Experimental and Numerical Investigations on the Electrostatics

Oct 9, 2012 - Electrostatics is an inevitable phenomenon in fluidization processes and granular flow systems where collisions between particulates and...
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Experimental and Numerical Investigations on the Electrostatics Generation and Transport in the Downer Reactor of a Triple-Bed Combined Circulating Fluidized Bed Yongpan Cheng,† Darren Yan Jun Lau,† Guoqing Guan,‡ Chihiro Fushimi,§ Atsushi Tsutsumi,⊥ and Chi-Hwa Wang*,† †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 North Japan Research Institute for Sustainable Energy (NJRISE), Hirosaki University, 2-1-3 Matsubara, Aomori 030-0813, Japan § Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo, 184-8588, Japan ⊥ Collaborative Research Center for Energy Engineering, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan ‡

ABSTRACT: Electrostatics is an inevitable phenomenon in fluidization processes and granular flow systems where collisions between particulates and walls with different materials occur. In this study, the electrostatic performance in the downer reactor of a triple-bed combined circulating fluidized bed was investigated through both experiments and numerical simulations. In numerical simulation the Discrete Element Method (DEM) was adopted to simulate the electrostatic charge generation and transfer occurring in the downer. Both experimental and numerical results showed that the averaged induced currents caused by electrostatics increased with increasing solids mass flux in the downer, and the electrostatics was the strongest near the entrance of the downer because of the highest frequency of collisions between particles and the wall caused by the largest solids holdup at that location. Numerical studies also revealed that the averaged induced currents increased with increasing initial particle velocity and particle size, and the averaged induced current caused by the charge transfer was much larger than those by charge generation through tribocharging. These results may help us better understand the mechanism of electrostatic phenomena, and better cope with challenges and problems caused by electrostatics.

1. INTRODUCTION When two particles approach each other in close contact, the transfer of electrons between atoms or molecules may occur, giving one particle a positive and the other a negative resultant charge. Such static electrification of particles involving contact and friction between particles is known as triboelectrification.1,2 In fluidization processes and granular flow systems, triboelectrification is inevitable due to the continuous and random motion of particles which results in numerous collisions, friction and rolling between particles and particles, particles and fluid, or particles and wall to occur constantly. The electrostatics on particles and walls may affect the hydrodynamics in the systems, and cause the formation of undesired byproducts. They can also result in inaccuracy in the measurement readings as well as malfunction of measurement instruments. In cases where the static charge is very high, hazardous discharge of the accumulated charge may cause sparks, fires, or even an explosion, affecting process performance and endangering the safety of operators.3 On the other hand, the proper usage of electrostatics can be beneficial in many industrial processes, such as printing and dust removal. Therefore, it is of great significance to study the electrostatic phenomenonin fluidization and granular flow system to mitigate its negative effects and utilize its positive effects. One of the earliest theoretical studies in this area was conducted by Cole,4 and he established theoretically that the electric charge generated on the containing wall by the charge © 2012 American Chemical Society

transfer was a decelerating exponential function of the number of particle−wall collisions. This was confirmed by experiments later by Masuda et al.,5 Artana et al.,6 and Saleh et al.7 Matsusaka et al.1 pointed out that the electrostatic charge distribution in gas−solids pipe flow was determined by (1) the number of collisions of a particle with the wall, (2) initial particle charge, and (3) the impact electrification factor characterizing the transferred charge. They proposed a model for tribocharging and validated it with experiments by using fly ash particles. By applying the method of images to a bubbling fluidized bed, Park et al.8 developed a mechanistic model to distinguish equivalent and transferred charges. Their results showed that the charge transfer during collision of particles surrounding a rising bubble with a probe was greatly affected by the particle velocity profile. However, the profile of equivalent charge vs time was insensitive to the thickness of the layer of charged particles at the bubble surface with a uniform charge density distribution. This model was improved by Chen et al.9 by considering the charge buildup on the particles remote from the bubble, and the particle charge density distribution in the vicinity of the bubble, thus significant improvement in Received: Revised: Accepted: Published: 14258

