Effect of admixed solid inertants on dispersibility of combustible dust clouds in a modified hartmann tube

https://doi.org/10.1016/j.psep.2019.12.017Get rights and content

Highlights

  • Actual particle size in dispersed dust clouds is much larger than the primary particle size

  • Micro Al2O3 can increase dust dispersibility by disrupting the combustible particles contact

  • Nano Al2O3 can decrease dust dispersibility by increasing cohesion of the powder mixtures

  • Degree of dust dispersion is related to particle-particle interactions

  • Nano-sized inertants show excellent promise in dust explosion prevention

Abstract

Reduction of dust explosion hazards can be achieved by processing materials in less hazardous forms. Two principles are often employed to moderate explosion potential. One approach is to increase dust particle size so as to decrease its reactivity. The other involves altering the dust composition by admixture with solid inertants. An essential prerequisite for dust explosions is the formation of airborne dust clouds. Dust dispersibility (i.e., the ease of dispersion of a dust and the tendency of the particulate matter to remain airborne once a dust cloud has been formed) is therefore an important safety characteristic, certainly requiring research attention. This paper presents experimental results using a modified Hartmann tube to determine the effect of admixed solid inertants on particle size distribution of dust clouds. Nine types of combustible dust and two sizes of inert Al2O3 were tested repeatedly five times. Generally, admixing small amounts of micro-sized solid inertants caused relative improvement on the dispersibility of combustible dust, forming a better dispersed cloud, and sometimes resulted in increased ignition sensitivity. However, the addition of agglomerate nano inert particles caused a multilayer coating of combustible particles, causing an increase in the effective surface area at the particle contact points, and thereby increasing cohesion in the powder mixtures. Higher inter-particle forces in turn enhanced the combustible particle agglomeration during dispersion, leading to an increase in effective particle size distribution.

Introduction

Dust explosions are a significant hazard in industries that process or transport combustible dusts, causing serious loss to human life and property (Yuan et al., 2015). Particle size and distribution are considered among the most important factors influencing dust explosions. Recent advancements in material science and engineering technology have resulted in a remarkable development in the use of nanomaterials. As a consequence, nanosafety issues have emerged as new areas of concern, covering explosion risk, environmental exposure risk, occupational risk and health risk at different release scenarios (Bressot et al., 2015, 2018a, 2018b; Morgeneyer et al., 2018). Much attention has been caught in understanding potential hazards related to exposures of ultrafine powdered materials (Warheit, 2018; Morgeneyer et al., 2019).

As for explosion risk, dust ignitability and explosivity generally increase with decreasing particle size (Eckhoff, 2003). The effects of a decrease in particle size of combustible dust have been well established and include decreasing minimum ignition energy (MIE), decreasing minimum ignition temperature (MIT), decreasing minimum explosible concentration (MEC), and increases in the maximum explosion pressure (Pmax) along with size-normalized maximum rate of pressure rise (Kst) (Mittal, 2014; Yuan et al., 2014a, 2014b; Azhagurajan et al., 2012; Bouillard et al., 2010; Wu et al., 2009; Iarossi et al., 2013; Nifuku et al., 2007; Boilard et al., 2013; Li et al., 2011, 2016; Yuan et al., 2012). Increasing particle size is an effective explosion prevention strategy, since larger-sized particles of explosible dusts are inherently safer than finer sizes of the same dust (Amyotte et al., 2007).

Another way to reduce or eliminate the risk of dust explosion is to mix solid inertants (i.e., non-combustible dusts) with combustible dust in sufficient quantity that the resulting mixture is rendered non-explosible (Amyotte, 2006). Generally, increasing the solid inertant concentration decreases ignition sensitivity (i.e., higher MIE, higher MIT, and higher MEC) and explosion severity (i.e., lower Pmax and lower Kst) until a threshold is reached where no ignition is obtained (Janès et al., 2014; Addai et al., 2016; Yu et al., 2018; Dastidar and Amyotte, 2002). With respect to inertant particle size, it is also well established that finer sizes of a given inertant are more effective at explosion inerting and suppression than coarser size fractions (Amyotte, 2006; Bu et al., 2020; Jiang et al., 2018).

