When solids meet solids: A glimpse into dust mixture explosions

Journal of Loss Prevention in the Process Industries 25 (2012) 853e861

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When solids meet solids: A glimpse into dust mixture explosions Olivier Dufaud*, Laurent Perrin, David Bideau, André Laurent Laboratoire Réactions et Génie des Procédés, Université de Lorraine, UPR 3349 CNRS, 1 rue Grandville, B.P. 20451, F-54001 Nancy, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 December 2011 Received in revised form 26 April 2012 Accepted 26 April 2012

Mixing an inert solid or a less flammable compound with a combustible dust can be regarded as a direct application of the inherent safety principle of moderation. An experimental investigation was carried out to determine the evolution of the ignition sensitivity and the explosion severity of such various mixtures as a function of their compositions. It demonstrates that the introduction of small amounts of highly combustible powders (such as sulphur or nicotinic acid) to a less flammable dust (such as microcrystalline cellulose or carbon black) can strongly influence the ignition sensitivity as well as the explosion severity. It has notably been shown that the ignition sensitivity of solid/solid mixtures significantly rises up when only 10e5%wt. of highly flammable dust is introduced. Simple models can often be applied to estimate the minimum ignition energy, minimum ignition temperature and minimum explosive concentration of such mixtures. Concerning the dust explosivity, three cases have been studied: mixtures of combustibles dusts without reaction, dusts with reactions between the powders, combustible dusts with inert solid. If the evolution of the maximum explosion pressure can be estimated by using thermodynamic calculations, the maximum rate of pressure rise is more difficult to predict with simple models, and both combustion kinetics and hydrodynamics of the dust clouds should be taken into account. These results were also extended to flammable dust/solid inertant mixture. They clearly show that the concentration of solid inertant at which the ignition is not observed anymore could reach 95%wt. As a consequence, the common recommendation of solid inertant introduction up to 50e80%wt. to prevent dust explosion/ignition should be reconsidered. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Solid/solid mixtures Dust explosion Ignition Solid inertant Explosion prevention

1. Introduction The management of risks related to a process is usually ensured by identifying means of prevention and by proposing protection barriers in order to lower both the probability of occurrence and the loss severity. The approach proposed by Trevor Kletz (Kletz, 1978, 1998) is fundamentally different and consists in controlling/reducing the risks at source and from the process design stage by using the inherent safety principles. According to the standard ISO 12100 (ISO, 2010), inherently safe design measures are the first and most important step in the riskreduction process. Conversely to the application of prevention and protection barriers, the inherent safety concept implies reducing the risk at the early stages of the plant design. It is based on four main principles: minimization, substitution, simplification and moderation (Kletz, 1978, 1998).

* Corresponding author. Tel.: þ33 (0)3 83 17 53 33; fax: þ33 (0)3 83 32 29 75. E-mail address: [email protected] (O. Dufaud). 0950-4230/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jlp.2012.04.011

However, if the potential benefits of inherent safety are generally well recognized, its systematic application remains marginal and sometimes difficult (Bollinger et al., 1996; Khan & Amyotte, 2003). The promotion of this concept must then be done both by industrialists and legislators, especially by the definition of new normative barriers based on these principles (Muñoz Giraldo, 2007). Since the early 2000s, Amyotte and Khan (2002) have applied these principles to the prevention of dust explosions. Amyotte, Pegg, and Khan (2009) described in great detail how the inherent safety principles can be implemented in practice to prevent and mitigate accidental dust explosions in process plants, notably by using the moderation principle (Amyotte, Pegg, Khan, Nikufu, & Yingxin, 2007). Thus, mixing an inert solid or a less flammable compound with a combustible dust can be regarded as a direct application of such a principle, as it allows the use of the hazardous material in a less hazardous form. Industrial products, and especially powders, are frequently encountered as mixtures: mixtures of excipients and/or active principles in the pharmaceutical industries, of cereals in grains industries, of pigments in paintings, etc. Thus, theoretically, the

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Table 1 Particles-size distribution of the combustible dusts (in mm). Powders (providers)

d10 (mm)

d50 (mm)

d90 (mm)

Magnesium stearate (Sigma Aldrich) Microcrystalline cellulose MCC (Avicel) Niacin (Fluka) Acetaminophen (Merck) Sulphur (VWR) Lycopodium (Prolabo) Carbon black (Prolabo) Zirconium (Alfa Aesar)

