Explosions of vapour/dust hybrid mixtures: A particular class O. Dufaud ⁎, L. Perrin, M. Traore, S. Chazelet, D. Thomas CNRS LSGC — ENSIC — 1, rue Grandville BP 20451 54001 Nancy Cedex, France ABSTRACT ARTICLE INFO Available online 30 April 2008 Keywords: Hybrid mixtures Dust explosion Vapours Explosion severity Explosivity limits The aim of this work is to demonstrate the particular aspects of the explosion of hybrid mixtures with respect to either vapour or dust explosions. This work is focused on pharmaceutical products from excipients to active drug substances and their associated solvents. Experiments with an explosion sphere has been used to determined the influence of dusts and vapours concentrations on the severity of explosions (maximum pressure P max and maximum rate of pressure rise (dP/dt) max ). Results clearly show that the explosion behaviour of such hybrid mixtures reveals significant differences with respect to either vapour or dust explosions; especially a promotion effect on the combustion kinetics and on the rate of pressure rise for poor mixtures. The influences of the introduction of small amount of vapour on the minimum explosible dust concentration (MEC) and of dust addition on the lower explosion limit (LEL) are noteworthy. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Due to their occurrence and gravity, dust and gas explosions have been widely studied for hundreds of years. Nevertheless, only few researches concern the explosion of hybrid, i.e. gas or vapours and dusts mixtures. In 1885, the experiments of Engler on coal dust explosivity in presence of methane could be considered as the first tests on hybrid mixtures [1]. One could also quote the researches of Pellmont on the effect of combustible gas introduction on the minimum ignition energy of dust clouds [2]. Methane, propane or more generally gaseous alcanes were used to study these special combustible mixtures [3–5]. More recently, Nifuku et al. have characterized the explosivity and ignitability of polyurethane as a function of the presence of cyclopentane vapours [6]; which is one of the only references dealing with vapours. In fact, hybrid mixtures are encountered in various industries such as paint factories (pigments and solvents), mining (dusts and gas), grain elevators (small grains and fermentation gases) or pharmaceu- tical industries; and accidents caused by their explosions are neither scarce nor minor (1994 — Groβ-Umstadt, Germany) (1906 — Courrières, France: methane explosion leading to secondary coal dusts explosions: 1100 fatalities). In spite of these facts, it would be naive to suppose that every risk analyses take this specific hazard into account. When detected, these explosible atmospheres are often treated the same way as for dust or gas taken individually. Moreover, data concerning hybrid dusts and vapours mixtures remain scarce. As a consequence, the aim of this work was to underline and identify the specific explosion characteristics of vapour–dust hybrid mixtures. This study has been mainly focused on pharmaceutical products. Firstly, the explosion severity and ignition sensitivity of the pure compounds have been determined. Secondly, solvent vapours and combustible powders have been associated in order to compare their explosivity to the previous results. 2. Experimental set-up and procedure 2.1. Powder characteristics and preparation Tests were notably carried out with three kinds of powders: an excipient (or lubricant) — magnesium stearate, a vitamin — B3, nicotinic acid, also called niacin, and an antibiotic. Magnesium stearate was supplied by Sigma-Aldrich and niacin was provided by Kuhner AG in order to perform the calibration tests of the 20 L sphere. The particle- size distribution of the dusts was determined by using a laser diffraction analyzer (Mastersizer, Malvern Instrument) or by SEM observations (antibiotic). The samples were characterized by the d 10 , d 50 and d 90 quantiles of the volumetric distribution as indicated on Table 1; the d x diameter, being defined as the diameter below which x% of volume of the particles lies (Table 1). Powder Technology 190 (2009) 269–273 ⁎ Corresponding author. E-mail address: dufaud@ensic.inpl-nancy.fr (O. Dufaud). Table 1 Particles sizes of the dusts Samples d 10 (μm) d 50 (μm) d 90 (μm) Magnesium stearate 3 6 15 Niacin 12 26 104 Antibiotic – 20 – 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.04.046 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec