- Email: [email protected]
Experimental investigation of the inerting effect of crystalline II type Ammonium Polyphosphate on explosion characteristics of micron-size Acrylates Copolymer dust
Journal of Hazardous Materials 344 (2018) 558–565
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Research Paper
Experimental investigation of the inerting effect of crystalline II type Ammonium Polyphosphate on explosion characteristics of micron-size Acrylates Copolymer dust Yuan Yu ∗ , Yunhao Li, Qingwu Zhang ∗ , Weishun Ni, Juncheng Jiang Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, College of Safety Science and Engineering, Nanjing Tech University, Mail Box 13, No. 200 North Zhongshan Rd., Nanjing, 210009, China
h i g h l i g h t s • • • • •
Explosion severity of ACR powders is confirmed to be class St–3. ACR powders exhibit MEC 20–30 g/m3 and MIE 10 mJ for d(50) = 4.4 m. There existed an effective MIC. And ACR dust explosion can be inerted completely by 80 wt% APP-II. 30 and 40 wt% APP-II had significantly inerting effect on the MIE of ACR dust. ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II.
a r t i c l e
i n f o
Article history: Received 13 April 2017 Received in revised form 1 September 2017 Accepted 29 October 2017 Available online 31 October 2017 Keywords: Inerting effect Crystalline II type Ammonium Polyphosphate Micron-size Acrylates Copolymer Inerting mechanism Chemical interaction
a b s t r a c t The inerting effect of crystalline II type Ammonium Polyphosphate (APP-II) on explosion characteristics of micron-size Acrylates Copolymer (ACR) powders was experimentally studied. The inerting mechanism was analysed by combining thermogravimetry (TG) and differential scanning calorimetry (DSC) tests. The results indicated that the maximum explosion pressure (Pmax ) and explosion index (Kst ) was 10.4 bar and 416 bar m/s, respectively for ACR powders. The minimum explosion concentration (MEC) of ACR powders ranged from 20 to 30 g/m3 , and the experimental minimum ignition energy (MIE) of the ACR dust cloud was 10 mJ. Therefore, ACR dust was determined to be severely combustible dust. There existed a minimum inerting concentration (MIC), and the explosion of ACR powders can be inerted completely by 80 wt% APP-II. Furthermore, 30 and 40 wt% APP-II had a significant inerting effect on the MIE of ACR dust. According to TG and DSC tests, thermal stability of ACR would be augmented by the introduction of APP-II. The addition of APP-II triggered lower maximum mass loss rate (MMLR), higher temperature corresponding to mass loss of 90% (T0.1 ), chars yield, and endothermic peaks. Consequently, the ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Dust explosions present substantial threats to people, assets, and the environment [1], such as the catastrophic aluminium-alloy dust explosion that killed 146 persons in China [2]. Table 1 lists several serious disasters caused by polymer dust explosions in the
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Yu), [email protected], [email protected] (Q. Zhang). https://doi.org/10.1016/j.jhazmat.2017.10.060 0304-3894/© 2017 Elsevier B.V. All rights reserved.
process industry [1,3]. Obviously, it is crucial to explore the explosion characteristics of combustible polymer dust. Polymers, which are polymerized by two or more than two monomers, are widely used in the chemical industry, aviation industry, synthetic rubber industry, and special materials. Currently, with the increasing utilization of polymers, and with many polymers being processed by cutting, grinding, or machining, polymer dust may accumulate to an explosive concentration if there are no effective prevention measures in workshops. Previous studies have proven that polymer dust clouds have explosion hazards if there has existed sufficient ignition energy and oxy-
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
559
Table 1 Accidents caused by polymer dust explosion [1,3]. date
country
Fatalities (F)/Injuries (I)
material
1993 1996 1999 2002 2003 2003 2003 2004 2011
USA USA USA USA USA USA USA USA USA
2 F/2 I 2F 2F 13 F 38 F/6 I 6 F/10 I 38 F/6 I 5 F/0 I 2 F/0 I
Plastic Rubber Plastic Rubber Rubber Powdered plastic Rubber Polyvinyl chloride Methomyl pesticide
gen concentration, such as fibrous nylon, fibrous polyethylene, polyamide 12 (PA12), 7-aminocephalosporanic acid, polymethyl methacrylate (PMMA), protein, polypropylene, cellulose ethers and cellulose acetates with different degrees of acetylation, Florfenicol, Tilmicosin, etc. [4–11]. Those studies indicated that polymer dust explosibility can be affected by particle shape, particle size, and dust concentration. Moreover, the maximum explosion pressure (Pmax ), minimum explosion concentration (MEC), and minimum ignition energy (MIE) of micron-size polymer dust can reach 9 bar, 10 g/m3 , and 10 mJ, respectively. Acrylates Copolymer (ACR), a type of polymer, is widely used as anti-blocking agents for films, anti-scratch and anti-matting agents for paintings or inks, special additives for cosmetics, and light diffusers for electronic displays, among other uses. Therefore, ACR dust can be generated in the process of shattering, classification, and meterage, etc. The study of explosion characteristics of ACR dust can give mitigative and preventive guidance of ACR dust explosion for relevant enterprises. To reduce the severity of dust explosions in confined spaces, venting, isolation, explosion suppression, and inerting have been proposed [12]. Explosion inerting, which is an inherent safety approach, consists of inert dust being premixed with the explosible dust before ignition. A dust explosion is thereby prevented or mitigated by inert dust. Such mixing has been proven to effectively reduce the risk of dust explosion. Mintz et al. [13] experimentally demonstrated that 50:50 Al-Mg alloy powders can be inerted completely by the weight between 70 and 75% of fine MgO dust. Chatrathi and Going [14] found the minimum inerting concentrations (MIC) of sodium bicarbonate (SBC), mono-ammonium phosphate (MAP) and rock dust (RD). Dastidar et al. [15] experimentally studied the inerting effect of rock dust in 1-m3 and 20-L vessels on coal dust. The results indicated that the inerting levels in the 1-m3 vessel were lower than that in the 20-L chamber. Myers [16] determined a proprietary flame retardant, an aqueous halogenated compound, was effective to residue created in an aluminium buffing operation to reduce the dust explosion hazard throughout the buffing process and within the dust collector systems. Kuai et al. [17], Du et al. [18] demonstrated the inerting effects of calcium carbonate (CaCO3 ), monoammonium phosphate (NH4 H2 PO4 ) on coal dusts. Yuan et al. [19,20] tested the inerting effect of nano TiO2 powders on MIE and minimum ignition temperature (MIT) of nano and micron Ti powder clouds. Janès et al. [21] investigated the MIE and MIT of several organic dusts in the presence of kieselguhr, alumina, and aerosol. Those studies provided a lot of useful information for safe design of dust explosion inerting. Therefore, we tried determining the inerting effect of crystalline II type Ammonium Polyphosphate (APP-II) what was widely used as a flame retardant on ACR dust. The current work is devoted to testing the explosion characteristics of ACR powders, along with the inerting effect of APP-II on ACR powders. The inerting mechanism of APP-II was studied according to thermogravimetry (TG) and differential scanning calorimetry (DSC) tests.
Fig. 1. The 20 L spherical experimental apparatus. Notes: 1–Water outlet, 2–Pressure sensor, 3–Manometer, 4–Dust container, 5–Air inlet, 6–Ignition source, 7–Rebound nozzle, 8–Fast acting valve, 9–Water inlet, 10–Outlet (air, reaction products).
Fig. 2. Pressure–time curve recorded during the dust explosion test for the dust concentration of 400 g/m3 .
