Investigation of electrostatic charge distribution in gas–solid fluidized beds

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Abstract

Over the past few decades there have been numerous attempts to measure electrostatic charges in gas–solid fluidized bed reactors; these charges have been prone to cause reactor downtime from electrostatic phenomena. In this study, a new system was developed that aimed to quantifying the electrostatic charge generation in three key areas within a gas–solid fluidized bed simultaneously: the bed particles, the particles that adhered to the column wall, and the particles that were entrained from the column. A unique online Faraday cup method was used to measure the electrostatic charge of the particles. The system was operated with dry air at two fluidizing gas velocities, one in the bubbling and the other in the slugging flow regime. An industrial polyethylene resin with a wide particle size range was utilized in all experiments. Results showed the occurrence of bi-polar charging in both flow regimes with entrained fines being mainly positively charged, whereas the bed particles and those attached to the column wall carrying a net negative charge. The charge-to-mass ratio (q/m) of the entrained fines in the bubbling regime was significantly higher than in the slugging regime. It was discovered that particles with a certain size range were predominantly adhering to the column wall with a significantly higher q/m than the other bed particles. These findings led to a proposed mechanism for the migration of particles within the fluidization column due to the effect of electrostatic charge generation.

Introduction

Gas–solid fluidization is a common process found in many chemical industries with applications in gas–solid reactions, drying, mixing, granulation and coating. One problem observed in some gas–solid fluidized beds is the occurrence of electrostatic charge generation. This problem arises due to particle–particle and particle–vessel wall contacts which results in triboelectrification and frictional charging (Cross, 1987).

Electrostatic charge generation has a profound impact on gas–solid fluidized bed hydrodynamics, resulting in several challenges associated with operating such systems. Common problems related to the electrification of gas–solid fluidized beds include particle–wall fouling and inter-particle cohesion. Such difficulties have been clearly observed in the production of polyethylene for many years (Goode et al., 1989; Hagerty et al., 2005; Hendrickson, 2006; Newton et al., 1999; Song et al., 1995; Yu et al., 2007). In these cases, large amounts of electrostatic charges are generated resulting in particles adhering to the reactor wall (known as “sheeting”), particle agglomeration, and disruption of electrical equipment. Sheeting is known to be a significant drawback of electrostatic charge generation since particle sheets can break off and cover the distributor plate, causing a long shut down period. In addition, finer particles, which can be either catalyst and/or finer polyethylene resins that entrain from the system, may cause fouling in reactor wall at the top, cycle gas line, and other downstream processes such as cyclones. Overall, challenges associated with electrostatic phenomena in gas–solid fluidization and other gas–solid processes include process shutdowns and economic losses due to decreased production and maintenance costs. Therefore, there is a great incentive to reduce or eliminate electrostatic effects, especially in gas–solid fluidized beds. An essential step to overcome such challenges is to determine the charging mechanisms, and therefore to accurately quantify electrostatic charge generation and its effects in these types of reactors.

Historically, two types of measurement techniques have been utilized to quantify the electrostatic effects in gas–solid fluidized beds. Many researchers have employed electrostatic probes, typically placed inside the fluidization column or along its wall to measure the bed charge indirectly through the absolute charge, current, or voltage (Bafrnec and Bena, 1972; Boland and Geldart, 1971/72; Ciborowski and Wlodarski, 1962; Fujino et al., 1985; Guardiola et al., 1992, Guardiola et al., 1996; Ham et al., 1992; Kisel’nikov et al., 1985; Moughrabiah et al, 2009; Park et al., 2002; Rojo et al., 1986; Tardos and Pfeffer, 1980). These probes provide the charge as a function of fluidization time. However, electrostatic probes have been known to lose their accuracy as particles adhere to the tip of the probe with fluidization time (Fujino et al., 1985). Others have removed small samples of the bed particles and placed them into a Faraday cup, which is capable of directly measuring the charge of particles (Ali et al., 1998, Ali et al., 1999; Calin et al., 2007; Chen et al., 2009; Fasso et al., 1982; Mehrani et al., 2005, Mehrani et al., 2007; Wolny and Opalinski, 1983; Wolny and Kazmierczak, 1989; Zhao et al., 2000). A Faraday cup is useful to determine the total net charge of a group of particles; however, it typically cannot undertake this measurement as a function of time. Furthermore, any handling of the bed particles prior to entering the Faraday cup can in turn introduce additional charging not present within the fluidization reactor. In all these works, due to the small sample size of the particles transferred to the Faraday cup, results have provided only very local electrostatic charge measurements. There have been other unconventional methods of measuring electrostatic charge generation in fluidized beds. Wolny and Kazmierczak (1989) attempted to measure the electric charge of granule particles by placing a pneumatic device inside the bed and shooting particles out of the bed and between a capacitor. This method, however, only measured the charge on a small amount of particles. Moreover, introducing the piping system for the pneumatic device would have changed the bed hydrodynamics and the results may have not been entirely reliable. Mehrani et al. (2005) constructed a fluidized bed that acted as a Faraday cup where the inner cup was the fluidization column. This system was restrictive as it could only measure the charge of the fines entrained from the column. An indirect method developed by Valverde et al. (2008) involved a 0.0254 m inner diameter polycarbonate column where an electric field was applied to the column, and particle movement was recorded with a high speed camera. The charge of agglomerated particles was found using Stoke's Law. Although this method proved to eliminate any contact with the charged particles, the system is limited to more local particle charge measurements, as well as a transparent and non-metallic column. Overall, in all of these reported cases which utilized either electrostatic probes or Faraday cups, focus has only been on measuring the electrostatic charges in one area of the fluidized bed, such as the bed particles or the entrained fines, and not on providing a good representation of the net charges generated in the fluidized bed as a whole.

