Effects of Geometric Parameters and Operating Conditions on

Nov 15, 2007 - of overflow gate, and stirrer on the deformation pattern of granular flow, the dynamic holdups, and residence time of particle in the r...
1 downloads 0 Views 362KB Size
Download PDF
Ind. Eng. Chem. Res. 2007, 46, 9263-9268

9263

Effects of Geometric Parameters and Operating Conditions on Granular Flow in a Modified Rotating Cone Guoxin Hu,* Xiwu Gong, Hao Huang, and Yanhong Li School of Mechanical and Power Engineering, Shanghai Jiaotong UniVersity, 200240, China

Experiments were performed to investigate granular flow in a modified rotating cone (MRC). The effects of geometric parameters and operating conditions, such as the cone rotational frequency, feed rate, dimension of overflow gate, and stirrer on the deformation pattern of granular flow, the dynamic holdups, and residence time of particle in the rotating vessel were examined. The results show that the existence of bed behavior as one active zone and one inactive zone is definitive for the granular flow in an MRC without a stirrer. Few particles are exchanged on the active-inactive interface while the cone rotational frequency is maintained during the running course. As the cone rotational frequency increases, the number of particles belonging to the active zone increases first, exhibits a maximum value, and decreases with increasing rotational frequency higher than the maximum value. The critical value of the rotational frequency is nearly independent of feed rate, and decreases with the increase of the dimension of overflow gates. The rotating vessel with a stirrer is helpful to enhance the granular fluidity and mixing rate. The particle holdups of the rotating vessel tend to increase with the increase of feed rate, or with the decrease of cone rotational frequency. The residence time of particles in the active zone is inversely proportional to the feed rate and cone rotational frequency. 1. Introduction To remedy the shortage of gas and electricity in many countries and the low utilization efficiency of coal, a gas-heatpower cogeneration technology has been developed, which combines coal pyrolysis and char combustion processes. The system consists of a fluidized bed combustion chamber (FBCC) and a pyrolysis reactor. High-temperature circulating solids from FBCC as the heat carrier are supplied into the pyrolysis reactor and provide the heat required for coal pyrolysis. The semi-coke produced in the pyrolysis reactor is introduced into the FBBC for further conversion. The heat produced in the FBBC is used to heat the circulating solids as well as to generate steam and power; in this way, gas, heat, and power cogeneration can be realized. For the gas-heat-power cogeneration technology, the pyrolysis process of coal is conducted usually in a fluidized bed, taking air, steam, or circulating coal gas as a possible fluidization medium, but this would make the process complicated.1 An extensive overview of existing biomass pyrolysis technologies is presented by Bridgwater and Bridge.2 All these reactor technologies require a large carrier gas stream, which reduces the thermal efficiency of the reactor, while larger downstream equipment is required. Recently, the rotating cone reactor type was developed by Wagenaar et al.3 in which feedstock particles are transported together with a heat carrier in a mechanical way, thus bypassing the need for carrier gas, while the construction is relatively simple. During the last decades the rotating cone technology has been drawing much attention.4-10 One of the most important applications is that the rotating cone reactor is applied to the flash pyrolysis of biomass to produce bio-oil. In such reactors, the biomass particles are heated rapidly and have a very short residence time (usually within several seconds) in the cone.11,12 Despite these promising advantages, the rotating cone reactor, which was applied for the flash pyrolysis of biomass, is not * To whom correspondence should be addressed. Tel./fax: +86 21 34206569. E-mail: [email protected].

