Article pubs.acs.org/IECR
Experimental Investigation of Electrostatic Reduction in a Gas−Solid Fluidized Bed by an in Situ Corona Charge Eliminator Kezeng Dong, Qing Zhang, Zhengliang Huang, Zuwei Liao, Jingdai Wang,* and Yongrong Yang State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: Accumulation of excess charges on insulated particles due to triboelectric charging can cause various powder industrial hazards. This work developed a novel in situ corona charge eliminator (ICCE) to neutralize the electrostatic charge in a gas−solid fluidized bed. In order to verify the feasibility of this method, variation of the electrostatic voltage, charge-to-mass ratio, and electric field before and after the application of ICCE was systemically investigated. The results showed that, under gas velocities of less than 2 U/Umf, particles were less charged, and the negative corona discharge caused a polarity reversal. As the gas velocity was increased, particles were more charged, and the charges on the particles were only partially neutralized by an identical corona discharge. Further analyses indicate that the charge neutralization efficiency was significantly influenced by both the triboelectric- and corona-charging processes. The superficial gas velocity was found to have a strong influence on the above two charging processes, and the charge neutralization efficiency favored a larger gas velocity. Besides, the equilibrium charge of the particles after the application of ICCE strongly depended on the initial charge accumulation in the fluidized bed.
■
INTRODUCTION Three mechanisms have been developed during materials charging, namely, triboelectric charging, corona charging, and induction charging.1,2 Among these, induction charging is only applicable for electrically conductive materials, while the insulated materials can be charged from triboelectric charging and/or corona charging. Triboelectric charging of insulated particles strongly depends on the particle material, particle size, surface property, friction rate, environmental conditions, and so on, which frequently leads to inconsistent results.3−6 Thus, triboelectric charging of insulated particles is not very predictable or easily controlled. In contrast, corona charging provides ions of single polarity with a reliable source and in a controlled way.1,7 Therefore, the corona-charging process of insulated particles can be controlled and has been intensively applied in many industrial powder processes, such as powder coating,7,8 electrostatic separation,9−11 electrostatic precipitation of dusts,12,13 and electrophotography.12 The accumulation of excess charges on insulated particles due to triboelectric charging can cause various powder industrial hazards. For instance, the presence of overly charged particles during particle handling and transportation can increase the pressure drop, cause agglomerates and blockage, and even lead to dust explosion.14−16 The excess accumulation of charges in the fluidized-bed reactors (FBRs) can cause problems such as agglomeration, wall sheeting, reduction of product quality, and even an unscheduled shutdown.4,17−19 Elimination or reduction of the electrostatic charge accumulation on insulated particles can prevent these issues. Because triboelectric charging of insulated materials can cause potential hazards and is not easily controlled, most powder industries should avoid excess accumulation of charges on insulated particles. The electrostatic charge of particles can be reduced by decreasing charge generation and/or accelerating charge dissipation. On the one hand, charge generation can be decreased by reducing the contact potential difference (CPD) © XXXX American Chemical Society
between friction materials, friction rate, and contact surface area.5,20,21 On the other hand, charge dissipation can be strengthened by increasing the environmental humidity,22−24 addition of antistatic agents,19 and surface pretreatment.3,25−27 Matsusaka et al.28 proposed a pneumatic-conveying system combining two different pipe materials in order to control the particle charge based on the particle-charging characteristics. This system worked because the polarity and the amount of particle charge are mainly controlled by the CPD between the particles and pipe materials. Wolny and Opaliǹski20 found that the addition of fine particles can reduce the friction surface area between the particles, which resulted in a decrease in the electrostatic charge in the fluidized bed. Park et al.23 indicated that an increase of the gas humidity to 40−80% can significantly reduce the accumulation of electrostatic charge. Sharma et al.26 showed that the surface resistivity of polymer particles obviously reduced after plasma treatment, especially in high humidity. Wang et al.19 studied the influence of fine metallic oxides on the electrostatic charge in the fluidized bed, and their results indicated that charge reduction strongly depended on the electronegativity of metallic ions. However, most methods of charge reduction mentioned above cannot be applied to the industrial FBRs directly. As for polyolefin FBRs, the materials of the particles and reactor wall are fixed and the variation ranges of operation conditions such gas velocity and pressure are very limited. What’s more, a trace of water and oxygen are poisonous to catalysts.17 Therefore, it is very difficult to eliminate the electrostatic charge in FBRs through either reduction of charge generation or acceleration of charge dissipation. Received: April 17, 2014 Revised: August 13, 2014 Accepted: August 24, 2014
A
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematic diagram of the experimental apparatus.