3 25 12 e 54 23 3 1

6 108 26 60 110 32 9 5

15 255 104 e 205 41 64 12

industrial applications of such principle are numerous (addition of silica, sodium bicarbonate, limestone, etc.) (Amyotte, 2006). Practically, they are limited because the presence of foreign solids in the process is often undesirable in terms of product quality or, for instance, food safety. Nonetheless, the simultaneous presence of various combustible dusts with different flammability or explosivity parameters is more frequently encountered. If the flammability of such mixtures depends notably on the ignition source and energy, on the turbulence, it also greatly depends on the dust chemical nature and on the mixture composition. However, only few risks analyses take the evolution of parameters as the minimum ignition energy (MIE), the minimum ignition temperature (MIT) or the minimum explosive concentration (MEC) as a function of the mixture proportions into account. Similarly, in the field of dusts explosions, numerous literature data (MIE, MIT, MEC, etc.) are available, but only a few concerns solid/solid hybrid mixtures (BIA, 1997; Denkevits & Dorofeev, 2004; Eckhoff, 2003). Due to this lack of data, the safety parameters of the most sensitive compound, or of the one having the largest proportion, is frequently assigned to the mixture. Nevertheless, experiments and industrial practice have shown that even small amounts of highly flammable powders (down to 5%wt.) can fundamentally modify the mixture ignitability. This theme could also be compared to the one of gas/ dusts hybrid mixtures (Dufaud, Perrin, & Traoré, 2008; GarciaAgreda, Di Benedetto, Russo, Salzano, & Sanchirico, 2011). This article synthesizes the various works carried out by our laboratory on the determination of the ignition sensitivity and the explosion severity of solid/solid hybrid mixtures. The results will be compared to those obtained by other authors. When possible, simple correlations will be provided in order to intrapolate the safety parameters of pure dusts to those of solid/solid mixtures. 2. Materials and methods 2.1. Samples characteristics and preparation In order to ensure the repeatability and reproducibility of the experiments, a precise protocol was proposed. The powders were

systematically dried at 50  C under vacuum during 2 h before handling, and the ambient temperature and relative humidity were recorded. Dusts were homogeneously mixed thanks to a Turbula chaotic shake-miker (T2F e GlenMills). A mechanic grinder (M20 IKA Universal mill) was specifically used to study the influence of particle-size distributions on the ignition sensitivity of such mixtures. Various dusts were used to study the influence of powders mixtures on explosion sensitivity and severity. They were chosen due to their particle-size characteristics and to their flammability and explosion properties. Some of the powders chosen for this study are used in the pharmaceutical industries as excipients (or lubricant) e magnesium stearate and MCC (microcrystalline cellulose), as vitamin e B3 or nicotinic acid also called niacin, and as an active principle e pure acetaminophen. Sulphur was chosen for its low melting temperature, whereas lycopodium and carbon black were chosen with regard to their particle-size distributions. Nonflammable dusts as silica and sodium bicarbonate were also added to combustible powders. The particle-size distribution of each dust was determined by using a laser diffraction analyzer (Mastersizer, Malvern Instrument). The samples were characterized by the d10, d50 and d90 quantiles of the volumetric distribution as indicated on Table 1; the dx diameter, being defined as the size at which x% of the particles are smaller. Table 1 shows that the particles-size distributions of MCC and sulphur, especially, that their specific diameters d50 and d90 are rather similar. Nevertheless, due to their different states of surface, the Sauter mean diameter d32 of MCC, defined as the diameter of a sphere having the same volume to surface area ratio as the particle, is significantly lower than the one of the sulphur, respectively 8.5 and 87 mm. In fact, it was observed by scanning electron microscopy (SEM) that MCC had a stick-like structure, with an irregular surface, whereas sulphur particles are generally composed of agglomerates (Fig. 1a). It should be noticed that acetaminophen has a rod-like shape with a mean diameter of 20 mm and a mean length of 80 mm. The surface of lycopodium spores is irregular with typical tetrahedral forms (Fig. 2a). Finally, due its lamella structure, the magnesium stearate exhibits a high specific surface area (Fig. 1b), and carbon blacks powders tend to form larger agglomerates (Fig. 2b). It should be pointed out that thermogravimetric analyses of pure sulphur, MCC and of a 50%wt. mixture of both compounds reveal that the oxidation of the mixture occurs at a significantly lower temperature than those of the pure compounds: Tonset (S) values 200  C and Tonset (MCC) reaches 290  C, whereas Tonset (S/ MCC) decreases down to 170  C. In this peculiar case, the interactions between both compounds could apparently not be neglected.

Fig. 1. Scanning electron microscopy (SEM) observations of a) sulphur and of b) microcrystalline cellulose (MCC).

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Fig. 2. SEM observations of a) lycopodium and of b) carbon black.

