Hydrodynamics of a Multistage Countercurrent Fluidized Bed Reactor

Aug 12, 2008 - In the present investigation, a multistage countercurrent gas-solid fluidized bed reactor with downcomer has been proposed. The basic ...
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Ind. Eng. Chem. Res. 2008, 47, 6917–6924

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Hydrodynamics of a Multistage Countercurrent Fluidized Bed Reactor with Downcomer for Lime-Dolomite Mixed Particle System C. R. Mohanty, Sivaji Adapala, and B. C. Meikap* Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721 302, West Bengal, India

In the present investigation, a multistage countercurrent gas-solid fluidized bed reactor with downcomer has been proposed. The basic operating parameters have been developed illustrating the mechanism of gas-solid contacting in the system. The system was operated in three stages in series one above the other in continuous regime for a two-phase system over a wide range of operating conditions including varying the weir height in order to determine the pressure drop. The data were generated under steady and stable operation of the column. The solids holdup of the proposed multistage system is higher than the single-stage system, which results in better mass-transfer characteristics. The results in this study assume importance from the standpoint of design and steady operation of a multistage fluidized bed reactor for control of gaseous pollutants. The fluidizing gas was air, and the solid phase was a mixture of dolomite and lime in varying proportions. Introduction Fluidized bed reactors are widely used in industry for different gas-solid operations such as drying, adsorption, calcinations, and combustion of coal. Sometimes baffles are used to partition the reactor to overcome certain disadvantages associated with single-stage fluidization.8,11The provision of internal baffles not only facilitates the continuous cocurrent or countercurrent flow of the phases at near plug flow condition but also establishes the temperature and concentration gradients along the length of the bed, in addition to limiting the formation and growth of bubbles. Besides these, this also offers other advantages such as reduction in axial mixing of phases, higher transfer rates, and reduction in size of equipment. This explains why multistage fluidized bed reactors are preferentially used for chemical applications as well as for flue gas treatment in industry. These multistage fluidized beds can be categorized basing on the transfer of the solids from one stage to next stage below: (i) solids passing through perforated plates and (ii) solids passing through downcomer, which are simply empty tubes allowing the transfer of solids from an upper fluidized bed to lower one.7 In the former, the diameters of perforated plate holes are large enough to allow simultaneous flow of solids and gas through them. Enough literature is available with studies carried out on the multistaged fluidization without downcomer. On the other hand, the literature of the stagewise operation of the gas-solid reactor with downcomer is very insufficient. These types of multistage fluidized bed reactors with downcomer have recently gained importance for recovery of gaseous pollutants from flue gas as an air pollution control system in the industries. Although there are some commercial units utilizing the principles of stagewise operation of a fluidized bed reactor as an air pollution control unit, the available data are insufficiently instructive. Therefore, in the present study, a multistage countercurrent gas-solid fluidized bed reactor operating in three stages has been designed, constructed, and investigated so that the hydrodynamic as well as the mass-transfer characteristics could be improved without substantially increasing energy dissipation. The staging effect has been achieved by exploiting the hydrodynamics of two-phase flow without using complicated internals. An attempt has therefore been made to acquire precise knowl* To whom correspondence should be addressed: Tel.: +91 3222 283958. Fax: +91 3222 282250. E-mail: [email protected].