instead of the expanded section of the fluidized bed, the compressed gas flow would strongly influence the hydrodynamics of the fluidized bed.42 Furthermore, the unipolar ions tend to dissipate more easily from the tube wall during pneumatic conveying, which will further decrease the charge neutralization efficiency. Although Vincenzi and Arletti40 and Revel et al.41 made their first trials of corona discharge on electrostatic reduction in the fluidized bed, the charge neutralization efficiency in their reports is relatively poor. The main cause of poor neutralization efficiency is the inappropriate position of the corona charge neutralizer and the dissipation of unipolar ions during pneumatic conveying, which results in a short residence time of unipolar ions and insufficient contact efficiency between the ions and charged particles. Therefore, we proposed an in situ corona charge eliminator (ICCE) in the fluidized bed to elongate the residence time of unipolar ions in the dense phase and enhance the electrostatic charge neutralization efficiency. Then the electrostatic levels in the fluidized bed before and after the application of ICCE were systematically characterized and compared in order to verify the feasibility of this method, by using electrostatic probes, a faraday cup electrometer, and an electric field meter.
Gas corona discharge has been widely applied in the charging of particles of different sizes.7,29−33 Besides, it can also be used in the neutralization of charged particles whether the charges come from triboelectric charging or corona charging.34−39 Kachi et al.39 studied the neutralization efficiency of polyethylene granules exposed to an alternating-current (ac) corona discharge from a wire-type electrode and indicated that, for samples charged in higher voltage, higher levels of neutralization voltages were necessary to obtain a better neutralization efficiency; otherwise, the corona discharge might not be sufficient to neutralize the material. Kodama et al.34 developed a passive-type electrostatic eliminator without a power supply in a pneumatic-transport system to prevent dust explosions caused by electrostatic discharge in a silo. Watano36 studied the mechanism and control of electrification in the pneumatic conveying of powders and found that the electrostatic charge can be well controlled by the corona discharge neutralizer. Although charge neutralization by corona discharge has been successfully used in silos and pneumatic-conveying processes, few studies have been reported on charge reduction in the fluidized bed using corona discharge.40,41 Vincenzi and Arletti40 set up a corona discharge neutralizer on the bed level of the fluidized bed in order to reduce sheeting through elimination of the charge on the bed level. However, the ions provided by the charge neutralizer were easily entrained by the fluidized gas into the dilute phase, thus decreasing the charge neutralization efficiency. Besides, the charge neutralizer on the bed level would also impact the hydrodynamics in the FBRs. Revel et al.41 set up an ac corona neutralizer on the expanded section of the fluidized bed and found that the positive and negative ions had an influence on the charge neutralization only on the top part of the fluidized bed. This is attributed to the fact that corona ions can only contact with the top particles directly and may be entrained by the fluidized gas. Therefore, the electrostatic charge on the middle and bottom parts of the bed did not change significantly. Besides, the corona neutralizer used by Revel et al.41 required a compressed gas flow as the ion carrier. If the corona neutralizer were set in the dense phase
■
METHOD AND MATERIALS
Figure 1 shows a schematic diagram of the experimental apparatus, which consists of a fluidization system, a charge eliminator system, and an electrostatic detection system. The cold model fluidized bed is made of a transparent Plexiglas column with an inner diameter of 420 mm and a height of 2000 mm. The iron slit nozzle distributor in the bottom of the column has an open area ratio of 2.6%. In order to comprehensively characterize the electrostatic charge in the fluidized bed, three measurement devices, including an electric field meter (Kasuga KSV-1000), a faraday cup electrometer (Monroe Electronics NanoCoulomb Meter 284), and a self-developed electrostatic probe, were used during the experiments. Among these, the electric field meter is B
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
the charge of the particles were generated to reduce the electrostatic charge in the fluidized bed. The variation of the electric field in the dilute phase, charge-to-mass ratios of particles, and electrostatic voltage before and after application of the charge eliminator was measured.
