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Experimental investigation of a gas–solid rotating bed reactor with static geometry
Chemical Engineering and Processing 50 (2011) 77–84
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Experimental investigation of a gas–solid rotating bed reactor with static geometry Rahul P. Ekatpure a , Vaishali U. Suryawanshi a , Geraldine J. Heynderickx a,∗ , Axel de Broqueville b , Guy B. Marin a a b
Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 (S5), B-9000 Ghent, Belgium Allée du bois de Bercuit, 109, B 1390 Grez-Doiceau, Belgium
a r t i c l e
i n f o
Article history: Received 6 November 2009 Received in revised form 15 November 2010 Accepted 25 November 2010 Available online 1 December 2010 Keywords: Centrifugal force Gas–solid flow Rotating bed Stable flow Static geometry Tangential Vortex
a b s t r a c t Hydrodynamics of a gas–solid rotating bed reactor (RBR) in static geometry are investigated. Tangential injection of a gas at mass flow rate of 0.4–1 kg/s in a cylindrical vessel with a diameter of 0.54 m generates a rotating gas phase flow field. Introduction of solid particles in this field results into an annular dense gas–solid rotating bed. A stable annular gas–solid rotating bed without solids particles loss is achieved over a wide range of operating conditions. Goal of the presented work is to investigate, by means of experiments, the window for a stable operation of the gas–solid RBR, as a function of the solid particle diameter and density, the geometry of the RBR and the gas flow rate. If the solid particle diameter is comparable to tangential gas injection slot width, the establishment of a stable flow is delayed due to an increased slugging tendency. The upper limit of the solids content is found to decrease with decreasing solid particle diameter. Obtained experimental cold flow results are the initial steps in assessing the potentials of a RBR as an efficient gas–solid processing reactor from a process intensification point of view. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Process intensification (PI) in chemical processes has gained considerable interest in recent years. The presented PI work uses centrifugal force to carry out multiphase operation, more specifically the contacting of a gas and a solid phase. A centrifugal field can be induced in two ways: (a) rotating the operating vessel itself and (b) the tangential injection of a primary phase (gas or liquid) into a static vessel. The lack of rotating reactor parts makes the latter option the most attractive one. If the tangential injection of the primary fluid is carried out at multiple positions on the circumference of a cylindrical vessel, a strong vortex in the fluid and a centrifugal force on the fluid are created in the vessel. This centrifugal force can be used to support a secondary phase (solid particles or liquid droplets) inside the static cylindrical vessel. The primary phase flows through the secondary phase towards the center of the vessel and is discharged through an outlet located on the vessel axis. By using gas as a primary phase and the solid particles as a secondary phase, an annular rotating gas–solid bed in a cylindrical vessel is achieved. Such a rotating gas–solid bed offers a higher slip
∗ Corresponding author. Tel.: +32 92644516; fax: +32 92644999. E-mail address: [email protected] (G.J. Heynderickx). 0255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.11.010
velocity (∼1–10 m/s) as compared to the gravitational field, accompanied by a higher solids volume fraction (∼0.3–0.6). As compared to gravitational static fluidized beds, rotating beds offer advantages such as high throughput operation, uniform gas–solid contacting and enhanced heat and mass transfer, thus intensifying the process at particle scale as well as reactor scale [1]. As a result, a reactor with a rotating gas–solid flow can potentially be used for a number of processes such as combustion of nuclear fuels [2], drying [3–5], fast gas–solid catalytic reactions [6], fluidization of nano-sized particles [7], granulation and coating of fine particles [8] and removal of NOx and soot from a diesel engine exhaust [9] and a number of HiGee applications [11–13]. The presented work is carried out in view of assessing the potential of gas–solid rotating beds as an efficient gas–solid contacting device to be used in the chemical process industry. As an initial step, a detailed fluid dynamic study of the gas–solid RBR behavior is carried out. A number of past fluid dynamic studies [10,15] has been dedicated to investigate the effect of different operating parameters on the performance of the RBR. The performance of the RBR can be evaluated by studying the behavior of some of its flow characteristics, such as pressure drop, PTotal , bed void fraction, εg , velocity of rotating solid particles, vs , minimum and maximum solids content, Ws,min and Ws,max , respectively, for stable operation. An increase in the solids content of the RBR will increase the RBR’s capacity to
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process a relatively higher content of solid particles. However, the pressure drop will increase as well, resulting in higher processing costs. Moreover, a higher gas flow rate will be required to maintain the centrifugal force of the rotating bed. An increase in the feed gas flow rate will increase the slip velocity. The latter will further enhance the heat and mass transfer in the RBR. However, the solid particles rotation velocity will increase correspondingly, resulting in possible attrition of the solid particles. Table 1 shows some of the past experimental studies on gas–solid RBRs in which gas is introduced tangentially into a static reactor. It can be seen that most of these reactors have a diameter in the range of 0.1–0.3 m. The 0.54 m diameter RBR configuration in the present work thus is the experimental set-up with the largest reactor diameter so far. One of the main objectives of the past experimental studies has been to find the ‘optimal configuration’ of the RBR. Kochetov et al. [10] defined the optimal configuration in terms of the pressure drop and the solid particles content. According to Kochetov et al. [10], the reactor configuration is said to be optimal, when the RBR is able to operate at the highest solid particles content, and the lowest possible pressure drop. Kochetov et al. [10] studied the effect of the reactor configuration, specially the effect of the ratio of the reactor length to the reactor diameter, LR /DR , and the ratio of the central reactor outlet diameter to the reactor diameter, DE /DR , on the pressure drop and the maximum solids content of the RBR. It was found that for an optimal configuration of the RBR, the ratio of the reactor length to the reactor diameter should be less than 0.5 [2,3] while the ratio of the reactor outlet diameter to the reactor diameter should be in the range of 0.3–0.5. Furthermore, the configuration of the tangential gas inlet distributor should be such that the incoming gas flow is uniformly distributed all over the periphery of the RBR while it provides a sufficiently high gas injection velocity vg,inj and thus momentum. Kochetov et al. [10] found that tangential gas inlet slots located in the bottommost position of a horizontal axis operated configuration only, resulted into a higher maximum solids content. The shape of the RBR end wall significantly affects the structure of the vortex, thereby affecting the solid particles flow field. For instance, for plane end walls the radial gas velocity is inversely proportional to the radial position [4] whereas for hyperbolic end walls the radial gas velocity is found to be almost independent of the radial position. Thus in case of hyperbolic end walls, solid particles entrainment with the gas phase is reduced resulting into more stable flow [2,3]. Another approach to increase the stability and the maximum solids content of the RBR is to reduce the strength of the vortex in the core region, that is near the central reactor outlet. Anderson et al. [2] used additional gas injection nozzles along the periphery of the centrally positioned outlet. Folsom et al. [16] used screens with a mesh size smaller than that of a single particle diameter at the central outlet of the reactor. In another approach, De Wilde and de Broqueville [14] inserted a chimney with single/multiple tangential slots at the central outlet of the reactor to have more axial flow stability. Additional rotation of this chimney [18] resulted into a reduction of the amount of solid particles entrainment with the gas via the centrally positioned reactor outlet. Thus there has been some research on the effect of the geometrical configuration of the reactor and its operating conditions on the performance of the RBR. However, there is still a lack of a comprehensive experimental study trying to take into account the broad variation in the number of operating conditions for the RBR geometry under consideration. More specifically there are no sufficient data on the ‘window of stable operation’ of the RBR for given geometrical conditions. In semi-batch mode of operation, the RBR is said to be under stable operation once the solid particles in the reactor are uniformly rotating along the periphery of the reactor without channeling or slugging effects. Furthermore, no or negligible losses of solid particles should leave the reactor through the
central outlet in the gas phase. For a gas–solid flow dominated by ‘channeling effects’, the gas entering the RBR through the tangential inlet slots forms annular channels. The solid particles are unable to penetrate these channels and separate channels of solid particles are formed. Thus, both phases are moving along separate parallel paths, resulting in an extremely non-uniform flow pattern [19]. A slugging gas–solid flow pattern implies that both phases rotate in the form of discrete chunks, referred to as ‘slugs’. The size of these slugs is comparable to the characteristic dimension of the reactor. Thus both phases hardly meet, resulting in ‘bypassing’ and a reduced performance of the reactor [19]. The main objective of the presented work is to investigate this window of stable operation of the RBR as a function of various operating conditions, in particular the diameter dp and density p of the solid particles and the tangential slot thickness IO . The investigation is carried out by means of a highly flexible experimental set-up. Within the window of stable operation, operating conditions, such as the gas mass flow rate GM , the solid particles content Ws , and the tangential inlet slot thickness IO , are studied to evaluate the corresponding pressure drop over the RBR. 2. Experimental work 2.1. Experimental set-up The schematic diagram of the cold-flow experimental set-up with all components is shown in Fig. 1. The set-up consists of a blower for the supply of air, 4, a solid particles feeding device, 7, the RBR apparatus, 1, a cyclone, 2, a solid particles collection silo, 3 and an electronic balance, 10, for the weighing of the solids inventory in the RBR. The main body of the RBR, as shown in Fig. 2, consists of two concentric cylinders, that is the air inlet jacket and the inner cylindrical reactor body with the central gas outlet. A blowercontrolled mass flow of air enters into the jacket through a number of inlets uniformly distributed over the jacket periphery. Table 2 lists the geometrical configuration parameters of the present RBR set-up. Air from the reactor jacket is allowed to enter under a given injection angle into the inner cylindrical reactor body through a number of tangential slots equally spaced over the entire periphery of the reactor. The width of the tangential gas injection slots can be varied from 2 mm to 6 mm. The plane end walls of the reactor are made of polycarbonate glass (Makrolon® ), allowing the visual observation of the two-phase flow. The plane end wall with central gas outlet is considered as ‘front end wall (see Fig. 2)’ and the other one is treated as the rear end wall. High density polyethylene (HDPE) particles with a density of 950 kg/m3 are used in the experiments. Three different HDPE particle diameters have been used. For comparative purposes, the behavior of the fluid catalytic cracking (FCC) catalyst particles with a density of 1500 kg/m3 and average diameter of 70 m is investigated. Solid particles are fed from the volumetric screw feeder to the main reactor body through an inlet inclined at 30◦ towards the front end wall of the reactor (see Fig. 2). The solid particles flow rate from the screw feeder is regulated using a controller and a sealing rotary valve to avoid back-pressure from the RBR. 2.2. Experimental procedure All the experiments are performed in the semi-batch mode of operation under a horizontal axis of rotation. The range of operating conditions is listed in Table 3. Initially, only gas continuously enters the main reactor body through the jacket inlets and exits through the central opening of the main reactor body, in order to obtain a steady rotating gas flow. This gas outlet is an opening at the center of the front end wall of the reactor (Fig. 2). A cyclone is connected to
Table 1 Overview of past and present experimental studies of gas–solid rotating bed reactor with static geometry. Ref.
