RUN AWAY REACTION β PRELIMINARY SCREENING & ASSESSMENT
Assessment of runaway reaction hazard using the experimental methods of the process safety laboratory
Characteristics of experimental methods
The experimental methods of the process safety laboratory in the field of thermal stability and runaway reaction hazard studies consist mostly of calorimetric determinations. The relevant methods can be divided into several types according to current laboratory practices or the type of apparatus used. Screening and other methods A screening technique is an experimental technique used for testing every sample submitted for experimental investigation. Differential thermal analysis (DTA) or differential screening calorimetry (DSC) is widespread as a screening method for testing thermal stability. Whether the accelerating rate calorimeter (ARC) or the vent-sizing package (VSP) are screening methods is more a question of laboratory practice. Any calorimetric technique, even the most unexpected one, can be used as a screening technique. Isothermal, pseudo-adiabatic and isoperibolic methods Isothermal calorimetric techniques are techniques where the apparatus imposes the sample temperature. This is best achieved in DTA experiments where the sample is small enough to exclude any significant temperature gradient. The operating mode may be a temperature scan or a constant temperature exposure. Typical isothermal techniques are DTA/DSC and C80 isothermal calorimetry. Pseudo-adiabatic techniques are those where the temperature of the test cell surroundings is controlled to be equal to the sample temperature, to exclude any heat loss from the test cell. In these techniques, the heat produced by the sample is spent to raise its own temperature and the temperature of the test cell. The experiment thermal inertia is characterized by the phi factor.
π= π―πππ ππππππππ ππ πππ ππππππ πππ ππππ πππππ―πππ ππππππππ ππ πππ ππππππ πππππ
In true adiabatic conditions for the sample, Ο = 1. In real experimental conditions, Ο > 1. If heat is transferred from the outside to the sample, Ο < 1. Typical pseudo-adiabatic techniques are:
Γ The closed Dewar
Γ Accelerating rate calorimetry (ARC)
Γ The vent-sizing package (VSP)
Γ The reactive system screening tool (RSST)
Determination of the pressure effect in addition to the thermal effect of runaway reactions, the pressure effect is of importance in hazard assessment. This information is obtained from:
Γ Autoclave experiments
Γ Closed Dewar experiments
Γ ARC experiments
Γ VSP experiments
Information on loss of weight during thermal decomposition Thermogravimetry is used to determine the loss of weight of a sample submitted to a temperature scan. Thermogravimetry can be coupled with DSC or DTA. This technique is used mainly in the field of explosives and, solid propellants testing. A normal chemical company should not make extensive use of it.
Nature of the experimental information required
The experimental data required in thermal stability screening are:
Γ The exotherm onset temperature
Γ The heat of reaction per weight of reacting sample for the exotherm considered
Γ The onset temperature for the production of decomposition gases
Γ The reaction kinetics for heat and gas production
Interpretation of experimental results
The principle of interpretation methods is described in this section using examples obtained in the laboratory. As pointed out earlier, the experimental data interpretation methods depend on:
Γ The experimental technique
Γ The type of apparatus
Γ The information required
Γ The experimental results obtained
Conclusions
The control of runaway reaction hazard in the chemical industry involves simple steps including:
Γ Screening of thermal stability
Γ Analysis of the process conditions
Γ Recognition of dangerous process conditions or process deviations
Γ Application of prevention and mitigation measures
The industrial situations are often complex, nevertheless the above methodology based on our own experience can be successfully applied to exclude the occurrence of accidents. The only limitation seems to be the extensive work necessary to obtain the required information for a safe and relevant risk assessment. This is the cost of process safety. Experience has proved that this cost is lower than the cost of accidents and that the companies with the lowest accident rates are the most profitable.
At Sigma-HSE, we provide a range of calorimetric solutions to aid you in identifying chemical reaction hazards. Our calorimetric solutions provide you with actionable insights to protect business, people, workplace, and the environment against potential deviations to your processes.