2.2. Ignition sensitivity

3. Results and discussion

The ignition sensitivity of a flammable dust is characterized by the following three parameters: MIE, MIT and MEC. The minimum ignition energy was determined by using the modified Hartmann tube (Kühner AG) in accordance with IEC standard 1241-2-3. The minimum ignition temperature of a dust cloud was determined using a Godbert-Greenwald furnace (Chilworth Technology) in accordance with the principles of IEC standards 1241-2-1, whereas the MIT of a 5 mm dust layer was measured by hot plate tests (IEC 1241-2-1). Both ignition delays for the modified Hartmann tube and air overpressure for the Godbert-Greenwald furnace have been set in order to obtain the best dispersion conditions, i.e. the worstcase scenario for dust explosions. MEC has not been specifically determined for this study, but examples of MEC evolution for solid/ solid mixtures will be given. The experimental test involves dispersing dust samples in a vessel and attempting to ignite the resulting dust cloud with an energetic ignition source. Trials are repeated for decreasing sample sizes until the MEC is determined. MEC test is performed using the 20-L sphere apparatus in accordance with ISO 6184/1 or ASTM E1515. The results of the various tests are listed in Table 2 for the pure compounds. It demonstrates that magnesium stearate, niacin and especially sulphur could be considered as highly ignitable dusts, whereas microcrystalline cellulose and carbon black have far lower ignition sensitivities. Thus, for instance, mixtures of MCC and sulphur particles or niacin will be used in order to cover a wide range of MIE.

3.1. Influence of solid/solid mixtures on the minimum ignition energy Concerning the sulphur/MCC mixtures, the minimum ignition energies of the pure compounds are strongly different: 6 mJ for sulphur and 590 mJ for MCC. If the thermal properties of the powders (thermal conductivity ls, heat capacity Cps and thermal diffusivity) are the only parameters which are considered, one would expect a nearly linear evolution of the MIE over the wide range of dusts proportions. Thus, by using the Kalkert and Schecker’s correlation quoted below (Eckhoff, 2003; Kalkert & Schecker, 1979), the MIE of a 50%wt. mixture, i.e. 27%vol. sulphur, would be of 80 mJ when it reaches 14 mJ experimentally:

MIE ¼

4p$lair rair $Cpair

!3=2 $rair $Cpair $

  ln 2 rs $Cps 3=2 $Tad;flame $d3p $ 12 lair (1)

3

where r is the density in kg m , dp is the particle diameter in m and Tad,flame is the adiabatic flame temperature in K, which has been calculated for both compounds thanks to CEA software (Gordon & McBride, 1994). Fig. 3 depicts the minimum ignition energy of MCCeniacin and MCCesulphur mixtures as a function of MCC volume concentration. It could be clearly seen that the evolution of the MIE is weak on the

2.3. Explosion severity The measurements of dust explosion severity, i.e. the maximum overpressure (Pmax), and the maximum rate of pressure rise (dP/ dt)max, were performed in a 20 L spherical vessel in accordance with the ISO 6184-1 standard (Bartknecht, 1989). The ignition delay, i.e. the time between the onset of dust dispersion and the ignition of the dust/air mixture, was set at 60 ms as standardized for dust testing. Tests were performed with two pyrotechnic ignitors of 5 kJ each as ignition source. Table 2 MIT of dusts clouds and MIE of pure powders. Powders

MIT/cloud ( C)

MIE (mJ)

Magnesium stearate Microcrystalline cellulose Niacin Acetaminophen Sulphur Lycopodium Carbon black Zirconium

420 510 580 680 270 410 315 213

4 590 2.6 12 6 12 >1000 e

Fig. 3. Minimum ignition energy of MCCeniacin and MCCesulphur mixtures as a function of MCC volume concentration. Fitting with a harmonic model. Ignition delay set at 120 ms.

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range of 0e94%vol. MCC (i.e. up to 70%wt.): respectively, from 2.6 to 26 mJ for niacin mixtures and from 6 to 42 mJ for sulphur ones. On the contrary, a strong increase of the minimum ignition energy is observed for MCC concentrations greater than 94 or 96%vol. As a consequence, the MIE of a hybrid mixture does not follow a linear relationship and seems strongly influenced by the proportion of the highly flammable compound. For the MCC/sulphur mixtures, settling velocities were calculated at each characteristic quantile of the particle-size distributions thanks to Stokes (0.0001 < Re < 1) and Van Allen’s laws (1 < Re < 1000) corrected by RichardsoneZaki’s equations for a swarm of spherical particles settling in air. It has been found that considering notably the slight differences between sulphur and MCC particles-size distributions, the threshold phenomena could not be exclusively explained by segregation between particles during the dispersion phase and that the chemical and thermal properties of the powders have strong influences on the MIE of hybrid mixtures. Similar behaviours are observed for other mixtures such as magnesium stearate/lycopodium (Fig. 4) and are even more marked for flammable dust/solid inertant mixtures (Dufaud et al., 2009). On Fig. 4, the trend seems to be less pronounced for those compounds due to the slight difference between the MIE of pure dusts, but also to the use of mass concentrations. On Fig. 3, a harmonic model based on the volume content of the least flammable compound has also been used to represent the evolution of the MIE of a solid/solid mixture (for instance, sulphur S and MCC):

