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Experimental study on electrostatic charges and discharges inside storage silo during loading of polypropylene powders
Accepted Manuscript Experimental study on electrostatic charges and discharges inside storage silo during loading of polypropylene powders
Kwangseok Choi, Yuta Endo, Teruo Suzuki PII: DOI: Reference:
S0032-5910(18)30190-6 doi:10.1016/j.powtec.2018.03.007 PTEC 13238
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
26 November 2017 2 February 2018 7 March 2018
Please cite this article as: Kwangseok Choi, Yuta Endo, Teruo Suzuki , Experimental study on electrostatic charges and discharges inside storage silo during loading of polypropylene powders. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Ptec(2017), doi:10.1016/j.powtec.2018.03.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Experimental study on electrostatic charges and discharges inside storage silo during loading of polypropylene powders
National Institute of Occupational Safety and Health, Tokyo, Japan
b
Kasuga Denki Inc., Kanagawa, Japan
*
Corresponding author; E-mail: [email protected]
US
CR
IP
a
T
Kwangseok Choi,*a Yuta Endo,a and Teruo Suzukib
AN
Abstract
As polymer powders have high resistivity, they can easily be charged due to the repeated
M
collision and separation of particles along with particle-wall friction in a pneumatic conveying
ED
system. This study experimentally investigated the electrostatic charges and discharges inside a conical-cylindrical silo during the loading of polypropylene (PP, 2 - 3 mm) powders. The silo
PT
was continuously loaded with PP powders at 0.68 kg/s to a total mass of approximately 800 kg.
CE
To measure electrostatic charges of the PP powder being loaded into the silo, a Faraday cage with a cover was set at a distance of 50 mm from the center of the loading pipe inside the silo. To
AC
observe electrostatic discharges inside the silo, an image-intensifier unit was set on the windowpane of the silo roof. The results show that the charge-to-mass ratio of the loading PP powder remained at a constant value of approximately -12 C/kg. The ring-shaped light, which is the electrostatic discharge, appeared clearly at the center of the silo approximately 7 s after initial loading. The diameter of the ring-shaped light grew larger as time passed after loading. This was because the diameter of the accumulated PP powders increased in the silo during the
1
ACCEPTED MANUSCRIPT running time, meaning that electrostatic discharge occurred between settled PP powders and the grounded metal silo wall. Additionally, the electrostatic discharges during the loading of powder in this study were clearly observed and classified into three kinds of discharges: brush, linear, or broad bulk surface discharges.
CR
IP
T
Keywords: electrostatic charges, electrostatic discharges, silo, powder, polypropylene
US
1. Introduction
AN
The process of moving bulk dry materials such as powders, granules, tablets using an air blower from one place to another specific place within a factory is called the pneumatic
M
conveying system. It has become one of the most popular means of transport in the food,
ED
chemical, and pharmaceutical industries. Powders moved in the pneumatic conveying system can usually be highly charged due to friction as well as pipe-particle and inter particle collisions.
PT
This charging phenomenon can lead to incendiary electrostatic discharges that can result in dust
CE
explosion with a high enough concentration [1].
AC
Many studies have been conducted on mitigation industrial accidents due to electrostatic charges and discharges on powders and its prevention. Our previous paper (Choi et al. 2014 [2]) introduced the relationship between the charge amount of polypropylene powders and the frequency of electrostatic discharges. The brush discharges and bulk surface discharges began to occur at the -1.16 C/kg and -2.23 C/kg points, respectively. Glor et al. have investigated the characteristics of discharges that may occur while filling silos with insulating powders or granules [3–5]. These include experimental tests under realistic conditions with industrial-scale
2
ACCEPTED MANUSCRIPT silos, laboratory investigations, and theoretical and computational model calculations. Boschung et al. introduced two methods (computational model calculations and laboratory investigations) related to a characterization of the electrostatic behavior of powders. In one important result, powders with resistivity of less than 107 •m exhibited no dangerous charge buildup during
IP
CR
dangerous level of charge accumulation may be reached [6].
T
storage in metal containers. However, in powders with resistivity greater than 1013 •m, a
Although many of the studies mentioned above have investigated electrostatic charges
US
and/or discharges from powder in a silo, the phenomenon is not yet well understood. This study focuses on the electrostatic charges and discharges inside a storage silo during the continuous
M
AN
loading of large amounts of powder.
ED
2. Experimental Apparatus, Methods, and Sample Powder
Pneumatic Powder Transport Facility
AC
2.1.
CE
PT
Some of the following apparatuses and sample powders were also used in our previous studies [2, 7, 8, 9].
