases, the particle size, and its density. The variation in solids concentration along the riser of the conventional CFB causes axial variation in bed characteristics including the contact time between the phases, which makes scale-up uncertain with respect to the hydrodynamics of circulating fluidized beds. To reduce or eliminate the inhomogeneity in solids concentration along the riser length, horizontal perforated plates were placed in the riser 400 mm apart. Figure 4 shows a typical variation of the pressure drop along the length of the riser with solids flow rate and gas flow rate when perforated plates of 30% free area are used in the riser. It is seen from the figure that the pressure drop increases with an increase in the
solids rate. It is further noticed from the experimental data that the solids holdup in the modified CFB is higher than that in the conventional CFB for the given flow rate of phases and particle characteristics. This is due to the additional resistance caused by the horizontal perforated plates to the flow of solids, resulting in longer residence time and higher holdup for solids. It is also noticed from the figure that the pressure drop (and hence suspension density) is uniform along the length of the riser for the given solids and gas flow rates versus its variation from a high to a low value in the conventional CFB. This is particularly attractive since, by a choice of gas and solids flow rates, the solids concentration as well as the loading ratio (viz. solids-to-gas ratio) may be maintained uniform and at the desired values along the length of the riser. Figure 5 shows the variation of total pressure drop with solids circulation rate for different free areas of the perforated plates. An increase in the plate free area decreases the solids holdup in the modified CFB. It is attempted in the present study to investigate the axial variation in solids concentration within the stage itself. Figure 6 shows typical variation for 30%, 40%, and 45% plate free area. It is noticed that, although the pressure drop does not vary from stage to stage for the entire length of the riser, it decreases from bottom to top within the stage when 30% free area plates are used. This variation within the stage is, however, negligible for plates having 40% and 45% free area. The pressure drop decreases from the bottom to the top of the riser for the plates having 67.6% free area. A similar observation was also noticed by Jiang et al. (1991). A close examination of the data reported for the conventional CFB reveals that, at very low solids rate, the solids concentration is almost uniform from the bottom to the top of the riser or the system operates under a pneumatic conveying region. Jiang et al. (1991) noted a similar linearity extended to a slightly higher solids rate when ring baffles were provided in the riser. The provision of horizontal perforated plates in the riser
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2551 Table 3. Comparison of Characteristic Velocities for Conventional CFB and Modified CFB with Transport Velocities and Terminal Velocities for the Particles Uo in eq 3 (m/s) material sand sand resin resin silica gel
Figure 6. Variation of pressure drop along the length within a stage for different free areas of perforated plates. Table 2. Extent of Axial Uniformity within the Stage in the Modified CFB Gs/Gg ratio modified material
dp (µm)
conventional
30%
45%
sand sand resin resin silica gel
412 177 530 385 384
1.25 1.76 1.53 1.88 1.27
2.23 2.46 2.06 2.18 1.89
3.79 3.98 3.88 4.01 3.25
of the CFB permits the use of a higher solids-to-gas flow ratio during the operation without encountering axial variation in the solids concentration either within the stage or from state to stage. Following are the results apparent from these studies. At low solids rate, the solids concentration decreases from the bottom to the top of the riser, albeit linearly along the riser. The provision of ring baffles improves the situation and permits the use of a higher solids rate. The ring baffles, however, are not sufficient to give uniform solids concentration for the entire length of the riser. Further, at high solids rate, the conventional CFB and the CFB with ring baffles show significant axial variation in solids concentration. These drawbacks are eliminated when perforated plates are provided in the riser. Additionally, the experimental data of the present study suggest that the axial uniformity in the solids concentration within the stage of modified CFB depends on the solids-to-gas flow ratio, the free area of the plates, and the material characteristics. This is shown in Table 2 for the range of materials, the flow rate of the phases, and the free area of the plates covered in this study. The provision of horizontal perforated plates in the riser of CFB increases solids holdup and the mean holding time of solids for a given solid rate. This implies that the desired contact time between the phases is achieved at a lower solids rate compared to the corresponding value in conventional CFB. Additionally the horizontal perforated plates give axial uniformity in solids concentration within the riser. This may be attributed to the following: 1. The presence of perforated plates disrupts and limits the solids downflow near the wall to the stage height. 2. Turbulent eddies formed due to the horizontal plates extend into the interplate distance and give radial uniformity in solids concentration, eliminating the predominant core flow for the gas.