edge of the hydrodynamic of the two-phase flow in the system. The study has been carried out for a mixture of solids with varying proportions. This reactor has been developed for possible use as a high-efficiency air pollution equipment to remove gaseous pollutants such as sulfur dioxide gas from the flue gas emitted from the sulfuric acid plants and cupper smelters by means of sorption onto calcium oxide. Experimental Setup and Technique Characterization of Solid Particles (Determining Flow Properties). The fluidization of solids depends on the properties of solids and gas. The gas properties (namely, that air) are dependent on temperature and pressure and could be estimated easily from the literature. These properties include air density and viscosity. In the case of fluidization of solids, the main properties affecting fluidization are solid particle size, density, and porosity. Before experiments on fluidization of solids were conducted, the characteristics of solids were determined. The solid phase, such as a mixture of lime and dolomite at two different proportions, was chosen for fluidization to determine the hydrodynamic properties of gas-solid phases before using the solids for sorption of sulfur dioxide gas as the second part of the objectives. The usage of dolomite in solid mixture with lime is to avoid the choking and agglomeration due to the fine lime particles in the bed and which allows the free flow. The calcium oxide (CaO) has been used for control of gaseous pollutants in a single-stage fluidization by Chiang et al.1 The lime in lumpy form was obtained from commercially available raw materials and then crushed, and the different flow properties were determined following the proper methods. The properties of dolomite used were obtained in a manner very similar to that of lime. The methods to determine the properties were as follows: (i) Particle size (dp). It was determined by standard sieve analysis in B.S. sieves to the investigated size (0.211-0.50 mm) and kept in desiccators to avoid being deliquesced. The only sharp cut fractions were used in the experimentation. The average of the diameters of the sieves through which the solids passed and the sieve on which these retained was taken as diameter of the particles. The average particle size was found to be 426.0 µm. (ii) Density (Fs). It was determined by kerosene to ensure that the wettability of the lime and dolomite by the liquid does not affect the results. The density of lime was found

10.1021/ie8007693 CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008

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Figure 1

to be 2040 kg/m3 and that of dolomite was 2160 kg/m3. (iii) Porosity (s). The porosity or void fraction of a solid material of a definite size is the ratio of the void volume to the volume of the bed and gives the porosity of the bed. To ensure accuracy in fluidization experiments, the cylinder diameter was taken to be the same as the diameter of the column. The porosity was found to be 0.48 for both the solid particles. (iv) Minimum fluidization velocity (µmf). The minimum fluidization velocity was determined by the ERGUN equation for the given properties of solids, and the value is 0.112 m/s. Experimental Setup and Procedure. Figure 1 is the schematic of the multistage fluidized bed reactor developed and used in this study. The configuration of this staged gas-solid fluidized bed reactor is similar to that of the sieve trays distillation column. The reactor consists of a three-stage fluidization column (FB1, FB2, FB3) having provision of solid feeding from the top and air supply system from the bottom along with other auxiliary equipment used for experimentation. Each stage of the column was constructed of a Perspex cylinder of 0.10-m internal diameter and 0.305-m length. The stainless steel plates of 0.002 m thick each (G1, G2, G3) were used as internal baffles between two stages and each plate was drilled with perforations of 0.002-m diameter on a triangular pitch having 8.56% total grid openings.2,10 The perforations were made accurately, avoiding possible burrs and protrusions during drilling process. The grid plates were covered with fine wire mesh (100 mesh size) to prevent the solids from the falling through the openings. Each section was provided with a downcomer of Perspex cylinder of 0.025-m internal diameter (D1, D2, D3), and the downcomers were fitted to the gas distributors by special threading arrangement having a provision for adjusting the weir height as desired. The downcomers were further fitted with a cone at the exit end in order to reduce the upflow of the gas through the downcomer and, consequently, widening the stable operating range. Figure 2 is the schematic of the downcomer developed and used in this study. The bottomopening diameter of the cone was 0.012 m for flow of solids to

