Many solids (powders) are capable of undergoing an
exothermic decomposition when exposed to elevated 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 very 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 chemical 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 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.
Chilworth Global 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.
HOW CHILWORTH CAN HELP
Laboratory testing (in GLP compliant facilities) to establish
thermal instability properties for powders and also liquids.
Test techniques for determination of thermal instability for
materials which undergo chemical 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)
- CLP and UN standard tests 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)
Determination of thermal instability properties such as onset
temperature for decomposition, safe storage temperature and maximum
duration time for safe storage for powders which undergo thermal
oxidation including tests for evaluating the thermal instability of
powders in:
- Bulk driers (spray, tray, flash driers) using the Bulk Powder
(Diffusion Cell) test and also the Basket test series which can be
used to extrapolate thermal limits for very large scale storage
facilities). The basket test approach can also be used under CLP
and transportation regulations for definition of self-heating
substances.
- Aerated environments such as fluid bed dryers or rotating drum
driers, using the Aerated Cell Test.
- Driers where thin layers or deposits are found such as smaller
tray driers, some spray driers or flash driers using the Air
Over Layer test approach
Chilworth Global can also assist by
providing:
- Independent Consultancy advice to carry out hazard and risk
assessments and auditing of thermal instability hazards.
- Incident investigations to establish the cause of thermal
instability incidents and how to prevent recurrence.
- Training courses to improve awareness of process hazards and
how to prevent thermal runaways.
Transportation Testing (including UN)
-
Burning Rate Test - DOT/UN Division 4.1
The Burning Rate or Fire Train Test is used to classify
materials as "flammable solids" in accordance with U.S. Department
of Transportation (DOT) and United Nations (UN) requirements.
Chilworth Global performs the Burning Rate Test in accordance with
the method described in 49 CFR 173 Appendix E and UN
Recommendations on the Transport of Dangerous Goods. The test
involves igniting a 250 mm x 20 mm x 10 mm powder train using a
suitable ignition source and measuring the burning rate (time).
-
Pyrophoric Solids Test - DOT/UN Division
4.2
The Pyrophoric Solids Test is used to classify solids materials
as "spontaneously combustible" in accordance with U.S. Department
of Transportation (DOT) and United Nations (UN) requirements.
Chilworth Global performs the Pyrophoric Solids Test in accordance
with the method described in 49 CFR 173 Appendix E and UN
Recommendations on the Transport of Dangerous Goods. The test
involves pouring a powder sample from a height of one meter onto a
non-flammable surface and observing whether the sample ignites
during its descent or after settling.
-
Self-Heating Substances Test - DOT/UN Division
4.2
The Self-Heating Substances Test is the second of two tests used
to classify materials as "spontaneously combustible" in accordance
with U.S. Department of Transportation (DOT) and United Nations
(UN) requirements. Chilworth Global performs the Self-Heating
Substances Test in accordance with the method described in 49 CFR
173 Appendix E and UN Recommendations on the Transport of Dangerous
Goods.
The Self-Heating Substances Test involves placing a powder
sample in a 100 mm x 100 mm x 100 mm stainless steel cubic mesh
basket and placing the basket in an oven where it is exposed to a
constant temperature of 140 °C for 24 hours. If necessary, a second
trial is performed using a 25 mm x 25 mm x 25 mm mesh basket. The
sample and oven temperatures are continuously monitored at several
locations for the duration of the test.
-
Dangerous When Wet Test - DOT/UN Division
4.3
The Dangerous When Wet Test is used to identify materials that
-- upon contact with water or moisture -- are liable to become
spontaneously flammable or evolve a flammable or toxic gas, as per
U.S. Department of Transportation (DOT) and United Nations (UN)
requirements. Chilworth Global performs the Dangerous When Wet Test
in accordance with the method described in 49 CFR 173 Appendix E
and UN Recommendations on the Transport of Dangerous Goods.
The Dangerous When Wet Test is comprised of four (4) parts, each
of which involves exposing a small amount of solid sample to a
limited amount of water. The samples are observed to determine
whether gas is evolved and whether such gas is flammable. The
volume of gas evolved is measured to determine the gas evolution
rate.
-
Solid Oxidizing Substances Test - DOT/UN
Division 5.1
The Solid Oxidizing Substances Test is used to classify
materials as such in accordance with U.S. Department of
Transportation (DOT) and United Nations (UN) requirements.
Chilworth Global performs the Solid Oxidizing Substances Test in
accordance with the method described in 49 CFR 173 Appendix F and
UN Recommendations on the Transport of Dangerous Goods. The test
involves mixing powder samples with sawdust in various proportions
and observing the combustion characteristics when a conical pile of
the sample/sawdust mixture is ignited by a hot loop ignition
source.
-
Oxidizing Liquids Test - UN/DOT Division
5.1
The Oxidizing Liquids Test is used to classify materials in
accordance with U.S. Department of Transportation (DOT) and United
Nations (UN) requirements. Chilworth Global performs the Oxidizing
Liquids Test in accordance with the method described in 49 CFR 173
Appendix F and the UN Recommendations on the Transport of Dangerous
Goods using the Time/Pressure Test Apparatus.
The test involves mixing a 2.5 gram sample of the suspected
liquid oxidizer with an equivalent sample of dried fibrous
cellulose. The mixture is then loaded into the test cell and
ignited using a fuse wire. A pressure transducer fitted to the test
cell is used to measure the maximum deflagration pressure and
maximum rate of pressure rise. The results for the sample and
mixture are then compared to results for testing of mixtures of
cellulose and known oxidizers to determine the classification and
packing group.
