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Risk assessment of the ignitability and explosivity of aluminum nanopowders
Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 304–310
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Risk assessment of the ignitability and explosivity of aluminum nanopowders ˜ b , J. Bouillard a , O. Dufaud c,∗ , L. Perrin c , A. Laurent c , A. Vignes a , F. Munoz c D. Thomas a
INERIS, Parc Technologique ALATA, B.P. 2, F-60550 Verneuil-en-Halatte, France Universidad de Los Andes, Departemento de Ingeniera Quimica, Carrera 1 E No 19A-40, Edificio Mario Laserna, Of. 322 Bogota, Colombia c Laboratoire Réactions et Génie des Procédés, Nancy-Université, CNRS,1 rue Grandville, B.P. 20451 F-54001 Nancy, France b
a b s t r a c t Previously, an extensive study has been carried out in order to assess the ignition sensitivity and explosivity of aluminum nanopowders. It showed notably that, as the particle size decreases, minimum ignition temperature and minimum ignition energy decrease, indicating higher potential inflammation. However, the explosion severity decreases for diameters lower than 1 m. As a consequence, this study leads to the conclusions that the ignition sensitivity and explosion severity of aluminum nanopowders may be affected by various phenomena, as pre-ignition, agglomeration/aggregation degree and the intrinsic alumina content. The presence of wall-quenching effects and the predominance of radiation compared to conduction in the flame propagation process have to be discussed to ensure the validity of the 20 L sphere and of the results extrapolation. Based on the peculiar behaviours that had been previously highlighted, a specific risk analysis has been developed in order to assess the fire and explosion risks of such materials. It has been applied to an industrial plant of aluminum nanopowders production. The hazard identification and the consequence modelling steps, especially the quantification of the likelihood and consequences, have been designed specifically. The application of this method has led to the definition of the most adequate safety barriers. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Nanopowders; Ignition; Dust explosion; Aluminum; Risk analysis
1.
Introduction
Nanomaterials are sometimes considered as the key to the next industrial revolution. Over the past ten years, many industrial applications have already been developed. Research in the field is growing very rapidly, and all the developed countries see potential for expansion and applications in numerous fields as well as great potential economic spin-offs. The fields with current commercial uses and producing the greatest revenue are mechanico-chemical polishing, magnetic recording tapes, sunscreens, automotive catalyst supports, bio-labelling, electro-conductive coatings and optical fibers. The biomedical and pharmaceutical fields, electronics, metallurgy, agriculture, textiles, coatings, cosmetics, energy and catalysts are
∗
other sectors with growing applications (Höck, 2007). As nanomaterial production and use are going to increase, there can be more and more associated hazards. A potential hazard of nanopowders that appears to have received little attention to date is their ignition sensitivity and their explosion severity. In fact, as well as combustible micropowders are considered in risks assessments, combustible nanopowders have also to be considered. So far, literature studies concerning the evaluation of explosion hazards of powders were essentially carried out on microsized powders (Bartknecht, 1989; Eckhoff, 2003), albeit there are the recent extensive studies performed by the EU NANOSAFE2 project (Schuster and Bouillard, 2008) during the 2005–2009 period and more recently by Wu in Taiwan (Wu et al., 2009, 2010) in 2009–2010 and
Corresponding author. Tel.: +33 3 83 17 53 33; fax: +33 3 83 32 29 75. E-mail address: [email protected] (O. Dufaud). Received 18 November 2010; Received in revised form 4 July 2011; Accepted 29 September 2011 0957-5820/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2011.09.008
Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 304–310
HSL (Holbrow, 2010). At the beginning of this work in 2005, in the frame of the EU NANOSAFE 2 project, there was only one study on nanomaterial explosivity published in the literature (Pritchard, 2004), which was more specifically devoted to an aluminum nanopowder. Thus, additional tests were performed within that project, and the main results were discussed in (Vignes, 2008) and (Bouillard et al., 2010). Both experimental results and the literature review showed that ignition sensitivity of aluminum nanopowders is higher than aluminum micropowders: the Minimum Ignition Temperature (MIT) of aluminum nanopowders may be as low as 550 ◦ C (Hull, 2002), Limiting Oxygen Concentration (LOC) in nitrogen of aluminum nanopowders may be lower than 5% (Kwok et al., 2002), Minimum Ignition Energy (MIE) of aluminum 200 nm is 7 mJ and is below 1 mJ in the case of aluminum 100 nm, which is lower than the MIE of typical Al micropowders. Finally, the experimental Minimum Explosive Concentration (MEC) is about 30 g m−3 for the tested aluminum nanopowders. Concerning the explosion severity characterised by the maximum explosion pressure and maximum rate of explosion pressure (Eckhoff, 2003), it tends to increase as the particle size decreases/the specific surface area increases, before reaching a peak for 1 m particle size in the case of aluminum powders (approximately 2 m2 .g−1 ). After this peak has been reached, the explosion severity finally tends to decrease as the particle size further decreases. Explanations and critics of these results are discussed elsewhere (Bouillard et al., 2010). Based on the assessment of the reliability of these results, a specific risk assessment methodology is proposed here in order to address explosion hazards within a representative plant manufacturing aluminum nanopowders by taking into account the various work areas, steps and phases of the process.
2.
Risk analysis
2.1.
General methodology
Assessing ignition and explosion risk related to the use and production of nanopowders implies to proceed in a first step to the hazard identification of the products at the different workplaces and manufacturing processes. Secondly, it is required to evaluate, rank and prioritise risks before choosing the actions needed to manage ignition and explosion risks. The proposed methodology is based on the ATEX 1999/92/CE directive and the INRS-CNPP methodology (Vincent et al., 2005) and is designed to be quite simple, synthetic and reliable. The explosion risk was assessed by taking into account of three main parameters, i.e. the explosion frequency, consequences and the level of confidence of the technical and organisational safety barriers. These three parameters were combined to obtain a risk score RS which will help the user to prioritise the requested actions to manage these specific risks:
RS =
Explosion consequences × Explosion frequency Level of confidence of the barriers
This methodology has been applied to an industrial plant of aluminum nanopowders production by plasma processing (Vignes, 2008). The various workshops, steps and phases of the process have been considered, such as normal operation, process cleaning, storage . . .
