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. Despite the best efforts of the chemical/pharmaceutical industries to be responsible, major incident casts a negative light on this industry as a whole. In order to prevent incidents from occurring there is a need for all R&D, process development, and batch production facilities to have an effective process safety strategy in place including sound safety-management systems. 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(s), defining 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 1910.119, Hazard Communication 1910.1200, and the General Duty Clause).
Causes of Thermal Runaway Reactions
Studies have determined that thermal runaway reactions occur due to the following four reasons:
- Insufficient understanding of the process chemistry and the energy/kinetics for the desired reactions,
- Improper design of the heat transfer capacity required at the plant level,
- Insufficient understanding of the adverse reaction and controls including plant-safety back-up systems, as well as adequate emergency venting, and
- Inadequate written batch procedures and poor operator training.
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. If your company does not have a testing facility, Fauske & Associates will be pleased to work with you to identify and conduct appropriate tests. 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. The interim process-safety information reports can then serve as a reference for technology-transfer purposes as the process scales from R&D, kilo-lab, pilot plant to commercial-production stage. 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.
Preliminary hazard assessment:
- Determine the thermal stability of all reaction components/mixtures within the minimum and maximum process temperatures attainable under a worst-case scenario,
- Identify unwanted interaction between reagents and solvents, and
- Identify potential reaction contaminants that may have an inhibitory or catalytic effect on the desired reaction.
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.e., coating heat transfer surface, and
- Consider the impact of temperature gradients and other issues, such as increased viscosity, freezing at reactor walls, fouling, and so on, in plant-scale equipment.
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. We use estimation techniques, differential scanning calorimetry (DSC), Advanced Reactive System Screening Tool (ARSST), and reaction calorimetry (RC), as needed. Identify any adverse or thermal runaway reactions and characterize them using adiabatic calorimetry, such as ARC (accelerating rate calorimetry), or ARSST. If required, the emergency vent size for a specific reactor can be determined using Design Institute for Emergency Relief Systems (DIERS, an AIChE industry alliance) methodology with data generated using an adiabatic Vent Sizing Package 2 (VSP2; a specialized adiabatic calorimeter that uses temperature, pressure, and rate data to allow for sizing emergency vents) or the 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.e., Thermal Activity Monitor (TAM) or AKTS Kinetics. This is an area where expertise is required and Fauske & Associates are able to help you plan and design these types of tests.
Training:
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. If we conduct hazard assessments for all processes, it develops a safety culture, avoids any confusion about how and why the tests were conducted and what other basic information is needed, allows for proper and consistent interpretation of test results/information, it ensures process safety information is comprehensive and can be used by a wide variety of end users. The reader is encouraged to add plant-specific items or issues to the guidelines in this article, as needed for their particular situation.
To learn more about how to scale-up chemical reactions in a safer manner, Fauske & Associates, LLC conducts training or please contact us at 630-323-8750 or info@fauske.com for additional information.
References
- Hendershot, Dennis C., “A Checklist for Inherently Safer Chemical Reaction Process Design and Operation,” Center for Chemical Process Safety International Conference and Workshop on Risk and Reliability, 2002.
- Kwasny, Richard S., "Hazard Assessment Strategies for Reduction Reactions," Southbank University, London, 1999.
- Barton, J. and Rogers, R., "Chemical Reaction Hazards," Second edition, Gulf Publishing, 1997.
- Bretherick, L., “Bretherick’s Handbook of Reactive Chemical Hazards," Seventh edition, Butterworth Heinemann, 2008.
- Stoessel, F., " Thermal Safety of Chemical Processes: Risk Assessment and Process Design," Wiley-VCH, 2008.
- Merritt, C. W., 2004. “Chemical Process Safety at a Crossroads,” Environmental Health Perspectives, 112:a332-a333. doi:10.1289/ehp.112-a332, 2004.