1 ð%vol:S Þ ð1  %vol:S Þ ¼ þ MIEmixture MIES MIEMCC

(2)

This kind of relationship is also used, like Maxwell or MaxwelleEucken’s equations, to represent the thermal conductivity of a solid/solid composite (Wang, Carson, North, & Cleland, 2006). It shows a good agreement with our experimental results. Within the context of gas/dust hybrid mixtures, some authors proposed to estimate the minimum ignition energy of such mixtures (HMIE) by assuming a semi-logarithmic relationship involving the gas concentration and the MIE ratio between the pure dust and the gas (Britton, 1998; Khalili, Dufaud, Poupeau, CuervoRodriguez, & Perrin, 2012):

     C MIEdust $ln HMIE ¼ exp lnðMIEdust Þ  C0 MIEgas

(3)

Fig. 4. Evolution of the minimum ignition energy of magnesium stearateelycopodium mixtures as a function of magnesium stearate concentration. Ignition delay set at 120 ms.

where MIEdust and MIEgas are respectively the MIE of the dust and the gas; C is the gas volume concentration and C0 is the gas concentration (%vol.) leading to the lowest MIE. This relationship is only valid for gas concentration lower than C0. If we extrapolate this relationship to solid/solid mixtures, C0 of approximately 5%vol. are obtained for both MCC/sulphur and MCC/niacin mixtures. It can then be considered that, from these concentrations, sulphur and niacin tend to impose their behaviour. 3.2. Effect of solid/solid mixtures on the minimum ignition temperature of a dust cloud Fig. 5 represents the influence of sulphur content on the minimum ignition temperature of a sulphur/MCC dust cloud. The MIT of pure powders are respectively of 270  C and 510  C, for sulphur and MCC. As previously observed for the minimum ignition energy, the same threshold phenomenon is described on Fig. 5. Concentrations thresholds of 2.5%vol. i.e. 15%wt. beyond which the most flammable compound imposes its ignition characteristic are also confirmed and the harmonic model could also be applied here, but to a much lesser extent. Such behaviours have already been quoted for Pittsburgh coal mixed with a solid inertant e Fuller’s earth (Dastidar & Amyotte, 2003; Dastidar, Amyotte, & Pegg, 1997; Dastidar, Amyotte, Going, & Chatrathi, 2001). Fig. 5 also shows that a representation as a function of the molar proportions tends to linearise this relationship, which confirms the importance of the chemical reactions towards thermal properties for such mixtures. In fact, with regard to the particle-size distribution of the cellulose and to its MIT, a combustion controlled by a diffusion regime limitation is likely. Moreover, due to the low MIT and melting temperature of sulphur (near 110  C), its combustion takes place in homogeneous gas phase, with chemical reaction limitation. After heating in the Godbert-Greenwald oven, the solid/ solid/air mixture is rapidly transformed into a gas/dust/air hybrid mixture. The influence of a flammable gaseous compound on the cloud ignition is important even for a small amount of gas (Khalili et al., 2012). It allows the transition from a diffusion regime to a chemical reaction limitation and leads to an ignition behaviour close to the one of the most flammable powder. It has already been noticed that the finer the inert particles are, the strongest the influence is (Dastidar et al., 1997). Hence, microcrystalline cellulose and sulphur powders have been ground to obtain dusts with approximately d50 of 10 mm (to be compared with

Fig. 5. Evolution of minimum ignition temperature of MCCesulphur mixture as a function of MCC volume, molar concentrations and particle-size distributions. Air overpressure set at 0.1 bar.

