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Chemical reaction hazards & thermally unstable substances

10 October 2016

The consequences of runaway exothermic chemical reactions can be devastating. Catastrophes at Bhopal and Seveso are stark reminders that all chemical processes require a thorough and rigorous assessment procedure to ensure that large scale manufacture can proceed without residual risk or threat of disaster. This article from DEKRA Insight looks at chemical reaction hazards and thermally unstable substances and links to guides that can help identify them.

When working with any manufacturing process it is always necessary to establish the hazards associated with its operation. This is most prominent with issues such as machine guards, tripping or slippery floor hazards, etc. but there could also be the potential for flammable materials to be present or a chemical reaction that may go out of control. Flammable gases and vapors are, in most cases, readily identified, especially with materials such as methanol, ethanol, propane, butane or hydrogen. Flammable dusts are less readily identified – and often no data will exist in published literature to identify potentially hazardous materials. The DEKRA Insight Guide to Dust Explosion Hazards and Thermally Unstable Solids provides a strategy for the identification and assessment of such materials along with information on international standard tests, their uses and limitations.

The understanding of chemical reactions and material reactivity is an equally critical element of safe processing. Can you think of an endothermic chemical process? – probably not – there aren’t too many common ones! Exothermic chemical processes on the other hand are much more abundant in manufacturing processes. Often these reactions are inherent in the transformation we are undertaking (e.g. the conversion of styrene to polystyrene) – on other occasions these may be unintended reactions which are not part of our processing plan (e.g. decomposition of a material due to contamination or over-temperature exposure).

The identification, assessment and characterization of both intended and, more importantly, unintended exothermic reactions, are critical for ensuring the safe scale-up and operation of a chemical process.

Incidents such as those at Seveso and Bhopal serve as a grim reminder of the potential consequences of runaway reactions and decompositions. In order to address this issue and to ensure safe operating conditions for companies using or producing these materials, European Regulations such as the Chemical Agents Directive (CAD, 1998/24/EC) highlight the need to obtain process safety data to complete a compulsory risk assessment. The ultimate aim of such studies is to specify and document a detailed basis of safety for the protection of personnel and plant from the consequences of a runaway reaction.

To help address these chemical reactions, a Chemical Reaction Hazards and Thermally Unstable Substances: A Strategic Guide to Reaction Hazard Assessment (Chemical Guide) was recently released and is intended to provide an overview of the strategy that should be employed to assess reaction hazards (mainly associated with exothermic and / or gas generating reactions) and thermally unstable substances to most foreseeable plant situations.

What is the Hazard?

When processing exothermic chemical reactions including thermally unstable substances and mixtures, it should be remembered that the hazard comes from PRESSURE generation. Pressure can be generated in a closed vessel (or inadequately vented vessel) from:
•  Permanent gas generation e.g. generation of nitrogen, carbon dioxide, etc. from the desired process or an unexpected event.
•  Vapor pressure effects caused by heating, possibly arising from an exothermic reaction or a process failure condition, thus raising a mixture above its boiling point.

These modes of pressure generation can arise from the desired reaction, a significant side reaction or a secondary decomposition reaction.

Identification of how pressure generation occurs is critically important for vent sizing, the most common basis of safety in the chemical industry, since the design calculations will require different data input.

Route Selection, Process Development and Optimization

The determination of an explosive potential in a substance may place severe transport restrictions on movement of the material and may require the site to be registered and licensed for explosives manufacture, handling or storage. Explosive properties in any material will require the adoption of extreme safety precautions to minimize any potential risk. In some cases, this may impact on the ability of the company to proceed with such chemistry (hence the need to identify this property as early as possible). Many contract manufacturers exist who are adept at processing and handling “highly energetic” substances and processes. Alternatively, conscious decisions can be made to prevent isolation of an energetic substance such that it is always processed in a phlegmatized (inerted) form.

Having completed adequate thermal stability testing, and with the application of adequate safety margins where applicable, it should be possible to define the maximum allowable exposure temperature of the process at all stages. This data should be used to define heating media and set trigger levels for vessel/process over-temperature protection.

Understanding the hazard potential of intended chemical reactions requires reaction calorimetry and associated gas evolution measurement if appropriate to identify and quantify any permanent gases formed. The resulting heat flow profile, heat of reaction and adiabatic temperature rise data can be used to assess the overall hazard potential of the reaction. If boiling or decomposition conditions are potentially initiated as a result of the temperature rise caused by the energy release, a review of safety systems and potential failure scenarios will be necessary to evaluate the probability of this happening in practice. Any significant reagent accumulation potential in the process should be addressed for safety – and potentially productivity – improvement. Any gas generation from the normal process should also be catered for by provision of adequate process venting facilities.

At the end of this stage of analysis, the operator should have a good understanding of the energetics of the normal process and thermal stability limits of process materials. Any potentially hazardous aspects of the process will be highlighted for further consideration on scale-up. Safety critical aspects of processing should be incorporated into the batch processing instructions such that operators are aware of critical phases and decisions to make under foreseeable circumstances.

This is the final stage where fundamental modifications can be made to the process with minimal cost implication. The process should be reviewed such that any obvious deviation scenarios which might create a hazard are identified and, if at all possible, either eliminated by changing the process conditions or understood so that appropriate protection measures can be incorporated into the process plant. This is the concept of prevention and protection, two concepts key to safe chemical manufacture. Typical prevention measures might involve increasing the solvent level, changing from batch to semi-batch operation or even continuous processing, etc. Any intrinsic hazard remaining in the process will need special engineering provisions on scale-up i.e. protection – this may prove much more expensive than eliminating the hazard at the previous stage.

Summary

The inherent exothermic reaction and/or uncontrolled gas evolution potential of all chemical processes require a thorough and rigorous assessment procedure in order to ensure that large scale manufacture can proceed with an acceptable level of residual risk.

Most decisions which impact on the inherent hazard of the process are made at the very early stages of process development – as early as the route selection stage – and this is the area where Chemists play a fundamental part in developing safer processes. It is critical that reaction hazard evaluation is integrated seamlessly into the process development lifecycle.

Chemists should be trained not just to look at quality, yield and productivity issues but also to understanding the criticality of their decisions on the safety of the process. This more holistic approach to reaction hazard evaluation will take at least some of the onus from engineers who are normally left to design and implement safety systems for scenarios which could possibly have been eradicated with forethought.

A strategic methodology must be employed in which the collection of safety data is intertwined with the development and scale-up of the process – rather than being an inconvenient add-on once the process is fully develop and optimized (and the hazards already inherently included).
Chemical Reaction Hazards and Thermally Unstable Substances: A Strategic Guide to Reaction Hazard Assessment outlines a cost-effective mechanism for assessing hazards present in the inherent exothermic reaction and uncontrolled gas evolution potential of chemical processes.


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