2.2.
305
Explosion frequency
The explosion frequency is evaluated by taking into account of the formation probability of an explosive atmosphere (see Section 2.2.1) and the frequency of the presence of an ignition source (see Section 2.2.2).The combination of the following scores gives an explosion frequency score which ranges from 1 to 15 (Vignes, 2008).
2.2.1.
Formation probability of an explosive atmosphere
This parameter is related to the ATEX zoning for a given working unit. The minimum explosive concentration (MEC), the limiting oxygen concentration (LOC), the nature of the process as well as the behaviour of nanopowders in the air enable to determine the occurrence frequency of an ATEX atmosphere. It is therefore, necessary to briefly evaluate the reliability of the safety data as classical apparatus used for micropowders may not provide reliable results for nanopowders. The MEC was determined to value 30 g m−3 which seems to be in accordance with literature data and with the fact that it is generally considered that MEC becomes nearly independent of particle size below a diameter of 30–50 m (Dufaud et al., 2010a,b; Jacobson et al., 1964; Lödel, 1992; Mannan, 2005). However, it must be reminded that experimental results may be influenced by both agglomeration, which tends to be important below 30–50 m (Geldart, 1973), and by the ignition temperature of the particles (Jaeckel, 1924), which tends to decrease as the particle size lowers (Huang et al., 2005). Thus it may be considered that the MEC value for aluminum nanopowders is quite reliable if nanopowders are released from storage or deposits, i.e. under the form of agglomerates. The value of LOC was determined to be 5% by (Kwok et al., 2002) which still remains coherent with the current tested aluminum nanopowders. In fact, by taking into account an empirical relationship determined by (Siwek and Cesana, 1995) depending on MIE and MIT for a given product, the limiting oxygen concentration was estimated at 3% in the case of 200 nm aluminum by considering a minimum ignition temperature for aluminum nanopowder around 550 ◦ C (Hull, 2002). However, it should be kept in mind that LOC value could be overestimated because the standard test is generally performed under nitrogen, whereas nanosized aluminum may be manufactured under argon - as nitrogen can react at high temperatures, especially in a plasma processing unit. Finally, agglomeration level may also influence the reliability of the LOC value. By taking into account the previous parameters, related remarks, industrial feedback within the NANOSAFE 2 project and by considering the classical ATEX classification proposed by ATEX 1999/92/CE directive, a qualitative frequency of an ATEX formation might be attributed to a work unit. Indeed, a qualitative occurrence frequency of the formation of an explosive atmosphere in a given working unit was scored from 1 to 3 in accordance with the Table 1 which link the score and related ATEX zone.
2.2.2.
Frequency of the presence of an ignition source
In order to assess the probability of ignition of an explosive atmosphere, the MIT as well as MIE should be considered. The reliability of these parameters should also be discussed. The minimum ignition temperature for aluminum nanopowder values around 550 ◦ C for 90 nm aluminum nanopowders (Hull, 2002). However, considering MIT to evaluate the frequency of presence of an ignition source does not
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Table 1 – Classification criteria of frequency of ATEX occurrence. Score
ATEX zones
0 1 2 3
Off-zone 22 21 20
appear to be relevant in the case of non passivated aluminum nanopowders, which may be considered as pyrophoric (Jacobson et al., 1964). The MIT value may be only relevant in the case of partially passivated aluminum nanopowders (with a slight layer of alumina around the core of aluminum). For 100 nm particle size, aluminum nanopowder dust cloud can be ignited by minimum ignition energy lower than 1 mJ (Bouillard et al., 2010), which means that even minor unearthed plant items of very low capacitances (of the order of 1 pF) may be hazardous (Randeberg, 2005). For 200 nm, aluminum nanopowder dust cloud can be ignited by minimum ignition energy higher than 7 mJ (Bouillard et al., 2010) which is also rather lower than microsized aluminum powders (Vignes, 2008). Such results were also found in the case of iron and titanium nanopowders (Wu et al., 2009). The risk of tribo-electric charging of unearthed metal objects may therefore imply that even quite small objects (screws, bolts, etc.) must be earthed to reduce the risk of ignition of very easily ignitable aluminum nanodust clouds. However, it should be also reminded that the MIE of aluminum nanopowders may be over-estimated as the modified Hartmann tube tends to create electrostatic charges on the nanoparticles during the dispersion process, lowering the ignition energy in the dust cloud (Glor, 1988). Even if the MIE measurements may be slightly distorted by the process measurements it-self, we advised to consider that the measures are quite representative of the ignition risks induced by metallic nanopowders. By considering these elements, it becomes possible to score the frequency of the presence of an ignition source, from 1 to 5 with the help of the classification proposed by INRS and CNPP for chemical risks (Vincent et al., 2005), which is in fact a modified and more accurate classification than the one usually employed in the classic ATEX method. This classification is presented in Table 2.
2.3.