A full-sized pneumatic powder transport facility is shown in Fig. 1. It had a conicalcylindrical silo (stainless steel; body length, 3.75 m; diameter, 1.5 m; capacity, 4.8 m 3; Fig. 2), hopper (stainless steel; body length, 0.7 m; capacity, 0.1 m3; opening size; 0.6 m ×0.6 m), a pipeline (stainless steel; diameter, 0.1 m; total length, 23 m), an air blower (10 m3/min) driven by an inverter motor, and an air-conditioning unit controlling the temperature (30ºC) and relative
3
ACCEPTED MANUSCRIPT humidity (30% RH) of the blowing air. The explosion relief panels were installed on the silo in order to reduce the damage caused by any dust explosion that may occur inside the silo during testing. The silo, hopper, and pipeline were electrically grounded. The hopper was provided with a rotary valve (29 rpm) driven by an inverter motor to discharge powders from its bottom. The
T
sample powder was loaded continuously into the silo until its total mass reached approximately
Measuring of Electrostatic Charges
AN
2.2.
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CR
IP
800 kg.
M
The charge to mass ratio, q [C/kg] of the falling powder was measured using the Faraday
ED
cage method. The Faraday cage consists of an inner cup (metal mesh, 0.27 m in diameter, 0.85 m in height), outer shield cup (metal, 0.33 m in diameter, 0.9 m in height), electrometer (Keithley,
PT
6514), and a digital balance scale (Shimadzu, UX8200S). The q [C/kg], of the powders was
CE
obtained by dividing the total charge measured, Q (C), by mass, m (kg), of the powders loaded
AC
into the inner cup, as shown in Eq.1: q =Q/m
----------------------- (1)
A Faraday cage with a metal cover was set at a distance of 50 mm from the center of the loading pipe inside the silo. The distance for testing was kept constant. When the thin wire was pulled, the cover slid open for 5 s, which is the sampling time shown in Fig. 3. The q was measured three times under the same conditions, and the average value was employed as the
4
ACCEPTED MANUSCRIPT final one. It should be noted that the inner cup was made of mesh so that the airflow would not be disturbed. The procedure was as follows: Load the powder into the silo while the cover of the Faraday cup closed.
(2)
Open the cover for 5 s and let the powder go into the Faraday cup.
(3)
Close the cover and measure the indication value of Faraday cup.
(4)
Stop the powder loading.
(5)
Take out the powders collected in the Faraday cup and measure the mass of
US
CR
IP
T
(1)
Repeat the first six steps 3 times.
M
(6)
AN
powder.
The test for the charge to mass ratio of powder particles was carried out separately from
Observation of Electrostatic Discharges
CE
2.3.
PT
ED
the test finding electrostatic discharges.
AC
In order to observe electrostatic discharges inside the silo, an image-intensifier unit was used in this study. It was set on the windowpane of the silo roof, as shown in Fig. 4. Blackout curtains were placed around the metal silo to prevent external light, such as sunlight and fluorescent lights, from entering. The image-intensifier unit consists of an image-intensifier head (Night Viewer, Hamamatsu Photonics, Ltd.; C9016-02; max. gain, 1,000,000), a remote controller, relay lens, a wide-angle lens (18–35 mm), a single-lens reflex camera (or a charged coupled device (CCD) video camera), a power supply, an AC adapter, a displayer, and a digital 5
ACCEPTED MANUSCRIPT image recorder. The image intensifier was operated and controlled with a remote control or a PC via the USB interface connector provided on the rear panel of the image-intensifier head.
Polymer Powder Sample
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2.4.
Since polypropylene (PP) has been widely used in chemical industries, approximately 800
CR
kg of PP powder (Fig. 5) was employed as a sample in this experiment. The PP had a typical
US
particle size of 2–3 mm. The apparent volume resistivity, R [•m], of the PP powder was on the
3. Results and Discussion
Electrostatic Charges of the Falling Powder
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3.1.
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order of 1015 •m.
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The result is shown in Table 1. The feeding mass of the PP powder, u [kg/s], was a
CE
constant 0.68 kg/s in this study. The q of the loading PP powder remained at a constant value of approximately -12 C/kg while the PP powder was loading. This value is high enough to
AC
generate incendiary electrostatic discharges inside a metal silo [2]. In this study, the PP powder was negatively charged with the contact surfaces of the stainless steel. This result relates to the work functions (or ionization potential) of each material in the triboelectric series. It is known that the work function of SUS is 4.64 eV [10]. The work function of PP materials, 5.43 eV, is higher than that of SUS [11]. The materials with higher work functions tend to appropriate electrons from materials with lower work functions.
6
ACCEPTED MANUSCRIPT
3.2.
Observation of Electrostatic Discharges
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Observing electrostatic discharges within silos is especially important to better understand potential electrostatic hazards. Figure 6 shows the image of an area inside the silo in low light
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before the test. Approximately 400 g of powder was deposited so as to insert the direction papers
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and then taken out before the test. Therefore, the silo was completely empty before the test.
AN
Figure 7 (a–d) shows electrostatic discharges generated inside the silo with between 0 and 100 kg of powder loaded. These photos were taken randomly during loading. This study showed
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that the electrostatic discharges appeared clearly as light in the shape of rings 7 s after loading
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the powder into the silo (Fig. 7 (b)). The electrostatic discharges were generated in the unloading pipe attached to the bottom of the silo, as shown in Fig. 2. The diameter of the ring grew larger
PT
as more loading time passed (see Fig. 7 (b–d)). This is because the diameter of the accumulated
CE
powders increased in the silo during the running time in the conical part of the silo. This kind of electrostatic discharge can be called a brush bulk surface discharge or simply a brush discharge.
AC
Brush discharges occurred conspicuously between settled PP powders and the grounded silo wall due to the strong electric fields occurring at the edges of the powder bed. This is in good agreement with the result of computational model simulation [12]. A multiplicity of very small discharge channels occurs. Even if experiments with flammable gases of known ignition energy indicate that the brush discharges dissipate a maximum energy that is equivalent to approximately 3–4 mJ, brush discharges have not been reported to ignite combustible dust [13]. However, in the chemical process, powders wetted with solvents must be carefully handled, as 7
ACCEPTED MANUSCRIPT these kinds of powders can be ignited with brush discharges. The ignitibility of powder due to brush discharges is still being discussed [14-16]. We also observed that linear bulk surface discharges are a stretched form of brush discharges. This kind of electrostatic discharge can be also called a brush streamer or super brush discharge. It discharges across a PP powder heap
T
when the electric fields across the surface of the heap due to charged powder compaction are
IP
sufficiently high. The energy of the linear bulk surface discharges will be discussed after
CR
mentioning broad bulk surface discharges.
US
Figure 8 shows the electrostatic discharges generated inside the silo with between 100 and 400 kg of powder loaded. The diameter of the ring-shaped light grew larger as more loading time
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passed, because the diameter of the accumulated powders increased during the running time in
M
the conical part of the silo. In an important finding of this study, the broad bulk surface discharges frequently occurred in places linear discharges did not reach. They started clearly
ED
appearing approximately 166 s after loading the powder into the silo (loaded powder: 112 kg;
PT
diameter of the accumulated powders: 0.9 m). These discharges were triggered by the ends of the linear discharges. The total charge amount of powders accumulated around the center of the silo
CE
was high even though the charge to mass ratio was low, because a lot of powder falls in that area.
AC
Several additional tests were carried out to investigate the relationship between broad bulk surface discharges and the generation time (or loaded powder mass, diameter of the accumulated powders). They all agreed with the results of the test above, showing a time of around 170 s. This detail, however, must be thoroughly tested in the future. Some publications do not differentiate between linear and broad bulk surface discharges, calling all of them cone (or bulking) discharges generated during powder loading [17-19]. It is well known that the energy released in cone discharges depends on the diameter of the silo and 8
ACCEPTED MANUSCRIPT the particle size of the products forming the powder heap. For silos with diameters in the range of 0.5–3.0 m and powders with a median range of 0.1–3.0 mm, the energy released in cone discharges can be estimated using the following Eq.2 [20]: W =5.22 D3.36 d1.46,
(2)
T
--------------------------------
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where W is the upper limit of the energy of the bulk surface discharge (mJ), D is the
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diameter of the silo (m), and d is the median of the particle size distribution (mm).
US
In this study, the D of the silo and the d of the PP were 1.5 m and 2–3 mm, respectively. Following Eq. (2) above, W was 56–101 mJ. These energy values can easily ignite combustible
AN
powders.
M
Figure 9 shows the electrostatic discharges generated inside the silo with between 400 and
ED
800 kg of powder loaded. The diameter of the ring-shaped light did not change, because the diameter of the accumulated powders was consistent in the cylindrical part of the silo during the
PT
running time. This is also evidence that electrostatic discharges are generated on the heap surface
CE
of the accumulated powders. As seen in Fig. 9 (b–d), there were stronger and longer streaks of linear bulk surface discharges heading toward the center, while the inner central broad bulk
AC
surface discharges disappeared, as compared to Fig. 8. In other words, the linear bulk surface discharges neutralize the surface of the heaped powder. Strong brush discharges were also found at two points, A (East) and B (South), which are in front of the side windows, as shown in Fig. 6. This was because the side windows had protruding bolts (12 mm in diameter, 13 mm in height), as shown in Fig. 10. In order to prevent
9
ACCEPTED MANUSCRIPT and mitigate industrial accidents due to electrostatic discharges, protrusions inside the silo must be removed. Many different testing conditions, such as the kinds, sizes, and feeding masses of powder; the size and geometry of the silo; and the size of protrusions, may affect the electrostatic
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discharging of granules. This needs to be examined in the future.
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4. Conclusions
AN
To achieve a better understanding of electrostatic hazards, the electrostatic charges and discharges inside a conical-cylindrical silo during the loading of polypropylene powders were
M
experimentally investigated in this study. The experimental results clearly led to the following
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conclusions:
PT
(1) The charge to mass ratio of the loading powders remained at a constant value of about -12
discharges.
