Study of the Severity of Hybrid Mixture Explosions and Comparison to Pure Dust-Air and Vapour-Air Explosions - Summary

Study of the Severity of Hybrid Mixture Explosions and Comparison to Pure Dust-Air and Vapour-Air Explosions – Summary

1-Sentence-Summary: The explosion severity of niacin dust/acetone gas mixtures decreases along lines of constant mixed stoichiometric ratio and have the largest value for pure acetone at its stoichiometric concentration.

Authors: R. Sanchirico, A. Di Benedetto, A. Garcia-Agreda, and P. Russo

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Study of the Severity of Hybrid Mixture Explosions and Comparison to Pure Dust-Air and Vapour-Air Explosions - Summary

The authors of this paper perform experimental explosion testing for hybrid mixtures of niacin dust (also called nicotinic acid or vitamin B3) and acetone gas. The experiments are performed in a 20-L explosion chamber with spark ignition. The turbulence level in the chamber is varied by using ignition delay times of 30, 60, and 120 ms.

The dust used in this testing has an average diameter of 32 µm. The dust concentrations are varied from 30 to 500 g/m3 and the gas concentrations from 0.6 to 7% by volume. The 20-L chamber is fitted with a water jacket to control the internal wall temperature.

Results are presented for maximum pressure and maximum rate of pressure rise (deflagration index) of the pure fuels at different turbulence levels. The deflagration index of hybrid mixtures is then explored based on fuel concentration and using the explosion regime diagram concept previously developed by these authors (see Garcia-Agreada et al., 2011).

This paper concludes by comparing the hybrid mixture flammability limits to Chatelier’s Law and Bartknecht’s curve. Literature data from Denkevits et al., 2007 and Pilao et al., 2006 is also plotted on explosion regime diagrams for reference.

Three of the main findings from this paper are:

  1. The turbulence level of hybrid mixtures must be held constant for the gas, dust, and hybrid tests in order to make accurate comparisons between the fuels.
  2. If measured along lines of constant stoichiometric ratio the explosion severity is highest for pure gas and lowest for pure dust mixtures.
  3. The explosion limits of acetone/niacin mixtures are independent of turbulence level and are narrower than predicted by 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: Turbulence plays a dominant role in gas, dust, and hybrid mixture explosion violence

Although maximum pressure of pure gaseous fuel is insensitive to turbulence level, the deflagration index is strongly influenced by this parameter. The maximum deflagration index throughout the gas concentration range increased by a factor of 7 moving from quiescent conditions to an ignition delay of 30 ms. Furthermore, the current authors were unable to reproduce deflagration indices of 65 bar-m/s for quiescent mixtures as reported in the NFPA standards. A quiescent deflagration index of almost 100 bar-m/s was found instead.

Turbulence level had a measurable, but small effect on maximum pressure of the pure dust explosions. However, the deflagration index increased by a factor of 3 moving from 120 to 30 ms ignition time delay. The small drop in maximum pressure suggests that dust sedimentation is occurring at the higher ignition delay times.

The explosion severity of hybrid mixtures was also greatly dependent on turbulence level. For 100 g/m3 and 4 % acetone, the deflagration index was approximately 500, 400, and 300 bar-m/s for 30, 60, and 120 ms ignition delay, respectively. The enhancing effect of adding acetone gas to niacin dust appeared to be largest at the highest turbulence level.

Finding #2: Explosion severity decreases with dust addition along lines of constant mixed stoichiometric ratio

From plotting the hybrid results as an explosion regime diagram the current authors demonstrated that the worst case explosion occurred for the pure gas at the stoichiometric concentration if turbulence was kept constant. This is in contrast to previous papers in the literature that showed “more than additive effects” (e.g., see Dufaud, 2008 and Dufaud, 2009).

The current authors also visually demonstrate that the deflagration index decreases as dust is substituted for gas in stoichiometric concentrations. They showed this by plotting lines of constant nominal stoichiometric ratios on the regime diagram. Moving from pure gas to pure dust along these lines showed decreasing deflagration indices. They also showed similar trends for other data previously published in the literature (href=”http://www.mydustexplosionresearch.com/explosibility-of-hydrogen-graphite-dust-hybrid-mixtures-summary”>Denkevits et al., 2007 and Pilao et al., 2006).

Finding #3: Explosion limits of acetone/niacin mixtures are narrower than predicted by Le Chatelier’s law .

The authors compared hybrid mixtures which resulted in no explosion to Le Chatelier’s Law and the curve proposed by Bartknecht (See Dust Explosion: Course, Prevention, and Protection). This comparison showed that niacin/acetone flammability limits are narrower than predicted by both of these approximations (i.e., more fuel is needed to have a flammable system). The turbulence level had little impact on this finding and one unique curve appear to delineate all of the tested results.

My Personal Take-Aways From
“Study of the Severity of Hybrid Mixture Explosions and Comparison to Pure Dust-Air and Vapour-Air Explosions”

This study is unique in the literature due to the shear amount of tests completed and quality of the data. Their findings suggest that explosion prevention for hybrid mixtures should concentrate on protecting against the stoichiometric gas concentration as the worst case scenario. A difficulty here is that this may be significantly over conservative if a case can be made for an alternative minimal credible accident scenario. Also, time based protection measures designed around this criteria (e.g., suppression in ducts) may occur much too fast if a slower explosion from a fuel-lean mixture occurs.

These authors also found no “more than additive” hybrid effects as seen by others in the literature (e.g., Dufaud, 2008 and Dufaud, 2009). This suggests that specific dust/gas mixtures may have an affinity for synergistic combustion kinetics. A detailed analysis of the combustion kinetics would be an interesting contribution to this area.

The results from this paper would be of interest to anyone studying hybrid explosion. Furthermore, acetone gas and niacin dust are both relevant materials for the pharmaceutical industry, and this paper gives some important implications for the role of turbulence level during explosion testing. The information on flammability limits is also useful for explosion prevention in industries using these materials.

Full Citation:

  • R. Sanchirico, A. Di Benedetto, A. Garcia-Agreda, and P. Russo, “Study of the severity of hybrid mixture explosions and comparison to pure dust-air and vapour-air explosions,” Journal of loss prevention in the process industries, vol. 24, pp. 648-655, 2011.
    [Bibtex]
    @ARTICLE{Sanchirico2011,
    title={Study of the severity of hybrid mixture explosions and comparison to pure dust-air and vapour-air explosions},
    author={Sanchirico, R. and Di Benedetto, A. and Garcia-Agreda, A. and Russo, P.},
    journal={Journal of Loss Prevention in the Process Industries},
    volume={24},
    pages={648--655},
    year={2011},
    link ={http://www.sciencedirect.com/science/article/pii/S095042301100074X},
    summary = {http://www.mydustexplosionresearch.com/study-severity-hybrid-mixture-explosions-summary},
    }

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