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108 mm for MCC and 110 mm for sulphur before grinding). The impact of the particle-size distribution on the MIT of hybrid mixtures is clearly seen on Fig. 5. As expected, the minimum ignition temperature of the pure compounds decreases from 270 to 260  C and from 510 to 480  C, respectively for sulphur and MCC. This evolution is due to a greater particle surface area, and thus to a greater reactivity, but also to an enhancement of their thermal properties combined with a better dispersion and stability in the atmosphere. However, a slight augmentation of MIT is noticed for sulphur concentrations lower than 50%mol. This modification shows the influence of the combustion regime on the MIT. In fact, by grinding sulphur from 110 to 10 mm, the combustion regime is not modified, which is not the case for the cellulose (evolution from a diffusion regime to a chemical reaction limitation). When MCC is ground, its pyrolysis gases mix with the sulphur in gaseous phase, which modifies the reactivity of the gases and thus, the ignition behaviour of the dust cloud. Hence, the contributions of each compound seem to be more balanced, which improves the applicability of the harmonic model. Fig. 6 depicts a somewhat different behaviour. The concentration threshold, if it can be identified, will probably be set at 50%wt. As well as for MIE of sulphur and acetaminophen mixture, the MIT increases rather progressively. An explanation of such evolution can perhaps be found in the rod-like shape of the acetaminophen but also in the compound reactivity. The combustion regime of both dusts being limited by diffusion control, the addition of either powder does not modify abruptly the ignitability of the mixture. 3.3. Influence of solid/solid mixtures on the minimum ignition temperature of a dust layer The minimum ignition temperature (MIT) of a 5 mm dust layer is one of the main characteristics which are used to characterize the risk of ignition of a dust deposit in contact with a hot surface. For MCC/sulphur mixtures, the MIT ranges from 240  C to 180  C, approximately, with 220  C values for mixtures containing 3.5 and 1.5%vol. of sulphur (Fig. 7). Even if the MIT of a pure sulphur layer has not been determined due to the powder melting at approximately 110  C, the trend shown on Fig. 7 confirms the peculiar and predominant influence of the most combustible compound on solid/solid hybrid mixtures ignition. Nevertheless, the harmonic model (eq. (2)) previously proposed for the MIE of solid/solid mixtures does not seem to be relevant here. Other tests were

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Fig. 7. MIT of a 5 mm dust layer of sulphur/MCC mixtures as a function of the sulphur content and of zirconium/alumina mixtures as a function of the alumina content (d50 ¼ 3 mm).

notably carried out by Sweis (1998) on oil shale and tar sand mixed with limestone, and Reddy, Amyotte, and Pegg (1998) on coals mixed with dolomite and limestone. The global trends are similar to those described on Fig. 7. The introduction of a small amount of inert compounds has only a limited impact on the MIT whereas this parameter rises significantly for concentrations in inert compound greater than 60%wt. Such behaviour is clearly visible on Fig. 7 for zirconium/alumina mixtures. An unsteady-state model, resuming some hypotheses made by Semenov (Bowes, 1984), has been developed to study the thermal behaviour of small dust deposits composed of two materials. It takes into account the distribution of temperature, species concentrations and their time evolution with regard to their oxidation kinetics. This approach has been applied to evaluate the ignition risk (ignition delay and temperature) of zirconium and zirconiumealumina layers and deposits, and has shown a good agreement with the tests performed in both static and dynamic modes (Bideau et al., 2011). 3.4. Effect of solid/solid mixtures on the minimum explosive concentration Minimum explosive concentration (MEC) of dust clouds is an important factor requiring special attention for hazard evaluation, particularly if a technological equipment is to be protected by the addition of solid inertants. For example, Denkevits and Dorofeev (2006) measured the lower explosion concentration limit of the tungsten (W)/graphite (C) mixture as a function of mixture composition. The results are plotted in Fig. 8. The lowest explosive molar concentration decreased monotonically with increasing tungsten content, from 5.8 mol m3 for pure graphite to 2.4 mol m3 for pure tungsten. On the same figure, we have tested a harmonic model based on the molar content of tungsten to represent the evolution of the MEC of such mixtures:

1 ð%mol:W Þ ð1  %mol:W Þ ¼ þ MECmixture MECW MECC

Fig. 6. Influence of acetaminophen weight concentration on the minimum ignition temperature of MCCeacetaminophen mixtures. Air overpressure set at 0.1 bar.

(4)

It shows a satisfactory agreement with the experimental results of Denkevits and Dorofeev (2006). Sweis (2006) also studied the MEC of oil shales and inert powders mixtures. By adding admixed inert material, the MEC

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Fig. 8. Evolution of the minimum explosive concentration of tungsten (W)/graphite (C) dust mixtures as a function of tungsten content.