Reactor dimensions
Tangential gas distributor configuration Characteristics of solid particles
Gas flow rate and solids contenta
Remarks/conclusion
DE (m)
LR (m)
IN (m)
IO (m)
dp (m)
p (kg/m3 )
GM (kg/s)
Ws (kg)
[10]
0.12
0.03
0.035
1
0.02
250–2500
1050
0.027
0–1
Ws,max is measured as a function of GM , DE and tangential gas distributor configuration 0.3 < DE /DR < 0.5 and 0.08 < IO,tot/ DR < 0.12 is found to be optimal
[2]
0.24 0.44 0.305
0.06 0.11 0.15
0.035 0.128 0.063
5b 6 12
0.004 0.0034 0.0003
20, 12, 10
2700, 7000, 19100
0–0.2
0.1–1
[16]c
0.305
0.041
0.035
–
–
1000, 3175
2500, 1140
0.04
– 0.15
Air injection nozzles at the central outlet More stable flow is observed with increase in GM. vs is found to decrease with increasing Ws Higher concentration of solid particles near the central outlet Increase in vg,inj along with increases in GM result in higher Ws,max Ptotal decreases with introduction of solid particles in the rotating gas flow
−4
[17]
0.16
0.03, 0.054, 0.12
0.20
18
8 × 10
40,160
2320, 2590
0.06
[4]
0.2
0.05
0.026
Aslots = 0.0322 AR
200–1000
1900
0.28
0–1
3500, 8000
1200
[14]
0.36
0.12
0.135
12
0.004
2000, 300
950, 2100
0.24
0–2 0–1.5
[18]
0.24
0.16
0.115
24
0.0023
2000
950
0.29
0–2
Present work
0.54
0.15
0.1
36
0.002–0.006
2400, 1600 900
950
0.48–1.08
0–7
a b c
Bed rotation velocity linearly increases with increase in vginj and GM Static chimney at outlet. Under identical operating conditions polymer solid particles were observed to form uniform rotating bed whereas alumina particles were found to be entrained through central outlet Rotating chimney at outlet Solid particles losses through chimney are observed Stable flow without any special device at outlet for wider range of operating conditions vg,inj at the constant GM has significant impact on reactor hydrodynamics
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DR (m)
Maximum values are reported. Located only in the lower part of the reactor. Liquid solid RBR.
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Fig. 1. Schematic diagram of gas–solid rotating bed reactor with static geometry setup: (1) main reactor body; (2) cyclone; (3) solid particles collection bin; (4) blower; (5) gas mass flow control system; (6) screw feeder; (7) solid particles mass flow control system; (8) high speed digital camera; (9) lamp; (10) electronic balance; (11) back pressure valve; (12) secondary air supply; (13) tangential gas distributor.
Fig. 2. Front view and side view of the main body of the gas–solid rotating bed reactor with static geometry showing stable gas–solid rotating flow.
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Table 2 Geometrical specification of gas–solid rotating bed reactor with static geometry. Geometrical variable
Value
Unit
Jacket diameter (DJ ) Main reactor diameter (DR ) Exhaust diameter (DE ) Reactor length (LR ) Number of jacket inlets (IJ ) Number of tangential inlet slots (IN ) Diameter of each jacket inlet (dJ,inlet ) Diameter of solid particles feeding (dsolids, inlet ) Width of tangential gas injection slot (IO ) Length of tangential gas injection slot (IO,L ) Injection angle ()
0.68 0.54 0.15 0.10 12 36 0.059 0.016 0.002; 0.006 0.10 10
m m m m – – m m m m
the downstream of the gas outlet. Solid particles in the gas stream are separated from the gas in this cyclone. Clean gas is discharged into the atmosphere. Next, the required amount of solid particles is fed to the RBR using the screw feeder. The latter results into an annular rotating dense bed of solid particles. The stability of the rotating annular gas–solid flow is the most critical aspect regarding the performance of the RBR. Depending upon the operating conditions, it takes only a few seconds to establish a stable uniform annular gas–solid rotating bed. Real time weighing of entrained solid particles during an experiment is performed to correctly determine the amount of solid particles in the reactor at each moment. The gas–solid flow behavior is studied using pressure measurements and visual observation. A pressure probe (P1), 14 (as shown in Fig. 1) which is connected to the pressure sensor is mounted on the wall of the jacket. At the end of an experiment, the solid particles in the RBR are removed through a solid particles outlet located at the lower part of the front end wall of the reactor and connected to the solid particles collection silo. In the present work, the flow behavior of the RBR is studied by varying the tangential slot thickness and the particle diameter. The tangential slot thickness represents a characteristic scale of the gas phase, while the particle diameter represents a similar characteristic scale of the solid phase. A few results of varying the particle density are also discussed. Experiments are performed for a tangential slot thickness of 6 mm (resulting in a gas injection velocity of 18–45 m/s) and a tangential slot thickness of 2 mm (resulting in a gas injection velocity of 55–135 m/s). For a given gas mass flow rate, the value of the centrifugal force in the RBR can thus be adapted by changing the gas velocity at the tangential slot by changing the tangential slot thickness. The corresponding minimum and maximum solids content, allowing a stable flow in the RBR, are determined. Within this window of operation, the solid particles content is varied and the corresponding pressure drop is determined.
Table 3 Range of operating conditions for gas–solid rotating bed reactor with static geometry. Operating variable
Value
Unit
Air mass flow rate (GM ) Solid material Particle density (p ) Particle size (dp )
0.48–1.2 HDPE FCC 950, 1500 2.4 1.6 0.9 0.07 101325 0.5–7
kg/s – kg/m3 mm mm mm mm Pa kg
Outlet pressure (Pout ) Mass of solid particles in RBR (Ws )
Fig. 3. (a) Time evolution of total pressure drop over the rotating bed reactor with static geometry; reactor configuration as listed in Table 2; GM = 0.87 kg/s; solid particles feeding started at t = 10 s and stopped at t = 350 s; IO = 6 mm; () dp = 2.4 mm; () dp = 1.6 mm; () dp = 0.9 mm. (b) Zooming in (a) to determine Ws,min and Ws,max .