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C/kg during powder loading. This value is high enough to cause incendiary electrostatic
AC
(2) The electrostatic discharges clearly appeared as light in the shape of rings 7 s after loading the powder into the silo. (3) The electrostatic discharges during the loading of powder were clearly classified as brush, linear, or broad bulk surface discharges.
Acknowledgment 10
ACCEPTED MANUSCRIPT The authors gratefully acknowledge Mr. Naoto Nogera for his assistance during the test.
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References T.H. Pratt, Electrostatic ignitions of fires and explosions, Center for chemical process safety of the American Institute of Chemical Engineers. (2000) 132–142.
[2]
K. Choi, T. Mogami, T. Suzuki, and M. Yamaguma, Experimental study on the relationship between the charge amount of polypropylene granules and the frequency of electrostatic discharges while silo loading, Journal of Loss Prevention in the Process Industries. 32 (2014) 1–4.
[3]
M. Glor, Ignition of gas/air mixtures by discharges between electrostatically charged plastic surfaces and metallic electrodes, Journal of Electrostatics, vol. 10, pp. 327–332, May 1981.
[4]
M. Glor, Conditions for the appearance of discharges during the gravitational compaction of powders, Journal of Electrostatics, vol. 15, no. 2, pp. 223–235, June 1984.
[5]
M. Glor and B. Maurer, Ignition tests with discharges from bulked polymeric granules in silos (cone discharges), Journal of Electrostatics, vol. 30, pp. 123–133, May 1993.
[6]
P. Boschung and M. Glor, Methods for investigating the electrostatic behaviour of powders, Journal of Electrostatics, vol. 8, no. 2–3, pp. 205–219, Feb. 1980.
[7]
K. Choi, T. Mogami, T. Suzuki, S. Kim, and M. Yamaguma, Charge reduction on polypropylene granules and suppression of incendiary electrostatic discharges by using a novel AC electrostatic ionizer, Journal of Loss Prevention in the Process Industries. 26 (2013) 255–260.
[8]
K. Choi, T. Mogami, and T. Suzuki, Experimental study on detection of electrostatic discharges generated by polymer granules inside a metal silo, Review of Scientific Instruments. 85.4 (2014) 045001-6.
[9]
K. Choi, T. Mogami, T. Suzuki, and M. Yamaguma, A novel bipolar electrostatic ionizer for charged polypropylene granules used in a pneumatic powder transport facility, Journal of Loss Prevention in the Process Industries. 40 (2016) 502–506.
[10]
T. Doi, M. Yamashita, and H. Nagano, Water adsorption behavior and kelvin potential change on metal surfaces, J. Japan Inst. Met. Mater. 62.1.64–70 (1998).
[11]
H.Masuda, K. Higashitani, and H. Yoshida, Powder Technology, CBC press. P. 152 (2006)
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AN
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[1]
[12] W. Azizi, Using electrostatic modelling to study cone discharges, Journal of Physics:
conference series 646 (2015) 012046. [13]
IEC50404, Electrostatics—Code of practice for the avoidance of hazards due to static electricity. (2003) 70. 11
ACCEPTED MANUSCRIPT M. Glor and A. Pey, Modelling of electrostatic ignition hazards in industry examples of improvements of hazard assessment and incident investigation, Journal of Electrostatics, vol. 71, pp. 362–367, June 2013.
[15]
K. Schwenzfeuer and M. Glor, Ignition tests with brush discharges, Journal of Electrostatics, vol. 51–52, pp. 402–408, May 2001.
[16]
S. Forestier, J.M. Dien, and M. Glor, Ignition of a cloud of dry powder using super brush discharges, Journal of Electrostatics, vol. 88, pp. 177–182, Aug. 2017.
[17]
G. Luttgens and N. Wilson, Electrostatic hazards, Butterworth Heinemann. (1997) 49–59.
[18]
T.H. Pratt, Electrostatic ignitions of fires and explosions, Center for Chemical Process Safety of the American Institute of Chemical Engineers. (2000) 34.
[19]
M. Glor and K. Schwenzfeuer, Occurrence of cone discharges in production silos, Journal of Electrostatics, vol. 40–41, pp. 511–516, June 1997.
[20]
IEC50404, Electrostatics—Code of practice for the avoidance of hazards due to static electricity. (2003) 71–72.
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[14]
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ACCEPTED MANUSCRIPT Table 1 Charge amount Q, mass M, and charge to mass q of loading PP powder. M [kg] 2.16 2.30 2.12 2.19
q [C/kg] -12.26 -11.43 -11.69 -11.78
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M
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Q [C] -26.5 -26.3 -24.8 -25.8
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1 2 3 Avg.
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No.
13
ACCEPTED MANUSCRIPT Highlights The electrostatic charge of the loading powder was constant at -12 C/kg.