2. Methodology 2.1. Experimental apparatus for explosion severity parameters A standardized 20-L stainless steel spherical vessel (Fig. 1), recommended by International Standard ISO 6184/1 [22], was selected to test the Pmax , maximum rate of pressure rise (dP/dt)max of ACR powders. Before an explosion test, an igniter was fixed at centre of the chamber. Moreover, the safely sealed explosion vessel was vacuumed in part to −0.6 bar (gauge) and the pressure of pressurized air in the dust storage tank was 20 bar (gauge). The ignition energy of chemical igniters which was ignited after delaying 60 ms, was 2 kJ. The measurement range of the pressure sensor was 0–250 psi (17 bar). A normal curve of pressure–time evolution noted during the ACR dust explosion is revealed in Fig. 2. Pressure evolution first began at −0.6 bar (gauge). The air blast applied for dispersing powders then began at 160 ms. Igniting finally started at 220 ms at an ambient condition of 1 bar (absolute). The value of Pex , for a single
560
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565 Table 2 Material properties of ACR powders. Supplier
Particle size distribution (m)
Specific surface area (m2 /g)
Suzhou Soken Chemical Co., Ltd
d(10) = 3.097 m d(50) = 4.339 m d(90) = 6.075 m
1.43
trol unit. The spacing between electrodes was 6 mm. In addition, the delay time between dispersion and the ignition was 90 ms. Based on EN 13821: 2002, if the dust was ignited and flame propagation appeared, a dust explosion was successfully induced. If the dust was non-ignitable in ten continual tests, this ignition energy could not induce a dust cloud explosion. 2.3. Material
Fig. 3. Schematic of the modified Hartmann tube.
test at a given concentration is the highest explosion overpressure (gauge). Moreover, (dP/dt)ex is the maximum rate of pressure rise in one test [23]. According to (dP/dt)max and the volume of the spherical chamber (V), Kst an international common value can be calculated by Eq. (1):
Kst = dP/dt
max
× V -3
(1)
ACR dust samples tested were produced by a supplier. Table 2 summarizes the material properties of ACR powders. Fig. 4 shows the shapes of ACR powders under a microscope. It can be seen that ACR powders are regular and spherical particles. Meanwhile, the particle size distribution varied within a narrow range. Ammonium polyphosphate (APP), a synthesis of polyphosphoric acid and ammonia is used as a kind of flame retardant due to its lower cost and prominent processability. Furthermore, owing to no halogen, most of all, APP does not create a supernumerary amount of smokes, which is environmental-friendly compared with halogenic flame retardants. Moreover, APP contains high levels of phosphorus (P) and nitrogen (N) [21,25]. In this study, APP-II is selected as the inert agent. APP-II is a type of flame retardant which is stable and almost insoluble. Table 3 lists the major properties of APP-II.
2.2. Experimental apparatus for MIE 2.4. Sample preparation for solid mixtures According to the European Standard EN 13821: 2002 [24], a modified 1.2-l Hartmann tube was used for ignition energy tests. Fig. 3 shows a schematic diagram of the modified Hartmann apparatus. The apparatus is comprised of a Hartmann tube, control system, high voltage unit, dust diffusion unit, and electronic pneumatic con-
ACR and APP-II powders were first dried by a vacuum drying box for eight hours at a low temperature of 30 ◦ C. Then, pre-weight ACR and APP-II powders for a given proportion were placed in a beaker. Finally, ACR and APP-II powders were fully mixed by
Fig. 4. Shapes of ACR powders of under the microscope.
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
561
Table 3 Properties of APP-II. P2 O5 content (wt%)
N content (wt%)
Density (g/cm3 )
Particle size (m)
Degree of polymerization (n)
≥72
≥14
1.9
5∼15
≥1000
manual stirring for five minutes, which has been justified to yield homogeneous mixtures effectively by Yuan et al. [19]. 3. Results and discussion 3.1. Inerting effect of APP-II on the explosion severity of ACR powders The explosion severity can be characterized by Pmax and Kst . It is vital for safety design and explosion prevention to understand the explosion severity parameters of ACR powders. The concentrations of ACR powders for tests were 200, 400, 600, 800, 1000, and 1200 g/m3 . Moreover, the ignition energy of chemical igniters was 2 kJ. The evolution of Pmax and Kst vs. dust concentration is illustrated in Figs. 5 and 6, respectively. It can be seen that the dust concentrations for the maximum values of Pmax and Kst are equal to 600 and 400 g/m3 respectively. Note that the maximum values of Pmax and Kst were 10.4 bar and 416 bar m/s, correspondingly for ACR powders. Pmax and Kst increased first and then decreased as dust concentration increased. The experimental results were in line with some common combustible dusts, such as aluminium, coals, and polyethylene [26,5,27]. Obviously, the reason is that the ACR powders participating in the explosion increased with the increase of dust concentration of ACR powders when the dust concentration was lower than 600 g/m3 . The ACR powders reacted fully with oxygen under the dust concentration of 600 g/m3 . Once the ACR dust concentration exceeded 600 g/m3 , there was too much ACR powder to react with oxygen. Therefore, the ACR powders that were not participating in the reaction absorbed the heat released from the chemical reaction. Furthermore, the temperature in the flame front decreased. Moreover, the heat release rate and burning rate both dropped. Accordingly, the explosion strength decreased as the ACR dust concentration increased.
Fig. 5. Pmax vs. dust concentration.
Table 4 summarizes the explosion characteristics of some common dusts with similar particle size. By contrast, it can be seen that the explosion severity of ACR powders was next only to the magnesium powders. Moreover, the explosion damage of ACR powders was much stronger than other dusts. Furthermore, it can also be seen that the experimentally measured MEC of ACR powders ranges from 20 to 30 g/m3 . Therefore, an ACR dust explosion can occur at a very low concentration. The MEC of ACR dust was almost smaller than any other common dust for similar particle size distribution. Table 5 presents the dust explosion class standard in accordance with ISO6184/1 [22]. According to the ISO6184/1, the explosion severity of ACR powders fell into class St–3, with severe explosive power. Due to the explosion risk of ACR dust, when designing the plants for producing and processing ACR products, preventive and
Table 4 Explosion characteristics for some dust samples summarized from the literatures. Dust
D50 , m
MEC, g/m3
Pmax , bar
Kst , bar m/s
Reference
B C-1 C-2 C-3 Mg Al-1 Al-2 Al-3 Si Cr Fe Ni Zn Mo Sn Hf Ta W-1 W-2 Mg Mg Graphite W ACR
∼3 ≤1 ∼1 ∼4 ∼16 ∼1 ∼7 ∼15 ∼4 ∼10 ∼4 ∼6 ∼4 ∼5 ∼8 ∼8 ∼10 ≤1 ∼10 1 10 4 1 ∼4
∼110 90 F NF 55 85 90 90 200 ∼F 220 NF 300 NF ∼450 ∼180 ∼400 ∼700 NF 30 40 70 ∼ 20∼30
7.0 5.5 ∼5 1.1 8.5 9.4 9.9 7.5 7.7 ∼3 4.5 1.0 4.4 1.0 4.3 5.2 ∼4 ∼3.3 1.0 14 13.2 6.6 5.7 10.4
∼ ∼ ∼ ∼ ∼ ∼ ∼250 ∼80 ∼ ∼ ∼28 ∼12 ∼ ∼ ∼ ∼ ∼ ∼ ∼ 510 482 70.6 86.9 416
[28]
NF: nonflammable or nonignitable; F: MEC could not be determined.
[23] [29] Present study
562
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
Fig. 6. Kst vs. dust concentration. Fig. 8. Overpressure vs. time for different mass fraction of APP-II. Table 5 Dust explosion class standard in accordance with ISO 6184/1 [22]. Dust explosion class
Kst /(bar m/s)
St–1 St–2 St–3
0 < Kst ≤ 200 200 < Kst ≤ 300 300 < Kst
Fig. 9. Pmax of ACR dust vs. mass fraction of APP-II.