3. Results and discussion 3.1. Minimum and maximum solids content (Ws,min , Ws,max ) During the time evolution of the gas–solid rotating bed, it is observed that the solid particles start to build up from one end wall of the RBR that is the end wall through which they are introduced into the reactor. As a result, the initial rotation of the solid particles is significantly non-uniform, characterized by channeling effects [14]. If the RBR configuration is not optimal with respect to the solid particles characteristics, the solid particles may rotate in the form of slugs giving rise to a non-uniform gas–solids contact and extensive gas bypassing. In the optimized RBR configuration, when there is a sufficient amount of solid particles in the reactor, also referred to as ‘minimum solids content, Ws,min ’, the gas–solid bed starts to rotate uniformly. Upon further addition of solid particles, the thickness of the solid particles bed starts to increase and the solid particles rotation velocity may increase or decrease depending on the inlet gas mass flow rate and corresponding gas velocity at the tangential slot. With an increasing thickness of the gas–solid bed in radial direction, there is a change in the relative magnitude of the centrifugal force and the radial drag force. As soon as the centrifugal force becomes smaller than the corresponding radial drag force, solid particles start to exit through the centrally positioned reactor outlet along with the gas phase until the point at which both forces become balanced again. The corresponding solids content in the reactor is referred to as ‘maximum solids content, Ws,max ’ of the RBR [10]. The solids content limits can be determined by the visual observation as well as by monitoring the total pressure drop over the RBR. Fig. 3 shows pressure drop over the RBR as a function of time for all three HDPE particle diameters used to perform experiments, viz. 2.4 mm; 1.6 mm and 0.9 mm. During the initial 10 s, only gas is allowed to flow in the RBR. When only gas phase is present in the
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RBR it forms a Rankine vortex like structure. The motion of the gas phase in such case can be described by:
vgt r k = const
(1)
In the peripheral region k ∼ 0.5–0.6 depending on the geometrical configuration of RBR, while k ∼ (−1) in the central region of the reactor [20]. In principle, if k > 0.5, the centrifugal force increases faster than the drag force, with decreasing radial position r. With a rotating solid particles bed in the RBR, there will be a gas vortex in the central core region of the RBR. Therefore, if solid particles are entrained into this vortex, they are likely to return to the free board of the rotating bed, provided the vortex is large enough. Next, the solid particle feeding is started at a mass flow rate Gs of 0.012 kg/s. For all three particle diameters the solid particles are continuously fed till t equals 350 s. From Fig. 3a it is clear that, as soon as the solid particles enter into the rotating gas flow field, the pressure drop suddenly drops [4,19]. As the solid particles enter into the RBR, the flow field inside the cylindrical reactor changes completely. The gas entering the RBR via the tangential inlet slots has to impart momentum to the solid particles. As a result, the velocity of gas phase is reduced. Furthermore, the rotating layer of solid particles acts like a distributor for the gas phase. As a result the gas flow entering the RBR via a given tangential inlet is dispersed in the presence of a rotating layer of solid particles. Thus the number of rotations made by the gas phase is reduced in the presence of the rotating solid particles, resulting into a decrease in the pressure drop. During the initial development of the gas–solid rotating bed, i.e. before the establishment of stable bed, the decrease in the pressure drop is found to be inversely proportional to the particle diameter. This is due to the fact that, as the particle size decreases, the specific surface area of the particle increases, thereby causing more reduction in the gas rotational speed. Thus, for larger particle diameters, the pressure drop over the RBR for a given solid particles content is higher before attaining the maximum solids content. In the present experimental set-up, all gas injection slots have the same length as the reactor length. On the other hand, solid particles are supplied from the solids feeder through an inlet at the front end wall of the RBR. When solid particles are thus allowed to flow from the solid particles feeder into the rotating gas flow field, annular channels are inevitably formed. A solid particles channel is observed at the front end wall while an annular gas channel is observed at the rear end wall of the RBR. Note that the central outlet is located in the front end wall of the RBR. For the two larger particle diameter (2.4 mm and 1.6 mm) slugging is also observed during the initial development of the rotating gas–solid bed. When the particles content in the RBR is sufficiently high, the non-uniform flow effects, such as channeling and/or slugging, disappear and a uniform stable flow is established. At this point the pressure drop is minimum and the solid particles content corresponds to the minimum solids content. Each experiment is repeated at least three times. A highly reproducible set of data is obtained during these experiments, with less than 3% error in the value of the solids content. Fig. 4 shows that, as the particle diameter increases, the minimum solids content increases. The stronger slugging tendency of larger particles may be at the origin of this behavior. For given operating conditions, the tangential slot thickness is found to have an effect on the minimum solids content for a given particle diameter as well. As the tangential slot thickness decreases a more optimal distribution of the gas phase results into a reduction of the channeling phenomena. However, next to the channeling phenomena observed for all three particle diameters, the slugging behavior has to be considered as well. When the tangential slot thickness is reduced from 6 mm to 2 mm, the tangential slot thickness becomes of the same order of magnitude as the larger particle diameters (2.4 mm and 1.6 mm). As a result, the slugging behav-
Fig. 4. Minimum solids content for various solid particle diameters; reactor configuration as listed in Table 2; GM = 0.75 kg/s; (), IO = 6 mm, p = 950 kg/m3 ; () IO = 2 mm, p = 950 kg/m3 ; (♦) IO = 2 mm, p = 1500 kg/m3 .
ior of these large particles in the rotating annular gas–solid bed increases and becomes more important than the channeling effect. As such, the minimum solids content increases with decreasing tangential slot thickness. However for the smaller particles (0.9 mm), the slugging behavior remains limited and the channeling effect remains the dominating effect. As a result, the minimum solids content decreases with decreasing tangential slot thickness. When the FCC catalyst particles with a density 1500 kg/m3 are used, a highly unstable gas–solid bed is formed for a tangential slot thickness of 6 mm. A large solid particles entrainment is observed, resulting in a fast emptying of the reactor. This is mainly due to the fact that the gas injection velocity is lower than 45 m/s at the given gas flow rate for a tangential slot thickness of 6 mm. However, when the tangential slot thickness is decreased to 2 mm, a stable bed of FCC catalyst particles is observed. The result of the minimum solids content in case of FCC catalyst particles is shown in Fig. 4. The observations are in-line with the description of the channeling and the characteristic dimension discussed in this section. For a given gas mass flow rate, when the mass of solid particles has increased beyond the minimum solids content, a stable gas–solid rotating bed can be observed. For a given solid particle content, the centrifugal force acting upon the rotating gas–solid bed is overcome by the radial drag force. As a result, the solid particles get carried away with the gas flow through the central outlet until both centrifugal force and radial drag force become balanced again. The mass of solid particles in the RBR corresponding to these conditions is the maximum amount of solid particles that can be retained by the RBR for the given gas mass flow rate and is referred as the maximum solids content of the RBR [10]. From Fig. 3b, showing the time evolution of the pressure drop, the maximum solids content can be determined for the different particle diameters. When the solid particles content equals the maximum solids content, a further feeding of solid particles does not result in a change of the pressure drop. The point in time at which the pressure drop becomes constant corresponds with the point in time at which it is observed that the solid particles start to get entrained by the gas through the central outlet. Due to the gravity effect and the horizontal axis of rotation of the reactor set-up, the solid particles are mainly entrained through the top-most position of the central outlet. Fig. 5 shows the maximum solids content for all the three particle diameters as a function of the gas mass flow rate and the gas injection velocity. For a given particle diameter and particle density an increase in the gas mass flow rate, with a corresponding increase in the gas injection velocity results in a slight increase in the maximum solids content. From a given gas mass flow rate, also referred to as ‘critical gas mass flow rate, GM,cr ’, the maximum solids content
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Fig. 5. Maximum solids content with varying gas mass flow rate; reactor configuration as listed in Table 2; () dp = 2.4 mm, p = 950 kg/m3 ; () dp = 1.6 mm, p = 950 kg/m3 ; () 0.9 mm, p = 950 kg/m3 ; (♦) p = 1500 kg/m3 ; filled symbols, IO = 6 mm; (open symbols) IO = 2 mm.
83
Fig. 6. Total pressure drop over the RBR with varying solids content; reactor configuration as listed in Table 2; GM = 0.9 kg/s; () dp = 2.4 mm; () dp = 1.6 mm; () 0.9 mm; (filled symbols) IO = 6 mm; (open symbols) IO = 2 mm.