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it was concluded that electrostatic effects played a dominant role in influencing particle behaviors during pneumatic conveying through vertical and horizontal pipes at low flow rates while drag forces became more important at high flow rates.19 To suppress the effects of electrostatics on the hydrodynamics, Zhang et al.20 reported that the addition of an antistatic agent (Larostat powder) caused cohesive particles belonging to Group C in the Geldart classification scheme to behave similarly to particles in Groups A and B. Zhu et al.21 also found that the addition of small amounts of such antistatic agent helped decrease the electrostatic charge generation by altering the dynamics of particle−particle and the particle−wall collisions. By adding fine powder and an antistatic agent (Larostat liquid) to the three-phase gas−liquid−solid fluidized bed, Park and Fan22 found that the charges inside the fluidized bed could be greatly reduced. More recently, Wu and Bi23 reported that particle size had significant effects on the accumulation of electrostatic charges in bulk powder. The addition of small particles to large bulk powders reduced electrostatic charge accumulation and it was observed that carbon nanotubes was a much more effective antistatic agent than other additives. Fluidized beds have been widely applied in energy-related areas due to the high mass and heat transfer with close contact among two or more phases. However, in the riser or bubbling fluidized bed reactor, the up-flowing gas streams often lead to unfavorable back-mixing of phases and heterogeneous flow structures, thus the downer reactor has attracted more and more attention from researchers and industries in the past decades.24,25 Different from the conventional gas−solid flows, a downer reactor is a cocurrent down-flow reactor, where gas and solid particles move vertically downward in the direction of gravity, hence very short residence times and narrow residence time distributions are possible in the downer reactor. These characteristics make the down-flow reactor ideal for certain classes of chemical reactions, such as the pyrolysis reaction, fluid catalytic cracking, and Fischer−Tropsch reactions. So far there have been extensive studies on the hydrodynamics in the downer reactor.26−34 However, little work has been devoted to the electrostatics study in the downer except for the experiment by Cheng et al.,35 who studied the electrostatics in the downer in terms of equivalent currents as well as in the riser, and found the variation of electrostatics magnitude to be greatly related to the flow patterns in the downer and riser. Furthermore, little work was devoted to electrostatics generation from the point of particle−particle and particle−wall collisions through numerical simulations. In this study, the electrostatic characteristics in the downer will be studied at different solids mass flux and different axial locations through experiments and the Discrete Element Method will be adopted to simulate the electrostatic charge generation process during particle−particle and particle−wall collisions. The influence of particle initial velocity and charge, particle diameter, and solids mass flux on the electrostatic performance in the downer will be investigated. These results will help us achieve a better understanding of the mechanism of electrostatic charge generation, and cope better with the challenges and problems posed by electrostatics.

