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Effect of operating conditions and particle properties on electrostatics and entrainment in gas-solid… Al-Smari, Turki A 2014

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1  Chapter 1: Introduction   In gas-solid fluidized beds, fluidization occurs when solid particles are transformed into a fluid-like state by being suspended in a gas (Kunii & Levenspiel, 1991). The fluid flows upwards through the solids at such a velocity that the gravitational force on the particles is counteracted, and the solids are supported by the upward-flowing fluid. Fluidized beds have many advantages, such as excellent gas-solid contacting and heat transfer. Due to these advantages, gas-solid fluidized beds have numerous industrial applications, such as catalytic cracking, drying, polymerization of olefins, heat exchange, acrylonitrile manufacture and coating.  One of the disadvantages of fluidized bed reactors is particle entrainment. The high gas velocities present in fluidized beds often result in fine particles becoming entrained in the fluid. Pollution control regulations and high solids costs in some cases (e.g. for catalytic reactors) make it necessary to recover the solid particles entrained from the fluidized bed (Briens et al., 1992). Proper design of solids recovery equipment, such as cyclones, requires accurate predictions of both the solid flux and the size distribution of the entrained solids.  Entrainment phenomena are still not well understood since many parameters affect the entrainment rate. Numerous empirical correlations have been proposed for predicting the flux of entrained solids. Figure 1.1 summarizes predictions of the commonly used correlations to predict the entrainment rate in fluidized beds for a particular set of experimental conditions. As indicated in this figures, predictions derived from these correlations may differ by several orders of magnitude, even when the particles, column and operating conditions are well within the normal range of fluidized bed operations. Theoretical approaches based purely on hydrodynamic principles have also tended to fail badly. None of the correlations shown in Figure 1.1 makes any allowance for electrostatic effects. One possibility is that the failure of these approaches to consider electrostatic forces is responsible, in part or to a large extent, for the wide range of results and for the discrepancies in predicting entrainment.  2   Figure 1.1 Correlations used to predict the entrainment rate (from George and Grace, 1981), rectangular column 0.254 m x 0.432 m, Silica sand particles, dp = 102 µm.       The nature of gas-solid fluidization processes produces continuous motion and rubbing among bed particles such that the generation of electrostatic charges is inevitable. The electrostatic charges in fluidized systems can interfere with the normal hydrodynamics of the bed, resulting in particle-wall adhesion, inter-particle cohesion, electrostatic discharges, wall sheeting and even explosions, all of which can affect plant safety and economics (Cross, 1987). Electrostatic phenomena in gas-solid fluidization have been reported for many years (Lewis et al., 1949, Miller and Logwinuk, 1951, Osberg and Charlesworth, 1951) and need to be better understood.  The aim of this study is to investigate the influence of particle properties, gas relative humidity (RH) and operating variables, such as pressure, temperature, fluidizing air velocity, simultaneously on both particle entrainment and electrostatic charges to gain a better understanding of how operating conditions and electrostatic influence particle entrainment in gas-solid fluidized beds.   3  1.1 Particle Entrainment  Entrainment refers to the removal of solid particles from a fluidized bed by transport of these solids through the freeboard and out of the vessel (Yang, 2003). Solids entrainment becomes approximately constant at a certain height above the bed; this height is denoted as the transport disengaging height (TDH) (e.g. Baron et al., 1988). Figure 1.2 illustrates the most important zones in a fluidized bed.    Figure 1.2 Fluidized bed zones (from Wouter, 2008)      In order for the solids to be entrained, particles must be ejected into the freeboard and then carried through it. Ejection of solid particles from the fluidized bed into the freeboard can occur by different mechanisms (Levy et al., 1983) as shown in Figure 1.3. There are two main mechanisms. The first occurs when the solids at the bed surface become part of the bubble roof and are thrown upward into the freeboard as the bubble breaks the bed surface (Nose model). The second mechanism occurs when some solids in the wake of rising bubbles are thrown upward into the freeboard as bubbles burst at the surface (Wake model). The main difference between bubble nose particles and bubble wake particles is that the particles ejected into the freeboard from the nose are highly dispersed, whereas those from the wake remain in denser clumps (Yang, 2003).  4   Figure 1.3 Particle ejection mechanisms (from Yang, 2003)       For environmental and financial reasons, it is necessary to recover the solid particles entrained from fluidized beds, requiring additional equipment and operational costs (Zenz & Weil, 1958). Accurate prediction of solid particles entrainment rates and size distribution in the freeboard are important for the proper design of cyclones and other gas-solid separation equipment (Alan & Hamdullahpur, 1993).   1.2 Parameters Affecting Particle Entrainment   1.2.1 Effect of Temperature   Temperature has a large effect on the fluid properties, as well as the properties of some solids, depending on the composition. George and Grace (1981) studied the effect of temperature on solids entrainment fluxes in a pilot scale fluidized bed over a temperature range of 300 to 445 K. They found that the temperature had little effect on entrainment rate over this limited range. Choi et al. (1989) reported that the particle entrainment rate decreased with increasing temperature. Other researchers (Merrick and Highley, 1974; Lee et al., 1990, 1992,; Park et al., 1991) obtained similar results in fluidized bed combustors. In contrast, Romanova et al. (1980) and Milne et al. (1993) reported that the particle entrainment rate increased as temperature increased. Knowlton et al. (1990) investigated the effect of temperature on entrainment in a pressurized fluidized bed. They found that particle entrainment rate increased with both gas viscosity and gas density. Choi et al. (1998) measured the effect of temperature on the entrainment rate from a gas-solid fluidized bed 5  with variations of gas velocity, bed temperature and particle properties. They found that entrainment rate increased after an initial decrease with increasing bed temperature. They concluded that the influence of temperature on entrainment rate decreased as the particle density or gas velocity increased.  1.2.2    Effect of Pressure  Increasing pressure increases the gas density, increasing the tendency of a gas to carry solid particles. May and Russel (1954) and Chan and Knowlton (1984) reported that the entrainment rate increased as pressure increased.   1.2.3    Effect of Superficial Gas Velocity  High superficial gas velocity in the fluidized bed reactors results in more carry-over of particles. If the superficial gas velocity is increased, bed expansion increases and larger bubbles rise more quickly. Therefore the ejection velocity of bubbles erupting at the bed surface also increases. In addition, upwards drag on the particles in the freeboard is increased, resulting in increased carry-over of solid particles from the fluidized bed (Kato and Ito, 1972).   Many researchers (e.g. Baron et al., 1990; Nakagawa et al., 1994; Ma and Kato, 1998; Choi et al., 2001) have reported that the entrainment rate increased strongly as superficial gas velocity increased, typically with entrainment proportional to U to the power of 3 to 4.   