Explosions of Vapour/Dust Hybrid Mixtures: A Particular Class - Summary

Explosions of Vapour/Dust Hybrid Mixtures: A Particular Class – Summary

1-Sentence-Summary: Hybrid mixtures from the pharmaceutical industry demonstrate non-linear and in some cases “more than additive” explosion consequence and reduced flammability limits.

Authors: O. Dufaud, L. Perrin, M. Traore, S. Chazelet, and D. Thomas

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Explosions of Vapour/Dust Hybrid Mixtures: A Particular Class - Summary

This is a follow on paper to a previous publication by these authors (Dufaud et al., 2008). In the current work they extend their previous explosion testing of niacin/diisopropyl ether mixtures and present experimental data for magnesium stearate/ethanol and antibiotic/toluene. The median particle diameters for the niacin, magnesium stearate, and antibiotic are 26 µm, 6 µm, and 20 µm, respectively.

The explosion experiments are completed in a 20-L chamber using two 5 kJ pyrotechnic ignitors. The explosion experiments were completed based on the standards ASTM E681-94 and ISO 6184-1, for the gas and dust, respectively. For the hybrid experiments two 5 kJ ignitors were used.

The authors of this article give contour plots summarizing maximum pressure and maximum rate of pressure rise for the different mixtures tested. They also test a “Le Chatelier” like law to predict iso-pressure and iso-pressure-rise contours in the data.

Three of the main findings from this paper are:

  1. More than additive effects are seen for explosion consequence, especially maximum rate of pressure rise.
  2. A “Le Chatelier” like law overpredicts the fuel necessary to achieve specific pressure-rise rates for lean mixtures and underpredicts the fuel necessary for rich mixtures.
  3. The explosibility limits of the hybrid mixtures are wider than predicted using Le Chatelier’s Law.

The following sections outline the main findings in more detail. The interested reader is encouraged to view the complete article at the link provided below.

Finding #1: Synergistic effects were found for maximum rate of pressure rise

Similar to their previous paper (Dufaud et al., 2008), the current authors found “more than additive” effects for maximum rate of pressure rise of pharmaceutical hybrid mixtures. This means that the maximum rate of pressure rise of a hybrid mixture can be larger than the maximum rate of pressure rise of the fuels individually. The authors suggest that the combustion kinetics during mixture of the gaseous and solid fuels is non-linear leading to the synergistic effects.

In general the peak maximum rate of pressure rise for the hybrid mixtures studied was over 1600 bar/s, where the individual gases and dusts tested ranged from 800 to 1400 bar/s. Furthermore, these maximum hybrid values tended to occur at quite a bit lower concentrations of the fuels than their worst case concentrations individually. This is important for the pharmaceutical industry as the hazard classification of the hybrid mixture may be significantly higher than that of the dust alone.

Finding #2: A “Le Chatelier” like mixing law can predict maximum pressure but not maximum pressure rise

The authors tested a “Le Chatelier” like mixing law (linear mixing) for determining the explosion parameters of hybrid mixtures. The authors state that this approach approach works for predicting the nearly linear “iso-pressure” curves. However, when applied to maximum rate of pressure rise, the linear mixing law overpredicts the amount of fuel necessary to achieve a given maximum rate of pressure rise for fuel lean mixtures, and underpredicts the amount of fuel needed for fuel rich mixtures.

Finding #3: Hybrid explosibility limits are wider than predicted by Le Chatelier’s Law

Contrary to other papers published in the literature such as Pialo et al., 2006, the current paper suggests that the explosion limits for the hybrid mixtures tested are wider than predicted using Le Chatelier’s law. It is currently unclear if the non-linear combustion kinetics seen for the explosion parameters play a role in the flammability limits. This paper also used 10 kJ ignitors where some of the papers that saw narrower limits than Le Chateliers law used spark ignition.

My Personal Take-Aways From
“Explosions of Vapour/Dust Hybrid Mixtures: A Particular Class”

This paper published some very interesting results for consequences of pharmaceutical hybrid mixture explosions. To my knowledge very few papers in the field presented truly “more than additive” effects for hybrid mixtures (e.g., Dufaud et al., 2008 and Khalil, 2013). This may be due to both the gases and dusts having on-board oxygen molecules that could further oxidize the fuel. This is in contrast to that hydrocarbon fuels typically studied which can only be oxidized from oxygen in the air. Computer simulation with detailed kinetics modelling may play a role in developing an understanding of the combustion kinetics.

Similar to the previous paper by these authors this paper would be useful to review for anyone involved with the pharmaceutical industry. Mixing of vapour solvents and excipient or active solids can lead to hybrid explosion scenarios. In contrast to the flammability limit work of Pilao et al, 2006, the flammability limits were narrower than predicted using Le Chateliers’s law. This also has important implications for hazard assessment.

Full Citation:

  • O. Dufaud, L. Perrin, S. TraorĂ©, S. Chazelet, and D. Thomas, “Explosion of vapour/dust hybrid mixtures: A particular class,” Powder technology, vol. 190, pp. 269-273, 2009.
    title={Explosion of vapour/dust hybrid mixtures: {A} particular class},
    author={Dufaud, O. and Perrin, L. and Traor\'{e}, S. and Chazelet, S. and Thomas, D.},
    journal={Powder Technology},

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