agreement between the model and experimental results was achieved. There have been many experimental studies conducted in this field to investigate the effects of various parameters on the electrostatic phenomenon. Most of the available papers are focused on the relationship between operating parameters and generated electrostatics. Guardiola et al.10 investigated the influence of particle size, fluidization velocity, and relative humidity on fluidized bed electrostatics, and found that the degree of electrification was increased with increasing particle size and air velocity, while the influence of relative humidity was quite complex. Nomura et al.11 studied in depth the humidity effect on the tribocharging of powder; they found that the absolute value of the saturated specific tribocharging of the powder decreased with increasing humidity, and the absorption hysteresis of the powder produced a large effect on the charging characteristic of powder. Moughrabiah et al.12 examined the effects of pressure, temperature, and gas velocity on electrostatics in gas−solid fluidized beds and found that with the increase of pressure or air velocity, the generation of electrostatic charges was increased due to more frequent particle−particle, particle−wall collisions and increased bubble rise velocity and flow rate, respectively, while with the increase of temperature, the charge generation was reduced for the limited range of conditions studied. Saleh et al.7 investigated the effect of several parameters, such as the chemical nature of the transport pipe, the particle mean size, the solids flow rate, the air velocity, and relative humidity, on the tribocharging of fine glass particles during pneumatic conveying. Greater electrostatic effects were observed for larger particles, high air velocities, high solids flow rate, and low relative humidity. Electrostatics had great influence on the hydrodynamics in the fluidization processes and granular systems. Joseph and Klinzing13 found that the pressure drop at choking conditions and the required gas velocity at minimum pressure in vertical pneumatic conveying were higher in the presence of electrostatic forces. Al-Adel et al.14 emphasized the importance of considering electrostatic effects in analyzing gas−solids flows in their study of radial segregation of particles in vertical risers. Their model involving the effects of an electric field captured important qualitative features of riser flows: core−annular particle distribution, annular particle downflow at low riser gas velocities, and annular upflow at high gas velocities. Similar phenomena were observed by Yao et al.15 for larger particles; with a diameter of 2.8 mm, they observed the formation of three characteristic aggregation patterns referred to as disperse flow, half-ring flow, and ring flow during pneumatic conveying through a vertical pipe in the presence of electrostatic effects. The electrostatic field strength in the pneumatic conveying system was determined by the amount of charges accumulated on the pipe wall at the state of electrostatic equilibrium. Yao et al.16 studied the electrostatic equilibrium, and showed that it was possible to evaluate the time-invariant electrostatic field strength if electrostatic equilibrium was established. They also developed a method to investigate the effects of granule size and shape on electrostatics generation in pneumatic conveying systems.17 With Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD), Lim et al.18 conducted numerical simulations on the pneumatic conveying in inclined pipe with significant electrostatic effects, and the eroding dunes regime observed experimentally by previous research workers could be reproduced computationally. On the basis of dynamic analysis of forces acting on individual particles,

2. EXPERIMENTAL SETUP 2.1. Experimental Apparatus and Particle Properties. The experimental apparatus was a lab-scale cold model of the Triple-Bed combined Circulating Fluidized Bed (TBCFB) for coal gasification, which consisted of a riser, a downer, and a 14259

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bubbling fluidized bed, as seen in Figure 1. The compressed ambient air from an air compressor was supplied to the bottom

Table 1. Geometrical Dimensions and Operating Conditions in Experiments material of TBCFB material of particles length of downer, m length of riser, m inner diameter of downer, mm inner diameter of riser, mm density of particle, kg/m3 diameter of particle, mm superficial velocity in riser Uf, m/s room temperature, °C air humidity, % pressure in downer, bar location of electrostatic device (from the entrance of downer), mm

acrylic polyethylene (PE) 2.0 4.0 40 40 941 2.7 12.6−15.9 25 5 1.0 150, 850, 1650

Figure 1. Schematic diagram of a lab-scale triple-bed combined circulating fluidized bed (TBCFB) for coal gasification.

of the riser after it passed through the dryer. To facilitate the comparison between numerical and experimental results, polyethylene (PE) particles with a relatively large diameter of 2.7 mm were used as the bed material. As the particle number is inversely proportional to the cube of the particle diameter, if the particle diameter is reduced to the order of micrometers, the simulation of a large amount of particles with the Discrete Element Method would be far beyond the capability of current computational resources. Under the aid of gravity the PE particles flowed down to the bottom of the riser from the bubbling fluidized bed and were then blown up along the riser by the compressed air. After a smooth elbow, the particles entered the cyclone for gas−solid separation, then the air escaped from the top of cyclone, and the particles flowed down to the distributor above the downer. After passing through the downer, the particles flowed back to the bubbling fluidized bed. The solids mass flux was measured with a piece of gauze netting at the bottom of the downer, which had a gauze size smaller than the particle size. The collected particles were loaded onto a weighing scale to measure its mass, and given the collection time, the solids mass flux could be obtained. In Table 1, the dimensions and properties of TBCFB and particles are shown in detail. 2.2. Electrostatic Device. The electrostatic device was selffabricated and attached onto the exterior of the downer and connected to an electrometer (Advantest R8252) to measure and record the induced currents on the wall during fluidization under various operating conditions. A top cross-sectional view is shown in Figure 2a to illustrate the different layers of materials used in fabricating the electrostatic device. The length of the electrostatic device was 300 mm, and a total of 3 electrodes were placed on the device, as seen in Figure 2b. The middle electrode was connected to the electrometer and it had copper wires connected to its conductive adhesive layer. The top and bottom electrodes were connected to the ground and their surfaces were in contact with the grounded aluminum foil

