Hi everyone,
I would like to share some brief notes I have made from my experiences in assessing Chemical Reaction Hazards (CRH) typically in HAZOP studies and designing relief systems.
These notes are brief and hopefully easy to digest.
Please comment, like and subscribe.
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| Figure 1 - Typical Control System DCS for a reactor |
1. There are 3 main types of reactivity hazards:
(1) Self-reactive (polymerisation, decomposing & rearranging types)
(2) Reactive with substances (e.g. air, water, etc)
(3) Incompatible materials
2. Complexities to consider in relief system design for thermal runaways (Chemical Reaction Hazards):
(1) Reaction rate varies exponentially with temperature
(2) Exothermic heat releases vary with time
(3) Volatile and non-condensable gas can be generated (decomposition)
(4) Composition change affects boiling point curve
(5) Viscosity increases due to polymerisation
3. Heat generation from a runaway reaction proportional to volume (mass), but heat removal is proportional to the surface area
(1) Elevated temperatures may also initiate secondary side reaction which is more rapid or energetic
(2) The speed of reaction does not change the potential energy released, a slower reactions may allow unreacted material to accumulate
4. Onset temperature for a reaction runaway is not an intrinsic property of the material reactants
(1) DO NOT treat it as an absolute value
(2) As onset temperature depends on test taken and safety margin – VERY IMPORTANT
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| Figure 2 - Generic reactor vessel heating up |
5. Colder reactions are not always safer than faster hotter reactions
(1) Accumulations can form and higher concentrations/amounts can react suddenly/unexpectedly even at colder temperatures.
(2) Thus restarting a failed stirrer can be very dangerous in this situation
(3) Stirrer malfunctions or restarting of stirrer is very common cause of a CRH incidents.
6. Chemical reactions that remove gases from the head space of a tank can cause a vacuum! – Make sure your vessel/reactor is designed for vacuum.
7. For storage of hazardous material - Risk is proportional to the size of the system components: e.g. number of valves, nozzles and lines on storage tank (e.g. leak points):
(1) Small storage vessels are usually deliberately overdesigned, than having one large vessel (which are more economical).
(2) But large connecting lines can also leak in a large storage tanks – thus bigger risk.
(3) This is an interesting trade off, especially with the supply chain issues in the world
8. Rupture disc alone should not be used on a tank with toxic material as they do not close after opening – and can lead to a continuous release into the area (risk people, assets and environment?)
Thank you for reading, please comment, like and subscribe to this blog
Hope you have enjoyed reading this
Chiraq