3.2. Total pressure drop (Ptotal ) remains constant for a given particle diameter. As stated earlier, the maximum solids content corresponds to the point in time where the centrifugal force acting on the bed is balanced by the radial drag force. The centrifugal force is proportional to the square of the angular velocity of the solid particles times the radius. At constant angular velocity, it decreases with a decrease in the radial position in the reactor. The radial drag force, which counterbalances the centrifugal force, is proportional to the nth power of the radial slip velocity, where n varies between 1 and 2. With an increase in gas mass flow rate, n approaches 2. As a result an increase in the gas mass flow rate gives an increase in the maximum solids content, which eventually becomes independent of the gas mass flow rate. For a given gas mass flow rate and a given particle diameter, when the gas injection velocity is increased by a factor of about three by decreasing the tangential slot thickness from 6 mm to 2 mm, the corresponding maximum solids content increases by 20–50%. This effect becomes more prominent for larger particle diameters. A possible explanation is that for lower gas injection velocities, the proportionality exponent n of the radial drag force is smaller than for higher gas injection velocities. For a given gas mass flow rate and for a given reactor configuration, the maximum solids content is observed to decrease as the particle diameter decreases. As the particle diameter decreases, the specific surface area of the particles and hence the radial drag force increases. At the same time however the increase in the centrifugal force due to this decrease in particle diameter does not equal the increase in the radial drag force. As a result, when using smaller solid particles, the balance of the radial drag force and the centrifugal force is already accomplished for a smaller total mass of solid particles in the RBR. Under the investigated operating conditions, the maximum solids content distinctly differs when varying the particle diameter at a constant gas mass flow rate. Kochetov et al. [10], on the other hand, observed that the maximum solids content is almost independent of the particle diameter for a gas mass flow rate smaller than the critical gas mass flow rate. The latter may be due to the smaller reactor diameter of their experimental set-up (see Table 1) and the relatively large diameter of their particles, resulting in more dominant gravitational effects. Thus, for the given RBR configuration, the width of the ‘stable operation window’ narrows down with a decrease in particle diameter, an increase in particle density or an increase in tangential slot thickness. The width of the stable operation window can be increased by increasing the centrifugal force. In the present work this is performed by increasing the gas injection velocity as a consequence of a decreasing tangential slot thickness.
For a given gas mass flow rate and a given reactor configuration, the solids content in the RBR is varied and the pressure drop is measured, once the limits of the solids content for a stable operation are determined. As shown in Eqs. (2a) and (2b): Ptotal = PRBR, main + Poutlet + Pcyclone
(2a)
PRBR, main = Pinlet slots + Pstatic, bed + Ptangential, bed + Pvortex (2b) The total pressure drop over the RBR includes, • The pressure drop over the inlet slots due to the tangential injection of gas, Pinlet,slots . This contribution can rise to 20–30% of the total pressure drop. • The static pressure drop over the gas–solid rotating bed, Pstatic, bed . Depending on the height of the gas solid bed this value may contribute up to 50% of the total pressure drop. • The pressure drop due to the tangential momentum transfer, Ptangential, bed . • The pressure drop due to the gas phase vortex in the central region of the RBR, Pvortex . Fig. 6 compares the total pressure drop over the RBR at a gas mass flow rate of 0.9 kg/s, for all three HDPE particle diameters, and for a reactor configuration with a tangential slot thickness of 6 mm, resulting in a gas injection velocity of 35 m/s. For a tangential slot thickness of 2 mm a gas injection velocity of 105 m/s is determined. For a given reactor configuration and gas mass flow rate, the effective bed height in the RBR increases, as the solids content is increased. Due to this increasing bed height, the static pressure drop and hence the radial gas–solid drag force over the gas–solid bed increases. Simultaneously, there is a reduction in the number of gas phase rotations causing a reduction in the centrifugal force. This results into the reduction of the tangential pressure drop and the pressure drop due to the gas vortex. Due to these compensating actions, the total pressure drop can somewhat increase or decrease. At a constant gas mass flow rate and an invariant solids content, an increase in the gas injection velocity by decreasing the tangential slot thickness results in a significant increase in the total pressure drop. As evident from the pressure data for single phase gas phase flow in the RBR, an increase in the gas injection velocity by decreasing the tangential slot thickness significantly increases the number of gas phase rotations, thereby increasing the pressure drop due to
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vortex formation in the RBR. When the particle diameter decreases while all other conditions remain invariant, the total pressure drop increases due to the increased contribution of the static pressure drop over the rotating gas–solid bed.
LR
4. Conclusions
Ws
The flow stability in a gas–solid rotating bed reactor (RBR) has been studied in a flexible set-up with a relatively large diameter, including the effects of tangential slot thickness and particle diameter. For the first time, the limits of the solids content, in the RBR at different operating conditions are reported. It is found that the interaction between tangential slot thickness and particle diameter may introduce slugging effects in the gas–solid rotating bed, if both tangential slot thickness and particle diameter have comparable values. It is observed that bed stability is improved by decreasing the tangential slot thickness. An increase in the centrifugal force implies an increase in the stability of the annular rotating bed. Hence, a higher gas injection velocity or a higher centrifugal force, allows a higher maximum solids content that can be retained in the RBR for a stable flow condition. However, there is a critical gas mass flow rate above which the maximum solids content will no longer increase. Thus, the tangential slot thickness needs to be optimized in view of the particle diameter, the particle density, and the gas mass flow rate. A change in particle diameter changes the specific surface area of the solid particles, thereby changing the gas–solid drag force. As result, for a constant solids content in the RBR, the use of smaller solid particles results in a higher total pressure drop over the RBR. Thus, under given operating conditions, the use of smaller solid particles will result in higher maximum solids content. Further studies will be made to investigate the effect of other geometrical and operating parameters on the flow behavior in the RBR. This will result in a better understanding of the two-phase flow behavior in the RBR in view of its application at industrial scale and allowing reliable scale-up.
Ws,min
Appendix A. Nomenclature
DE DJ DR
diameter of central gas outlet (m) diameter of jacket (m) diameter of the rotating bed reactor with static geometry (m) dJ,inlet diameter of each jacket feeding line (m) dsolids, inlet diameter of solids feeding line (m) particle diameter (mm) dp FCC fluid catalytic cracking volumetric gas flow rate (m3 /s) GV GM gas mass flow rate (kg/s) critical gas mass flow rate (kg/s) GM,cr GS solid particles mass flow rate (kg/s) HDPE high density polyethylene number of inlets IN IO tangential inlet slot thickness (mm) IO,L length of tangential inlet slot (m)
P
v vg,inj
Ws,max
axial length of the rotating bed reactor with static geometry (m) pressure (Pa) velocity (m/s) gas injection velocity (GV /IO IN LR ) (m/s) mass of solid particles in the rotating bed reactor with static geometry (kg) minimum mass of solid particles required for stable flow (kg) maximum mass of solid particles retained under stable flow (kg)
Greek symbols ε volume fraction density (kg/m3 ) injection angle (◦ ) References [1] H. Zhao, L. Shao, J.F. Chen, High-gravity process intensification technology and application, Chem. Eng. J. 156 (3) (2010) 588–593. [2] L.A. Anderson, S.H. Hasinger, B.N. Turman, Two-component vortex flow studies of the colloid core nuclear rocket, J. Spacecraft 9 (1972) 311–317. [3] L.M. Kochetov, B.S. Sazhin, E.A. Karlik, Experimental determination of the optimal ratios of structural dimensions in the whirl chamber for drying granular materials, Invest. Calculation Thermochem. Processes 2 (1969) 106–108. [4] E.P. Volchkov, V.I. Terekhov, A.N. Kaidanik, A.N. Yadykin, Aerodynamics and heat and mass transfer of fluidized particle beds in vortex chambers, Heat Trans. Eng. 14 (1993) 36–47. [5] A.S. Belousov, B.S. Sazhin, Application of guided vortex breakdown for drying and separation of the powder in vortex cyclone, in: Proc. 2nd Nordic Drying Conf., Copenhagen, 2003, pp. 475–479. [6] M.A. Goldshtik, F. Hussain, R.J. Yao, The vortex liquid piston engine and some other vortex technologies, Sadhana 22 (1998) 323–367. [7] H. Nakamura, S. Watano, Fundamental particle fluidization behavior and handling of nano-particles in a rotating fluidized bed, Powder Technol. 183 (2008) 324–332. [8] S. Watano, H. Nakamura, K. Hamada, Y. Wakamatsu, Y. Tanabe, R.N. Dave, R. Pfeffer, Fine particle coating by a novel rotating fluidized bed coater, Powder Technol. 141 (2004) 172–176. [9] A. Tsutsumi, W. Ju, K. Yoshida, Reduction of NO in diesel exhaust by soot using a centrifugal fluidized bed, AIChE Symp. Ser. 92 (1996) 86–90. [10] L.M. Kochetov, B.S. Sazhin, E.A. Karlik, Hydrodynamics and heat exchange in vortex drying chambers, Invest. Calculation Thermochem. Processes 9 (1969) 10–11. [11] D.G. Kroger, E.K. Levy, J.C. Chen, Flow characteristics in packed and fluidized rotating beds, Powder Technol. 24 (1979) 9–18. [12] C.C. Lin, Y.S. Chen, H.S. Liu, Prediction of liquid holdup in countercurrent-flow rotating packed bed, Chem. Eng. Res. Des. 78 (2000) 397–403. [13] G.Q. Wang, Z.C. Xu, Y.L. Yu, J.B. Ji, Performance of a rotating zigzag bed—a new HIGEE, Chem. Eng. Process. 47 (2008) 2131–2139. [14] J. De Wilde, A. de Broqueville, Rotating fluidized beds in a static geometry: experimental proof of concept, AIChE J. 53 (2007) 793–810. [15] E.P. Volchkov, N.A. Dvornikov, A.N. Yadykin, Characteristic features of heat and mass transfer in a fluidized bed in a vortex chamber, Heat Trans. Res. 34 (2003) 486–498. [16] B.A. Folsom, Two-phase flow in vertical and annular fluidized beds, California Institute of Technology, Ph.D. Dissertation, 1974. [17] J.J. Smulsky, The weighted layer of particles in the cylindrical vortex chamber, J. Appl. Chem. 8 (1983) 1782–1789. [18] J. De Wilde, A. de Broqueville, Experimental study of fluidization of 1G-Geldart D-type particles in a rotating fluidized bed with a rotating chimney, AIChE J. 54 (2008) 2029–2044. [19] J. De Wilde, A. de Broqueville, Experimental investigation of a rotating fluidized bed in a static geometry, Powder Technol. 183 (2008) 426–435. [20] B.S. Sazhin, L.M. Kochetov, A.S. Belousov, Retention capacities and flow patterns of vortex contactors, Theor. Found. Chem. Eng. 42 (2008) 125–135.