Figure 2. Self-fabricated electrostatic device for measurement of electrostatics. Panel (a): (1) interior flow region (empty space), (2) acrylic tube wall, (3) conductive adhesive copper sheet (copper wires are connected to this layer), (4) nonconductive adhesive polymer film, and (5) aluminum foil shield (grounded). Panel (b): (1) connected to ground, (2) connected to electrometer, and (3) connected to ground.

shield. Charges generated from the tribocharging would be transferred from particles to the acrylic wall to the conductive copper sheet, then finally to the ground, creating an induced current. The induced current was measured by the electrometer, which was a composite value resulting from a balance between the electrostatic charges on the particle surface and the 14260

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pipe wall.15 The measured induced current I could then be integrated over time t to give the charge value Q from the following relation. Q=

∫ I dt

Nq(t ) = Q (t ) = Nqs(1 − e−αt )

3.3. Calculation of Electrostatic Force. When an electrostatic force model is incorporated into EDEM, it brings the requirement of a long-range model that works together with existing contact force calculations.38 The long-range model is based on Coulomb’s law where the electrostatic force, FE, is given by 1 q1q2 FE = r̂ 4πε0 r 2 (6)

(1)

During the experiment, the supplied air to the riser could be varied to change the solids mass flux in the TBCFB. The electrostatic device was shifted along different axial positions along the downer to investigate the electrostatic profile along the downer. Each reading was taken and recorded over a period of at least 90 min, to ensure that electrostatic charging had reached a stable state and the initial charge on the particles and walls had no influence on the measured induced currents.

where q1 and q2 are the charges of two particles, r is the distance between their centers, and ε0 is the permittivity of free space. The Coulomb’s law model does not account for the effects of other nearby charged particles within the vicinity of the target particle. A screening term is usually added to introduce these effects. The following terms are involved in deriving the screened Coulomb force Fs. qq Ue = 1 2 e−κr 4πε0r (7)

3. NUMERICAL SIMULATIONS 3.1. Mathematical Model. The numerical simulation was implemented with the Discrete Element Method (DEM) in the commercially available software EDEM (DEM solutions, Inc.). DEM is commonly used in many industrial processes and granular flow to track the dynamics of particulate systems by modeling the dynamics of each particle. This model considers the mechanical and inertial properties of the particles, as well as the particles interactions with other particles, boundary surfaces, and other force fields,36 such as surrounding fluid, gravity, and electrostatic fields. The electrostatic forces of particles are considered as body force, and will be introduced in detail in the following section. Once the forces acting on the particles are computed, the linear and angular accelerations are calculated for each particle by using Newton’s second law of motion, and the velocity and position of the particles are updated. The process repeats until the required simulation time is reached. 3.2. Tribocharging of Particles. In our system there was only one kind of PE particle, as the particles were of the same material, the charge transfer between particles could be assumed to be negligible because the effective work function was the same. Instead, most of the charge exchange occurred between the particles and the containing wall during particle− wall collisions, which were of different materials. The equation describing the triboelectrification process is shown below, which consists of two terms, charge generation and charge dissipation terms:37 dq = α(qs − q) − βq dt

where Ue is the electrostatic potential and ⎛ 1 κ = qe ⎜⎜ ⎝ εε0KBT

1 [1 − e−(α + β)t ] 1 + β /α

i

⎠

(8)

