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The effect of the generation and handling in the acquired electrostatic charge in airborne particles
Powder Technology 191 (2009) 299–308
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
The effect of the generation and handling in the acquired electrostatic charge in airborne particles Wiclef D. Marra Jr. a, Marcos V. Rodrigues b, Rosilene G.A. Miranda b, Marcos A.S. Barrozo c, José R. Coury b,⁎ a b c
Departamento de Hidráulica e Saneamento, Universidade de São Paulo, São Carlos/SP, Brazil Departamento de Engenharia Química, Caixa Postal 676, CEP 13565-905, São Carlos/SP, Brazil Faculdade de Engenharia Química, Universidade Federal de Uberlândia, Uberlândia/MG, Brazil
a r t i c l e
i n f o
Article history: Received 5 September 2007 Received in revised form 22 October 2008 Accepted 23 October 2008 Available online 5 November 2008 Keywords: Charged particles Electrostatics Powder handling Charge distribution
a b s t r a c t The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 × 10− 6 m to 1.4 × 10− 5 m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 × 10− 6 m and size range between 1.5 × 10− 7 m and 8.0 × 10− 6 m was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles, the median charges of the particles varied between − 3.0 × 10− 16 C and −5.0 × 10− 18 °C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Small particles may carry significant electrical charge, and can therefore be subjected to considerable forces if exposed to an electric field. Electrical forces on a charged particle can far exceed gravitational forces, and may in some circumstances even exceed aerodynamic forces in a moving air stream. It is well known that controlled handling of particles is exceptionally difficult to achieve [1]. Electrostatic particle charging is therefore an important phenomenon related to powder handling. Numerous industrial applications have evolved utilizing electrostatic charging of particles. In several processes such as drying, filtration, milling or storage of powder or granules, where the formation, dispersion and transport of particles occur, electrostatic charges can arise spontaneously due to friction or mechanical shock between particles (triboelectrification or impact). The presence of charges is desirable for some processes, as in electrostatic precipita⁎ Corresponding author. Tel.: +55 16 33518264. E-mail address: [email protected] (J.R. Coury). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.10.018
tors and electrostatic coatings; however there are other operations where it is highly inconvenient, as in storage silos and in the packing of very fine powders, causing difficulty in the flow and in the manipulation of the product. The presence of charges can also be dangerous, as in the transport and storage of cereal grains or dry powders, were they may promote ignition and explosion of the container. The importance of electrostatic charges in gas cleaning can be illustrated by the work of Coury and co-workers. Coury et al. [2] carried out extensive work on the measurement of electrostatic charges in airborne particles and the effect of those charges on the filter behavior. The equipment utilized in the measurements of charge distribution was developed by the authors, and constitutes a previous version of the one used in this work. Tests were performed at laboratory level as well as at bag-houses in power plants in Australia. In both cases, linear dependence between particle size and charge were obtained. The filtration tests showed that the electrostatic charges on the particles had a marked influence on the filter efficiency as well as on the cake formation. Duarte et al. [3] analyzed the effects of the level of electrostatic charges on filtration in a granular bed. The removal
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plate generator (TSI model 3433). In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. In the rotating plate generator, a phosphate rock concentrate was utilized as powder material. The particle charge distribution was measured experimentally immediately after generation or following corona or impact chargers. 2. Particle electrostatics
Fig. 1. Deflection of a charged particle under an electric field.
efficiency of the filter and the pressure drop were monitored at the granular bed. The particles were electrified by triboelectrification and impact, and the overall charges were measured with a Faraday cage that included the filter. The authors concluded that the formation of the filter cake was very much dependent on the level of particle charging. It was also observed that increase of the particle charges induces the formation of looser deposits, with lower resistance to gas flow. Whichever is the case, information on the size distribution and on the electrostatic charges is of paramount importance for the adequate handling of processes involving particles. Johnston et al. [4] and Forsyth et al. [5] have pointed out the influence of the generating conditions on the particle charge. In this study, the relationship between the generating conditions on the magnitude and distribution of the particle charge was investigated. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator (TSI model 3450) and a rotating
Most aerosol particles carry some electric charge, and some may be highly charged. For highly charged particles, the electrostatic force can be much greater than the gravity force. The motion induced by electrostatic force forms the basis for important types of air-cleaning equipment and aerosol sampling and measuring instruments. A charged particle experiences trajectory deviations in the vicinity of charged surfaces or other charged particles or in the presence of an electric field. The electrical force acts remotely through the air or vacuum and does not require a fluid flow. The charge on a particle can be negative or positive depending on whether the particle has an excess or deficiency of electrons. The velocity of motion resulting from this force can be determined in a manner similar to the terminal settling velocity [6]. 2.1. Charging mechanisms of particles In the handling of airborne suspensions, particles can acquire electrostatic charges spontaneously or charging can be induced by various charging mechanisms. Thorough reviews on this subject can be found in Lowell and Rose-Innes [7], Bailey [8], Cross [9], Flagan [10] and Matsusaka and Masuda [11]. There are three basic mechanisms by which aerosol particles acquire charge: static electrification, diffusion charging and field charging. The latter two require the production of unipolar ions, usually by corona discharge, and are used to produce highly charged aerosols [6]. 2.1.1. Static electrification Static electrification causes particles to become charged as they are separated from the bulk material or other surfaces. Particles are
Fig. 2. General view of the Electrostatic Charge Classifier (ECC).
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Fig. 3. Scheme of the impact charger device.
usually charged by this mechanism during formation, re-suspension, or high-velocity transport. The three mechanisms of static electrification that can charge aerosol particles during generation are electrolytic charging, spray electrification, and contact charging [6]. Electrolytic charging results when liquids with high dielectric constants are separated from solid surfaces. During atomization these liquids strip off charge from the surfaces of the atomizer and produce slightly to moderately charged droplets as they are separated from these surfaces. Pure water is a high dielectric liquid that can become charged during atomization [5]. Spray electrification results from the disruption of charged liquid surfaces. Some liquids, due to surface effects, have a charged surface layer, and when this surface is disrupted during the formation of droplets by atomization or bubbling, charged droplets are produced. Generation of aerosols by atomization promotes spray electrification. Due to forces in the outer surface of dielectric liquids, the dipole layer has its negative charge pointing to the outer surface and its positive charge pointing toward the diffuse liquid bulk, which creates a dipole moment. Impurities or additional ions may enter and interact with the dipole layer, causing an electrical potential change in the inner charge region. As dissolved ions interact with this layer, a destabilization of the dipole layer occurs that can reduce or reverse the charge of atomized aerosol particles. The mean particle charge is a function of the liquid dielectric constant and the solute concentration in solution and its dielectric properties. However, the magnitude of the aerosol particle charge is affected not only by the properties of the dispersed material (i.e., the dielectric constant and ion concentration) but also by the nature of the generation method. In addition, if the particles share frequent collisions during dispersion and transport, the net particle charge will also undoubtedly be altered [5]. Contact charging occurs during the separation of dry nonmetallic particles from solid surfaces. When the particles contact the surface there is a charge transfer to equalize the Fermi levels of the two materials. When the particles are separated from the surface they have an excess or deficiency of electrons. Polarity and amount of charge depends on the materials involved and their position in the triboelectric series. This phenomenon is often called “contact electrification” or “contact charging”. When they are rubbed, it can be called “frictional electrification” or “tribo-charging”. In case of short contact, it can be called “impact charging”. Because it requires dry surfaces, it becomes ineffective at relative humidity greater than about 65%. Most methods of re-suspending dry powders involve some friction between the powder and the apparatus and consequently produce charged aerosols [11].
Contact charging occurs due to the disruption of a dust or powder surface during aerosol generation. A triboelectric or contact charge develops along the particle surface because of mechanical friction from surface or particle contact during dispersion. The nature of the powder material will affect the resulting particle charge after generation. Johnston et al. [4,12] and Forsyth et al. [5] discovered a strong dependence between the resulting particle charge and the powder type and generation method. 2.1.2. Corona discharge The corona discharge is capable of producing ions in air. It can produce unipolar ions at a sufficiently high concentration to be useful for aerosol charging. To produce a corona discharge, one must establish a non-uniform electrostatic field such as that between a needle and a plate or a concentric wire and a tube. At normal conditions, air is a good insulator, but in a region of high field strength it may undergo an electrical breakdown and become conductive. Depending on the geometry and strength of the field, this breakdown can be a corona discharge. Regarding the wire and tube geometry the only region with enough field strength is a thin layer at the wire surface [6]. In the corona region, electrons are accelerated to a velocity sufficient to knock an electron from an air molecule upon collision, thereby creating a positive ion and an electron. Within the corona region, this process takes place in a self-sustaining flood that produces a discharge called corona discharge. The process is initiated by electrons and ions created by natural radiation. If the wire is negative, the positive ions will be attracted to it and the electrons will be repelled toward the tube. Introduction of aerosol particles into the space between the wire and the tube will result in field charging of the particles to the same polarity of the wire [9]. Mori et al. [13] studied the effects of a positive corona precharger on performance of a fabric filter in air with controlled humidity. The authors observed that, with the corona precharger on, the charge of particles were independent of the air relative humidity, whilst with the corona precharger off the particle charge decreased with increase in the relative humidity. 2.2. Measuring electrostatic charge in particles In all applications related to charged particle technology, the measurement of electrical parameters is of paramount importance.
Table 1 Distances C in the impact charger (see Fig. 3) utilized in the tests Impact condition
C (mm)
C1 C2 C3 C4
60 30 20 10
Fig. 4. The corona charger device (dimensions in cm).
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Fig. 5. Scheme of the experimental apparatus.
Measuring the charge of particles is very useful in practice as it immediately gives an indication of how well a particular particle charging system is working, which in turn may be used to assess the performance and efficiency of a given system. A number of electrostatic charge measurement devices have been developed in the last three decades and thorough reviews can be found in the literature [9,10]. The electrostatic charge classifier (ECC) utilized in this work measures the trajectory deviation that a particle experiences due to the electrical forces promoted by an electric field to which it is subjected [2,14–17]. The trajectory of a particle with a charge Q can be altered by the presence of an electric field, E, which promotes an electric force, Fe, given by: Fe = Q E
ð1Þ
As a result of this electric force, the particle moves with velocity Ux in the direction of the force lines, as illustrated in Fig. 1. As the Reynolds number of the particle is usually small and the fluid and particle relaxation times are also sufficiently small, the validity of Stokes law can be assumed in calculating the drag force, Fd, that counterbalances the electric force: Fd =
3μπdp Ux Fs
ð2Þ
For parallel plates separated by a distance L and with a potential difference ΔV applied between them, the electric field in the direction x may be calculated as: Ex =
ΔV L
distance traveled by the particle, X, measured from the center (Fig. 1), by the following expression: Ux =
U0 X Z
ð4Þ
Assuming that Fe = Fd, the particle charge, Q, can be obtained as follows: Q=
3μπdp U0 XL Fs ZΔV
ð5Þ
Eq. (5) is strictly valid for the particles that cross all the electric field lines in a uniform field and for a stable velocity profile in direction z. A detailed discussion of theory of the ECC can be found elsewhere [2,9,15–19]. 3. Experimental equipment and procedures 3.1. The Electrostatic Charge Classifier (ECC) The particle charge distribution was measured utilizing the Electrostatic Charge Classifier, ECC. A brief description of the equipment is given below. The ECC was built in transparent acrylic and the general view of the equipment can be seen in Fig. 2. The apparatus can be divided in three sections: A: aerosol insertion region; B: particle deflection region; and C: particle collection region.
ð3Þ
The velocity of the particle, Ux, can be determined as a function of the gas velocity, U0, of the length of the deflection plates, Z, and of the
Table 2 Chemical composition of the phosphate rock concentrate Main constituents
Mass (%)
CaO SiO2 P2O5 Al2O3 Fe2O3 F− K2O MgO Na2O Cl−
32.20 28.10 24.80 5.00 2.44 2.00 1.24 0.51 0.13 0.11
Fig. 6. Typical result from the ECC (methylene blue particles of 7.1 µm): number or of particles as a function of position X, with and without an applied voltage of − 5 kV.
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Fig. 7. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 8.8 µm particle of methylene blue generated by vibrating orifice generator (monodispersed aerosol).
The classifier is placed in a horizontal orientation with the test aerosols introduced from the upstream to prevent any gravitational effects on the relative particle trajectory to the copper plates. Once introduced into the classifier through the central slit, the particle crosses section B, and suffers a deflection in its path caused by an electric field applied between the copper plates, generated by a Spellman SL30PN300 high-voltage supply. The migration of each particle between the plates is proportional to its charge and to the applied deflection voltage. The equipment design provides a flat profile for the gas velocity to the full extent of region B, with U0 = 0.28 ± 0.05 m/s [15]. Finally a mobile isokinetic probe collects the particle, in section C, where the deflection is determined, directs the particle to a particle size analyzer, the Aerodynamic Particle Sizer (TSI model 3320). The particle charge is determined with the use of Eq. (5). This equipment has been extensively used for measuring particle charge distribution [2,14–16,18] and the results are in accordance with values from the literature. A detailed description of the charge determination procedure can be seen elsewhere [2,17]. 3.2. Mono-dispersed particle generator A TSI vibrating orifice generator, model 3450, capable of producing aerosols from various materials in the form of solutions was used to
generate the mono-dispersed aerosol. A solution contained in a plastic syringe is expelled at a constant and adjustable velocity and is injected through a small orifice, producing a liquid spray. The orifice is fixed on a device with a piezoelectric crystal, which produces high frequency vibrations when subjected to an electric current. Those vibrations “break” the liquid spray in droplets that are dragged by an ascendant air column, where the liquid portion of the droplet evaporates, leaving the non-volatile solute as a particle with a uniform and nearly spherical form. When the solvent evaporates from the droplets, solute particles are obtained. 3.3. Poly-dispersed particle generator For the poly-dispersed aerosol production, a TSI Powder Disperser, model 3433 (DP-3433) was a utilized. This is an appropriate instrument to disperse small quantities of dry powder with diameters between 1 and 50 µm, using the venturi aspiration technique. The powder is placed over the surface of a rotating disc using a brush, forming a uniform layer, which is then aspirated by a venturi through a capillary tube. The lower and upper sections of the capillary tube are located very close to the surface of the rotating disc and in the narrow portion of the venturi, respectively. A low-pressure region is created by the air velocity increase in the narrow portion of the venturi and, consequently, the particles are aspirated through the capillary tube.
Fig. 8. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 8.8 µm particle of methylene blue generated by vibrating orifice generator (monodispersed aerosol) followed by the corona charger.
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Fig. 9. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 4.0 µm particle of phosphatic rock generated in the rotating disk aerosol generator, with no extra charging device.
3.4. The impact charger The impact charger utilized in this work is shown in Fig. 3. In this device, the particles are charged by impact as the aerosol is forced to flow through the small duct (1), with an internal diameter of 6.35 mm, and directed against the grounded copper disk (2), that has a diameter of 12 mm. The impact conditions can be varied by changing the distance C between the exit of the duct and the surface of the disk and also by changing the air flow rate. In the experiments conducted in this work, the air flow rate was kept constant at 2.55 × 10− 3 m3/s. Table 1 lists the distances C used in the tests. 3.5. The corona charger The corona charger employed in this work is shown in Fig. 4. It consists of a cylindrical tube of PVC holding a grounded copper ring, with tapered ends to facilitate its connection with the aerosol generator and with the ECC. The cylindrical copper electrode has an internal diameter of 4.8 cm and a length of 10 cm. The discharge electrode is located at the central portion of the charger, in the axial position, and consists of a steel wire of diameter 0.25 mm. The wire is connected to a high voltage supply EXACTUS, model EAT 22 2012-B. The source allows the application of an electric potential ranging from 0 to −20 kV and current of 0 to 10 mA, and produces corona current positive as well as negative.
For the present work, the applied potential was kept at −15 kV and the measured current escaping to the ground was of 0.42 mA. 3.6. The experimental system The complete experimental set up used in the analysis of the charged particle behavior can be seen in Fig. 5. It can be observed that the aerosol generators under study were connected to the particle chargers and followed by the ECC. With this configuration, the dustladen gas could be introduced into the ECC immediately after dispersion, or after passing through the particle charger. Therefore, the electrostatic charges measured in the tests could be those generated in the dispersion technique itself or those superimposed by the chargers. The particle diameters after deflection were measured using the TSI-APS, Model 3320 particle counter. 3.7. The test aerosols Methylene blue, with a density of 1.57 g/cm3 and a dielectric constant of 7.6 [20] was used in the mono-dispersed aerosol generator, fed as solution in water and ethanol alcohol. After atomization and drying, it resulted in mono-dispersed particles with the diameter adjusted to the desired value. The poly-dispersed powder utilized in this work consisted of a fine fraction of phosphate concentrate, with a Stokes mean diameter of 1.3 µm, measured in a TSI Aerodynamic Particle Sizer model 3320, a
Fig. 10. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 6.0 µm particle of phosphatic rock generated in the rotating disk aerosol generator, followed by the impactor charging device.
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305
Fig. 11. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 4.0 µm particle of phosphatic rock generated in the rotating disk aerosol generator, followed by the corona charging device.
density of 2.94 g/cm3, measured in a Micromeritics Accupic helium pycnometer and a dielectric constant of 6.5 [21]. A phosphate rock concentrate is a non-cohesive and non-hygroscopic powder, with the chemical composition given in Table 2. Prior to dispersion, the samples of phosphate rock were placed in a stove at 105 °C for 24 h. 4. Results and discussion 4.1. Particle charge in aerosols generated from liquid solutions All tests were performed in controlled laboratory conditions. The air humidity and temperature were kept at 44±3% and 23±2 °C, respectively. Fig. 6 illustrates a typical result from the ECC: a 7.1 µm methylene blue particle displacement due to an applied voltage of −10 kV. The displacement X is used in Eq. (5) for calculating the particle charge. A Gaussian curve was adjusted to the experimental points, so that the median charge and the standard deviation, S.Dev., could be estimated. Fig. 7(a) shows the median charge and the standard deviation S.Dev. versus particle diameter, using the vibrating orifice aerosol generator alone. In Fig. 7(b) is detailed the charge distribution obtained in the ECC for the 8.8 µm particles, taken as an example. It can be noticed that, although each particle diameter presents a charge distribution, the S. Dev. is relatively small (the Gaussian curves are quite narrow). The point shown in black in Fig. 7(a) refers to a test in which the aerosol was driven through an aerosol electrostatic neutralizer (Kr-85) placed before the ECC. In all the other tests, the neutralizer was not utilized. The results
Fig. 12. The relative spread S.Dev./Qmedian as a function of particle diameter, for the three conditions studied.
show that the neutralizer acts effectively, as the median charge is virtually zero. For the remaining tests, most of the particles are negatively charged (with the exception of the 13.6 µm particles, that show a distribution with some positively charged). The median charge values fluctuate between 0.5 and 4.5 × 10− 16 °C, and have no clear dependence on particle size. Fig. 8(a) and (b) shows the equivalent results for the monosized aerosol after passing through the corona charger, where a −15 kV potential was applied between the wire and the wall. The results show that the corona charger causes an increase in the S.Dev. of the particle charge (see Fig. 8(b)). On the other hand, the scattering of the median values of charge with particle diameter was reduced (see Fig. 8(a)) and a trend of charge increasing with diameter is noticeable. As suggested by Forsyth et al. [5], the median particle charge can be correlated to particle diameter by a power law expression as: jQ=ej = AdBp
ð6Þ
where dp is the particle diameter in micrometers, A is the median number of elementary charges of magnitude e on a particle of diameter 1 µm and B is a constant. In this case, A = 191.03 and B = 0.78. Table 3 Power law fit, |Q/e|=AdBp, to aerosol charge distribution Particle
Aerosol generator
From dust generator Phosphate Corona charger + concentrate turntable-Venturi Phosphate Impact charger + concentrate turntable-Venturi Phosphate Turntable-Venturi concentrate Coal Turntable-Venturi Quartz Turntable-Venturi Mica Turntable-Venturi Fly ash Wright dust feeder Alumina Fluidized bed Road dust Fluidized bed fine From liquid solution Methylene Vibrating orifice blue Methylene Vibrating orifice + blue corona charger NaCl Collision nebulizer KCl 0.1% () Spray atomizer KCl 1% Spray atomizer NaCl 0.1% Spray atomizer NaCl 10% Spray atomizer
A
B
259.9 1.08
R2
Diameter range (µm)
1.00 0.15–8.0
125.9 0.99 1.00 0.15–8.0 38.3
1.24
0.97 0.15–8.0
28.8 39.8 34.1 175.8 44.2 67.8
1.32 1.25 1.10 1.05 1.00 1.05
0.99 0.99 1.00 0.99 0.97 0.96
0.6–7.5 0.6–7.5 0.6–7.5 0.7–12 0.15–0.3 0.26–2.6
⁎
⁎
⁎
6.0–14.0
191.0
0.79 0.39 6.0–14.0
4.6 14.1 6.7 19.1 6.2
1.32 0.70 0.45 0.80 0.60
0.92 0.96 0.93 0.96 0.89
0.6–2.1 0.15–0.3 0.15–0.3 0.15–0.75 0.15–0.65
Results from This work This work This work [4] [4] [4] [2] [5] [5]
This work This work [4] [5] [5] [5] [5]
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The results of this work are compared to those of other authors in Table 2. It can be seen that the exponent B is comparable in all cases, while the parameter A is considerably higher in the results of this work, probably due to the corona overcharging. 4.2. Aerosol charge from powder dispersion In the tests discussed in this section, the phosphate rock concentrate dispersed in the rotating disk feeder was the test powder. Firstly, the charges acquired by the particles with no charging device are compared to those acquired with the impact and corona devices. Also, the median charges obtained in this work are compared to those of other authors. In the second part, the effect of the impact charger was evaluated, by varying the charger configuration.
Fig. 13. Comparison of the results of this work with those from Johnston et al. [4], for similar dispersion conditions.
The complexity of the process of particle generation in the vibrating orifice, in which several mechanisms are acting in the generation of particle charges, contributes to the instability of the electrostatic charging process and can explain the erratic behavior of the particle charging when no corona is used. According to Forsyth et al. [5], the charge distribution of an aerosol is affected by the nature of the dipole layer existing at the liquid–air interface. The methylene blue concentration in water/ethanol solution can affect the ion field intensity so as to increase or decrease the dielectric constant of the solution, affecting the charge magnitude. It is well known (see, for example, Refs. [9,22]) that in most practical cases, the corona current promotes an ionic avalanche that is superimposed on the initial charging, and causes a direct dependence between particle size and charge. However, in Figs. 7 and 8, the charge acquired as a function of particle diameter for the methylene blue aerosol, this effect was not clearly noticeable: the dependence between particle size and charge was very weak, both with and without corona. Also, the median charge promoted by the corona did not exceed the median charge spontaneously acquired: the cloud of points with corona (Fig. 8) is contained within the cloud of points without corona (Fig. 7). This indicates that other mechanism, probably due to the particle generation technique, have a greater effect than the corona (see discussion at the end of Section 4.2.1).
4.2.1. Comparison of the charge acquired in different cases Figs. 9–11 show the phosphate rock concentrate particle charge distribution (median charge and standard deviation) under three conditions: (a) after dispersion by the rotating disc aerosol generator (Fig. 9); (b) after dispersion and charging by the impacting device (Fig. 10); and (c) after dispersion and charging by the corona charger in condition C4 (Fig. 11). It can be observed, in all cases, that there is a clear dependence between the particle charge and its diameter. The measurements indicate that the median charge on the particles increases from −1.3 × 10− 17 °C to −6.0 × 10− 17 °C in case (a); from −3.0 × 10− 17 °C to −10.0 × 10− 17 °C in case (b) and from −10.0 × 10− 17 °C to −40.0 × 10− 17 °C in case (c). The impact charger increased the median particle charge by a factor of two and corona charger by a factor of six. It can also be seen, in all cases, that although the median charges are negative, an appreciable amount of particles are positively charged. This spread in particle charge is also depicted in Fig. 12, where a relative spread, defined as the ratio S.Dev./Qmedian, is plotted as a function of particle diameter. The results show that the particle charge spreads over 3.5 times its median value when no charging device was used, whereas this spreading drops to approximately 1.6 times when impact charging occurred and 1.3 times when the corona charger was utilized. No clear dependence between the relative spread and particle diameter is noticeable. The relation between particle size and median charge (expressed in number of elementary charges) can be expressed in terms of the power law correlation, given by Eq. (6), for B varying between 1.0 and 1.3, as can be seen in Table 3. Values of A was approximately 40 for the aerosol with no extra charging, 125 for impact charging and 260 to corona charging. Table 3 also lists the experimental results obtained by Johnston et al. [4], Coury et al. [2] and Forsyth et al. [5] for comparable aerosols. The values and trends are similar, as illustrated in Fig. 13. Noticeably, the corona charger was very effective in charging the particles, individually increasing the particle charges by approximately five times. However, it did not affect the nearly linear dependence between particle size and charge. It is interesting to note the different responses of the aerosols to the corona current: it had little effect in the charge acquisition of the
Table 4 The A and B coefficients for the power law fit of particle charge as a function of its diameter, for the four impact conditions investigated Condition
Parameters
Corr. coeff.
C1
A = 10.52 B = 1.75 A = 24.37 B = 1.42 A = 40.42 B = 1.15 A = 72.02 B = 1.24
r2 = 0.994
C2 C3 C4 Fig. 14. Charge acquired with the use of the impact.
r2 = 0.998 r2 = 0.999 r2 = 0.995
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methylene blue aerosol (see Figs. 7 and 8) and a strong charging effect in the phosphate rock aerosol (Figs. 9 and 11). In the experimental conditions of this work, two possible explanations to this distinct behavior arise: (a) the different chemical composition of the tested aerosols; and (b) the distinct dispersion techniques utilized. It is well known that differences in chemical composition can affect particle charging [22]. This is normally accounted for in the corona charging correlations through the particle dielectric constant, ɛ. In the widely used correlation for corona charging by Cochet [23], this dependence can be expressed as: Q α ðe−1Þ=ðe + 2Þ
ð7Þ
For the particles utilized in this work (dielectric constants of 7.6 and 6.5 for the methylene blue and the phosphate rock, respectively), this would result in very similar charge acquisition by both particles, with a slight advantage to the methylene blue(!). Therefore, the difference in chemical composition cannot, in principle, be responsible for the observed charging behavior. On the other hand, the differences in the aerosol dispersion/ generation technique can result in quite distinct residual charges in the particles. The methylene blue particle, generated in a vibrating orifice generator, is originally in the form of a solution that passes though severe shearing, vibration, electric field, and then a drying (solidification) process. These numerous steps can generate residual charging that can possibly be trapped in the bulk of the particle. Conversely, the phosphate rock particle is dispersed in the solid phase where the residual charging occurs by friction. In this case, the charges are situated on the particle external surface. These different dispersion characteristics may explain the distinct interaction of the particle with the corona current. 4.2.2. The charge acquisition by impaction A set of experiments was carried out in order to verify the effect of the impact charger on the particle charging and the results are shown in Fig. 14. There, the acquired charge, in number of elementary units, is plotted as a function of particle diameter, for four different distances (C1 to C4) between the aerosol duct (1) and the copper disk (2) (see Fig. 3 and Table 1). It can be noticed that the charging is very sensitive to the impact conditions, as the charge level increases approximately three times when changing from C1 to C4. Considering that the distance between the aerosol duct and the impact disk decreases from 60 mm at C1 to 10 mm at C4, this means that the particle impact velocity also increases from C1 to C4. This indicates that the impact energy, directly related to the particle velocity, is likely to be the governing mechanism of charge generation here. It is also worth noting that the dependence between particle charge and size can be well represented by a power law relation linear in all cases. The slopes of the curves C1 to C3 are fairly similar, but the slope increases substantially in curve C4. Table 4 lists the power law coefficients of the fitted curves for each case. 5. Conclusions Electrostatic charge in the particles was quantifiable in all tests and they responded according to the applied electric field as expected. The values of the charge measured in the particles are of the same order of magnitude as the ones reported in the literature, indicating the adequacy of the equipment and of the method employed. No evident dependence between the particle electric charge and its diameter was observed for the tests using the mono-dispersed aerosol, generated in the vibrating orifice generator. When these particles where subjected to a corona current, a weak power law dependence was noticed. For the tests using the poly-dispersed aerosol, a fairly linear dependence between median charge and diameter was noticed.
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Although predominantly negative, an appreciable amount of particles are positively charged, as their charge distribution spreads over a Gaussian curve. The impact charger and the corona charger were effective in increasing the median charge of the particles for the polydispersed aerosol by approximately two and five times, respectively, of the typical charge value obtained by the aerosol generated in the rotating disk generator. Conversely, the impact charger and the corona charger caused a decrease in the relative spread of the charge distribution to approximately 0.5 and 0.4 of the aerosol generated in the rotating disk generator. The particle charging was very sensitive to the impact conditions. The generating conditions therefore determine the distribution of the charges acquired by the particles in an aerosol. Acknowledgements The authors are indebted to FAPESP, FAPEMIG and CNPq for the financial support given to this work. References [1] J.F. Hughes, Electrostatic Particle Charging: Industrial and Health Care Applications, John Wiley & Sons, New York, 1997. [2] J.R. Coury, J.A. Raper, D. Guang, R. Clift, Measurement of electrostatic charge on gasborne particles and the effect of charges on fabric filtration, Process Safety and Environmental Protection (Trans IChemE, part B) 69 (B2) (1991) 97–106. [3] O.B. Duarte Fo., W.D. Marra Jr., G.C. Kachan, J.R. Coury, Filtration of electrified solid particles, Industrial and Engineering Chemistry Research 39 (10) (2000) 3884–3895. [4] A.M. Johnston, J.H. Vincent, A.D. Jones, Electrical charge characteristics of dry aerosols produced by a number of laboratory mechanical dispensers, Aerosol Science and Technology 6 (1987) 115–127. [5] B. Forsyth, B.Y.H. Liu, F.J. Romay, Particle charge distribution measurement for commonly generated laboratory aerosols, Aerosol Science and Technology 28 (1998) 489–501. [6] W.C. Hinds, Aerosol Technology, 2nd ed. Wiley-Interscience, New York, 1999. [7] J. Lowell, A.C. Rose-Innes, Contact electrification, Advances in Physics 29 (6) (1980) 947–1023. [8] A.G. Bailey, Electrostatic phenomena during powder handling, Powder Technology 37 (1984) 71–85. [9] J.A. Cross, Electrostatics: Principles, Problems and Applications, Adam Hilger, Bristol, 1987. [10] R.C. Flagan, History of electrical aerosol measurements, Aerosol Science and Technology 28 (1998) 301–380. [11] S. Matsusaka, H. Masuda, Electrostatics of particles, Advanced Powder Technology 14 (2) (2003) 143–166. [12] A.M. Johnston, J.H. Vincent, A.D. Jones, Measurement of electric charge for workplace aerosols, Annals of Occupational Hygiene 29 (2) (1985) 271–284. [13] Y. Mori, T. Shiomi, N. Katada, H. Minamide, K. Iinoya, Effects of corona precharger on performance of fabric filter, Journal of Chemical Engineering of Japan 15 (3) (1982) 211–216. [14] D. Guang, (1991) In-Situ Measurement of Electrostatic Charge and Distribution on Fly Ash Particles in Power Station Exhaust Stream. Ph.D. Tesis., University of New South Wales. 315 pp. [15] W.D. Marra Jr., J.R. Coury, Measurement of the electrostatic charge in airborne particles: I – Development of the equipment and preliminary results, Brazilian Journal of Chemical Engineering 17 (01) (2000) 39–50. [16] W.D. Marra Jr., J.R. Coury, Electrostatic charge measurement on airborne particles, Key Engineering Materials 189–191 (2001) 412–417. [17] W.D. Marra, Jr., (2000).The Development of Equipment for Measuring Electrostatic Charge Distribution in Aerosols. PhD Thesis. Universidade Federal de São Carlos. 200 pp. [18] M.V. Rodrigues, W.D. Marra Jr., R.G. Almeida, J.R. Coury, Measurement of the electrostatic charge in airborne particles: II – Particle charge distribution of different aerosols, Brazilian Journal of Chemical Engineering 23 (01) (2006) 125–133. [19] J.R. Coury, (1983) Electrostatic Effects in the Granular Bed Filtration of Gases. PhD Thesis. University of Cambridge. 234 pp. [20] V.R.K. Murthy, T.A. Prasada Rao, J. Sobhanadri, Dielectric properties of some dyes in the radio-frequency region, Journal of Physics D: Applied Physics 10 (1977) 2405–2409. [21] M.C.R. Falaguasta, S.W. Nóbrega, J.R. Coury, Scale up investigation of a wire plate geometry electrostatic precipitator, Particulate Science and Technology 24 (4) (2006) 453–465. [22] C. Riehle, Basic and theoretical operation of ESPs, in: K.R. Parker (Ed.), Applied Electrostatic precipitation, Chapman & Hall, London, 1997, Ch.3. [23] R. Cochet, Lois Charge des Fines Particules (submicroniques) Études Théoriques: Controles Rècents Spectre de Particules, Coll. Int. la Physique dês Forces Electrostatiques et Leurs Application, Centre National de la Recherche Scientifique, Paris, 1961, pp. 331–338.
