|Vahid Ebadat, Chilworth North America|
While dust explosions grab the lion’s share of the network TV news because of their utter devastation and loss of life, an insidious calamity could be heating up in your facility, just waiting to put your plant on TV and out of business. Self-heating of materials—somewhat analogous to the spontaneous combustion of stored hay or grain—can start a fire that could even set off a dust explosion if the conditions are right in your plant. If your plant has had a “near-miss,” it’s time to investigate further and mitigate before a breaking-news event happens at your facility.
FE asked Vahid Ebadat, CEO of Chilworth North America, to explain the dangers of self-heating. Ebadat has worked as a process and operational hazards consultant for the chemical, pharmaceutical and food industries, and is a regular speaker at training courses on gas and vapor flammability, dust explosions and controlling electrostatic hazards. He is a member of the NFPA 77 Technical Committee on Static Electricity, NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particular Solids, and ASTM E27 Committee on Hazard Potential of Chemicals.
FE: Are food manufacturers aware of self-heating hazards in their plants?
Vahid Ebadat: Whenever self-heating incidents are investigated, we find that a common root cause is a lack of understanding of the self-heating phenomenon. Many particulate solid materials can exhibit self-heating, which—if unchecked—may affect the quality of the product and progress to a fire or an explosion.
FE: How do food processors prevent self-heating during processing of dry materials such as sugar or flour?
Ebadat: To prevent self-heating, they need to know under what conditions it could occur, so controls can be put in place to avoid those conditions. The self-heating hazard of solid materials that are subjected to heat during drying operations can be determined by conducting appropriate laboratory test(s). The test(s) will be selected based on the type of heating/drying process the solid material undergoes, for example, tray drying or fluidized-bed drying. The test results can then be used to determine safe heating/drying temperatures and durations, using sufficient safety margins.
FE: How does self-heating arise during drying operations?
Ebadat: When a material undergoes exothermic chemical reaction(s) or decomposes exothermically, the temperature of the material will rise if the rate of heat generation exceeds the rate of heat loss to the environment. Further, the temperature rise of the material due to the exothermic reaction will exponentially increase the chemical reaction rate, resulting in a faster increase in temperature. This unstable process is referred to as self-heating. The temperature at which the rate of heat generation is greater than the rate of heat loss is called the exothermic onset temperature.
In an exothermic (heat-releasing) chemical reaction, the chemical reaction(s) are often an oxidation reaction with air, similar to what occurs during a fire or explosion. At the start of the self-heating process, the reaction is very slow, like steel that oxidizes (corrodes) with atmospheric oxygen to form rust.
FE: How is an exothermic reaction different from exothermic decomposition?
Ebadat: For unstable materials, decomposition results in less complex molecules and sometimes gases, while releasing heat. However, unlike an exothermic reaction between a product and oxygen (air), decomposition does not require additional reactants and is, therefore, largely independent of the environment making it simpler to predict its occurrence without detailed experimental studies. In fact, some materials can self-heat at ambient temperatures and spontaneously ignite in large-scale storage such as grain.
Self-heating of solids and powders may result in smoldering which can set the material on fire or cause dust explosions, particularly when a “smoldering nest” is disturbed and exposed to air. Many plants that have experienced self-heating incidents have had a history of “near misses” where some self-heating occurs but does not progress to full-blown ignition. In such cases, there may be a hot batch, black spots in an otherwise light-colored product, or a lump of charred product found within. It is important to recognize such occurrences as indications of a potentially serious problem, and not just a “near-miss.”
Self-heating reactions may also produce flammable gases, which may lead to gas/air explosions or pressure/volume explosions in closed process vessels, and also compromise product quality.
FE: How do you determine the exothermic onset temperature for self-heating of a particular process?
Ebadat: The exothermic onset temperature for self-heating is influenced by the chemical and physical properties of a substance such as chemical-reaction kinetics, thermal conductivity and heat of reaction, as well as by other factors, including:
- Dimensions and shape of the material—solid or powder
- Ambient airflow over the material
- Availability of oxygen within the bulk, or porosity, and
- Additives or contaminants.
Usually, the material has to be exposed to a temperature near the onset temperature for an induction time, which is reduced with an increase in temperature.
FE: Are there any tests that can be done to simulate self-heating behavior?
Ebadat: Several laboratory tests have been developed to simulate the conditions where a powder could be heated above the onset temperature: bulk form, layer form (with air flowing over the powder) and aerated form, where air is passing through the bulk of the product, which increases the oxygen availability for the reaction, but also removes heat from the reacting material.
For large-scale storage situations, tests are carried out on different scales so the effect of the size of the bulk material can be assessed. All tests are carried out in temperature-controlled ovens that allow screening tests (with the temperature ramped up at a defined rate) and isothermal testing (with a constant temperature controlled within narrow margins). Because of the potential for violent reactions during the self-heating process, all equipment has explosion protection.
About Vahid Ebadat
Vahid Ebadat, CEO of Chilworth North America, is a member of the NFPA 77 Technical Committee on Static Electricity, NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particular Solids, and ASTM E27 Committee on Hazard Potential of Chemicals.
Chilworth Global provides expert consulting services relating to process safety, and has ISO 17025- and GLP-accredited dust explosion, gas and vapor, flammability, and electrostatic testing laboratories and specialist facilities for the evaluation of thermal runaway reactions and energetic materials.