Hydrodynamics of Electrostatic Charge in Polypropylene Fluidized

Article pubs.acs.org/IECR

Hydrodynamics of Electrostatic Charge in Polypropylene Fluidized Beds Piyawan Tiyapiboonchaiya,a Dimitri Gidaspow,b and Somsak Damronglerd*,a a

Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, United States

b

ABSTRACT: Experiments were performed in a double jacket gas−solid fluidized bed to investigate the process and degree of electrification in polypropylene fluidized beds. Five copper rings attached to the inner column wall were connected to a picoammeter for measuring the electrical current at the different vertical levels. It was found that both positive and negative electrical currents were traced in the whole bed. The magnitude of positive current value was observed at the bottom of the bed while at the top part of the bed the magnitude of negative current value was found. The influence of initial bed height, fluidization velocity, and gas temperature on the degree of electrification was investigated. The effect of gas temperature played a significant role in the charge dissipation. Also, the degree of electrification increased with initial bed height and gas velocity. Sowinski et al.9 proposed the hypothesis that the particles adhere to the wall due to their positive charge. Fang et al.10 found that the voltage polarity of static charge resulted in a Zshaped axial profile by using static probe to measure the axial electrostatic potential distribution. The reverse values took place near the interface between dense and dilute phases. The main objective of this work was to study the triboelectric process generated and dissipated in the polypropylene fluidized bed by using the copper strips connected to the picoammeter. Using this method, the understanding of the mechanism of electrostatic charge in fluidized beds was improved. Also, the effects of initial bed height, gas velocity, and gas temperature on electrostatic charge generation in fluidized beds were investigated.

1. INTRODUCTION In 1968 the first gas-phase fluidized bed polymerization reactor was commercialized by Union Carbide for producing highdensity polyethylene (HDPE). They extended this process for producing linear low-density polyethylene (LLDPE) in 1975 and for polypropylene (PP) in 1985.1,2 Although there were several advantages of using gas−solid fluidized bed reactors, there were also some problems.3 Naturally in the process of fluidization, continuous contacts among bed particles are common, thus the occurrence of electrostatic charge in insulating materials is almost unavoidable. The electrostatic force may cause the particles to stick to the wall, known as wall sheeting, the formation of large aggregate particles, and changing of the hydrodynamics. Hendrickson4 has given a thorough of review of electrostatics in gas-phase fluidized bed polymerization reactors. The mechanisms of static charge generation are quite complicated and still poorly understood. There are various ways to produce a charge on material, such as triboelectric charging, induction, frictional charging, and ion bombardment, but the most common mechanism is triboelectrification. Tribocharging occurs when materials are placed in contact with one another and then separated, causing charges to be pulled from one material surface and relocated on the other material surface. Previously Yao et al.5 found that an increase in relative humidity of fluidizing gas enhances the charge dissipation due to increases in the surface conductivity of particles. Wolny and Opalinski6 added fine particles to large dielectric particles to neutralize the electric charge. Boland and Geldart7 investigated the charge in fluidized beds. They found that the charge was generated around gas bubbles in the wake region and that the amount of charge increases with particle size because of enhancing interparticle contact. Recently Giffin and Mehrani8 used Faraday cups to measure particle charge in bubbling and slugging fluidized bed columns filled with polyethylene particles. They found that the fines were predominantly positively charged, while the wall and dropped particles were predominantly negatively charged. © 2012 American Chemical Society

2. FLUIDIZATION EXPERIMENT 2.1. Experimental Apparatus. The experiment was performed in double jacket columns fabricated from acrylic. The outside jacket is to prevent ambient static electricity from disturbing the bed. The height of column is 1 m, the inside diameter of inner column is 15 cm, and the outside column is 20 cm. The upper part of the column has a large diameter of 32 cm for reducing the gas velocity. The polypropylene particles were blown out the column and collected in the cyclone when the experiment was over. The polypropylene powders used in this study were supplied by Thai Polyethylene Co., Ltd. The particles were classified as Geldart B material of a mean diameter of 830.8 μm and particle density of 910 kg/m3. The particle size distribution is shown in Table 1. There are 5 strips of copper plate with the height of 3.81 cm located inside the column at 5.72, 13.34, 20.96, 28.58, and 36.20 cm above the distributor plate. The schematic of Received: Revised: Accepted: Published: 8661