suitable for the pyrolysis process of coal. For coal pyrolysis with a heat carrier in a moving packed or fixed bed, a dominant percentage of gas product is yielded during the first 1-3 min, although it would last for as long as 10 min.1,13,14 If the rotating cone reactor were employed for the pyrolysis process of large coal particles, the particles would move very rapidly through the reactor and have a very short residence time in the reactor. The conversion process of coal pyrolysis would be limited in the rotating cone. To obviate this disadvantage, a modified rotating cone reactor was developed by Shanghai Jiaotong University. The reactor is applied to the gasification and pyrolysis of coal. An upright ring wall with some overflow gates is introduced to set on the upper edge of the cone. The overflow gates are arranged symmetrically on the upright ring wall. A stationary flat-bladed stirrer placed above the inclined inner wall of the vessel is used to force particles to mix and flow across the vessel. For the granular flow in the rotating cone, they are forced to move upward along the cone wall in a spiral way. Most particles would be headed off by the upright ring wall, and cannot pass the edge of it; only a few particles can pass the overflow gate and be thrown out from the designed cone. Therefore, the residence time of particles remaining in the rotating vessel would be prolonged. To understand the knowledge of granular flow performance in such a modified rotating cone reactor, a real-time observation system was used to record the granular flow and mixing process, and experiments were conducted to examine the effects of geometric parameters and operating conditions, such as the cone rotational frequency, feed rate, and dimensions of the overflow gate and stirrer on the deformation pattern of granular flow, the dynamic holdups, and the residence time of particles in the rotating vessel. 2. Background Figure 1 illustrates the flow of particles in the modified rotating cone. Particles fed near the center of the bottom plate of the vessel will be forced by the centrifugal action to move

10.1021/ie071014h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/15/2007

9264

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007

Figure 1. Schematic diagram of particle flow in the rotating cone.

Figure 2. Zones of particle flow in three different rotating cones.

upward along the cone wall in a spiral way. Most of them would be headed off by the upright ring wall, and cannot pass the edge of it; only a few particles can pass the overflow gate or the edge of upright ring wall and be thrown out from the designed cone, so the residence time of particles in the rotating cone would be prolonged. The structure of the granular bed is an important issue about this modified rotating cone. Shown in Figure 2 is a schematic representation of the structure constructed from our own experimental results using the real-time observation technique and experimental results of other workers.15-17 For the modified rotating cone without a stirrer, the granular bed can be divided into two zones, namely an “inactive” zone where particles are carried up by the vessel wall and move as a rigid body with negligible mixing, and an “active” zone where particle mixing occurs while the particles are forced by the centrifugal action to move upward on the “inactive” particle zone in a spiral way. In a conventional rotating cone, three hydrodynamic flow regimes exist for flow over the cone wall, where the forced flow regime, the free flow regime, and the slide flow regime appear sequentially with the increase of cone angle or rotational frequency.11 In the free flow regime, the centrifugal force is sufficiently high to transport particles in an upward direction along the cone wall by itself, so particles move in plug flow without an interparticle stress in the r-direction (no solids pressure), which is called the “active” zone. In the forced flow regime, particle flow can only be induced by exerting an external force on the particles in the r-direction, which results in an interparticle stress. Particles in this regime would not be mixed and flow out from the vessel if the external force ceased to zero, and this is in correspondence to the “inactive” zone. Let “I” represent the mass ratio of particles in the “active” zone to that in the whole rotating vessel. There would be three operating modes according to the value of I: (1) for I ) 0, the active zone is absent in this mode; (2) for I ) 1, all the particles in the rotating vessel belong to the active zone; (3) 0 < I < 1

Figure 3. Schematic diagram of the experimental system.

describes most situations of the experiment in which all particles are separated into two portions, the active zone and the inactive zone. 3. Experimental Procedures A schematic diagram of the experimental system is shown in Figure 3. The modified rotating cone reactor consists of the cone, upright ring wall, stirrer, rotating shaft, overflow gate, etc. The rotation cone, 45° in the half cone top angle θ, is fixed at a revolution shaft driven by the electromotor. The height of the cone upright ring wall is 100 mm. The maximum diameter and bottom diameter are 300 mm and 100 mm, respectively, as shown in Figure 3. A stationary flat-bladed stirrer installed above the inclined wall of the cone spans nearly the entire length of the inclined cone wall, and forms an angle with the rotation axis. Apparently, it can exert an effort to force particles to mix and flow across the vessel. In this experiment, the cone with no overflow gate or one, two, or four overflow gates (height 60 mm and width 60 mm) arranged symmetrically are used respectively. Silica gel particles, mung beans, and ash particles are used as the test materials in this study. Their physical properties are presented in Table 1.

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9265

Figure 4. Particle flow at different operating times in the rotating cone (t, time; rotational frequency n ) 10 Hz). Table 1. Physical Properties of the Test Materials material

length (mm)

breadth (mm)

height (mm)

bulk density, Fb (kg m-3)

particle density, Fs (kg m-3)

interparticle friction factor11

silica gel ash particle mung bean

5.578 5.453 5.256

4.127 3.751 4.068

3.639 3.556 3.888

586 773 722

1037 1486 1431

0.38 0.78 0.30

In order to observe the granular flow in the rotating vessel, an observation system, which consists of the modified rotating cone, electromotor, shoe plate, support frame, camera, feeder machine, etc., is used. As shown in Figure 3, the rotation shaft connects the electromotor and the shoe plate. The cone is fixed at the middle of the shoe plate, which is arranged on the support frame. A Canon digital camera, which is installed on the upper part of the support frame, is placed above the cone. The stationary feed pipe is focused to the core of the cone, and at 300 mm from the bottom of the cone. As the electromotor works, the whole setup, including the shoe plate, cone, support frame, and camera, will rotate together. The particles supplied by the feed pipe will flow in the modified rotating cone. Through the camera, the real-time pictures of particle mixing and transportation are captured. The total uncertainty mainly comes from the measurement of holdups and feed rate and the reproducibility of the results. The holdups are measured by the electronic balance. Its minimum accuracy is 5 g. The feed rate can be read from the feeder machine with an uncertainty about 2%. To investigate the reproducibility of the results in the experiment, replicates are made of randomly selected experiments. From these tests the reproducibility values for the holdups and feed rate were within 4.6% and 5.9% of their respective reported values. Through repeated experiments, it is possible to control the error in the range that is permitted. 4. Results and Discussion Experiments were carried out for the modified rotating cone with or without a stirrer at room temperature and atmospheric pressure. For every condition, some parameters, such as the cone rotational frequency, feed rate, and dimension of overflow gate, are altered to investigate the granular flow or the change of dynamic holdups and residence time of the particles. During the course of an experiment, particles supplied by the feed pipe enter the vessel and, rotating, finally flowed out from the

overflow gate or the upper edge of the upright ring wall. For a steady running of revolution, the net mass of mixing particles in the active zone, which can be thrown out from the rotating vessel, is considered as dynamic holdups. The ratio of dynamic holdups to feed rate in certain experimental parameters is defined as the residence time of a particle in the rotating vessel. 4.1. Granular Flow without Stirrer. For the modified rotating cone without a stirrer, the bed behaviors were examined. The experiment shows that no particle can flow out from the modified rotating cone if the rotational frequency is smaller than 2 Hz. This is because the centrifugal force is too small to overcome the friction and other resistance force. Under this condition, particles fill the whole volume of the vessel. The whole bed is a dead zone where particles are carried up by the vessel wall and move as a rigid body with no mixing. To observe the structure of the particle bed for the modified rotating cone with no overflow gate, photographs were taken during real-time observation of the mixture of bed materials at different operating times with a Canon digital camera, shown in Figure 4. Two kinds of granules, silica gel and mung bean, of which the main difference appears to be the color, were used as the feed materials. During the experiment, the granules of silica gel are fed into the rotating vessel through the feed pipe by a feeder machine. When it runs steadily, the transient holdups of granules in the rotating vessel reach dynamic balance while the flow-out rate is equal to the feed rate. Then, the feeding is stopped until no silica gel flows out from the rotating vessel. Obviously, Figure 4a shows that quite an amount of silica gel lay still on the bottom or wall of the vessel and could not flow out of it when the feeding interruption lasted more than 10 s. Under this condition, particles are carried up by the vessel wall and collectively rotate as a rigid body with no mixing, which is the inactive zone. At t ) 14 s, mung bean instead of silica gel is fed into the rotating vessel while the cone rotational frequency is maintained. Figure 4b shows that an active zone presents on the inactive