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Glossary dp E
Fd Fe Fs L Q Qmedian
Particle Stokes diameter, m Electric field, Vm− 1 Drag force, N Electric force, N Cunningham slip factor Distance between the parallel plates, m Particle charge, C Particle median charge, C
Ux
X Z ΔV ε µ
Standard deviation, C Gas velocity, ms− 1 Particle velocity in direction x, ms− 1 Particle deflection, m Length of the deflection plates, m Electrical potential difference, V particle dielectric constant Gas viscosity, kg s− 1m− 1
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
The effect of the generation and handling in the acquired electrostatic charge in airborne particles Wiclef D. Marra Jr. a, Marcos V. Rodrigues b, Rosilene G.A. Miranda b, Marcos A.S. Barrozo c, José R. Coury b,⁎ a b c
Departamento de Hidráulica e Saneamento, Universidade de São Paulo, São Carlos/SP, Brazil Departamento de Engenharia Química, Caixa Postal 676, CEP 13565-905, São Carlos/SP, Brazil Faculdade de Engenharia Química, Universidade Federal de Uberlândia, Uberlândia/MG, Brazil
a r t i c l e
i n f o
Article history: Received 5 September 2007 Received in revised form 22 October 2008 Accepted 23 October 2008 Available online 5 November 2008 Keywords: Charged particles Electrostatics Powder handling Charge distribution
a b s t r a c t The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 × 10− 6 m to 1.4 × 10− 5 m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 × 10− 6 m and size range between 1.5 × 10− 7 m and 8.0 × 10− 6 m was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles, the median charges of the particles varied between − 3.0 × 10− 16 C and −5.0 × 10− 18 °C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Small particles may carry significant electrical charge, and can therefore be subjected to considerable forces if exposed to an electric field. Electrical forces on a charged particle can far exceed gravitational forces, and may in some circumstances even exceed aerodynamic forces in a moving air stream. It is well known that controlled handling of particles is exceptionally difficult to achieve [1]. Electrostatic particle charging is therefore an important phenomenon related to powder handling. Numerous industrial applications have evolved utilizing electrostatic charging of particles. In several processes such as drying, filtration, milling or storage of powder or granules, where the formation, dispersion and transport of particles occur, electrostatic charges can arise spontaneously due to friction or mechanical shock between particles (triboelectrification or impact). The presence of charges is desirable for some processes, as in electrostatic precipita⁎ Corresponding author. Tel.: +55 16 33518264. E-mail address: [email protected] (J.R. Coury). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.10.018
tors and electrostatic coatings; however there are other operations where it is highly inconvenient, as in storage silos and in the packing of very fine powders, causing difficulty in the flow and in the manipulation of the product. The presence of charges can also be dangerous, as in the transport and storage of cereal grains or dry powders, were they may promote ignition and explosion of the container. The importance of electrostatic charges in gas cleaning can be illustrated by the work of Coury and co-workers. Coury et al. [2] carried out extensive work on the measurement of electrostatic charges in airborne particles and the effect of those charges on the filter behavior. The equipment utilized in the measurements of charge distribution was developed by the authors, and constitutes a previous version of the one used in this work. Tests were performed at laboratory level as well as at bag-houses in power plants in Australia. In both cases, linear dependence between particle size and charge were obtained. The filtration tests showed that the electrostatic charges on the particles had a marked influence on the filter efficiency as well as on the cake formation. Duarte et al. [3] analyzed the effects of the level of electrostatic charges on filtration in a granular bed. The removal
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plate generator (TSI model 3433). In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. In the rotating plate generator, a phosphate rock concentrate was utilized as powder material. The particle charge distribution was measured experimentally immediately after generation or following corona or impact chargers. 2. Particle electrostatics
Fig. 1. Deflection of a charged particle under an electric field.
efficiency of the filter and the pressure drop were monitored at the granular bed. The particles were electrified by triboelectrification and impact, and the overall charges were measured with a Faraday cage that included the filter. The authors concluded that the formation of the filter cake was very much dependent on the level of particle charging. It was also observed that increase of the particle charges induces the formation of looser deposits, with lower resistance to gas flow. Whichever is the case, information on the size distribution and on the electrostatic charges is of paramount importance for the adequate handling of processes involving particles. Johnston et al. [4] and Forsyth et al. [5] have pointed out the influence of the generating conditions on the particle charge. In this study, the relationship between the generating conditions on the magnitude and distribution of the particle charge was investigated. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator (TSI model 3450) and a rotating
Most aerosol particles carry some electric charge, and some may be highly charged. For highly charged particles, the electrostatic force can be much greater than the gravity force. The motion induced by electrostatic force forms the basis for important types of air-cleaning equipment and aerosol sampling and measuring instruments. A charged particle experiences trajectory deviations in the vicinity of charged surfaces or other charged particles or in the presence of an electric field. The electrical force acts remotely through the air or vacuum and does not require a fluid flow. The charge on a particle can be negative or positive depending on whether the particle has an excess or deficiency of electrons. The velocity of motion resulting from this force can be determined in a manner similar to the terminal settling velocity [6]. 2.1. Charging mechanisms of particles In the handling of airborne suspensions, particles can acquire electrostatic charges spontaneously or charging can be induced by various charging mechanisms. Thorough reviews on this subject can be found in Lowell and Rose-Innes [7], Bailey [8], Cross [9], Flagan [10] and Matsusaka and Masuda [11]. There are three basic mechanisms by which aerosol particles acquire charge: static electrification, diffusion charging and field charging. The latter two require the production of unipolar ions, usually by corona discharge, and are used to produce highly charged aerosols [6]. 2.1.1. Static electrification Static electrification causes particles to become charged as they are separated from the bulk material or other surfaces. Particles are
Fig. 2. General view of the Electrostatic Charge Classifier (ECC).
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301
Fig. 3. Scheme of the impact charger device.
usually charged by this mechanism during formation, re-suspension, or high-velocity transport. The three mechanisms of static electrification that can charge aerosol particles during generation are electrolytic charging, spray electrification, and contact charging [6]. Electrolytic charging results when liquids with high dielectric constants are separated from solid surfaces. During atomization these liquids strip off charge from the surfaces of the atomizer and produce slightly to moderately charged droplets as they are separated from these surfaces. Pure water is a high dielectric liquid that can become charged during atomization [5]. Spray electrification results from the disruption of charged liquid surfaces. Some liquids, due to surface effects, have a charged surface layer, and when this surface is disrupted during the formation of droplets by atomization or bubbling, charged droplets are produced. Generation of aerosols by atomization promotes spray electrification. Due to forces in the outer surface of dielectric liquids, the dipole layer has its negative charge pointing to the outer surface and its positive charge pointing toward the diffuse liquid bulk, which creates a dipole moment. Impurities or additional ions may enter and interact with the dipole layer, causing an electrical potential change in the inner charge region. As dissolved ions interact with this layer, a destabilization of the dipole layer occurs that can reduce or reverse the charge of atomized aerosol particles. The mean particle charge is a function of the liquid dielectric constant and the solute concentration in solution and its dielectric properties. However, the magnitude of the aerosol particle charge is affected not only by the properties of the dispersed material (i.e., the dielectric constant and ion concentration) but also by the nature of the generation method. In addition, if the particles share frequent collisions during dispersion and transport, the net particle charge will also undoubtedly be altered [5]. Contact charging occurs during the separation of dry nonmetallic particles from solid surfaces. When the particles contact the surface there is a charge transfer to equalize the Fermi levels of the two materials. When the particles are separated from the surface they have an excess or deficiency of electrons. Polarity and amount of charge depends on the materials involved and their position in the triboelectric series. This phenomenon is often called “contact electrification” or “contact charging”. When they are rubbed, it can be called “frictional electrification” or “tribo-charging”. In case of short contact, it can be called “impact charging”. Because it requires dry surfaces, it becomes ineffective at relative humidity greater than about 65%. Most methods of re-suspending dry powders involve some friction between the powder and the apparatus and consequently produce charged aerosols [11].
Contact charging occurs due to the disruption of a dust or powder surface during aerosol generation. A triboelectric or contact charge develops along the particle surface because of mechanical friction from surface or particle contact during dispersion. The nature of the powder material will affect the resulting particle charge after generation. Johnston et al. [4,12] and Forsyth et al. [5] discovered a strong dependence between the resulting particle charge and the powder type and generation method. 2.1.2. Corona discharge The corona discharge is capable of producing ions in air. It can produce unipolar ions at a sufficiently high concentration to be useful for aerosol charging. To produce a corona discharge, one must establish a non-uniform electrostatic field such as that between a needle and a plate or a concentric wire and a tube. At normal conditions, air is a good insulator, but in a region of high field strength it may undergo an electrical breakdown and become conductive. Depending on the geometry and strength of the field, this breakdown can be a corona discharge. Regarding the wire and tube geometry the only region with enough field strength is a thin layer at the wire surface [6]. In the corona region, electrons are accelerated to a velocity sufficient to knock an electron from an air molecule upon collision, thereby creating a positive ion and an electron. Within the corona region, this process takes place in a self-sustaining flood that produces a discharge called corona discharge. The process is initiated by electrons and ions created by natural radiation. If the wire is negative, the positive ions will be attracted to it and the electrons will be repelled toward the tube. Introduction of aerosol particles into the space between the wire and the tube will result in field charging of the particles to the same polarity of the wire [9]. Mori et al. [13] studied the effects of a positive corona precharger on performance of a fabric filter in air with controlled humidity. The authors observed that, with the corona precharger on, the charge of particles were independent of the air relative humidity, whilst with the corona precharger off the particle charge decreased with increase in the relative humidity. 2.2. Measuring electrostatic charge in particles In all applications related to charged particle technology, the measurement of electrical parameters is of paramount importance.
