Electrostatics in Gas-Solid Fluidized Beds: A Review

Accepted Manuscript Review Electrostatics in Gas-Solid Fluidized Beds: A Review Farzam Fotovat, Xiaotao T. Bi, John R. Grace PII: DOI: Reference:

S0009-2509(17)30506-7 http://dx.doi.org/10.1016/j.ces.2017.08.001 CES 13744

To appear in:

Chemical Engineering Science

Received Date: Revised Date: Accepted Date:

3 May 2017 29 June 2017 2 August 2017

Please cite this article as: F. Fotovat, X.T. Bi, J.R. Grace, Electrostatics in Gas-Solid Fluidized Beds: A Review, Chemical Engineering Science (2017), doi: http://dx.doi.org/10.1016/j.ces.2017.08.001

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Electrostatics in Gas-Solid Fluidized Beds: A Review Farzam Fotovat, Xiaotao T. Bi*, John R. Grace*

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, Canada V6T 1Z3

Abstract Gas-solid fluidized beds, by their nature, are associated with intense and frequent collisions of solid particles with each other and with the vessel wall, causing tribo-electrification. Accumulation of electrostatic charges in fluidized bed reactors can result in severe problems such as agglomeration, wall fouling, nuisance and hazardous discharge, all reducing the process performance and raising significant safety concerns. Tribo-charging of particles in fluidized beds has also been exploited in a number of useful applications. In this review, the characterization methods of electrostatics and the mechanisms of charge generation and distribution in fluidized beds are presented, followed by an account of the interplay between the hydrodynamics and electrostatic phenomena. Furthermore, techniques of electrostatic charge control in fluidized beds are reviewed, and applications of tribo-electrostatic fluidization systems are summarized. Finally, computational fluid dynamics simulations of the electrostatic effects on the hydrodynamic characteristics of fluidized beds are outlined.

Keywords: Fluidization, Electrostatics, Hydrodynamics, Triboelectric charging, Application, Simulation

1 *

Corresponding authors: Tel.: +1 604-822-4408 E-mail addresses: [email protected], [email protected], [email protected]

Contents 1.

Introduction ................................................................................................................................ 3

2.

Characterization of electrostatics in fluidized beds ...................................................................... 5 2.1.

Direct methods ......................................................................................................................... 5 2.1.1.

2.2.

3.

Faraday cups ............................................................................................................. 5

Indirect methods....................................................................................................................... 9 2.2.1.

Electrostatic probes and sensors ................................................................................ 9

2.2.2.

Particle trajectory tracking ...................................................................................... 14

Charge generation and distribution in fluidized beds.................................................................. 14 3.1.

Charge generation .................................................................................................................. 14

3.2.

Bipolar charging..................................................................................................................... 16

3.3.

Charge distribution ................................................................................................................. 18

4.

Relationship between electrostatic phenomenon and hydrodynamics in fluidized beds............... 22 4.1.

Electrostatic force vs. other forces acting on fluidized particles............................................... 22

4.2.

Influence of fluidized bed hydrodynamics on electrostatics..................................................... 26

4.3.

Influence of electrostatics on hydrodynamics of fluidized beds ............................................... 30

5.

Electrostatic charge control ....................................................................................................... 34

6.

Applications.............................................................................................................................. 37 6.1.

Powder coating ...................................................................................................................... 37

6.2.

Solids separation .................................................................................................................... 38 6.2.1.

Coal and fly ash beneficiation ................................................................................. 39

6.2.2.

Separation of granular plastic waste ........................................................................ 40

6.2.3.

Protein enrichment in a tribo-electrification bio-separation process ......................... 41

6.3.

Modifying hydrodynamics of fluidized beds ........................................................................... 42

6.4.

Enhancing fluidization of nanoparticles .................................................................................. 43

6.5.

Measuring fluidized bed hydrodynamics ................................................................................ 45 6.5.1.

Measurement of particle mean velocity ................................................................... 46

6.5.2.

Measurement of bed level ....................................................................................... 47

7.

Simulation including electrostatic charges ................................................................................. 47

8.

Summary and recommendations ................................................................................................ 57

Nomenclature ........................................................................................................................................ 58 Acknowledgements ............................................................................................................................... 59 References............................................................................................................................................. 60

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1. Introduction Fluidization is associated with solid particles being transformed into a fluid-like state by a flowing fluid. It arrived on the industrial scene in a major way in the early 1940s with Fluid Catalytic Cracking (FCC) (Jahnig et al., 1980) and has since been implemented in many other industrial applications, including solid-catalyzed gas-phase reactions, non-catalytic reactions and physical processes. Advantageous features of gas-solid fluidized beds such as excellent gas-solid contacting, efficient and uniform heat transfer, temperature uniformity, and suitability for processing a wide range of feedstocks, have led to widespread industrial applications including coal/biomass combustion/gasification/pyrolysis, drying, coating, ore roasting, catalytic processes such as acrylonitrile, aniline and Fischer-Tropsch synthesis, and gas-phase polyolefin production (Grace et al., 2006; Kunii and Levenspiel, 1991). Electrostatic charging of particles in gas-solid fluidized beds was first reported about 60 years ago in connection with anomalous behavior encountered in experiments on subjects as diverse as heat transfer (Miller and Logwinuk, 1951), elutriation (Osberg and Charlesworth, 1951), and characteristics of fluidized particles (Lewis et al., 1949). Problems associated with fluidized bed electrification include particle-wall adhesion, inter-particle cohesion and electrostatic discharges. The charged particles can coat vessel walls, requiring frequent cleaning. The electrostatic charges on particles and vessel walls, as well as the high-voltage electrical fields arising from them, can affect hydrodynamics and cause the formation of undesired byproducts (Cheng et al., 2012a). They can also interfere with sensors and bed internals, leading to malfunction of measurement instruments and operation (Zhang et al., 2013). For instance, when electrical capacitance tomography (ECT) is applied in a particulate process, electrification can result in measurement errors and even malfunction of some ECT systems (Gao et al., 2012; Zhang et al., 2014). Electrostatic charges are also responsible for potentially severe problems in commercial gas-solid fluidized bed facilities due to agglomeration (Ciborowski and Wlodarski, 1962), sheeting (Hendrickson, 2006), shank (fusion of solid particles into solid shapes resulting from overheating particles residing on the reactor wall in a reactive environment) (Moughrabiah, 2009), nuisance discharges and product handling (Chen et al., 2003). All of the obstacles owing to electrostatics, especially sheeting in fluidized bed polymerization reactors, may cause serious operational problems and production losses (Hendrickson, 2006). Unintentional charge accumulation and resultant hazardous discharges can cause sparks, fires, and even explosions, affecting process performance and endangering the operators (Jones et al., 1991; Nifuku and Katoh, 2003; Ohsawa, 2003). On the other hand, since electrostatic forces can affect the motion of charged particles, the exploitation of electrostatics in fluidized beds can be beneficial in some industrial processes such as powder coating (Yang et al., 2016a), coal beneficiation (Zhao et al., 2014) and separation of solid wastes (Wu et al., 2013). In addition, useful information on the dynamic characteristics

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of bubble and particles can be obtained by processing electrostatic signals acquired from different locations in fluidized beds (He et al., 2016a). In granular flow systems including fluidized beds, tribo-electrification is inevitable due to the motion of particles which results in continuous particle-particle, particle-wall, and particle-fluid interactions, friction and rolling (Alsmari, 2014; Mehrani et al., 2007a). The net change in particle charge level in a gas-solids fluidized bed or transport line results from the balance of charge generation and dissipation occurring simultaneously (Bi, 2005). Electrostatics in gas fluidized beds is complex across multiple scales, affected by many factors influencing electrostatic charge generation, accumulation, transfer and dissipation, e.g. roughness and condition of surfaces, particle properties, relative velocity of particles, fluid physical properties and operating variables such as pressure and temperature (Moughrabiah, 2009). Electrostatic phenomena in gas-solids fluidized beds are further complicated by the heterogeneous flow structure due to the presence of gas bubbles and particle clusters, so that charged particle beds cannot be treated as homogeneous media (Jalalinejad, 2013). Accordingly, the heterogeneous flow structure at the bubble scale must be considered in order to understand the electrostatic forces and fields associated with charged particles. Moreover, the charge on particles may be non-uniformly distributed, making the estimation of the electrostatic charges and process control more difficult (Matsusaka et al., 2010). As a result of the incentive to mitigate the negative impacts of electrostatic in fluidized bed reactors and to exploit it effectively in developing useful physical processes, there has been a reawakening of interest in this topic in recent years (Fotovat et al., 2017a). In 2006, Hendrickson (2006) reviewed electrostatics phenomena in gas phase polymerization fluidized bed reactors with a focus on commercial issues and mitigation techniques. Powder charging mechanisms were reviewed by Matsusaka et al. (2010) in 2010. Mehrani et al. (2017) have recently reviewed advances in understanding charge buildup in gas-solid fluidized beds. To capture the significant progress in understanding the electrostatic phenomena in fluidized beds, this paper reviews recent advances in measurement techniques of electrostatics in fluidized beds. The interplay between electrostatics and hydrodynamics in fluidized beds is then outlined by focusing on the underlying mechanisms and associated phenomena. Applications, mitigation techniques and computational simulations of electrostatics in fluidized beds are summarized next. Finally, we provide a brief outlook on future challenges and required research in this area.