Explosion consequences
In order to determine the consequences of an explosion, it is necessary to consider the quantity of aluminum
Table 2 – Proposed score of the frequency of the presence of an ignition source. Score 1 2 3
4 5
Frequency of presence of the ignition source External accidental source or a natural event (e.g. malevolence, lightning) Ignition source due to a malfunction, a wear, a handling error (e.g. electric stoppage) Presence due to maintenance’s operations (e.g. hot spot working) Ignition source due to static electricity (e.g. load of organic material or inflammable materials) Occasional operation (e.g. presence of accumulator charge port, add-on heating appliance) Occasional presence due to the process Occasional presence not due to the process (e.g. smoker) Permanent presence of an ignition source (e.g. hot surface in the process’ equipment)
Classification criteria No presence Unlikely and only for short periods (never in normal operation) Occasionally likely in normal operation Frequently likely or for long periods or continuously
Table 3 – Proposed score of the violence index. Kst (bar m s−1 )
Score 1 2 3
0–200 201–300 >300
nanopowders, their explosion severity as well as the specificities of the process. The explosion severity is the combination of maximum explosion pressure and maximum rate of explosion pressure rise (Eckhoff, 2003). The maximum explosion pressure rise of a given nanopowder can be related to the explosion violence index Kst. The proposed classification is based on the St Classes of powders. This parameter is already used in the ATEX methodology and is summarized in the Table 3. The maximum explosion pressure obtained in the 20 L explosion sphere is mainly under the dependence of thermodynamics and thus can be roughly related to the combustion enthalpy of aluminum, which is of 31 MJ per kilogram of aluminum. It is then possible to combine the combustion enthalpy with the estimated mass of nanopowders to catch a hazard profile index related to the potential energy that can be released by the explosion of a nanodust cloud. In order to get a reliable qualitative index, we have considered as a reference the energy released by TNT to establish a non-dimensional ratio closed to an equivalent TNT mass. It should be underlined that this equivalent TNT mass will not be used in order to quantify an explosion pressure, as a contrary to the equivalent TNT method. By estimating this ratio, it becomes then possible to score specific range of equivalent TNT mass as illustrated in Table 4. The consequences of an aluminum nanopowders explosion can then be scored as the combination of TNT equivalent weight and explosion severity. The score attribution is partially based on the explosion characteristics of aluminum nanopowders, which have to be also carefully discussed. As a matter of fact, if the current results seem to be in accordance with the literature as far as low specific surface areas are considered, they do not plateau for nanosized particles but tend to decrease (Bouillard et al., 2010). This decrease, obtained for particles sizes lower than 1 m, also corresponds to a change of the combustion regime: from diffusion controlled, for large size particles to kinetically controlled, for small size particles (Bouillard et al., 2010). As no literature feedback enables us to validate experimental data, it is necessary to evaluate the validity of the 20 L explosion sphere, i.e. its ability to determine reliable
Table 4 – Criticity score linked to the equivalent TNT mass of aluminum nanopowders mass. Score Equivalent TNT mass [kg]
1
2
3
4
5
<0.5
0.5–5
5–10
10–50
>50
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Fig. 1 – Flame propagation of aluminum dust explosion in a modified Hartmann tube. 900 g m−3 of powder at 30 mJ for an ignition time tv of 120 ms. (a) 100 nm nanoparticles, (b) 3 m particles. results. These aspects were studied in details in (Vignes et al., 2009). This study concluded that the explosion severity of nanopowders may be affected by the following phenomena/parameters: (i) The pre-ignition phenomenon: it was observed that aluminum nanopowders can be pre-ignited during the injection process into the 20 L explosion sphere. Preignition phenomenon may lead to an increase of the initial temperature as well as an increase of alumina content. Consequently, it may be assumed that the measured explosion severity may be lowered due to the pre-ignition phenomena. (ii) The intrinsic alumina content. (iii) The agglomeration/aggregation level of the nanopowders. The validity of the 20 L sphere has thus been investigated. The explosion pressure Pm and rate of pressure rise (dP/dt)m were combined through the semi-empirical relation of Silvestrini et al. (2008) in order to calculate an experimental laminar flame velocity SUL,exp . This experimental flame velocity was compared to a theoretical one SUL,calc , determined by Huang’s model (Huang et al., 2005) which was initially established to predict laminar flame velocity in an aluminum dust cloud. This model assumes that the propagation of the flame is run mainly by conduction. By assuming that the main difference between the experimental and the theoretical flame velocity depends on the aggregation structure of the nanopowders (indirectly related to the specific surface area SBET ) and on the concentration C0 of the dust cloud, a semi-empirical relationship was established as SUL,calc = f(C0 , SBET ) · SUL,exp (Vignes et al., 2009). A Huang-like model could be used to represent our experimental data presented elsewhere (Bouillard et al., 2010), which means that the propagation flame is mainly conducted by conduction within the 20 L sphere. However, it should be reminded that it was experimentally and theoretically demonstrated that the flame propagation within nanodust may be mainly conducted by radiation (Proust, 2004). Two hypotheses can thus be made. One the one hand, it can be reasonably assumed that the 20 L sphere does not enable to get reliable data as it probably disturbs the flame propagation and thermal mechanisms by absorbing radiation (quenching due to wall effect). On the other hand, it has been observed, thanks to the use of a high speed camera and the visualization of the propagating flame edge structure (Fig. 1) that the
preheated zone is smaller for nanopowders (100 nm) than for micro-particles. It could notably be explained by the fact that the flame radiation is absorbed by the cloud of unburnt aluminum nanopowders. A bichromatic pyrometer could be used in order to check if this modification of the flame behaviour in the visible range could be due to a change in the radiation wavelengths (flame temperature modification) (Dufaud et al., 2010a,b).
2.4.
Level of confidence of the existing safety barriers
At this point, the relevance and effectiveness of some existing equipments which lower the severity or the probability of a nanopowder explosion should be taken into account. However, it should be kept in mind that the current relevance and effectiveness of the technical prevention and protection barriers is not evaluated yet. Technical barriers that are efficient to prevent and protect micropowder explosion may be irrelevant in the case of nanopowders. The worst case would, for instance, be that dispersed nanopowder induces detonation. Fortunately, explosion risk can also be lowered by organisational barriers. Despite the previously quoted lack of information, it is necessary to evaluate qualitatively the probability of failure on demand/confidence of identified safety barriers/independent protection layers. Moreover, multiple factors as the reliability, robustness, availability and efficiency have to be taken into account (Sklet, 2006). Thus, for simplicity, a safety barrier risk reduction score based on the barrier confidence level was considered. Such a ranking is proposed in the Table 5.
2.5.
Risk ranking
The methodology developed in Section 3 enables to obtain a final risk score from 1 to 225, which leads to a risk ranking. The determination of a risk score related to the explosion of nanopowders at a given working unit is then used to
Table 5 – Risk reduction scores for safety prevention and protection barriers. Score Confidence level of safety barriers
1
2
3
4
Lack of barriers
Low
Medium
High
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Table 6 – ATEX risk assessment for a given working unit.
