Influence of liquid and vapourized solvents on explosibility of pharmaceutical excipient dusts

Hybrid mixtures of a combustible dust and flammable gas are found in many industrial processes. Such fuel systems are often encountered in the pharmaceutical industry when excipient (non-active ingredient) powders undergo transfer in either a dry or solvent prewetted state into an environment possibly containing a flammable gas. The research described in this paper simulated the conditions of the above scenarios with microcrystalline cellulose (MCC) and lactose as excipients, and methanol, ethanol and isopropanol as solvents. Standardized dust explosibility test equipment (Siwek 20-L explosion chamber, MIKE 3 apparatus and BAM oven) and ASTM test protocols were used to determine the following explosibility parameters: maximum explosion pressure (Pmax), size-normalized maximum rate of pressure rise (KSt), minimum explosible concentration (MEC), minimum ignition energy (MIE), and minimum ignition temperature (MIT). Because the MIKE 3 apparatus and BAM oven are not closed systems, only baseline excipient-alone testing and excipient pre-wetted with solvent testing were possible for MIE and MIT determination. With the Siwek 20-L chamber (a closed system), it was feasible to conduct Pmax, KSt and MEC testing for all three cases of the dust alone, prewetted with solvent, and with solvent admixed to the combustion atmosphere at 80 % of the lower flammability limit for each solvent prior to dust dispersal. The experimental results demonstrate the significant enhancements in explosion likelihood and explosion severity brought about by solvent admixture in either mode. The extent of solvent influence was found to be specific to the given excipient and method of solvent addition. Solvent burning velocity considerations help to account for some of the experimental observations but for others, a more rigorous evaluation of solvent and excipient physical property data is needed.