increases more significantly for mixtures containing limestone than for stone dust. It should be underlined that the relationships between the composition of the dust mixture and the MEC were non-linear. Nikufu et al. (2007) obtained similar results by mixing calcium carbonate or calcium oxide with magnesium. Amyotte, Mintz, and Pegg (1995) illustrated the existence of a mean particle diameter above which the inerting level remains essentially constant. This characteristic diameter is about 65 mm for fine coal and 80 mm for coarse coal in presence of limestone. Similarly, Eckhoff and Pedersen (1988) showed that the MEC of the polyester/epoxy resins used in electrostatic powder coating increased systematically by increasing the pigment content. Thus, for dusts containing 50%wt. of inert powders, the MEC was nearly the same for all dusts, i.e. 65e70 g m3. At present, if the MEC of a solid/solid mixture seems to follow well-identified trends, the existing mathematical models available for the prediction of the MEC are not successful particularly for organic dust clouds (Mitsi & Tanaka, 1973; Mittal, 1997). Finally, tests carried out on pea flour and Tropolar have demonstrated that the influence of inert solid addition is more pronounced on the upper explosivity limit than on the MEC (Bartknecht, 1989). 3.5. Influence of solid/solid mixtures on the explosion severity Various configurations can be encountered in the process industries handling multiple powders: mixtures of combustibles dusts without reaction, occurrence of reactions between the powders, combustible dusts with inert solid, etc. These cases will be illustrated by some examples in the following paragraphs. 3.5.1. Combustibles solids without reaction between the solids Figs. 9and 10 show some examples of such mixtures with magnesium stearate/lycopodium and magnesium stearate/carbon black powders. It should be stressed that each point Pmax and (dP/ dt)max corresponds to the maximum value of Pm or (dP/dt)m obtained on a 30e1500 g m3 concentration range. The maximum explosion pressure is a thermodynamical parameter which can be estimated thanks to the calculation of the adiabatic flame temperature. Hence, the evolution of this parameter seems rather linear as a function of the molar composition of the mixture. Concerning the maximum rate of pressure rise, this evolution is greatly influenced by the presence of a small amount of the less flammable compound (here, lycopodium or carbon black). Thus, for

Fig. 9. Influence of the magnesium stearate proportion on the explosion severity of magnesium stearate/lycopodium mixtures.

magnesium stearate/lycopodium mixtures, (dP/dt)max decreases from 25% for an addition of 25%wt. of lycopodium (Fig. 9). This decrease is more pronounced for magnesium stearate/carbon black mixtures: (dP/dt)max drops from 1035 to 772 bar s1 for only 2%wt. of carbon black (Fig. 10). Models have been tested in order to represent the explosion severity of such mixtures. However, if the evolution of the maximum explosion pressure can be estimated by calculating the adiabatic flame temperature of the solid/solid mixture, the maximum rate of pressure rise is more difficult to predict with simple models as the harmonic one. Models based on the evolution of the Spalding number, which represents the ratio of the heat released by the combustion to the heat required to gasify and heat the fuel to ignition, have also been unsuccessfully tested. It confirms that, in addition to the thermodynamic approach, both combustion kinetics and hydrodynamics of the dust clouds should be taken into account. This can only be done by developing more elaborate models based on the combustion/pyrolysis mechanisms of each compound and on a set of differential equations describing the heat and mass balances in the burning dust clouds (Dufaud et al., 2011). 3.5.2. Combustibles solids with reaction between the solids The tests carried out with MCC/sulphur mixtures have shown a non-monotonic evolution of the explosion severity. Table 3 summarizes the values of Pmax and of (dP/dt)max obtained by using a 20 L sphere.

Fig. 10. Influence of the carbon proportion on the explosion severity of magnesium stearate/carbon mixtures.

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Table 3 Evolutions of Pmax and (dP/dt)max for MCC/sulphur mixtures as a function of the sulphur content. Sulphur content (%wt./%vol.)

0/0

25/4

50/13

75/30

100/100

Pmax (bar(g)) (dP/dt)max (bar s1)

7.1 317

7.0 307

6.6 238

5.0 332

5.6 342

For pure compounds and for solid/solid mixtures, typical trends are observed for lean mixtures (fuel concentration lower than stoichiometry) and especially the increase of the severity parameters up to 750 g m3, followed by a slight decrease of Pm and (dP/dt)m for richer mixtures, from 750 g m3 to 1500 g m3. Nevertheless, for mixtures having higher combustible contents both Pmax and (dP/dt)max tend to increase slightly (Dufaud et al., 2009). Such peculiar behaviour could be explained by different assertions. Thermogravimetric analyses of the pure powders and of a 50/50 wt. mixture have demonstrated that the onset temperature is the lowest for the mixture: Tonset (S) is 200  C and Tonset (MCC) is 290  C, whereas Tonset (MCC/S) decreases down to 170  C. As a consequence, interactions between sulphur and cellulose cannot be neglected, which modifies the explosive behaviour of their mixtures. 3.5.3. Mixtures of solid inertants and combustible dusts Fig. 11 shows the effect of sodium bicarbonate and silica insertion on the maximum explosion pressure of magnesium stearate dusts. These powders have notably been chosen with regard to their particles sizes, compatible with those of the combustible dusts. One should notice that sodium bicarbonate is a well-known flame extinguisher (Chelliah, Wanigarathne, Lentati, Krauss, & Fallon, 2003). The amount of inertant was set at 8%wt. for each sample. A slight decrease of Pm is observed for both inertants on the entire range of dusts concentration (Perrin, Dufaud, Traoré, & Laurent, 2007). At 60 g m3, the maximum pressure drop is of 20%wt. for silica and of 30%wt. for sodium bicarbonate. The inerting effect is also observable on the maximum rate of pressure rise as seen on Fig. 12. The impact of inertant introduction is more pronounced for (dP/ dt)m than for Pm. Reductions of maximum rate of pressure rises up to 70% are obtained with sodium bicarbonate and up to 50% with

Fig. 11. Influence of inertant insertion on the maximum explosion pressure of magnesium stearate powders.

