Key Considerations During a Reactive Hazard Analysis (RHA)

Evaluating potential chemical hazards is a crucial part of process safety. Reactive chemical hazards are a unique and possibly devastating subset of chemical hazards and can be present whether the reaction is intended or not.  Therefore, it is critical to study desired and undesired reactions to ensure that the proper safeguards, engineering controls, procedures, and safety-related equipment are installed to provide adequate protection during process operations. This strategy is needed for both the desired and undesired reactions and must include consideration of the worst-case scenarios. 

Identifying potential reactive hazards is achieved through a detailed Reactive Hazard Analysis (RHA).  An RHA is dedicated to identifying reactive chemical hazards and can be considered a more focused version of a Process Hazard Analysis (PHA).  The overall strategy for developing a robust process generally entails a cycle of steps including identification, evaluation, implementation, and then documentation.  Here is an example RHA procedure with key considerations when developing a safer process: 

Process and Material Characterizations

• Generate a thorough understanding of the chemistry (e.g., balanced equations and material properties of involved components) and thermochemistry (e.g., theoretical heat of reaction).
  • Conduct a desktop review of the process by consulting the available literature for all materials involved such as the sample SDS, DIPPR, or the NIST Webbook. 
  • Complete a literature search for similar chemistry incidents through resources like the US Chemical Safety and Hazard Investigation Board (CSB), or AIChE PSID. Keep in mind that the lack of a previously reported incident is not an indication the chemistry is safe to scale up.
• Perform small-scale screening tests to characterize the process materials and streams for thermal, mechanical, and friction sensitivity.
  • Tools such as the DSC, ARSST, ARC, and TGA are excellent screening tools requiring small amounts of material.
    
• Complete dust explosivity, gas/vapor flammability, or other experiments on materials with unknown material properties or hazards to identify “red flags” that may limit large-scale production.
  

Calorimetric Testing

• Calorimetric tools such as the RC1 or FAI VSP2 can be used with the DSC, ARSST, ARC, or TGA to perform more advanced studies on the desired process or to simulate an upset scenario to further identify and evaluate potential reactive hazards.
  • Consider Reaction Calorimetry to obtain a heat flow profile (heat of reaction, instantaneous heat flow, rate of gas generation, and heat of reaction) of the intended (desired) reaction as it proceeds through initiation to completion.  These experiments can also support the selection of optimal process procedures such as the ideal process temperature, rate of addition, catalyst quantity, or other parameters to ensure there is minimal heat accumulation.
  • Sometimes testing indicates the potential for a thermal runaway reaction or the possibility to reach the decomposition temperature of the reaction mass. In that case, the next step is to conduct adiabatic testing using instruments such as the ARSST, VSP2, or the ARC. The ARSST and VSP2 have low thermal inertia and good stirring, facilitating the collection of directly scalable data, which is ideal for practical emergency vent sizing. FERST Powered by CHEMCAD is a useful software tool for vent sizing by such direct scaleup methods.

Data Interpretation and Setting a Reaction Hazard Class

• Once the desktop review, screening tests, and other calorimetric tests are completed, the next step is to interpret the data for the plant equipment that has been assigned to this scale-up.
  • Consideration must be given to materials of construction, available cooling capacity, normal process venting and scrubbing, emergency overpressurization protection, etc.
  • Prof. Francis Stoessel developed one of the most common ways to classify the safety of a chemical reaction. The “Stoessel diagram” helps determine the hazard class of the reaction and provide a safer scale up analysis.
• Other important safety parameters that can be identified from calorimetric testing include adiabatic time-to-maximum-rate (TMR), and self-accelerating decomposition or polymerization temperatures.  These values can guide the selection of safe processing, safe storage, safe transportation, and alarm temperatures.
  

Conduct a Process Hazard Analysis of the Assigned Equipment

• Once the process is understood from a mass and energy balance, we need to apply this knowledge to determine the effects of a process deviation.
• The current practice is to perform a Process Hazard Analysis (PHA) to identify potential process deviations, determine the consequences, and quantify the risk to determine if it is acceptable.
• If the current safeguards are adequate, no further recommendation is needed. However, if the consequence cannot be mitigated to a level where the risk is acceptable, the PHA team needs to make recommendations to improve the safeguards.
• A comprehensive PHA must include both the desired and worst-case scenarios.

Process Redesign

• Based on the risk level, identify the best available technology to provide layers of protection (e.g., emergency relief systems, operating procedures, training, secondary containment, etc.). It may not be possible to scale up the reaction “as is” for engineering reasons.
  • Conduct an emergency relief system evaluation to ensure it can adequately protect the process equipment from overpressurization using a tool like FERST Powered by CHEMCAD.
  • Ensure the existing cooling capacity or process vents are adequate to handle the process at scale.
• FAI can work with you to determine if a change in solvents, reagents, or reaction conditions can be implemented to make the reaction safer for the available plant equipment.
  

Documentation and Management of Change

• FAI can produce a comprehensive process safety report that clearly documents the assessment results. In addition, we can provide training on the process control so that any change is evaluated before implementation.
• Contact us to identify and characterize reactive hazards and support the development of an inherently safe design and scale-up.  We would be happy to support your RHA, or any of the steps therein. 

References

1. Stoessel, Francis, “Thermal Safety of Chemical Processes: Risk Assessment and Process Design,” 2020.
2. Crowl, D. et al., “Chemical Process Safety: Fundamentals with Applications," 2019.
3. Center for Chemical Process Safety, “Guidelines for Risk-Based Process Safety,” 2017.
4. U.S. Department of Labor and Occupational Safety and Health Administration, “Process Safety Management,” 2012.
5. Occupational Safety and Health Administration, “Process Safety Management of Highly Hazardous Chemicals,” 29 CFR 1910.119. Occupational Safety and Health Administration, “General Duty Clause,” Section 5. Duties. OSH Act of 1970.
6. Fauske, H.K., “Managing Chemical Reactivity – Minimum Best Practice,” 2006, Process Safety Progress, AIChE.
7. DIPPR: Design Institute for Physical Properties: https://www.aiche.org/dippr
8. US Chemical Safety and Hazard Investigation Board (CSB): https://www.csb.gov/
9. PSID: Process Safety Incident Database: https://www.aiche.org/ccps/resources/psid-pro cess-safety-incident-database
10. NIST Webbook: National Institute of Standards and Technology: https://webbook.nist.gov/