attached to the upper column wall at 800 mm above the distributor, which is used to measure the electric field intensity induced by all of the charged particles in the dense phase of the fluidized bed and fine particles in the dilute zone. The electric field was mainly influenced by the charge carried by the particles and the distance between the charged particles and field meter. Compressed air of 0.05 MPa was supplied to the field meter to prevent the deposition of powder particles. The charged particles were sampled and the relevant charge-to-mass ratios were measured by the faraday cup electrometer. The sampling port was set at 100 mm above the gas distributor, and the charged particles from the sampling port went directly into the faraday cup because of the pressure difference between the inside and outside of the fluidized bed. Considering the possible influence of the sampling process on the measurements, all experimental runs were repeated at least three times to ensure reproducibility of the results. The self-developed electrostatic voltage measurement consists of a spherical electrostatic probe, a voltage/current converter (Adtech MVX106), a data acquisition card (NI USB-6351), and a computer.4,19 The electrostatic probes were set at 100 and 200 mm above the gas distributor, and each probe was located 5 mm from the inner column wall. Output voltage signals were sampled at 200 Hz, and the sample time depends on the detailed experiments. The charge eliminator system includes a needle electrode and a high voltage module (LSL801, China) with a 24 V directcurrent power supply. The output current of the high voltage module is 0.2 mA, and the output voltage can be continuously adjusted in the range of 1−12 kV. The needle electrode was embedded into the cylindrical poly(tetrafluoroethene) (PTFE) and installed in the fluidized-bed wall, with the 2.5 mm needle point exposed to the fluidized gas and particles directly. Six corona electrodes in total were symmetrically installed on the wall at heights of 100, 200, and 300 mm above the distributor, respectively. The fluidized gas around the needle electrode with a high-voltage power supply was ionized and caused the generation of ions of single polarity. The polarity and number concentration of the ions depend on the polarity and amplitude of the high voltage supplied. Because the needle electrode is installed in the fluidized bed, the ions generated by corona discharge of the fluidized gas can have direct contact with most charged particles from bottom to top with the drag of the fluidized gas. Thus, the ions of single polarity have a long residence time in the fluidized bed and lead to a better charge neutralization efficiency. Compressed air was used as the fluidized gas and predried to a relative humidity of 20−30%, and the air temperature was around 15−20 °C. A linear low-density polyethylene (Sinopec, China) with a density of 920 kg/m3 and a relative permittivity of 2.3 was used as fluidized particles. The mean diameter of the fluidized particles is 0.608 mm, and they belong to Geldart B particles. The fluidized bed was operated with a static bed height of 300 mm. The minimum fluidization velocity (Umf) was determined experimentally by the classical pressure method, and the value of Umf for polyethylene particles was 0.2 m/s. During each experiment, the particles were fluidized for 30 min first to make sure the particles are fully charged due to triboelectric charging through particle−particle and particle−wall impact.4,21 Then, the electrostatic charge in the fluidized bed was measured by an electric field meter, a faraday cup electrometer, and an electrostatic probe, respectively. Ions of opposite polarity to
■
RESULTS AND DISCUSSION Variation of the Electrostatic Voltage with ICCE. Figure 2 shows variation of the electrostatic voltage at 200 mm above
Figure 2. Variation of the electrostatic voltage at H = 200 mm before and after negative corona discharge (0.6 m/s).
the gas distributor before and after the application of negative corona discharge at a superficial gas velocity of 0.6 m/s. Before the onset of corona discharge, the fluidized particles were only charged as a result of friction charging. The electrostatic voltage increased rapidly from 0 to 400 V and leveled off. Because the voltage was positive, a negative high voltage of 12 kV was applied on the needle electrode at 150 s, and negative ions were generated to neutralize the positive charge. As Figure 2 shows, the electrostatic voltage quickly decreased after the onset of negative corona discharge and the voltage fell back to 0 V after t = 350 s. The negative ions moved to the positively charged particles under the electric field caused by both the high voltage on the needle electrode and the field due to the positive charge of the particles. Therefore, the positively charged particles were neutralized, and the electrostatic voltage measured obviously decreased. Variation of the equilibrium electrostatic voltage at 200 mm above the gas distributor with increasing superficial gas velocity before and after the onset of ICCE is displayed in Figure 3. When the gas velocity was small, friction charging between fluidized particles was weak because of the slow relative motion of the particles. Thus, the electrostatic voltage detected by the electrostatic probe was small. After the onset of ICCE, the electrostatic voltage quickly decreased from +55 to −170 V. The polarity of the electrostatic voltage reversed when excess negative corona discharge was applied at low gas velocities. The impact frequency between insulated particles increased with increasing gas velocity, thus leading to a larger electrostatic voltage compared to that at low gas velocities. When the electrostatic voltage at 200 mm above the distributor reached a plateau, ICCE of the same configuration was applied under various gas velocities. The results in Figure 3 show that the electrostatic voltages under various gas velocities significantly C
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
more negative after ICCE ceased. Under both conditions, the electrostatic voltage varied only because of triboelectric charging of the particles. It is interesting to find that the equilibrium voltage of the particles under both conditions was almost identical. In other words, the equilibrium charge of the particles due to triboelectric charging is independent of the initial charge. Matsusaka et al.5 have reported that the equilibrium charges of the particles are independent of their initial charges during triboelectric charging. This conclusion has been verified in pneumatic conveying15 but has not been verified in the fluidized bed. The initial charges of the particles can be adjusted through corona charging, and their influence on the equilibrium charges can be investigated. The conclusion can be drawn from Figure 4 that the equilibrium charges of the particles due to triboelectric charging in the fluidized bed are also independent of their initial charges. Besides, the growth of the electrostatic voltage due to triboelectric charging was more rapid with a negative initial charge than with a positive initial charge. This may be caused by the relatively large CPD between charged particles and an electrostatic probe.5 Variation of the Charge-to-Mass Ratio of the Particles with ICCE. The equilibrium charge-to-mass ratio of the particles at 100 mm above the distributor was measured by a faraday cup electrometer before and after the application of ICCE. The variation of the charge-to-mass ratios under various gas velocities is displayed in Figure 5.
Figure 3. Variation of the voltage at H = 200 mm with different superficial gas velocities before and after negative corona discharge.
reduced after the application of ICCE. Furthermore, the electrostatic voltage remained positive at larger gas velocities and changed to negative at lower gas velocities. This is attributed to the fact that the positive charge accumulation in the fluidized bed was larger at higher gas velocities than at lower gas velocities. Thus, it is reasonable that the polarity of the electrostatic voltage reversed at lower gas velocities but remained unchanged at higher gas velocities with the same configuration of corona discharge. As has been previously analyzed, the electrostatic voltage in the bed significantly decreased after the application of ICCE. Figure 4 shows variation of the electrostatic voltage at 200 mm
Figure 5. Variation of the charge-to-mass ratios with the gas velocities before and after application of ICCE (H = 100 mm).
Particles in the fluidized bed were positively charged at 100 mm above the distributor due to triboelectric charging, and the charge-to-mass ratio increased with increasing gas velocity. After the onset of ICCE, particles were involved in both the triboelectric-charging and corona-charging processes and the charge-to-mass ratio started to decrease. At a low gas velocity (0.3 m/s), the charge-to-mass ratio of the particles decreased from +2.5to −2.5 μC/kg with negative corona discharge. At a high gas velocity (0.7 m/s), the charge-to-mass ratio decreased from 11.3 to 1.5 μC/kg. The negative corona discharge caused a decrease of the particle charge and a polarity reversal in lower gas velocities, and only a decrease of charge was observed in higher gas velocities. In other words, the equilibrium charge of the particles during triboelectric and corona charging is dependent on the initial charge of the particles before ICCE.
Figure 4. Comparison of the variations of the electrostatic voltage with time before and after ICCE ceased.
above the distributor after ICCE ceased. The voltage was found to increase from negative to positive progressively until the equilibrium voltage before ICCE was reached and then leveled off. When ICCE was stopped, the negative ions generated by previous corona discharge may be neutralized by triboelectric charging of the particles or dissipated with fluidizing gas. Thus, the electrostatic voltage recovered gradually to its equilibrium voltage before ICCE. As can be seen from Figure 4, the initial voltage before ICCE is a bit positive, while the voltage seems D
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
particles with increasing gas velocity. Once a negative discharge was applied, the electric field significantly reduced. Variation of the electric field strength before and after ICCE was similar to that of the electric voltage and charge-to-mass ratio. Besides, as Figure 6 shows, the electric field strength was almost zero after the application of ICCE at 0.5 m/s. Because the electric field strength in the dilute phase represented the sum of the electric field induced by all of the particles, zero electric field did not mean that all of the particles in the fluidized bed were uncharged. Because the particles in the fluidized bed were possibly bipolar charged, the zero electric field at 80 cm above the distributor only stood for neutralization of the total electric field induced by both positively and negatively charged particles. As can be seen from Figures 4 and 5, when the gas velocity was 0.5 m/s, the particles after negative corona discharge at 10 cm were still negatively charged, while the particles at 20 cm were positively charged. The bipolar charge of the particles at different locations may cause a zero electric field at the dilute zone.
The number concentration of the ions generated by ICCE is strongly dependent on the needle electrode configuration and high voltage. In higher gas velocities, the negative ions generated by the same condition of ICCE with lower gas velocities were not sufficient to neutralize the increased positive charge on the particles due to intensified particle collision. The results of variation of the charge-to-mass ratios before and after ICCE coincide with those of variation of the electrostatic voltage. Besides, a comparison of the results in Figures 4 and 5 shows that both the electrostatic voltage at H = 200 mm and the charge-to-mass ratio at H = 100 mm were positive before application of ICCE. However, after the application of a negative corona discharge, the charge-to-mass ratio changed from positive to negative, while the electrostatic voltage still remained positive. This may be caused by stronger attrition at 200 mm than at 100 mm because of the bubble growth in the vertical direction, and the particles at 200 mm were more charged. The measurement showed that the charge-to-mass ratio of the particles at 200 mm was 5.67 μC/kg, which was significantly larger than 3.17 μC/kg at 100 mm. Therefore, it is possible that particles with smaller positive charge at 100 mm were negatively charged while particles at 200 mm were still positively charged after the application of a negative corona discharge. Variation of the Electric Field Strength with ICCE. Except for the electrostatic voltage and charge-to-mass ratio, the electric field strength in the dilute phase of the fluidized bed was also measured before and after ICCE. The results of variation of the electric field strength with the gas velocity are displayed in Figure 6. When particles were charged solely due
■
DISCUSSION Variation of the electrostatic voltage, charge-to-mass ratio, and electric field strength before and after the application of ICCE has been previously investigated and analyzed. In general, the equilibrium charge of the particles after the application of ICCE strongly depends on the initial charge. When particles are less charged at low gas velocities, negative corona discharge causes a polarity reversal. When particles are more charged at higher gas velocities, the negative corona discharge only causes a decrease of the charge on the particles without polarity change. Upon comparison to the corona neutralizer setting in the expanded section by Revel et al.,41 the ICCE developed in this work had a higher charge neutralization efficiency. Because the corona electrodes were inserted in the dense zone, the unipolar ions caused by ICCE had a better contact efficiency with charged particles within the gas flow and a longer residence time within the fluidized bed. Therefore, it is predictable to find that the charges of the particles at both the bottom and top of the fluidized bed were partially or fully neutralized. Figure 7 shows the schematic mechanism of corona discharge in the fluidized bed under different charges. Particles are charged through particle−particle and particle−wall frictions. The fluidized gas around the needle electrode with high-voltage power supply was ionized, and unipolar ions were produced. The negative ions migrated to the positively charged particles under the electric field caused by corona discharge and charged particles. Thus, the positive charge on the particles was neutralized. Further, the charge neutralization efficiency depends on the negative ion concentration and the charges on the particles. When the gas velocities were low, the generation rates of positive charge on the particles due to triboelectric charging are smaller than those of the negative ions generated by negative corona discharge. Therefore, particles tended to be negatively charged after the application of ICCE, as described in Figure 7a. The generation rates of positive charge on the particles increased due to stronger friction under higher gas velocities. As a result, the particles were still positively charged after the application of ICCE, within a significant decrease of charge, as shown in Figure 7b. It is worth noting that while the positive charges on the particles were neutralized by the negative ions, particles also obtained charges through triboelectric charging. If the positive charges generated by triboelectric charging outweigh the negative ions generated
Figure 6. Variation of the electric field strength (H = 80 cm) with the gas velocity before and after ICCE.
to triboelectric charging, the electric field strength in the dilute phase was positive and almost increased linearly with increasing gas velocity. On the one hand, the charge carried by the particles increased with increasing gas velocity because of intensive collision. On the other hand, the dynamic bed height also increased with increasing gas velocity; thus, a decreased distance between the charged particles and field meter was found. Therefore, the electric field strength significantly increased as a result of increasing charge on the particles and decreasing distance between the field meter and charged E
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
Figure 7. Schematic mechanism of corona discharge in the fluidized bed: (a) low gas velocities; (b) large gas velocities.
Figure 8. Decreasing amplitudes of the charge-to-mass ratio, electrostatic voltage, and electric field strength before and after the application of ICCE under various gas velocities.
by corona discharge, the particles are still positively charged. In other words, charge neutralization by corona discharge is a dynamic process. Therefore, either change of triboelectric charging or corona discharge will cause an unbalanced charge equilibrium. A new charge equilibrium will be established under new conditions. Previous analyses have showed that charge reduction is influenced by both the triboelectric- and corona-charging processes. Figure 8 shows the decreasing amplitudes of electrostatic charge (electrostatic voltage, charge-to-mass ratio, and electric field strength) before and after the application of ICCE. As the gas velocity increases, the decreasing amplitudes of all three electrostatic parameters increased after the same configuration of negative corona discharge. In other words, the
neutralization efficiency of the electrostatic charge in the fluidized bed increases with increasing gas velocity. The dependence of the charge neutralization efficiency on the superficial gas velocity is further analyzed in the following section. For single gas corona discharge, the corona electric field is mainly controlled by the needle electrode configuration and high voltage. In gas−solid mixtures, except for the electrode configuration and high voltage, the corona electric field is also dependent on the solid content and the relative permittivity of two phase mixtures.33 Corona discharge of two phase mixtures is harder to initiate; i.e., a higher onset corona voltage is needed at higher solid content. On the one hand, increasing gas velocity leads to more charges on the particles. On the other F
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
charging processes, and the charge neutralization efficiency increased with increasing gas velocity. Besides, the dielectric particles in the fluidized bed are possibly bipolar charged, but so far only one polarity of corona discharge was applied in this work. A future study will focus on the application of ICCE of double polarities, and a theoretical model of corona discharge will also be developed.
hand, the gas voidage also increases with increasing gas velocity. Both of these factors will influence the final equilibrium charge after the application of ICCE. Figure 9 shows variation of the
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful for financial support of this work by the National Natural Science Foundation of China (Grant 21236007), National Basic Research Program of China (Grant 2012CB720500), Specialized Research Fund for the Doctoral Program of Higher Education (Grant 20130101110063), and Zhejiang Provincial Natural Science Foundation of China (Grants LQ13B060002 and R14B060003).
Figure 9. Variation of the onset corona voltage in the gas−solid fluidized bed with the gas velocity.
■
onset voltage of negative corona discharge with increasing gas velocity in the gas−solid fluidized bed. The onset voltage was found to be lower with increasing gas velocity, which means that the negative corona discharge prefers a higher gas velocity. This is due to the fact that the bed voidage increased with increasing gas velocity, which resulted in a smaller onset voltage. Therefore, under an identical high voltage and needle configuration, the number concentration of the ions generated by the needle electrode increased with increasing gas velocity. This caused higher neutralization efficiencies of electrostatic charge at higher gas velocities. This advantage can never be achieved if the corona discharger is attached in the dilute phase or the expanded section.
REFERENCES
(1) Cross, J. Electrostatics: principles, problems and applications; Science Publishers: Boca Raton, FL, 1987. (2) Hogue, M. D.; Mucciolo, E. R.; Calle, C. I. Triboelectric, corona, and induction charging of insulators as a function of pressure. J. Electrost. 2007, 65 (4), 274−279. (3) Trigwell, S.; Grable, N.; Yurteri, C. U.; Sharma, R.; Mazumder, M. K. Effects of surface properties on the tribocharging characteristics of polymer powder as applied to industrial processes. IEEE Trans. Appl. Ind. 2003, 39 (1), 79−86. (4) Wang, F.; Wang, J.; Yang, Y. Distribution of electrostatic potential in a gas−solid fluidized bed and measurement of bed level. Ind. Eng. Chem. Res. 2008, 47, 9517−9526. (5) Matsusaka, S.; Maruyama, H.; Matsuyama, T.; Ghadiri, M. Triboelectric charging of powders: A review. Chem. Eng. Sci. 2010, 65 (22), 5781−5807. (6) Sow, M.; Lacks, D. J.; Sankaran, R. M. Effects of material strain on triboelectric charging: Influence of material properties. J. Electrost. 2013, 71 (3), 396−399. (7) Xiangbo, M.; Hui, Z.; Jingxu, Z. The characteristics of particle charging and deposition during powder coating processes with coarse powder. J. Phys. D: Appl. Phys. 2008, 41 (19), 195207. (8) Bailey, A. G. The science and technology of electrostatic powder spraying, transport and coating. J. Electrost. 1998, 45 (2), 85−120. (9) Higashiyama, Y.; Asano, K. Recent progress in electrostaic separation technology. Part. Sci. Technol. 1998, 16 (1), 77−90. (10) Miloudi, M.; Dascalescu, L.; Jia, L.; El-Mossouess, S.; Medles, K. Tribo-aero-electrostatic separator for coarse granular insulating materials. IEEE Trans. Dielectr. Electr. Insul. 2013, 20 (5), 1510−1515. (11) Zelmat, M.; Tilmatine, A.; Rizouga, M.; Gouri, R.; Medles, K.; Dascalescu, L. Experimental analysis of a cyclone tribocharging device for free-fall triboelectric separation of plastic particles. IEEE Trans. Dielectr. Electr. Insul. 2013, 20 (5), 1584−1589. (12) Mazumder, M. K.; Sims, R. A.; Biris, A. S.; Srirama, P. K.; Saini, D.; Yurteri, C. U.; Trigwell, S.; De, S.; Sharma, R. Twenty-first century research needs in electrostatic processes applied to industry and medicine. Chem. Eng. Sci. 2006, 61 (7), 2192−2211. (13) Jaworek, A.; Krupa, A.; Czech, T. Modern electrostatic devices and methods for exhaust gas cleaning: A brief review. J. Electrost. 2007, 65 (3), 133−155. (14) Glor, M. Ignition hazard due to static electricity in particulate processes. Powder Technol. 2003, 135−136, 223−233.