November 1, 2011 May 25, 2012 June 1, 2012 June 1, 2012 dx.doi.org/10.1021/ie202496t | Ind. Eng. Chem. Res. 2012, 51, 8661−8668

Industrial & Engineering Chemistry Research

Article

Imax| > |Imin|). On the other hand, the magnitude electrical current equal to the minimum value of electrical current (Imag = Imin) when the absolute value of minimum electrical current was larger than the absolute value of maximum electrical current (| Imin| > |Imax|). Hence, the magnitude of electrical current can have either a positive or a negative value.

Table 1. Particle-Size Distribution of Polypropylene Particles wt %

841 μm 97.2

500 μm 2.4

381.6 μm 0.3

250 μm 0.1

experimental equipment is depicted in Figure 1. The air from a blower was passed through the five stages of air dryer column which was filled with molecular sieve pellets in each stage. The moisture was reduced to less than 40% relative humidity and then the bed was heated to the desired temperature in an electrical heater column. The controlled air was fed into the fluidizing column to fluidize the polypropylene particles. 2.2. Current Measured and Analyzed. The measured electrostatic charge in the fluidized bed was represented in a terms of electrical current which was measured by connecting copper strips to a picoammeter (Keithley model 6485 picoammeter). For transferring the electrical current from copper strips to the picoammeter and logging signals from picoammeter to computer, Bayonet Neill−Concelman (BNC) connectors were used to minimize the disturbance caused by outside charge and distortion of outside potential field. Every 10 min in our fluidization experiment, the electrical current data at each copper strip were measured by using the Keithley model 6485 picoammeter and logged into a computer using ExcelLINK software with sampling frequency of 0.5 Hz for 90 s. Hence, data from the picoammeter were “instantaneous electrical current” which showed both positive and negative values. The positive value was called “positive current” and the negative value was called “negative current”. Then these data were averaged for obtaining the mean electrical current data every 10 min. The values were separated as “average positive current” (Iavg,pos) and “average negative current” (Iavg,neg). We called the electrical current which represented the major charge transfer in each zone and in each period as “magnitude electrical current” (Imag). The magnitude electrical current is equal to the maximum value of electrical current (Imag = Imax) when the absolute value of maximum electrical current was larger than the absolute value of minimum electrical current (|

3. RESULTS AND DISCUSSION The experiments investigated the electrostatic charge generation in a bubbling regime. In each experiment, the fluidization was maintained at 1 atm and the relative humidity was kept at less than 40%. Guardiola et al.11 conducted a study of parameters that effected the electrostatic charge on glass beads. Fluidizing glass beads of diameter of 250−350 μm with the relative humidity below 40% and beads of 350−420 μm with relative humidity below 75%, the quantity of electrostatic charge occurring did not change as much in these periods. Hence, to prevent the effect of the relative humidity on the degree of electrification, our experiments were carried out with a relative humidity lower than 40%. 3.1. Electrification Phenomenon. The polypropylene powders were fluidized by air supplied from a blower which was injected to the column at the bottom of the bed. In the beginning, small bubbles formed at the bottom before coalescencing in the upper part, where bubbles became bigger (Figure 2a). After conducting the experiment for 10 min, we were able to observe the temporary adhesion of some particles on the walls of both the fluidized bed and above the bed surface (Figure 2b and 2c). This indicated that the polypropylene powders had an electrostatic charge occurring on the particle surfaces. At the beginning, polypropylene powders formed little clusters, with the approximate diameter of 2−3 cm, on the bed surface, as shown in Figure 2b. The powders continued adhering to the wall and formed a thick layer. However, permanent adhesion of powders on the walls appeared after 30 min. 3.2. Electrification Measurement. The amount of electrostatic current was measured in an empty column by passing air with humidity less than 40%. The experiment was

Figure 1. Schematic diagram of the fluidization unit. 8662

dx.doi.org/10.1021/ie202496t | Ind. Eng. Chem. Res. 2012, 51, 8661−8668


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Figure 2. Fluidization of 831-μm polypropylene powders in three-dimensional column.

