Determining the Influence of Particle Size and Surface Area on the Measured Minimum Ignition Energy of Pharmaceutical Powders

Dust explosions constitute a significant risk in many industries. To identify materials for which mitigation strategies are required, information about the relative risk for each material is required. Such information will include characteristics such as the minimum ignition energy (MIE), but material availability represents a significant challenge for the pharmaceutical industry at early stages of development. These challenges contrast with the relatively high material requirements for risk characterization. To this end, there is significant interest in the application of models to predict MIE. The aims of this study were 2-fold. The first stage was to assess the predictive strength of a published MIE prediction model for a range of pharmaceutical powders. The second stage of the study was to investigate the role of particle size for a series of samples of ‘constant chemistry’ and varying size. The results demonstrate that the model provided ‘safe’ results for half the materials tested while the accuracy of the model was unsatisfactory. The results showed that the risk was often overestimated; thereby, the work required for safety mitigation would not add value to the process, or underestimated, raising the risk of inadequate safety mitigation. When the chemistry of the materials was maintained constant, significant differences in the relationship of particle size and surface area with the measured MIE were demonstrated. Overall, the work suggests that the relative influence of chemistry and particle properties on MIE shows notable differences between materials, thereby affecting the ability of the assessed model to accurately predict MIE.