Table 1 Distances C in the impact charger (see Fig. 3) utilized in the tests Impact condition
C (mm)
C1 C2 C3 C4
60 30 20 10
Fig. 4. The corona charger device (dimensions in cm).
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Fig. 5. Scheme of the experimental apparatus.
Measuring the charge of particles is very useful in practice as it immediately gives an indication of how well a particular particle charging system is working, which in turn may be used to assess the performance and efficiency of a given system. A number of electrostatic charge measurement devices have been developed in the last three decades and thorough reviews can be found in the literature [9,10]. The electrostatic charge classifier (ECC) utilized in this work measures the trajectory deviation that a particle experiences due to the electrical forces promoted by an electric field to which it is subjected [2,14–17]. The trajectory of a particle with a charge Q can be altered by the presence of an electric field, E, which promotes an electric force, Fe, given by: Fe = Q E
ð1Þ
As a result of this electric force, the particle moves with velocity Ux in the direction of the force lines, as illustrated in Fig. 1. As the Reynolds number of the particle is usually small and the fluid and particle relaxation times are also sufficiently small, the validity of Stokes law can be assumed in calculating the drag force, Fd, that counterbalances the electric force: Fd =
3μπdp Ux Fs
ð2Þ
For parallel plates separated by a distance L and with a potential difference ΔV applied between them, the electric field in the direction x may be calculated as: Ex =
ΔV L
distance traveled by the particle, X, measured from the center (Fig. 1), by the following expression: Ux =
U0 X Z
ð4Þ
Assuming that Fe = Fd, the particle charge, Q, can be obtained as follows: Q=
3μπdp U0 XL Fs ZΔV
ð5Þ
Eq. (5) is strictly valid for the particles that cross all the electric field lines in a uniform field and for a stable velocity profile in direction z. A detailed discussion of theory of the ECC can be found elsewhere [2,9,15–19]. 3. Experimental equipment and procedures 3.1. The Electrostatic Charge Classifier (ECC) The particle charge distribution was measured utilizing the Electrostatic Charge Classifier, ECC. A brief description of the equipment is given below. The ECC was built in transparent acrylic and the general view of the equipment can be seen in Fig. 2. The apparatus can be divided in three sections: A: aerosol insertion region; B: particle deflection region; and C: particle collection region.
ð3Þ
The velocity of the particle, Ux, can be determined as a function of the gas velocity, U0, of the length of the deflection plates, Z, and of the
Table 2 Chemical composition of the phosphate rock concentrate Main constituents
Mass (%)
CaO SiO2 P2O5 Al2O3 Fe2O3 F− K2O MgO Na2O Cl−
32.20 28.10 24.80 5.00 2.44 2.00 1.24 0.51 0.13 0.11
Fig. 6. Typical result from the ECC (methylene blue particles of 7.1 µm): number or of particles as a function of position X, with and without an applied voltage of − 5 kV.
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Fig. 7. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 8.8 µm particle of methylene blue generated by vibrating orifice generator (monodispersed aerosol).
The classifier is placed in a horizontal orientation with the test aerosols introduced from the upstream to prevent any gravitational effects on the relative particle trajectory to the copper plates. Once introduced into the classifier through the central slit, the particle crosses section B, and suffers a deflection in its path caused by an electric field applied between the copper plates, generated by a Spellman SL30PN300 high-voltage supply. The migration of each particle between the plates is proportional to its charge and to the applied deflection voltage. The equipment design provides a flat profile for the gas velocity to the full extent of region B, with U0 = 0.28 ± 0.05 m/s [15]. Finally a mobile isokinetic probe collects the particle, in section C, where the deflection is determined, directs the particle to a particle size analyzer, the Aerodynamic Particle Sizer (TSI model 3320). The particle charge is determined with the use of Eq. (5). This equipment has been extensively used for measuring particle charge distribution [2,14–16,18] and the results are in accordance with values from the literature. A detailed description of the charge determination procedure can be seen elsewhere [2,17]. 3.2. Mono-dispersed particle generator A TSI vibrating orifice generator, model 3450, capable of producing aerosols from various materials in the form of solutions was used to
generate the mono-dispersed aerosol. A solution contained in a plastic syringe is expelled at a constant and adjustable velocity and is injected through a small orifice, producing a liquid spray. The orifice is fixed on a device with a piezoelectric crystal, which produces high frequency vibrations when subjected to an electric current. Those vibrations “break” the liquid spray in droplets that are dragged by an ascendant air column, where the liquid portion of the droplet evaporates, leaving the non-volatile solute as a particle with a uniform and nearly spherical form. When the solvent evaporates from the droplets, solute particles are obtained. 3.3. Poly-dispersed particle generator For the poly-dispersed aerosol production, a TSI Powder Disperser, model 3433 (DP-3433) was a utilized. This is an appropriate instrument to disperse small quantities of dry powder with diameters between 1 and 50 µm, using the venturi aspiration technique. The powder is placed over the surface of a rotating disc using a brush, forming a uniform layer, which is then aspirated by a venturi through a capillary tube. The lower and upper sections of the capillary tube are located very close to the surface of the rotating disc and in the narrow portion of the venturi, respectively. A low-pressure region is created by the air velocity increase in the narrow portion of the venturi and, consequently, the particles are aspirated through the capillary tube.
Fig. 8. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 8.8 µm particle of methylene blue generated by vibrating orifice generator (monodispersed aerosol) followed by the corona charger.
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Fig. 9. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 4.0 µm particle of phosphatic rock generated in the rotating disk aerosol generator, with no extra charging device.
3.4. The impact charger The impact charger utilized in this work is shown in Fig. 3. In this device, the particles are charged by impact as the aerosol is forced to flow through the small duct (1), with an internal diameter of 6.35 mm, and directed against the grounded copper disk (2), that has a diameter of 12 mm. The impact conditions can be varied by changing the distance C between the exit of the duct and the surface of the disk and also by changing the air flow rate. In the experiments conducted in this work, the air flow rate was kept constant at 2.55 × 10− 3 m3/s. Table 1 lists the distances C used in the tests. 3.5. The corona charger The corona charger employed in this work is shown in Fig. 4. It consists of a cylindrical tube of PVC holding a grounded copper ring, with tapered ends to facilitate its connection with the aerosol generator and with the ECC. The cylindrical copper electrode has an internal diameter of 4.8 cm and a length of 10 cm. The discharge electrode is located at the central portion of the charger, in the axial position, and consists of a steel wire of diameter 0.25 mm. The wire is connected to a high voltage supply EXACTUS, model EAT 22 2012-B. The source allows the application of an electric potential ranging from 0 to −20 kV and current of 0 to 10 mA, and produces corona current positive as well as negative.
For the present work, the applied potential was kept at −15 kV and the measured current escaping to the ground was of 0.42 mA. 3.6. The experimental system The complete experimental set up used in the analysis of the charged particle behavior can be seen in Fig. 5. It can be observed that the aerosol generators under study were connected to the particle chargers and followed by the ECC. With this configuration, the dustladen gas could be introduced into the ECC immediately after dispersion, or after passing through the particle charger. Therefore, the electrostatic charges measured in the tests could be those generated in the dispersion technique itself or those superimposed by the chargers. The particle diameters after deflection were measured using the TSI-APS, Model 3320 particle counter. 3.7. The test aerosols Methylene blue, with a density of 1.57 g/cm3 and a dielectric constant of 7.6 [20] was used in the mono-dispersed aerosol generator, fed as solution in water and ethanol alcohol. After atomization and drying, it resulted in mono-dispersed particles with the diameter adjusted to the desired value. The poly-dispersed powder utilized in this work consisted of a fine fraction of phosphate concentrate, with a Stokes mean diameter of 1.3 µm, measured in a TSI Aerodynamic Particle Sizer model 3320, a
Fig. 10. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 6.0 µm particle of phosphatic rock generated in the rotating disk aerosol generator, followed by the impactor charging device.