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2. Characterization of electrostatics in fluidized beds In view of the serious technical issues caused by electrostatic charges in industrial fluidized beds, quantification of electrostatic charges is essential for monitoring and control. Moreover, based on the interplay between the hydrodynamic and electrostatic phenomena in fluidized beds, measurements and analysis of electrostatic charges are helpful when characterizing the motion of particles and bubbles, an approach that has been extensively adopted in pneumatic transport pipelines to measure the velocity, concentration and mass flow rate of solids (Gajewski, 2006; Ma and Yan, 2000; Masuda et al., 1994; Matsusaka and Masuda, 2006; Qian et al., 2012, 2014; Xu et al., 2010a; Yan, 1996; Yan et al., 1995, 2006). Particle charge-to-mass ratio, current or the electric potential induced by charged particles are commonly measured to quantify electrification phenomena in fluidized beds. The instruments employed to measure these parameters are 1) Faraday cup, the most common method to measure particle charge density (Bi, 2011); and 2) electrostatic sensors, including both contacting (or collision) probes and induction sensors. Contacting probes, in the form of a ball, hemisphere or rod, are inserted into the bed to measure charge, current or voltage signals arising from a combination of charges transferred between particles and the probe and those induced by particles. In spite of the intrusiveness of these probes, they have been applied more commonly than induction sensors in gas-solid fluidized beds (Sun and Yan, 2016). The wide application of the contacting probes reflects their usefulness in determining local charges and their spatial distributions (Chen et al., 2003; Chen et al., 2006b), particle charge density (Chen et al., 2003; He et al., 2015a), and even bubble characteristics (He et al., 2014, 2015a, 2015b).

2.1. Direct methods 2.1.1.

Faraday cups

The sign and density of charges on particles provide crucial information on the degree of particle charging and the magnitude of electrostatic forces acting on individual particles. Charged particles in fluidized beds can be removed by a sampling tube or a scooper, then poured into a Faraday cage, as illustrated in Fig. 1. Such simple sampling methods have been applied to directly measure the particle charge density in the dense bed (Ali et al., 1999; Fujino et al., 1985; Tardos and Pfeffer, 1980; Zhao et al., 2003) and freeboard (Alsmari, 2014; Alsmari et al., 2015a, 2015b; Fasso et al., 1982; Fotovat et al., 2016a, 2016c, 2016d) regions of bubbling/slugging fluidized beds and circulating fluidized bed (CFB) risers (Jiang et al., 1997; Tucholski and Colver, 1998). These methods can be localized by sampling from different

5

locations of the fluidized bed (Ali et al., 1999; Salama et al., 2013; Song et al., 2016; Song and Mehrani, 2017; Sowinski et al., 2012, 2011, 2010; Zhang et al., 2016). Powder in Fluidized Bed Grounded Cup

Faraday Cup Electrometer ++ + + + ++ + +

+ + + Electrometer Insulation + Faraday Cup + + Fig. 1. Schematic showing Faraday cup charge density measurement system. + h Since wide particle size distributions j are commonly encountered in fluidized beds, it is important to be + able to distinguish differential charging of particles of different sizes. To determine the charge density on

particles of different sizes sampled from fluidized beds, multi-compartment Faraday cup systems may be used which separate particles of different charge densities into several Faraday cups, arranged horizontally (Sharmene Ali et al., 1998) or vertically (Zhao et al., 2003), as shown in Fig. 2, based on the principle that charged particles of the same polarity repulse each other while falling.

b)

a)

Fluidized bed Plug Powders Metal sampling tube 1 2 3 4

Vertical array of Faraday cup sensors

5 6 7

Fig. 2. (a) Horizontal and (b) vertical multi-compartment Faraday cups (Sharmene Ali et al., 1998; Zhao et al., 2003).

6

Faraday cups have mostly been used offline, measuring the charge density of particles withdrawn from different locations of the fluidized bed. The charge density of particles is then obtained by dividing the charge by the mass of particles which have entered the Faraday cup. Despite the simplicity of Faraday cups, charge generation or dissipation during particle sampling may reduce the measurement accuracy. To avoid this drawback, Faraday cups equipped with a filter can be used to capture airborne particles and measure their charge densities in the freeboard of a fluidized bed or in gas-solid pipe flows (Alsmari, 2014; Fotovat et al., 2016c). Also, to minimize the additional electric charging of bed components, entrained and bed material particles can be discharged into two separate Faraday cups, one at the top and the other at the bottom of the bed (Salama et al., 2013; Song and Mehrani, 2017; Sowinski et al., 2012). Also, to minimize the additional electric charging of bed components, entrained particles can be directly collected into Faraday cups at the top of the bed to measure the charge density, while the charge density of the bed material is determined by dropping the bed particles into a Faraday cup located underneath the gas distributor, immediately after the flow of fluidizing gas is terminated (Fig. 3a) (Sowinski et al., 2009, 2010, 2012). By placing a charge separator and a horizontal array Faraday cup system below the fluidized bed, the charge distribution of dropped particles can also be determined (Salama et al., 2013; Song and Mehrani, 2017) (Fig. 3b). A novel in-situ Faraday cup fluidized bed method was developed by Mehrani (2005) and Mehrani et al. (2005, 2007b) to measure the charge density of entrained fine powders from an electrically isolated copper fluidization column, which serves as a Faraday cup. As shown in Fig. 4, when charged fine particles are elutriated from the fluidized bed, an equal but opposite charge is registered by the electrometer connected to the isolated copper fluidization column. To monitor the change of charge density of entrained fine powders with time, the entrained particles are captured by a bag filter and weighed by a sensitive balance so that the transient charge density of the entrained fines can be obtained (Omar et al., 2010). Unlike conventional Faraday cups, this fluidized bed Faraday cup is not susceptible to strong interferences of the flow field, generation of extra charges or discharging while handling particles, or variations in environmental factors, such as atmospheric air humidity and electromagnetic noise. However, this approach is unable to provide information on the charge distribution of bed particles, since it only measures total net charges in the bed.

7

b)

a)

Top Faraday cup Top Faraday cup

Charge separator

Bottom Faraday cup cup

Fig. 3. Schematic of (a) a fluidization column equipped with bottom and top Faraday cups; (b) electrostatic charge separator placed below the fluidized bed (adapted from (Salama et al., 2013)).

Fig. 4. Schematic of an in-situ Faraday cup fluidized bed (Mehrani et al., 2005).

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2.2. Indirect methods 2.2.1.