The electrostatic discharges during the powder loading were observed successfully.
The discharges were classified into brush, linear or broad bulk surface discharges.
AC
CE
PT
ED
M
AN
US
CR
IP
T
14
Graphics Abstract
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Kwangseok Choi, Yuta Endo, Teruo Suzuki PII: DOI: Reference:
S0032-5910(18)30190-6 doi:10.1016/j.powtec.2018.03.007 PTEC 13238
To appear in:
Powder Technology
Received date: Revised date: Accepted date:
26 November 2017 2 February 2018 7 March 2018
Please cite this article as: Kwangseok Choi, Yuta Endo, Teruo Suzuki , Experimental study on electrostatic charges and discharges inside storage silo during loading of polypropylene powders. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Ptec(2017), doi:10.1016/j.powtec.2018.03.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Experimental study on electrostatic charges and discharges inside storage silo during loading of polypropylene powders
National Institute of Occupational Safety and Health, Tokyo, Japan
b
Kasuga Denki Inc., Kanagawa, Japan
*
Corresponding author; E-mail: [email protected]
US
CR
IP
a
T
Kwangseok Choi,*a Yuta Endo,a and Teruo Suzukib
AN
Abstract
As polymer powders have high resistivity, they can easily be charged due to the repeated
M
collision and separation of particles along with particle-wall friction in a pneumatic conveying
ED
system. This study experimentally investigated the electrostatic charges and discharges inside a conical-cylindrical silo during the loading of polypropylene (PP, 2 - 3 mm) powders. The silo
PT
was continuously loaded with PP powders at 0.68 kg/s to a total mass of approximately 800 kg.
CE
To measure electrostatic charges of the PP powder being loaded into the silo, a Faraday cage with a cover was set at a distance of 50 mm from the center of the loading pipe inside the silo. To
AC
observe electrostatic discharges inside the silo, an image-intensifier unit was set on the windowpane of the silo roof. The results show that the charge-to-mass ratio of the loading PP powder remained at a constant value of approximately -12 C/kg. The ring-shaped light, which is the electrostatic discharge, appeared clearly at the center of the silo approximately 7 s after initial loading. The diameter of the ring-shaped light grew larger as time passed after loading. This was because the diameter of the accumulated PP powders increased in the silo during the
1
ACCEPTED MANUSCRIPT running time, meaning that electrostatic discharge occurred between settled PP powders and the grounded metal silo wall. Additionally, the electrostatic discharges during the loading of powder in this study were clearly observed and classified into three kinds of discharges: brush, linear, or broad bulk surface discharges.
CR
IP
T
Keywords: electrostatic charges, electrostatic discharges, silo, powder, polypropylene
US
1. Introduction
AN
The process of moving bulk dry materials such as powders, granules, tablets using an air blower from one place to another specific place within a factory is called the pneumatic
M
conveying system. It has become one of the most popular means of transport in the food,
ED
chemical, and pharmaceutical industries. Powders moved in the pneumatic conveying system can usually be highly charged due to friction as well as pipe-particle and inter particle collisions.
PT
This charging phenomenon can lead to incendiary electrostatic discharges that can result in dust
CE
explosion with a high enough concentration [1].
AC
Many studies have been conducted on mitigation industrial accidents due to electrostatic charges and discharges on powders and its prevention. Our previous paper (Choi et al. 2014 [2]) introduced the relationship between the charge amount of polypropylene powders and the frequency of electrostatic discharges. The brush discharges and bulk surface discharges began to occur at the -1.16 C/kg and -2.23 C/kg points, respectively. Glor et al. have investigated the characteristics of discharges that may occur while filling silos with insulating powders or granules [3–5]. These include experimental tests under realistic conditions with industrial-scale
2
ACCEPTED MANUSCRIPT silos, laboratory investigations, and theoretical and computational model calculations. Boschung et al. introduced two methods (computational model calculations and laboratory investigations) related to a characterization of the electrostatic behavior of powders. In one important result, powders with resistivity of less than 107 •m exhibited no dangerous charge buildup during
IP
CR
dangerous level of charge accumulation may be reached [6].
T
storage in metal containers. However, in powders with resistivity greater than 1013 •m, a
Although many of the studies mentioned above have investigated electrostatic charges
US
and/or discharges from powder in a silo, the phenomenon is not yet well understood. This study focuses on the electrostatic charges and discharges inside a storage silo during the continuous
M
AN
loading of large amounts of powder.
ED
2. Experimental Apparatus, Methods, and Sample Powder
Pneumatic Powder Transport Facility
AC
2.1.
CE
PT
Some of the following apparatuses and sample powders were also used in our previous studies [2, 7, 8, 9].