9266

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007

zone of silica gel as the granules of mung bean move and flow out of the vessel. At t ) 45 s, the flow-out rate is equal to the feed rate for the active zone. At t ) 55 s, the feeding is again stopped. The active zone gradually vanishes as the granules of mung bean flow out of the vessel, shown in Figure 4d,e. At t ) 130 s, almost no particles flow out from the rotating vessel (Figure 4f), and a thorough inactive zone of silica gel is viewable again by throwing out the “active” particles from the upper part of the rotating vessel. The experimental results demonstrate clearly that the transport performance as one active zone and one inactive zone exists for the granular flow in the modified rotating cone. In observing those granules flowing out from the rotating vessel after the mung beans are fed, almost no silica gel but mung beans are found in the off-load materials, indicating that a few particles are exchanged on the active-inactive interface during the running course while keeping the cone rotational frequency. If the running time after the feeding stop is longer than the residence time of the particles in the active zone, theoretically, all of them in the active zone have the chance to overflow from it, whereas the granules of the inactive zone would remain in it. On the basis of what was mentioned above, quantitative experiments were conducted to measure the mass of particles in the active and inactive zones for the modified rotating cone with or without overflow gates. The mass ratio of particles in the active zone to that in the whole rotating vessel is quantitatively demonstrated in Figure 5. It can be seen obviously that the value of I increases with the increase of feed rate. As the cone rotational frequency increases, the value of I increases first and exhibits its maximum value, and then decreases with increase of the cone rotational frequency higher than the maximum value. This can be explained by the analysis of force balance. When the cone rotational frequency is increased, the centrifugal force can become high enough to overcome the other resistance. The number of active particles in the rotating vessel increases. On the other hand, the frictional force that prevents particle movement would rise along with the increase of cone rotational frequency. Sooner or later, the frictional force increases to gain balance with the momentum that makes particles move. At that moment, if the cone rotational frequency increases further, the mass ratio of particles in the active zone to that in the whole vessel would tend to reduce. The critical value of the cone rotational frequency for the maximum value of I is 12-13 Hz. As shown in Figure 5, the critical value of the rotational frequency is nearly independent of the feed rate, and is about 8 Hz for two overflow gates and 6 Hz for four overflow gates. This fact indicates that, with the increase of the dimension of overflow gates (or number of overflow gates), the critical value of the cone rotational frequency decreases, whereas the number of particles in the active zone increases under the same operating conditions. It was found that the active zone depth or its number of particles is minimum for the vessel without overflow gates compared with having overflow gates on the upright ring wall of the vessel. Presumably, the inactive zone would never vanish for the granular flow in the modified rotating cone, which results in a value of I always less than 1. As a consequence, the existence of bed behavior as one active zone and one inactive zone is definitive for the modified rotating cone in the experimental conditions used in our work. As the dimension of overflow gates enlarges to the limiting case in correspondence with the conventional rotating cone, no inactive zone would present even though the cone rotational frequency is larger than the critical value.11 As shown in Figure 6, the value of I is equal

Figure 5. Influence of cone rotational frequency and feed rate on the mass of particles in the active zone.

to 1 as the whole granular bed is changed into a free flow regime for the conventional rotating cone. 4.2. Effect of Stirrer on Granular Flow. In general, an inactive zone that is the unmixed parts of granular bed would be persistent in the modified rotating cone without a stirrer. Therefore, a stirrer should be adopted to ensure good mixing and transportation of particles. The effects of the stirrer on the dynamic holdups and residence time at different feed rates and rotational frequencies were examined. With the help of a stationary flat-bladed stirrer, the granules formerly in the inactive zone would be forced to move into the

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 9267

Figure 6. Bed behavior diagram for granular flow in the rotating cone.