the next stage.5 Pressure tapings were provided just below the grid plate and the near the air outlet, and four manometers were provided to measure the pressure drop at every stage as well as the total pressure drop. A compressor (E) of 5.0 KW was used to supply the air as fluidizing gas and its flow rate was measured using a calibrated rotameter (F). A gas distributor of 0.150 m length was provided at the bottom of the column for uniform distribution of gas to the column. A conical hopper was attached at the bottom of the column for storage of solids coming out from the bottom stage through the downcomer. The gas leaving the column from the top stage was passed through a 0.15-m-diameter standard cyclone (C) and then into the exhaust system. A cloth bag was attached at the bottom of the cyclone to collect the fines, if any, carried over. A Perspex hopper of 0.150-m internal diameter and 0.4-m length called a feeding funnel was used to hold the lime and was attached to the screw feeder. The solids from the screw feeder was fed through a Perspex tube of 0.012-m internal diameter to the first-stage downcomer of the reactor. The screw feeder was fitted to a motor of 0.25 kW, and the speed of the motor was controlled by a variable rheostat. A compressor was used to provide the air through a predistributor underneath the last stage. The gas flow rate was adjusted by a rotameter. Necessary precautions were made to ensure that no air from outside intruded into the column during operation. The solids were fed into the column at a rate from 1.2 to 2.4 kg/h with gas flow rate varying from 0.2625 to 0.4725 m/s. The weir height of the downcomer was kept at 40, 50, and 60 mm, and the gap between the downcomer bottom and the grid plate were kept 20, 25, and 30 mm, respectively. The pressure drops across each stage and across the entire column were recorded. In the experiment, for a given apparatus condition, the effect of gas flow rate and solid flow rate on pressure drop was observed. Mechanism of Solids Flow in Multistage Fluidization. Figure 3 is the pictorial representation of flow conditions progressively existing on a multistage fluidization column, beginning with a fixed bed of lime and dolomite mixture of

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Figure 1. (a) Schematic diagram of the experimental setup of a three-stage countecurrent fluidized bed reactor. (b) Photographic view of the experimental setup of a three-stage countercurrent fluidized bed reactor.

particles with gas flowing upward, and then gradually changing over to the continuous operation in the column with gas and solid flowing countercurrently at constant flow rates. At very low gas flow rate, solids filled the downcomer and piled over the distributor plate of the top stage just below the downcomer. Air flow rate was then increased from the bottom of the column. As the gas rate was increased in small increments, the solids dispersed and started distributing in the top stage due to the difference in the solids gradient developed across the stage from the left (location of the upper downspout) to the right of the stage, resulting in the cross-flow of the fluidized solids on the stage. As the flow rate of gas was gradually increased, the fluidized particles flowed over the weir to the next lower stage,

as shown in Figure 3. The solids were then transferred from stage to stage fluidizing in each stage and finally transferred to the solids outlet storage. The operation of the column was usually smooth and steady when all the stages had sufficient material. Thus, the overall operation of the multistaged fluidized bed column may be considered to be countercurrent; there is apparently cross-fluidization of the solids on stages, with gas flowing in the upward direction and the fluidized solids flowing horizontally across the stage. Results and Discussion The results on the pressure drop with and without solids are discussed in this section.

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Figure 2. Schematic of the downcomer.

Figure 3. Schematic of solids flow from upper stage to next stage.

Pressure Drop in the Multistaged Fluidized Bed Reactor (without Solid Flow). Prior to the experiment carried out on the multistage column operating under countercurrent gas-solid flows, several pressure drop measurements were taken without any solids flow to determine pressure drop due to distributor plate and wall at different gas flow rates. The pressure drops across each stage and across the entire column were recorded. No noticeable difference in the pressure drop across the stage was noticed from stage to stage. It was observed that the pressure drop due to the distributor and wall across each stage was found to be almost same. The effect of superficial gas velocity (U0) on empty column pressure drop (PP + PW) across a single stage is given in Figure 4, which shows that increase in superficial gas velocity increases the pressure drop due to the distributor plate and wall. Similar behavior of empty column pressure drop with superficial gas velocity has been reported by Krishnaiah and Verma.4 Pressure Drop in the Multistaged Fluidized Bed Reactor (with Solid Flow). The system was operated with two varying proportions of lime and dolomite mixture such as (i) 25% dolomite and 75% lime and (ii) 50% dolomite and 50% lime. While operating the system with solids, it was observed that all the stages of the reactor were identical in their operation as well as performance and the pressure drops across each stage and across the entire column were recorded. No discernible difference in the pressure drop across the stage was noticed from