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Experimental investigation of a gas–solid rotating bed reactor with static geometry Rahul P. Ekatpure a , Vaishali U. Suryawanshi a , Geraldine J. Heynderickx a,∗ , Axel de Broqueville b , Guy B. Marin a a b
Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 (S5), B-9000 Ghent, Belgium Allée du bois de Bercuit, 109, B 1390 Grez-Doiceau, Belgium
a r t i c l e
i n f o
Article history: Received 6 November 2009 Received in revised form 15 November 2010 Accepted 25 November 2010 Available online 1 December 2010 Keywords: Centrifugal force Gas–solid flow Rotating bed Stable flow Static geometry Tangential Vortex
a b s t r a c t Hydrodynamics of a gas–solid rotating bed reactor (RBR) in static geometry are investigated. Tangential injection of a gas at mass flow rate of 0.4–1 kg/s in a cylindrical vessel with a diameter of 0.54 m generates a rotating gas phase flow field. Introduction of solid particles in this field results into an annular dense gas–solid rotating bed. A stable annular gas–solid rotating bed without solids particles loss is achieved over a wide range of operating conditions. Goal of the presented work is to investigate, by means of experiments, the window for a stable operation of the gas–solid RBR, as a function of the solid particle diameter and density, the geometry of the RBR and the gas flow rate. If the solid particle diameter is comparable to tangential gas injection slot width, the establishment of a stable flow is delayed due to an increased slugging tendency. The upper limit of the solids content is found to decrease with decreasing solid particle diameter. Obtained experimental cold flow results are the initial steps in assessing the potentials of a RBR as an efficient gas–solid processing reactor from a process intensification point of view. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Process intensification (PI) in chemical processes has gained considerable interest in recent years. The presented PI work uses centrifugal force to carry out multiphase operation, more specifically the contacting of a gas and a solid phase. A centrifugal field can be induced in two ways: (a) rotating the operating vessel itself and (b) the tangential injection of a primary phase (gas or liquid) into a static vessel. The lack of rotating reactor parts makes the latter option the most attractive one. If the tangential injection of the primary fluid is carried out at multiple positions on the circumference of a cylindrical vessel, a strong vortex in the fluid and a centrifugal force on the fluid are created in the vessel. This centrifugal force can be used to support a secondary phase (solid particles or liquid droplets) inside the static cylindrical vessel. The primary phase flows through the secondary phase towards the center of the vessel and is discharged through an outlet located on the vessel axis. By using gas as a primary phase and the solid particles as a secondary phase, an annular rotating gas–solid bed in a cylindrical vessel is achieved. Such a rotating gas–solid bed offers a higher slip
∗ Corresponding author. Tel.: +32 92644516; fax: +32 92644999. E-mail address: [email protected] (G.J. Heynderickx). 0255-2701/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.11.010
velocity (∼1–10 m/s) as compared to the gravitational field, accompanied by a higher solids volume fraction (∼0.3–0.6). As compared to gravitational static fluidized beds, rotating beds offer advantages such as high throughput operation, uniform gas–solid contacting and enhanced heat and mass transfer, thus intensifying the process at particle scale as well as reactor scale [1]. As a result, a reactor with a rotating gas–solid flow can potentially be used for a number of processes such as combustion of nuclear fuels [2], drying [3–5], fast gas–solid catalytic reactions [6], fluidization of nano-sized particles [7], granulation and coating of fine particles [8] and removal of NOx and soot from a diesel engine exhaust [9] and a number of HiGee applications [11–13]. The presented work is carried out in view of assessing the potential of gas–solid rotating beds as an efficient gas–solid contacting device to be used in the chemical process industry. As an initial step, a detailed fluid dynamic study of the gas–solid RBR behavior is carried out. A number of past fluid dynamic studies [10,15] has been dedicated to investigate the effect of different operating parameters on the performance of the RBR. The performance of the RBR can be evaluated by studying the behavior of some of its flow characteristics, such as pressure drop, PTotal , bed void fraction, εg , velocity of rotating solid particles, vs , minimum and maximum solids content, Ws,min and Ws,max , respectively, for stable operation. An increase in the solids content of the RBR will increase the RBR’s capacity to
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process a relatively higher content of solid particles. However, the pressure drop will increase as well, resulting in higher processing costs. Moreover, a higher gas flow rate will be required to maintain the centrifugal force of the rotating bed. An increase in the feed gas flow rate will increase the slip velocity. The latter will further enhance the heat and mass transfer in the RBR. However, the solid particles rotation velocity will increase correspondingly, resulting in possible attrition of the solid particles. Table 1 shows some of the past experimental studies on gas–solid RBRs in which gas is introduced tangentially into a static reactor. It can be seen that most of these reactors have a diameter in the range of 0.1–0.3 m. The 0.54 m diameter RBR configuration in the present work thus is the experimental set-up with the largest reactor diameter so far. One of the main objectives of the past experimental studies has been to find the ‘optimal configuration’ of the RBR. Kochetov et al. [10] defined the optimal configuration in terms of the pressure drop and the solid particles content. According to Kochetov et al. [10], the reactor configuration is said to be optimal, when the RBR is able to operate at the highest solid particles content, and the lowest possible pressure drop. Kochetov et al. [10] studied the effect of the reactor configuration, specially the effect of the ratio of the reactor length to the reactor diameter, LR /DR , and the ratio of the central reactor outlet diameter to the reactor diameter, DE /DR , on the pressure drop and the maximum solids content of the RBR. It was found that for an optimal configuration of the RBR, the ratio of the reactor length to the reactor diameter should be less than 0.5 [2,3] while the ratio of the reactor outlet diameter to the reactor diameter should be in the range of 0.3–0.5. Furthermore, the configuration of the tangential gas inlet distributor should be such that the incoming gas flow is uniformly distributed all over the periphery of the RBR while it provides a sufficiently high gas injection velocity vg,inj and thus momentum. Kochetov et al. [10] found that tangential gas inlet slots located in the bottommost position of a horizontal axis operated configuration only, resulted into a higher maximum solids content. The shape of the RBR end wall significantly affects the structure of the vortex, thereby affecting the solid particles flow field. For instance, for plane end walls the radial gas velocity is inversely proportional to the radial position [4] whereas for hyperbolic end walls the radial gas velocity is found to be almost independent of the radial position. Thus in case of hyperbolic end walls, solid particles entrainment with the gas phase is reduced resulting into more stable flow [2,3]. Another approach to increase the stability and the maximum solids content of the RBR is to reduce the strength of the vortex in the core region, that is near the central reactor outlet. Anderson et al. [2] used additional gas injection nozzles along the periphery of the centrally positioned outlet. Folsom et al. [16] used screens with a mesh size smaller than that of a single particle diameter at the central outlet of the reactor. In another approach, De Wilde and de Broqueville [14] inserted a chimney with single/multiple tangential slots at the central outlet of the reactor to have more axial flow stability. Additional rotation of this chimney [18] resulted into a reduction of the amount of solid particles entrainment with the gas via the centrally positioned reactor outlet. Thus there has been some research on the effect of the geometrical configuration of the reactor and its operating conditions on the performance of the RBR. However, there is still a lack of a comprehensive experimental study trying to take into account the broad variation in the number of operating conditions for the RBR geometry under consideration. More specifically there are no sufficient data on the ‘window of stable operation’ of the RBR for given geometrical conditions. In semi-batch mode of operation, the RBR is said to be under stable operation once the solid particles in the reactor are uniformly rotating along the periphery of the reactor without channeling or slugging effects. Furthermore, no or negligible losses of solid particles should leave the reactor through the