Electrostatic probes and sensors

Electrostatic charge buildup inside fluidized beds has been measured by electrostatic probes of three major types: capacitance probes, induction probes and collision probes. Unlike the Faraday cup, which is a static measurement tool, probe signals contain dynamic information on particle charging and hydrodynamics inside the bed. The output of an electrostatic probe is in the form of an induced charge signal (Chen et al., 2007, 2006b), current signal (Dong et al., 2015b) or voltage signal (Coombes and Yan, 2015; Zhang et al., 2015), from which the charge level and charge distribution in a gas–solid fluidized bed are obtained. When particles in the fluidized bed are charged, whether or not the reactor wall is grounded, an electrical field is created. By placing a metal probe connected to an electrometer inside the fluidized bed, a potential relative to a grounded reference surface (reactor wall, metal distributor or another metal probe) will be registered by the electrometer. The magnitude of the potential relative to a grounded metal wall or distributor generally increases with time, finally reaching a steady state value. The final potential is then believed to reflect the degree of electrification of fluidized particles at steady state when equilibrium between charging and discharging is reached. In this technique, the metal probe and the grounded reference probe are considered to act like plates of a capacitor, while the section of the bed located between the probes acts as a capacitor dielectric medium over which the average charges are obtained (Bi, 2011). The other type of electrostatic probe, most commonly used to measure electrostatics in industry, is the collision type current probe, or so-called ball probe. The collision probe is installed in the fluidized bed and connected to a resistor, with the current measured by an electrometer (Fig. 5). These probes receive both charges transferred from particles colliding with the probe surface and charges induced when particles pass the probe (Tardos and Pfeffer, 1980). Park et al. (2002b) and Chen et al. (2003) mounted collision ball probes to measure charges induced and transferred by particles surrounding rising bubbles in a two-dimensional fluidized bed. Moughrabiah (2009) and Liu et al. (2010) also measured the axial distribution of electrostatic current from a number of ball probes located both at the axis and near the wall of a fluidized bed containing polyethylene powders.

9

Brass tube Stainless steel ball

6.7 mm O.D.

Alumel wire lead to electrometer

3.2 mm dia.

Glass sleeve 5.8 mm O.D.

Fig. 5. Collision ball probe (Adapted from (Park et al., 2002b)). To eliminate probe interference with the motion of particles and gas in the bed and charge transfer due to collisions between particles and the probe, shielded induction probes have also been deployed to characterize the electrification of fluidized beds (Demirbas et al., 2008). Induction sensors with disc-, ring-, arc- or stud-shape electrodes are also used to measure electrostatics in multiphase systems (Sun and Yan, 2016). Non-contacting induction sensors are independent of net charge accumulation, such as with contacting probes, and have the advantage of not disturbing the flow since they are not directly exposed to the fluidized material. Furthermore, compared to contacting probes, it is easier to reconstruct the charge distribution from the signals generated by non-contacting sensors, since the influence of contact charging can be reasonably ignored (Sun and Yan, 2016). However, they are unsuitable for obtaining local information on non-homogenous flow systems since particle-wall interactions, rather than particleparticle interactions, dominate the signal output. In the case of contacting probes, the properties of the particles and probe tip, e.g., work function, dielectric constant, electrical conductivity, particle density and size distribution, probe tip size and shape, affect current signals (He et al., 2016b). For a given combination of a probe and a bed material, the average magnitude of electrical current from collision probes depends not only on the charge density of particles colliding with the probe, but also on the particle velocity and collision frequency, because charge transfer from charged particles to the probe is a function of the relative velocity or contact time of the contacting surfaces (Matsusaka et al., 2010). Since bubbles are known to be mainly responsible for particle motion and therefore collision with the ball probe in bubbling fluidized beds (Tiyapiboonchaiya et al., 2012), some attempts have been made to decouple hydrodynamic contributions from the probe current so as to extract the particle charge density from ball probe current signals. It has been observed (Yao et al., 2002) that the probability density distributions of local pressure drop and electrostatic voltage are similar, and that the amplitudes of voltage signals from a ball probe are mainly induced by passing bubbles. Moreover, the power spectra of the pressure drop and the cumulative electrostatic charge signals

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show a characteristic frequency of about 1 Hz at relatively high superficial gas velocities, indicating that the periodicity of both signals is dominated by large bubbles near the bed surface (Liu et al., 2010). For a single bubble passing a current probe in a two-dimensional fluidized bed, Park et al. (2002b) and Chen et al. (2003) developed a combined charge transfer and induction model to interpret transient current signals. The change in net current is related to the charge transfer from particle-probe collisions, while the fluctuations are induced by passing bubbles. Chen et al. (2007) further demonstrated that the charge density of the particles in the dense phase surrounding the bubble could be obtained by fitting a single bubble model to the measured current signals for given particle electrical properties based on the assumption of a uniformly charged dense phase. These findings suggest that it may be possible to estimate local particle charge-to-mass ratios from electrical current and local voidage and/or pressure signals. By developing a novel dual-tip electrostatic probe, He (2015) exploited this potential to simultaneously measure the in-situ charge density of particles and the size and rise velocity of bubbles in a bubbling fluidized bed. As shown in Figs. 6a and 6b, this novel dual-tip probe consists of two tips separated vertically by a known distance, Δz. The working principle of this probe is that when a bubble passes the probe, each tip registers a current signal, with maximum and minimum peaks corresponding to arrival of the bubble nose and wake, respectively. The time lag between the peaks from the two tips, Δt, can then be used to estimate the bubble rise velocity, Ub. The bubble size, Db, represented by the vertical pierced length, can be obtained from the time difference Δτ between the times corresponding to arrival of the bubble nose and wake from the signals of either tip.

Ub 

z t

(1)

Db  U b 

(2)

The time lag, Δt, can be obtained either from the cross-correlation of the current signals registered by each tip, or from the time difference between maximum and minimum peak corresponding times (tmax,1, tmin,1, tmax,2, tmin,2) from the upper and lower tips as demonstrated in Fig. 6c.

t 

(tmax ,1  tmax ,2 )  (tmin ,1  tmin ,2 ) 2

(3)

11

 

(tmin ,1  tmax ,1 )  (tmin ,2  tmax ,2 ) 2

(4)

The total current measured by the probe arises from a combination of current transferred from charged particles to the probe tip and the current induced by charged particles. The transferred current can be related to charge density and particle velocity (He et al., 2014), while the induced current is correlated with the bubble rise velocity and particle charge density (Chen et al., 2003). Accordingly, He et al. (2015a) proposed an equation of the form

I peak,i   i qmU b   p (1   mf )U b Ap (iU b   i ) 2

(5)

to represent the current peaks from either of the probe tips, where qm, εmf and Ap are the particle charge density, voidage at minimum fluidization and probe tip surface area, respectively. αi, βi and γi are fitted constants obtained by calibrating the probe, related to the properties of the probe materials and particles, such as the dielectric constant of particles, work function difference between the probe tip material and the bed particles, probe tip size and particle size and shape (He et al., 2016b). Calibration was achieved by injecting bubbles into a two-dimensional fluidized bed. Synchronization experiments were performed with different particle charge densities and bubble rise velocities. The charge density (qm) was varied by changing the superficial gas velocity (Ug), and measured by discharging particles into a Faraday cup. Single bubbles passing a dual-material probe in vertical alignment were selected from recorded videos to obtain the bubble rise velocity (Ub), and corresponding current peaks (Ipeak) were selected from the synchronized probe signals. Eq. (5) was then fitted to measured data for bubbles passing the probe from both single bubble injection and freely bubbling experiments. This method seems practical for calibration of the novel probe for use in commercial-scale fluidized beds. Eq. (5), representing the total current when the bubble nose or wake reaches the probe, relates the current peak values (Ipeak) to in-bed particle charge density and bubble rise velocity. Once the bubble rise velocity has been determined from Eq. (1), the particle charge density can be estimated by inserting the bubble rise velocity into Eq. (5). As shown in Fig. 7, the charge density obtained indirectly from this novel dual probe gave reasonable agreement with those measured directly by a Faraday cup (He et al., 2015b).

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a)

b)

c)

Fig. 6. (a) Schematic of dual-tip electrostatic probe developed by He et al. (2015a, 2015b). (b) Configuration of probe tips inserted into a fluidization column. (c) Principle of probe to determine bubble rise velocity and size (He et al., 2015a).

a)

b)

Fig. 7. Decoupled charge densities from dual-tip probe and Faraday cup in 0.3 m diameter fluidized bed column with polyethylene particles. (a) Steady state operation; (b) time-on-stream monitoring with stepwise velocity changes (He et al., 2015b).

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2.2.2.

Particle trajectory tracking

The particle trajectory method is a non-contact measurement method by which two parallel metal plates are installed in the freeboard region of a transparent fluidized bed (Wolny and Kaźmierczak, 1989). When single particles are ejected into the space between the parallel plates by a single gas nozzle located inside the dense bed immediately underneath the parallel plates, the trajectory of the single particles can be captured by a high-speed video camera, enabling the charge density of individual particles to be determined by analyzing the trajectory of the particles subject to the electrical field. The accuracy of this method is affected by the interference of the electric field imposed by the charged column walls and the bed particles, unless the space between the parallel plates is well isolated (Bi, 2011).