conventional modified dp (µm) Ut (m/s) Utr (m/s) CFB CFB 412 177 530 385 384
3.43 1.47 2.99 2.17 1.28
3.20 2.50 3.00 2.55 2.35
2.95 2.29 3.16 2.57 2.29
3.20 2.69 3.30 2.60 2.50
3. The solids clusters are broken into individual particles as they pass through the perforated plates, and the process improves the contact between the phases. The performances of the conventional CFB and the modified CFB are analyzed using the slip velocity concept. The slip velocity is defined as
Us )
Ug Ud 1-
(2)
The slip velocity and bed porosity are related as
Us ) Uon
(3)
where n ) -1.5 for the conventional CFB and n ) -0.6 for the modified CFB. The slip velocity as defined in eq 2 represents the true relative velocity between the particle and the medium in a medium of finite population of particles. However, it differs in a medium of infinite particle population. The slip velocity is smaller than Uo if the particles are retarded due to the other particles or due to the wall. On the other hand, Uslip is higher than Uo if the particles have a tendency to form agglomerates and attain a higher velocity than that for a single particle. Uslip/Uo is the ratio of true fall velocity of the particle in a finite medium to particle velocity in an infinite medium. This is related to bed voidage which accounts for the buoyancy effect (Balasubramaniam and Srinivasa kannan, 1997). Uslip, predicted using eq 2, is plotted against bed voidage . The slope of the plots remains the same for all the materials while the intercept (Uo) is found to vary with material characteristics (Table 3). From the present analysis it is noticed that Uo is found to correspond to transport velocity Utr. The transport velocity is the true transition velocity from a turbulent fluidized bed to a circulating bed, in a medium of a multiparticle system, which is obtained for the present experimental data using a linear extrapolation technique (Schnitzlein and Weinstein, 1988). The lower value for n and the close correspondence of Uo to the transport velocity for the CFB with horizontal perforated plates, compared to the value for conventional CFB, suggest less bypassing of phases in the former mode of operation. High free area plates offer negligible pressure drop for air flow, compared to the pressure drop due to the solids; the distribution of air at each plate is uniform with no pressure fluctuations during operation. With a decrease in the plate free area, plate resistance for the flow of phases increases, a solid bed forms over each plate, and the entire riser operates as a multistage fluidization column (Srinivasa kannan et al., 1994). Likewise, too close a plate placement gives higher pressure drop due to the plates alone, while too large a
2552 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
plate placement is likely to create axial variation in solids concentration within each stage. Summary and Conclusions The operation of a conventional circulating fluidized bed gives rise to a dense region at the lower portion and a dilute region at the upper portion of the riser. The axial variation in solids concentration in the riser can be minimized using horizontal perforated plates in the riser. Perforated plates limit the annular downflow of solids to the stage height and simultaneously disturb the core upflow of gas to bring in radial uniformity in fluid and particle concentrations. Further, the horizontal perforated plates minimize the formation of clusters for improved contact between the phases. The provision of perforated plates increases the solids holdup and hence the mean holding time of solids within the riser. The increase in either of them, however, depends on the free area of the plate; the smaller the free area, the larger will be the increase in solids holdup. However, with plates of low free area, the solids concentration varies axially within the stage itself; this is in spite of the fact that the solids concentration remains uniform from stage to stage. The axial uniformity in solids concentration within the stage depends on solids-to-gas flow ratio, the free area of the plate, and the material characteristics. Nomenclature A ) cross-sectional area of the column, m2 dp ) particle size range, µm Gs ) solids circulation rate, kg m-2 s-1 Ud ) superficial solid velocity, m s-1 Ug ) superficial gas velocity, m s-1 Us ) slip velocity, m s-1 Uo ) characteristic velocity, m s-1 Ut ) terminal velocity, m s-1 Utr ) transport velocity, m s-1 L ) riser length, m S ) free area, % W ) solids holdup, kg z ) axial coordinate, m ∆L ) distance between the pressure taps, m ∆P ) pressure drop for the entire bed, kg m-2 ) bed porosity Fg ) density of the gas, kg m-3 Fs ) density of the solid particle, kg m-3
Literature Cited Balasubramaniam, N.; Srinivasa kannan, C. Slip velocity characteristics in the riser of the circulating fluidised beds. Chem. Eng. Technol. 1997, 20, 491. Hartge, E. U.; Li, Y.; Werther, J. Analysis of local structure of the two-phase flow in a fast fluidised bed. In Circulating Fluidised Bed Technology; Basu, P., Ed.; Pergamon Press: Toronto, Canada, 1986; p 153. Jiang, P.; Bi, H.; Jean, R. H.; Fan, L. S. Baffle effects on performance of catalytic circulating fluidised bed reactor. AIChE J. 1991, 37, 1392. Li, Y.; Kwauk, M. The dynamics of fast fluidised bed. In Fluidisation; Grace, J. R., Matson, J. M., Eds.; Plenum Press: New York, 1980; p 537. Rhodes, M. J.; Laussmann, V. S.; Geldart, D. Measurement of radial and axial solids flux variation in the riser of a circulating fluidised bed. In Circulating Fluidised Bed Technology; Basu, P., Ed.; Pergamon Press: Toronto, Canada, 1988; p 155. Schnitzlein, M. G.; Weinstein, H. Flow characterisation in high velocity fluidised beds using pressure fluctuations. Chem. Eng. Sci. 1988, 43, 2605. Srinivasa kannan, C.; Subba Rao, S.; Varma, Y. B. G. A study on stable range of operation in multistage fluidised beds. Powder Technol. 1994, 78, 203. Van der Ham, A. G. J.; Prins, W.; Van Swaaij, W. P. M. Hydrodynamics of pilot plant scale regularly packed circulaing fluidised bed. AIChE Symp. Ser. 1993, 89, 53. Van der Ham, A. G. J.; Prins, W.; Van Swaaij, W. P. M. A small scale regularly packed circulating fluidised bed: Part I Hydrodynamics. Powder Technol. 1994a, 79, 17. Van der Ham, A. G. J.; Prins, W.; Van Swaaij, W. P. M. A small scale regularly packed circulating fluidised bed: Part II Mass transfer. Powder Technol. 1994b, 79, 29. Weinstein, H. M.; Shao, J.; Schnitzlein, M. Radial variation in void fraction in high velocity fluidisation. In Fluidisation V; Basu, P., Osteragard, K., Sorenson, A., Eds.; Engineering Foundation: New York, 1986; p 329. Yerushalmi, J.; Cankurt, N. T. Further studies on regimes of fluidisation. Powder Technol. 1979, 24, 189. Yerushalmi, J.; Avidan, A. High velocity fluidisation. In Fluidisation; Davidson, J. F., Clift, R., Harrison, D., Eds.; Academic Press: London, 1985; pp 225-291. Zheng, C.; Dong, Y.; Zhang, W.; Zhang, J. Impact of internal components on radial distribution of solids in circulating fluidised bed. Inst. Chem. Metall. 1990, 11 (4), 296.
Received for review May 19, 1997 Revised manuscript received January 8, 1998 Accepted February 22, 1998 IE970358C