3.4. Simulation Conditions. DEM was used for conducting simulations for the electrostatics study in the downer and the simulations were run on a PC with Windows 7 Professional 64-bit Intel Xeon CPU 2.67 GHz processor and 24 gigabyte RAM. Simulation time varied widely from 3 h to as long as 24 h, depending on the operational settings and parameters for simulation. A unit at the top of the downer was created to generate the particles. With the aid of gravity, the particle flowed downward along the downer. During this process, the particles collided with the walls of the downer, leading to generation of electrostatic charges, as seen in Figure 3. The particle generation rate, particle diameter, particle initial velocity, and initial charge could be varied to examine their influence on the electrostatic charges. The details on the force models, particle properties, operation conditions, and simulations are given in Table 2.

(2)

(3)

The simulation time was set to be at most 5 s; charge dissipation was a slow process relative to this short time frame, as it usually occurred through atmospheric ion impingement. Therefore, the charge dissipation could be assumed to be negligible, i.e., β ≈ 0. q(t ) = qs(1 − e−αt )

⎞

∑ nizi2⎟⎟

where κ is the inverse of the Debye length, λD, and is based on the local charge concentration (where n is the number of particles of charge z) around a target particle, qe is the charge of an electron (1.602 × 10−19 C), ε is the relative permittivity of the medium, ε0 is the permittivity of free space (8.854 × 10−12 F m−1), KB is the Boltzmann’s constant (1.38 × 10−23 J K−1), and T is the temperature in Kelvin.38 The screened Coulomb force, Fs, is then given by the following q q ⎛κ dU 1⎞ Fs = − e = 1 2 ⎜ + 2 ⎟e−κr ⎝ dr 4πε0 r r ⎠ (9)

where q is the charge on the sphere at time t, qs is the saturation charge, and α and β are the time constants of the charge generation and dissipation, respectively. Integration of this equation gave q(t ) = qs

(5)

4. RESULTS AND DISCUSSION 4.1. Experimental Results. 4.1.1. Bypass Velocity and Solids Mass Flux. As most of the air was able to escape from the top of the cyclone, there was only a small amount of air to flow into the downer. To examine the influence of this bypass air on the electrostatics in the downer, the bypass air velocity in the downer was measured with the rotary meter under different

(4)

To determine the total charge of all particles, we multiplied eq 4 by the total number of particles, N 14261