1.2.4    Effect of Freeboard and Expanded Bed Height  As stated above, the entrainment flux becomes approximately constant at a certain height above the bed surface, the TDH. The freeboard zone of fluidization columns provides space for disengagement of particles. Baron et al. (1988) found that the entrainment rate decreased exponentially with freeboard height. Anderson and Leckner (1989) and Tannous et al. (2008) obtained similar results and, in addition, concluded that the mean particle size decreased exponentially with height in the freeboard. Baron et al. (1990) investigated the effect of dense bed height on the rate of solid particle entrainment. They reported that the entrainment rate increased as the bed height increased. This could be due to larger bubbles and reduced freeboard heights for a given overall equipment height.  6  1.2.5    Effect of Particle Properties   Solid particles in fluidized bed industrial applications cover a very wide range of properties. Entrainment and elutriation rates are affected by these properties. It is well known that the terminal velocity for a single particle decreases as the particle diameter decreases. Therefore, for smaller particles, a lower terminal velocity is expected, resulting in greater entrainment from the fluidized bed at a given gas velocity.  Kato and Ito (1972) and Tasirin and Geldart (1999) reported that elutriation increased as the particle size decreased in a gas-solid fluidized bed. Different results were reported by Baeyens et al. (1992), Ma and Kato (1998) and Nakazato et al. (2004) who investigated the effect of adding very fine particles on the elutriation rate constant. They concluded that the elutriation rate constant increased with decreasing particle size; however, below a critical particle size the elutriation rate constant no longer increased. They attributed this to interparticle adhesion forces. Baeyens et al. (1992) proposed a method for calculating the particle critical size at which adhesion forces become negligible compared with other forces. Choi et al. (2001) investigated the effect of fine particles on the entrainment of coarse particles. They found that the rate of carryover of coarse particles increased with the increasing proportion of fine particles in the bed. They also reported that the effect of fine particles on the elutriation rate of coarse particles decreased as the gas velocity increased, whereas the bed particle size distribution had only a minor effect on the elutriation rate of fine particles. The effect of coarse particle density on the entrainment from a gas-solid fluidized bed was evaluated by Nakazato et al. (2004). They found that the entrainment of fine and coarse particles decreased as the coarse particle density increased. Kato and Li (2001) studied the effect of bed coarse particle diameter on the entrainment rate. They reported that the elutriation rate constant for Geldart group C particles in a fluidized bed of fine-coarse particle mixtures decreased with increasing mean diameter of the coarse particles in the bed for a constant superficial gas velocity.     7  1.2.6    Effect of Electrostatics  In a fluidized bed, interparticle interactions occur due to different forces, such as van der Waals, electrostatics, capillary and collision forces. Interparticle forces have been reported to have a significant effect on the entrainment rate of fine particles from a fluidized bed (Kato and Li, 2001). Extensive studies (Geldart and Wong, 1987; Baron et al., 1992; Baeyens et al., 1992; Ma and Kato, 1998) have been conducted on the effect of van der Waals forces on the entrainment of fine particles. These forces are only appreciable when the particles come sufficiently close together, and they then produce significant attractive atomic interactions. These forces are strongly influenced by the size of the entrained particles. Adhesion forces become important when the entrained particles are smaller than a critical size (Geldart and Wong, 1987). The effect of electrostatics on entrainment has received little attention. The generation of static charges is quite complex. Electrons or ions can transfer between two bodies in contact, forming electrical double layers of charges of opposite sign. If the bodies are suddenly pulled apart, the original electronic equilibrium cannot be re-established, and one of the surfaces gains more electrons or ions than before the contact, while the other has a deficit (Cross, 1987). Geldart and Wong (1985) studied the effect of gas relative humidity (RH) on particle entrainment from a fluidized bed. They found that the entrainment rate decreased when the gas RH increased to above 60%. They believed that powder cohesivity was responsible for this phenomenon, and that electrostatic forces, which are influenced by gas humidity, had a negligible effect in their system. On the other hand, Baron et al. (1987) concluded that the entrainment rate decreased by 45% when the gas RH was reduced from 30 to 10% in a bubbling fluidized bed. They attributed this to an increase in pressure drop in the freeboard (~ 50% of the total pressure drop) due to electrostatic interactions between the particles and column wall. Briens at al. (1992) investigated the effect of electrostatics in a bubbling fluidized bed. They concluded that electrostatic forces had no effect on the size distribution of the entrained particles; consequently, these forces were not responsible for particle agglomeration. However, they found that electrostatic forces greatly affected particle entrainment. On the other hand, Mehrani and Giffin (2013) reported that RH had little or no 8  effect on particle entrainment rate. They attributed this result to the hydrophobic nature of the polyethylene particles tested in their study.  The effect of electrostatic forces on the elutriation of fine particles from a gas-solids fluidized bed was also studied by Kato and Li (2001). They proposed three zones to explain the variation of interparticle forces with gas RH, as shown in Figure 1.4. At low RH (RHe:RH≤32%), electrostatic forces were determined to be dominant forces, and the elutriation rate constant increased as the RH of the gas increased due to the decrease in the electrostatic potential of bed particles. At high RH (RHc:RH≥66%), the elutriation rate constant was found to decrease with increasing gas RH due to the effect of the capillary and van der Waals forces, whereas electrostatic forces were believed to have a minor affect.    Figure 1.4 Relative humidity effect zones (adapted from Kato and Li, 2001). RHe and RHc denote electrostatic and capillary relative humidity of gas, whereas K is the entrainment rate constant  Mehrani (2005) and Mehrani et al. (2012) measured electrostatic charge generation in a bubbling fluidized bed based on a Faraday cup. They observed that entrained fine particles carried charges, leaving a net charge behind. Also, they found that the charge density of entrained fines is higher than the charge density of large particles in the bed. They concluded that the entrainment of fine particles may contribute significantly to net charge build-up in a 9  fluidized bed. In an elevated-pressure unit, Moughrabiah et al. (2009) reported that the charge polarity in the freeboard region was opposite to that at all three measurement levels inside the dense bed, indicating that the fine particles entrained from the fluidized bed carried charges, leaving behind a net charge of opposite polarity inside the bed. Rokkam et al. (2010a) developed a CFD model to study the effect of electrostatics on entrainment of fine polymer particles in fluidized bed reactor. In their simulation, lower entrainment rate was observed when fine particles were charged. In another study (Rokkam et al, 2010b) the rate of entrainment was predicted to be lower for charged catalyst particles than for uncharged particles.  