Fig. 12. Effect of 8%wt. inertant insertion on the maximum rate of pressure rise of magnesium stearate powders.

silica. This difference of efficiency is due to the chemical nature of the non combustible powders. With regard to combustion phenomenon, silica is an inert compound and has an impact on dust explosion severity by decreasing the combustible dust concentration, reducing the thermal transfers, increasing the heat capacity and modifying the dust dispersion; whereas sodium bicarbonate NaHCO3 also generates carbon dioxide when heated, which also leads to a drop of the oxygen content. It could also been observed that the inerting effect of silica and sodium bicarbonate is greater at small dusts concentrations near stoichiometry. It should also be noted that tests carried out on pea flour, cellulose and Tropolar (inert solid) have demonstrated the impact of the ignition energy on the inerting conditions of the dusts and lower concentrations of inert dusts are needed in case of less potent ignition sources (Bartknecht, 1989). The previous experiments have been conducted with a fixed amount of inertant. Fig. 13 presents the influence of inertant concentration on the explosion severity of magnesium stearateesodium bicarbonate mixtures. Both Pm and (dP/dt)m strongly decrease when the concentration of sodium bicarbonate increases up to 10%wt. Nevertheless, this drop seems to be less important for higher inertant concentrations. However, the threshold phenomena observed with the ignition sensitivity do not appear clearly when considering the

Fig. 13. Evolution of the maximum explosion pressure and of the maximum rate of pressure rise of magnesium stearateeNaHCO3 explosions as a function of the sodium bicarbonate concentration.

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explosion severity. Such gradual trends have already been observed for other solid/solid mixtures, especially for the maximum rate of pressure rise (Cashdollar, 2000; Denkevits & Dorofeev, 2004). Nevertheless, threshold effects on maximum explosion pressure have been quoted for mine-size coal dusts and rock dusts mixtures (Dastidar et al., 2001). A peculiar attention should also be paid to the fact that, at 65%wt. of sodium bicarbonate, the dusts mixture is far from being inert. Values of 75e85%wt. of inert content can reasonably be expected (Mintz, Bray, Zuliani, Amyotte, & Pegg, 1996). 4. Conclusions Industrial processes are never definitely fixed and often evolve during their life cycle. However, adding control equipment or protection measures to existing processes sometimes leads to the emergence of unexpected hazards and, always, to significant costs. Pertinent examples are given by Kletz, by notably reporting that collecting dusts to reduce pollution and collecting them dry to avoid water pollution can lead to an increase of dust explosion occurrence (Kletz, 2001). As a consequence, inherent safety must be implemented in the early stages of the process design. Dealing with solid/solid mixtures can provide a good opportunity to apply the inherent safety principle of moderation. This work, dealing with the flammability and the explosivity of solid/solid mixtures, underlines the presence of threshold phenomena for the minimum ignition energy and minimum ignition temperature; especially when the properties of the dusts are strongly different. As a consequence, the common recommendation of solid inertants introduction up to 50e80%wt. to eliminate the dust explosion risk should be reconsidered (Eckhoff, 2003). The thermal/thermodynamic properties of each dust are not the only parameters to be considered to understand the ignition sensitivity of such mixtures and the presence of a most flammable compound (due to its chemical composition or particle-size distribution) can allow the transition from a diffusion regime to a chemical reaction limitation and leads to an increased flammability. The predominant influence of the most combustible compound on solid/solid hybrid mixtures ignition is often less pronounced when the dust explosivity is considered. Various behaviours can be encountered when two reactive dusts are mixed. They can react together more or less directly to modify their explosivity characteristics (with sulphur and microcrystalline cellulose for instance), their properties can be uniformly ranged between those of the pure compounds (as for lycopodium/magnesium stearate or carbon black/magnesium stearate) or a decrease of their explosivity can be observed (when sodium bicarbonate or solid inertant is used, for instance). Unfortunately, this diversity is seldom taken into account during dust explosion risk assessments. With regard to these results, five direct industrial applications of the inherent safety principle of moderation have been made. In pet food industries, various additives as vitamins and mineral premixes are added to the bulk solid product, especially to improve the palatability of the food. If the mineral compounds are introduced at an early stage of the process, the ignitability of the premixes can then be significantly reduced. During the removal and recovery of metals from polluted soils or wastes, dusts clouds can be generated at various stages of the processes. Knowing the evolution of the combustible concentration at each stage is essential to set the inerting level and define additional prevention measures. In coal-fired thermal power stations, cleaning of certain boilers can be achieved by blasting soft abrasive powders such as walnut shell. The addition of such particles, having a rather low ignition sensitivity, to coal dusts can modify the dust explosion risk, even if the global heating power must not be lowered. As a consequence,