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305
Fig. 11. (a) Median charge and S.Dev. vs. particle diameter and (b) charge distribution for the 4.0 µm particle of phosphatic rock generated in the rotating disk aerosol generator, followed by the corona charging device.
density of 2.94 g/cm3, measured in a Micromeritics Accupic helium pycnometer and a dielectric constant of 6.5 [21]. A phosphate rock concentrate is a non-cohesive and non-hygroscopic powder, with the chemical composition given in Table 2. Prior to dispersion, the samples of phosphate rock were placed in a stove at 105 °C for 24 h. 4. Results and discussion 4.1. Particle charge in aerosols generated from liquid solutions All tests were performed in controlled laboratory conditions. The air humidity and temperature were kept at 44±3% and 23±2 °C, respectively. Fig. 6 illustrates a typical result from the ECC: a 7.1 µm methylene blue particle displacement due to an applied voltage of −10 kV. The displacement X is used in Eq. (5) for calculating the particle charge. A Gaussian curve was adjusted to the experimental points, so that the median charge and the standard deviation, S.Dev., could be estimated. Fig. 7(a) shows the median charge and the standard deviation S.Dev. versus particle diameter, using the vibrating orifice aerosol generator alone. In Fig. 7(b) is detailed the charge distribution obtained in the ECC for the 8.8 µm particles, taken as an example. It can be noticed that, although each particle diameter presents a charge distribution, the S. Dev. is relatively small (the Gaussian curves are quite narrow). The point shown in black in Fig. 7(a) refers to a test in which the aerosol was driven through an aerosol electrostatic neutralizer (Kr-85) placed before the ECC. In all the other tests, the neutralizer was not utilized. The results
Fig. 12. The relative spread S.Dev./Qmedian as a function of particle diameter, for the three conditions studied.
show that the neutralizer acts effectively, as the median charge is virtually zero. For the remaining tests, most of the particles are negatively charged (with the exception of the 13.6 µm particles, that show a distribution with some positively charged). The median charge values fluctuate between 0.5 and 4.5 × 10− 16 °C, and have no clear dependence on particle size. Fig. 8(a) and (b) shows the equivalent results for the monosized aerosol after passing through the corona charger, where a −15 kV potential was applied between the wire and the wall. The results show that the corona charger causes an increase in the S.Dev. of the particle charge (see Fig. 8(b)). On the other hand, the scattering of the median values of charge with particle diameter was reduced (see Fig. 8(a)) and a trend of charge increasing with diameter is noticeable. As suggested by Forsyth et al. [5], the median particle charge can be correlated to particle diameter by a power law expression as: jQ=ej = AdBp
ð6Þ
where dp is the particle diameter in micrometers, A is the median number of elementary charges of magnitude e on a particle of diameter 1 µm and B is a constant. In this case, A = 191.03 and B = 0.78. Table 3 Power law fit, |Q/e|=AdBp, to aerosol charge distribution Particle
Aerosol generator
From dust generator Phosphate Corona charger + concentrate turntable-Venturi Phosphate Impact charger + concentrate turntable-Venturi Phosphate Turntable-Venturi concentrate Coal Turntable-Venturi Quartz Turntable-Venturi Mica Turntable-Venturi Fly ash Wright dust feeder Alumina Fluidized bed Road dust Fluidized bed fine From liquid solution Methylene Vibrating orifice blue Methylene Vibrating orifice + blue corona charger NaCl Collision nebulizer KCl 0.1% () Spray atomizer KCl 1% Spray atomizer NaCl 0.1% Spray atomizer NaCl 10% Spray atomizer
A
B
259.9 1.08
R2
Diameter range (µm)
1.00 0.15–8.0
125.9 0.99 1.00 0.15–8.0 38.3
1.24
0.97 0.15–8.0
28.8 39.8 34.1 175.8 44.2 67.8
1.32 1.25 1.10 1.05 1.00 1.05
0.99 0.99 1.00 0.99 0.97 0.96
0.6–7.5 0.6–7.5 0.6–7.5 0.7–12 0.15–0.3 0.26–2.6
⁎
⁎
⁎
6.0–14.0
191.0
0.79 0.39 6.0–14.0
4.6 14.1 6.7 19.1 6.2
1.32 0.70 0.45 0.80 0.60
0.92 0.96 0.93 0.96 0.89
0.6–2.1 0.15–0.3 0.15–0.3 0.15–0.75 0.15–0.65
Results from This work This work This work [4] [4] [4] [2] [5] [5]
This work This work [4] [5] [5] [5] [5]
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The results of this work are compared to those of other authors in Table 2. It can be seen that the exponent B is comparable in all cases, while the parameter A is considerably higher in the results of this work, probably due to the corona overcharging. 4.2. Aerosol charge from powder dispersion In the tests discussed in this section, the phosphate rock concentrate dispersed in the rotating disk feeder was the test powder. Firstly, the charges acquired by the particles with no charging device are compared to those acquired with the impact and corona devices. Also, the median charges obtained in this work are compared to those of other authors. In the second part, the effect of the impact charger was evaluated, by varying the charger configuration.
Fig. 13. Comparison of the results of this work with those from Johnston et al. [4], for similar dispersion conditions.
The complexity of the process of particle generation in the vibrating orifice, in which several mechanisms are acting in the generation of particle charges, contributes to the instability of the electrostatic charging process and can explain the erratic behavior of the particle charging when no corona is used. According to Forsyth et al. [5], the charge distribution of an aerosol is affected by the nature of the dipole layer existing at the liquid–air interface. The methylene blue concentration in water/ethanol solution can affect the ion field intensity so as to increase or decrease the dielectric constant of the solution, affecting the charge magnitude. It is well known (see, for example, Refs. [9,22]) that in most practical cases, the corona current promotes an ionic avalanche that is superimposed on the initial charging, and causes a direct dependence between particle size and charge. However, in Figs. 7 and 8, the charge acquired as a function of particle diameter for the methylene blue aerosol, this effect was not clearly noticeable: the dependence between particle size and charge was very weak, both with and without corona. Also, the median charge promoted by the corona did not exceed the median charge spontaneously acquired: the cloud of points with corona (Fig. 8) is contained within the cloud of points without corona (Fig. 7). This indicates that other mechanism, probably due to the particle generation technique, have a greater effect than the corona (see discussion at the end of Section 4.2.1).
4.2.1. Comparison of the charge acquired in different cases Figs. 9–11 show the phosphate rock concentrate particle charge distribution (median charge and standard deviation) under three conditions: (a) after dispersion by the rotating disc aerosol generator (Fig. 9); (b) after dispersion and charging by the impacting device (Fig. 10); and (c) after dispersion and charging by the corona charger in condition C4 (Fig. 11). It can be observed, in all cases, that there is a clear dependence between the particle charge and its diameter. The measurements indicate that the median charge on the particles increases from −1.3 × 10− 17 °C to −6.0 × 10− 17 °C in case (a); from −3.0 × 10− 17 °C to −10.0 × 10− 17 °C in case (b) and from −10.0 × 10− 17 °C to −40.0 × 10− 17 °C in case (c). The impact charger increased the median particle charge by a factor of two and corona charger by a factor of six. It can also be seen, in all cases, that although the median charges are negative, an appreciable amount of particles are positively charged. This spread in particle charge is also depicted in Fig. 12, where a relative spread, defined as the ratio S.Dev./Qmedian, is plotted as a function of particle diameter. The results show that the particle charge spreads over 3.5 times its median value when no charging device was used, whereas this spreading drops to approximately 1.6 times when impact charging occurred and 1.3 times when the corona charger was utilized. No clear dependence between the relative spread and particle diameter is noticeable. The relation between particle size and median charge (expressed in number of elementary charges) can be expressed in terms of the power law correlation, given by Eq. (6), for B varying between 1.0 and 1.3, as can be seen in Table 3. Values of A was approximately 40 for the aerosol with no extra charging, 125 for impact charging and 260 to corona charging. Table 3 also lists the experimental results obtained by Johnston et al. [4], Coury et al. [2] and Forsyth et al. [5] for comparable aerosols. The values and trends are similar, as illustrated in Fig. 13. Noticeably, the corona charger was very effective in charging the particles, individually increasing the particle charges by approximately five times. However, it did not affect the nearly linear dependence between particle size and charge. It is interesting to note the different responses of the aerosols to the corona current: it had little effect in the charge acquisition of the
Table 4 The A and B coefficients for the power law fit of particle charge as a function of its diameter, for the four impact conditions investigated Condition
Parameters
Corr. coeff.