1.3    Electrostatic Phenomena   Since the 1940s, a number of researchers (e.g. Lewis et al., 1949; Miller and Logwinuk, 1951; Osberg and Charlesworth, 1951) have encountered electrostatic effects in fluidized beds by observing the adhesion of particles to reactor walls and their influence on fluidization behaviour (Jones, 1997). Problems associated with fluidized bed electrification include particle-wall adhesion, inter-particle cohesion and electrostatic discharges. Particles of dielectric materials, such as glass and polyolefin, tend to generate significant electrostatic charges in fluidized beds. The charged particles can coat vessel walls, requiring frequent cleaning. Frequent shutdowns and mechanical cleaning can negatively affect plant economics and safety. The charged particles can also interfere with sensors and bed internals, and they can significantly alter bed hydrodynamics.  1.4    Electrostatic Charge Generation Mechanism  Fluidization by its nature is associated with continuous contact and separation, as well as with friction of particles against each other and against the vessel wall, leading to electrostatic charge generation. The charge generation mechanism is not well understood due to its complexity. In gas-solid fluidized beds, particles can be charged due to triboelectrification, frictional charging and thermionic emissions in high-temperature processes.  1.4.1    Triboelectrification  Triboelectrification involves the generation of electrical charges due to rubbing of materials. 10  As shown in Figure 1.5, when two solid bodies come into contact with each other, charges can move from one to the other based on the energy of electrons and ions at the surfaces until charge equilibrium is reached (Fan and Zhu, 1998). Triboelectrification, also known as contact electrification, occurs due to the difference in the initial Fermi energy levels of the materials at the contact surface until the energy levels are equalized. The Fermi energy level is the highest occupied energy level at an absolute temperature of 0 K (Cross, 1987). The energy required to move an electron from the top of the energy distribution, out of the metal to infinity, is called the work function (Cross, 1987). Upon separation, particles that lose electrons become positively charged, whereas those that gain electrons become negatively charged. The charge polarity of a material is affected by different factors, such as material purity, surface finish, particle shape and moisture content (Cross, 1987).    Figure 1.5 Triboelectrification charging mechanism (adapted from Jones, 1997)      1.4.2    Frictional Charging  Frictional charging occurs when the surfaces of solid particles rub against each other and/or against the reactor walls. In industrial gas-solid fluidized beds, frictional charging is often identified as the main charging mechanism. In cases where the reactor diameter is sufficiently large to neglect wall effects, particle rubbing becomes the dominant source of charge generation inside the reactor. According to Cross (1987), charge generation between similar materials due to frictional charging can be as great as for dissimilar materials. Boland and Geldart (1971) attributed the charges generated in a gas-solid fluidized bed to the motion of solid particles in the wakes of gas bubbles. Montgomry (1959) reported that charge generation increased with rubbing velocity and as the force at contact increased. Cross 11  (1987) concluded that the charge transfer is influenced by friction energy more than by the nature of the material. In large industrial gas-solid fluidized beds, frictional charging is the main source of charge generation since wall effects can be neglected (Park et al., 2002a).    1.4.3    Thermionic Emission Charging  Thermal electrification can occur when solid particles are exposed to very high temperatures (>1,000 K). The electrons inside solid particles can gain energy from a high temperature field to overcome the energy barriers and be freed. Therefore, the bed becomes thermally electrified (Fan and Zhu, 1998).  1.5    Electrostatic Measurement Techniques  Different experimental measurement techniques have been employed by previous researchers to measure electrostatic charges in gas-solid fluidized beds. The two main techniques are Faraday cups and electrostatic probes. Electrostatic probes measure the cumulative charges generated inside the bed, whereas Faraday cups determine the charges on particle surfaces. A third method, discussed in this section, is a current detecting pipe technique that has been used by previous researchers in gas-solid pipe flow to measure the charges transferred from the particles to the pipe wall. In this study, all three techniques were employed in a gas-solid fluidized bed to measure the electrostatic charges generated in the fluidized bed.  1.5.1    Electrostatic Probes  Electrostatic charges that build up inside a gas-solid fluidized bed and on its walls have been measured by previous researchers using three types of electrostatic probes:  1.5.1.1   Induction Probes  Induction probes have been used by many researchers, e.g. Boland and Geldart (1971/1972) and Chen et al. (2006). Figure 1.6 shows the induction probe utilized by Boland and Geldart (1971). The principle behind these probes is that a real charge induces an image of itself on conducting surfaces. This type of probe is generally mounted on the wall of the column, with part of the wall between the tip of the probe and the inner surface of the column. These non-contacting probes have the advantage of not disturbing the flow because they are not in direct 12  contact with the fluidized bed. However, a disadvantage is that they primarily measure the charge build-up on the column walls rather than the charges inside the bed.   3.2 mm inner face(of 2-D bed) 1.27 cm back-up wallNuts and washer forelectrical connectionMetal tubeProbe head2.5 mm dim Coaxial cable   Figure 1.6 Induction probe (from Boland and Geldart, 1971)    1.5.1.2   Capacitance Probes  Capacitance probes have been used by a number of researchers, e.g. Wolny and Kazmierczak (1989) and Guardiola et al. (1992). Figure 1.7 shows the capacitance probe developed by Guardiola et al. (1992) to measure the degree of electrification by determining the potential difference between a metallic probe in contact with the bed and a metallic distributor. The probe placed along the axis of the column was 0.052 m in diameter and constructed from a Perspex tube; the capacitance probe was 5 mm in diameter and made from a copper tube. The distributor plate was made from stainless steel and was grounded. The disadvantages of this method include disturbing the flow and averaging the effect of electrostatic charges over the entire fluidized bed, rather than obtaining localized measurements.    13    Figure 1.7 Capacitance probe (from Guardiola et al., 1992)  1.5.1.3   Collision Probes  Collision probes have been employed in a number of studies (e.g. Ciborowski and Wlodarski, 1962; Fujino et al., 1985; Park et al., 2002b; Chen et al., 2003a; Moughrabiah et al., 2009). The contacting probes are made of highly conductive materials. These were either suspended or mounted in the column, and electrically connected to electrometers to measure the potential or current generated inside the fluidized bed. Low accuracy due to particles adhesion to the tip of the probe and disturbing the flow are major drawbacks of collision probes. The most common type of collision probes is a ball probe. Figure 1.8 shows the collision ball probe used by Park et al., (2002b) to measure charge inducement and transfer due to bubble movement in a two-dimensional fluidized bed.      14     Figure 1.8 Collision ball probe (adapted from Park et al., 2002b)  1.5.2    Faraday Cup  Faraday cups have been used by previous researchers (e.g. Tardos and Pfeffer, 1980; Wolny and Opalinski, 1983; Ali et al., 1998; Zhao et al., 2000; Wang et al., 2004; Mehrani, 2005; Mehrani and Giffin, 2013) to measure net charge build-up on particle surfaces directly. A Faraday cup is a double-walled vessel of any suitable shape, as illustrated in Figure 1.9. The outer cup is grounded and functions as a screen, preventing external fields from affecting the measurements. The inner cup is connected to an electrometer to measure charges by detecting the voltage across a known capacitor. When a charged object enters the inner cup, an equal and opposite charge is induced on the wall of the inner cup. This charge is stored on the capacitor in the electrometer and measured (Cross, 1987). The Faraday cup method has some disadvantages, such as the possibility additional charging during handling of solid particles before entering the inner cup and the ability to measure only overall charges of withdrawn samples.       