Many substances are capable of exothermic reaction when exposed to elevated temperatures.
In materials handling operations, it is crucial that the conditions required to initiate this activity are understood such that these conditions can be avoided by the clear specification of maximum safe processing or storage temperatures.
Thermal instability hazards can be manifested in a range of ways including "black bits" in product after drying, yield decrease after prolonged reaction or drying times, propensity for smouldering (often leading to fire), etc.
The definition of a maximum safe temperature is complicated by the impact of a number of variables such as scale, geometry, particle size and air availability. Whilst some materials decompose ("fall apart") at the molecular level at elevated temperatures, others (such as milk powder, coal dust, citrus peel and dried sewage sludge) undergo exothermic reaction with oxygen in air (ie. burning). For finely divided powders, the surface area exposed for reaction with air can be huge and can lead, in extreme cases, to pyrophoric behaviour (ie. near-spontaneous combustion).
Determining whether exothermic behaviour at elevated temperature is due to pure decomposition or oxidation is absolutely crucial in selecting an appropriate test to measure the initiation conditions. 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 inadequate to detect oxidation processes and massively overstate safe processing temperatures. Alternative test methods specific for powders in air available environments are available to overcome this limitation and provide sound and conservative data for any processing condition.
DEKRA has many years of experience in evaluating the properties of thermally unstable substances and providing consultancy advice and support to help operating companies prevent thermal decomposition processes in industrial situations.
How DEKRA Can Help
- Laboratory testing (in GLP compliant facilities) to establish thermal stability properties of liquids, solids and mixtures
- Determination of thermal stability for materials which undergo thermal decomposition
- Differential Scanning Calorimetry (DSC)
- Differential thermal analysis (Carius Tube)
- Adiabatic calorimetry (using the Accelerating Rate Calorimeter (ARC) or low phi factor Adiabatic Calorimeter system)
- UN standard tests for classification of self-reactive substances (UN Class 4.1) including test methods H.2 (Adiabatic Storage Test) and H.4 (Heat Accumulation Storage Test)
- Determination of thermal stability for powders which undergo thermal oxidation including tests for evaluating the stability of powders in:
- Bulk driers and large scale storage facilities (using the Diffusion cell thermal stability test and the Basket test series which can be used to extrapolate thermal limits for very large scale storage facilities)
- Aerated environments such as spray driers, fluid bed dryers (using the Aerated cell thermal stability test) layers such as tray driers (using the Air Over Layer thermal stability test)
- UN standard tests for classification of self-heating substances (UN Class 4.2)
- Independent Consultancy advice to carry out hazard and risk assessments and auditing of thermal stability hazards.
- Incident investigations to establish the cause of thermal stability incidents and how to prevent recurrence.
- Training courses to improve awareness of process hazards and how to prevent thermal runaways.