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Effects of Pressure, Temperature, and Gas Velocity on Electrostatics in Gas-Solid Fluidized Beds Wajeeh O. Moughrabiah, John R. Grace,* and Xiaotao T. Bi Fluidization Research Center Department of Chemical and Biological Engineering, UniVersity of British Columbia 2360 East Mall, VancouVer, Canada, V6T 1Z3
The influences of operating pressure, temperature, and gas velocity on the degree of electrification in a fluidized bed of glass beads and different grades of polyethylene resin were investigated in a fluidization column of 150-mm inner diameter and 2.0-m height. Eight collision ball probes at different levels and radial positions measured the degree of electrification in the bed. Faraday cups also measured the charge density in the bed by taking samples from three different online sampling ports at different levels. The degree of electrification increased as pressure increased from 1.0 to 8.0 bar, probably due to an increase in bubble rise velocity, frequency, and volume fraction. The maximum static charges were found at approximately two-thirds of the bed height and near the axis. As the pressure increased, particle-particle and particle-wall collisions near the distributor and wall contributed heavily to static charge generation. At higher temperatures (up to 75 °C), the bed exhibited smoother fluidization. Temperature played a significant role in determining electrostatic charging. As the superficial gas velocity increased from 0.23 to 0.40 m/s, the degree of electrification increased. However, at higher gas velocities, the polarity in the freeboard region was opposite to that in the bed, indicating that fines entrained from the column carried charges, resulting in a net charge of polarity opposite to that inside the bed. 1. Introduction Fluidizing a dielectric material, such as glass beads, polystyrene. and polyolefin particles, tends to generate significant electrostatic charges within the reactor. The charged particles and electric fields arising from them can affect the bed hydrodynamics and cause severe problems in commercial gas-solid fluidized bed facilities, such as agglomeration, sheeting (fusion of solid particles into solid shapes), nuisance discharges, and product handling. In addition, unintentional charge accumulation can cause problems in industry ranging from a minor nuisance to severe explosions.1 Electrostatic charging in fluidized beds was first reported 60 years ago in connection with anomalous bed behavior encountered in experiments on subjects as diverse as heat transfer,2 elutriation,3 types of fluidization,4 and characteristics of fluidized particles.5 The mechanism of static charge generation is complex. Electrons or ions can transfer between bodies in contact, forming an electrical double layer consisting of two layers of charges of opposite sign. If the bodies are suddenly pulled apart, the original electronic equilibrium cannot be re-established and one surface retains more electrons or ions than before the contact, while the other has a deficit. The total charges of the two surfaces remain constant. However, if one of the surfaces loses its charge (for instance, because it is a better conductor or is grounded), the global result is a net electrical charge.1 Fluidization is, by its very nature, associated with continuous particle contact and separation, in addition to friction between particles, as well as with the wall. In gas-solid fluidized beds, various mechanisms such as triboelectrification, frictional charging, ion collision, and high-temperature thermionic emission can all generate electrostatic charges.1 In gas-solid fluidized beds, particle-particle collisions tend to be much more energetic among particles surrounding bubbles than for particles elsewhere in the bed, leading to enhanced * To whom correspondence should be addressed. Phone: +1 604 822 3121. Fax: +1 604 822 6003. E-mail:
[email protected].
charge generation.6 This appears to be of key importance in charge generation and buildup.6 In bubbling fluidized beds, relatively small bubbles form at the distributor plate and coalesce as they rise. Darton et al.7 showed that the mean bubble size at a given height increases with increasing gas velocity and the catchment area for bubbles generated at the distributor plate. Several researchers6,8-12 have studied the influence of the fluidizing gas velocity and bubble size on the generation/ accumulation of electrostatic charges. Boland and Geldart9 concluded that electrification in fluidized beds is generated by particle motion around bubbles, and they showed that the voltage associated with the passage of the wake is higher than that associated with the nose. They speculated that most particleparticle charging occurs in bubble wakes because particle motion is much smoother at the nose than in the wake. They measured the potential generated around bubbles passing a conduction probe mounted on the wall of the column. The induced charge on the probe was found to increase with bubble diameter because larger bubbles rise more quickly, causing more particle motion than smaller bubbles. It was speculated that the charges had opposite polarity at the wake and nose regions of the bubbles. However, Park et al.8 showed that their results can be ascribed to induction effects. Ciborowski and Wlodarski10 suggested that increases in charge generation in their experiments were directly proportional to the fluidizing gas velocity. Chen et al.6 suggested that the increase in generated and transferred charges with increasing bubble volume resulted mainly from the higher velocities of particles surrounding faster-rising bubbles. Guardiola et al.12 reported that the degree of electrostatic charging in fluidized beds increases with increasing superficial gas velocity. Guardiola et al.11 explained the increase in electrostatic charge generation with increasing fluidization velocity by the larger bubbles that cause increased particle movement. This trend is limited by the onset of slugging, since slugs cause a net reduction in particle motion.11 Each of the above studies on electrostatics was conducted at atmospheric pressure, without considering the effect of the
10.1021/ie800556y CCC: $40.75 2009 American Chemical Society Published on Web 09/17/2008
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Figure 1. Overall layout of high-pressure fluidization unit.