central outlet in the gas phase. For a gas–solid flow dominated by ‘channeling effects’, the gas entering the RBR through the tangential inlet slots forms annular channels. The solid particles are unable to penetrate these channels and separate channels of solid particles are formed. Thus, both phases are moving along separate parallel paths, resulting in an extremely non-uniform flow pattern [19]. A slugging gas–solid flow pattern implies that both phases rotate in the form of discrete chunks, referred to as ‘slugs’. The size of these slugs is comparable to the characteristic dimension of the reactor. Thus both phases hardly meet, resulting in ‘bypassing’ and a reduced performance of the reactor [19]. The main objective of the presented work is to investigate this window of stable operation of the RBR as a function of various operating conditions, in particular the diameter dp and density p of the solid particles and the tangential slot thickness IO . The investigation is carried out by means of a highly flexible experimental set-up. Within the window of stable operation, operating conditions, such as the gas mass flow rate GM , the solid particles content Ws , and the tangential inlet slot thickness IO , are studied to evaluate the corresponding pressure drop over the RBR. 2. Experimental work 2.1. Experimental set-up The schematic diagram of the cold-flow experimental set-up with all components is shown in Fig. 1. The set-up consists of a blower for the supply of air, 4, a solid particles feeding device, 7, the RBR apparatus, 1, a cyclone, 2, a solid particles collection silo, 3 and an electronic balance, 10, for the weighing of the solids inventory in the RBR. The main body of the RBR, as shown in Fig. 2, consists of two concentric cylinders, that is the air inlet jacket and the inner cylindrical reactor body with the central gas outlet. A blowercontrolled mass flow of air enters into the jacket through a number of inlets uniformly distributed over the jacket periphery. Table 2 lists the geometrical configuration parameters of the present RBR set-up. Air from the reactor jacket is allowed to enter under a given injection angle into the inner cylindrical reactor body through a number of tangential slots equally spaced over the entire periphery of the reactor. The width of the tangential gas injection slots can be varied from 2 mm to 6 mm. The plane end walls of the reactor are made of polycarbonate glass (Makrolon® ), allowing the visual observation of the two-phase flow. The plane end wall with central gas outlet is considered as ‘front end wall (see Fig. 2)’ and the other one is treated as the rear end wall. High density polyethylene (HDPE) particles with a density of 950 kg/m3 are used in the experiments. Three different HDPE particle diameters have been used. For comparative purposes, the behavior of the fluid catalytic cracking (FCC) catalyst particles with a density of 1500 kg/m3 and average diameter of 70 m is investigated. Solid particles are fed from the volumetric screw feeder to the main reactor body through an inlet inclined at 30◦ towards the front end wall of the reactor (see Fig. 2). The solid particles flow rate from the screw feeder is regulated using a controller and a sealing rotary valve to avoid back-pressure from the RBR. 2.2. Experimental procedure All the experiments are performed in the semi-batch mode of operation under a horizontal axis of rotation. The range of operating conditions is listed in Table 3. Initially, only gas continuously enters the main reactor body through the jacket inlets and exits through the central opening of the main reactor body, in order to obtain a steady rotating gas flow. This gas outlet is an opening at the center of the front end wall of the reactor (Fig. 2). A cyclone is connected to
Table 1 Overview of past and present experimental studies of gas–solid rotating bed reactor with static geometry. Ref.
Reactor dimensions
Tangential gas distributor configuration Characteristics of solid particles
Gas flow rate and solids contenta
Remarks/conclusion
DE (m)
LR (m)
IN (m)
IO (m)
dp (m)
p (kg/m3 )
GM (kg/s)
Ws (kg)
[10]
0.12
0.03
0.035
1
0.02
250–2500
1050
0.027
0–1
Ws,max is measured as a function of GM , DE and tangential gas distributor configuration 0.3 < DE /DR < 0.5 and 0.08 < IO,tot/ DR < 0.12 is found to be optimal
[2]
0.24 0.44 0.305
0.06 0.11 0.15
0.035 0.128 0.063
5b 6 12
0.004 0.0034 0.0003
20, 12, 10
2700, 7000, 19100
0–0.2
0.1–1
[16]c
0.305
0.041
0.035
–
–
1000, 3175
2500, 1140
0.04
– 0.15
Air injection nozzles at the central outlet More stable flow is observed with increase in GM. vs is found to decrease with increasing Ws Higher concentration of solid particles near the central outlet Increase in vg,inj along with increases in GM result in higher Ws,max Ptotal decreases with introduction of solid particles in the rotating gas flow
−4
[17]
0.16
0.03, 0.054, 0.12
0.20
18
8 × 10
40,160
2320, 2590
0.06
[4]
0.2
0.05
0.026
Aslots = 0.0322 AR
200–1000
1900
0.28
0–1
3500, 8000
1200
[14]
0.36
0.12
0.135
12
0.004
2000, 300
950, 2100
0.24
0–2 0–1.5
[18]
0.24
0.16
0.115
24
0.0023
2000
950
0.29
0–2
Present work
0.54
0.15
0.1
36
0.002–0.006
2400, 1600 900
950
0.48–1.08
0–7
a b c
Bed rotation velocity linearly increases with increase in vginj and GM Static chimney at outlet. Under identical operating conditions polymer solid particles were observed to form uniform rotating bed whereas alumina particles were found to be entrained through central outlet Rotating chimney at outlet Solid particles losses through chimney are observed Stable flow without any special device at outlet for wider range of operating conditions vg,inj at the constant GM has significant impact on reactor hydrodynamics
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DR (m)
Maximum values are reported. Located only in the lower part of the reactor. Liquid solid RBR.
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Fig. 1. Schematic diagram of gas–solid rotating bed reactor with static geometry setup: (1) main reactor body; (2) cyclone; (3) solid particles collection bin; (4) blower; (5) gas mass flow control system; (6) screw feeder; (7) solid particles mass flow control system; (8) high speed digital camera; (9) lamp; (10) electronic balance; (11) back pressure valve; (12) secondary air supply; (13) tangential gas distributor.
Fig. 2. Front view and side view of the main body of the gas–solid rotating bed reactor with static geometry showing stable gas–solid rotating flow.
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81
Table 2 Geometrical specification of gas–solid rotating bed reactor with static geometry. Geometrical variable
Value
Unit
Jacket diameter (DJ ) Main reactor diameter (DR ) Exhaust diameter (DE ) Reactor length (LR ) Number of jacket inlets (IJ ) Number of tangential inlet slots (IN ) Diameter of each jacket inlet (dJ,inlet ) Diameter of solid particles feeding (dsolids, inlet ) Width of tangential gas injection slot (IO ) Length of tangential gas injection slot (IO,L ) Injection angle ()
0.68 0.54 0.15 0.10 12 36 0.059 0.016 0.002; 0.006 0.10 10
m m m m – – m m m m
the downstream of the gas outlet. Solid particles in the gas stream are separated from the gas in this cyclone. Clean gas is discharged into the atmosphere. Next, the required amount of solid particles is fed to the RBR using the screw feeder. The latter results into an annular rotating dense bed of solid particles. The stability of the rotating annular gas–solid flow is the most critical aspect regarding the performance of the RBR. Depending upon the operating conditions, it takes only a few seconds to establish a stable uniform annular gas–solid rotating bed. Real time weighing of entrained solid particles during an experiment is performed to correctly determine the amount of solid particles in the reactor at each moment. The gas–solid flow behavior is studied using pressure measurements and visual observation. A pressure probe (P1), 14 (as shown in Fig. 1) which is connected to the pressure sensor is mounted on the wall of the jacket. At the end of an experiment, the solid particles in the RBR are removed through a solid particles outlet located at the lower part of the front end wall of the reactor and connected to the solid particles collection silo. In the present work, the flow behavior of the RBR is studied by varying the tangential slot thickness and the particle diameter. The tangential slot thickness represents a characteristic scale of the gas phase, while the particle diameter represents a similar characteristic scale of the solid phase. A few results of varying the particle density are also discussed. Experiments are performed for a tangential slot thickness of 6 mm (resulting in a gas injection velocity of 18–45 m/s) and a tangential slot thickness of 2 mm (resulting in a gas injection velocity of 55–135 m/s). For a given gas mass flow rate, the value of the centrifugal force in the RBR can thus be adapted by changing the gas velocity at the tangential slot by changing the tangential slot thickness. The corresponding minimum and maximum solids content, allowing a stable flow in the RBR, are determined. Within this window of operation, the solid particles content is varied and the corresponding pressure drop is determined.