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influence of bypass air in the downer on electrostatics could be neglected. Table 3 also showed the variation of solids mass flux on superficial air velocity in the riser. It was clear that the solids mass flux increased dramatically with increasing air velocity in the riser, which was caused by the high solids carrying capability under high flow rate. It was observed during the experiment that if the air velocity in the riser was increased further beyond 15.9 m/s, the high pressure at the bottom of the riser exerted by the high air flow rate would cause the air to flow back to the bubbling fluidized bed from the bottom of the riser, thus reducing greatly the amount of available particles from the bubbling fluidized bed, i.e., the solids mass flux in a circulating fluidized bed system. 4.1.2. Influence of Solids Mass Flux on Accumulated Wall Charges. Figure 4 shows the accumulated charges on the wall under different solids mass flux at three measured locations along the downer (z = 150, 850, and 1650 mm). It was noticed that the accumulated wall charge increased over time. This was because the electrostatic device was earthed via the electrometer, and the electrostatic charging on the wall was always occurring. The charge acquired by the acrylic downer wall was positive during collisions between polyethylene particles and the acrylic wall. This was in accordance with the theory of triboelectric series where acrylic is higher in position than polyethylene. Acrylic has a lower work function of 4.7 eV than polyethylene that has a work function of 4.9 eV, thus it was preferable for acrylic to acquire a positive charge or lose electrons than polyethylene. From Figure 4 it was obvious that the accumulated wall charges in the downer increased with increasing solids mass flux, regardless of the position of the electrostatic device. This was attributed to a greater amount of particles for particle−wall collisions under higher solids mass flux, and for charge generation to occur. These results were consistent with the results by Saleh et al.7 It was also noticed that the charge vs time graphs showed more unsteady increase in wall charge with more jagged patterns, especially those found at the bottom of the downer. A possible explanation was that the particles upon collisions with the wall at higher axial positions would acquire a resultant negative charge. This could have caused an increase in the discharging process at the subsequent sections at lower axial positions, resulting in these jagged patterns observed in Figure 4c. 4.1.3. Influence of Axial Positions on Accumulated Wall Charges. Figure 5 shows that the accumulated wall charges were larger at the higher position of the downer (closer to the entrance) regardless of solids mass flux. This was because at a higher axial position, the solids holdups were higher and collisions between particles and walls were more frequent. As particles progressed downward along the downer, the particles started to fall at higher velocities, leading to reduced solids holdup at the constant solids mass flux, thus there would be less-frequent particle−particle and particle−wall collisions, and less electrostatic charge generation. It was noted that at the beginning of electrostatic charging the accumulated charges might not be the largest at the highest axial location, which was attributed to the influence of initial charges on the wall and particles. Therefore, in our experiment a 90 min record time was adopted to eliminate the influence of initial charge on the accumulated charges on the wall. 4.1.4. Definition of Averaged Induced Current. Electrostatic phenomenon can be characterized by the total charge accumulated on the wall over a period of time or by the

Figure 3. Tribocharging of polyethylene (PE) particles with the wall in the downer at Gs = 46.3 kg/(m2 s).

Table 2. Geometrical Dimensions and Operating Conditions in Simulation force model of particle to particle force model of particle to downer particle body force gravity, m/s2 Poisson’s ratio of acrylic shear modulus of acrylic, Pa density of acrylic, kg/m3 work function of acrylic, eV Poisson’s ratio of polyethylene (PE) shear modulus of polyethylene, Pa density of polyethylene, kg/m3 work function of polyethylene, eV coefficient of restitution coefficient of static friction coefficient of rolling friction diameter of downer, mm length of downer, m diameter of particles, mm time step, s

Hertz−Mindlin (no-slip) tribocharging (α = −1) Hertz−Mindlin (no-slip) electrostatics 9.81 0.3 3 × 109 1200 4.7 0.4 1 × 108 941 4.9 0.9 0.5 0.01 40 2.0 2.7 1 × 10−5

air superficial velocities in the riser, as shown in Table 3. It was found that the superficial air velocity in the downer increased mildly with that in the riser, and the value was constantly less than 2% of the superficial velocity in the riser, hence the Table 3. Bypass Air Velocity in Downer and Solid Mass Flux Versus Superficial Velocity in Riser Ufa (m/s)

Udb (m/s)

Gsc (kg/(m2 s))

12.6 13.3 13.9 14.6 15.9

0.25 0.26 0.28 0.28 0.30

13.42 16.53 18.47 24.03 30.03

a

Superficial air velocity in riser. bBypass air velocity in downer. cSolid mass flux. 14262