Data Score
TNT mass
Explosion severity
ATEX zoning
Presence of ignition source
Level of confidence
Risk score
>50 kg 5
St3 3
Zone 22 1
Rare 2
Medium 3
10
characterise the priorities of the needed actions in order to reduce the risk to an acceptable level. The definition of thresholds for risk acceptability has been an essential step due to the peculiarity of this emerging risk. The risk acceptability has been defined as a function of the risk perception by the workers, the society, but also with regard to the risk management policy of the company, the regulatory context and the legal obligations. It has led to the development of suitable action plans aiming at reducing the risk criticality to an acceptable value for the workers. An example of risk ranking is given on Table 6 for a given work unit where nanopowders are handled. Every workstation of a plant can be examined in that way and prevention and protection measures analyzed. Additional safety barriers can be set up until the explosion risks have been finally found to be acceptable.
2.6.
Additional safety barriers
In order to reduce the dust explosion hazard, general safety measures could be taken, safety barriers could be placed (Sklet, 2006) to provide multiple devices, systems or actions that reduce the likelihood of an undesirable event according to the key principles of inherent safety: minimization, substitution, moderation and simplification (Amyotte et al., 2009;
Kletz, 1998) in an approach focused on elimination or reduction of intrinsic hazards associated to a set of conditions for a determined substance or process. The reduction of the volume of the process equipment can minimize the risks in operation by decreasing the quantity of material or energy contained in a manufacturing process. It is also appropriate to perform reliability-centered maintenance techniques to increase inherent safety by downtime, thus the intermediate storage. Substitution is the replacement of a hazardous material or process with an alternative that reduces or eliminates the hazard. The moderation strategies can provide less hazardous conditions for handling of combustible solid materials. Some alternatives consist in the reduction of nanoparticles concentrations at workplaces, in the modification of the process equipment or the operation mode as manipulation in liquid form. Simplification consists of a design of the process that eliminates unnecessary complexity and in this way reduces the opportunities for error and misoperation. In addition to such general actions, some specific safety barriers could be proposed. For instance, specific cleaning devices should be used as HEPA-ULPA vacuum cleaners, earthing of booths, tables, devices are compulsory to prevent sparks generation due to static electricity; the filtration/vacuum cleaning processes should be adapted to the particles size. . .
Table 7 – Various types of barriers and their applications in this case study. Barriers classification
Definition
Examples
Human: activated barrier – procedural
Barrier with human detection/diagnostic/action
Symbolic: activated-warned
Human action based on passive warning
Activated barrier – manual
Manual action in response to alarm/detection Barrier activated by a mechanism. Need a preliminary diagnostic. Barrier put in place by a person
ATEX regulations, nanoparticles regulation at a local level, cleaning procedures, appropriate waste processing, predictive maintenance procedure . . . Safety signs, plans and instructions posters, up-to-date safety documents/MSDS, adequate GHS labelling . . . Manual system shutdown, water deluge systems, manual tank draining . . . Ventilation, adequate media filters (ULPA), inerting, dust collecting (catch tank) . . .
Permanent active barriers
Temporary passive barriers
Passive barriers – inherent
Based on design properties
Passive barriers – add-on
Passive components added to a system because of safety considerations
Adequate personal protective equipment (nitrile gloves, airline hood, non-woven coverall, FFP3/N100 dust masks . . .) Isolated pumps, vacuum lines, cleanrooms, laminar flow booths . . . Containments, dust extraction by exhaust ventilation, electrical grounding . . .
Proportion in %
Inherent safety principle
40
Minimize Simplify
6
Minimize Moderate
5
Moderate
14
Moderate
10
Moderate
15
Minimize Simplify
10
Moderate
Process Safety and Environmental Protection 9 0 ( 2 0 1 2 ) 304–310
The validity of ingress protection ratings protection should also be discussed. Moreover, nitrile gloves are recommended in order to avoid dermal exposure. Protection measures should also be taken as the installation of explosion panels and explosion venting to minimize the severity of the explosion; the use of vessels able to withstand the maximum force generated by an explosion. . . In order to formalize and optimize the use of safety barriers, the classification of Sklet (2006) has been slightly modified and applied to this case (Guldenmund et al., 2006). The level of confidence of such additional barrier has also been assessed as done for the existing barriers (see Section 2.4). Such application and safety barrier placement thanks to a fault tree analysis have already been done in the case of a laboratory handling nanopowders (Dufaud et al., 2006) Examples of barriers are given in Table 7.
3.
Conclusion
Two kinds of aluminum nanopowders were tested in order to evaluate their flammability and explosivity characteristics and provide accurate and reliable data for occupational and major risks analyses. Experimental and theoretical investigations lead to the conclusions that the minimum ignition energy of such nanoparticles could be lower than 1 mJ, which induces a very high ignition sensitivity. The explosion severity has also been determined and the influence of several parameters has been highlighted. In fact, the safety parameters may be affected by agglomeration, wall-quenching and pre-ignition phenomena, especially for highly flammable aluminum nanopowders. The question of the leading phenomenon of the heat transfer in the flame propagation is also put forward. If, as proposed by certain authors, the predominance of radiative phenomena over conductive transfer is established in the case of well-dispersed nanoparticles (Proust, 2006), then the risk of detonation could be theoretically founded and the validity of the cubic law used to extrapolate the explosion severity values to other volumes questioned (Vignes et al., 2009); as this semi-empirical relationship assumes that the flame thickness is relatively thin. If these points are correctly addressed, it will be possible to get reliable explosion severity data and detect potential detonation risk or, at least, high explosion risk. Finally, relying on our experimental study, the proposed risk analysis method has been successfully applied to an industrial plant in order to define the best safety barriers which had to be positioned to ensure the best occupational safety level to all workers. By highlighting the main issues related to the assessment of the explosion risks within a facility handling nanopowders, this work provides useful information to the QHSE engineers concerned about the use and the production of combustible nanoparticles. This approach is consistent with the European Directives on occupational risk assessment 89/391/EEC and on explosive atmospheres 1999/92/EC and 94/9/EC.