■
CONCLUSIONS In this work, a novel ICCE was developed to neutralize the electrostatic charge in the fluidized bed. In order to verify the feasibility of this method, variation of the electrostatic voltage, charge-to-mass ratio, and electric field before and after the application of ICCE was investigated, respectively. The results indicated that the triboelectric- and corona-charging processes had a joint influence on the charge neutralization efficiency. The optimal way to improve the charge neutralization efficiency is to apply an appropriate corona voltage to balance the charges of the particles because an excess amount of corona ions can cause a polarity reverse of charges. Because of the in situ setup of the corona electrodes, both charged particles in the bottom and top of the fluidized bed can be neutralized or partially reduced. ICCE has shown great potential in reducing and controlling the electrostatic charge in the industrial FBRs. The equilibrium charge of the particles after the application of ICCE strongly depends on the initial charge in the fluidized bed. When particles were less charged at low gas velocities (less than 2 U/Umf), negative corona discharge caused a polarity reversal. When particles were more charged at high gas velocities, the charge on the particles was only partially neutralized. What’s more, the superficial gas velocity had a significant influence on both the triboelecric- and coronaG
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
(35) Mayr, M. B.; Barringer, S. A. Corona Compared with Triboelectric Charging for Electrostatic Powder Coating. J. Food Sci. 2006, 71 (4), E171−E177. (36) Watano, S. Mechanism and control of electrification in pneumatic conveying of powders. Chem. Eng. Sci. 2006, 61 (7), 2271−2278. (37) Kimoto, S.; Mizota, K.; Kanamaru, M.; Okuda, H.; Okuda, D.; Adachi, M. Aerosol Charge Neutralization by a Mixing-Type Bipolar Charger using Corona Discharge at High Pressure. Aerosol Sci. Technol. 2009, 43 (9), 872−880. (38) Kachi, M.; Dascalescu, L.; Herous, L.; Nemamcha, M. Experimental study of charge neutralization at the surface of granular layers of insulating materials. Industry Applications Society Annual Meeting (IAS); IEEE: New York, 2011; pp 1−8. (39) Kachi, M.; Nemamcha, M.; Lazhar, H.; Dascalescu, L. Neutralization of charged insulating granular materials using AC corona discharge. J. Electrost. 2011, 69 (4), 296−301. (40) Vincenzi, P.; Arletti, A. Process for the production of polymer. U.S. Patent6,252,015, 1999. (41) Revel, J.; Gatumel, C.; Dodds, J. A.; Taillet, J. Generation of static electricity during fluidisation of polyethylene and its elimination by air ionisation. Powder Technol. 2003, 135−136, 192−200. (42) Li, T.; Pougatch, K.; Salcudean, M.; Grecov, D. Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed. Powder Technol. 2008, 184 (1), 89−99.