Figure 3. Intensity of electrical current generated from copper ring and gas flowing through the unfilled bed column.

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Figure 4. Evolution of static electrical current intensity with time.

run to collect raw data in the range of picoamperes to nanoamperes. Then we calculated the quantity of electrostatic charge, as shown in Figure 3. The positive current value was due to the electrons transferring from the system to the picoammeter, while the negative current value was due to the electrons transferring from the picoammeter to the system. There are both positive and negative values in the range of 66− 660 picoamperes for positive values and in the range of 63−620 picoamperes for negative values. The electrical current from copper rings connected to a picoammeter produced complete positive values from all data. Copper is a high conducting material which lies between a valence band and a conducting band causing the free electrons to move. The average current values from each copper ring were in the range of 20−30 picoamperes. This indicated that the airflow inside the bed created the process of charge transfer onto the copper surface. The charge generation was determined by feeding air through the polypropylene particle bed. Figure 4 shows the electrification current values of fluidized 831-μm polypropylene powders in a bubbling regime at gas flow rate of 170 m3/h and gas temperature of 348 K. The electrical currents were measured both in positive and negative range, at the level of 10−1 microamperewhich were larger than the values shown in Figure 3 by approximately 1 000 to 10 000 times. An indication that the charge generation took place in the fluidized bed was mostly from the particles which occurred for both positive and negative charges in the whole bed. Figure 5 shows that the maximum average electrical current values took place at the lower part of the bed and afterward the values continued to decrease with an increase in bed’s height. Because of the gas inlet at the bottom of the bed, the turbulent movement of gas and solids in this region was higher than at the upper part of the bed, causing more chance of collision between solids, solids and walls, or friction between solid and gas. Hence, the zone near the fluid inlet was a dominant zone in the electrification process in the fluidized bed. The average values of positive current at CH1, CH2, CH3, CH4, CH5 were 0.376, 0.379, 0.289, 0.129, and 0.077 μA and the average values of negative current were −0.363, −0.386, −0.340, −0.116, and −0.097 μA, respectively. The electrical charge accumulated on the polypropylene surfaces via the triboelectric process according to the law of the conservation of charge as shown in Figure 5. The summations of the average positive current and the average negative current were near zero because the electrons lost from one particle would be collected by the other.

Figure 5. Electrification current generated in the polypropylene fluidized bed.

3.3. Effect of Initial Bed Height. Figure 6 shows that the positive electrical current values were generated at different initial bed heights of 2, 4, and 6 in., which were equivalent to the mass of polypropylene particles of 400, 800, and 1200 g, respectively. The bed height played a significant role in the degree of electrostatic charge generated in the fluidized bed. It

Figure 6. Influence of initial bed height on the electrostatic charge current intensity in the fluidized bed. 8664



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high air feeding rate. The increase in size and velocity of bubbles formed at the distributor was due to the increase of fluidizing gas. This resulted in a decline in solid circulation and collision frequency in the shallow bed, causing less charge generation and transfer between particle−particle. 3.5. Effect of Gas Temperature. The study of the temperature effect on electrostatic charge process was conducted at a gas flow rate of 170 m3/h and a freeboard of 101 kPa. To achieve a steady state, polypropylene powder was preheated for 1 h before the fluidization process. As shown in Figure 8a, it is found that increasing the values of the positive

can be seen that the values at each height increase with the initial bed height. The observation at the 6 in. height found that the maximum electrification current value was located at CH2. This was because the second vortex ring took place in the deeper fluidized bed and resulted in the flow direction of solids at the top part of the bed, which was reversed from the flow of solids from the lower part. The quantity and the contact time of the solids in this zone were more elevated than in the other regions causing the high development of electrostatic charge. We call the zone in this region a stagnant zone.12 Furthermore, we were able to understand clearly why the high initial bed height produced a higher electrical current value than the low initial bed height. This was because the number of contacts among the particles was certainly higher than for the low initial bed height. The maximum values of average current with initial bed height of 4 and 6 in. were 2.44 and 3.94 times larger, respectively, compared to the 2 in. initial bed height. 3.4. Effect of Gas Velocity. The average electrification current was measured as a function of the gas flow rate, as shown in Figure 7. To study the effect of gas velocity on the