A full-sized pneumatic powder transport facility is shown in Fig. 1. It had a conicalcylindrical silo (stainless steel; body length, 3.75 m; diameter, 1.5 m; capacity, 4.8 m 3; Fig. 2), hopper (stainless steel; body length, 0.7 m; capacity, 0.1 m3; opening size; 0.6 m ×0.6 m), a pipeline (stainless steel; diameter, 0.1 m; total length, 23 m), an air blower (10 m3/min) driven by an inverter motor, and an air-conditioning unit controlling the temperature (30ºC) and relative
3
ACCEPTED MANUSCRIPT humidity (30% RH) of the blowing air. The explosion relief panels were installed on the silo in order to reduce the damage caused by any dust explosion that may occur inside the silo during testing. The silo, hopper, and pipeline were electrically grounded. The hopper was provided with a rotary valve (29 rpm) driven by an inverter motor to discharge powders from its bottom. The
T
sample powder was loaded continuously into the silo until its total mass reached approximately
Measuring of Electrostatic Charges
AN
2.2.
US
CR
IP
800 kg.
M
The charge to mass ratio, q [C/kg] of the falling powder was measured using the Faraday
ED
cage method. The Faraday cage consists of an inner cup (metal mesh, 0.27 m in diameter, 0.85 m in height), outer shield cup (metal, 0.33 m in diameter, 0.9 m in height), electrometer (Keithley,
PT
6514), and a digital balance scale (Shimadzu, UX8200S). The q [C/kg], of the powders was
CE
obtained by dividing the total charge measured, Q (C), by mass, m (kg), of the powders loaded
AC
into the inner cup, as shown in Eq.1: q =Q/m
----------------------- (1)
A Faraday cage with a metal cover was set at a distance of 50 mm from the center of the loading pipe inside the silo. The distance for testing was kept constant. When the thin wire was pulled, the cover slid open for 5 s, which is the sampling time shown in Fig. 3. The q was measured three times under the same conditions, and the average value was employed as the
4
ACCEPTED MANUSCRIPT final one. It should be noted that the inner cup was made of mesh so that the airflow would not be disturbed. The procedure was as follows: Load the powder into the silo while the cover of the Faraday cup closed.
(2)
Open the cover for 5 s and let the powder go into the Faraday cup.
(3)
Close the cover and measure the indication value of Faraday cup.
(4)
Stop the powder loading.
(5)
Take out the powders collected in the Faraday cup and measure the mass of
US
CR
IP
T
(1)
Repeat the first six steps 3 times.
M
(6)
AN
powder.
The test for the charge to mass ratio of powder particles was carried out separately from
Observation of Electrostatic Discharges
CE
2.3.
PT
ED
the test finding electrostatic discharges.
AC
In order to observe electrostatic discharges inside the silo, an image-intensifier unit was used in this study. It was set on the windowpane of the silo roof, as shown in Fig. 4. Blackout curtains were placed around the metal silo to prevent external light, such as sunlight and fluorescent lights, from entering. The image-intensifier unit consists of an image-intensifier head (Night Viewer, Hamamatsu Photonics, Ltd.; C9016-02; max. gain, 1,000,000), a remote controller, relay lens, a wide-angle lens (18–35 mm), a single-lens reflex camera (or a charged coupled device (CCD) video camera), a power supply, an AC adapter, a displayer, and a digital 5
ACCEPTED MANUSCRIPT image recorder. The image intensifier was operated and controlled with a remote control or a PC via the USB interface connector provided on the rear panel of the image-intensifier head.
Polymer Powder Sample
IP
T
2.4.
Since polypropylene (PP) has been widely used in chemical industries, approximately 800
CR
kg of PP powder (Fig. 5) was employed as a sample in this experiment. The PP had a typical
US
particle size of 2–3 mm. The apparent volume resistivity, R [•m], of the PP powder was on the
3. Results and Discussion
Electrostatic Charges of the Falling Powder
ED
3.1.
M
AN
order of 1015 •m.
PT
The result is shown in Table 1. The feeding mass of the PP powder, u [kg/s], was a
CE
constant 0.68 kg/s in this study. The q of the loading PP powder remained at a constant value of approximately -12 C/kg while the PP powder was loading. This value is high enough to
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generate incendiary electrostatic discharges inside a metal silo [2]. In this study, the PP powder was negatively charged with the contact surfaces of the stainless steel. This result relates to the work functions (or ionization potential) of each material in the triboelectric series. It is known that the work function of SUS is 4.64 eV [10]. The work function of PP materials, 5.43 eV, is higher than that of SUS [11]. The materials with higher work functions tend to appropriate electrons from materials with lower work functions.
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3.2.
Observation of Electrostatic Discharges
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Observing electrostatic discharges within silos is especially important to better understand potential electrostatic hazards. Figure 6 shows the image of an area inside the silo in low light
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before the test. Approximately 400 g of powder was deposited so as to insert the direction papers
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and then taken out before the test. Therefore, the silo was completely empty before the test.