However, the addition of solid inertants does not always reduce explosion risk, but may in fact facilitate ignition under special circumstances. Janès et al. (2014) reported a decrease in both MIE and MIT after adding small amounts of inert powder. Bu et al. (2019a, 2020) reported an increase in flame spread velocity in Ti powder clouds in the presence of 10 % by wt of inert TiO2 powder, and a decrease in MIE when micro-sized inert Al2O3 was admixed at 5 or 10 % by weight with 1000−1500 g/m3 Al mixtures. These results indicate that within inert powder concentration ranges too low to enable efficient inerting, the admixed solid inertant may actually increase the dust explosion hazard by ameliorating powder dispersibility and thus forming a better dispersed dust cloud. Thus, solid inertants not only act as a thermal inhibitor or chemical suppressant in dust combustion, but also may impact the dust dispersion process. The effect of admixed inertant on dust dispersibiliy requires further clarification.

In dust/air mixtures, dust particles are strongly influenced by gravity, an essential prerequisite for dust explosions is therefore the formation of dust/oxidant suspensions. Due to the continuously falling of dust particles, the particle size distribution of a certain dust cloud can vary considerably during different stages of dust dispersion (Murillo et al., 2013). In addition to primary particle size, the degree of agglomeration of the powder also plays a significant role in dust explosion potential. If agglomerates are not broken up during dispersion, they tend to behave as large single particles. For very small particles, especially nano powders, inter-particle attractive forces (i.e., Van der Waals forces) are not negligible, allowing incomplete dispersion, and thereby impacting dust ignition (Eckhoff, 2003). Indeed, dust ignition has been strongly linked to the small particles in a cloud, and the widely used mean volume diameter (i.e., D50) cannot always represent the real dust explosion hazard (Murillo et al., 2013). In a previous study, it was found that adding nano-sized Al2O3 increased the degree of agglomeration in Al dust clouds by binding Al particles together forming larger-sized aggregates that reduced dispersion, and thus alleviated the explosion hazard (Bu et al., 2020). It may be possible to ameliorate dust explosions by means of introducing very cohesive nano-sized inertants, assuming the addition actually decreases dispersibility in all types of combustible dust clouds.

This research was designed to further clarify the effect of admixed solid inertant on the dispersibility of dust clouds, by determining the effective particle size distributions of combustible/inert powder mixtures in a modified Hartmann tube. Nine types of combustible dust along with alumina at two particle sizes were selected as combustible and inert media, respectively. The results of flowability/cohesiveness of the mixtures are also presented to extend the scope of solid inertant application.

Section snippets

Materials

Dusts of activated charcoal, anthraquinone, ascorbic acid, cornstarch, microcrystalline cellulose (MCC), nicotinic acid, polyethylene, titanium, and wood were selected as representing the various industries (e.g., mining, chemical, pharmaceutical, food, and processing) worldwide in which combustible dust explosions are an issue (Yuan et al., 2015). All samples except wood dust were provided by commercial manufacturers. Wood dust was obtained by sanding red palm wood taken from the manufacturing

Effective particle size distribution in the modified Hartmann tube

Hartmann tubes are used worldwide to determine the MIE values of dust clouds. The degree of dust dispersion in a Hartmann tube may affect the MIE results. Currently, there is no practical experimental method that can produce an optimal dust cloud for MIE determinations. Either cloud turbulence is too high or the degree of dust dispersion is inadequately low. Excellent dust cloud dispersion requires a powerful, intense blast of air which results in high turbulence. However, dust clouds of low

Conclusions

This study presents experimental results on the effective particle size distribution of combustible dust clouds in the presence of low loading inertant powder in a Hartmann tube. Activated charcoal, anthraquinone, ascorbic acid, cornstarch, microcrystalline cellulose (MCC), nicotinic acid, polyethylene, titanium, and wood dusts were used as combustible materials, while 2.5-μm and 50-nm Al2O3 were used as inert additives.

Admixed 2.5-μm Al2O3 slightly increased the dispersibility of combustible

Conflict of interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work entitled “Effect of admixed solid inertants on dispersibility of combustible dust clouds in a modified Hartmann tube”.

Acknowledgements

The authors gratefully acknowledge financial support from the National Key Research and Development Program (No. 2017YFC0804703), the National Natural Foundation of China (Nos. 51974189, 51604175, 51874070 and 51774068), the Fundamental Research Funds for the Central Universities (N180701011, N170108029), and the Shenyang Science and Technology Program (19-109-4-19). This research was also supported by the opening project of State Key Laboratory of Explosion Science and Technology (Beijing

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