Acknowledgments This study was carried out with the financial support of the European Commission through the Sixth Framework program for Research and Technological Development NMP2-CT-2005515843 contract “NANOSAFE2”.
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Risk assessment of the ignitability and explosivity of aluminum nanopowders ˜ b , J. Bouillard a , O. Dufaud c,∗ , L. Perrin c , A. Laurent c , A. Vignes a , F. Munoz c D. Thomas a
INERIS, Parc Technologique ALATA, B.P. 2, F-60550 Verneuil-en-Halatte, France Universidad de Los Andes, Departemento de Ingeniera Quimica, Carrera 1 E No 19A-40, Edificio Mario Laserna, Of. 322 Bogota, Colombia c Laboratoire Réactions et Génie des Procédés, Nancy-Université, CNRS,1 rue Grandville, B.P. 20451 F-54001 Nancy, France b
a b s t r a c t Previously, an extensive study has been carried out in order to assess the ignition sensitivity and explosivity of aluminum nanopowders. It showed notably that, as the particle size decreases, minimum ignition temperature and minimum ignition energy decrease, indicating higher potential inflammation. However, the explosion severity decreases for diameters lower than 1 m. As a consequence, this study leads to the conclusions that the ignition sensitivity and explosion severity of aluminum nanopowders may be affected by various phenomena, as pre-ignition, agglomeration/aggregation degree and the intrinsic alumina content. The presence of wall-quenching effects and the predominance of radiation compared to conduction in the flame propagation process have to be discussed to ensure the validity of the 20 L sphere and of the results extrapolation. Based on the peculiar behaviours that had been previously highlighted, a specific risk analysis has been developed in order to assess the fire and explosion risks of such materials. It has been applied to an industrial plant of aluminum nanopowders production. The hazard identification and the consequence modelling steps, especially the quantification of the likelihood and consequences, have been designed specifically. The application of this method has led to the definition of the most adequate safety barriers. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Nanopowders; Ignition; Dust explosion; Aluminum; Risk analysis
1.
Introduction
Nanomaterials are sometimes considered as the key to the next industrial revolution. Over the past ten years, many industrial applications have already been developed. Research in the field is growing very rapidly, and all the developed countries see potential for expansion and applications in numerous fields as well as great potential economic spin-offs. The fields with current commercial uses and producing the greatest revenue are mechanico-chemical polishing, magnetic recording tapes, sunscreens, automotive catalyst supports, bio-labelling, electro-conductive coatings and optical fibers. The biomedical and pharmaceutical fields, electronics, metallurgy, agriculture, textiles, coatings, cosmetics, energy and catalysts are
∗
other sectors with growing applications (Höck, 2007). As nanomaterial production and use are going to increase, there can be more and more associated hazards. A potential hazard of nanopowders that appears to have received little attention to date is their ignition sensitivity and their explosion severity. In fact, as well as combustible micropowders are considered in risks assessments, combustible nanopowders have also to be considered. So far, literature studies concerning the evaluation of explosion hazards of powders were essentially carried out on microsized powders (Bartknecht, 1989; Eckhoff, 2003), albeit there are the recent extensive studies performed by the EU NANOSAFE2 project (Schuster and Bouillard, 2008) during the 2005–2009 period and more recently by Wu in Taiwan (Wu et al., 2009, 2010) in 2009–2010 and
Corresponding author. Tel.: +33 3 83 17 53 33; fax: +33 3 83 32 29 75. E-mail address: [email protected] (O. Dufaud). Received 18 November 2010; Received in revised form 4 July 2011; Accepted 29 September 2011 0957-5820/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.psep.2011.09.008
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HSL (Holbrow, 2010). At the beginning of this work in 2005, in the frame of the EU NANOSAFE 2 project, there was only one study on nanomaterial explosivity published in the literature (Pritchard, 2004), which was more specifically devoted to an aluminum nanopowder. Thus, additional tests were performed within that project, and the main results were discussed in (Vignes, 2008) and (Bouillard et al., 2010). Both experimental results and the literature review showed that ignition sensitivity of aluminum nanopowders is higher than aluminum micropowders: the Minimum Ignition Temperature (MIT) of aluminum nanopowders may be as low as 550 ◦ C (Hull, 2002), Limiting Oxygen Concentration (LOC) in nitrogen of aluminum nanopowders may be lower than 5% (Kwok et al., 2002), Minimum Ignition Energy (MIE) of aluminum 200 nm is 7 mJ and is below 1 mJ in the case of aluminum 100 nm, which is lower than the MIE of typical Al micropowders. Finally, the experimental Minimum Explosive Concentration (MEC) is about 30 g m−3 for the tested aluminum nanopowders. Concerning the explosion severity characterised by the maximum explosion pressure and maximum rate of explosion pressure (Eckhoff, 2003), it tends to increase as the particle size decreases/the specific surface area increases, before reaching a peak for 1 m particle size in the case of aluminum powders (approximately 2 m2 .g−1 ). After this peak has been reached, the explosion severity finally tends to decrease as the particle size further decreases. Explanations and critics of these results are discussed elsewhere (Bouillard et al., 2010). Based on the assessment of the reliability of these results, a specific risk assessment methodology is proposed here in order to address explosion hazards within a representative plant manufacturing aluminum nanopowders by taking into account the various work areas, steps and phases of the process.
2.
Risk analysis
2.1.
General methodology
Assessing ignition and explosion risk related to the use and production of nanopowders implies to proceed in a first step to the hazard identification of the products at the different workplaces and manufacturing processes. Secondly, it is required to evaluate, rank and prioritise risks before choosing the actions needed to manage ignition and explosion risks. The proposed methodology is based on the ATEX 1999/92/CE directive and the INRS-CNPP methodology (Vincent et al., 2005) and is designed to be quite simple, synthetic and reliable. The explosion risk was assessed by taking into account of three main parameters, i.e. the explosion frequency, consequences and the level of confidence of the technical and organisational safety barriers. These three parameters were combined to obtain a risk score RS which will help the user to prioritise the requested actions to manage these specific risks:
RS =
Explosion consequences × Explosion frequency Level of confidence of the barriers
This methodology has been applied to an industrial plant of aluminum nanopowders production by plasma processing (Vignes, 2008). The various workshops, steps and phases of the process have been considered, such as normal operation, process cleaning, storage . . .