Figure 8. Influence of fluidizing gas temperature on the electrostatic charge current intensity: (a) positive electrical current and (b) negative electrical current.

electrical current at copper strip numbers 1, 2, 3, 4, and 5 with the gas temperature of 328 K were 7.22, 10.17, 30.84, 14.48, and −1.64%, respectively, compared with the values at gas temperature of 308 K. The percentage of the positive electrical current value at the gas temperature of 348 K was higher than the values at gas temperature of 328 K of 4.11, 7.44, 8.72, −1.14, and 9.64 at copper strip numbers of 1, 2, 3, 4, and 5, respectively. The experimental result shows that the increase of gas temperature in the low level led to a significant increase in the degree of the charge generated. This was because of the increasing kinetic energy of electrons. The results in Figure 8b show that the gas temperature also played an important role on the negative electrical current. The quantity of electrons transferred from the picoammeter to the particles in the zone above the stagnant region goes along with the increasing temperature. The negative current values at CH3 of gas temperatures at 308:328:348 K were 1:1.30:1.64, respectively. For the bed when the gas temperature was raised from 328 to 348 K would be lower than the increased positive electrical current for the bed with increasing gas temperature from 308 to 328 K. This agrees with the results by Moughrabiah et al.13

Figure 7. Influence of fluidizing gas velocity on the electrostatic charge current intensity with the initial bed height of (a) 4 in. and (b) 6 in.

degree of charge generation in the fluidized bed, air was used as the fluidizing medium at three different flow rates at the constant temperature of 328 K. With greater velocity, it is believed that the turbulence of bubbles and particles inside the bed would increase which enhances the frequency and velocity of particle−particle and particle−wall collision. As a result, the electrical current value rises with increasing gas velocity due to the heightened circulation rate, bubble size, and contact frequency between solids. With initial bed height of 6 in., the electrical current value increased with gas velocity at all vertical bed heights and had more effect at the lower part of the bed. The maximum positive current value at CH2 of gas flow rate 150:160:170 m3/h was 1:1.09:1.16. However, at initial bed height of 4 in., the degree of electrical current dropped at the 8665



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Figure 9. Magnitude electrification current occurring in 831 μm polypropylene fluidized bed with gas flow rate of 160 m3/h and gas temperature of 328 K.

They demonstrated that the degree of charge electrification decreased with increasing temperature. 3.6. Charge Mechanism and Charge Distribution. The results in Figure 4 showed that both positive and negative electrical current occurred in every part of the fluidized bed. The magnitude of electrical currents has reversed signs between the top part and the bottom part of the bed, as shown in Figure 9. At the bottom of the bed (CH1 and CH2), the magnitude of electrical current had positive values, while the negative electrical current values occurred at the top part of the bed (CH3 and CH4). So, the static charge occurring in fluidized beds could be expressed by two mechanisms. First, the gas flowed through each hole of distributor as a jet stream and then formed small bubbles. These bubbles moved violently, which produced good contact between the gas and solids in the dense zone at the bottom, causing charge generation in this zone.14 The second mechanism was bipolar charging process15,16 with two contacting particles having different work functions.17,18 Many investigators found that particles have positive polarity on large particles and negative polarity on small particles.19−21 In theory, for particles of the same material and dielectric constant, the work function of the particles would decrease with increase in the diameter of particle.17 However, some studies showed that the negative charge could form on large particles and the positive charge could form on small particles.10,22 The effective work function would rise if the particles had impurities on their surface.23 Hence, the coarse particles may have a higher work function than the fine particles, when the impurities are on their surface because of a high surface. These mechanisms may explain why both particle charges can be found everywhere in the polypropylene fluidized bed. These results agreed very well with the experiments of Day,24 which were reviewed by Hendrickson.4 They explained that the transfer of positive charge to the wall occurred near the top part of the bed, while the transferring of negative charge took place near the distributor plate. Their report agrees well with these experimental results. The scheme of the charging mechanism and transport is shown in Figure 10. Furthermore, the measuring technique of electrostatic charge in this study was rather efficient and of a high confidence because it can express the instantaneous charging of the fluidized bed, while the

Figure 10. Scheme of the electric charge transporting process during the fluidization: (a) occurred charging via particle−gas friction, (b) occurred charging via smooth and purities particle−particle contact, and (c) occurred charging via roughness and impurities particle− particle contact.