Within the works presented in literature in the area of electrostatic phenomenon in gas–solid fluidization only a few have focused on the charge distribution with relation to particle size. Zhao et al. (2000) attempted to measure the charge of different sized particles with an array of seven vertical Faraday cups placed below a fluidized bed. A sampling rod with holes was placed in the middle of the column and was used to remove a few particles from the bed and drop them into the Faraday cups, with the larger and finer particles settling in the bottom and top cups, respectively. Although this method provided the charge distribution of the particles, the sampling rod in the centre of the fluidized bed interferes with bed hydrodynamics and samples were limited to the centre of the bed providing a very local measurement with very small sample sizes. Ali et al. (1998) attempted to measure the charge distribution by fluidizing small particles for a period of time, sampling the particles, then dropping them over a grid of nine small Faraday cups. As particles fell into different cups, their charges were recorded and the particle size distribution from each individual cup was determined. Ali et al. (1999) removed samples from the bed particles systematically at different locations using a scoop, and placed them into an external Faraday cup. They also performed particle size distribution analysis on deposits along different heights of the column wall to correlate electrostatic charging to the migration of certain sized particles along the reactor wall. In all their works, the real electrostatic charge of the particles could have been affected by the sampling method. Other groups measured the distribution of electrostatic charges along the axial and radial directions of the fluidization column, which gave a distribution of charge as a function of column dimensions (Fang et al., 2008; Fujino et al., 1985; Gajewski, 1985; Moughrabiah et al., 2009; Murtomaa et al., 2003). Gajewski (1985) used conductive copper rings at different column heights to measure the charge distribution along the wall. The rings, however, were placed inside the column where they would have come into contact with the particles causing a possible charge transfer between the sensor and the particles. Murtomaa et al. (2003) developed a non-intrusive ring system on a miniature fluidized bed that was able to measure the charge along the column wall which may have limitation to other column dimensions and materials (i.e., metallic columns). Fang et al. (2008) employed an electrostatic probe and measured the charge of the particles close to the reactor wall, as well as at different axial locations. They were able to produce an axial charge distribution of the particles that adhered to the column wall. Similar work was done by Moughrabiah et al. (2009) with a high pressure system. However, like any system using electrostatic probes within the fluidized bed, the probe is prone to particle adhesion and subsequently a reduction in the accuracy of the measurement with fluidization time (Fujino et al., 1985).

In order to fully understand the charge generation mechanisms inside industrial gas–solid fluidized beds which typically contain a large particle size distribution, it is important to quantify the amount of electrostatic charges in the fluidization system at different locations simultaneously. In addition, it is vital to determine the migration of particles of different sizes within the bed due to the influence of electrostatic forces. Moreover, these investigations must be conducted using an adequate online measurement technique. This study explored the charge and particle size distribution in a gas–solid fluidized bed in three key areas, concurrently, the bed particles, the particles which adhere to the column wall, and the entrained fines by utilizing an online Faraday cup method. In addition, the effect of the fluidization regime (bubbling and slugging) on particle charge generation and migration was studied.

Section snippets

Experimental setup and material

The overall experimental setup was the same as Sowinski et al. (2009), except for the top section of the bed (Fig. 1). This system consisted of a fluidization column constructed of carbon steel to mimic typical polyethylene fluidized bed reactors. The inner diameter and height of the column were 0.10 and 1.3 m, respectively. The column was fitted with a removable distributor plate (i.e., modified knife gate valve), and a windbox housing a Faraday cup that allowed the measurement of the total

Experimental procedure and data collection

Before each run, the inner wall of the fluidization column was rigorously cleaned to remove any particles left from the previous run. The initial charge of the polyethylene particles was measured in each run using a bench-scale Faraday cup, before being placed inside the column. The top Faraday cup was assembled, followed by grounding the main fluidization column wall and the distributor plate for a short period of time to remove any excess charge. The fluidizing gas velocity was then slowly

Results and discussion

Experiments were conducted to investigate and quantify electrostatic charge generation in two fluidization flow regimes (bubbling and slugging). Two fluidizing gas velocities were selected at different factors of the minimum fluidization velocity (umf=0.131 m/s); u/umf=1.5 and 4 to represent the bubbling and slugging regimes, respectively. In each experiment, particles were fluidized for a period of 60 min. A new batch of particles was used for each run and all experiments were repeated at least

Conclusions

The new system that was developed in this work proved to adequately quantify electrostatic effects at various zones of gas–solid fluidized beds by allowing simultaneous measurement of electrostatic charge of three groups of particles, which in turn also provided an insightful look into the migration of particles within the system as a function of particle size. In addition, this work aided in describing the nature of the column wall fouling within a highly electrostatically charged fluidization

Acknowledgements

The support and input from Univation Technologies LLC (Texas, USA), and the financial assistance from the Natural Sciences and Engineering Research Council of Canada (NSERC) are acknowledged with gratitude.

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