(15) Bunchatheeravate, P.; Curtis, J.; Fujii, Y.; Matsusaka, S. Prediction of particle charging in a dilute pneumatic conveying system. AIChE J. 2013, 59 (7), 2308−2316. (16) Fath, W.; Blum, C.; Glor, M.; Walther, C.-D. Electrostatic ignition hazards due to pneumatic transport of flammable powders through insulating or dissipative tubes and hosesNew experiments and calculations. J. Electrost. 2013, 71 (3), 377−382. (17) Hendrickson, G. Electrostatics and gas phase fluidized bed polymerization reactor wall sheeting. Chem. Eng. Sci. 2006, 61 (4), 1041−1064. (18) Cao, Y.; Wang, J.; He, Y.; Liu, W.; Yang, Y. Agglomeration detection based on attractor comparison in horizontal stirred bed reactors by acoustic emission sensors. AIChE J. 2009, 55 (12), 3099− 3108. (19) Wang, J.; Xu, Y.; Li, W.; Yang, Y.; Wang, F. Electrostatic potentials in gas−solid fluidized beds influenced by the injection of charge inducing agents. J. Electrost. 2009, 67, 815−826. (20) Wolny, A.; Opaliǹski, I. Electric charge neutralization by addition of fines to a fluidized bed composed of coarse dielectric particles. J. Electrost. 1983, 14 (3), 279−289. (21) Yu, X.; Li, W.; Xu, Y.; Wang, J.; Yang, Y.; Xu, N.; Wang, H. Effect of polymer granules on the electrostatic behavior in gas−solid fluidized beds. Ind. Eng. Chem. Res. 2010, 49, 132−139. (22) Nieh, S.; Nguyen, T. Effects of humidity, conveying velocity, and particle size on electrostatic charges of glass beads in a gaseous suspension flow. J. Electrost. 1988, 21 (1), 99−114. (23) Park, A.-H.; Bi, H.; Grace, J. R. Reduction of electrostatic charges in gas−solid fluidized beds. Chem. Eng. Sci. 2002, 57 (1), 153− 162. (24) Burgo, T. A. d. L.; Rezende, C. A.; Bertazzo, S.; Galembeck, A.; Galembeck, F. Electric potential decay on polyethylene: Role of atmospheric water on electric charge build-up and dissipation. J. Electrost. 2011, 69 (4), 401−409. (25) Sharma, R.; Sims, R. A.; Mazumder, M. K. Charge decay enhancement in polymer powders using plasma treatment. Conference Record of the 2002 IEEE Industry Applications Conference. 37th IAS Annual Meeting, Pittsburgh, PA, Oct 13−18, 2002; IEEE: New York, 2002; Vol. 1, pp 610−613. (26) Sharma, R.; Trigwell, S.; Biris, A. S.; Sims, R. A.; Mazumder, M. K. Effect of ambient relative humidity and surface modification on the charge decay properties of polymer powders in powder coating. IEEE Trans. Appl. Ind. 2003, 39 (1), 87−95. (27) Wang, H.; Cai, B.; Chen, Q.; Ge, L.; Chen, S. The effects of chemical pretreatment on tribocharging of density fraction coal. J. Electrost. 2014, 72 (1), 65−69. (28) Matsusaka, S.; Oki, M.; Masuda, H. Control of electrostatic charge on particles by impact charging. Adv. Powder Technol. 2007, 18 (2), 229−244. (29) Zhang, B.; Zhou, Y.; Wang, N.; Liang, X.; Guan, Z.; Takada, T. Polarity reversal charging of polypropylene films under DC corona discharge. J. Electrost. 2005, 63 (6−10), 657−663. (30) Intra, P.; Tippayawong, N. Progress in unipolar corona discharger designs for airborne particle charging: A literature review. J. Electrost. 2009, 67 (4), 605−615. (31) Leong, F. Y.; Smith, K. A.; Wang, C.-H.; Matsusaka, S.; Hua, J. Electrostatic effects on inertial particle transport in bifurcated tubes. AIChE J. 2009, 55 (6), 1390−1401. (32) Huang, C.-H.; Alonso, M. Nanoparticle electrostatic loss within corona needle charger during particle-charging process. J. Nanopart. Res. 2011, 13 (1), 175−184. (33) Wenjun, Y.; Zhenghao, H.; Heming, D.; Jing, L.; Jun, M. Experimental Investigation of Two-Phase Mixture Discharges under DC Voltage from Effects of Macroparticle Sizes. IEEE Trans. Plasma Sci. 2011, 39 (3), 856−864. (34) Kodama, T.; Suzuki, T.; Mogami, T.; Watano, S. A new passivetype electrostatic eliminator used in a powder storage silo. Powder Technol. 2003, 135−136, 209−215. H
dx.doi.org/10.1021/ie501584v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