Faraday cage method can express only the net charge accumulated on the particles. The observation from experiments found that the surfaces of the copper rings, which were connected to the picoammeter (ground system), had no polypropylene powders on them. The inside ground of the column played an important role in transferring the charge to the powders. Hence adhesion of polypropylene particles was absent in this region. Figure 2d shows the bed after the air supply was turned off. The powders stick on the wall only in the region of the acrylic column. In industry, a commercial-size reactor would be grounded from 8666



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the outside surface wall of the reactor. As a result, the charge produced inside the bed cannot be transferred to the outside. The charge inside the column had to share or to combine the charge with the reactor wall or other particles. This phenomenon will cause the particles to attach and form layers on the reactor wall or consequently form large aggregations of particles inside the bed. It had been demonstrated that the adhesion of particles on the upper section of the bed surface, above copper strip number 5, was observed. Hence, it may be concluded that the charging process took place by solid particles in the region of wake and/ or nose. Due to the fact that particles adhered at the upper part of copper ring sheet CH5 came from the bursting of bubbles at the bed surface, they bounced off the solids to the top region of the bed surface. The polymer is a high insulating material. Thus the electrical charge generated in the fluidized bed gradually accumulated on the polymer surface and the electrical force acting to the solid particles increased over time. This can be seen from the observation of polymer adhesion on the wall, as described previously. It takes quite a long time to measure the amount of electrical current because the charge gradually accumulates on the polypropylene particles. Bartilucci et al.25 reported that the magnitude of the current typically lies in the range of 0.1−10 microamperes per square meter of reactor surface area. In this experiment when we operated with an initial bed height of 6 in., we could obtain the surface current density of about 44 microamperes per square meter of reactor surface area.