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4.2. Numerical Results on Averaged Induced Current. 4.2.1. Validation of Numerical Model. To achieve an approximation check and obtain a substantial proof for the model simplification, a CFD-DEM simulation was run to obtain an exact solution in order to compare it to the approximate solution from the simplified DEM model. The accumulated wall charges were compared between the simplified DEM model and coupled DEM-CFD model at particle initial velocity Up = 3 m/s and solids mass flux Gs = 30.8 kg/(m2 s); it was found that the maximum deviation between the two models was only about 6.9%. Furthermore, the dynamic forces exerted on a single particle were calculated, which found that the drag force was 1 order of magnitude lower than that of the gravitational force, hence the drag force was negligible, and the simplified DEM model was applicable. The numerical simulation was carried out based on the same geometrical dimension of the downer, particle size, and solids mass flux as in the experiment. The averaged induced currents near the entrance and outlet of downer are presented in Figure 7. It was found that the numerical results were qualitatively in agreement with the experimental results in Figure 6, i.e., with increasing solids mass flux, the averaged induced currents would increase, and the averaged induced currents near the entrance of downer would be higher than those near the outlet. As electrostatic charging is a very complex phenomenon, some operating parameters could not be completely controlled in the experimental setup, and it was quite difficult to make numerical and experimental results agree well. For example, initial charges in the particles and walls had a significant effect on the tribocharging, and this will be analyzed in a later section. In the experiments, tribocharging was continuous during the fluidization process, while in the simulation the initial charges were all zero. This was why the experimental induced currents were much higher than those in the numerical simulations. The qualitative agreement proved that the numerical model for the simulation was valid. 4.2.2. Influence of Axial Locations. To obtain the averaged induced currents at different axial positions, the downer was split into 5 sections of equal length, with the center point to represent its axial location in the downer. Figure 8 shows the results of averaged induced currents at particle initial velocity Up = 1 m/s, solids mass flux Gs = 23.2 kg/(m2 s), and particle diameter dp = 2.7 mm. z refers to the sectional distance from the entrance of the downer. It was found that the averaged induced current decreased along the downer, which was consistent with the experimental results. The high averaged induced currents near the entrance of the downer were attributed to frequent particle−particle and particle−wall collisions due to high solids holdup. Along the downer, the averaged induced currents decreased dramatically near the entrance and then decreased mildly downstream. This was in accordance with the variation of solids holdup along the downer. 4.2.3. Influence of Initial Particle Velocity. Figure 9 shows the variation of averaged induced currents over initial particle velocity near the entrance of the downer at particle diameter dp = 2.7 mm. It could be seen that with increasing initial particle velocity, the averaged induced currents increased, while the increase rate became milder. The high particle velocity corresponded to the high gas velocity in the downer. Therefore, the findings agreed with the experimental work conducted by Masuda et al.,5 Saleh et al.,7 and Guardiola.10 It was known that at high particle velocities, the force of impact with the walls

Figure 4. Accumulated charge Q on the wall of downer under different solids mass flux Gs.

averaged current induced on the wall. The averaged induced current could be calculated as follows: Im =

∫ I dt /Δt =Q /Δt

(10)

The averaged induced current can be taken as the averaged gradient of the charge vs time graph. Thus the variation of averaged induced currents over the solids mass flux and axial position along the downer could be provided, as shown in Figure 6. It was found that similar trends could be observed as accumulated wall charge. The averaged induced currents increased monotonically with solids mass flux, and decreased along the downer, with increasing distance from the downer entrance. 14263

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Figure 5. Accumulated charge Q on the wall of downer at different axial locations along the downer.

experimental work on horizontal pneumatic flow done by Saleh et al.,7 who found that particle charge-to-mass ratio increased with mean particle size, indicating greater electrostatic phenomenon. With the increase of particle size, the contact area during particle−wall collisions would increase, and this effect was more prominent in the downer where the flow was dilute. This hypothesis was also supported by the work of Masuda and Iinoya,40 who used Hertz’s theory to show that contact surface during elastic collisions varied positively with particle size. 4.2.5. Influence of Solids Mass Flux. Figure 11 shows the variation of averaged induced currents over solids mass flux at particle diameter dp = 2.7 mm. It was found that the averaged induced current increased linearly with increasing solids mass flux. This was supported by the experimental study of Saleh et

would be large due to the large particle momentum. The larger force of impact was more able to excite electrons to be released. In addition, particles tended to collide more frequently with the wall of the downer at higher velocities, which also led to greater electrostatic charging. 4.2.4. Influence of Particle Diameter. Figure 10 shows the variation of averaged induced currents over particle diameter for initial particle velocity Up = 1 m/s. It was found that as the particle diameter increased, the averaged induced currents increased dramatically. This result agreed with the experimental findings by Marra et al.,39 who studied the tribocharging of aerosols of fine phosphate rock concentrations and observed that the generated charge of particles increased accordingly with the mean particle size, implying a greater extent of electrostatics. In addition, this trend was also supported by 14264