Figure 4. Effect of superficial gas velocity (U0) on empty column pressure drop (PP + PW).

stage to stage. In view of the identical performance, the pressure drop due to solids across each stage has been obtained from the difference between the pressure drop with and without solids. The variations of pressure drop for the first mixture of particles across a single stage under various flow conditions have been presented in Figures 5-7. Figures 5-7 describes the pressure drop due to the first mixture of solids, i.e., 25% dolomite and 75% lime mixture measured across each stage varying the gas, solid flow rates, and weir heights. It is seen from the figures that the pressure drop due to solids, Ps, decreases with increase in the gas flow rate and increases with increase in the solids flow rate. It is also seen from the figures that the pressure drop due to solids, Ps, decreases with increase in the gas flow rate and increases with increase in the solids flow rate. Similar observations were reported by Pillay and Varma,8 Krishnaiah and Verma,4 and Kannan et al.3 The minimum pressure drops occurred in the column at high gas flow rate (0.4725 m/s), corresponding to minimum solid flow rate (1.2 kg/h), and are 56.3, 66.1, and 79.7 N/m2 at 0.04-, 0.05-, and 0.06-m weir height, respectively. The maximum pressure drops occurred in the column at low gas flow rate (0.2625 m/s), corresponding to maximum solid flow rate (2.4 kg/h), and are 112.7, 135.2, and 151.6 N/m2 at 0.0-4, 0.05-, and 0.06-m weir height, respectively. It appears that the reason may be that an increase in gas rate increases the porosity of the bed in the system, resulting in decrease in the solids concentration and hence the pressure drops across the stage. The effect of gas velocity on pressure drop at different weir heights for varying solid flow rates is shown in Figure 5. From this figure it has been found that the pressure drop decreases with gas velocity at constant solid flow rate. The decrease in the pressure drop is due to the decrease in the bed height, which in turn decreases the resistance caused by the solids. The effect of gas velocity on pressure drop at different solid flow rates for varying weir heights is shown in Figure 6. It is clear from Figure 6 that the pressure drop increases with weir height due to an increase in the bed height, which offers more resistance toward gas flow. The effect of solid flow rate on pressure drop at different gas velocities for varying weir heights is shown in Figure 7. The increase in the solid flow rate increases the bed height depending on the variable weir height. As the bed height

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Figure 5. Effect of mass velocity of gas (Ug) on pressure drop (Ps) at (a) 40-, (b) 50-, and (c) 60-mm weir height for 25% dolomite and 75% lime.

Figure 6. Effect of weir height (hw) on pressure drop (Ps) at (a) 1.2, (b) 1.8, and (c) 2.4 kg/h solid flow rate for 25% dolomite and 75% lime.

increases. the resistance created to the gas flow rate by the bed thickness increases thereby increasing the pressure drop. However, the increase in the gas flow rate increases the solid flow rates to the underlying stages of the multistage fluidized bed reactor through the downcomer. Thus, the increase in the gas flow rate decreases the bed height, leading to the decrease in the pressure drop as shown in Figure 7. Figures 8-10 describe the pressure drop due to second mixture of solids, i.e., 50% dolomite and 50% lime measured across each stage varying the gas, solid flow rates, and weir heights. It has been observed from the figures that the pressure drop trend was similar to that of the first mixture of solids and the only difference is pressure drops are slightly more than that of first mixture. The reason for higher pressure drops with this mixture was higher proportion of dolomite, which is denser than lime where dolomite density is 2160 kg/m3 and that of lime is 2040 kg/m3. It is seen from the figures that the pressure drop due to solids, Ps, decreases with increase in the gas flow rate and increases with increase in the solids flow rate. It is also seen from the figures that the pressure drop due to solids, Ps,

