Reactive chemical hazards are a special subset of chemical hazards that can be present whether the reaction is intended or not, and the results can have catastrophic consequences such as explosion, fires, or harmful chemical releases. Some materials are even thermally unstable in the absence of an intentional chemical reaction. Examples of thermally unstable materials includes self-reactive materials, organic peroxides, explosives, and uninhibited monomers. Important tools for characterizing the hazard associated with the thermal stability of materials are adiabatic calorimeters. These tools provide important information such as temperature and pressure generation that can be used to extract many important safety parameters. This article provides example adiabatic calorimetry data on a thermally unstable organic peroxide (TBHP in decane) and a self-reactive material (benzene sulfonyl hydrazide) [1].
Self-Reactive Materials and Organic Peroxides
Per the UN Model Regulations [2], self-reactive (or self-polymerizing) materials are designated as 4.1 materials, and organic peroxides are designated as 5.2 materials, and both are subsets of hazardous goods that take precedence over many other hazardous groups.
A self-reactive substance (e.g. a liquid, solid, solution, or mixture) is a material that is thermally unstable and that can exhibit a strongly exothermic decomposition even without the presence of oxygen. These energetic decomposition reactions can be
initiated by heat, friction, impact, or due to catalytic interaction with impurities.
The UN Model Regulations define organic peroxides as substances which contain the bivalent -O-O- structure and may be considered derivatives of hydrogen peroxide, where one or both of the hydrogen atoms have been replaced by organic radicals and are considered thermally unstable substances which may undergo exothermic self-accelerating decomposition.
The decomposition may be initiated by heat, contact with impurities, friction, or impact. In addition, they may have one or more of the following properties:
- Liable to explosive decomposition
- Burn rapidly
- Sensitive to impact or friction
- React dangerously with other substances
- Cause damage to the eyes
The thermal instability associated with organic peroxides makes it very important to adequately assess the material so that proper handling, storage, and shipping procedures and equipment are in place.
Advanced Reactive System Screening Tool (ARSST™)
One way of assessing the thermal stability of materials like organic peroxides is using adiabatic calorimetry. The ARSST is a low-phi adiabatic calorimeter typically used to quickly and safely identify potential chemical hazards by measuring directly scalable rates of temperature and pressure generation during a runaway reaction. These data are typically applied to the design or evaluation of emergency relief systems and effectively track the types of reactions associated with self-reactive, self-polymerizing, and organic peroxide materials.
The basic components of the ARSST (Figure 1) include a 10 ml open spherical glass test cell with a ¼” diameter neck, heater, insulation, thermocouple(s), pressure transducer, and a 350 ml or 450ml stainless steel containment vessel that serves as both a pressure simulator and safety vessel. A small magnetic stir bar is typically placed in the test cell and driven by an external magnetic stirrer. The external bottom heater is secured directly to the test cell.
The apparatus has a low effective heat capacity relative to that of the sample, which may be expressed as a capacity ratio,
or phi-factor, of about 1.04. This key feature allows the measured data to be directly applied to process scale. Sample
temperature is measured using a type K thermocouple, and typically a 500-psig transducer is used to measure pressure. The
containment vessel is hydrostatically tested to 3000 psig and is fitted with a Hastelloy rupture disk rated at about 900 psig for the standard 350 ml vessel.
The ARSST’s primary mode of operation is called polynomial control mode. In this pseudo-adiabatic mode of operation,
a second-order polynomial is used to both raise the sample temperature and to compensate for the increased heat losses at increased sample temperatures.
The polynomial coefficients are calibrated to yield a steady temperature rise rate throughout the tested temperature region for a nonreactive material. During an experiment with a reactive material, an exothermic reaction is observed when the temperature rise rate increases above the imposed background heating rate. Endotherms (e.g. melting) which may occur prior to an exotherm are readily accommodated as well.
The ARSST lends itself well to measuring rapid exothermic reactions that generate non-condensable gas because the
thermocouple is in direct contact with the sample and the large containment volume minimizes pressure accumulation due
to non-condensable gas generation. This makes it an ideal instrument for quantifying the heat and gas generation rates of
many energetic materials.
Test B102-07.5: Tert-Butyl Hydroperoxide Solution in 5.0-6.0 M Decane
Sample: 8.061 g of TBHP in 5.0-6.0 M decane
Test Cell: Pyrex glass construction, 1/4" diameter neck, large Teflon-coated stir bar, extension tube, and glass lined
thermocouple located approximately 1/8" from bottom of test cell. Test cell volume: 12 ml. Tare mass (with stir bar): 1.6725 g.
Containment Vessel: 450 mL volume, 301.2 psig initial nitrogen pressure
Comments: The heater was secured on the test cell, and the test cell was placed inside the insulation sheath. The entire assembly was placed into the ARSST containment vessel, and the necessary connections were made. The specified amount of TBHP in 5.0-6.0 M decane was added directly to the test cell. The containment vessel was sealed and pressurized with 301.2 psig of nitrogen. The standard magnetic stirrer was enabled. Data acquisition began. The heater was enabled with a polynomial generated using pentadecane. The heater power was decreased at 29.0°C (7.8 minutes) and again at 49.1°C (22.4 minutes). The resulting background heating rate was approximately 1.4°C/min. Exothermic activity was observed beginning at approximately 65°C, heating the sample to a maximum temperature and pressure of 350.3°C at 51.8 minutes and 489.6 psig at 51.7 minutes. The maximum temperature and pressure rise rates were 5,691°C/min, and 11,874 psi/min at 51.7°C. The rates are based on a 5-point regression of the collected data. Following the peaks, the temperature and pressure decreased, and above this the test cell was likely empty. The heater remained enabled and increased the temperature of the test cell to a maximum temperature of 399.9°C at which point the heater was disabled, and cooldown data were collected. The data from this test are presented in Figure 2 through 7. The pure component vapor pressure in Figure 6 is from DIPPR [3].
Post Test Observations: The cooldown pressure in the containment vessel was 319.7 psig at 30.7°C, indicating 0.010 moles of non-condensable gas generation. The pH of the gas was not measured. The final sample mass in the test cell
was 0.0142 g, indicating 99.8% mass loss.
Test B102-07.5: Benzene Sulfonyl Hydrazide
Sample: 7.029 g of benzene sulfonyl hydrazide
Test Cell: Pyrex glass construction, 1/4" diameter neck, large Teflon-coated stir bar, extension tube, and glass lined
thermocouple located approximately 1/8" from bottom of test cell. Test cell volume: 12 ml. Tare mass (with stir bar): 1.7898 g.
Containment Vessel: 450 mL volume, 302.2 psig initial nitrogen pressure
Comments: The heater was secured on the test cell, and the test cell was placed inside the insulation sheath. The entire assembly was placed into the ARSST containment vessel, and the necessary connections were made. The specified amount of benzene sulfonyl hydrazide was added directly to the test cell. The containment vessel was sealed and pressurized with 302.2 psig of nitrogen. The standard magnetic stirrer was enabled. Data acquisition began. The heater was enabled with a polynomial generated using pentadecane. The heater power was increased at 62.7°C (43.4 minutes) and again at 88.2°C (67.0 minutes). Melting of the sample likely occurred beginning around 100°C with exothermic activity immediately following, heating the sample to a maximum recorded temperature and pressure of 373.2°C and 588.1 psig at 91.7 minutes. The maximum recorded temperature and pressure rise rates were 30,726°C/min at 221.0°C, and 175,038 psi/min at 187.5°C. The rates are based on a 5-point regression of the collected data. The test cell ruptured during the experiment at the peak rates, and therefore cooldown data are not presented. At 93.7 minutes, the heater was disabled. The data from this test are presented in Figure 8 through 13.
Post Test Observations: The cooldown pressure in the containment vessel was 340.0 psig at 30.1°C, indicating 0.018 moles of non-condensable gas generation. The pH of the gas was not measured. A final mass was not recorded due to the breaking of the test cell.
Conclusion
Reactive chemical hazards, such as thermally unstable materials like self-reactive substances and organic peroxides pose significant risks due to their potential for catastrophic consequences, including explosions, fires, and harmful chemical releases. Adiabatic calorimeters, like the Advanced Reactive System Screening Tool (ARSST), play a crucial role in characterizing the thermal stability of such materials. These tools provide vital data on temperature and pressure generation during runaway reactions, aiding in the assessment of the hazards associated with these substances. Proper handling, storage, and shipping procedures, along with suitable safety equipment, are essential to mitigate the risks posed by thermally unstable materials and ensure the safety of handling such substances.
The two experiments presented in this article were conducted to assess the thermal stability of different materials. TBHP in 5.0-6.0M decane presented exothermic activity at around 65°C with maximum temperature and pressure rise rates of 5,691°C/min and 11,874 psi/min at 51.7°C. Benzene Sulfonyl Hydrazide resulted in an exothermic activity with maximum temperature and pressure rise rates of 30,726°C/min at 221.0°C and 175,038 psi/min at 187.5°C/min. Understanding the maximum recorded temperature and pressure, as well as, the temperature and pressure rise rates, aids in assessing the safety of reactive materials. Various calculations and analyses can be performed to assess the behavior of exothermic reactions and evaluate safety measures such as reaction kinetics, adiabatic temperature rise, heat generation rate, pressure generation rate, time-to-maximum rate, critical temperature, evaluation of cooling systems and many more. These calculations are essential in process safety evaluations, emergency response planning, and the design of equipment and systems to handle exothermic reactions safely and efficiently.
References
- Raines, E., Doup, B., “An alternative methodology addressing United Nations classification type for self-reactive
substances,” Process Safety Progress, Volume 42, Issue 1, Pages 12-20, March 2023. - United Nations. Recommendations on the Transport of Dangerous Goods. Model Regulations. Vol 1. 22nd
Revision ed. New York and Geneva; 2021. - The DIPPR Information and Data Evaluation Manager for the Design Institute for Physical Properties, Version
11.2.0, Database Date 2016, Brigham Young University.