Table 3 Range of operating conditions for gas–solid rotating bed reactor with static geometry. Operating variable
Value
Unit
Air mass flow rate (GM ) Solid material Particle density (p ) Particle size (dp )
0.48–1.2 HDPE FCC 950, 1500 2.4 1.6 0.9 0.07 101325 0.5–7
kg/s – kg/m3 mm mm mm mm Pa kg
Outlet pressure (Pout ) Mass of solid particles in RBR (Ws )
Fig. 3. (a) Time evolution of total pressure drop over the rotating bed reactor with static geometry; reactor configuration as listed in Table 2; GM = 0.87 kg/s; solid particles feeding started at t = 10 s and stopped at t = 350 s; IO = 6 mm; () dp = 2.4 mm; () dp = 1.6 mm; () dp = 0.9 mm. (b) Zooming in (a) to determine Ws,min and Ws,max .
3. Results and discussion 3.1. Minimum and maximum solids content (Ws,min , Ws,max ) During the time evolution of the gas–solid rotating bed, it is observed that the solid particles start to build up from one end wall of the RBR that is the end wall through which they are introduced into the reactor. As a result, the initial rotation of the solid particles is significantly non-uniform, characterized by channeling effects [14]. If the RBR configuration is not optimal with respect to the solid particles characteristics, the solid particles may rotate in the form of slugs giving rise to a non-uniform gas–solids contact and extensive gas bypassing. In the optimized RBR configuration, when there is a sufficient amount of solid particles in the reactor, also referred to as ‘minimum solids content, Ws,min ’, the gas–solid bed starts to rotate uniformly. Upon further addition of solid particles, the thickness of the solid particles bed starts to increase and the solid particles rotation velocity may increase or decrease depending on the inlet gas mass flow rate and corresponding gas velocity at the tangential slot. With an increasing thickness of the gas–solid bed in radial direction, there is a change in the relative magnitude of the centrifugal force and the radial drag force. As soon as the centrifugal force becomes smaller than the corresponding radial drag force, solid particles start to exit through the centrally positioned reactor outlet along with the gas phase until the point at which both forces become balanced again. The corresponding solids content in the reactor is referred to as ‘maximum solids content, Ws,max ’ of the RBR [10]. The solids content limits can be determined by the visual observation as well as by monitoring the total pressure drop over the RBR. Fig. 3 shows pressure drop over the RBR as a function of time for all three HDPE particle diameters used to perform experiments, viz. 2.4 mm; 1.6 mm and 0.9 mm. During the initial 10 s, only gas is allowed to flow in the RBR. When only gas phase is present in the
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RBR it forms a Rankine vortex like structure. The motion of the gas phase in such case can be described by:
vgt r k = const
(1)
In the peripheral region k ∼ 0.5–0.6 depending on the geometrical configuration of RBR, while k ∼ (−1) in the central region of the reactor [20]. In principle, if k > 0.5, the centrifugal force increases faster than the drag force, with decreasing radial position r. With a rotating solid particles bed in the RBR, there will be a gas vortex in the central core region of the RBR. Therefore, if solid particles are entrained into this vortex, they are likely to return to the free board of the rotating bed, provided the vortex is large enough. Next, the solid particle feeding is started at a mass flow rate Gs of 0.012 kg/s. For all three particle diameters the solid particles are continuously fed till t equals 350 s. From Fig. 3a it is clear that, as soon as the solid particles enter into the rotating gas flow field, the pressure drop suddenly drops [4,19]. As the solid particles enter into the RBR, the flow field inside the cylindrical reactor changes completely. The gas entering the RBR via the tangential inlet slots has to impart momentum to the solid particles. As a result, the velocity of gas phase is reduced. Furthermore, the rotating layer of solid particles acts like a distributor for the gas phase. As a result the gas flow entering the RBR via a given tangential inlet is dispersed in the presence of a rotating layer of solid particles. Thus the number of rotations made by the gas phase is reduced in the presence of the rotating solid particles, resulting into a decrease in the pressure drop. During the initial development of the gas–solid rotating bed, i.e. before the establishment of stable bed, the decrease in the pressure drop is found to be inversely proportional to the particle diameter. This is due to the fact that, as the particle size decreases, the specific surface area of the particle increases, thereby causing more reduction in the gas rotational speed. Thus, for larger particle diameters, the pressure drop over the RBR for a given solid particles content is higher before attaining the maximum solids content. In the present experimental set-up, all gas injection slots have the same length as the reactor length. On the other hand, solid particles are supplied from the solids feeder through an inlet at the front end wall of the RBR. When solid particles are thus allowed to flow from the solid particles feeder into the rotating gas flow field, annular channels are inevitably formed. A solid particles channel is observed at the front end wall while an annular gas channel is observed at the rear end wall of the RBR. Note that the central outlet is located in the front end wall of the RBR. For the two larger particle diameter (2.4 mm and 1.6 mm) slugging is also observed during the initial development of the rotating gas–solid bed. When the particles content in the RBR is sufficiently high, the non-uniform flow effects, such as channeling and/or slugging, disappear and a uniform stable flow is established. At this point the pressure drop is minimum and the solid particles content corresponds to the minimum solids content. Each experiment is repeated at least three times. A highly reproducible set of data is obtained during these experiments, with less than 3% error in the value of the solids content. Fig. 4 shows that, as the particle diameter increases, the minimum solids content increases. The stronger slugging tendency of larger particles may be at the origin of this behavior. For given operating conditions, the tangential slot thickness is found to have an effect on the minimum solids content for a given particle diameter as well. As the tangential slot thickness decreases a more optimal distribution of the gas phase results into a reduction of the channeling phenomena. However, next to the channeling phenomena observed for all three particle diameters, the slugging behavior has to be considered as well. When the tangential slot thickness is reduced from 6 mm to 2 mm, the tangential slot thickness becomes of the same order of magnitude as the larger particle diameters (2.4 mm and 1.6 mm). As a result, the slugging behav-
Fig. 4. Minimum solids content for various solid particle diameters; reactor configuration as listed in Table 2; GM = 0.75 kg/s; (), IO = 6 mm, p = 950 kg/m3 ; () IO = 2 mm, p = 950 kg/m3 ; (♦) IO = 2 mm, p = 1500 kg/m3 .