operating pressure. However, many industrial fluidized beds operate at high pressures (e.g., 10-80 bar). Well-known examples include pressurized fluidized bed combustion, gasification of coal, catalytic processes such as acrylonitrile, analine, and Fischer-Tropsch synthesis, and gas-phase polyolefin processes.13 High pressures, by increasing gas density, affect gas-particle and particle interactions and flow patterns, affecting gas-solid contacting efficiency.14 Pressurized beds exhibit smoother fluidization and smaller bubbles.15-19 This could be due to (i) a larger portion of gas flowing through the emulsion phase due to increased emulsion-phase voidage or (ii) a decrease in bubble stability.20 Olowson and Almstedt21 showed that, despite the decreased bubble size, their rise velocity, frequency, volume fraction, and visible bubble flow rate increase with increasing pressure and excess gas velocity. It has also been reported that bubble flow is increasingly concentrated toward the center of the bed at high pressures.22 For group B particles, the bubble size was reported to first increase and then to decrease as the pressure increased.23,24 This is consistent with the effect of pressure on the mean pierced length of bubbles observed by Olowson and Almstedt.21 Fiorention and Newton25 reported that elevated pressures cause the mean bubble diameter to decrease and the bubble velocity to increase. Li and Kuipers14 found that elevated pressures enhanced gas-solid interaction and reduced the frequency of particle collisions, effectively suppressing the formation of large bubbles. There are a number of factors influencing electrostatic generation and dissipation in gas-solid fluidized beds, e.g., bubble movement, particles rubbing against each other in the region surrounding rising bubbles, particle motion adjacent to the wall, the condition of surfaces, particle morphology (size and shape), relative velocity of particles, fluid physical properties, and operating conditions. All of the above, together with the complexity of fluidized media, explain the dearth of studies on fluid bed electrostatics. In this paper, the influences of
pressure, temperature, and fluidized gas velocities are investigated in order to characterize and assist in the control of electrostatic charge generation and dissipation in gas-solid fluidized beds. 2. Experimental Equipment Freely bubbling fluidization experiments were conducted in a three-dimensional high-pressure (up to 10 bar) fluidization vessel constructed of stainless steel of 150-mm inner diameter and 2.0-m height (straight section), expanding to 200-mm inner diameter over a 0.75-m-tall expanded section. A schematic diagram of the overall layout is shown in Figure 1. The gas distributor consists of two stainless steel perforated plates, each containing 50 aligned holes, those in the upper plate being 4 mm in diameter, while those in the bottom plate had a diameter of 5.6 mm. The perforated plates were designed with an open area ratio of 3.8% (based on the top plate) and a pressure drop through the plates of 4.5 mbar at a superficial gas velocity of 0.2 m/s to ensure a uniform distribution of the fluidizing gas. A steel screen with 38-µm openings is sandwiched between the plates to prevent fine particles from dropping into the windbox. An air compressor, Kaeser model SK19, pressurizes the vessel to the required operating pressure and continuously supplies the fluidizing gas. The vessel is equipped with several pressure-sampling ports at different heights. Pressure transducers measure differential pressure fluctuations and local and overall pressure drops. The absolute pressure of the vessel is set by a control valve downstream. A pressure relief valve downstream of the vessel protects it from excessive pressure buildup. The temperature is controlled by electrical heating tape coiled around the pipes upstream of the vessel. Air is dried by passing through a refrigerating unit and vapor removal filters. A valve at the entrance of the vessel establishes the gas velocity by controlling the mass flow of the
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Figure 2. Schematic diagram of electrostatic collision ball probe, a stainless steel ball of db ) 5.3 mm. Table 1. Properties of Particles in This Study at 20 °C Fp, djp, Fb, Umf at 379 kPa, µm kg/m3 kg/m3 m/s material linear low-density polyethylene resin, LLDPE high-density polyethylene resin, HDPE glass beads
600
918
340
0.19
450
965
385
0.14
321
2458
1478
0.33
Table 2. Particle Size Distribution particle diameter (mm)
LLDPE wt %
HDPE
glass beads
cumulative cumulative cumulative wt % wt % wt % wt % wt %
1.140 1.99 0.707-1.410 28.33 0.354-0.707 49.57 0.177-0.354 19.36