2.2.
305
Explosion frequency
The explosion frequency is evaluated by taking into account of the formation probability of an explosive atmosphere (see Section 2.2.1) and the frequency of the presence of an ignition source (see Section 2.2.2).The combination of the following scores gives an explosion frequency score which ranges from 1 to 15 (Vignes, 2008).
2.2.1.
Formation probability of an explosive atmosphere
This parameter is related to the ATEX zoning for a given working unit. The minimum explosive concentration (MEC), the limiting oxygen concentration (LOC), the nature of the process as well as the behaviour of nanopowders in the air enable to determine the occurrence frequency of an ATEX atmosphere. It is therefore, necessary to briefly evaluate the reliability of the safety data as classical apparatus used for micropowders may not provide reliable results for nanopowders. The MEC was determined to value 30 g m−3 which seems to be in accordance with literature data and with the fact that it is generally considered that MEC becomes nearly independent of particle size below a diameter of 30–50 m (Dufaud et al., 2010a,b; Jacobson et al., 1964; Lödel, 1992; Mannan, 2005). However, it must be reminded that experimental results may be influenced by both agglomeration, which tends to be important below 30–50 m (Geldart, 1973), and by the ignition temperature of the particles (Jaeckel, 1924), which tends to decrease as the particle size lowers (Huang et al., 2005). Thus it may be considered that the MEC value for aluminum nanopowders is quite reliable if nanopowders are released from storage or deposits, i.e. under the form of agglomerates. The value of LOC was determined to be 5% by (Kwok et al., 2002) which still remains coherent with the current tested aluminum nanopowders. In fact, by taking into account an empirical relationship determined by (Siwek and Cesana, 1995) depending on MIE and MIT for a given product, the limiting oxygen concentration was estimated at 3% in the case of 200 nm aluminum by considering a minimum ignition temperature for aluminum nanopowder around 550 ◦ C (Hull, 2002). However, it should be kept in mind that LOC value could be overestimated because the standard test is generally performed under nitrogen, whereas nanosized aluminum may be manufactured under argon - as nitrogen can react at high temperatures, especially in a plasma processing unit. Finally, agglomeration level may also influence the reliability of the LOC value. By taking into account the previous parameters, related remarks, industrial feedback within the NANOSAFE 2 project and by considering the classical ATEX classification proposed by ATEX 1999/92/CE directive, a qualitative frequency of an ATEX formation might be attributed to a work unit. Indeed, a qualitative occurrence frequency of the formation of an explosive atmosphere in a given working unit was scored from 1 to 3 in accordance with the Table 1 which link the score and related ATEX zone.
2.2.2.
Frequency of the presence of an ignition source
In order to assess the probability of ignition of an explosive atmosphere, the MIT as well as MIE should be considered. The reliability of these parameters should also be discussed. The minimum ignition temperature for aluminum nanopowder values around 550 ◦ C for 90 nm aluminum nanopowders (Hull, 2002). However, considering MIT to evaluate the frequency of presence of an ignition source does not
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Table 1 – Classification criteria of frequency of ATEX occurrence. Score
ATEX zones
0 1 2 3
Off-zone 22 21 20
appear to be relevant in the case of non passivated aluminum nanopowders, which may be considered as pyrophoric (Jacobson et al., 1964). The MIT value may be only relevant in the case of partially passivated aluminum nanopowders (with a slight layer of alumina around the core of aluminum). For 100 nm particle size, aluminum nanopowder dust cloud can be ignited by minimum ignition energy lower than 1 mJ (Bouillard et al., 2010), which means that even minor unearthed plant items of very low capacitances (of the order of 1 pF) may be hazardous (Randeberg, 2005). For 200 nm, aluminum nanopowder dust cloud can be ignited by minimum ignition energy higher than 7 mJ (Bouillard et al., 2010) which is also rather lower than microsized aluminum powders (Vignes, 2008). Such results were also found in the case of iron and titanium nanopowders (Wu et al., 2009). The risk of tribo-electric charging of unearthed metal objects may therefore imply that even quite small objects (screws, bolts, etc.) must be earthed to reduce the risk of ignition of very easily ignitable aluminum nanodust clouds. However, it should be also reminded that the MIE of aluminum nanopowders may be over-estimated as the modified Hartmann tube tends to create electrostatic charges on the nanoparticles during the dispersion process, lowering the ignition energy in the dust cloud (Glor, 1988). Even if the MIE measurements may be slightly distorted by the process measurements it-self, we advised to consider that the measures are quite representative of the ignition risks induced by metallic nanopowders. By considering these elements, it becomes possible to score the frequency of the presence of an ignition source, from 1 to 5 with the help of the classification proposed by INRS and CNPP for chemical risks (Vincent et al., 2005), which is in fact a modified and more accurate classification than the one usually employed in the classic ATEX method. This classification is presented in Table 2.
2.3.
