Process Safety Strategy - Chemical Reaction Hazard

It has been some time since I wrote my last post. I was on leave for my marriage. I have returned to the office now and I am back with another post on Chemical Reaction Hazards. Many times, I have come across people asking us what steps should be followed for identifying and mitigating the chemical reaction hazards. Today we will be talking on the same.

First, of all, I would like to thank Fauske & Associates LLC (FAI) for their continuous support and for publishing my last blog post on their website. If you have missed the same, you can access it by clicking on the link below

https://www.fauske.com/blog/should-set-point-for-pressure-relieving-device-be-equivalent-to-design-pressure

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.

Concept Sciences Inc. Explosion

Hence, it is imperative to develop a process safety strategy to address these four gaps. A process safety strategy should include the following:

Should set point for Pressure Relieving Device be equivalent to Design Pressure?

Hope all of you are doing well. Today's topic is focusing on the general practice sometimes followed in industry related to the set point of Pressure Relief Devices of process equipment. Many times during PHA, I have come across a general philosophy of keeping the set point of the reactor's pressure relieving devices equivalent to its design pressure. This might be adequate for a non-reactive system. But the question is, is it adequate for a reactive system?

Here, the reactive system is one where there are hazards due to a chemical reaction, including the possibility of decomposition or polymerization or some side reaction, etc. We know based on normal kinetics that the rate of reaction is directly proportional to the temperature which in turn is proportional to pressure. In simple terms, it is said that the rate of a chemical reaction doubles with every 10 deg C rise in the reaction temperature. Hence it can be said that:

Higher set pressure leads to a correspondingly higher "set" temperature (i.e. the relieving temperature).  This, in turn, leads to a higher rate of reaction which results in higher self-heating or higher temperature rise rate (dT/dt i.e. deg C/min) and a higher pressure generation rate (dP/dt i.e. bar/min). This is very important because for a chemically reactive system the required pressure relief area depends directly on the self-heating and pressure rise rates at the relief conditions.