Figure 9. Influence of feed rate on holdups for the rotating cone with two overflow gates and a flat-bladed stirrer.

Figure 10. Influence of feed rate on residence time for the rotating cone with one overflow gate and a flat-bladed stirrer.

Figure 7. Influence of stirrer on holdups for the rotating cone with two overflow gates.

Figure 11. Influence of feed rate on residence time for the rotating cone with two overflow gates and a flat-bladed stirrer.

Figure 8. Influence of feed rate on holdups for the rotating cone with one overflow gate and a flat-bladed stirrer.

active zone, which means that the whole granular bed is changed into one active zone through the action of the stirrer, so that all of them have the chance to flow out of the modified rotating cone. The effect of the stirrer on dynamic holdups at different rotating frequencies and feed rates are given in Figure 7. It can be seen from Figure 7 that the dynamic holdups of the rotating cone with a stirrer are obviously smaller than that with no stirrer in the vessel. The reason is that the relative turning movement

between the stationary flat-bladed stirrer and the granular bed once per revolution causes the enhancement of granular fluidity and mixing rate, which results in a shortened residence time of particles in the vessel. According to Figures 8 and 9, the dynamic holdups increase with the increase of feed rate and decrease with the increase of rotational frequency. This is because the higher the cone rotational frequency is, the stronger the centrifugal force would be, and the more particles can flow out from the overflow gate, which results in a decrease of the dynamic holdups. Figures 10 and 11 show the influences of feed rate and rotational frequency on the residence time of particles in the modified rotating cone with the stirrer, respectively. It can be seen that the residence time of particles in the rotating vessel tends to decrease with the increase of feed rate, as well as with

9268

Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007

the cone rotational frequency. Observed from the experimental data of the vessel with one overflow gate (Figure 8) and two overflow gates (Figure 9), at the same feed rate and rotational frequency, the dynamic holdup of the single overflow gate is more than that of two overflow gates. Therefore, the residence time of particles in conditions of one gate is larger than that of two gates. Comparing these data with that in the literature,13,14 obviously, the residence time of particles is prolonged due to welding the upright ring wall on the upper edge of the conventional cone 5. Conclusion (1) The existence of bed behavior as one active zone and one inactive zone is definitive for granular flow in the modified rotating cone without a stirrer. Few particles are exchanged on the active-inactive interface while keeping the cone rotational frequency during the running course. There exists a critical value of cone rotational frequency; under the critical value, the number of particles belonging to the active zone increases with the increase of cone rotational frequency, and above the critical value, it decreases with the increase of cone rotational frequency. (2) For the modified rotating cone without a stirrer, under all operating conditions, the critical value of the rotational frequency is nearly independent of feed rate, and decreases with the increase of the dimension of overflow gates. If the dimension of overflow gates enlarges to the limiting case in correspondence with the conventional rotating cone, no inactive zone would present even though the cone rotational frequency is larger than the critical value as the whole granular bed is changed into one free flow regime. (3) For the modified rotating cone with a stirrer, the active zone is formed by the whole granular bed due to the action of the stirrer, which causes the enhancement of granular fluidity and mixing rate. The particle holdup increases with the increase of feed rate or the decrease of rotational frequency. The residence time of particles in the rotating vessel tends to decrease with increasing feed rate, as well as the cone rotational frequency. The holdups and the residence time of particle are inversely proportional to the dimension of the overflow gate. Acknowledgment This work was supported by the National Science Foundation of China under Grant 50376033. Nomenclature I ) ratio of particle mass n ) rotational frequency of cone (Hz) Q ) dynamic holdup (kg) t ) operating time (s)

