Designing reactive processes for highly hazardous reactive materials (HHRMs) presents a complex set of challenges that go well beyond typical process engineering. At the core of these challenges is the inherent instability and high reactivity of HHRMs, which can lead to violent runaway reactions, explosions, toxic releases or fires if not rigorously controlled.
Accurately predicting the behavior of HHRMs under various conditions is at the core of the process challenges faced in designing safe and cost-effective facilities.
HHRMs are substances with toxic, flammable, or explosive properties that can react unpredictably under certain conditions. Some examples include ethylene oxide, butadiene, hydrogen cyanide, MDI, phosgene, hydrofluoric acid and chlorine. The Occupational Safety and Health Administration (OSHA) provides a comprehensive list of regulated reactive materials in 29 CFR 1910.119 Appendix A.
Accurately forecasting upset scenarios can be particularly difficult if thermodynamic and kinetic data are incomplete. This data gap often requires extensive laboratory testing and modeling, which can be both time-consuming and dangerous.
Designing HHRM Systems
When dealing with HHRMs, it is very important to understand the risks of each substance and how best practices could mitigate those risks. Information on HHRM risks and associated mitigations can be obtained from:
- Technology providers.
- Producers and material Safety Data Sheet (SDS).
- Operational experience.
- Historical incidents such as documented cases from the Chemical Safety Board (CSB).
- Reactivity matrices.
Implementing robust process safety measures is vital. HHRM processes must be designed with multiple layers of protection, including emergency relief systems, inerting, temperature and pressure control, and real-time monitoring. However, it can be a delicate balance to integrate these safety systems without compromising process efficiency or creating new hazards. Risks during scale-up from laboratory testing to the pilot phase or full-scale operations also may be difficult to anticipate, particularly for materials that are prone to exothermic reactions or decomposition.
Human factors and organizational culture also play a critical role. Operations personnel should be thoroughly trained so that safety procedures — including detailed hazard assessments, documentation and inspections — are followed precisely in accordance with regulatory standards.
Operational experience is key, but historical practices and designs should not preclude following the best practices or improved safety measures. Different sites or facilities will not behave in exactly the same way and past performance does not validate that an incident can’t occur in the future.
TPC Butadiene Incident
In 2019, a pipe rupture involving an HHRM at the TPC Group plant in Port Neches, Texas, released a butadiene vapor cloud that migrated to an ignition source and triggered an explosion of this highly flammable and reactive gas. The incident started when butadiene began expanding in a 16-inch pump suction line, forming a toxic popcorn polymer that began pushing against metal pipe walls until the pressure finally ruptured the pipe. The resulting explosion was felt 30 miles away, injured three plant workers, damaged nearby homes and businesses, and forced thousands of residents from surrounding areas to evacuate.
An investigation by the U.S. Chemical Safety Board found that aging equipment and piping, inadequate leak detection and inspection systems, delayed maintenance, and inadequate storage and handling controls were among the key contributing factors for the incident.
Risk Properties of HHRMs
HHRMs can undergo rapid, violent, and often unpredictable chemical reactions under certain conditions. Their risk properties stem from a combination of intrinsic factors that make them particularly dangerous to handle in industrial settings. These properties include:
- Thermal instability in which HHRMs decompose or react exothermically when exposed to heat. If not properly controlled, this can lead to runaway reactions with a rapid release of energy.
- Sensitivity to contaminants or impurities even at trace levels of moisture, air or incompatible chemicals.
- High reactivity with common substances such as water, air or even materials used in pipes or containment systems. These reactions may result in releases of toxic gases, overpressurization or creation of heat.
- Gas generation and overpressurization that results in rapid increases of pressure, potentially leading to vessel ruptures if relief systems are inadequate.
- Incompatibility with safe handling conditions, potentially initiating hazardous events if strict environmental controls over inert atmospheres, sub-ambient temperatures or other factors are not maintained.
- Low activation energy in which even small increases in temperature or pressure can dramatically accelerate reaction rates.
Because of these properties, HHRMs demand careful hazard evaluation, precise process control and stringent safety protocols to mitigate the risk of catastrophic incidents.
Engineering Design Considerations
Designing reactive processes begins with assessment of the potential hazards. These hazards could include auto-decomposition, auto-polymerization, or other violent reactions with incompatible substances including inadvertent contamination. These can occur due to operational excursions or slowly developed over time such as catalyzed reaction from metal corrosion products in dead legs.
During the preliminary development of a reactive design, the following should be considered:
- Thermal stability of reactants across all expected temperatures.
- Interactions across all potential materials in the process.
- Any possible contamination including residues from construction or fabrication of equipment.
- Develop a Chemical Reactivity Matrix including classification of the reactions.
Desired reactions must be quantified as follows:
- Evaluate the maximum heat of reaction and off-gas rates.
- Determine the maximum adiabatic temperature achievable.
- Understand relative reaction rates across all potential conditions.
For adverse reactions, consider the mixture stability over a large range of temperatures, pH, and concentrations/conversions along with any other undesired side reactions, including the vapor phase.
Once the chemistry is understood, the overall mass and energy balance and process across all operations should be assessed, especially in settings where energy is added or removed. Consider all operational scenarios where deviations to normal operation could occur and if there are safety concerns. This should not only consider high temperature but also low-temperature scenarios including localized freezing or high viscosities impacting reactions and localized heat transfer.
Some golden rules for managing these types of materials are to keep the material moving or de-inventoried in a system and keeping the material cool (but not too cold). Also follow the specifications, don’t allow contamination and keep the material inhibited or stabilized.
Some additional design concepts to consider in reactive designs include:
- Avoid batch reaction systems where possible. Continuous operation provides more steady operation and monitoring with fewer changes in conditions.
- Consider the impact of vessel size to heat and mass transfer when scaling systems and associated impact to safety risks.
- Consider more than thermal control measures to control reaction systems such as pressure or inerting.
- Design the system to be as inherently safe as possible prior to implementation of high-integrity relief systems. This could be using more inert or lower-boiling-point solvents to help self-quench reactive systems.
- Incorporate higher reliability and redundant validation of key reactants, inhibitors, and catalyst additions.
Disciplined Approach Is Demanded
Designing HHRMs systems demands a disciplined, informed approach that integrates understanding of the chemistry and potential hazards along with strong understanding of industrial process, and robust safety evaluations. By learning from past failures and applying quantitative, scenario-based design strategies, chemical facilities can build resilient systems that protect lives and assets.
When tackling complex capital projects, a structured knowledge transfer can reduce risks and keep things on track.