REFERENCES

(1) Yang, W. C. Handbook of Fluidization and Fluid-Particle Systems; Marcel Dekker, Inc.: New York, 2003. (2) Xie, T.; McAuley, K. B.; Hsu, J. C.C.; Bacon, D. W. Gas Phase Ethylene Polymerization: Production Processes, Polymer Properties, and Reactor Modeling. Ind. Eng. Chem. Res. 1994, 33, 449. (3) Rokkam, R. G.; Fox, R. O.; Muhle, M. E. Computational fluid dynamics and electrostatic modeling of polymerization fluidized-bed reactors. Powder Technol. 2010, 203, 109. (4) Hendrickson, G. Electrostatics and gas phase fluidized bed polymerization reactor wall sheeting. Chem. Eng. Sci. 2006, 61, 1041. (5) Yao, L.; Bi, H. T.; Park, A. H. Characterization of electrostatic charges in freely bubbling fluidized beds with dielectric particles. J. Electrost. 2002, 56, 183. (6) Wolny, A.; Opaliński, I. Electric charge neutralization by addition of fines to a fluidized bed composed of coarse dielectric particles. J. Electrost. 1983, 14, 279. (7) Boland, D.; Geldart, D. Electrostatic Charging in Gas Fluidized Beds. Powder Technol. 1971, 5, 289. (8) Giffin, A.; Mehrani, P. Comparison of Influence of Fluidization Time on Electrostatic Charge Buildup in Bubbling vs. Slugging Flow Regimes in Gas-Solid Fluidized Beds. J. Electrost. 2010, 68, 492. (9) Sowinski, A.; Miller, L.; Mehrani, P. Investigation of electrostatic charge distribution in gas-solid fluidized beds. Chem. Eng. Sci. 2010, 65, 2771. (10) Fang, W.; Jingdai, W.; Yongrong, Y. Distribution of Electrostatic Potential in a Gas-Solid Fluidized Bed and Measurement of Bed Level. Ind. Eng. Chem. Res. 2008, 47, 9517. (11) Guardiola, J.; Rojo, V.; Ramos, G. Influence of particle size, fluidization velocity and relative humidity on fluidized bed electrostatics. J. Electrost. 1996, 37, 1. (12) Kunii, D.; Levenspiel, O. Fluidization Engineering; ButterworthHeinemann: Newton, MA, 1991. (13) Moughrabiah, W. O.; Grace, J. R.; Bi, X. T. Effects of Pressure, Temperature, and Gas Velocity on Electrostatics in Gas-Solid Fluidized Beds. Ind. Eng. Chem. Res. 2009, 48, 320. (14) Buzanov, V. I.; Abramyan, V. K.; Grishina, T. F.; Krapivin, L. E. Probe measurement of electrostatic fields in particle bearing gas flows. Meas. Tech. 1978, 21, 692. (15) Ali, F. S.; Ali, M. A.; Ali, R. A.; Inculet, I. I. Minority charge separation in falling particles with bipolar charge. J. Electrost. 1998, 45, 139. (16) Bi, H. T. Electrostatic phenomena in gas-solids fluidized beds. China Particuol. 2005, 3, 395. (17) Gallo, C. F.; Lama, W. L. Classical Electrostatic Description of the Work Function and Ionization Energy of Insulators. IEEE Trans. Ind. Appl. 1976, IA-12, 7. (18) 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. Ind. Appl. 2003, 39, 79. (19) Zhao, H.; Castle, G. S. P.; Inculet, I. I.; Bailey, A. G. Bipolar Charging in Polydisperse Polymer Powders in Industrial Processes. In Conference Record of the 2000 IEEE Industry Applications Conference, Rome, Italy, Oct 8−12, 2000; IEEE Press: New York, 2000. (20) Zhao, H.; Castle, G. S. P.; Inculet, I. I.; Bailey, A. G. Bipolar Charging of Polydisperse Polymer Powders in Fluidized Beds. IEEE Trans. Ind. Appl. 2003, 39, 612. (21) Forward, K. M.; Lacks, D. J.; Sankaran, R. M. Charge Segregation Depends on Particle Size in Triboelectrically Charged Granular Materials. Phys. Rev. Lett. 2009, 102, 028001. (22) Mehrani, P.; Bi, H. T.; Grace, J. R. Electrostatic charge generation in gas-solid fluidized beds. J. Electrost. 2005, 63, 165. (23) Lowell, J.; Rose-Innes, A. C. Contact electrification. Adv. Phys. 1980, 29, 947. (24) Day, D. R. New process instrumentation for metallocene-based resin production. Presented at 4th International Congress on Metallocene Polymers: Metallocenes Asia ‘97; Singapore, May, 1997.

4. CONCLUSIONS Initially, the generation of electrostatic charging was investigated in empty column by passing air with less than 40% humidity through it. Both the positive charge and the negative charge were about 10−1 nanoamperes. Then polypropylene particles were put in the column and fluidized with air with a humidity of less than 40%. The electric current was found to be 10−1 microamperes, or 1000 times higher than in the empty column. The average electrical current had a maximum value in the stagnant zone for a deep bed. Increasing the values of initial bed height and gas velocity resulted in an increase in the degree of electrification of the fluidized bed. Both variables induced more solids circulation and frequency of solids collision. The gas temperature also had an effect on the electrostatic charge occurring in the fluidized bed, with increased charge dissipation when gas temperature increased. This is reasonable for the dielectric material. Positive magnitude electrical currents were found at the bottom part of the bed, while negative magnitude electrical currents were found at the top part of the bed.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (66)2-218-7515. Fax: (66)2-255-5831. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support of this study by the Thai Polyethylene Co., Ltd., and also would like to thank the Department of Chemical Technology, Faculty of Science, Chulalongkorn University, for the instrument support. 8667



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(25) Bartilucci, M. P.; Davis Jr., E. R.; Egan, B. J.; Hagerty, R. O.; Husby, P. K. Method and apparatus for reducing static charges during polymerization of olefin polymers. WO 02/30993 A2, 2002.

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Hydrodynamics of Electrostatic Charge in Polypropylene Fluidized

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