15     Figure 1.9 Faraday cup (adapted from Cross, 1987)  1.5.3    Current Detecting Pipe  A current detecting pipe has been used by several previous researchers (e.g. Masuda et al.1976, 1994; Cartwright et al., 1985; Nieh and Nguyen, 1988; Gajewski, 1989; Wang et al., 2004, Matsusaka and Masuda, 2006) to measure the electric current generated from a metal pipe in a gas-solids pipe flow system directly. In gas-solids pipe flow, particles are charged as a result of repeated collisions with the pipe inner wall. The amount of charges transferred from solid particles to the wall per unit time is nearly equal to the electric current flowing from the wall to earth. This generated current is measured by an electrometer connected to the detecting pipe surface. These detecting pipes can be used in dilute phase gas-solids pipe flow when particles can freely collide with the pipe inner wall, and the effect of particle-particle interactions is neglected; however, for dense phase gas-solids pipe flow, these detecting pipes give lower efficiency because the surrounding particles prevent free particle-pipe interactions (Matsusaka et al., 2010). Such detecting pipes could be made of different materials, such as copper, stainless steel and conductive PTFE, with electrical isolation and an electric shielding around the detecting pipe to prevent electrical noise, as shown in Figure 1.10.  16  An advantage of current detecting pipes is that they do not disturb the flow because they are not in direct contact with the gas-solids flow. However, a disadvantage is low accuracy due to particle adhesion to the pipe inner wall at low gas velocities.    Figure 1.10 Current detecting pipe (adapted from Matsusaka and Masuda, 2006)  1.6 Parameters Affecting Electrostatics in Bubbling Fluidized Beds Most industrial gas-solids fluidized bed reactors operate at high pressures, high temperatures and over a wide ranges of gas velocities. The gasification of coal and polyolefin generation are good examples of industrial processes that operate at elevated temperature and pressure. It is therefore very useful to understand how fluidized bed reactors perform at elevated temperature and pressure and with different gas and particles properties. Some researchers have investigated the influence of these parameters on bubble behaviour and electrostatic charge generation in bubbling fluidized beds.  1.6.1 Influence of Gas Velocity It has been reported that collisions between particles surrounding bubbles results in charge generation in bubbling fluidized beds. Boland and Geldart (1971) noted that electrostatic charges in gas-solid fluidized beds are generated by the motion of particles around the bubbles. Darton et al. (1977) found that the bubble size increases as gas velocity increases. 17  Several researchers (e.g. Ciborowski and Wlodarski, 1962; Guardiola et al., 1996; Yao et al., 2002, Chen et al., 2003) reported increases in electrostatic charge generation due to increases in fluidizing gas velocity. They explained the increase in electrostatic charge generation as a result of an increase in bubble size and rise velocity. All the above studies were conducted at atmospheric pressure. Moughrabiah et al. (2009) studied the influence of superficial gas velocity on electrostatic charge generation in an elevated-pressure bubbling fluidized bed. They performed their experiments with polyethylene particles and glass beads in a three-dimensional fluidized bed. They concluded that the bed electrification increases as the superficial gas velocity increases. They explained their results by the formation of bigger bubbles and higher bubble rise velocities that enhanced particle motion in the bed.  1.6.2 Influence of Operating Pressure and Temperature Limited work has been devoted to study the effect of pressure and temperature on electrostatic charge generation in gas-solid fluidized beds. Most previous experiments were conducted at ambient temperature and atmospheric pressure. Several researchers (e.g. Botterill and Desai, 1972; Barreto et al., 1983; Chitester et al., 1984; Li and Kuipers, 2002) have studied the influence of pressure on bubble behaviour in gas-solid fluidized beds and reported that higher pressure resulted in smoother fluidization and smaller bubbles. Kawabata et al. (1981) studied the effect of pressure (up to 800 kPa) in a two-dimensional fluidized bed containing sand particles. They found that bubble size was not influenced by elevated pressure. Olowson and Almstedt (1990) conducted free bubbling experiments at elevated pressures (up to 1600 kPa) in a fluidization column containing silica sand. They observed that the rise velocity and mean bubble frequency increased, whereas bubble size decreased with increasing pressure. Newton et al. (2001) showed that the mean bubble diameter decreased and bubble velocity increased as the pressure increased. Boland and Geldart (1971) reported that the degree of charging in a fluidization column containing glass beads particles increased as the bubble size increased. Yao et al. (2002) studied the influence of bubble behaviour on electrostatic charges in a Plexiglas fluidization column of 0.089 m inner diameter containing polyethylene particles. They used a collision ball probe to measure the local electrostatic charges. Their results showed that electrostatic charges in a gas-solid fluidized bed are affected by the bubble behaviour. 18  Moughrabiah et al. (2009) investigated the effect of operating pressure (up to 724 kPa) on the degree of bed electrification in a three-dimensional fluidized bed containing glass beads. Their experiments showed that, as the pressure increased, the degree of electrification increased. They attributed these results to the increase in bubble rise velocity, frequency and volume fraction. The effect of temperature is still not well understood. Some researchers (Mii et al., 1973; Yoshida et al., 1974) studied the effect of temperature on fluidization behaviour. They concluded that both the frequency of bubble formation and the quality of fluidization increased as temperature increased. On the other hand, Newton et al. (2001) investigated the effect of temperature on bubble behaviour in a gas-solid fluidized bed, finding that the bubble frequency decreased and bubbles became larger as temperature increased. Moughrabiah et al. (2009) investigated the influence of temperature (up to 90°C) on the degree of electrification inside a gas-solid fluidized bed by conducting freely bubbling experiments with glass beads and low-density polyethylene particles. They found that as the bed temperature increased, the degree of bed electrification decreased. They attributed these results to the smaller and slower bubbles as temperature increased.    1.6.3 Influence of Gas Properties – Relative Humidity Controlling gas humidity is a common technique to prevent or reduce electrostatic charges in gas-solid fluidized beds. The influence of gas RH on electrostatic charges inside fluidized beds has been investigated by several researchers. Ciborowski and Wlodarski (1962) concluded that increasing the fluidizing gas RH results in a decrease in bed potential. They attributed these results to increases in the rate of charge dissipation. Other researchers (Boland and Geldart, 1972; Tardos and Pfeffer, 1980; Wolny and Kazmierczak, 1989; Guardiola at al., 1996; Park et al., 2002; Mehrani, 2005; Moughrabiah et al., 2009; Mehrani and Giffin, 2013) reported that increasing the gas RH reduces electrostatic effects in a gas-solid fluidized bed. They attributed their results to the increase in the solids surface conductivity and higher charge dissipation rate. Mehrani et al. (2007) investigated the influence of fluidizing gas RH on the charge density (charge-to mass ratio) of fine glass beads particles. They found that as the gas RH increased, the charge density decreased. This technique has several issues. First, high gas humidity (> 75%) tends to result in excessive capillary forces that cause defluidization (Guardiola et al., 1996). Second, 19  humidification is not feasible for some gas-solids industrial fluidized bed reactors where humidity poisons the catalyst. Finally, this technique has been found to be ineffective in high-temperature processes (Ciborowski and Wlodarski, 1962).  