Explosion consequences
In order to determine the consequences of an explosion, it is necessary to consider the quantity of aluminum
Table 2 – Proposed score of the frequency of the presence of an ignition source. Score 1 2 3
4 5
Frequency of presence of the ignition source External accidental source or a natural event (e.g. malevolence, lightning) Ignition source due to a malfunction, a wear, a handling error (e.g. electric stoppage) Presence due to maintenance’s operations (e.g. hot spot working) Ignition source due to static electricity (e.g. load of organic material or inflammable materials) Occasional operation (e.g. presence of accumulator charge port, add-on heating appliance) Occasional presence due to the process Occasional presence not due to the process (e.g. smoker) Permanent presence of an ignition source (e.g. hot surface in the process’ equipment)
Classification criteria No presence Unlikely and only for short periods (never in normal operation) Occasionally likely in normal operation Frequently likely or for long periods or continuously
Table 3 – Proposed score of the violence index. Kst (bar m s−1 )
Score 1 2 3
0–200 201–300 >300
nanopowders, their explosion severity as well as the specificities of the process. The explosion severity is the combination of maximum explosion pressure and maximum rate of explosion pressure rise (Eckhoff, 2003). The maximum explosion pressure rise of a given nanopowder can be related to the explosion violence index Kst. The proposed classification is based on the St Classes of powders. This parameter is already used in the ATEX methodology and is summarized in the Table 3. The maximum explosion pressure obtained in the 20 L explosion sphere is mainly under the dependence of thermodynamics and thus can be roughly related to the combustion enthalpy of aluminum, which is of 31 MJ per kilogram of aluminum. It is then possible to combine the combustion enthalpy with the estimated mass of nanopowders to catch a hazard profile index related to the potential energy that can be released by the explosion of a nanodust cloud. In order to get a reliable qualitative index, we have considered as a reference the energy released by TNT to establish a non-dimensional ratio closed to an equivalent TNT mass. It should be underlined that this equivalent TNT mass will not be used in order to quantify an explosion pressure, as a contrary to the equivalent TNT method. By estimating this ratio, it becomes then possible to score specific range of equivalent TNT mass as illustrated in Table 4. The consequences of an aluminum nanopowders explosion can then be scored as the combination of TNT equivalent weight and explosion severity. The score attribution is partially based on the explosion characteristics of aluminum nanopowders, which have to be also carefully discussed. As a matter of fact, if the current results seem to be in accordance with the literature as far as low specific surface areas are considered, they do not plateau for nanosized particles but tend to decrease (Bouillard et al., 2010). This decrease, obtained for particles sizes lower than 1 m, also corresponds to a change of the combustion regime: from diffusion controlled, for large size particles to kinetically controlled, for small size particles (Bouillard et al., 2010). As no literature feedback enables us to validate experimental data, it is necessary to evaluate the validity of the 20 L explosion sphere, i.e. its ability to determine reliable
Table 4 – Criticity score linked to the equivalent TNT mass of aluminum nanopowders mass. Score Equivalent TNT mass [kg]
1
2
3
4
5
<0.5
0.5–5
5–10
10–50
>50
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Fig. 1 – Flame propagation of aluminum dust explosion in a modified Hartmann tube. 900 g m−3 of powder at 30 mJ for an ignition time tv of 120 ms. (a) 100 nm nanoparticles, (b) 3 m particles. results. These aspects were studied in details in (Vignes et al., 2009). This study concluded that the explosion severity of nanopowders may be affected by the following phenomena/parameters: (i) The pre-ignition phenomenon: it was observed that aluminum nanopowders can be pre-ignited during the injection process into the 20 L explosion sphere. Preignition phenomenon may lead to an increase of the initial temperature as well as an increase of alumina content. Consequently, it may be assumed that the measured explosion severity may be lowered due to the pre-ignition phenomena. (ii) The intrinsic alumina content. (iii) The agglomeration/aggregation level of the nanopowders. The validity of the 20 L sphere has thus been investigated. The explosion pressure Pm and rate of pressure rise (dP/dt)m were combined through the semi-empirical relation of Silvestrini et al. (2008) in order to calculate an experimental laminar flame velocity SUL,exp . This experimental flame velocity was compared to a theoretical one SUL,calc , determined by Huang’s model (Huang et al., 2005) which was initially established to predict laminar flame velocity in an aluminum dust cloud. This model assumes that the propagation of the flame is run mainly by conduction. By assuming that the main difference between the experimental and the theoretical flame velocity depends on the aggregation structure of the nanopowders (indirectly related to the specific surface area SBET ) and on the concentration C0 of the dust cloud, a semi-empirical relationship was established as SUL,calc = f(C0 , SBET ) · SUL,exp (Vignes et al., 2009). A Huang-like model could be used to represent our experimental data presented elsewhere (Bouillard et al., 2010), which means that the propagation flame is mainly conducted by conduction within the 20 L sphere. However, it should be reminded that it was experimentally and theoretically demonstrated that the flame propagation within nanodust may be mainly conducted by radiation (Proust, 2004). Two hypotheses can thus be made. One the one hand, it can be reasonably assumed that the 20 L sphere does not enable to get reliable data as it probably disturbs the flame propagation and thermal mechanisms by absorbing radiation (quenching due to wall effect). On the other hand, it has been observed, thanks to the use of a high speed camera and the visualization of the propagating flame edge structure (Fig. 1) that the
preheated zone is smaller for nanopowders (100 nm) than for micro-particles. It could notably be explained by the fact that the flame radiation is absorbed by the cloud of unburnt aluminum nanopowders. A bichromatic pyrometer could be used in order to check if this modification of the flame behaviour in the visible range could be due to a change in the radiation wavelengths (flame temperature modification) (Dufaud et al., 2010a,b).
2.4.
Level of confidence of the existing safety barriers
At this point, the relevance and effectiveness of some existing equipments which lower the severity or the probability of a nanopowder explosion should be taken into account. However, it should be kept in mind that the current relevance and effectiveness of the technical prevention and protection barriers is not evaluated yet. Technical barriers that are efficient to prevent and protect micropowder explosion may be irrelevant in the case of nanopowders. The worst case would, for instance, be that dispersed nanopowder induces detonation. Fortunately, explosion risk can also be lowered by organisational barriers. Despite the previously quoted lack of information, it is necessary to evaluate qualitatively the probability of failure on demand/confidence of identified safety barriers/independent protection layers. Moreover, multiple factors as the reliability, robustness, availability and efficiency have to be taken into account (Sklet, 2006). Thus, for simplicity, a safety barrier risk reduction score based on the barrier confidence level was considered. Such a ranking is proposed in the Table 5.
2.5.
Risk ranking
The methodology developed in Section 3 enables to obtain a final risk score from 1 to 225, which leads to a risk ranking. The determination of a risk score related to the explosion of nanopowders at a given working unit is then used to
Table 5 – Risk reduction scores for safety prevention and protection barriers. Score Confidence level of safety barriers
1
2
3
4
Lack of barriers
Low
Medium
High
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Table 6 – ATEX risk assessment for a given working unit.