Fig. 7. Inerting envelope for ACR dust with APP-II.
mitigative measures for ACR dust explosion must be seriously considered. Meanwhile, avoiding the formation of flammable ACR dust clouds is a crucial task during production. According to the previous study by Dastidar et al. [30], there was not always a MIC for a type of combustible dust. To determine the mass fraction of completely inerting which can be applied for the safe design of explosion suppression, ACR dust explosion tests with various mass fraction of APP-II (the ratio of weight of APP-II powders and total dust weight) were carried out. Fig. 7 shows the inerting envelope for ACR dust. The ACR dust concentrations were between 200 and 800 g/m3 . It can be seen that the APP-II concentrations increased first and then decreased with the concentration of ACR dust increasing. There existed a MIC occurring at approximately 2400 g/m3 of APP-II corresponding to about 600 g/m3 of ACR dust. Fig. 8 depicts the overpressure evolution of 600, 3000 g/m3 of ACR dust and 600 g/m3 of ACR dust with different mass fraction of APP-II. Figs. 9 and 10 portray the variety of Pmax and Kst of 600 g/m3 of ACR powders with various mass fraction of APP-II respectively. It
Fig. 10. Influence of mass fraction of APP-II on the Kst of ACR dust.
can be seen that APP-II had a great inerting effect on ACR powders. As in Fig. 8, the time from ignition to overpressure peak increased for 600 g/m3 of ACR dust with the mass fraction of APP-II increasing. The result indicated that the burning rate dwindled as the
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
563
Table 6 Experimental results of MIE for ACR powders with different mass fraction of APP-II. Mass fraction of APP-II (wt%)
0 10 20 30 40
MIE (mJ) for different sample mass 240 mg
480 mg
720 mg
960 mg
1200 mg
1440 mg
190 300 800 >1000 >1000
90 95 120 590 >1000
60 80 85 480 >1000
30 50 60 220 990
10 20 55 230 1000
50 55 65 235 1000
Table 7 Ignition sensitivity for flammable dust [31]. MIE of a dust
Ignition sensitivity
500
Low sensitivity to ignition: Earth plant when ignition energy is at or below this level. Consider earthing personnel when ignition energy is at or below this level. The majority of ignition incidents occur when ignition energy is below this level. The hazard from electrostatic discharges from dust clouds should be considered. High sensitivity to ignition. Take the above precautions and consider restrictions on the use of high resistivity materials (plastics). Electrostatic hazard from bulk powders of high resistivity should be considered. Extremely sensitive to ignition. Precautions should be as for flammable liquids and gases when ignition energy is at or below this level.
100 25
10
1
Fig. 11. Effect of mass fraction of APP-II on MIEs of ACR dust.
mass fraction of APP-II increased. Moreover, the Pmax and Kst of ACR powders also reduced with the increase of mass fraction of APP-II. Additionally, an explosion was not triggered for ACR/APP-II mixture when the mass fraction of APP-II increased to 80 wt%. This means that the ACR powders were completely inerted by APP-II.
3.2. Inerting effect of APP-II on MIE of ACR powders Ignition sensitivity can be characterized scientifically by the MIE of a dust cloud. To understand the inerting effect of APP-II on MIE of ACR powders, the MIEs of ACR dust in the range of 240–1440 mg with different mass fraction of APP-II were experimentally investigated. Experimental results of MIE for ACR powders with different mass fraction of APP-II are summarized in Table 6. Fig. 11 shows the effect of mass fraction of APP-II on MIEs of ACR dust. It can be seen that the MIE of ACR powders decreased first and then increased with the increase of sample mass in the range of 240–1440 mg. Moreover, The ACR dust cloud can be ignited by the ignition energy of 10 mJ corresponding to the sample of 1200 mg, which is low enough to be sensitive to static electricity (Table 7 [31]). MIEs of ACR dust mixed with 10, 20, 30, and 40 wt% of APP-II were also experimentally tested. The results indicated that APPII had an inerting effect on ACR dust. For the mixture with 10 and 20 wt% of APP-II, MIEs of mixtures for the ACR sample mass ranging from 480 to 1440 mg increased slightly. While the mass fraction of APP-II increased to 30 wt%, MIEs of mixtures increased sharply. Mixtures for the ACR sample mass of 240 mg was nonignitable under the spark energy of 1000 mJ. Furthermore, no ignition occurred for the ACR sample mass of 240–720 mg with 40 wt% of APP-II. What’s more, the minimum value of MIE for ACR/APP-II mixtures was 990 mJ corresponding to the ACR sample mass of 960 mg with 40 wt% of APP-II.
Fig. 12. Calculated MIEs vs. mass fraction of APP-II.
To compare the inerting effects between different mass fraction of APP-II, the MIEs of ACR/APP-II mixtures were calculated by Eq. (2) [19]. MIE = 10log IE-NI ×
log IE-log NIE N+1
(2)
N and NI denote the test number and combustion number, correspondingly, at energy level IE. Fig. 12 reveals the inerting effect of the mass fraction of APP-II on ACR dust. It can be seen that the calculated MIEs of mixtures with 10 wt% APP-II was close to that of ACR dust. According to Table 7, the hazard triggered by electrostatic discharges for ACR dust clouds with 10 wt% of APP-II also should be considered. Although 30 and 40 wt% APP-II had a significant inerting effect on the calculated MIEs of mixtures, explosion severity parameters were still severe (Figs. 9 and 10). Therefore, explosibility still should be considered carefully.
564
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
Fig. 13. Mass loss of ACR powders vs. temperature with various mass fraction of APP-II.
Fig. 14. Derivative mass to loss of ACR powders vs. temperature with various mass fraction of APP-II.
3.3. Mechanism analysis Fig. 13 shows the mass loss results of ACR powders, APP-II powders and ACR/APP-II mixtures according to TG tests under air condition. Derivative mass to loss for ACR powders vs. temperature with various mass fraction of APP-II is plotted in Fig. 14. It can be seen that ACR powders had a mass loss of approximately 255 ◦ C. All the ACR powders decomposed completely at around 500 ◦ C. Furthermore, the temperature of maximum mass loss rate (Tmax ) was 314 ◦ C. Moreover, the mass loss of APP-II powders mainly occurred in two stages. The first stage was triggered from about 275 ◦ C to 425 ◦ C. While the second stage occurred in the range of 500–675 ◦ C. Besides, Tmax for two stages were 346 and 615 ◦ C, correspondingly. The residue of APP-II was 28% at 785 ◦ C. It can also be seen that ACR/APP-II mixtures had two stages of mass loss. The temperature corresponding to mass loss of 90% (T0.1 ) was commonly selected as a criterion for judging the thermal stability [32]. The T0.1 for ACR, 20 wt% APP-II, and 40 wt% APP-II was 286, 305, and 311 ◦ C, respectively. In addition, it was clearly observed that the maximum mass loss rate (MMLR) dropped when a different amount of APP-II was added. The amount of residue increased when the mass fraction of APP-II increased. The results indicated that the thermal stability of ACR could be augmented by the introduction of APP-II. To evaluate the mutual interactions of ACR/APP-II mixtures, a comparison of experimental and theoretical mass losses of ACR/APP-II mixtures was conducted. Assuming that ACR and APP-
Fig. 15. Experimental and theoretical TG curves of ACR with 40 wt% APP-II in air atmosphere.
II decompose independent of each other, the theoretical mass loss was calculated by a linear combination of the experimental values of both materials, in accordance with their ratio [32]. It can be seen from Fig. 15 that the onset temperature for theoretical mass loss was higher than that for the experimental result. Meanwhile, the theoretical T0.1 was 17 ◦ C higher than the experimental value. The experimental mass loss was greater than the theoretical result until approximately 555 ◦ C. The result indicates that APP-II was chemically interacting with the ACR during this decomposition stage. Furthermore, the residue for the experimental value was greater than the theoretical value, denoting a significant catalytic effect on the char formation of ACR. In the case of the condensed phase action of phosphate containing systems, the better the charring properties, the higher the char yield. Thus, more carbonaceous char is left behind during decomposition [33]. Consequently, the char layer can prevent contact between O2 and ACR powders. The thermal conduction was decreased by the char layer, too. Moreover, the decomposition and volatilization of ACR powders was prevented during explosion. To discuss the thermal impact of APP-II on ACR decomposition, as shown in Fig. 16, the TG and DSC tests of ACR and ACR/APPII mixtures were carried out. It can be seen that there were two endothermic decomposition peaks for ACR and ACR/APP-II mixtures. The first endothermic peak may be the decomposition of side groups of ACR. The second endothermic peak was most likely due to the decomposition of the backbone of ACR. After the introduction of APP-II, the first endothermic peak remained at 309 ◦ C. However, the endothermic peak was sharper and stronger. The second endothermic peak increased greatly from 356 to 420 ◦ C compared to the neat ACR, resulting from the decomposition of APP-II, which absorbed significant heat. The results indicate that the thermal decomposition of ACR was inerted by APP-II. Above all, the results demonstrated that the introduction of APP-II into ACR triggered lower MMLR, higher T0.1 , char yield, and endothermic peaks. Consequently, the ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II. 4. Conclusions The Pmax and Kst were 10.4 bar and 416 bar m/s, respectively for the ACR powders. The MEC of ACR powders ranged from 20 to 30 g/m3 . Therefore, ACR powders were a kind of combustible dust. Moreover, the explosion severity of ACR powders fell into class St–3, with severe explosive power. ACR/APP-II mixtures showed an effective MIC occurring at 2400 g/m3 of APP-II in the concentra-
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
Fig. 16. TG and DSC curves for ACR and ACR with 40 wt% APP-II in N2 atmosphere: (a) TG; (b) DSC.
tion ranges tested. The explosion of ACR powders can be inerted completely by 80 wt% APP-II. The MIE of the ACR dust cloud was 10 mJ, which is low enough to be sensitive to static electricity. Furthermore, 30 and 40 wt% APPII had a significant inerting effect on the ACR dust explosion. The minimum value of MIE for mixtures with 40 wt% APP-II was 990 mJ. After the introduction of APP-II, the T0.1 increased from 286 to 305 and 311 ◦ C for 20 and 40 wt% APP-II respectively. MMLR decreased as the ratio of APP-II increased. Meanwhile, the chemical interaction between ACR and APP-II led to higher char yield. The char layer acted as a thermal barrier. Furthermore, the second endothermic peak was increased greatly from 356 to 420 ◦ C due to the thermal inerting of the endothermic decomposition of APP-II. Acknowledgements The authors are grateful for the financial support given by the National Key Research and Development Plan under grant no. 2016YFC0800102 and the State Key Program of National Natural Science of China under grant no. 2143 6006, along with the open project of Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control. References [1] Z. Yuan, N. Khakzad, F. Khan, P. Amyotte, Dust explosions: a threat to the process industries, Process Saf. Environ. Prot. 98 (2015) 57–71. [2] G. Li, H.X. Yang, C.M. Yuan, R.K. Eckhoff, A catastrophic aluminium-alloy dust explosion in China, J. Loss Prev. Process Ind. 39 (2016) 121–130.
565
[3] S.Y. Lin, T.H. Lin, Y.C. Cheng, K.-H. Hsueh, C.-M. Shu, Assessment of dust explosion with adipic acid and p-terephthalic acid in the powdered resin process, J. Loss Prev. Process Ind. 43 (2016) 92–97. [4] L. Marmo, D. Cavallero, Minimum ignition energy of nylon fibres, J. Loss Prev. Process Ind. 21 (2008) 512–517. [5] P.R. Amyotte, C.T. Cloney, F.I. Khan, R.C. Ripley, Dust explosion risk moderation for flocculent dusts, J. Loss Prev. Process Ind. 25 (2012) 862–869. [6] S. Bernard, L. Youinou, P. Gillard, MIE determination and thermal degradation study of PA12 polymer powder used for laser sintering, J. Loss Prev. Process Ind. 26 (2013) 1493–1500. [7] J. Zhang, X. Liu, Y. Wang, Y. Li, Q. Li, Experimental research on minimum ignition temperature of 7-ACA dust cloud, Procedia Eng. 84 (2014) 467–471. [8] W. Gao, J. Yu, X. Zhang, J. Li, B. Wang, Characteristics of vented nano-polymethyl methacrylate dust explosions, Powder Technol. 283 (2015) 406–414. [9] E.K. Addai, D. Gabel, M. Kamal, U. Krause, Minimum ignition energy of hybrid mixtures of combustible dusts and gases, Process Saf. Environ. Prot. 102 (2016) 503–512. [10] W. Kowhakul, H. Shibahara, H. Masamoto, M. Shigematsu, Dust explosion characteristics of cellulose ethers and cellulose acetates with various degrees of acetylation, J. Loss Prev. Process Ind. 44 (2016) 544–550. [11] X. Zhang, Q. Shen, X. Shen, Z. Zhang, S. Xu, S. Ye, Minimum ignition energy of medicinal powder e Florfenicol and Tilmicosin, J. Loss Prev. Process Ind. 39 (2016) 30–38. [12] P.R. Amyotte, Solid inertants and their use in dust explosion prevention and mitigation, J. Loss Prev. Process Ind. 19 (2006) 161–173. [13] K.J. Mintz, M.J. Bray, D.J. Zuliani, P.R. Amyotte, M.J. Pegg, Inerting of fine metallic powders, J. Loss Prev. Process Ind. 9 (1996) 77–80. [14] K. Chatrathi, J. Going, Dust deflagration extinction, Process Saf. Prog. 19 (2000) 146–153. [15] A. Dastidar, P. Amyotte, J. Going, K. Chatrathi, Inerting of coal dust explosions in laboratory- and intermediate-scale chambers, Fuel 80 (2001) 1593–1602. [16] T.J. Myers, Reducing aluminum dust explosion hazards: case study of dust inerting in an aluminum buffing operation, J. Hazard. Mater. 159 (2008) 72–80. [17] N. Kuai, W. Huang, J. Yuan, B. Du, Z. Li, Y. Wu, Experimental investigations of coal dust-inertant mixture explosion behaviors, Procedia Eng. 26 (2011) 1337–1345. [18] B. Du, W. Huang, N. Kuai, J. Yuan, Z. Li, Y. Gan, Experimental investigation on inerting mechanism of dust explosion, Procedia Eng. 43 (2012) 338–342. [19] C. Yuan, P.R. Amyott, M.N. Hossain, C. Li, Minimum ignition energy of nano and micro Ti powder in the presence of inert nano TiO2 powder, J. Hazard. Mater. 274 (2014) 322–330. [20] C. Yuan, P.R. Amyotte, M.N. Hossain, C. Li, Minimum ignition temperature of nano and micro Ti powder clouds in the presence of inert nano TiO2 powder, J. Hazard. Mater. 275 (2014) 1–9. [21] A. Janès, A. Vignes, O. Dufaud, D. Carson, Experimental investigation of the influence of inert solids on ignition sensitivity of organic powders, Process Saf. Environ. Prot. 92 (2014) 311–323. [22] ISO 6184/1, Explosion Protection Systems–Part 1: Determination of Explosion Indices of Combustible Dusts in Air, International Organization for Standardization, 1985. [23] M. Mittal, Explosion characteristics of micron- and nano-size magnesium powders, J. Loss Prev. Process Ind. 27 (2014) 55–64. [24] BS EN 13821, Potentially Explosive Atmosphere. Explosion Prevention Andprotection. Determination of Minimum Ignition Energy of Dust/air Mixtures, British Standards Institution, 2002. [25] K.S. Lim, S.T. Bee, L.T. Sin, T.T. Tee, C.T. Ratnam, D. Hui, A.R. Rahmat, A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites, Compos. Part B 84 (2016) 155–174. [26] Q. Li, B. Lin, W. Li, C. Zhai, C. Zhu, Explosion characteristics of nano-aluminum powder-air mixtures in 20L spherical vessels, Powder Technol. 212 (2011) 303–309. [27] D. Castellanos, V.H. Carreto-Vazquez, C.V. Mashuga, R. Trottier, A.F. Mejia, M.S. Mannan, The effect of particle size polydispersity on the explosibility characteristics of aluminum dust, Powder Technol. 254 (2014) 331–337. [28] K.L. Cashdollar, I.A. Zlochower, Explosion temperatures and pressures of metals and other elemental dust clouds, J. Loss Prev. Process Ind. 202 (2007) 337–348. [29] A. Denkevits, S. Dorofeev, Dust explosion hazard in ITER: explosion indices of fine graphite and tungsten dusts and their mixtures, Fusion Eng. Des. 75–79 (2005) 1135–1139. [30] A.G. Dastidar, P.R. Amyotte, J. Going, K. Chatrathi, Flammability limits of dusts: minimum inerting concentrations, Process Saf. Prog. 18 (1999) 56–63. [31] BS 5958-1-1991, Code of Practice for Control of Undesirable Static Electricity. General Considerations, 1991. [32] K. Dai, L. Song, S. Jiang, B. Yua, W. Yang, R.K.K. Yuen, Y. Hu, Unsaturated polyester resins modified with phosphorus-containing groups: effects on thermal properties and flammability, Polym. Degrad. Stab. 98 (2013) 2033–2040. [33] J.R. Ebdon, D. Price, B.J. Hunt, P. Joseph, F. Gao, G.J. Milnes, L.K. Cunliffe, Flame retardance in some polystyrenes and poly(methyl methacrylate)s with covalently bound phosphorus-containing groups: initial screening experiments and some laser pyrolysis mechanistic studies, Polym. Degrad. Stab. 69 (2000) 267–277.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Research Paper
Experimental investigation of the inerting effect of crystalline II type Ammonium Polyphosphate on explosion characteristics of micron-size Acrylates Copolymer dust Yuan Yu ∗ , Yunhao Li, Qingwu Zhang ∗ , Weishun Ni, Juncheng Jiang Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control, College of Safety Science and Engineering, Nanjing Tech University, Mail Box 13, No. 200 North Zhongshan Rd., Nanjing, 210009, China
h i g h l i g h t s • • • • •
Explosion severity of ACR powders is confirmed to be class St–3. ACR powders exhibit MEC 20–30 g/m3 and MIE 10 mJ for d(50) = 4.4 m. There existed an effective MIC. And ACR dust explosion can be inerted completely by 80 wt% APP-II. 30 and 40 wt% APP-II had significantly inerting effect on the MIE of ACR dust. ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II.
a r t i c l e
i n f o
Article history: Received 13 April 2017 Received in revised form 1 September 2017 Accepted 29 October 2017 Available online 31 October 2017 Keywords: Inerting effect Crystalline II type Ammonium Polyphosphate Micron-size Acrylates Copolymer Inerting mechanism Chemical interaction
a b s t r a c t The inerting effect of crystalline II type Ammonium Polyphosphate (APP-II) on explosion characteristics of micron-size Acrylates Copolymer (ACR) powders was experimentally studied. The inerting mechanism was analysed by combining thermogravimetry (TG) and differential scanning calorimetry (DSC) tests. The results indicated that the maximum explosion pressure (Pmax ) and explosion index (Kst ) was 10.4 bar and 416 bar m/s, respectively for ACR powders. The minimum explosion concentration (MEC) of ACR powders ranged from 20 to 30 g/m3 , and the experimental minimum ignition energy (MIE) of the ACR dust cloud was 10 mJ. Therefore, ACR dust was determined to be severely combustible dust. There existed a minimum inerting concentration (MIC), and the explosion of ACR powders can be inerted completely by 80 wt% APP-II. Furthermore, 30 and 40 wt% APP-II had a significant inerting effect on the MIE of ACR dust. According to TG and DSC tests, thermal stability of ACR would be augmented by the introduction of APP-II. The addition of APP-II triggered lower maximum mass loss rate (MMLR), higher temperature corresponding to mass loss of 90% (T0.1 ), chars yield, and endothermic peaks. Consequently, the ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Dust explosions present substantial threats to people, assets, and the environment [1], such as the catastrophic aluminium-alloy dust explosion that killed 146 persons in China [2]. Table 1 lists several serious disasters caused by polymer dust explosions in the
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Yu), [email protected], [email protected] (Q. Zhang). https://doi.org/10.1016/j.jhazmat.2017.10.060 0304-3894/© 2017 Elsevier B.V. All rights reserved.
process industry [1,3]. Obviously, it is crucial to explore the explosion characteristics of combustible polymer dust. Polymers, which are polymerized by two or more than two monomers, are widely used in the chemical industry, aviation industry, synthetic rubber industry, and special materials. Currently, with the increasing utilization of polymers, and with many polymers being processed by cutting, grinding, or machining, polymer dust may accumulate to an explosive concentration if there are no effective prevention measures in workshops. Previous studies have proven that polymer dust clouds have explosion hazards if there has existed sufficient ignition energy and oxy-
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
559
Table 1 Accidents caused by polymer dust explosion [1,3]. date
country
Fatalities (F)/Injuries (I)
material
1993 1996 1999 2002 2003 2003 2003 2004 2011
USA USA USA USA USA USA USA USA USA
2 F/2 I 2F 2F 13 F 38 F/6 I 6 F/10 I 38 F/6 I 5 F/0 I 2 F/0 I
Plastic Rubber Plastic Rubber Rubber Powdered plastic Rubber Polyvinyl chloride Methomyl pesticide
gen concentration, such as fibrous nylon, fibrous polyethylene, polyamide 12 (PA12), 7-aminocephalosporanic acid, polymethyl methacrylate (PMMA), protein, polypropylene, cellulose ethers and cellulose acetates with different degrees of acetylation, Florfenicol, Tilmicosin, etc. [4–11]. Those studies indicated that polymer dust explosibility can be affected by particle shape, particle size, and dust concentration. Moreover, the maximum explosion pressure (Pmax ), minimum explosion concentration (MEC), and minimum ignition energy (MIE) of micron-size polymer dust can reach 9 bar, 10 g/m3 , and 10 mJ, respectively. Acrylates Copolymer (ACR), a type of polymer, is widely used as anti-blocking agents for films, anti-scratch and anti-matting agents for paintings or inks, special additives for cosmetics, and light diffusers for electronic displays, among other uses. Therefore, ACR dust can be generated in the process of shattering, classification, and meterage, etc. The study of explosion characteristics of ACR dust can give mitigative and preventive guidance of ACR dust explosion for relevant enterprises. To reduce the severity of dust explosions in confined spaces, venting, isolation, explosion suppression, and inerting have been proposed [12]. Explosion inerting, which is an inherent safety approach, consists of inert dust being premixed with the explosible dust before ignition. A dust explosion is thereby prevented or mitigated by inert dust. Such mixing has been proven to effectively reduce the risk of dust explosion. Mintz et al. [13] experimentally demonstrated that 50:50 Al-Mg alloy powders can be inerted completely by the weight between 70 and 75% of fine MgO dust. Chatrathi and Going [14] found the minimum inerting concentrations (MIC) of sodium bicarbonate (SBC), mono-ammonium phosphate (MAP) and rock dust (RD). Dastidar et al. [15] experimentally studied the inerting effect of rock dust in 1-m3 and 20-L vessels on coal dust. The results indicated that the inerting levels in the 1-m3 vessel were lower than that in the 20-L chamber. Myers [16] determined a proprietary flame retardant, an aqueous halogenated compound, was effective to residue created in an aluminium buffing operation to reduce the dust explosion hazard throughout the buffing process and within the dust collector systems. Kuai et al. [17], Du et al. [18] demonstrated the inerting effects of calcium carbonate (CaCO3 ), monoammonium phosphate (NH4 H2 PO4 ) on coal dusts. Yuan et al. [19,20] tested the inerting effect of nano TiO2 powders on MIE and minimum ignition temperature (MIT) of nano and micron Ti powder clouds. Janès et al. [21] investigated the MIE and MIT of several organic dusts in the presence of kieselguhr, alumina, and aerosol. Those studies provided a lot of useful information for safe design of dust explosion inerting. Therefore, we tried determining the inerting effect of crystalline II type Ammonium Polyphosphate (APP-II) what was widely used as a flame retardant on ACR dust. The current work is devoted to testing the explosion characteristics of ACR powders, along with the inerting effect of APP-II on ACR powders. The inerting mechanism of APP-II was studied according to thermogravimetry (TG) and differential scanning calorimetry (DSC) tests.
Fig. 1. The 20 L spherical experimental apparatus. Notes: 1–Water outlet, 2–Pressure sensor, 3–Manometer, 4–Dust container, 5–Air inlet, 6–Ignition source, 7–Rebound nozzle, 8–Fast acting valve, 9–Water inlet, 10–Outlet (air, reaction products).
Fig. 2. Pressure–time curve recorded during the dust explosion test for the dust concentration of 400 g/m3 .
2. Methodology 2.1. Experimental apparatus for explosion severity parameters A standardized 20-L stainless steel spherical vessel (Fig. 1), recommended by International Standard ISO 6184/1 [22], was selected to test the Pmax , maximum rate of pressure rise (dP/dt)max of ACR powders. Before an explosion test, an igniter was fixed at centre of the chamber. Moreover, the safely sealed explosion vessel was vacuumed in part to −0.6 bar (gauge) and the pressure of pressurized air in the dust storage tank was 20 bar (gauge). The ignition energy of chemical igniters which was ignited after delaying 60 ms, was 2 kJ. The measurement range of the pressure sensor was 0–250 psi (17 bar). A normal curve of pressure–time evolution noted during the ACR dust explosion is revealed in Fig. 2. Pressure evolution first began at −0.6 bar (gauge). The air blast applied for dispersing powders then began at 160 ms. Igniting finally started at 220 ms at an ambient condition of 1 bar (absolute). The value of Pex , for a single
560
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565 Table 2 Material properties of ACR powders. Supplier
Particle size distribution (m)
Specific surface area (m2 /g)
Suzhou Soken Chemical Co., Ltd
d(10) = 3.097 m d(50) = 4.339 m d(90) = 6.075 m
1.43
trol unit. The spacing between electrodes was 6 mm. In addition, the delay time between dispersion and the ignition was 90 ms. Based on EN 13821: 2002, if the dust was ignited and flame propagation appeared, a dust explosion was successfully induced. If the dust was non-ignitable in ten continual tests, this ignition energy could not induce a dust cloud explosion. 2.3. Material
Fig. 3. Schematic of the modified Hartmann tube.
test at a given concentration is the highest explosion overpressure (gauge). Moreover, (dP/dt)ex is the maximum rate of pressure rise in one test [23]. According to (dP/dt)max and the volume of the spherical chamber (V), Kst an international common value can be calculated by Eq. (1):
Kst = dP/dt
max
× V -3
(1)
ACR dust samples tested were produced by a supplier. Table 2 summarizes the material properties of ACR powders. Fig. 4 shows the shapes of ACR powders under a microscope. It can be seen that ACR powders are regular and spherical particles. Meanwhile, the particle size distribution varied within a narrow range. Ammonium polyphosphate (APP), a synthesis of polyphosphoric acid and ammonia is used as a kind of flame retardant due to its lower cost and prominent processability. Furthermore, owing to no halogen, most of all, APP does not create a supernumerary amount of smokes, which is environmental-friendly compared with halogenic flame retardants. Moreover, APP contains high levels of phosphorus (P) and nitrogen (N) [21,25]. In this study, APP-II is selected as the inert agent. APP-II is a type of flame retardant which is stable and almost insoluble. Table 3 lists the major properties of APP-II.
2.2. Experimental apparatus for MIE 2.4. Sample preparation for solid mixtures According to the European Standard EN 13821: 2002 [24], a modified 1.2-l Hartmann tube was used for ignition energy tests. Fig. 3 shows a schematic diagram of the modified Hartmann apparatus. The apparatus is comprised of a Hartmann tube, control system, high voltage unit, dust diffusion unit, and electronic pneumatic con-
ACR and APP-II powders were first dried by a vacuum drying box for eight hours at a low temperature of 30 ◦ C. Then, pre-weight ACR and APP-II powders for a given proportion were placed in a beaker. Finally, ACR and APP-II powders were fully mixed by
Fig. 4. Shapes of ACR powders of under the microscope.
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
561
Table 3 Properties of APP-II. P2 O5 content (wt%)
N content (wt%)
Density (g/cm3 )
Particle size (m)
Degree of polymerization (n)
≥72
≥14
1.9
5∼15
≥1000
manual stirring for five minutes, which has been justified to yield homogeneous mixtures effectively by Yuan et al. [19]. 3. Results and discussion 3.1. Inerting effect of APP-II on the explosion severity of ACR powders The explosion severity can be characterized by Pmax and Kst . It is vital for safety design and explosion prevention to understand the explosion severity parameters of ACR powders. The concentrations of ACR powders for tests were 200, 400, 600, 800, 1000, and 1200 g/m3 . Moreover, the ignition energy of chemical igniters was 2 kJ. The evolution of Pmax and Kst vs. dust concentration is illustrated in Figs. 5 and 6, respectively. It can be seen that the dust concentrations for the maximum values of Pmax and Kst are equal to 600 and 400 g/m3 respectively. Note that the maximum values of Pmax and Kst were 10.4 bar and 416 bar m/s, correspondingly for ACR powders. Pmax and Kst increased first and then decreased as dust concentration increased. The experimental results were in line with some common combustible dusts, such as aluminium, coals, and polyethylene [26,5,27]. Obviously, the reason is that the ACR powders participating in the explosion increased with the increase of dust concentration of ACR powders when the dust concentration was lower than 600 g/m3 . The ACR powders reacted fully with oxygen under the dust concentration of 600 g/m3 . Once the ACR dust concentration exceeded 600 g/m3 , there was too much ACR powder to react with oxygen. Therefore, the ACR powders that were not participating in the reaction absorbed the heat released from the chemical reaction. Furthermore, the temperature in the flame front decreased. Moreover, the heat release rate and burning rate both dropped. Accordingly, the explosion strength decreased as the ACR dust concentration increased.
Fig. 5. Pmax vs. dust concentration.
Table 4 summarizes the explosion characteristics of some common dusts with similar particle size. By contrast, it can be seen that the explosion severity of ACR powders was next only to the magnesium powders. Moreover, the explosion damage of ACR powders was much stronger than other dusts. Furthermore, it can also be seen that the experimentally measured MEC of ACR powders ranges from 20 to 30 g/m3 . Therefore, an ACR dust explosion can occur at a very low concentration. The MEC of ACR dust was almost smaller than any other common dust for similar particle size distribution. Table 5 presents the dust explosion class standard in accordance with ISO6184/1 [22]. According to the ISO6184/1, the explosion severity of ACR powders fell into class St–3, with severe explosive power. Due to the explosion risk of ACR dust, when designing the plants for producing and processing ACR products, preventive and
Table 4 Explosion characteristics for some dust samples summarized from the literatures. Dust
D50 , m
MEC, g/m3
Pmax , bar
Kst , bar m/s
Reference
B C-1 C-2 C-3 Mg Al-1 Al-2 Al-3 Si Cr Fe Ni Zn Mo Sn Hf Ta W-1 W-2 Mg Mg Graphite W ACR
∼3 ≤1 ∼1 ∼4 ∼16 ∼1 ∼7 ∼15 ∼4 ∼10 ∼4 ∼6 ∼4 ∼5 ∼8 ∼8 ∼10 ≤1 ∼10 1 10 4 1 ∼4
∼110 90 F NF 55 85 90 90 200 ∼F 220 NF 300 NF ∼450 ∼180 ∼400 ∼700 NF 30 40 70 ∼ 20∼30
7.0 5.5 ∼5 1.1 8.5 9.4 9.9 7.5 7.7 ∼3 4.5 1.0 4.4 1.0 4.3 5.2 ∼4 ∼3.3 1.0 14 13.2 6.6 5.7 10.4
∼ ∼ ∼ ∼ ∼ ∼ ∼250 ∼80 ∼ ∼ ∼28 ∼12 ∼ ∼ ∼ ∼ ∼ ∼ ∼ 510 482 70.6 86.9 416
[28]
NF: nonflammable or nonignitable; F: MEC could not be determined.
[23] [29] Present study
562
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
Fig. 6. Kst vs. dust concentration. Fig. 8. Overpressure vs. time for different mass fraction of APP-II. Table 5 Dust explosion class standard in accordance with ISO 6184/1 [22]. Dust explosion class
Kst /(bar m/s)
St–1 St–2 St–3
0 < Kst ≤ 200 200 < Kst ≤ 300 300 < Kst
Fig. 9. Pmax of ACR dust vs. mass fraction of APP-II.
Fig. 7. Inerting envelope for ACR dust with APP-II.
mitigative measures for ACR dust explosion must be seriously considered. Meanwhile, avoiding the formation of flammable ACR dust clouds is a crucial task during production. According to the previous study by Dastidar et al. [30], there was not always a MIC for a type of combustible dust. To determine the mass fraction of completely inerting which can be applied for the safe design of explosion suppression, ACR dust explosion tests with various mass fraction of APP-II (the ratio of weight of APP-II powders and total dust weight) were carried out. Fig. 7 shows the inerting envelope for ACR dust. The ACR dust concentrations were between 200 and 800 g/m3 . It can be seen that the APP-II concentrations increased first and then decreased with the concentration of ACR dust increasing. There existed a MIC occurring at approximately 2400 g/m3 of APP-II corresponding to about 600 g/m3 of ACR dust. Fig. 8 depicts the overpressure evolution of 600, 3000 g/m3 of ACR dust and 600 g/m3 of ACR dust with different mass fraction of APP-II. Figs. 9 and 10 portray the variety of Pmax and Kst of 600 g/m3 of ACR powders with various mass fraction of APP-II respectively. It
Fig. 10. Influence of mass fraction of APP-II on the Kst of ACR dust.
can be seen that APP-II had a great inerting effect on ACR powders. As in Fig. 8, the time from ignition to overpressure peak increased for 600 g/m3 of ACR dust with the mass fraction of APP-II increasing. The result indicated that the burning rate dwindled as the
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
563
Table 6 Experimental results of MIE for ACR powders with different mass fraction of APP-II. Mass fraction of APP-II (wt%)
0 10 20 30 40
MIE (mJ) for different sample mass 240 mg
480 mg
720 mg
960 mg
1200 mg
1440 mg
190 300 800 >1000 >1000
90 95 120 590 >1000
60 80 85 480 >1000
30 50 60 220 990
10 20 55 230 1000
50 55 65 235 1000
Table 7 Ignition sensitivity for flammable dust [31]. MIE of a dust
Ignition sensitivity
500
Low sensitivity to ignition: Earth plant when ignition energy is at or below this level. Consider earthing personnel when ignition energy is at or below this level. The majority of ignition incidents occur when ignition energy is below this level. The hazard from electrostatic discharges from dust clouds should be considered. High sensitivity to ignition. Take the above precautions and consider restrictions on the use of high resistivity materials (plastics). Electrostatic hazard from bulk powders of high resistivity should be considered. Extremely sensitive to ignition. Precautions should be as for flammable liquids and gases when ignition energy is at or below this level.
