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Hybrid Mixtures - Is Your Explosive Dust Testing Giving You Applicable Data?

Posted by Clayton R. Johnson, PhD on 10.01.20

It is necessary, and often mandated, that explosive dusts be characterized in order to help develop a mitigation plan for explosion hazards, which in turn ensures increased process safety. Typically, dust explosivity testing is performed under standardized, ambient conditions as outlined by ASTM and other international standards. However, numerous industrial processes do not occur under these standard conditions. For example, the process may occur in the presence of a fuel vapor, fuel gas, and/or at an elevated temperature, all of which could affect the dust explosivity properties and thus effect the requirements for mitigating the hazard.


Combustible dust clouds that form in the presence of a fuel vapor or gas are often referred to as hybrid mixtures. Hybrid mixtures are commonly present in situ when a process requires the use of a volatile solvent during chemical synthesis (e.g. alcohols, benzene derivatives), gases from decomposition products (carbon monoxide from smoldering material), or if a natural fuel gas is present in the environment (e.g. mining). Currently, it is not common practice to determine the explosivity characteristics of hybrid mixtures. Instead, the explosivity of the dust alone is tested, which can lead to much different results compared to the hybrid mixture. Although current testing of hybrid mixtures is limited, preliminary data suggests that the presence of fuel vapors in the presence of explosive dusts likely decreases the minimum explosible concentration (MEC), increases the max explosion overpressure (Pmax), increases the max normalized rate of pressure rise of explosion (Kmax), and decreases the limiting oxygen concentration (LOC) for an explosion.[1–3] all of which would affect the hazard assessment. Thus, explosivity testing should be performed on hybrid mixtures, and not on explosive dusts alone, to ensure the test results better reflect the hazards associated with the specific process. This will ultimately lead to improved hazard mitigation and enhanced process safety. Fauske & Associates, LLC (FAI) has the capabilities to perform hybrid mixture testing using modified ASTM and EN standards that parallel industrial processes.


To demonstrate our capabilities, we have chosen to perform explosivity testing on a hybrid mixture containing Creatine HCl, a common dietary supplement, and ethanol. This hybrid mixture is likely to form in situ during the last step in Creatine HCl synthesis when the product is washed with ethanol to remove impurities. We have determined the MEC, explosion severity data, and LOC for pure ethanol vapor, pure Creatine HCl, and then a hybrid mixture containing an ethanol atmosphere and Creatine HCl. We then use this data to assess the differences in hazards between pure Creatine HCl and the hybrid mixture.

hybrid mixtures testing apparatus

Figure 1. Testing apparatus including: 20 L Siwek chamber with jacket, temperature controllable water recirculator for the jacket, custom front panel, and dispersion gas storage tank with controllable heating wrap.

Testing was performed using a standard 20-L Siwek chamber with slight modifications (Figure 1). A fuel introduction port, a gas addition/sampling port, and a thermocouple port was fixed to the front of the chamber. The internal chamber temperature was controlled using a temperature controlled water recirculator for the 20-L vessel’s jacket and by wrapping a dispersion gas storage vessel with a controllable heating wrap. Internal chamber temperature was monitored using a calibrated thermocouple. A partial ethanol vapor atmosphere was generated in the chamber by evacuating the chamber to a pressure below 100 mbara, fixing a syringe containing a desired amount of ethanol to the air-tight front fuel port, injecting the ethanol, and observing the coinciding desired pressure rise. Testing was conducted in accordance to modified ASTM standards.


Reagent grade, pure ethanol purchased from Sigma-Aldrich was and pure Creatine HCl purchased from pure Creatine HCl purchased from was used for testing. The Creatine HCl was found to have a percent weight moisture content of 0.26%, a mean particle diameter of 193 µm, with 31.11% of the particles were less than 75 µm.


The MEC, Pmax, Kmax, average peak explosivity concentration, and LOC for pure ethanol were determined at 36°C ± 3°C (Table 1). 36°C was chosen because large volumes of ethanol were rapidly vaporized at this temperature under the pressure conditions during introduction. All measured explosivity values for ethanol are in general agreement with literature values [1–3], although the literature values were determined under ambient temperature conditions.

Explosivity data for pure ethanol vapor

Given the MEC of the pure ethanol under the elevated temperature conditions, a 1% ethanol atmosphere was chosen for hybrid mixture testing to ensure that any explosivity exhibited by the mixture could be attributed to the vapor-dust hybrid mixture and not ethanol vapor alone. Testing of Creatine HCl and the hybrid mixture was performed at 32°C ± 3°C because the desired amount of ethanol was rapidly volatilized under these conditions. Additionally, a 1% ethanol atmosphere at 32°C also reflects a plausible process condition.


The MEC, Pmax, Kmax, average peak explosivity concentration, and LOC for Creatine HCl and hybrid mixture of Creatine HCl and 1% ethanol atmosphere were determined at 32°C ± 3°C (Table 2). In comparison to pure Creatine HCl, the hybrid mixture was determined to have a lower MEC, higher Pmax, higher Kmax, lower average peak explosivity concentration, and lower LOC. These general trends agree with previously published data for hybrid mixtures of various dusts and alcohol vapors [1–3].


The addition of a 1% ethanol vapor atmosphere decreased the MEC of Creatine HCl from 750 to 40 g/m3. This result suggests that extra care should be taken to ensure small Creatine HCl dust clouds do not form as even small dust clouds have explosive properties in the presence of ethanol vapor. While the Pmax and Kmax values for the hybrid mixture are slightly higher than values for pure Creatine HCl, both the pure sample and hybrid mixture would be classified as St1 explosions. While St1 explosions typically require similar risk management systems, the hybrid mixture exhibits peak explosivity characteristics at a concentration ~5 times lower than the pure compound. Again, this serves as further evidence that extra precautions should be taken to ensure that even small concentration Creatine HCl dust clouds do not form in the presence of an ethanol atmosphere. The LOC of the hybrid mixture was determined to be 5% lower than the LOC of Creatine HCl alone. Meaning, in the presence of an ethanol atmosphere, additional inerting gas would be needed to mitigate any possible explosivity hazards. In total, the differences in test results suggest the hybrid mixture is likely to present an increased and more complex explosion hazard risk compared to the pure dust alone.


Our hybrid mixture testing results further substantiate the idea that explosion testing should be performed on hybrid mixtures, and not dust alone, when specific processes may lead to hybrid mixture formation. It is extremely important to perform testing on hybrid mixtures when relevant, because the presence of a fuel vapor or gas can lead to much more dangerous explosivity characteristics, thus it will affect the hazard mitigation strategies needed to maintain process safety. Luckily, we at FAI have the tools, knowledge, and experience needed to perform modified explosion testing methods on hybrid mixtures under conditions that match your process. Contact us today!

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  1. E.K. Addai, M. Clouthier, P. Amyotte, M. Safdar, U. Krause, Experimental investigation of limiting oxygen concentration of hybrid mixtures, Journal of Loss Prevention in the Process Industries. 57 (2019) 120–130.
  2. E.K. Addai, A. Aljaroudi, Z. Abbas, P. Amyotte, A. Addo, U. Krause, Investigation of the explosion severity of multiphase hybrid mixtures, Process Safety Progress. n/a (n.d.) e12139.
  3. E.K. Addai, H. Ali, P. Amyotte, U. Krause, Experimental and theoretical investigation of the lower explosion limit of multiphase hybrid mixtures, Process Safety Progress. 38 (2019) e12045.

Topics: dust hazards, dust explosibility, Combustible Dust Hazards Analysis


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