3. Charge generation and distribution in fluidized beds 3.1. Charge generation By its very nature, fluidization is associated with continuous solid-solid contact and separation, as well as friction, as particles rub against each other and against the wall. These circumstances lead to electrostatics through triboelectric charging, or tribo-electrification, which is the process of charge transfer between two materials that are brought into contact and then separated. Triboelectric charging is a non-equilibrium process. However, when charged surfaces are close to one other, a charged state may represent a quasiequilibrium, as the Columbic attraction between the oppositely charged surfaces stabilizes the charged system (Lacks and Sankaran, 2016). Triboelectric charging can be caused by electron transfer, ion transfer, and material transfer (Lacks and Sankaran, 2011; Matsusaka et al., 2010). Once electrons are responsible for charge transfer, triboelectrification between dissimilar materials is characterized by the surface work function, defined as the work/energy needed to pull an electron away from the surface of a material. In general, metals have lower work functions than non-metals, so it is easier for them to lose electrons when in contact with other materials. The work function is also closely correlated with the dielectric constant of the material, being higher for materials with higher dielectric constants. When two dissimilar materials are in contact, electrons flow from the surface of lower work function to that of a higher work function. Thus, in contact with a neutral metal surface, a neutral dielectric particle will extract electrons from the metal surface. Upon separation, the dielectric particle then becomes negatively charged. When a charged particle contacts a metal surface, however, the net charge exchange depends on the pre-charge level (Matsuyama et al., 2003; Watanabe et al., 2006), collision speed (Watanabe et al., 2006), collision angle (Ema et al.,

14

2003), distribution of charge on the particle surface (Matsuyama et al., 2003), and time interval between impacts (Matsusaka et al., 2000). Matsusaka et al. (2010) thoroughly reviewed the extensive work carried out on the effect of particle pre-charge level, surface pre-charge level, collision angle and collision speed on the degree of charge transfer and separation. As depicted in Fig. 8, charge transfer occurs when two surfaces that come into contact with each other acquire charges of similar polarity after contact. In charge transfer contact between a charged particle and a neutral surface, or vice versa, results in charge being transferred to the neutral body (Ireland, 2010). Charge separation occurs when the charge polarities of contacting surfaces differ after contact (Fig. 9). In charge separation contact between a neutral particle and a neutral surface results in each acquiring an equal charge, but with opposite polarities. Charge separation can explain the opposite polarity of fine and coarse particles of the same material when they come into contact, known as bipolar charging (Mehrani et al., 2007a). This phenomenon is discussed further in section 3.2. The dynamics of electrostatic charges in fluidized beds are complex and depend on several parameters, including the particle material and size distribution, column wall material and diameter, fluidization time, fluidizing gas velocity, relative humidity, pressure and temperature, among others (Salama et al., 2013).

a)

b)

Surface

c) Surface

Surface P (neutral particle)

P (Charge Transfer)

+ + + +

P (neutral particle)

P + (Charge Transfer)

-

P (neutral particle)

P - (Charge Transfer)

Fig. 8. Charging transfer between particles and a surface with (a) zero initial charge, (b) positive initial charge, (c) negative initial charge.

a)

b)

Surface

c)

Surface P (neutral particle)

P + (Charge Separation) OR P - (Charge Separation)

+ + + +

Surface P (neutral particle)

P - (Charge Separation)

-

P (neutral particle)

P + (Charge Separation)

Fig. 9. Charge separation between particles and a surface with (a) zero initial charge, (b) positive initial charge, (c) negative initial charge.

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3.2. Bipolar charging As noted above, contact charging between particles of the same material, but different sizes, leading to opposite charge polarity of large and small particles, is known as bipolar charging. Three mechanisms have been proposed to explain triboelectric charging of systems composed of particles of identical material. The first is the particle-size dependence of the work function, which can result in transfer of charge species (electrons and ions) from larger particles (with smaller work function) to smaller particles (with larger work function) (Gallo and Lama, 1976). However, the influence of particle size on the work function is negligible for particles larger than 1 µm. (The size correction to the work function for a 1 µm sphere would be ∼5 × 10−4 eV, compared with the typical magnitude of the work function (∼5 eV) (Lacks and Sankaran, 2011)). A second mechanism proposed to explain particle tribocharging of identical materials is that the material surfaces of particles of a single material are not truly identical since there is a statistical distribution of the properties of materials around the mean values. These statistical variations can significantly affect charge transfer since only a very small number of charged species are required to bring about an electrostatically charged surface, e.g. a very highly charged surface has only approximately one excess species per 105 surface atoms (Lacks and Sankaran, 2016)). The number of charge donor states on two surfaces of identical materials may differ due to statistical variations, with a width of variation proportional to square root of the surface area (Apodaca et al., 2010). This difference can lead to net transfer of charged species from the surface with a larger number of donor states to that with a smaller number of donor states. In the presence of an external electric field surfaces of identical materials can be polarized leading to the breakage of the symmetry between the surfaces and consequently to bipolar charging of surfaces contact with each other. As discussed in section 4.2, the interplay between hydrodynamics and electrostatics in gas-solid fluidized beds results in an electrical potential distribution along the bed, which results in tribocharging of particles of identical materials in fluidized beds. Asymmetric contact is another mechanism proposed by Lowell and Truscott (1986a) to explain triboelectric charging between surfaces of identical materials. This theory is based on the existence of charge transfer species on the contacting surfaces that are out-of-equilibrium, i.e., trapped in high-energy states. Assuming an equal density of species trapped in high-energy states for contacting surfaces, contact between two surfaces leads to equilibration, with species in high-energy states on one surface relaxing to low-energy states on the other surface. According to Lowell and Truscott (1986a, 1986b), in the case of contact between two surfaces with unequal surface areas, the surface contacted over a larger area will lose

16

charge species, while that contacted over a smaller area gains species. Lacks and Levandovsky (2007) extended the Lowell and Truscott theory to polydisperse granular systems by attributing bipolar charging of identical material particles to particle-size differences. Their model shows that the collision of particles of different sizes leads to the accumulation of charged species on the smaller particles in the system and a depletion of charged species on the larger particles (Lacks et al., 2008). Bipolar charging is often used in a general sense once particles with opposite polarities exist in a system, whether or not the system contains chemically identical particles. It has been found that roughness or asperities in the range of 0.01-1 µm (Baytekin et al., 2011) on the surface of particles give rise to uneven surface energies at different locations on the surface of a single particle that can result in different polarities upon contact (Cross, 1987). Most studies on electrostatic charging of fluidized beds before the 1990s assumed a single polarity of bed materials, although bipolar charging had been identified as early as 1950 (Kunkel, 1950; Turner and Balasubramanian, 1976). The first direct evidence of bipolar charging in fluidized beds was provided by Wolny and Kaźmierczak, (1989) who measured the charge density of individual particles using a particle trajectory tracking technique installed in the freeboard of the fluidized bed. They reported a probability distribution of charge density, revealing both negatively and positively charged particles. Bipolar charging of the fine and coarse fluidized particles in binary systems has been identified in a number of studies. However, there is no consensus on the relationship between polarity and the size of particles. While some researchers have identified negatively charged fines and positively charged coarse particles (Forward et al., 2009; Mehrani et al., 2007b; Omar et al., 2010; Sharmene Ali et al., 1998; Zhao et al., 2003), others have observed the opposite (Alsmari, 2014; Alsmari et al., 2015b; Fotovat et al., 2017b, 2016c). A plausible reason for this discrepancy could be the contribution of particle-wall contacts to the reported charge and polarity of the particles tested in fluidization vessels of different construction materials and dimensions. In addition to type and size distribution of fluidized particles, relative humidity has also been shown to influence the polarity of particles. In the experiments conducted by Mehrani et al. (2007b), addition of different proportions of fines to the polyethylene powder bed showed a strong dependence of the charge level and polarity on the relative humidity of the fluidizing gas. Fine Larostat 519 powder (an antistatic compound) was negatively charged in dry nitrogen, but positively at 60% relative humidity. Sign reversal was also observed for fine catalyst powder and silver-coated glass beads. Bipolar charging is responsible for most electrostatic-related phenomena in fluidized beds such as particle agglomeration (Taillet, 1993) and wall fouling (Salama et al., 2013). Trapping catalyst particles in electrostatic-induced agglomerates can be beneficial. However, the lower heat transfer rate from agglomerates can be detrimental since the particle temperature may exceed the sintering temperature of

17

the bed materials, resulting in defluidization (Hendrickson, 2006; Wei and Gu, 2015). The identification and confirmation of bipolar charging changed the perception of a uniformly charged fluidized bed of particles of singular polarity, thus opening the door to examine charge distributions in fluidized beds.