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Figure 9. Variation of averaged induced current Im at dp = 2.7 mm.

Figure 6. Averaged induced current Im under different solids mass flux Gs and axial location z along the downer.

Figure 10. Variation of averaged induced currents Im over particle diameter dp at Up = 1 m/s.

Figure 7. Numerical results on the local induced currents Im under different solids mass flux Gs at dp = 2.7 mm and Up = 1 m/s.

Figure 11. Variation of averaged induced current Im over solids mass flux Gs at dp = 2.7 mm.

Figure 8. Variation of averaged induced current Im along the downer at dp = 2.7 mm, Gs = 23.2 kg/(m2 s), and Up = 1 m/s.

induced currents between experimental and numerical results was mainly attributed to the influence of initial charges. In the electrostatic phenomenon, there were two main processes, electrostatic charge generation and electrostatic charge transfer. In the previous simulation the initial charges on the particles and wall were all zero, the electrostatic charges were generated mainly through the triboelectrification. As the acrylic wall had a lower work function than the polyethylene particles, the wall would be charged positively. However, when the particles were given negative initial charges, the acrylic wall was found to be charged negatively, as seen from Figure 12. This meant that the

al.,7 who found that charge transfer increased almost linearly with solids mass flux as long as the transport regime remained very dilute, due to the increase in the number of particles. This result could be explained by the fact that with more particles flowing in the downer, the frequency of particle−wall collisions would be higher, and therefore greater electrostatic charge generation could occur during particle flow in the downer over the same time period. 4.2.6. Influence of Initial Particle Charge on Accumulated Wall Charge. As discussed previously, the deviation of averaged 14265

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effects of charge transfer than that due to charge generation through triboelectrification.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +65 6516-5079. Fax: +65 6779-1936. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Economic Development Board (EDB) of Singapore under grant no. R-261-501-003-414 through the Minerals, Metals and Materials Technology Center (M3TC) of the National University of Singapore (NUS).

Figure 12. Accumulated wall charge Q over time t at dp = 2.7 mm, Gs = 23.2 kg/(m2 s), and Up = 1 m/s.

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electrostatic charge transfer became dominant instead of charge generation. Figure 13 shows the variation of averaged induced

REFERENCES

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Figure 13. Variation of averaged induced current Im over initial particle charge Qp at dp = 2.7 mm and Up = 1 m/s.

currents with initial charges on each particle. It was found that initial charges had significant influence on the averaged induced currents. With the increase of initial charge on the particles, the averaged induced current increased dramatically, and the magnitude of averaged induced currents with initial charges could be one or two orders higher than those without initial charges.

5. CONCLUSIONS To investigate the electrostatics phenomenon in the downer reactor of a triple-bed combined circulating fluidized bed, both experimental and numerical works were carried out to quantify the electrostatic charge generation and transfer in terms of averaged induced currents. Both experimental and numerical studies showed that the solids mass flux in the downer had a positive influence on the averaged induced currents, and the electrostatic charging was strong near the entrance of the downer due to the frequent collisions between particles and the wall. Electrostatic charging then became weaker along the downer. Numerical simulations also revealed that the averaged induced currents increased with increasing initial particle velocities, and particle size. In addition, the initial charges on particles or walls had significant influence on electrostatic charging. Electrostatic strength was much larger due to the 14266

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Industrial & Engineering Chemistry Research

Article

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