T ) residence time (s) Um ) feed rate (kg min-1) Literature Cited (1) Hu, G.; Fan, H.; Liu, Y. Experimental Studies on Pyrolysis of Datong Coal with Solid Heat Carrier in a Fixed Bed. Fuel Process. Technol. 2001, 69, 221-228. (2) Bridgwater, A. V.; Bridge, S. A. A Review of Biomass Pyrolysis and Pyrolysis Technologies. Biomass Pyrolysis Liquids Upgrading and Utilisation; Elsevier: New York, 1991. (3) Wagenaar, B. M.; Prins, W.; van Swaaij, W. P. M. Pyrolysis of biomass in the rotating cone reactor: modeling and experimental justification. Chem. Eng. Sci. 1994, 49, 5109-5126. (4) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Recycling of polyethene and polyprolene in a novel bench-scale rotating cone reactor by high-temperature pyrolysis. Ind. Eng. Chem. Res. 1998, 37, 2293-2300. (5) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Development of a continuous rotating cone reactor pilot plant for the pyrolysis of polyethene and polypropene. Ind. Eng. Chem. Res. 1998, 37, 2316-2322. (6) Wagenaar, B. M.; Kuipers, J. A. M.; van Swaaij, W. P. M. Particle dynamics and gas-phase hydrodynamics in a rotating cone reactor. Chem. Eng. Sci. 1994, 49, 927-936. (7) Noui-Mehidi, M. N.; Bouabdallah, A. Hydrodynamics between a rotating cylinder and a fixed cone. Modell., Measure. Control B: Solid Fluid Mech. Thermics, Mech. Syst. 1994, 54, 55-63. (8) Beunder, E. M.; van Olst, K. A.; Rem, P. C. Shape separation on a rotating cone. Int. J. Miner. Process. 2002, 67, 145-160. (9) Anilkumar, D.; Roy, S. Unsteady mixed convection flow on a rotating cone in a rotating fluid. Appl. Math. Comput. 2004, 155, 545-561. (10) Roy, S.; Anilkumar, D. Unsteady mixed convection from a rotating cone in a rotating fluid due to the combined effects of thermal and mass diffusion. Int. J. Heat Mass Transfer 2004, 47, 1673-1684. (11) Janse, A. M. C.; Biesheuvel, P. M.; Prins, W.; van Swaaij, W. P. M. Granular flow in a rotation cone partly submerged in a fluidized bed. AIChE J. 2000, 46, 499-508. (12) Janse, A. M. C.; Biesheuvel, P. M.; Prins, W.; van Swaaij, W. P. M. A novel interconnected fluidized bed for the combined flash pyrolysis of biomass and combustion of char. Chem. Eng. J. 2000, 76, 77-86. (13) Guo, R.; Yang, J.; Liu, Z. Behavior of trace elements during pyrolysis of coal in a simulated drop-tube reactor. Fuel 2004, 83, 639643. (14) Carsky, M.; Kuwornoo, D. K. Neural network modelling of coal pyrolysis. Fuel 2001, 80, 1021-1027. (15) Ding, Y. L.; Forster, R.; Seville, J. P. K.; Parker, D. J. Granular motion in rotating drums: bed turnover time and slumping-rolling transition. Powder Technol. 2002, 124, 18-27. (16) Wightman, C.; Muzzio, F. J. Mixing of granular material in a drum mixer undergoing rotational and rocking motions I. Uniform particles. Powder Technol. 1998, 98, 113-124. (17) Stewart, R. L.; Bridgwater, J.; Parker, D. J. Granular flow over a flat-bladed stirrer. Chem. Eng. Sci. 2001, 56, 4257-4271.

ReceiVed for reView July 25, 2007 ReVised manuscript receiVed September 18, 2007 Accepted September 20, 2007 IE071014H