3.3. Charge distribution Fluidized beds are non-uniform in terms of the electrostatic field strength and polarity of particles. The highest electrical potential may occur at the bottom (grid zone) (Buzanov et al., 1978) or at the top (near the bed surface) (Servais and Bernot, 2000), depending the charge density distribution of the fluidized particles. The polarity of the electrostatic voltage may also change from the bottom to the top of the bed (Fujino et al., 1985; Goode et al., 2000), if there is axial segregation of positively and negatively charged particles. The grid zone and upper part of the fluidized bed are likely the regions of maximum charge generation (associated with high-speed gas jets and rubbing of particles on the distributor plate) and dissipation (associated with eruption of large bubbles at the bed surface), respectively (Buzanov et al., 1978; Ciborowski and Wlodarski, 1962; Gajewski, 1985). The variation of the polarity of the charged particles has been supported by the measured opposite polarities of the particles in the dense bed and the entrained fine particles (Ali et al., 1999) and varying polarity of wall deposits in the dense bed region and above the bed surface (Salama et al., 2013; Sowinski et al., 2009, 2010). In the radial direction, the electrical field is strongest near the wall and zero at the axis of the vessel (Fang et al., 2008; Fujino et al., 1985; Fulks et al., 1985), which is expected even if the particles in the bed are uniformly charged and distributed, not necessarily caused by the radial non-uniform distributions of local void fraction or particle charge density, as speculated by Fang et al. (2008). Gajewski (1985) measured the axial profiles of the average electrostatic current from multiple isolated copper rings embedded inside a glass fluidized bed with polypropylene powders as the bed material. Tiyapiboonchaiya et al. (2012) found positive current in the bottom dense bed region, and negative current in the upper bed and freeboard region, i.e. reversal of current flow in the system. Moughrabiah et al. (2009) and Liu et al. (2010) measured the axial distribution of electrostatic current from eight ball probes located at both the axis and near the wall of a fluidized bed of polyethylene powders. The average currents from those probes were always negative in the lower dense bed region, and positive in the upper and the freeboard region, confirming the polarity reversal with increasing height in the bed. Rojo et al. (1986), Servais and Bernot (2000), and Fang et al. (2008) measured the axial distribution of electrical potential based on potential probes immersed in the fluidized bed. Fang et al. (2008) showed that regardless of the static bed height and particle size distribution, negative potentials were present in

18

the lower dense bed region, and positive potentials in the upper bed and freeboard region. According to Fang et al. (2008) the polarities at the top and bottom of the bed are largely determined by the net charges on small and large particles, respectively. As illustrated in Fig. 10, in a bed of linear low-density polyethylene (LLDPE) particles fluidized at 0.6 m/s, the polarities in the dense and dilute phases are opposite, and, as expected, the magnitude of the electrostatic potential at the same height increases with increasing radial distance from the center of the bed (Fang et al., 2008).

a)

b)

Fig. 10. Distribution of (a) electrostatic potential and (b) equipotential lines in a bed of LLDPE particles fluidized at 0.6 m/s (Fang et al., 2008). Ali et al. (1999) studied the charge distribution in fluidized beds of polymer powders, with samples removed from different locations of the fluidized bed by a scooper and poured into a Faraday cup. Their results showed that the charge density inside the bed was quite uniform, except in the near-wall region where some fine powders deposited onto the walls. The mean size and the charge density of the wall deposit were also analyzed. It was found that wall deposits were positively charged in the dense bed region, with the charge density increasing with increasing height above the distributor. Above the bed surface, however, the deposit polarity was negative. In the region where the charge density passed through zero, there was no deposit on the wall. The maximum charge density of the deposit corresponded to the maximum field potential for a uniformly charged bed. Ali et al. (1999) noted that wall deposit particles below and above the dense bed differed in mean volume size and charge polarity. They supposed that the wall deposit particles above the dense bed were originally positively charged, but became negatively charged due to the electric field induced by fluidized charged particles. However, they indicated that this was not the case for wall-deposit particles in the dense bed since these particles were

19

continually bombarded or replaced by other particles in the positively charged dense bed, leading to the maintenance of their positive net charge. Sowinski et al. (2009, 2010, 2012) carried out similar tests on polyethylene particles with a wide particle size range in a fluidized bed equipped with two separate Faraday cups described in section 2.1.1 (Fig. 3a). They visually inspected and sampled the fine powders deposited on the column wall so that the charge density and particle size distribution could be determined. Sowinski et al. (2010) observed bipolar charging with entrained fines being mainly positively charged, whereas the bed particles and those attached to the column wall carried net negative charges. In their experiments the smallest particles were positively charged and entrained from the column. A small fraction of these particles adhered to the column wall above the dense bed. Thus, the polarity of the wall deposit above the dense bed was positive. In the dense bed, the smaller bed particles adhered to the wall followed by the larger bed particles. Moreover, overlap of the particle size distribution of the wall particles and fines with positive charge implied that there was, to an extent, some fines trapped within the wall particles. Overall, the polarity of wall particles in the dense bed was negative. Despite an overlap, the size distributions of wall particles differed below and above the bed surface, likely explaining their different polarities. Again there was a clean wall region between the dense bed and freeboard regions. These results are in general consistent with those of Ali et al. (1999), although the deposits in different regions were not examined in the latter case. As indicated by Sowinski et al (2010), the polarity of fine particles in their work differed from those fluidized by Ali et al. (1999). The polarity of charged particles in fluidized beds depends not only on particle size, but also on the particle and column wall materials, as well as the operating conditions such as relative humidity. Ali et al. (1999) fluidized polyamide particles smaller than 150 µm in a steel vessel with a rectangular cross section. Sowinski et al (2010) fluidized polyethylene particles with a particle size distribution ranging from 20 to 1500 µm in a 0.10 m diameter carbon steel column. While the former group used air with 6-10% RH (relative humidity), the latter used dry air. These differences are likely responsible for different polarities of wall deposit particles observed in these two studies, and further investigation is still warranted. Salama et al. (2013) utilized an electrostatic charge separator below the fluidized bed to measure the charge distribution of bed material (dropped through the distributor) and wall particles. The dominance of negative charge in the wall region was shown to result from a small number of highly negatively charged particles, tribo-charged by contact with the carbon steel column wall. Giffin and Mehrani (2010) showed that the charge density (charge-to-mass ratio) of wall particles was two orders of magnitude larger than that of dropped particles. As shown in Fig. 11, these highly charged particles adhered strongly to the

20

positively charged wall due to the image force and attracted positively charged polyethylene particles due to the electrostatic force. Layerwise formation of wall fouling with positive net specific charge was observed by Song et al. (2016) and Song and Mehrani (2017), who fluidized polyethylene particles at various operating pressures, ranging from atmospheric to 2600 kPa, with the latter being similar to the operating pressure of a commercial gas-phase polyethylene process. Generation of electrostatic charges in the bottom of the bed where high-velocity gas jets impinge into the dense bed before bubbles form and charge dissipation at the bed surface where bubbles burst and eject particles into the freeboard at high speed, bipolar charging of segregating coarse and fine particles, and the possibility of charge transfer to or from the vessel wall are among the factors which contribute to the axial and radial charge distribution in fluidized beds (Fang et al., 2008; Hendrickson, 2006; Tiyapiboonchaiya et al., 2012). The size distribution of fluidized particles and the fraction of a certain particle size can also affect the electrostatic charging behavior of the system (Tian and Mehrani, 2015). There is some evidence (Krauss et al., 2003; Zheng et al., 2003) that polydispersity of particle sizes enhances charge transfer and electric field intensities of charged particles, presumably due to greater degree of particle charging (Lacks and Sankaran, 2011). Addition of antistatic agents has been shown to be able to completely change the axial and radial charge distribution profiles in fluidized beds (Goode et al., 2000; Servais and Bernot, 2000).

Image force

Electrostatic force

Fig. 11. Schematic of wall coating formation mechanism in a fluidized bed of polyethylene particles (Song and Mehrani, 2017).

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4. Relationship between electrostatic phenomenon and hydrodynamics in fluidized beds Continual collision of particles with each other or with the vessel wall is known to be the main mechanism of tribo-electrification in fluidized beds. At the same time, electrostatic forces generated by charged particles can influence the hydrodynamic behavior of fluidized beds and other granular systems. Understanding the interplay between electrostatics and hydrodynamics is vitally important to optimize the operation and performance of fluidized beds experiencing tribo-electrification. In bubbling fluidized beds, particle-gas contacting has negligible effect on tribo-charging of particles (Mehrani et al., 2005). By exploring the charge distribution around a single bubble rising in a twodimensional fluidized bed based on a number of induction probes, Chen et al. (2006a, 2006b) showed that the wakes of bubbles could be more negatively charged than the remainder of the dense phase, and that the charge density inside the bubbles was nearly zero because of the extremely low solids concentration inside the bubble. Accumulation of electrostatic charges in fluidized beds can be attributed to entrainment of charged fines, leaving net positive or negative charges behind in the bed (Mehrani et al., 2005). In the case of fluidization with negligible entrainment in a grounded conductive column, the net specific charge generated within the bed has been attributed to contacts between fluidizing particles and the column wall because the net charge from bipolar charging due to particle-particle contacts would be zero (Song and Mehrani, 2017).

4.1. Electrostatic force vs. other forces acting on fluidized particles The motion of particles in fluidized beds results from a balance between gravitational force, drag and inter-particle forces such as electrostatic and van der Waals forces exerted on the particles. Assessment of the relative magnitude of the electrostatic forces with respect to other forces is therefore crucial to determine the contribution of electrostatics to the motion of particles in fluidized beds. Hendrickson (2006) provided a detailed comparison of the electrostatic forces with drag force and van der Waals force in fluidized beds containing polymer particles. Due to highly non-uniform distributions of particle charges and resulting electric field in fluidized beds, it is difficult, if not impossible, to reliably estimate the electrostatic forces exerted on a particle in a fluidized bed. Instead, the maximum electrostatic force, linked to the maximum particle charge that can accumulate before ionization (breakdown) of the surrounding fluid, is generally calculated and used as a reference point to compare the electrostatic forces with the other forces. However, in practice, charge densities of particles are significantly less than the

22

maximum theoretical values (Chen et al., 2003a; Hendrickson, 2006) due to the influence of experimental conditions, e.g., relative humidity, or the measurement techniques (Hendrickson, 2006). Among the three methods developed to estimate the theoretical maximum particle charge based on Gauss’ law, Hendrickson (2006) showed that the method developed by Revel et al. (2003) gives a more reasonable result. Revel et al. (2003) applied the Gauss’ law to a cylindrical-shaped fluidized bed to obtain the maximum theoretical particle charge, |q|max, at the condition of incipient breakdown in air:

dp 2 0 d p E d | q | max   5.56  10 5 3 (1   ) D (1   ) D 3

3

(6)

where ε0, dp, Ed, ε, and D are permittivity of free space (8.854×10-12 F/m), particle diameter, breakdown potential in air (3×106 V/m), fluidized bed void fraction, and fluidized bed diameter, respectively. Since the electrostatic force acting on a particle of charge q in an electric field E is F=Eq, the maximum electrostatic force, |Fe|max, is the product of |q|max and the electric breakdown potential of air. Thus | Fe | max 

3 2 d p3 2 0  p d p E d  167 3 b D (1   ) D

(7)

Dong et al. (2015b) compared the electrostatic force and the drag force exerted on polypropylene (PP) particles ( d p = 1.85 mm, ρp= 900 kg/m3) for various gas velocities. As seen in Fig. 12, the maximum electrostatic force and drag were of the similar order of magnitude in their experiments. The drag force increased significantly with increasing superficial gas velocity so that when superficial gas velocity increased from 0.7 to 0.9 m/s, the drag became the dominant force, considering that the charge density of PP particles never exceeded 50% (q/m)max in the system studied. Fig. 12 shows that when the PP particles were charged to 50% of (q/m)max, the electrostatic force was of the same order of magnitude as the fluid drag at a low gas velocity (Ug=0.7 m/s). For lower particle charge densities, e.g. 25% of (q/m)max, the electrostatic force decreased correspondingly and the drag was dominant, except at superficial gas velocities 1, indicating that the electrostatic forces on these particles often exceed the gravity force, which must be overcome for entrainment to occur. Therefore, when analyzing the elutriation of fine particles, it is crucial to consider the effect of the electrostatic forces on entrainment. This is in accord with the findings of Maurer et al. (2016), who noted that the maximum electrostatic forces can be relevant for particle adhesion and hence for the particle elutriation and attrition in fluidized beds. Because the electrostatic forces are inter-particle forces, they primarily affect entrainment by influencing whether particles travel individually or as aggregates/clusters. As shown by Maurer et al. (2016), electrostatic forces may be complemented by van der Waals forces in this role.

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Fig. 13. Comparison of drag-to-gravity force ratio at (Ug-Ut = 0.3 m/s) with electrostatic-to-gravity force ratio for (a) fine glass beads ( d p = 38 µm, ρp= 2700 kg/m3), (b) alumina ( d p =35 µm, ρp= 3200 kg/m3), (c) polyethylene furanoate (PEF) ( d p = 41 µm, ρp= 940 kg/m3), (d) cork ( d p =171 µm, ρp= 664 kg/m3) and (e) porcelain ( d p = 138 µm, ρp= 2403 kg/m3) (Fotovat et al., 2016b).

According to Feng and Hays (2003), both van der Waals forces and the image electrostatic force exerted on a particle with net charge of about 15 fC in the vicinity of a material surface can be manipulated at

25

least in a range over one order of magnitude without encountering any physical limitations. It is generally believed that the electrostatic forces can be more important than van der Waals forces for large particles (Feng and Hays, 2003; Hendrickson, 2006; Walton, 2008). However, the relative importance of electrostatic and van der Waals forces depends highly on the surface characteristics of fines, such as roughness, and the distance between adjacent particles. Irregular particles or particles with a dusting of nano-scale fines may have very low effective van der Waals adhesion forces. As a very rough estimate for such cases, the electrostatic and image forces can exceed surface energy forces for particles considerably larger than 10 microns (Walton, 2008).

4.2. Influence of fluidized bed hydrodynamics on electrostatics The generation, dissipation and accumulation of electrostatic charges are closely related to the hydrodynamic behavior of fluidized beds (Wei and Gu, 2015). The maximum level of charge accumulation in fluidized beds is determined by the bed size and fluidization parameters, such as particle size, bubble size, as well as superficial gas velocity (Chen et al., 2003). In general, the degree of electrification (charge per particle or per unit mass of particles) increases with increasing superficial gas velocity in bubbling fluidized beds (Alsmari et al., 2015a; Guardiola et al., 1996; He et al., 2015a; Liu et al., 2010; Moughrabiah et al., 2009; Park et al., 2002a). This is attributed to enhanced particle movement by large bubbles and increased contact frequency between particle and wall surfaces (Tiyapiboonchaiya et al., 2012). Under bubbling conditions, the fluidized bed usually contains two circulation zones – core and annulus. Positive and negative electrical currents can be traced in different locations of the bed, with opposite polarities associated with different circulation zones, demonstrating the interplay between hydrodynamic and electrification phenomena. The electrical potential distribution can be manipulated by altering the gas-inlet configuration (Tiyapiboonchaiya et al., 2012; Zhou et al., 2013). For instance, by strengthening a core-annulus structure, Zhou et al. (2013) enlarged the upper circulation zone by increasing the inlet gas velocity in the core zone. Conversely, the bottom circulation zone was expanded by increasing the gas velocity in the annulus zone. These alterations led to expansion of the electropositive and electronegative zones, respectively. Fig. 14 compares the electrostatic potential distributions in a fluidized bed of LLDPE particles with three different gas-entering modes. One should note that the local electrical potential may not directly reflect the local charge density because charged particles in the whole fluidized bed determine the distribution of electrical potential fields.