C1
A = 10.52 B = 1.75 A = 24.37 B = 1.42 A = 40.42 B = 1.15 A = 72.02 B = 1.24
r2 = 0.994
C2 C3 C4 Fig. 14. Charge acquired with the use of the impact.
r2 = 0.998 r2 = 0.999 r2 = 0.995
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methylene blue aerosol (see Figs. 7 and 8) and a strong charging effect in the phosphate rock aerosol (Figs. 9 and 11). In the experimental conditions of this work, two possible explanations to this distinct behavior arise: (a) the different chemical composition of the tested aerosols; and (b) the distinct dispersion techniques utilized. It is well known that differences in chemical composition can affect particle charging [22]. This is normally accounted for in the corona charging correlations through the particle dielectric constant, ɛ. In the widely used correlation for corona charging by Cochet [23], this dependence can be expressed as: Q α ðe−1Þ=ðe + 2Þ
ð7Þ
For the particles utilized in this work (dielectric constants of 7.6 and 6.5 for the methylene blue and the phosphate rock, respectively), this would result in very similar charge acquisition by both particles, with a slight advantage to the methylene blue(!). Therefore, the difference in chemical composition cannot, in principle, be responsible for the observed charging behavior. On the other hand, the differences in the aerosol dispersion/ generation technique can result in quite distinct residual charges in the particles. The methylene blue particle, generated in a vibrating orifice generator, is originally in the form of a solution that passes though severe shearing, vibration, electric field, and then a drying (solidification) process. These numerous steps can generate residual charging that can possibly be trapped in the bulk of the particle. Conversely, the phosphate rock particle is dispersed in the solid phase where the residual charging occurs by friction. In this case, the charges are situated on the particle external surface. These different dispersion characteristics may explain the distinct interaction of the particle with the corona current. 4.2.2. The charge acquisition by impaction A set of experiments was carried out in order to verify the effect of the impact charger on the particle charging and the results are shown in Fig. 14. There, the acquired charge, in number of elementary units, is plotted as a function of particle diameter, for four different distances (C1 to C4) between the aerosol duct (1) and the copper disk (2) (see Fig. 3 and Table 1). It can be noticed that the charging is very sensitive to the impact conditions, as the charge level increases approximately three times when changing from C1 to C4. Considering that the distance between the aerosol duct and the impact disk decreases from 60 mm at C1 to 10 mm at C4, this means that the particle impact velocity also increases from C1 to C4. This indicates that the impact energy, directly related to the particle velocity, is likely to be the governing mechanism of charge generation here. It is also worth noting that the dependence between particle charge and size can be well represented by a power law relation linear in all cases. The slopes of the curves C1 to C3 are fairly similar, but the slope increases substantially in curve C4. Table 4 lists the power law coefficients of the fitted curves for each case. 5. Conclusions Electrostatic charge in the particles was quantifiable in all tests and they responded according to the applied electric field as expected. The values of the charge measured in the particles are of the same order of magnitude as the ones reported in the literature, indicating the adequacy of the equipment and of the method employed. No evident dependence between the particle electric charge and its diameter was observed for the tests using the mono-dispersed aerosol, generated in the vibrating orifice generator. When these particles where subjected to a corona current, a weak power law dependence was noticed. For the tests using the poly-dispersed aerosol, a fairly linear dependence between median charge and diameter was noticed.
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Although predominantly negative, an appreciable amount of particles are positively charged, as their charge distribution spreads over a Gaussian curve. The impact charger and the corona charger were effective in increasing the median charge of the particles for the polydispersed aerosol by approximately two and five times, respectively, of the typical charge value obtained by the aerosol generated in the rotating disk generator. Conversely, the impact charger and the corona charger caused a decrease in the relative spread of the charge distribution to approximately 0.5 and 0.4 of the aerosol generated in the rotating disk generator. The particle charging was very sensitive to the impact conditions. The generating conditions therefore determine the distribution of the charges acquired by the particles in an aerosol. Acknowledgements The authors are indebted to FAPESP, FAPEMIG and CNPq for the financial support given to this work. References [1] J.F. Hughes, Electrostatic Particle Charging: Industrial and Health Care Applications, John Wiley & Sons, New York, 1997. [2] J.R. Coury, J.A. Raper, D. Guang, R. Clift, Measurement of electrostatic charge on gasborne particles and the effect of charges on fabric filtration, Process Safety and Environmental Protection (Trans IChemE, part B) 69 (B2) (1991) 97–106. [3] O.B. Duarte Fo., W.D. Marra Jr., G.C. Kachan, J.R. Coury, Filtration of electrified solid particles, Industrial and Engineering Chemistry Research 39 (10) (2000) 3884–3895. [4] A.M. Johnston, J.H. Vincent, A.D. Jones, Electrical charge characteristics of dry aerosols produced by a number of laboratory mechanical dispensers, Aerosol Science and Technology 6 (1987) 115–127. [5] B. Forsyth, B.Y.H. Liu, F.J. Romay, Particle charge distribution measurement for commonly generated laboratory aerosols, Aerosol Science and Technology 28 (1998) 489–501. [6] W.C. Hinds, Aerosol Technology, 2nd ed. Wiley-Interscience, New York, 1999. [7] J. Lowell, A.C. Rose-Innes, Contact electrification, Advances in Physics 29 (6) (1980) 947–1023. [8] A.G. Bailey, Electrostatic phenomena during powder handling, Powder Technology 37 (1984) 71–85. [9] J.A. Cross, Electrostatics: Principles, Problems and Applications, Adam Hilger, Bristol, 1987. [10] R.C. Flagan, History of electrical aerosol measurements, Aerosol Science and Technology 28 (1998) 301–380. [11] S. Matsusaka, H. Masuda, Electrostatics of particles, Advanced Powder Technology 14 (2) (2003) 143–166. [12] A.M. Johnston, J.H. Vincent, A.D. Jones, Measurement of electric charge for workplace aerosols, Annals of Occupational Hygiene 29 (2) (1985) 271–284. [13] Y. Mori, T. Shiomi, N. Katada, H. Minamide, K. Iinoya, Effects of corona precharger on performance of fabric filter, Journal of Chemical Engineering of Japan 15 (3) (1982) 211–216. [14] D. Guang, (1991) In-Situ Measurement of Electrostatic Charge and Distribution on Fly Ash Particles in Power Station Exhaust Stream. Ph.D. Tesis., University of New South Wales. 315 pp. [15] W.D. Marra Jr., J.R. Coury, Measurement of the electrostatic charge in airborne particles: I – Development of the equipment and preliminary results, Brazilian Journal of Chemical Engineering 17 (01) (2000) 39–50. [16] W.D. Marra Jr., J.R. Coury, Electrostatic charge measurement on airborne particles, Key Engineering Materials 189–191 (2001) 412–417. [17] W.D. Marra, Jr., (2000).The Development of Equipment for Measuring Electrostatic Charge Distribution in Aerosols. PhD Thesis. Universidade Federal de São Carlos. 200 pp. [18] M.V. Rodrigues, W.D. Marra Jr., R.G. Almeida, J.R. Coury, Measurement of the electrostatic charge in airborne particles: II – Particle charge distribution of different aerosols, Brazilian Journal of Chemical Engineering 23 (01) (2006) 125–133. [19] J.R. Coury, (1983) Electrostatic Effects in the Granular Bed Filtration of Gases. PhD Thesis. University of Cambridge. 234 pp. [20] V.R.K. Murthy, T.A. Prasada Rao, J. Sobhanadri, Dielectric properties of some dyes in the radio-frequency region, Journal of Physics D: Applied Physics 10 (1977) 2405–2409. [21] M.C.R. Falaguasta, S.W. Nóbrega, J.R. Coury, Scale up investigation of a wire plate geometry electrostatic precipitator, Particulate Science and Technology 24 (4) (2006) 453–465. [22] C. Riehle, Basic and theoretical operation of ESPs, in: K.R. Parker (Ed.), Applied Electrostatic precipitation, Chapman & Hall, London, 1997, Ch.3. [23] R. Cochet, Lois Charge des Fines Particules (submicroniques) Études Théoriques: Controles Rècents Spectre de Particules, Coll. Int. la Physique dês Forces Electrostatiques et Leurs Application, Centre National de la Recherche Scientifique, Paris, 1961, pp. 331–338.
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Glossary dp E
Fd Fe Fs L Q Qmedian
Particle Stokes diameter, m Electric field, Vm− 1 Drag force, N Electric force, N Cunningham slip factor Distance between the parallel plates, m Particle charge, C Particle median charge, C
Ux
X Z ΔV ε µ
Standard deviation, C Gas velocity, ms− 1 Particle velocity in direction x, ms− 1 Particle deflection, m Length of the deflection plates, m Electrical potential difference, V particle dielectric constant Gas viscosity, kg s− 1m− 1