Chemical engineering[ edit ] Thermal runaway is also called thermal explosion in chemical engineeringor runaway reaction in organic chemistry. It is a process by which an exothermic reaction goes out of control: This has contributed to industrial chemical accidentsmost notably the Texas City disaster from overheated ammonium nitrate in a ship's hold, and the explosion of zoalenein a drier, at King's Lynn.
Posted by AnnMarie Fauske on Serious incidents can result in death, injury, capital loss, and business interruptions.
Prior to scale-up, it is critical to have a clear understanding of the reactivity of all process chemicals as well as the energetics of both desired reaction sdefining a worst-case scenario, characterizing the resulting adverse reaction, and understanding how to mitigate the process safety impact.
Processes that cannot be adequately controlled must be redesigned if possible or less hazardous materials used. This article attempts to provide guidelines that can be used as a basis for developing and designing safer new processes.
It can also be used to identify process safety information gaps when existing processes undergo periodic reviews, as required in part by OSHA Process Safety Management Causes of Thermal Runaway Reactions Studies have determined that thermal runaway reactions occur due to the following four reasons: Never assume a chemical is not hazardous because of a low-hazard rating.
Many incidents involve materials that have NFPA hazard ratings of 0 and 1. It is best to develop a proper testing program to identify and characterize all reactive materials and reaction mixtures under a variety of process conditions. Subsequently a process hazard analysis can then be used to assign appropriate controls and safeguards to reduce risk of an adverse event.
It is important to remember to update the process safety information, as a process undergoes changes during its lifecycle.
Once the process has been set, the final process safety report can then be used by a variety of end users either in-house or by outsource facilities. When developing safety documentation, it is important to keep in mind that it must comply with company policies and procedures as well as country and local regulations.
Process Safety Considerations The following items should be considered in relation to a process safety hazard evaluation. Quantification of desired reactions: Determine the heat of reaction and off-gas rates for the desired and quench reactions, including the heat resulting from accumulation of reagents or slow forming intermediates, Determine the maximum adiabatic temperature for the reaction, and determine the basis of safety relative to the estimated boiling point of the reaction mixture, and Understand the relative rates of all chemical reactions.
Quantification of adverse reactions: Assess the thermal stability of the reaction mixture over a wide temperature range When optimizing the robustness of the process, consider other reaction variables, such as pH, concentration, conversion rate, off-gas rate, stability of starting and product substrates in solution and as a slurry Consider the potential and impact of unwanted vapor-phase reactions, and Develop a chemical-interaction matrix for materials present in the reaction mixture, classify the reactivity, and communicate this information to operational personnel Plant considerations: Conduct a basic energy balance to consider the heats during various additions, heat generated during the chemical reaction, and the heat removal capability of the plant reactor system.
Remember to include reactor agitation as a source of energy, Consider the impact of possible deviations from the intended reactant charges and operating conditions, Identify all heat sources connected to a reaction vessel and assume the maximum possible worst-case scenario, Determine the effect of the lowest possible temperature to which the reactor heat-transfer fluid could cool the reaction mixture, i.
General chemistry and engineering design concepts: Design reactions that occur fairly rapidly, if possible, Avoid batch reactions in which all the potential chemical energy is present at the onset of the reaction, unless absolutely necessary, Use semi-batch processes for exothermic reactions in which the batch temperature and any off-gassing can be maintained through controlled addition of the reagent, For highly exothermic reactions, avoid using temperature control of the reaction mixture as the only means for limiting the reaction rate, and When scaling up a reaction, account for the impact of vessel size on heat generation and heat removal: The volume of the reaction mixture increases by the cube of the vessel radius but the wetted heat-transfer area increases only by the square of the radius.
A comprehensive hazard evaluation should be conducted using appropriate estimation and experimental techniques to identify potential reaction hazards in materials, as well as the desired and adverse reactions. Identify any adverse or thermal runaway reactions and characterize them using adiabatic calorimetrysuch as ARC accelerating rate calorimetryor ARSST.
Sometimes, it is necessary to use specialized equipment or techniques to obtain kinetic information on reaction or decomposition rates under unique or specific plant conditions, i. It is important to have clear, concise and unambiguous batch directions with appropriate hazards warnings to clearly explain what must be done at each step in the process including the identification of required safeguards.
The directions should be reviewed and approved by a team consisting of the chemist, engineer, process safety, operational and environmental personnel including plant management. All operators must be properly trained in the directions, including specific engineering controls, working practices, and personal protective equipment, as needed.
Training must be upgraded as the process is revised and it must be documented.
Conclusion Having a documented process safety strategy or procedure in place allows for a standardized approach to hazard identification and organizing process safety information in a uniform manner. The reader is encouraged to add plant-specific items or issues to the guidelines in this article, as needed for their particular situation.The process for evaluating runaway reactions and sizing your relief devices for this case is complex enough that it requires its own project.
And because of their complexity, runaway reaction contingencies are often ignored or over simplified. to runaway and with a view to highlighting particular areas for attention by those, particularly chemists, chemical engineers and managers, who research, devise, design or operate existing, new or modified products, processes, plant Runaway Reactions in Batch Reactors.
Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result.
It is a kind of uncontrolled positive feedback.. In other words, "thermal runaway" describes a process which is accelerated by increased temperature, in turn releasing energy that further increases temperature.
Preventing Runaway Reactions thermal stability criteria [1, 4] As a guideline, three levels are sufficient to characterize the severity and probability of a runaway reaction, as shown in the Table.
How to Prevent Runaway Reactions August Phenol-formaldehyde reactions are common industrial processes. The reaction of phenol or substituted phenol with an aldehyde, such as formaldehyde, in the presence of an acidic or basic catalyst is used to prepare phenolic resins. A General Strategy for the Safer Scale-Up of Batch and Semi-Batch Reactions -with Richard Kwasny, Ph.D., Senior Consulting Engineer, Fauske & Associates, LLC.
Thermal runaway incidents continue to occur in batch production facilities in the chemical and pharmaceutical industries.
Serious incidents can result in death, injury, capital loss, and business interruptions.