Data Score
TNT mass
Explosion severity
ATEX zoning
Presence of ignition source
Level of confidence
Risk score
>50 kg 5
St3 3
Zone 22 1
Rare 2
Medium 3
10
characterise the priorities of the needed actions in order to reduce the risk to an acceptable level. The definition of thresholds for risk acceptability has been an essential step due to the peculiarity of this emerging risk. The risk acceptability has been defined as a function of the risk perception by the workers, the society, but also with regard to the risk management policy of the company, the regulatory context and the legal obligations. It has led to the development of suitable action plans aiming at reducing the risk criticality to an acceptable value for the workers. An example of risk ranking is given on Table 6 for a given work unit where nanopowders are handled. Every workstation of a plant can be examined in that way and prevention and protection measures analyzed. Additional safety barriers can be set up until the explosion risks have been finally found to be acceptable.
2.6.
Additional safety barriers
In order to reduce the dust explosion hazard, general safety measures could be taken, safety barriers could be placed (Sklet, 2006) to provide multiple devices, systems or actions that reduce the likelihood of an undesirable event according to the key principles of inherent safety: minimization, substitution, moderation and simplification (Amyotte et al., 2009;
Kletz, 1998) in an approach focused on elimination or reduction of intrinsic hazards associated to a set of conditions for a determined substance or process. The reduction of the volume of the process equipment can minimize the risks in operation by decreasing the quantity of material or energy contained in a manufacturing process. It is also appropriate to perform reliability-centered maintenance techniques to increase inherent safety by downtime, thus the intermediate storage. Substitution is the replacement of a hazardous material or process with an alternative that reduces or eliminates the hazard. The moderation strategies can provide less hazardous conditions for handling of combustible solid materials. Some alternatives consist in the reduction of nanoparticles concentrations at workplaces, in the modification of the process equipment or the operation mode as manipulation in liquid form. Simplification consists of a design of the process that eliminates unnecessary complexity and in this way reduces the opportunities for error and misoperation. In addition to such general actions, some specific safety barriers could be proposed. For instance, specific cleaning devices should be used as HEPA-ULPA vacuum cleaners, earthing of booths, tables, devices are compulsory to prevent sparks generation due to static electricity; the filtration/vacuum cleaning processes should be adapted to the particles size. . .
Table 7 – Various types of barriers and their applications in this case study. Barriers classification
Definition
Examples
Human: activated barrier – procedural
Barrier with human detection/diagnostic/action
Symbolic: activated-warned
Human action based on passive warning
Activated barrier – manual
Manual action in response to alarm/detection Barrier activated by a mechanism. Need a preliminary diagnostic. Barrier put in place by a person
ATEX regulations, nanoparticles regulation at a local level, cleaning procedures, appropriate waste processing, predictive maintenance procedure . . . Safety signs, plans and instructions posters, up-to-date safety documents/MSDS, adequate GHS labelling . . . Manual system shutdown, water deluge systems, manual tank draining . . . Ventilation, adequate media filters (ULPA), inerting, dust collecting (catch tank) . . .
Permanent active barriers
Temporary passive barriers
Passive barriers – inherent
Based on design properties
Passive barriers – add-on
Passive components added to a system because of safety considerations
Adequate personal protective equipment (nitrile gloves, airline hood, non-woven coverall, FFP3/N100 dust masks . . .) Isolated pumps, vacuum lines, cleanrooms, laminar flow booths . . . Containments, dust extraction by exhaust ventilation, electrical grounding . . .
Proportion in %
Inherent safety principle
40
Minimize Simplify
6
Minimize Moderate
5
Moderate
14
Moderate
10
Moderate
15
Minimize Simplify
10
Moderate
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The validity of ingress protection ratings protection should also be discussed. Moreover, nitrile gloves are recommended in order to avoid dermal exposure. Protection measures should also be taken as the installation of explosion panels and explosion venting to minimize the severity of the explosion; the use of vessels able to withstand the maximum force generated by an explosion. . . In order to formalize and optimize the use of safety barriers, the classification of Sklet (2006) has been slightly modified and applied to this case (Guldenmund et al., 2006). The level of confidence of such additional barrier has also been assessed as done for the existing barriers (see Section 2.4). Such application and safety barrier placement thanks to a fault tree analysis have already been done in the case of a laboratory handling nanopowders (Dufaud et al., 2006) Examples of barriers are given in Table 7.
3.
Conclusion
Two kinds of aluminum nanopowders were tested in order to evaluate their flammability and explosivity characteristics and provide accurate and reliable data for occupational and major risks analyses. Experimental and theoretical investigations lead to the conclusions that the minimum ignition energy of such nanoparticles could be lower than 1 mJ, which induces a very high ignition sensitivity. The explosion severity has also been determined and the influence of several parameters has been highlighted. In fact, the safety parameters may be affected by agglomeration, wall-quenching and pre-ignition phenomena, especially for highly flammable aluminum nanopowders. The question of the leading phenomenon of the heat transfer in the flame propagation is also put forward. If, as proposed by certain authors, the predominance of radiative phenomena over conductive transfer is established in the case of well-dispersed nanoparticles (Proust, 2006), then the risk of detonation could be theoretically founded and the validity of the cubic law used to extrapolate the explosion severity values to other volumes questioned (Vignes et al., 2009); as this semi-empirical relationship assumes that the flame thickness is relatively thin. If these points are correctly addressed, it will be possible to get reliable explosion severity data and detect potential detonation risk or, at least, high explosion risk. Finally, relying on our experimental study, the proposed risk analysis method has been successfully applied to an industrial plant in order to define the best safety barriers which had to be positioned to ensure the best occupational safety level to all workers. By highlighting the main issues related to the assessment of the explosion risks within a facility handling nanopowders, this work provides useful information to the QHSE engineers concerned about the use and the production of combustible nanoparticles. This approach is consistent with the European Directives on occupational risk assessment 89/391/EEC and on explosive atmospheres 1999/92/EC and 94/9/EC.
Acknowledgments This study was carried out with the financial support of the European Commission through the Sixth Framework program for Research and Technological Development NMP2-CT-2005515843 contract “NANOSAFE2”.
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