decreases with increase in the gas flow rate and increases with increase in the solids flow rate. Similar observations were reported by Pillay and Varma,8 Krishnaiah and Verma,4 and Kannan et al.3 The minimum pressure drops occurred in the column at high gas flow rate (0.4725 m/s), corresponding to minimum solid flow rate (1.2 kg/h), and are 64.1, 71.9, and 83.6 N/m2 at 0.04-, 0.05-, and 0.06-m weir heigh,t respectively. The maximum pressure drops occurred in the column at low gas flow rate (0.2625 m/s), corresponding to maximum solid flow rate (2.4 kg/h), and are 143.8, 151.6, and 178.8 N/m2 at 0.04-, 0.05-, and 0.06-m weir height, respectively. It appears that the reason may be that an increase in gas rate increases the porosity of the bed in the system, resulting in decrease in the solids concentration and hence the pressure drop across the stage, as the height of the fluidized bed in the system corresponds to downcomer weir height. The effect of gas velocity on pressure drop at different weir heights for varying solid flow rates is shown in Figure 8. From this figure, it has been found that the pressure drop decreases with gas velocity at constant solid flow rate. The decrease in

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Figure 7. Effect of solid flow rate (Qs) on pressure drop (Ps) at (a) 40-, (b) 50-, and (c) 60-mm weir height for 25% dolomite and 75% lime.

Figure 8. Effect of mass velocity of gas (Ug) on pressure drop (Ps) at (a) 40-, (b)50-, and (c) 60-mm weir height for 50% dolomite and 50% lime.

the pressure drop is due to the decrease in the bed height, which in turn decreases the resistance caused by the solids. The effect of gas velocity on pressure drop at different solid flow rates for varying weir heights is shown in Figure 9. It is clear from Figure 9 that pressure drop increases with weir height due to increase in the bed height, which offers more resistance toward gas flow. The effect of solid flow rate on pressure drop at different gas velocities for varying weir heights is shown in Figure 10. The increase in the solid flow rate increases the bed height, depending on the variable weir height. As the bed height increases, the resistanced created to the gas flow rate by the bed thickness increases thereby increasing the pressure drop. However, the increase in the gas flow rate increases the solid flow rates to the underlying stages of the multistage fluidized bed reactor through the downcomer. Thus, the increase in the gas flow rate decreases the bed height, leading to the decrease in the pressure drop as shown in Figure 10.

The solids height in the downcomer found increases from 20 to 40 mm with solid flow rate from 1.2 to 2.4 kg/h, respectively, at constant gas flow rate and decreases from 40 to 15 mm with gas flow rate from 0.4725 to 0.265 m/s, respectively, at constant solid flow rate. The dolomite particles affect the pressure drop substantially for every operating variable in the multistage fluidized bed reactor. The maximum pressure drop occurred with the first set of particles, where dolomite proportion is 25%, and is 151.6 N/m2 at minimum solid flow rate (1.2 kg/h) and maximum gas flow rate (0.4725 m/s). If we consider the maximum pressure drop occurring with the second set of particles, where dolomite proportion is 50%, and is 178.8 N/m2 at the same operating conditions as in the previous case. The increase in pressure drop is due to the increase in dolomite ratio, where dolomite is denser than that of lime. For higher dolomite ratio, the choking is observed to be minimum in which the particles are freely flowing in the bed. Optimization is to be done on dolomite ratio

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Figure 9. Effect of weir height (hw) on pressure drop (Ps) at (a) 1.2, (b) 1.8, and (c) 2.4 kg/h solid flow rate for 50% dolomite and 50% lime.

