By Himanshu Chichra, Principal Process Safety Engineer and host of ‘Process Safety and Risk Blog’ www.staub-ex.blogspot.com
First, of all, I would like to thank Fauske & Associates LLC (FAI) for their continuous support and for publishing my last blog post "Should Set Point For Pressure Relieving Device Be Equivalent to Design Pressure?" on their website.
As per the research conducted by Dr Phil Nolan (South Bank University, UK) and Dr. John Barton (UK Health and Safety Executive) and graduate students based on data analysis, the following four gaps have contributed equally i.e. 25% each to thermal runaway reactions leading to multiple incidents in past:
1. Lack of proper understanding of the thermochemistry (heat of reaction) and chemistry (balanced chemical equation)
2. Insufficient engineering design for reactor heat transfer system
3. Inadequate control and safety back-up systems including emergency relief systems, process vent, and other engineering controls
4. Poorly written batch procedures and insufficient operator training
Hence, it is imperative to develop a process safety strategy to address these four gaps. A process safety strategy should include the following:
1. Preliminary Hazard Analysis (Screening)
- Review the molecular structure of known reactive groups and the balanced chemical equations
- Conduct literature searches (SDS, NFPA, Bretherick's, Heat of Reaction Theoretical Calculations, CHETAH calculations, chemical compatibility, etc.)
- Perform small-scale thermal stability tests (Example: DSC)
2. Assess the Desired Reaction
- Conduct Reaction Calorimetry testing on the reaction of interest and the subsequent quench reaction
- How much heat (or gas) is generated? Is cooling adequate?
- If a scale-up assessment suggests a much longer addition is needed to allow the available cooling capacity to maintain temperature control, does the desired reaction still perform to standard (i.e. produce the same quality product as was made in the lab)?
3. Assess the Undesired or Decomposition Reaction
- Conduct Adiabatic Calorimetry testing (Example: VSP2) to obtain data on temperature and pressure vs. time
- From the Adiabatic data evaluate the rates of temperature and pressure rise.
- How fast is heat (or gas) generated?
- Is the pressure relief system adequate, both in terms of the relief vent area and set pressure?
Dr. James P. Burelbach, Director Process Safety at Fauske & Associates LLC says, "Reaction Calorimetry and Adiabatic Calorimetry are two distinctly different things that both have their place in a process safety strategy. Sometimes, it is a challenge to help people understand the difference and sometimes people think that by getting an RC1 or similar they are covered as far as process safety calorimetry goes. I think this can be a very dangerous approach. I try to simplify by saying that reaction calorimetry helps you understand the desired chemical reactions (for example to design process heat removal systems) while adiabatic calorimetry helps you understand (and prepare for) undesired chemical reactions (for example to ensure there is an adequate pressure relief system if process cooling is lost)."
4. Review Batch Directions and Operator Training
- Batch review meetings to ensure procedures are correct
- Operator training to ensure a clear understanding of the procedures
5. Reduce risk or redesign process
- Mitigation by having an adequately sized pressure relief system (might include dump tank)
- Redesigning to make it inherently safer using elimination or substitution
Process Safety Strategy: CRH (Ref: FAI)
A strategy such as outlined here helps to formalize the process for easy identification of hazards and enables companies to opt for appropriate measures to mitigate the risk to an acceptable level.
Hope you enjoyed reading this post and that you can relate to understand the steps one should follow while identifying chemical reaction hazards. If you would like to share on the topic, your experience, your questions or future blog topics, you can write to me on firstname.lastname@example.org.
Mr. Chichra is a guest blogger with whom Fauske & Associates, LLC (FAI) has recently worked to support customers in India. Read more of his posts related to process safety at www.staub-ex.blogspot.com and subscribe to FAI's blog to never miss out on any new process safety content.