the concentration of organic abrasive has to be chosen carefully and as early as possible. In the pharmaceutical industries, mixtures of excipients and/or active principles are encountered. As a function of the ignition sensitivity of the dusts mixture, wet or dry granulation can be considered. Then, from a safety point of view, the choice of the process depends on the composition and the nature of the blend. Finally, dust separation from an air stream is often achieved by using filter bags. In waste recycling processes, for instance during tyres recycling, the filter cakes are composed of various dusts: carbon blacks, rubber particles, organic fibres, mineral loads, etc. During clogging and cleaning cycles, the composition of the dust cloud evolves as a function of time (Simon, Chazelet, Thomas, Bémer, & Régnier, 2007). This composition, and thus the explosion risk, depends especially on the cut off diameter which has been determined during the process design phase. The previous examples underline the importance of implementing inherent safety concepts in the earliest stages of the process. Concerning dusts mixtures explosions, it is essentially possible if the ignition and explosion parameters of the mixtures are well known. However, there exists no general correlation for determining such parameters for solid/solid mixtures and, often, they had to be determined for every application (Bartknecht, 1989). In this purpose, developing simple correlations or more complex models is then necessary. Acknowledgements The authors would like to thank CEA, the Atomic Energy and Alternative Energies Commission, for its contribution to the study of zirconium/alumina mixtures. References Amyotte, P. R. (2006). Solid inertants and their use in dust explosion prevention and mitigation. Journal of Loss Prevention in the Process Industries, 19, 161e173. Amyotte, P. R., & Khan, F. I. (2002). An inherent safety framework for dust explosion prevention and mitigation. Journal of Physics IV France, 12(7), 189e196. Amyotte, P. R., Mintz, K. J., & Pegg, M. J. (1995). Effects of rock dust particle size on suppression of coal dust explosions, process safety and environmental protection. Transactions of the Institute of Chemical Engineers, Part B, 73, 89e100. Amyotte, P. R., Pegg, M. J., & Khan, F. I. (2009). Application of inherent safety principles to dust explosion prevention and mitigation. Process Safety and Environmental Protection, 87(1), 35e39. Amyotte, P. R., Pegg, M. J., Khan, F. I., Nikufu, M., & Yingxin, T. (2007). Moderation of dust explosion. Journal of Loss Prevention in the Process Industries, 20(4e6), 675e687. Bartknecht, W. (1989). Dust explosions e Course, prevention, protection. Berlin: Springer Verlag. BIA. (1997). Combustion and explosion characteristics of dusts. Report. Sankt Augustin, Germany: HVBG. Bideau, D., Dufaud, O., Le Guyadec, F., Perrin, L., Génin, X., Corriou, J. P., et al. (2011). Self ignition of layers of powders mixtures: effect of solid inertants. Powder Technology, 209, 81e91. Bollinger, R. E., Clark, D. G., Dowell, A. M., Ewbank, R. M., Hendershot, D. C., Lutz, W. K., et al. (1996). Inherently safer chemical processes e A life cycle approach. New York: Center for Chemical Process Safety, AIChE. Bowes, P. C. (1984). Self-heating: Evaluating and controlling the hazards. New York: Elsevier. Britton, L. G. (1998). Short communication: estimating the minimum ignition energy of hybrid mixtures. Process Safety Progress, 17, 124e126. Cashdollar, K. (2000). Overview of dust explosibility characteristics. Journal of Loss Prevention in the Process Industries, 13(3e5), 183e199. Chelliah, H. K., Wanigarathne, P. C., Lentati, A. M., Krauss, R. H., & Fallon, G. S. (2003). Effect of sodium bicarbonate particle size on the extinction condition of nonpremixed counterflow flames. Combustion and Flame, 134, 261e271. Dastidar, A. G., & Amyotte, P. R. (2003). Determination of minimum inerting concentrations for combustible dusts in a laboratory-scale chamber. Transactions of the Institute of Chemical Engineers, Part B, 80(7), 287e297. Dastidar, A. G., Amyotte, P. R., Going, J., & Chatrathi, K. (2001). Inerting of coal dust explosions in laboratory and intermediate-scale chambers. Fuel, 80, 1593e1602. Dastidar, A. G., Amyotte, P. R., & Pegg, M. J. (1997). Factors influencing the suppression of coal dust explosions. Fuel, 76(7), 663e670.

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