ior of these large particles in the rotating annular gas–solid bed increases and becomes more important than the channeling effect. As such, the minimum solids content increases with decreasing tangential slot thickness. However for the smaller particles (0.9 mm), the slugging behavior remains limited and the channeling effect remains the dominating effect. As a result, the minimum solids content decreases with decreasing tangential slot thickness. When the FCC catalyst particles with a density 1500 kg/m3 are used, a highly unstable gas–solid bed is formed for a tangential slot thickness of 6 mm. A large solid particles entrainment is observed, resulting in a fast emptying of the reactor. This is mainly due to the fact that the gas injection velocity is lower than 45 m/s at the given gas flow rate for a tangential slot thickness of 6 mm. However, when the tangential slot thickness is decreased to 2 mm, a stable bed of FCC catalyst particles is observed. The result of the minimum solids content in case of FCC catalyst particles is shown in Fig. 4. The observations are in-line with the description of the channeling and the characteristic dimension discussed in this section. For a given gas mass flow rate, when the mass of solid particles has increased beyond the minimum solids content, a stable gas–solid rotating bed can be observed. For a given solid particle content, the centrifugal force acting upon the rotating gas–solid bed is overcome by the radial drag force. As a result, the solid particles get carried away with the gas flow through the central outlet until both centrifugal force and radial drag force become balanced again. The mass of solid particles in the RBR corresponding to these conditions is the maximum amount of solid particles that can be retained by the RBR for the given gas mass flow rate and is referred as the maximum solids content of the RBR [10]. From Fig. 3b, showing the time evolution of the pressure drop, the maximum solids content can be determined for the different particle diameters. When the solid particles content equals the maximum solids content, a further feeding of solid particles does not result in a change of the pressure drop. The point in time at which the pressure drop becomes constant corresponds with the point in time at which it is observed that the solid particles start to get entrained by the gas through the central outlet. Due to the gravity effect and the horizontal axis of rotation of the reactor set-up, the solid particles are mainly entrained through the top-most position of the central outlet. Fig. 5 shows the maximum solids content for all the three particle diameters as a function of the gas mass flow rate and the gas injection velocity. For a given particle diameter and particle density an increase in the gas mass flow rate, with a corresponding increase in the gas injection velocity results in a slight increase in the maximum solids content. From a given gas mass flow rate, also referred to as ‘critical gas mass flow rate, GM,cr ’, the maximum solids content
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Fig. 5. Maximum solids content with varying gas mass flow rate; reactor configuration as listed in Table 2; () dp = 2.4 mm, p = 950 kg/m3 ; () dp = 1.6 mm, p = 950 kg/m3 ; () 0.9 mm, p = 950 kg/m3 ; (♦) p = 1500 kg/m3 ; filled symbols, IO = 6 mm; (open symbols) IO = 2 mm.
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Fig. 6. Total pressure drop over the RBR with varying solids content; reactor configuration as listed in Table 2; GM = 0.9 kg/s; () dp = 2.4 mm; () dp = 1.6 mm; () 0.9 mm; (filled symbols) IO = 6 mm; (open symbols) IO = 2 mm.
3.2. Total pressure drop (Ptotal ) remains constant for a given particle diameter. As stated earlier, the maximum solids content corresponds to the point in time where the centrifugal force acting on the bed is balanced by the radial drag force. The centrifugal force is proportional to the square of the angular velocity of the solid particles times the radius. At constant angular velocity, it decreases with a decrease in the radial position in the reactor. The radial drag force, which counterbalances the centrifugal force, is proportional to the nth power of the radial slip velocity, where n varies between 1 and 2. With an increase in gas mass flow rate, n approaches 2. As a result an increase in the gas mass flow rate gives an increase in the maximum solids content, which eventually becomes independent of the gas mass flow rate. For a given gas mass flow rate and a given particle diameter, when the gas injection velocity is increased by a factor of about three by decreasing the tangential slot thickness from 6 mm to 2 mm, the corresponding maximum solids content increases by 20–50%. This effect becomes more prominent for larger particle diameters. A possible explanation is that for lower gas injection velocities, the proportionality exponent n of the radial drag force is smaller than for higher gas injection velocities. For a given gas mass flow rate and for a given reactor configuration, the maximum solids content is observed to decrease as the particle diameter decreases. As the particle diameter decreases, the specific surface area of the particles and hence the radial drag force increases. At the same time however the increase in the centrifugal force due to this decrease in particle diameter does not equal the increase in the radial drag force. As a result, when using smaller solid particles, the balance of the radial drag force and the centrifugal force is already accomplished for a smaller total mass of solid particles in the RBR. Under the investigated operating conditions, the maximum solids content distinctly differs when varying the particle diameter at a constant gas mass flow rate. Kochetov et al. [10], on the other hand, observed that the maximum solids content is almost independent of the particle diameter for a gas mass flow rate smaller than the critical gas mass flow rate. The latter may be due to the smaller reactor diameter of their experimental set-up (see Table 1) and the relatively large diameter of their particles, resulting in more dominant gravitational effects. Thus, for the given RBR configuration, the width of the ‘stable operation window’ narrows down with a decrease in particle diameter, an increase in particle density or an increase in tangential slot thickness. The width of the stable operation window can be increased by increasing the centrifugal force. In the present work this is performed by increasing the gas injection velocity as a consequence of a decreasing tangential slot thickness.
For a given gas mass flow rate and a given reactor configuration, the solids content in the RBR is varied and the pressure drop is measured, once the limits of the solids content for a stable operation are determined. As shown in Eqs. (2a) and (2b): Ptotal = PRBR, main + Poutlet + Pcyclone
(2a)
PRBR, main = Pinlet slots + Pstatic, bed + Ptangential, bed + Pvortex (2b) The total pressure drop over the RBR includes, • The pressure drop over the inlet slots due to the tangential injection of gas, Pinlet,slots . This contribution can rise to 20–30% of the total pressure drop. • The static pressure drop over the gas–solid rotating bed, Pstatic, bed . Depending on the height of the gas solid bed this value may contribute up to 50% of the total pressure drop. • The pressure drop due to the tangential momentum transfer, Ptangential, bed . • The pressure drop due to the gas phase vortex in the central region of the RBR, Pvortex . Fig. 6 compares the total pressure drop over the RBR at a gas mass flow rate of 0.9 kg/s, for all three HDPE particle diameters, and for a reactor configuration with a tangential slot thickness of 6 mm, resulting in a gas injection velocity of 35 m/s. For a tangential slot thickness of 2 mm a gas injection velocity of 105 m/s is determined. For a given reactor configuration and gas mass flow rate, the effective bed height in the RBR increases, as the solids content is increased. Due to this increasing bed height, the static pressure drop and hence the radial gas–solid drag force over the gas–solid bed increases. Simultaneously, there is a reduction in the number of gas phase rotations causing a reduction in the centrifugal force. This results into the reduction of the tangential pressure drop and the pressure drop due to the gas vortex. Due to these compensating actions, the total pressure drop can somewhat increase or decrease. At a constant gas mass flow rate and an invariant solids content, an increase in the gas injection velocity by decreasing the tangential slot thickness results in a significant increase in the total pressure drop. As evident from the pressure data for single phase gas phase flow in the RBR, an increase in the gas injection velocity by decreasing the tangential slot thickness significantly increases the number of gas phase rotations, thereby increasing the pressure drop due to
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vortex formation in the RBR. When the particle diameter decreases while all other conditions remain invariant, the total pressure drop increases due to the increased contribution of the static pressure drop over the rotating gas–solid bed.
LR
4. Conclusions
Ws
The flow stability in a gas–solid rotating bed reactor (RBR) has been studied in a flexible set-up with a relatively large diameter, including the effects of tangential slot thickness and particle diameter. For the first time, the limits of the solids content, in the RBR at different operating conditions are reported. It is found that the interaction between tangential slot thickness and particle diameter may introduce slugging effects in the gas–solid rotating bed, if both tangential slot thickness and particle diameter have comparable values. It is observed that bed stability is improved by decreasing the tangential slot thickness. An increase in the centrifugal force implies an increase in the stability of the annular rotating bed. Hence, a higher gas injection velocity or a higher centrifugal force, allows a higher maximum solids content that can be retained in the RBR for a stable flow condition. However, there is a critical gas mass flow rate above which the maximum solids content will no longer increase. Thus, the tangential slot thickness needs to be optimized in view of the particle diameter, the particle density, and the gas mass flow rate. A change in particle diameter changes the specific surface area of the solid particles, thereby changing the gas–solid drag force. As result, for a constant solids content in the RBR, the use of smaller solid particles results in a higher total pressure drop over the RBR. Thus, under given operating conditions, the use of smaller solid particles will result in higher maximum solids content. Further studies will be made to investigate the effect of other geometrical and operating parameters on the flow behavior in the RBR. This will result in a better understanding of the two-phase flow behavior in the RBR in view of its application at industrial scale and allowing reliable scale-up.
Ws,min
Appendix A. Nomenclature
DE DJ DR
diameter of central gas outlet (m) diameter of jacket (m) diameter of the rotating bed reactor with static geometry (m) dJ,inlet diameter of each jacket feeding line (m) dsolids, inlet diameter of solids feeding line (m) particle diameter (mm) dp FCC fluid catalytic cracking volumetric gas flow rate (m3 /s) GV GM gas mass flow rate (kg/s) critical gas mass flow rate (kg/s) GM,cr GS solid particles mass flow rate (kg/s) HDPE high density polyethylene number of inlets IN IO tangential inlet slot thickness (mm) IO,L length of tangential inlet slot (m)
P
v vg,inj
Ws,max
axial length of the rotating bed reactor with static geometry (m) pressure (Pa) velocity (m/s) gas injection velocity (GV /IO IN LR ) (m/s) mass of solid particles in the rotating bed reactor with static geometry (kg) minimum mass of solid particles required for stable flow (kg) maximum mass of solid particles retained under stable flow (kg)
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