Crucial to safer material handling is a thorough understanding of maximum safe processing and storage temperatures to avoid uncontrolled initiation of exothermic activity from oxidation/combustion or molecular decomposition.
Many solids (powders) are capable of undergoing an exothermic decomposition or oxidation when exposed to elevated temperatures. Thermal instability hazards can be manifested in a range of ways including simple "black bits" in product after drying, through yield decrease after prolonged reaction or drying times to propensity for smouldering and fires.
If products are to be processed at elevated temperature then consideration must be given to evaluating the safety implications from the bulk storage or drying of powders. Thermal stability data for powders are used to define safe operating temperatures for drying applications or where powders are otherwise subjected to elevated temperatures.
It is not uncommon for powders to be processed at elevated temperatures and then stored often in very large quantities for prolonged periods of time. It is therefore essential to understand the thermal stability characteristics of the materials being processed and stored, together with the operating conditions under in which they are handled; otherwise it may be possible to initiate a dangerous exotherm arising from self-heating or self-reaction
The definition of a maximum safe operating temperature is complicated by the impact of a number of variables such as scale, geometry, particle size and air availability. Whilst some materials decompose at the molecular level at elevated temperatures, that is they don’t require oxygen to exotherm, others need it. This needs to be taken into account as part of the risk assessment as does the fact that with very finely divided powders, the surface area exposed for reaction with air, can be huge and can lead, in extreme cases, to pyrophoric behaviour .
This means that selection of the correct test method for determining maximum safe handling temperatures is critical. In many cases in industry, companies blindly use Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA) methods for establishing thermal stability limits. However, these sealed test methods are normally inadequate to detect oxidation processes and massively overstate safe processing temperatures. Alternative test methods that are specific to powders, that take into consideration scale and also air availability are available to overcome these small scale test limitation and to provide sound and conservative data for any processing condition.
How Chilworth Can Help
Chilworth has many years of experience in evaluating thermal instability properties for a wide range of materials as well as being able to provide expert consultancy advice for the selection of the most appropriate test and to provide assistance and support for companies wishing to prevent hazardous thermal decomposition in an industrial situation.
The range of tests offered by Chilworth for thermal stability determinations cover two distinct phenomena; those materials which do not require oxygen for decomposition (Chemical Decomposition) and those materials which do require oxygen (Thermal Oxidation):
- Differential Scanning Calorimetry (DSC)
- Differential Thermal Analysis (Carius Tube)
- Adiabatic calorimetry (using the Accelerating Rate Calorimeter (ARC) or ADC II Calorimeter system)
- CLP and UN test methods for classification of self-reactive substances (UN Class 4, Division 4.1) including test methods H.2 (Adiabatic Storage Test) and H.4 (Heat
Accumulation Storage Test)
- Bulk Powder (Diffusion Cell) for examining decomposition in
Bulk dryers (spray, tray, flash dryers)
- Basket test series used to extrapolate thermal limits for very large scale storage facilities.
- Aerated Cell Test for examining powders in aerated environments such as fluid bed dryers or rotating drum driers
- Air Over Layer test for dryers where thin layers or deposits are found or to examine the thermal stability of powder layers in duct work
Independent Professional Advice
It doesn’t just stop with measuring a parameter or identifying a flammability characteristic of powder, it is what that data means in the context of a manufacturing process. Our Process Safety Specialists will interpret the data and provide independent consulting advice on fire prevention and guidance on choice and design of explosion protection systems, be it relief venting, containment, suppression, use of inert gas, or isolation techniques.
Because of our unique capabilities and experience, our laboratories in the UK are not only able to undertake a wide range of tests to internationally recognised standards, but we can also undertake custom projects for testing of materials that may sit more closely with real world activities.
We are here to support all your laboratory testing needs, contact us today for a demonstration of our industry respected, GLP accredited service.