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Figure 7 (a–d) shows electrostatic discharges generated inside the silo with between 0 and 100 kg of powder loaded. These photos were taken randomly during loading. This study showed
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that the electrostatic discharges appeared clearly as light in the shape of rings 7 s after loading
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the powder into the silo (Fig. 7 (b)). The electrostatic discharges were generated in the unloading pipe attached to the bottom of the silo, as shown in Fig. 2. The diameter of the ring grew larger
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as more loading time passed (see Fig. 7 (b–d)). This is because the diameter of the accumulated
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powders increased in the silo during the running time in the conical part of the silo. This kind of electrostatic discharge can be called a brush bulk surface discharge or simply a brush discharge.
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Brush discharges occurred conspicuously between settled PP powders and the grounded silo wall due to the strong electric fields occurring at the edges of the powder bed. This is in good agreement with the result of computational model simulation [12]. A multiplicity of very small discharge channels occurs. Even if experiments with flammable gases of known ignition energy indicate that the brush discharges dissipate a maximum energy that is equivalent to approximately 3–4 mJ, brush discharges have not been reported to ignite combustible dust [13]. However, in the chemical process, powders wetted with solvents must be carefully handled, as 7
ACCEPTED MANUSCRIPT these kinds of powders can be ignited with brush discharges. The ignitibility of powder due to brush discharges is still being discussed [14-16]. We also observed that linear bulk surface discharges are a stretched form of brush discharges. This kind of electrostatic discharge can be also called a brush streamer or super brush discharge. It discharges across a PP powder heap
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when the electric fields across the surface of the heap due to charged powder compaction are
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sufficiently high. The energy of the linear bulk surface discharges will be discussed after
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mentioning broad bulk surface discharges.
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Figure 8 shows the electrostatic discharges generated inside the silo with between 100 and 400 kg of powder loaded. The diameter of the ring-shaped light grew larger as more loading time
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passed, because the diameter of the accumulated powders increased during the running time in
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the conical part of the silo. In an important finding of this study, the broad bulk surface discharges frequently occurred in places linear discharges did not reach. They started clearly
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appearing approximately 166 s after loading the powder into the silo (loaded powder: 112 kg;
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diameter of the accumulated powders: 0.9 m). These discharges were triggered by the ends of the linear discharges. The total charge amount of powders accumulated around the center of the silo
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was high even though the charge to mass ratio was low, because a lot of powder falls in that area.
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Several additional tests were carried out to investigate the relationship between broad bulk surface discharges and the generation time (or loaded powder mass, diameter of the accumulated powders). They all agreed with the results of the test above, showing a time of around 170 s. This detail, however, must be thoroughly tested in the future. Some publications do not differentiate between linear and broad bulk surface discharges, calling all of them cone (or bulking) discharges generated during powder loading [17-19]. It is well known that the energy released in cone discharges depends on the diameter of the silo and 8
ACCEPTED MANUSCRIPT the particle size of the products forming the powder heap. For silos with diameters in the range of 0.5–3.0 m and powders with a median range of 0.1–3.0 mm, the energy released in cone discharges can be estimated using the following Eq.2 [20]: W =5.22 D3.36 d1.46,
(2)
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where W is the upper limit of the energy of the bulk surface discharge (mJ), D is the
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diameter of the silo (m), and d is the median of the particle size distribution (mm).
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In this study, the D of the silo and the d of the PP were 1.5 m and 2–3 mm, respectively. Following Eq. (2) above, W was 56–101 mJ. These energy values can easily ignite combustible
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powders.
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Figure 9 shows the electrostatic discharges generated inside the silo with between 400 and
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800 kg of powder loaded. The diameter of the ring-shaped light did not change, because the diameter of the accumulated powders was consistent in the cylindrical part of the silo during the
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running time. This is also evidence that electrostatic discharges are generated on the heap surface
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of the accumulated powders. As seen in Fig. 9 (b–d), there were stronger and longer streaks of linear bulk surface discharges heading toward the center, while the inner central broad bulk
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surface discharges disappeared, as compared to Fig. 8. In other words, the linear bulk surface discharges neutralize the surface of the heaped powder. Strong brush discharges were also found at two points, A (East) and B (South), which are in front of the side windows, as shown in Fig. 6. This was because the side windows had protruding bolts (12 mm in diameter, 13 mm in height), as shown in Fig. 10. In order to prevent
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ACCEPTED MANUSCRIPT and mitigate industrial accidents due to electrostatic discharges, protrusions inside the silo must be removed. Many different testing conditions, such as the kinds, sizes, and feeding masses of powder; the size and geometry of the silo; and the size of protrusions, may affect the electrostatic
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discharging of granules. This needs to be examined in the future.
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4. Conclusions
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To achieve a better understanding of electrostatic hazards, the electrostatic charges and discharges inside a conical-cylindrical silo during the loading of polypropylene powders were
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experimentally investigated in this study. The experimental results clearly led to the following
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conclusions:
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(1) The charge to mass ratio of the loading powders remained at a constant value of about -12
discharges.