26

Cheng et al. (2012b) studied electrostatics in the fully developed regions of the riser and downer of a large-scale circulating fluidized bed. They found that at relatively low superficial gas velocities the typical core-annulus flow pattern in the riser maintained a balance between negative and positive induced currents generated due to the motion of charged particles. When the flow pattern approached dilute phase transport or the dense suspension up-flow regime at high superficial velocities in the riser, positive induced current became dominant, likely associated with reduced back-mixing or down-flow of sand particles near the wall. The electrostatic characteristics also differed in the fully developed regions of a riser and a downer. While increasing the air superficial velocity resulted in an increase in the average induced current in the riser, it led to a decrease in the induced current in the downer at a given solids circulation rate (Cheng et al., 2012b). Both the experimental observation of Cheng et al. (2012a) and the results of numerical simulations showed that the average induced currents increased with increasing solids mass flux in a downer at a given gas velocity, because of the increased number of particles colliding with the probe.

a)

b)

c)

Fig. 14. Electrostatic potential distribution in a fluidized bed of LLDPE particles with three different gas-entering configurations (Ug = 0.3 m/s): (a) normal uniform gas-inlet mode, (b) gas entering core zone exclusively, and (c) gas entering annulus zone exclusively (Zhou et al., 2013). Particle size has a significant effect on the accumulation of electrostatic charges in bulk powder flows (Wu and Bi, 2011). Boland and Geldart (1972a) and Guardiola et al. (1996) found that the charge density

27

increased with increasing particle size, likely because of augmented inter-particle contacts. Addition of large polymer granules to a fluidized bed containing particles of the same chemical composition, but different sizes, had no discernable effect on the electrostatic level, other than a little fluctuation of the electrostatic potentials (Yu et al., 2010). However, injection of small granules could increase or decrease the electrostatic potential of the bed, depending on the amount added (Moughrabiah et al., 2012; Yu et al., 2010). This was attributed to the role of small particles in altering the contact mode between particles and in affecting the generation, transfer and neutralization of electrostatic charges. For instance, as illustrated in Fig. 15, in the presence of fine particles which tend to adhere to surfaces of opposite polarity, the contact surfaces between coarse particles themselves and between coarse particles and the column wall are reduced. As the fines concentration increases, the contact between fine particles and between fine particles and the wall increases to a certain extent and then levels off. In general, the finer the granules, the stronger the influence on fluidized bed behavior (Yu et al., 2010).

a)

c)

b)

d)

Fig. 15. Transition of the structure of a bed of coarse LLDPE particles by adding fine particles (Yu et al., 2010). Moughrabiah (2009) and Moughrabiah et al. (2009) explored the influence of operating pressure on the degree of electrification in bubbling fluidized beds of glass beads and high-density polyethylene resin (HDPE) particles (see Fig. 16). As the pressure increased, the degree of electrification, as reflected by the average current from ball probes and particle charge density from a Faraday cup, increased, probably due to an increase in bubble rise velocity, frequency, and volume fraction. This is consistent with a higher degree of fluidized bed wall coating observed in the experiments of Song et al. (2016) as a consequence of elevating the system pressure from atmospheric to 2600 kPa. Moughrabiah et al. (2009), Alsmari (2014), and Alsmari et al. (2015a) reported a decrease in electrostatic current, associated with charge polarity reversal, as the bed temperature increased from 20 to 75 °C, likely due to increased charge dissipation at higher temperatures. Fig. 17 shows the effect of temperature on cumulative transferred charge to the ball probe as a function of time in a bed of LLDPE particles.

28

Increasing the initial bed height resulted in an increase in the degree of electrification of fluidized polypropylene (PP) particles because of increased bubble size enhancing solids circulation and frequency of particle collisions (Tiyapiboonchaiya et al., 2012).

b)

a)

Fig. 16. (a) Effect of pressure on cumulative transferred charge to ball probes as a function of time measured by a collision probe located at the center of a fluidized bed of glass beads ~0.3 m above distributor,

=321 µm, T = 19 °C, RH= 9-12%, and Ug -Umf= 0.05 m/s. Numbers on the curves denote

absolute pressures (in kPa). (b) Charge density of HDPE particles measured by Faraday cup,

= 450

μm, T = 22 °C, RH = 9−13%, and U−Umf = 0.05 m/s, particles sampled 0.15 m above distributor (Moughrabiah et al., 2009).

Fig. 17. Effect of temperature on cumulative charge, measured by a collision probe located at the center of a fluidized bed of LLDPE beads about 0.3 m above distributor, as a function of time. dp =600 µm, P = 379 kPa, and Ug-Umf= 0.05 m/s (Moughrabiah et al., 2009).

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4.3. Influence of electrostatics on hydrodynamics of fluidized beds Electrostatic charges can change the forces on particles (Dong et al., 2015a; Jiang et al., 1994; Revel et al., 2003), thereby inducing particle-wall adhesion, inter-particle cohesion, and agglomeration (Hendrickson, 2006; Wang et al., 2009; Wolny and Kaźmierczak, 1993), which can further affect electrostatic phenomena in the bed. For instance, in fluidized beds of polyethylene (PE), small agglomerates and falling sheets significantly affect the electrostatic behavior of the bed by changing the particle concentration, as well as the surface charge and polarity of particles (Yang et al., 2016b). Experimental and numerical studies have reported that the frequency and size of bubbles decrease due to particle charging in fluidized beds (Dong et al., 2015a, 2015b; Hassani et al., 2013; Jalalinejad et al., 2015a, 2012). Moreover, the bubble rising zone in the bed shrinks as a consequence of electrostatic accumulation (Dong et al., 2015b). Bubble elongation and the tendency of bubbles to rise more towards the axis of the column in the presence of charged particles have also been predicted by computational simulations (Jalalinejad, 2013; Jalalinejad et al., 2015a, 2012) (see Figs. 18a and 18b). Bubble shape and stability have been shown to be influenced by the particle charge density distribution in the bed (Jalalinejad et al., 2016). Coalescence of a pair of bubbles in a bed of charged particles was predicted to be asymmetric (see Figs. 18c and 18d), with the leading bubble wobbling, and particles raining from the roof of the trailing bubble. Numerical simulations of Jalalinejad et al. (2015) also predicted higher rise velocity of elongated bubbles in the lower part of a 2D column containing charged glass beads, compared to a corresponding uncharged system. This was in line with experimental work of Grace and Harrison (1967), who found that elongated bubbles rise at higher velocity. However, with respect to uncharged conditions, Dong et al. (2015) inferred the reduced bubble rise velocity from less fluctuating dynamic height of a bed of charged coarse polypropylene particles, fluidized in a cylindrical column. The discrepancy in the impact of electrostatics on the bubble rise velocity in studies of Jalalinejad et al. (2015) and Dong et al. (2015) may be due to different wall effects on bubble properties in 2D and 3D columns. Moreover, Jalalinejad et al. (2016) showed that the distribution of charge density along the bed affects the bubble shape and stability, thereby influencing the bubble rise velocity. Accordingly, the different charge density distributions of particles in these studies may explain the different conclusions drawn about the effect of electrostatic charges on bubble rise velocity in fluidized beds.

30

a)

c)

b)

d)

Fig. 18. Effect of charge density distribution on single simulated bubbles for (a) uncharged particles and (b) uniformly charged particles with charge density of −1 μC/kg (dp = 520 µm, ρp = 2500 kg/m3, Ujet = 0.64 m/s)

(Jalalinejad et al., 2016). Coalescence of two simulated bubbles in horizontal

alignment in (c) uncharged (d) charged particles with charge density of −0.33 μC/kg (dp = 300 µm, ρp = 2450 kg/m3, Ug = 0.084 m/s) (Jalalinejad et al., 2015b).