1.6.4 Influence of Particles Properties In triboelectrification charging, the generation of electrostatic charges is due to rubbing between materials. The charge polarity of the materials can be influenced by several factors, such as purity, particle shape, moisture content and surface finish (Cross, 1987). Guardiola et al. (1996) investigated the effect of particle size on bed electrification and found that as the particle size increases, electrification in the fluidized bed increases. On the other hand, Mehrani et al. (2007) conducted experiments in a Faraday cup fluidized bed with a binary mixture of particles consisting of large and fine glass beads. It was found that the fine particles carried higher charges per unit mass than the larger particles. Several researchers (Ali et al., 1998; Zhao et al., 2000; Mehrani, 2005; Inculet et al., 2006; Mehrani at al., 2007; Moughrabiah et al., 2009) have reported bipolar charging. Bipolar charging has been described as contact charging between solid particles of the same material but different sizes. Ali et al. (1998) found that for one type of particles, small particles charged negatively and large ones charged positively. Similar results were reported by Zhao et al. (2000). Mehrani (2005) performed experiments in a fluidized bed with binary mixtures of large and fine glass beads particles. They found that the entrained fines (30 µm mean diameter) were positively charged, whereas larger particles (566 µm mean diameter) were negatively charged. Mehrani et al. (2007) concluded that the charges carried by the fine particles were more likely due to charge separation. Moughrabiah et al. (2009) measured the electrostatic charges of entrained fine glass beads particles using a collision ball probe mounted in the freeboard zone of a fluidized bed. It was found that the polarity in the freeboard was opposite to that of the dense bed of large glass beads below.  1.7 Solid Particles Classification In fluidized beds, different types of solid particles fluidize differently depending on such factors as drag, particle interactions and physical properties. Based on particle diameter and density, Geldart (1973) classified different types of solids in air at atmospheric temperature and pressure into four groups, namely A, B, C and D particles. Figure 1.11 illustrates the 20  Geldart classification of particles for air at ambient conditions. For extension to other gases, temperatures and pressures, see Grace (1986). In this study, groups A (fine) and B (coarse) particles are used.   Figure 1.11 Geldart solids classification (adapted from Geldart, 1973)  1.8 Fluidization Flow Regimes In gas-solid fluidized beds, as the gas velocity increases, different flow regimes are encountered, as illustrated in Figure 1.12. For Geldart group B and D particles in gas-solid fluidized beds, the bed is transformed from a fixed bed into a bubbling fluidized bed when the superficial gas velocity (Ug) exceeds the minimum fluidization velocity (Umf). For Geldart group A particles, the bubbles appear when the gas velocity exceeds the minimum bubbling velocity (Umb> Umf) (Yang, 2003). In this study, only the bubbling flow regime is investigated.  21    Figure 1.12 Gas-solid flow regimes (from Grace, 1986)  1.9 Thesis Objectives The main goal of this research project is to gain a better understanding of the particle entrainment and how it is influenced by operating conditions, electrostatic and particle properties in gas-solid fluidized beds. This was investigated by simultaneously measuring the particle entrainment flux, electrostatic charges inside a fluidized bed and the charge on entrained particles as a function of operating conditions and bed properties.  Specific objectives contributing to this goal are:  • To develop adequate measurement techniques to determine the particle entrainment flux from the fluidized bed and the electrostatic charge on the entrained particles. • To determine the effect of different operating variables, such as pressure, temperature and gas velocity on particle entrainment and electrostatics.         • To identify the effect of fluidizing gas relative humidity on particle entrainment and electrostatic charges. • To investigate the influence of particle properties (such as size, type and density) on particle entrainment and electrostatic charges in fluidized beds. 22  • To study the relationship between entrainment and electrostatics and investigate whether it is important to include the effect of electrostatic when modeling or correlating solids entrainment from fluidized beds.     1.10 Thesis Outline Chapter 2 describes the experimental equipment and methodology. The first section describes the fluidization column air system, including the air compressor, refrigeration unit, dryers and buffer tank. The second section provides details of the fluidization column. The third provides details of the instrumentation and measurement techniques, such as pressure and temperature sensors, pressure control valve, pressure transducers, collision ball probes, freeboard sampling pipe and current detection pipe. The fourth section provides details of the properties of the particles studied. The final section describes the experimental procedure.  Chapter 3 presents the experimental results for the effect of operating variables on the particle entrainment flux and electrostatic charge inside a fluidized bed. Results and analysis of experiments performed at different gas velocity, temperature and pressure are given.  Chapter 4 focuses on the influence of gas relative humidity (RH) on particle entrainment flux and electrostatic charge in a fluidized bed. Results of experiments carried out at different gas RH for different fine particle types are presented and analyzed.  Chapter 5 provides details of the experimental results on the influence of coarse and fine particle properties on the entrainment flux and electrostatic charge in fluidized beds. Results and analysis of experiments performed with different coarse and fine particles, sizes, concentrations and densities are presented.  Chapter 6 is devoted to the influence of electrostatic on particle entrainment flux at different operating conditions in a fluidized bed. Results and analysis are presented.  Chapter 7 presents the overall conclusions and recommendations for future work.  Additional experimental results, detailed engineering drawings and photographs of the equipment are given in the appendices. 23  Chapter 2: Experimental Equipment and Procedure    Chapter 2 describes the experimental equipment, including the high-pressure air system, elevated-pressure fluidization column, instrumentation for measuring pressure, flow and temperature, control systems, collision ball probes, freeboard-sampling pipe (based on Faraday cup principle) and current-detecting pipe. It also presents the properties of the solid particles used in the experiments and describes the experimental procedures.    2.1 High-Pressure System – Air Compressor High-pressure air was supplied by a rotary screw compressor (KAESER Model SK19) to pressurize the column to the required operating pressure. The compressor is equipped with a Sigma Control System to provide the option of automatic control of the discharged air pressure. It includes safety pressure relief valves and an emergency stop button. The high-pressure air was dried by passing through a refrigeration unit (KAESER Model TA11) and vapour-removal filters, located downstream of the compressor. The refrigeration unit and filters can be totally or partially bypassed, which helps to control the relative humidity (RH) of the fluidizing air.   