100 25
10
1
Fig. 11. Effect of mass fraction of APP-II on MIEs of ACR dust.
mass fraction of APP-II increased. Moreover, the Pmax and Kst of ACR powders also reduced with the increase of mass fraction of APP-II. Additionally, an explosion was not triggered for ACR/APP-II mixture when the mass fraction of APP-II increased to 80 wt%. This means that the ACR powders were completely inerted by APP-II.
3.2. Inerting effect of APP-II on MIE of ACR powders Ignition sensitivity can be characterized scientifically by the MIE of a dust cloud. To understand the inerting effect of APP-II on MIE of ACR powders, the MIEs of ACR dust in the range of 240–1440 mg with different mass fraction of APP-II were experimentally investigated. Experimental results of MIE for ACR powders with different mass fraction of APP-II are summarized in Table 6. Fig. 11 shows the effect of mass fraction of APP-II on MIEs of ACR dust. It can be seen that the MIE of ACR powders decreased first and then increased with the increase of sample mass in the range of 240–1440 mg. Moreover, The ACR dust cloud can be ignited by the ignition energy of 10 mJ corresponding to the sample of 1200 mg, which is low enough to be sensitive to static electricity (Table 7 [31]). MIEs of ACR dust mixed with 10, 20, 30, and 40 wt% of APP-II were also experimentally tested. The results indicated that APPII had an inerting effect on ACR dust. For the mixture with 10 and 20 wt% of APP-II, MIEs of mixtures for the ACR sample mass ranging from 480 to 1440 mg increased slightly. While the mass fraction of APP-II increased to 30 wt%, MIEs of mixtures increased sharply. Mixtures for the ACR sample mass of 240 mg was nonignitable under the spark energy of 1000 mJ. Furthermore, no ignition occurred for the ACR sample mass of 240–720 mg with 40 wt% of APP-II. What’s more, the minimum value of MIE for ACR/APP-II mixtures was 990 mJ corresponding to the ACR sample mass of 960 mg with 40 wt% of APP-II.
Fig. 12. Calculated MIEs vs. mass fraction of APP-II.
To compare the inerting effects between different mass fraction of APP-II, the MIEs of ACR/APP-II mixtures were calculated by Eq. (2) [19]. MIE = 10log IE-NI ×
log IE-log NIE N+1
(2)
N and NI denote the test number and combustion number, correspondingly, at energy level IE. Fig. 12 reveals the inerting effect of the mass fraction of APP-II on ACR dust. It can be seen that the calculated MIEs of mixtures with 10 wt% APP-II was close to that of ACR dust. According to Table 7, the hazard triggered by electrostatic discharges for ACR dust clouds with 10 wt% of APP-II also should be considered. Although 30 and 40 wt% APP-II had a significant inerting effect on the calculated MIEs of mixtures, explosion severity parameters were still severe (Figs. 9 and 10). Therefore, explosibility still should be considered carefully.
564
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
Fig. 13. Mass loss of ACR powders vs. temperature with various mass fraction of APP-II.
Fig. 14. Derivative mass to loss of ACR powders vs. temperature with various mass fraction of APP-II.
3.3. Mechanism analysis Fig. 13 shows the mass loss results of ACR powders, APP-II powders and ACR/APP-II mixtures according to TG tests under air condition. Derivative mass to loss for ACR powders vs. temperature with various mass fraction of APP-II is plotted in Fig. 14. It can be seen that ACR powders had a mass loss of approximately 255 ◦ C. All the ACR powders decomposed completely at around 500 ◦ C. Furthermore, the temperature of maximum mass loss rate (Tmax ) was 314 ◦ C. Moreover, the mass loss of APP-II powders mainly occurred in two stages. The first stage was triggered from about 275 ◦ C to 425 ◦ C. While the second stage occurred in the range of 500–675 ◦ C. Besides, Tmax for two stages were 346 and 615 ◦ C, correspondingly. The residue of APP-II was 28% at 785 ◦ C. It can also be seen that ACR/APP-II mixtures had two stages of mass loss. The temperature corresponding to mass loss of 90% (T0.1 ) was commonly selected as a criterion for judging the thermal stability [32]. The T0.1 for ACR, 20 wt% APP-II, and 40 wt% APP-II was 286, 305, and 311 ◦ C, respectively. In addition, it was clearly observed that the maximum mass loss rate (MMLR) dropped when a different amount of APP-II was added. The amount of residue increased when the mass fraction of APP-II increased. The results indicated that the thermal stability of ACR could be augmented by the introduction of APP-II. To evaluate the mutual interactions of ACR/APP-II mixtures, a comparison of experimental and theoretical mass losses of ACR/APP-II mixtures was conducted. Assuming that ACR and APP-
Fig. 15. Experimental and theoretical TG curves of ACR with 40 wt% APP-II in air atmosphere.
II decompose independent of each other, the theoretical mass loss was calculated by a linear combination of the experimental values of both materials, in accordance with their ratio [32]. It can be seen from Fig. 15 that the onset temperature for theoretical mass loss was higher than that for the experimental result. Meanwhile, the theoretical T0.1 was 17 ◦ C higher than the experimental value. The experimental mass loss was greater than the theoretical result until approximately 555 ◦ C. The result indicates that APP-II was chemically interacting with the ACR during this decomposition stage. Furthermore, the residue for the experimental value was greater than the theoretical value, denoting a significant catalytic effect on the char formation of ACR. In the case of the condensed phase action of phosphate containing systems, the better the charring properties, the higher the char yield. Thus, more carbonaceous char is left behind during decomposition [33]. Consequently, the char layer can prevent contact between O2 and ACR powders. The thermal conduction was decreased by the char layer, too. Moreover, the decomposition and volatilization of ACR powders was prevented during explosion. To discuss the thermal impact of APP-II on ACR decomposition, as shown in Fig. 16, the TG and DSC tests of ACR and ACR/APPII mixtures were carried out. It can be seen that there were two endothermic decomposition peaks for ACR and ACR/APP-II mixtures. The first endothermic peak may be the decomposition of side groups of ACR. The second endothermic peak was most likely due to the decomposition of the backbone of ACR. After the introduction of APP-II, the first endothermic peak remained at 309 ◦ C. However, the endothermic peak was sharper and stronger. The second endothermic peak increased greatly from 356 to 420 ◦ C compared to the neat ACR, resulting from the decomposition of APP-II, which absorbed significant heat. The results indicate that the thermal decomposition of ACR was inerted by APP-II. Above all, the results demonstrated that the introduction of APP-II into ACR triggered lower MMLR, higher T0.1 , char yield, and endothermic peaks. Consequently, the ACR dust explosion was inerted by the chemical interaction of ACR/APP-II mixtures and endothermic decomposition of APP-II. 4. Conclusions The Pmax and Kst were 10.4 bar and 416 bar m/s, respectively for the ACR powders. The MEC of ACR powders ranged from 20 to 30 g/m3 . Therefore, ACR powders were a kind of combustible dust. Moreover, the explosion severity of ACR powders fell into class St–3, with severe explosive power. ACR/APP-II mixtures showed an effective MIC occurring at 2400 g/m3 of APP-II in the concentra-
Y. Yu et al. / Journal of Hazardous Materials 344 (2018) 558–565
Fig. 16. TG and DSC curves for ACR and ACR with 40 wt% APP-II in N2 atmosphere: (a) TG; (b) DSC.
