Hybrid mixture explosion research is typically conducted with three possible approaches: (i) gaseous solvent at room temperature existing in the combustion atmosphere prior to dust dispersal, (ii) liquid solvent at room temperature requiring flashing-off for admixture to the combustion atmosphere prior to dust dispersal, and (iii) liquid solvent at room temperature admixed as a liquid with the dust prior to dust dispersal.
The current project utilizes the latter two of the above approaches with common pharmaceutical excipients and solvents – lactose and microcrystalline cellulose (MCC), and methanol, ethanol and isopropanol, respectively. All materials tested are pharmaceutical-grade in terms of composition and, in the case of the solids, particle size distribution.
Explosibility parameters investigated include 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). ASTM protocols are being followed using standard dust explosibility test equipment (Siwek 20-L explosion chamber, MIKE 3 ignition energy apparatus, and BAM oven for ignition temperature measurement).
Because the MIKE 3 apparatus and BAM oven are not closed systems, only baseline excipient-alone testing and excipient pre-wetted with solvent testing are possible for MIE and MIT determination in our laboratory. With the Siwek 20-L chamber (i.e., a closed system), however, it is feasible to conduct Pmax, KSt and MEC testing for all three cases of the dust alone, pre-wetted with solvent, and with solvent admixed to the combustion atmosphere prior to dust dispersal.
At the time of writing, Pmax, KSt, MEC, MIE and MIT tests have been conducted (with ongoing data analysis) for lactose and MCC, and also for each dust pre-wetted with methanol, ethanol and isopropanol at 80 % of the lower flammability limit for each solvent. Testing is currently underway for Pmax, KSt and MEC of the dusts with solvent admixed directly to the combustion atmosphere.
As preliminary commentary, we offer the following thoughts on data trends. In all cases, pre-wetting of MCC and lactose with solvent had a measurable impact on each explosibility parameter (Pmax, KSt, MEC, MIE and MIT). As expected, the influence was generally an enhancement of the particular parameter (e.g., increase in KSt, decrease in MIE, etc.); the lone exception was Pmax for MCC which displayed a decrease of 0.6-0.8 bar(g) with solvent admixture. Additionally, while the magnitude of the effect of solvent admixture to MCC was generally distinguishable for the different solvents, this was not the case for lactose. Pre-wetting of lactose with each of the three solvents resulted in similar values of Pmax, KSt and MIE.
It therefore appears that burning velocity considerations alone may adequately account for the enhanced KSt values brought about by solvent pre-wetting of microcrystalline cellulose (with laminar burning velocities of 56 cm/s for methanol, 42 cm/s for ethanol and 41 cm/s for isopropanol). As indicated above, this does not appear to be the case for solvent pre-wetting of lactose. Consideration of physical properties such as solvent latent heat of vaporization, and excipient heat capacity and solubility, may be required to attempt a phenomenological explanation of the observed data trends. It is anticipated that further insight will be provided by the ongoing experiments with solvent admixture by flashing-off into the partially evacuated 20-L chamber prior to dust dispersal.
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