To deliver the high-pressure air to the column at constant pressure, a high-pressure buffer tank (120 gallon) was installed upstream of the column inlet. A silica gel dryer was also installed between the buffer tank and the column inlet. This dryer can be totally or partially bypassed to maintain the RH of the fluidizing air in the required range.  The overall layout of the elevated-pressure fluidization unit is shown in Figure 2.1.          24  Air FlowmeterPR Air DryerBufferTankPSVCurrent Detecting PipeCycloneFluidized BedElectrometersHeater TapesSampling VesselTo VentTo atm. Pressure ControllerPCV 3-way Diverter ValveCompressorPre FilterRefrigerating UnitCoalescing Filter Vapor Removal FilterCollision Probes Figure 2.1 Schematic of overall layout of fluidization unit. PR: pressure regulator, PVC: pressure control valve.   2.2 Elevated-Pressure Fluidization Column The fluidization experiments were performed in a modified three-dimensional elevated-pressure (up to 1000 kPa) fluidization column constructed of stainless steel, with an inner diameter of 0.15 m and a height of 2.0 m. The modified fluidization column originally built for Moughrabiah (2009), is equipped with a distributor consisting of two stainless steel perforated plates, each containing 50 aligned holes. The holes in the upper and lower plates are 4 and 5.5 mm in diameter, respectively. A steel screen with 15 µm openings is installed between the two plates to prevent fine particles from dropping into the windbox. The distributor plates are designed to have an open area ratio of 3.8%.  25  The particles entrained from the vessel by gas at the top of the column pass through an external cyclone. For a wider range of superficial gas velocities, two alternative cyclones (inside diameter 75 and 51 mm) constructed from stainless steel were employed to capture and return entrained particles via a dip-leg through a port whose centre was 150 mm above the distributor plate. A 1-inch (25 mm) high-pressure rated three-way diverting valve with 60 degree outlets ports, specially manufactured by Quality Controls, Inc. was installed on the dip-leg, downstream of the cyclone. In normal operation, the solid particles recovered by the cyclone are directed via the three-way valve into the fluidization column. For entrainment rate measurements, they are diverted into a cone-shaped sampling vessel, 100 mm inner diameter and 254 mm height. A sintered metal filter (provided by Mott Corporation) with 90% collection efficiency of 10 µm openings was installed in the sampling vessel at the exit line to prevent losses of fines during sampling.   The fluidization column is equipped with three sight glasses, each 25.4 mm in diameter, to allow visual observation inside the column. Figure 2.2 shows a schematic diagram of the fluidization column.  Detailed engineering drawings and photographs of the fluidization unit are provided in the Appendix A.     26   Figure 2.2 Schematic of fluidization column.    2.3 Instrumentation and Measurements The fluidization column was equipped with several instruments and control systems, such as pressure transducers, pressure control valve, pressure safety valve, pressure and temperature sensors, flowmeters, hygrometers, electrical heating tapes, electrostatic collision ball probes, freeboard sampling pipe, and current detecting pipe. 27  2.3.1 Fluidization Column Operating Variables and Measurements The bed temperature was measured at different levels by two bi-metal dial thermometers immersed inside the column. The bed temperature was controlled by electrical heating tapes (Omega HTWC) wrapped around the fluidization column and the inlet pipe. The air flowrate was measured by a mass flowmeter located on the column inlet pipe, equipped with a valve to adjust the flowrate to the desired value. The RH of the incoming air was monitored by a hygrometer (Vaisala Model HMP238) immersed in the bed. This hygrometer can measure RH from 0 to 100% and with ±1% accuracy. The RH of the exit stream was also monitored using a portable digital hygrometer (Omega Model RH71, ±2% accuracy). The pressure inside the fluidization column was measured by pressure transducers (Omega Model PX4200, ±0.25% accuracy). These were frequently back-flushed by high-pressure air to prevent blockage by fine particles. The pressure of the column was controlled by a pressure control valve (Fisher-Rosemount 24000C-series) located on the cyclone gas outlet line. To protect the fluidization column from excessive pressure build-up, a pressure relief valve was installed downstream of the column. Differential pressure transducers (Omega Model PX750, ±0.25% accuracy) were used to measure the pressure drops.   2.3.2 Electrostatic Charge Measurement Techniques 2.3.2.1   Collision Ball Probes The degree of electrification in the bed was measured by four collision ball probes, identical to those described and employed by Moughrabiah et al. (2009). Each probe consisted of a stainless steel ball, 5.3 mm in diameter, connected to a stainless steel wire. Ceramic and polyethylene tubes 5.5 and 8.75 mm in diameter, respectively, were used to protect the probe and maintained high resistance to the ground. In addition, a brass tube 12.7 mm in diameter was used to eliminate any disturbances due to build-up of charges on the fluidization column walls.  A schematic of the electrostatic collision ball probe is provided in Figure 2.3.      28   Stainless steel ball5.3 mm diaBrass tube to reduce background current by eliminatingdisturbances due to build-up of charges on column walls   12.7 mm O.D. Copper wire leads to electrometer   Teflon tube isolating S. S. wirePolyethylene tube protecting the ceramic tube 8.75 mm O.D.  Ceramic tube maintaining a high resistance to the ground5.5 mm O.D.  Figure 2.3 Collision ball probe (from Moughrabiah et al., 2009)  The degree of electrification in the bed was characterized by measuring the cumulative charges induced and transferred through the collision ball probes. Four collision probes were mounted so that they were centred on the axis of the column and at different levels, 0.15, 0.31, 0.55 and 0.97 m above the distributor plate, as shown in Figure 2.4. Each probe was connected directly to an electrometer (Kistler model 5010B), able to measure charges from ±10.0 pC to ±1.0 µC. The electrometer was connected to a computer through a DAS08 data acquisition card to record the measured signals.             29                                              Figure 2.4 Locations of collision ball probes.  2.3.2.2   Freeboard Sampler  To measure the charge density of entrained particles in the freeboard, a sampling pipe inspired by the Faraday cup concept was developed. The sampling pipe was mounted in the freeboard at the centre of the fluidization column. It consists of two copper pipes insulated from each other by Black Epoxy (832B), as shown in Figure 2.5. The sampling pipe intake was designed with a cone shape and a maximum inner diameter of 51 mm to enhance the solid particle collection efficiency from the dilute phase stream. The inner pipe was 19 mm in inner diameter and 127 mm in length, whereas the outer pipe was 25.4 mm in inner 30  diameter. The entrained particles were collected in a filter (provided by IMP CO Ltd.) with 0.0063 m2 surface area and 89% collection efficiency of 10 µm particulates placed at the end of sampling pipe. The mass of fine particles collected was determined by weighing the filter before and after each run by an analytical balance with an accuracy of ±0.0002 g. A high-pressure air tube was connected to the sampler to facilitate flushing of the sampling pipe and to provide small positive flow to prevent blockage by fine particles.         Figure 2.5 Schematic of freeboard sampler.    2.3.2.3   Current Detecting Pipe  To measure the degree of electrification of entrained particles in the fluidization column exit line, a current detecting pipe similar to those used by Matsusaka and Masuda (2006) was developed. This is a 17-4 PH stainless steel pipe, 25.4 mm in diameter, 4 mm in thickness and 400 mm long, installed at the column exit line, as shown in Figure 2.6. It is coated internally with Ni (work function=5.35 eV). The principle underlying this technique is that, in a gas-solid pipe flow, particles are charged by collisions with the interior pipe walls. The charge transferred from the solid particles to the wall per unit time is essentially equal to the electric current flowing from the wall to the earth. 31  The degree of entrained particles electrification was characterized by measuring the electrical current transferred from entrained fine particles to the metal pipe by collisions. To prevent electrical noise, an electric shield (grounded) made from copper was installed around the detecting pipe and insulated by a small piece of Teflon. The current detecting pipe was connected directly to an electrometer (Kistler model 5010B) by a triaxial cable to protect signals against external interference. The electrometer was also connected to a computer through a DAS08 data acquisition card to record the measured signals. Photographs of the collision ball probe, freeboard sampling pipe, current detecting pipe and electrometer are provided in the Appendix A.                               Figure 2.6 Schematic of current detecting pipe  2.4 Bed Materials 2.4.1 Coarse Particles The particles described in this section will for convenience be referred to as “coarse”. The coarse particles tested in this study included glass beads (GB) of different sizes, high-density polyethylene (HDPE) and low-density polyethylene (LDPE) of different properties. The glass beads were supplied by Potters Industries Inc. The polyethylene particles provided by NOVA Chemicals Inc. and Saudi Basic Industries Corporation (SABIC), sieved to the 32  desired size ranges, are non-smooth and non-spherical. They represent typical industrial conditions, whereas the glass beads with smooth and spherical surface represent ideal particles.  The key physical properties of the coarse particles used in this study are shown in Table 2.1.   Table 2.1 Relevant properties of the coarse particles.  GBS GBM GBL HDPE LDPE Particle density, ρp (kg/m3) 2500 2500 2500 965 797 Size range (µm) 106-212 212-425 425-600 500-600 500-600 Volume-weighted mean dia (µm) 155 357 560 553 573 Sphericity (-) ~1 ~1 ~1 ~0.75 ~0.77 Dielectric constant (-) 5-10 5-10 5-10 2.3 2.3      The coarse particle densities were provided by the suppliers. The size distributions and the volume weighted mean diameter were measured by a Malvern Mastersizer 2000 (see Appendix B for size distribution graphs). The sphericity of the coarse particles was provided by the suppliers. The dielectric constants were obtained from Reitz et al. (1993) and Jiang et al. (1994). The physical surface structure, such as sphericity and roughness of all coarse particles, was analyzed by Scanning Electron Microscopy (SEM). Figures 2.7a, 2.7b and 2.7c show that the glass beads are closely spherical whereas Figure 2.7d establishes that the polyethylene particles are non-spherical and have uneven surfaces.  33   (a) GBL  (b) GBM  (c) GBS  (d) LDPE  Figure 2.7 SEM images of coarse particles used in this study.   2.4.2 Added Fine Particles  The fines particles employed in this study were fine glass beads (GBF), fine polyethylene (PEF) and fine Aluminium oxide (Al2O3). Their key physical properties are provided in Table 2.2.       34  Table 2.2 Properties of fine particles employed in this study.  GBF Al2O3 PEF Particle density, ρp (kg/m3) 2700 3200 940 Size range (µm) 25-50 18-73 23-72 Volume-weighted  mean diameter (µm) 38 42 45 Particle sphericity ~1 ~0.9 ~0.78 Dielectric constant 5-10 4-11 ~2.3     The fines densities and sphericities were provided by the suppliers. The size range and volume-weighted mean diameters were measured by using a Malvern Mastersizer 2000 (see the Appendix B for size distribution graphs). The fine glass beads were supplied by Potters Industries Inc in the desired size range. The polyethylene powder was provided by Sigma-Aldrich Co. The spherical alumina (Al2O3) was provided by Industrial Powder Inc. Typical applications of Al2O3 include acting as a filler for molding materials, base powder for baking ceramic and blasting materials, and spacers.   The shape and physical surface structure, such as sphericity and roughness of all fine particles, were analyzed by SEM. Figures 2.8a and 2.8b show that GBF and Al2O3 are closely spherical, whereas Figure 2.8c provides evidence that the PEF particles are non-spherical and have uneven surfaces.    35   (a) GBF  (b) Al2O3    (c) PEF   Figure 2.8 SEM images of fine particles tested during this study   According to the Geldart (1973) classification of powders, the coarse particles (GBS, GBM, GBL, HDPE and LDPE) were all group B particles, whereas the fine particles (PEF, Al2O3 and GBF) were all group A particles.     36  2.4.3 Binary Mixtures Different binary mixtures were tested to investigate the influence of operating conditions, electrostatics and particle properties on entrainment in fluidized bed. The coarse particles, such as glass beads and polyethylene, were used as the mono-sized particles, and different fine particles, as showed in Table 2.2, were added to provide binary mixtures. The key physical properties of the mixtures are provided in Table 2.3.   Table 2.3 Properties of binary particle mixtures.  Mixture  Vol.-weighted mean dia. (µm) Loosely packed voidage Specific surface area (m2/g) Umf at 20 °C and 414 kPa  (m/s) Coarse Fines (wt %) M1 GBS GBF (5%) 136 0.368 0.0182 0.0154 M2 GBM GBF (5%) 257 0.372 0.0096 0.0528 M3 GBL GBF (5%) 342 0.375 0.0075 0.0883 M4 GBL GBF (10%) 246 0.373 0.0104 0.0484 M5 GBL GBF (15%) 191 0.371 0.0134 0.0299 M6 GBL Al2O3 (5%) 377 0.376 0.0083 0.1211 M7 GBL Al2O3 (10%) 282 0.374 0.0121 0.0724 M8 GBL Al2O3 (15%) 225 0.370 0.0159 0.0470 M9 GBL PEF (5%) 233 0.377 0.0072 0.0170 M10 HDPE GBF (5%) 442 0.622 0.0176 0.1311 M11 LDPE GBF (5%) 472 0.466 0.0208 0.1440  The size distributions, volume-weighted mean diameters and specific surface areas were measured by a Malvern Mastersizer 2000 (see Appendix B for size distribution graphs). Loose- packed voidages were determined by adding a known-weight of the particles to a known-volume container. Then, water (or ethanol for polyethylene particles) was added into the container to fill the total volume. The total mass (container + water + particles) was measured, and by knowing the density of water, the volume of added water and loose packed bed was then calculated.  The minimum fluidization velocities, Umf, of binary mixtures were estimated using the correlation given by Goossens et al. (1971). Several trials were made to measure Umf experimentally as recommended by Kunii and Levenspiel (1991); however, due to frequent 37  blockages in differential pressure taps, it was difficult to obtain conventional fluidization curves (pressure drop vs. gas velocity) for the binary mixtures. Chiba et al. (1979) found that the experimental procedure used to measure the Umf for mono-size particles is not applicable to binary mixtures, and Umf could not be determined accurately. Goossens et al. (1971) modified the correlation of Wen and Yu (1966) to account for mixture particle density and mixture particle mean diameter, and found that the new correlation could be employed for binary mixtures. For detailed calculations of Umf for the binary mixtures in this study, see the Appendix C.  2.5 Experimental Methodology  The following methods were used to study the effect of operating conditions, gas and particle properties on solids entrainment flux, electrostatic charges inside the fluidized bed and charges on entrained particles.  • Gas Velocity Variation Experiments To investigate the influence of gas velocity on the electrostatic charges generated inside the fluidized bed, entrainment flux and charge density of fine particles, free-bubbling experiments were performed with binary mixtures M3 and M10. The superficial gas velocity, Ug was varied from 0.2 to 0.6 m/s. To isolate the effect of other operating parameters, these experiments were conducted at 207 kPa and ambient temperature. The RH of the air was maintained at 12±2%. The collision ball probe locations and static bed height (~ 0.54 m) were the same for all experiments. The bed was fluidized for around 1.0 h to achieve steady state before data were collected. The degree of electrification in the bed was then characterized by the four collision ball probes. The online freeboard sampler (Faraday cup), located in the freeboard was utilized to measure the fines charge density by withdrawing samples from particles within the freeboard. The entrainment flux of solid particles recovered by the cyclone was measured directly by diverting the solid particles via a three-way diverter valve into a stainless steel sampling container. Then, the collected solids were weighed and the entrainment flux was calculated. The volume of collected samples was small enough, that the withdrawal of each sample did not influence the fines concentration in the bed appreciably. The entrainment flux and fines charge density were measured at 38  least four times for different experimental conditions. After each run, the collected solid particles were returned to the fluidization column. Also, the detection stainless pipe coated with Ni (see section 2.3.2.3) was used to measure the current generated by entrained fine particles passing through the reactor outlet line.    • Temperature Variation Experiments To study the effect of temperature on the electrostatic charges generated inside the fluidized bed, entrainment flux and charge density of fine particles in the freeboard, free bubbling experiments were performed with binary mixtures M3, and M10 at a freeboard pressure of 414 kPa and at superficial gas velocity Ug = 0.3 m/s. The experimental procedures were the same as those described previously. The temperature of the bed was varied from 25 to 75 ºC.  • Pressure Variation Experiments Free bubbling fluidization was performed at room temperature with superficial gas velocity, Ug = 0.3 m/s and the RH of the gas maintained at 12±2%. In these experiments, the same procedures described previously were repeated. The air pressure was varied from 207 to 517 kPa to determine the influence of pressure on electrostatic charges generation inside a fluidized bed, flux and charge density of entrained particles. Binary mixtures M3 and M10 were tested in these experiments.  • Relative Humidity Variation Experiments To investigate the effect of fluidizing gas RH, free bubbling experiments were conducted at room temperature with a freeboard pressure of 414 kPa and bed fluidization at a superficial gas velocity, Ug = 0.3 m/s. The previous runs were repeated at a freeboard pressure of 210 kPa, and higher superficial gas velocity, Ug = 0.6 m/s. Binary mixtures M3, M6 and M9 were used in these experiments. In this set of experiments, the RH of the fluidizing air was varied from 3 to 38% by passing the air through a compressor refrigerator dryer or a silica gel dryer.        39  • Coarse Particles Properties Experiments To investigate the effect of coarse particle size on electrostatic charge generation, entrainment flux and charge density of fine particles in the freeboard, free-bubbling experiments were conducted with the M1, M2 and M3 binary mixtures of different sizes of coarse glass beads (see Table 2.3). These experiments were performed at room temperature with a freeboard pressure of 414 kPa and a superficial gas velocity, Ug of 0.3 m/s. The RH of the gas was maintained at 12±2%. To study the effect of coarse particle type, binary mixtures M3 and M10 having glass beads and polyethylene particles, respectively, but fine glass beads of the fines were used. In addition, binary mixtures M10 and M11 of polyethylene with different densities (LLDPE and HDPE) were tested to investigate the effect of coarse particle density.   • Fine Particle Properties Experiments To determine the effect of fines wt%, binary mixtures M3, M4, M5, M6, M7 and M8 were studied, with the fines (glass beads in M3 to M5 or alumina in M6 to M8) varied from 5 to 15 wt%. Free bubbling experiments were conducted at room temperature with a freeboard pressure of 414 kPa. The bed was fluidized at a superficial gas velocity, Ug of 0.3 m/s. The RH of the gas was maintained at 12±2%. In addition, binary mixtures M3, M6 and M9 with different fines types and densities were tested to investigate the effect of the fines particle density and type.   40 Chapter 3: Results and Discussion: Effect of Operating Parameters   The elevated-pressure fluidization column portrayed in the previous chapter was used to measure the entrainment flux and electrostatic charges generated inside the bed at different levels and in the freeboard region. The effect of operating parameters, such as superficial gas velocity, operating pressure and bed temperature, were investigated for binary particle mixtures of glass beads and polyethylene (whose particle properties are provided in Tables 2.2 and 2.3). This chapter presents and discusses the experimental results.  3.1 Collision Ball Probes Location and Sensitivity  To characterize the electrostatic charges at different locations in a fluidized bed, three collision ball probes (A, B and C) were immersed at different axial locations in the bed (see Figure 2.4). In addition, a fourth probe (D) was normally mounted in the freeboard, 1.1 m above the distributor plate, also shown in Figure 2.4. However, in preliminary experiments, probe D was mounted at a lower level, 0.89 m above the distributor plate, and it was observed that, at high superficial gas velocities, negative charges were measured by probe D, as shown in Figure 3.1. This is probably because, at high superficial gas velocities and as bubbles erupted at the bed surface, more negatively charged coarse particles were ejected into the freeboard region and collided with probe D. As a result, negative charges were transferred to the probe. Since the function of probe D was to measure the electrostatic charges carried by fine particles in the freeboard, probe D was relocated to the higher level, and positive charges were then measured for the fine particles in the freeboard region.   The induced electrical field in the bed could influence the signals measured from each of the collision ball probes; therefore, the sensitivity of the probe was set to 1 pC/Mechanical Unit (MU) to localize the measurements for each probe (Moughrabiah, 2009).     41  Figure 3.1 Net cumulative charges measured by probe D in freeboard as a function of time for binary mixture M3. P = 207 kPa, T = 20±2ºC, Ug = 0.6 m/s, RH = 12±2%. For physical properties of particles, see Tables 2.1, 2.2 and 2.3. Numbers on curves denote probe D distance above distributor plate (in m).   The collision ball probes and current detection pipe (see section 2.3.2) were connected directly to an electrometer (Kistler model 5010B). This electrometer is able to measure charges from ±10.0 pC to ±1.0 µC. To verify that the fluidizing air alone does not carry electrostatic charges that could influence the binary mixture results, air alone was passed through the empty fluidization column while the charges were measured, by the probes in the bed and freeboard regions, and by the current detection pipe at the column exit. The column absolute pressure was maintained constant at 207 kPa, while the temperature and relative humidity of the air were nearly constant (T = 20 ±2ºC and RH = 12±2%). The fluidizing air was introduced to the column at a superficial gas velocity of 0.3 m/s for ~300 s. The electrostatic charges measured by the ball probes and current detection pipe are plotted versus time in Figure 3.2. It can be seen that the fluidizing air alone carries negligible charges.     42  Figure 3.2 Cumulative charge as a function of time 

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