tion ranges tested. The explosion of ACR powders can be inerted completely by 80 wt% APP-II. The MIE of the ACR dust cloud was 10 mJ, which is low enough to be sensitive to static electricity. Furthermore, 30 and 40 wt% APPII had a significant inerting effect on the ACR dust explosion. The minimum value of MIE for mixtures with 40 wt% APP-II was 990 mJ. After the introduction of APP-II, the T0.1 increased from 286 to 305 and 311 ◦ C for 20 and 40 wt% APP-II respectively. MMLR decreased as the ratio of APP-II increased. Meanwhile, the chemical interaction between ACR and APP-II led to higher char yield. The char layer acted as a thermal barrier. Furthermore, the second endothermic peak was increased greatly from 356 to 420 ◦ C due to the thermal inerting of the endothermic decomposition of APP-II. Acknowledgements The authors are grateful for the financial support given by the National Key Research and Development Plan under grant no. 2016YFC0800102 and the State Key Program of National Natural Science of China under grant no. 2143 6006, along with the open project of Jiangsu Key Laboratory of Hazardous Chemicals Safety and Control. References [1] Z. Yuan, N. Khakzad, F. Khan, P. Amyotte, Dust explosions: a threat to the process industries, Process Saf. Environ. Prot. 98 (2015) 57–71. [2] G. Li, H.X. Yang, C.M. Yuan, R.K. Eckhoff, A catastrophic aluminium-alloy dust explosion in China, J. Loss Prev. Process Ind. 39 (2016) 121–130.
565
[3] S.Y. Lin, T.H. Lin, Y.C. Cheng, K.-H. Hsueh, C.-M. Shu, Assessment of dust explosion with adipic acid and p-terephthalic acid in the powdered resin process, J. Loss Prev. Process Ind. 43 (2016) 92–97. [4] L. Marmo, D. Cavallero, Minimum ignition energy of nylon fibres, J. Loss Prev. Process Ind. 21 (2008) 512–517. [5] P.R. Amyotte, C.T. Cloney, F.I. Khan, R.C. Ripley, Dust explosion risk moderation for flocculent dusts, J. Loss Prev. Process Ind. 25 (2012) 862–869. [6] S. Bernard, L. Youinou, P. Gillard, MIE determination and thermal degradation study of PA12 polymer powder used for laser sintering, J. Loss Prev. Process Ind. 26 (2013) 1493–1500. [7] J. Zhang, X. Liu, Y. Wang, Y. Li, Q. Li, Experimental research on minimum ignition temperature of 7-ACA dust cloud, Procedia Eng. 84 (2014) 467–471. [8] W. Gao, J. Yu, X. Zhang, J. Li, B. Wang, Characteristics of vented nano-polymethyl methacrylate dust explosions, Powder Technol. 283 (2015) 406–414. [9] E.K. Addai, D. Gabel, M. Kamal, U. Krause, Minimum ignition energy of hybrid mixtures of combustible dusts and gases, Process Saf. Environ. Prot. 102 (2016) 503–512. [10] W. Kowhakul, H. Shibahara, H. Masamoto, M. Shigematsu, Dust explosion characteristics of cellulose ethers and cellulose acetates with various degrees of acetylation, J. Loss Prev. Process Ind. 44 (2016) 544–550. [11] X. Zhang, Q. Shen, X. Shen, Z. Zhang, S. Xu, S. Ye, Minimum ignition energy of medicinal powder e Florfenicol and Tilmicosin, J. Loss Prev. Process Ind. 39 (2016) 30–38. [12] P.R. Amyotte, Solid inertants and their use in dust explosion prevention and mitigation, J. Loss Prev. Process Ind. 19 (2006) 161–173. [13] K.J. Mintz, M.J. Bray, D.J. Zuliani, P.R. Amyotte, M.J. Pegg, Inerting of fine metallic powders, J. Loss Prev. Process Ind. 9 (1996) 77–80. [14] K. Chatrathi, J. Going, Dust deflagration extinction, Process Saf. Prog. 19 (2000) 146–153. [15] A. Dastidar, P. Amyotte, J. Going, K. Chatrathi, Inerting of coal dust explosions in laboratory- and intermediate-scale chambers, Fuel 80 (2001) 1593–1602. [16] T.J. Myers, Reducing aluminum dust explosion hazards: case study of dust inerting in an aluminum buffing operation, J. Hazard. Mater. 159 (2008) 72–80. [17] N. Kuai, W. Huang, J. Yuan, B. Du, Z. Li, Y. Wu, Experimental investigations of coal dust-inertant mixture explosion behaviors, Procedia Eng. 26 (2011) 1337–1345. [18] B. Du, W. Huang, N. Kuai, J. Yuan, Z. Li, Y. Gan, Experimental investigation on inerting mechanism of dust explosion, Procedia Eng. 43 (2012) 338–342. [19] C. Yuan, P.R. Amyott, M.N. Hossain, C. Li, Minimum ignition energy of nano and micro Ti powder in the presence of inert nano TiO2 powder, J. Hazard. Mater. 274 (2014) 322–330. [20] C. Yuan, P.R. Amyotte, M.N. Hossain, C. Li, Minimum ignition temperature of nano and micro Ti powder clouds in the presence of inert nano TiO2 powder, J. Hazard. Mater. 275 (2014) 1–9. [21] A. Janès, A. Vignes, O. Dufaud, D. Carson, Experimental investigation of the influence of inert solids on ignition sensitivity of organic powders, Process Saf. Environ. Prot. 92 (2014) 311–323. [22] ISO 6184/1, Explosion Protection Systems–Part 1: Determination of Explosion Indices of Combustible Dusts in Air, International Organization for Standardization, 1985. [23] M. Mittal, Explosion characteristics of micron- and nano-size magnesium powders, J. Loss Prev. Process Ind. 27 (2014) 55–64. [24] BS EN 13821, Potentially Explosive Atmosphere. Explosion Prevention Andprotection. Determination of Minimum Ignition Energy of Dust/air Mixtures, British Standards Institution, 2002. [25] K.S. Lim, S.T. Bee, L.T. Sin, T.T. Tee, C.T. Ratnam, D. Hui, A.R. Rahmat, A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites, Compos. Part B 84 (2016) 155–174. [26] Q. Li, B. Lin, W. Li, C. Zhai, C. Zhu, Explosion characteristics of nano-aluminum powder-air mixtures in 20L spherical vessels, Powder Technol. 212 (2011) 303–309. [27] D. Castellanos, V.H. Carreto-Vazquez, C.V. Mashuga, R. Trottier, A.F. Mejia, M.S. Mannan, The effect of particle size polydispersity on the explosibility characteristics of aluminum dust, Powder Technol. 254 (2014) 331–337. [28] K.L. Cashdollar, I.A. Zlochower, Explosion temperatures and pressures of metals and other elemental dust clouds, J. Loss Prev. Process Ind. 202 (2007) 337–348. [29] A. Denkevits, S. Dorofeev, Dust explosion hazard in ITER: explosion indices of fine graphite and tungsten dusts and their mixtures, Fusion Eng. Des. 75–79 (2005) 1135–1139. [30] A.G. Dastidar, P.R. Amyotte, J. Going, K. Chatrathi, Flammability limits of dusts: minimum inerting concentrations, Process Saf. Prog. 18 (1999) 56–63. [31] BS 5958-1-1991, Code of Practice for Control of Undesirable Static Electricity. General Considerations, 1991. [32] K. Dai, L. Song, S. Jiang, B. Yua, W. Yang, R.K.K. Yuen, Y. Hu, Unsaturated polyester resins modified with phosphorus-containing groups: effects on thermal properties and flammability, Polym. Degrad. Stab. 98 (2013) 2033–2040. [33] J.R. Ebdon, D. Price, B.J. Hunt, P. Joseph, F. Gao, G.J. Milnes, L.K. Cunliffe, Flame retardance in some polystyrenes and poly(methyl methacrylate)s with covalently bound phosphorus-containing groups: initial screening experiments and some laser pyrolysis mechanistic studies, Polym. Degrad. Stab. 69 (2000) 267–277.