Figure 10. Effect of solid flow rate (Qs) on pressure drop (Ps) at (a) 40-, (b) 50-, and (c) 60-mm weir height for 50% dolomite and 50% lime.

to get the required pressure drop in suitable range and at the same time choking should be minimized. Conclusions A multistage fluidized bed reactor have been designed and fabricated to use as an air pollution control device. An appropriate range of solid and gas flow rates was chosen for continuous operation of the multistage fluidized bed reactor with solid and gas flowing countercurrently under steady-state conditions. During steady operation, fluidized particles move across the stage to the next stage through the downcomer, as gas flows upward through the distributors followed by voidage of the solid particles. Flooding limits were obtained for the stable operation of the column. The maximum pressure drops occurring in the column at low gas flow rate (0.2625 m/s) corresponding to maximum solid flow rate (2.4 kg/h) are 143.8, 151.6, and 178.8 N/m2 at 0.04-, 0.05-, and 0.06-m weir height, respectively. The maximum pressure drop occurred in the column at low gas flow rate corresponding to maximum solid flow rate. The

maximum pressure drops were obtained for higher dolomite ratio mixture where choking was minimized. The hydrodynamic data presented in this study assume significance from the perspective of design and stable operation of staged fluidized bed reactors. Nomenclature A ) area of cross section of the column or bed (m2) D ) diameter of column (m) dp ) diameter of the particle (m) dw ) diameter of downcomer (m) F ) free area of distributor (%) G ) acceleration due to gravity (m/s2) Ug ) velocity of air (kg/ (m2 s)) Qs ) mass velocity of solids (kg/m2 s) hw ) weir height (m) 4p ) pressure drop across the bed (N/m2) Ps ) pressure drop due to solids across each bed (N/m2) Pp ) Pressure drop due to plate/distributor (N/m2) Pw ) Pressure drop due to wall (N/m2)

6924 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 Umf ) superficial velocity of air at the onset of fluidization (m/s) Uo ) superficial velocity of air at the empty condition (m/s) Us ) superficial velocity of solids (m/s) Greek symbols ε ) bed porosity Fg ) density of air (kg/m3) Fs ) bulk density of solid materials (kg/m3) Subscripts f ) fluid (air) mf ) minimum fluidization o ) superficial p ) distributor/plate s ) solid w ) wall

(3) Kannan, C. S.; Rao, S. S.; Varma, Y. B. G. A study of stable range of operation in multistage fluidized beds. Powder Technol. 1994, 78, 203. (4) Krishnaiah, K.; Verma, Y. B. G. Pressure drop, solids concentration and mean holing time in multistage fluidization. Can. J. Chem. Eng. 1982, 60, 346–352. (5) Krishnaiah, K.; Verma, Y. B. G. A downcomer for gas-solid multistage fluidized bed. Ind. J. Technol. 1988, 26, 265. (6) Kunni, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth- Heinemann: Boston, MA, 1991. (7) Martin-Gullon, I.; Marcilla, A.; Font, R.; Asensio, M. Stable operating velocity range for multistage fluidized bed reactors with downcomers. Powder Technol. 1995, 85, 193. (8) Pillay, P. S.; Varma, Y. B. G. Pressure drop and solid holding time in multistage fluidization. Powder Technol. 1983, 35, 223. (9) Pisani, R., Jr.; Moraes, D., Jr. Removal of sulfur dioxide with particles of dolomite lime stone powder in a binary fluidized bed reactor with bubbling fluidization. Br. J. Chem. Eng. 2003, 20, 95. (10) Sathiyamoorthy, D.; Horio, M. On the influence of aspect ratio and distributor in gas fluidized beds. Chem. Eng. J. 2003, 93, 151. (11) Verma, Y. B. G. Pressure drop of the fluid and the flow patterns of the phases in multistage fluidization. Powder Technol. 1975, 12, 167.

Literature Cited (1) Chiang, B. C.; Wey, M.; Yeh, C. Control of acid gases using a fluidized bed adsorber. J. Hazad. Mater. 2003, B101, 259. (2) Geldart, D. The design of distributors for gas fluidized beds. Pow. Tech. 1985, 42, 67.

ReceiVed for reView May 13, 2008 ReVised manuscript receiVed June 7, 2008 Accepted July 11, 2008 IE8007693