When particles are mainly charged with one polarity, repulsion forces dominate (Dong et al., 2015a). Then the void fraction in the emulsion phase increases, explaining shrinkage of bubbles (Dong et al., 2015a; Hassani et al., 2013). As a consequence of a reduction in the bubble size and velocity and also because of the charged particles which adhere to the wall and form immobile close-packing aggregates, the bed height slightly decreases and bed level fluctuations are damped (Dong et al., 2015a; Lim, 2013). Increasing the degree of electrification of particles can also delay fluidization by increasing the minimum fluidization velocity (Dong et al., 2015a). Electrostatics accumulation in fluidized beds also restrains particle motion and reduces particle impact velocities, which may augment wall sheeting and particle agglomeration. However, at high gas velocities, where the effect of the drag force on particles is dominant, the impact of electrostatic force on the particle impact velocity was found to be insignificant (Dong et al., 2015b). Computational simulations of fluidized beds with electrostatic effects have indicated that charged particles are subject to axial and lateral segregation because of the non-uniform potential distribution and particle charging in the bed (Bi, 2005). Due to the presence of adhesive forces between particles and the

31

walls arising from electrostatic forces, particle motion is significantly hindered. Consequently, fluidization is less vigorous in beds with strong electrostatic effects, and the mixing efficiencies of such systems are less than those with weak electrostatic effects (Lim, 2013). Fig. 19 compares the time evolution of Lacey mixing indices of three systems differing in the charge densities of particles and the column wall. Electrostatic forces between particles and walls during the fluidization process were observed to be stronger on average than both fluid drag forces and particle-particle collision forces when strong electrostatic effects were present (Lim, 2013). Particle segregation due to different charges is more pronounced for the particle fraction that is at lower concentration, because of the higher frequency of collisions with particles of a different size (Bilici et al., 2014). Monopolar charged particles are subject to lateral segregation which is increased by particle charges (Kolehmainen et al., 2016a). Size segregation in fluidized beds can be suppressed in the presence of bipolar electrostatic charges, since small and large particles having opposite polarities are likely to attach to each other and form agglomerates (Yang et al., 2017b). There is a lack of experimental studies on the mutual effects of mixing or segregation of particles and their electrostatic characteristics in fluidized beds.

a)

b)

c)

Fig. 19. Time evolution of Lacey mixing indices for simulated fluidized bed systems at various fluidizing velocities with particle charge density and wall charge density of (a) 1 µC/kg and 1 µC/m (b) 1 µC/kg and 10 µC/m, and (c) 10 µC/kg and 10 µC/m (Lim, 2013). There is extensive evidence that the entrainment of charged fines is reduced relative to uncharged particles due to particle-particle and particle-wall attractive interactions hindering free and independent particle motion (Alsmari et al., 2015b; Baron et al., 1987; Briens et al., 1992; Fotovat et al., 2016b, 2016c; Tardos et al., 1983; Wolny and Kaźmierczak, 1989; Yang et al., 2017b). Fig. 20 portrays the normalized entrainment flux of fines vs. electrostatic-to-gravity-force (Fe/Fg) ratio for a variety of binary

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mixtures composed of fine and coarse particles. Fig. 20 suggests that, regardless of the ratio between superficial gas velocity and terminal settling velocity of fine particles (Ug/Ut), the entrainment flux decreases with increasing electrostatic force. Such a trend cannot be predicted by any of the many empirical correlations proposed to estimate particle entrainment from fluidized beds, exclusively based on hydrodynamic mechanisms, which have been shown to be prone to extremely wide discrepancies in predictions, up to 20 orders of magnitude (Chew et al., 2015), in part due to ignoring electrostatic effects. By incorporating the effect of electrostatic forces on the entrainment process and developing a new correlation, Fotovat et al. (2016b) showed that the predictability of the correlations for estimating entrainment rates can be greatly improved. Fotovat et al. (2016c, 2016d, 2017) also demonstrated that both the particle conductivity and the column wall material properties have major impacts on the entrainment of fines because of their effects on the electrostatic force exerted on particles in the freeboard zone.

Fig. 20. Normalized entrainment flux vs. electrostatic-to-gravity-force ratio of entrained fine particles, with electrostatic forces varied by varying the relative humidity (from 5% to 35%). Coarse particles, 90– 95% of bed inventory, were coarse glass beads of average size 528 μm in all cases (Fotovat et al., 2016b). xi denotes the weight fraction of the ith cut of particle size distribution, with di as average diameter.

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5. Electrostatic charge control As discussed above, electrostatic charges in gas–solid fluidized beds can lead to problems such as agglomeration, wall sheeting, reduction of product quality, and unscheduled shutdowns (Dong et al., 2014; Wang et al., 2009). Therefore, there is great incentive to control electrostatic charge generation and accumulation in fluidized beds. Most relevant studies have been carried out on fluidized bed polymerization reactors, which are prone to plugging of the reactor product discharge system or loss of fluidization as a consequence of significant wall sheeting (Hendrickson, 2006). The methods adopted to control electrostatic charges in fluidized beds are categorized as charge generation rate reduction, charge dissipation rate enhancement and charge neutralization (Hendrickson, 2006). Table 1 provides some methods pertaining to each mechanism found to be effective in mitigating electrification in fluidized beds. Grounding the column wall is commonly ineffective in reducing electrostatic charges in fluidized beds (Park et al., 2002a; Sowinski et al., 2011), possibly because of limited charge transfer from poorly conductive dielectric particles to the reactor wall. However, coating the inner column wall with an appropriate antistatic or charge-neutralizing agent is a proven method to mitigate electrostatic charges (Mihan et al., 2002; Muhle et al., 2005). In Plexiglas laboratory-scale columns operating at ambient temperature, coating the inner surface with a layer of cellotape or Scotch tape has been widely used to prevent particles from adhering to the wall (Grace and Baeyens, 1986; Park et al., 2002a). There is extensive evidence that increasing the relative humidity of the fluidizing gas decreases the level of electrostatic charging in fluidized beds (Fujino et al., 1985; Jiang et al., 1997; Tardos et al., 1983; Tardos and Pfeffer, 1980; Wolny and Kaźmierczak, 1989). The increase in humidity of the fluidizing gas may lower the resistivity and the break-down potential of the gases, helping to dissipate particle surface charges (Bi, 2011). At relatively high relative humidities, the reduction in bed charge level is attributed to the increased surface conductivity of moisture-coated particles (Boland et al., 1969). Adsorption hysteresis of the powder has a large effect on the charging characteristics (Nomura et al., 2003). Thus the impact of relative humidity on tribo-electrification of particles in fluidized beds also depends on particle surface properties. This is supported by observations of Giffin and Mehrani (2013) and Alsmari (2014), who found that relative humidity has no effect on the charge density of hydrophobic fine particles, which are unlikely to be coated with moisture. Fotovat et al. (2016d) observed that the charge density of conductive fine particles was increased by increasing the relative humidity, presumably because the electrical conductivity of moisture coating was lower than that of the conductive particles, hindering charge dissipation. Fig. 21 illustrates the opposite trends of change in particle charge density with increasing relative humidity for dielectric uncoated and conductive silver-coated fine glass beads of the same size, density and shape, fluidized in stainless steel and acrylic columns.

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In general, injection of antistatic or charge inducing agents is the most successful technique to eliminate electrostatic charges in commercial fluidized bed polymerization reactors (Wang et al., 2009). Selection of an appropriate electrostatic charge inducing agent in these reactors depends on the polarity of the electrostatic charge induced and the effects on the catalyst activity (Hendrickson, 2006). When solid metal oxides are used as the charge inducing agent, their chemical composition, especially the electronegativity of the metal ions, can greatly influence the final electrostatic potential distribution (Wang et al., 2009). In addition to the approaches listed in Table 1, a few other techniques have been proposed to control tribo-charging in particulate systems, though their effectiveness in fluidized beds needs to be explored experimentally. Physisorption and chemisorption of electronegative species, such as oxide layers, and environmental contamination on the surface of particles have been shown to decrease the chargeability of particles upon tribo-electrification by changing their surface work functions, resulting in a reduced charge generation rate (Trigwell et al., 2003). Surface modification by oxygen plasma treatment is another method suggested for tailoring the work function of polymeric powders to provide selective unipolar charging (Trigwell et al., 2003). Plasma treatment has also been exploited to increase the charge decay rate of acrylic and epoxy polymer powders by increasing the hydrophilicity and decreasing the electrical resistivity of the particle surface (Sharma et al., 2003, 2002).

a) 140

b) 25

Acrylic column Stainless steel column

120

20

qm (µC/kg)

qm (µC/kg)

100 80 60

15 Acrylic column Stainless steel column

10

40

5

20 0

0 5

15

25

35

5

RH (%)

15 RH (%) 25

35

Fig. 21. Comparison of charge density for (a) fine glass beads (b) silver-coated fine glass beads in acrylic column (circular symbols) and stainless steel column (square symbols) at different relative humidities, with Ug= 0.56 m/s (Fotovat et al., 2017b).

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Table 1. Summary of methods adopted for electrostatic charge mitigation in fluidized beds. Mechanism

Reducing the charge generation rate

Approach

Example

Reducing contact surface area

Adding 0.1 vol% fines (dp