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C/kg during powder loading. This value is high enough to cause incendiary electrostatic
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(2) The electrostatic discharges clearly appeared as light in the shape of rings 7 s after loading the powder into the silo. (3) The electrostatic discharges during the loading of powder were clearly classified as brush, linear, or broad bulk surface discharges.
Acknowledgment 10
ACCEPTED MANUSCRIPT The authors gratefully acknowledge Mr. Naoto Nogera for his assistance during the test.
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References T.H. Pratt, Electrostatic ignitions of fires and explosions, Center for chemical process safety of the American Institute of Chemical Engineers. (2000) 132–142.
[2]
K. Choi, T. Mogami, T. Suzuki, and M. Yamaguma, Experimental study on the relationship between the charge amount of polypropylene granules and the frequency of electrostatic discharges while silo loading, Journal of Loss Prevention in the Process Industries. 32 (2014) 1–4.
[3]
M. Glor, Ignition of gas/air mixtures by discharges between electrostatically charged plastic surfaces and metallic electrodes, Journal of Electrostatics, vol. 10, pp. 327–332, May 1981.
[4]
M. Glor, Conditions for the appearance of discharges during the gravitational compaction of powders, Journal of Electrostatics, vol. 15, no. 2, pp. 223–235, June 1984.
[5]
M. Glor and B. Maurer, Ignition tests with discharges from bulked polymeric granules in silos (cone discharges), Journal of Electrostatics, vol. 30, pp. 123–133, May 1993.
[6]
P. Boschung and M. Glor, Methods for investigating the electrostatic behaviour of powders, Journal of Electrostatics, vol. 8, no. 2–3, pp. 205–219, Feb. 1980.
[7]
K. Choi, T. Mogami, T. Suzuki, S. Kim, and M. Yamaguma, Charge reduction on polypropylene granules and suppression of incendiary electrostatic discharges by using a novel AC electrostatic ionizer, Journal of Loss Prevention in the Process Industries. 26 (2013) 255–260.
[8]
K. Choi, T. Mogami, and T. Suzuki, Experimental study on detection of electrostatic discharges generated by polymer granules inside a metal silo, Review of Scientific Instruments. 85.4 (2014) 045001-6.
[9]
K. Choi, T. Mogami, T. Suzuki, and M. Yamaguma, A novel bipolar electrostatic ionizer for charged polypropylene granules used in a pneumatic powder transport facility, Journal of Loss Prevention in the Process Industries. 40 (2016) 502–506.
[10]
T. Doi, M. Yamashita, and H. Nagano, Water adsorption behavior and kelvin potential change on metal surfaces, J. Japan Inst. Met. Mater. 62.1.64–70 (1998).
[11]
H.Masuda, K. Higashitani, and H. Yoshida, Powder Technology, CBC press. P. 152 (2006)
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[12] W. Azizi, Using electrostatic modelling to study cone discharges, Journal of Physics:
conference series 646 (2015) 012046. [13]
IEC50404, Electrostatics—Code of practice for the avoidance of hazards due to static electricity. (2003) 70. 11
ACCEPTED MANUSCRIPT M. Glor and A. Pey, Modelling of electrostatic ignition hazards in industry examples of improvements of hazard assessment and incident investigation, Journal of Electrostatics, vol. 71, pp. 362–367, June 2013.
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K. Schwenzfeuer and M. Glor, Ignition tests with brush discharges, Journal of Electrostatics, vol. 51–52, pp. 402–408, May 2001.
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S. Forestier, J.M. Dien, and M. Glor, Ignition of a cloud of dry powder using super brush discharges, Journal of Electrostatics, vol. 88, pp. 177–182, Aug. 2017.
[17]
G. Luttgens and N. Wilson, Electrostatic hazards, Butterworth Heinemann. (1997) 49–59.
[18]
T.H. Pratt, Electrostatic ignitions of fires and explosions, Center for Chemical Process Safety of the American Institute of Chemical Engineers. (2000) 34.
[19]
M. Glor and K. Schwenzfeuer, Occurrence of cone discharges in production silos, Journal of Electrostatics, vol. 40–41, pp. 511–516, June 1997.
[20]
IEC50404, Electrostatics—Code of practice for the avoidance of hazards due to static electricity. (2003) 71–72.
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[14]
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ACCEPTED MANUSCRIPT Table 1 Charge amount Q, mass M, and charge to mass q of loading PP powder. M [kg] 2.16 2.30 2.12 2.19
q [C/kg] -12.26 -11.43 -11.69 -11.78
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Q [C] -26.5 -26.3 -24.8 -25.8
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1 2 3 Avg.
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ACCEPTED MANUSCRIPT Highlights The electrostatic charge of the loading powder was constant at -12 C/kg.
The electrostatic discharges during the powder loading were observed successfully.
The discharges were classified into brush, linear or broad bulk surface discharges.
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Graphics Abstract
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